DE102019131696A1 - Method and device for the multi-dimensional direction measurement of gamma radiation in the far field - Google Patents

Abstract

The invention relates to a method and a device for the multi-dimensional direction measurement of gamma radiation in the far field by means of a group of several energy-discriminating, mutually synchronized detectors for detecting radiation. Measured values are acquired in list mode, selected according to coincidence events in defined detector pairs and the coincidence events are associated with an identification number ID. A defined function value f (E1, E2) is then calculated from two coincident energy values E1, E2 per selected coincidence event and data is recorded in one or more frequency distributions Y, with a separate frequency distribution Y being available for each radionuclide. From this, one or more directional distributions X are calculated using a statistical image reconstruction method of emission tomography.

Description

The invention relates to a method and a device for radiation detection, in particular a direction-measuring detector system for measuring the directional distribution of the radiation intensity. The method supports the 3-dimensional as well as the 2-dimensional direction measurement of one and several radiation sources in the far field.

In NBC and radiation protection there is a demand for handy, compact radiation measuring devices that can provide directional information. In contrast to imaging in medicine, where activity concentrations are displayed, direction measurement here means determining the directions of one or more radiation sources in order to help locate the radiation sources.

Compact gamma spectrometers and nuclide identification devices are state of the art. However, such devices cannot determine the angle of incidence of the radiation.

It is currently only possible to locate radiation sources in the area if emergency services walk or drive across the entire area with a radiation measuring device in order to create an intensity distribution. The procedure is very complex and cannot always be used if, for example, radiation sources are to be located in inaccessible terrain.

The method according to the invention applies to compact, directionally resolving radiation measuring devices which are becoming increasingly popular. It can be used for composite systems consisting of several radiation detectors and for segmented radiation detectors.

Complex devices composed of many individual detectors, which are used for medical imaging, are known from the clinical environment. These devices are complex and require complex algorithms for data evaluation, which require considerable computing capacity. The algorithms calculate the activity concentrations of a radiopharmaceutical in a patient in the immediate vicinity of the radiation detectors (near field). In principle, these algorithms can also be used for the compact, directionally resolving devices mentioned, provided they are adapted to the operating conditions in NBC and radiation protection.

To do this, the method must be switched from the near field to the far field. The computing effort must be reduced to such an extent that data processing is possible in real time parallel to the data acquisition, so that the results are available live during the measurement. There are also differences in terms of radiation intensity. When detecting radioactive radiation sources, the radiation intensities registered at the detection location are often far below those commonly used in medicine. This places increased demands on the algorithms, which should deliver reliable and reproducible results despite statistical restrictions on the data sets. The direction measurement should function reliably even if the radiation intensity of the sources to be measured is below the level of the natural radiation background. It is therefore necessary that the natural radiation background is taken into account in the evaluation.

For the localization of radiation sources in the field, fast methods are required that reconstruct a radiation far field, can also be used with low meter statistics and take the influence of natural radiation into account.

The US 2012/0043467 describes a method for single plane Compton cameras that is suitable for measuring the direction of a radiation source. However, this method has the disadvantage that it is limited to the direction measurement of one radiation source per radionuclide and cannot measure directional distribution.

The invention therefore aims to overcome the disadvantages of the methods from the prior art. In particular, reliable and efficient localization of several radiation sources should be provided without restriction to the number of radiation sources.

Another property of the invention is that the method is not restricted to special detection systems, but can be used universally for a large number of devices and systems which are composed, for example, of several segments or detectors.

The objects are achieved by the method according to independent claim 1 and a device according to independent claim 14. Advantageous developments and preferred designs are specified in the subclaims.

1. a) Akquirieren von Messwerten im List-Mode, wobei die Messwerte einer Strahlungsverteilung im Fernfeld entstammen, die Strahlung von einem oder mehreren Radionukliden ausgeht und die Messwerte die in den Detektoren auftretenden Wechselwirkungsenergien der Strahlung sind;
2. b) Selektieren der Messwerte nach Koinzidenzereignissen in definierten Detektorpaaren aus je zwei Detektoren;
3. c) Assoziieren von selektierten Koinzidenzereignissen mit einer Identifikationsnummer ID;
4. d) Berechnen des definierten Funktionswertes f(E1,E2) aus zwei koinzidenten Energiewerten E1, E2 pro selektiertem Koinzidenzereignis;
5. e) Erfassen der selektierten Koinzidenzereignisse entsprechend ihrer Identifikationsnummer ID und ihrer Funktionswerte f(E1,E2) in einer oder mehreren Häufigkeitsverteilungen Y, wobei für jedes Radionuklid eine separate Häufigkeitsverteilung Y vorliegt,
6. f) Berechnen einer oder mehrerer Richtungsverteilungen X aus den Häufigkeitsverteilungen Y mit einem statistischen Bildrekonstruktionsverfahren der Emissionstomographie unter Nutzung von Lookup-Tabellen LUT^SK, wobei für jedes Radionuklid eine separate Richtungsverteilung X vorliegt.

The method according to the invention for the multi-dimensional direction measurement of gamma radiation in the far field by means of a group of several energy-discriminating, mutually synchronized detectors for detecting radiation is characterized in that the method lookup tables LUT ^SK , a defined function value f (E1, E2), a list of defined detector pairs with an identification number ID for defined detector pairs and one or more frequency distributions Y are used to record the measured values. The method according to the invention is described in more detail by the following steps:

1. a) Acquiring measured values in list mode, the measured values originating from a radiation distribution in the far field, the radiation emanating from one or more radionuclides and the measured values being the interaction energies of the radiation occurring in the detectors;
2. b) selecting the measured values according to coincidence events in defined detector pairs from two detectors each;
3. c) associating selected coincidence events with an identification number ID;
4. d) calculating the defined function value f (E1, E2) from two coincident energy values E1, E2 per selected coincidence event;
5. e) recording the selected coincidence events according to their identification number ID and their function values f (E1, E2) in one or more frequency distributions Y, with a separate frequency distribution Y being available for each radionuclide,
6. f) Calculating one or more directional distributions X from the frequency distributions Y with a statistical image reconstruction method of emission tomography using lookup tables LUT ^SK , a separate directional distribution X being available for each radionuclide.

The method according to the invention for measuring the direction of a radiation distribution in the far field applies equally to composite systems comprising a plurality of radiation detectors and to segmented radiation detectors. In one case a group of several radiation detectors is interconnected, in the other case the active detector medium is divided into several units. The group of radiation detectors or the segmented individual detector each form a time-synchronized system. Each detector or each segment can measure the energy of the radiation which is deposited there through interaction and can be synchronized with other detectors or segments. In the context of the invention, a detector is understood to be a radiation detector or a detector segment which detects radiation and can be read out individually.

It is assumed that the measuring system is equipped with data acquisition in list mode and can process the data of each detector or each segment.

The method according to the invention is particularly suitable for stationary or quasi-stationary direction measurement, with mainly two versions, namely the 3-dimensional direction measurement and the 2-dimensional direction measurement, being of particular advantage.

The 2-dimensional direction measurement is a special case of direction measurement in which all radiation sources lie in one plane, for example in the horizontal plane. Groups of planar radiation detectors represent device types suitable for 2-dimensional direction measurement. The device is oriented in such a way that the detection plane is identical to the plane of the radiation sources.

The method according to the invention for direction measurement is also called directionally resolved coincidence spectroscopy. In coincidence spectroscopy, coincidences are investigated in which interactions occur simultaneously in two detectors (or segments) of the measuring device. Pairs of two detectors each are the imaging basic elements of the direction measurement. The energy inputs E1 and E2 registered in a coincident detector pair are considered together with the spatial orientation of the detector pair. What matters here is the relationship between the measured energy values E1 and E2 and the directions in which the coincidences are observed.

First of all, a suitable coordinate system must be defined that applies to both the radiation sources and the radiation detectors. With the 3-dimensional direction measurement this is a Spherical coordinate system, the origin of the coordinates being in the spatial center of the detector arrangement. In the 2-dimensional case, a polar coordinate system is used.

Directional coincidence spectroscopy makes use of two interaction processes of gamma radiation: Compton scattering and photoelectric absorption.

The measuring device is seen as a combination of detector pairs. First, some definitions should be made for the detector pairs.

Those pairs of detectors are selected that are to be used for the direction measurement. When making a selection, it is helpful to first consider the materials from which the detectors are constructed. It is sufficient to divide the materials into two classes. A group is made up of materials with a medium to high atomic number. In the context of the invention, a number Z _eff of greater than 30 is understood by a medium to high atomic number. If the detector materials are present as chemical compounds, Z _{eff is} understood to mean the effective atomic number, ie the mean atomic number of all elements contained in the compound, taking into account the atomic mass of the elements and their stoichiometric composition.

The detector materials with a medium to high atomic number Z _eff > 30 include, for example, Nal, a material frequently used in scintillation detectors, and also rare earth halides such as CeBr ₃ , LaBr ₃ , LaCl ₃ and La (Br _x Cl _1-x ) ₃ . Among the semiconductor materials with a medium to high atomic number, Ge, GaAs, CdTe and CdZnTe should be mentioned in particular. These materials are characterized by their high probability of photoelectric absorption in the energy range from a few 10 keV to 3 MeV.

The other group includes detectors made of materials with a low atomic number, whose atomic number Z _{eff is} less than or equal to 30. These include plastic scintillators, organic crystals such as anthracene, stilbene or p-terphenyl, inorganic crystals such as CaF ₂ and the semiconductor material silicon. The materials with a low atomic number Z _eff ≤ 30 have a high probability of Compton scattering in the energy range from 100 keV to 3 MeV.

From these two groups of detectors, two types of detector pairs can be put together, which have to be examined with regard to the direction of the incident and the scattered radiation.

A unidirectional pair of detectors contains both a detector from the group with a low atomic number and a detector from the group with a medium to high atomic number. If, for example, a plastic detector and a cerium bromide detector are combined, this is a unidirectional pair of detectors. In a unidirectional pair of detectors, each detector has a unique physical function. If a coincidence event with the radiation energy E1 + E2 belonging to the nuclide is observed, it can be determined without a doubt that the incident radiation was first scattered in the plastic detector before it was absorbed in the cerium bromide detector. The opposite case, that the radiation was first scattered in the cerium bromide detector before it was absorbed in the plastic detector, has a probability close to zero.

In a unidirectional pair of detectors, the direction of the scattered radiation is known. The radiation is scattered in the detector with a low atomic number in the direction of the detector with a high atomic number.

Bidirectional detector pairs are created from the combination of two detectors each from the group with medium and high atomic number, e.g. cerium bromide with cerium bromide or germanium with germanium. When using segmented semiconductor detectors, pairs of two segments each are also bidirectional, since both segments are made of the same material.

In a bidirectional pair, if no further information is available, it cannot be determined which of the two detectors first scattered the incident radiation. The direction of the scattered radiation remains unknown. The radiation can have been scattered from detector 1 to detector 2, but also from detector 2 to detector 1.

It should be noted that both unidirectional and bidirectional detector pairs have a restriction in determining the direction of the incident radiation. The direction of the incident radiation cannot be uniquely reconstructed with a single coincidence event. For the 2-dimensional case, the direction of incidence is set on a V-line, for the 3-dimensional case on a cone surface. The Measurement data of a single event do not allow any statement on which arm of the V-line or in which direction on the surface of the cone the radiation source is located.

With regard to the direction reconstruction of the incident radiation, unidirectional and bidirectional detector pairs provide equivalent information. In both cases, the measurement data from many coincidence events have the statistical properties required for direction measurement. The fact that the direction of the scattered radiation is unknown in a bidirectional pair of detectors does not represent any experimental restriction with regard to the determination of the direction of the incident radiation.

As will be explained in more detail below, the technology of directionally resolved coincidence spectroscopy has as an essential distinguishing feature that it can be used for both unidirectional and bidirectional pairs of detectors.

The detectors of a device can be divided several times into different pairs. The number of possible combinations is again related to the detector materials. Detectors from the group with a low atomic number can be combined with any detector from the other group with a medium or high atomic number. Detectors from the medium or high atomic number group can even be paired with any other detector in the device.

However, the group of detectors with a low atomic number is not suitable for putting together pairs within the group.

A measurement process can use a combination of unidirectional and bidirectional detector pairs or use exclusively unidirectional or exclusively bidirectional pairs for the direction measurement.

Many of the radiation detectors in use do not provide any information about the point of interaction of the radiation in the detector material. It is therefore necessary to make an assumption about the connecting line which connects the two points of interaction with one another in a detector pair. An obvious assumption is to choose the line that connects both center points of the active detector media (the scintillator or the semiconductor). For each detector pair there is then a connecting line which runs through the two detector centers.

The line connecting a pair of detectors is regarded as a directed quantity. A vector is assigned to the line. The vector is defined in that each detector in a pair is uniquely assigned a number 1 or 2. If a pair contains different detector materials, the detectors of the material with the lower atomic number are designated as 1, the detectors of the material with the higher atomic number than 2. If the pairs consist of detectors of the same material, the decision as to which detector in a pair is designated as 1 or 2, can be taken arbitrarily, provided that it is consistently maintained in the data evaluation.

The vector has its place of origin in the center of detector 2 and points to the center of detector 1.

The spatial directions of the detector pairs are now to be determined. It is assumed that radiation is incident on the measuring device in parallel (far field). The incidence of radiation is approximately parallel if the distances between all radiation sources are much larger than the dimensions of the measuring device.

Imagine that the coordinate system can be shifted axially parallel to the Cartesian coordinate directions in space. For each pair of detectors, the coordinate system can be shifted to the center of detector 2.

The three-dimensional case is considered first: the spherical coordinates of detector 1 in this shifted coordinate system, the origin of which is in detector 2, define a characteristic azimuth and elevation angle for this pair of detectors. Each detector pair i thus has such a characteristic azimuth angle φ _i and elevation angle β _i . _{Both angles φ i} and β _i associated with the detector pair are stored in the software. They are used in the evaluation process.

In the two-dimensional case, it is a polar coordinate system that is shifted into the center of detector 2 for each detector pair i, axially parallel to the Cartesian coordinate directions. The Azimuth angle φ _i of detector 1 in this shifted coordinate system is stored in software for each detector pair i.

It is advantageous that every radiation detector of a measuring device is calibrated for the energy measurement. For radiation detectors from the group with a medium to high atomic number, the photoelectric effect can be used for energy calibration. For such detectors, the energy calibration is carried out on the basis of the photo peaks in the pulse height spectrum. The procedure is well known to the experts in the field of radiation detection and will not be discussed further here.

aus der Strahlungsenergie E_y des jeweiligen Radionuklids, mit mc² = 511 keV, der Ruhenergie des Elektrons. Die Position der Compton-Kante lässt sich experimentell bestimmen, indem die erste Ableitung des Pulshöhen-Spektrums gebildet wird. Die Minimalstelle in der ersten Ableitung des Pulshöhen-Spektrums liefert in guter Näherung die Compton-Kante. Die Pulshöhe an der Minimalstelle kann dann gemäß Gl. (1) als Energie kalibriert werden.For detectors from the group with a low atomic number, however, energy measurement is less common, as these are often operated in pulse counting mode. Several calibration options are available for detectors with Z _eff ≤ 30. The pulse height spectra of these detectors can be examined for the presence of photo peaks. If photo peaks can be detected, they can be used for energy calibration. If no photo peaks can be detected, the end point of the Compton energy spectrum represents a suitable alternative. The maximum energy E _{C, max} that can be observed in Compton scattering events, which is also referred to as the Compton edge, results from $${\mathrm{E.}}_{\mathrm{C.},max}=\frac{{\mathrm{E.}}_{\gamma }}{1+\frac{\mathrm{<figure-callout\; id="2"\; label="m\; c"\; filenames="DE102019131696A1\_0045.png,DE102019131696A1\_0046.png"\; state="\{\{state\}\}">m</figure-callout>}{c}^{}{}^2}{2{\mathrm{E.}}_{\gamma }}}$$

from the radiation energy E _{y of} the respective radionuclide, with mc ² = 511 keV, the rest energy of the electron. The position of the Compton edge can be determined experimentally by taking the first derivative of the pulse height spectrum. The minimum point in the first derivative of the pulse height spectrum provides the Compton edge as a good approximation. The pulse height at the minimum point can then be calculated according to Eq. (1) to be calibrated as energy.

In the following it is assumed that all radiation detectors are calibrated for energy measurement. In the measurement process, coincidence events are acquired which, according to their energy values E1 and E2 and the spatial orientation of the detector pair, form a triple. The spatial orientation is mapped via an identification number (ID) of the detector pair. The two energy values E1 and E2 contain the information that firstly identifies the nuclide whose radiation is being observed, and secondly also measures the direction from which the radiation reaches the detector pair. Due to the conservation of energy, the sum of the registered energy inputs E1 + E2 is set to the radiant energy of the nuclide. Both measured values E1 and E2 are strongly correlated with one another and the direction information is mainly contained in the difference E2-E1. It is therefore possible to reduce the number of variables by defining a new measured value f (E1, E2) from the two energy values E1 and E2, which approximately represents the directional information. The measurement data (ID, E1, E2) initially organized as a triplet can thus be reduced to a double with the two features ID and f (E1, E2).

Since stationary or quasi-stationary measurement conditions are assumed, the measurement data can be grouped as a 2-dimensional frequency distribution Y. Each event is sorted into the frequency distribution Y according to its two characteristics, ID and f (E1, E2). From the observed frequency distribution Y, the directions of the radiation sources can finally be reconstructed using statistical methods.

berechnet werden. Die Asymmetrie A ist ein Beispiel für den Funktionswert f(E1, E2), wie die beiden Messwerte E1 und E2 zu einer neuen Größe A kombiniert werden können. Alternativ kann z.B. auch der Kosinus des Compton-Streuwinkels CCW $$CCW=1-\frac{m{c}^2}{E2}+\frac{m{c}^2}{E1+E2}$$

mit mc² = 511 keV, der Ruhenergie des Elektrons, berechnet werden.A histogram is created for each detector pair to record the measurement data. For the histograms, the value range of f (E1, E2) is to be divided into classes. If a coincidence event is registered, then from the two energy values E1 and E2 in the detectors 1 and 2, a suitable direction-dependent variable such as the asymmetry A $$\mathrm{A.}=\frac{\mathrm{E.}2-\mathrm{E.}1}{\mathrm{E.}1+\mathrm{E.}2}$$

be calculated. The asymmetry A is an example of the function value f (E1, E2), how the two measured values E1 and E2 can be combined to form a new variable A. Alternatively, for example, the cosine of the Compton scattering angle CCW $$\mathrm{C.}\mathrm{C.}\mathrm{W.}=1-\frac{\mathrm{<figure-callout\; id="2"\; label="m\; c"\; filenames="DE102019131696A1\_0045.png,DE102019131696A1\_0046.png"\; state="\{\{state\}\}">m</figure-callout>}{c}^{}{}^2}{\mathrm{E.}2}+\frac{\mathrm{<figure-callout\; id="2"\; label="m\; c"\; filenames="DE102019131696A1\_0045.png,DE102019131696A1\_0046.png"\; state="\{\{state\}\}">m</figure-callout>}{c}^{}{}^2}{\mathrm{E.}1+\mathrm{E.}2}$$

with mc ² = 511 keV, the rest energy of the electron.

According to its value for the direction-dependent variable A or CCW, a coincidence event is sorted into the histogram that belongs to the detector pair in which the coincidence occurred. The totality of the histograms for all detector pairs forms the 2-dimensional frequency distribution Y, which is continuously filled during a measurement.

When recording the 2-dimensional frequency distribution Y, it is advisable to select the coincidence events with a selection condition for the energy sum E1 + E2. Only those values of the direction-dependent quantity A or CCW should be sorted into the distribution Y whose energy sum E1 + E2 lies within a predefined range around the energy value which corresponds to the characteristic nuclide energy whose radiation is detected. With the help of such a selection condition it is possible to evaluate separate frequency tables for each nuclide.

If the directions of sources of a nuclide that emits radiation on several lines are measured, it is at the discretion of the user whether the coincidence events are to be recorded in one or more frequency distributions. For example, Co-60 has two closely spaced lines at 1173 keV and 1332 keV. Coincidence events of both emissions can be sorted into the same frequency distribution without any problems. Since both emission lines are so close to each other, the 2-dimensional frequency distributions are also very similar and can be evaluated together without difficulty. However, if a nuclide has several widely spaced gamma lines, it is advisable to record them separately. The frequency distributions belonging to a nuclide are later combined again in the evaluation process.

As a further component, the method according to the invention uses a system matrix that describes the response behavior of the device to a point radiation source. Since the computational effort of data processing scales with the size of the system matrix, it is helpful to use the symmetry properties of the measuring device in order to keep the system matrix as small as possible. For this purpose, the detector pairs of a measuring device are divided into symmetry classes.

Those detector pairs that are mapped onto other identically constructed detector pairs when they are rotated or shifted belong to a symmetry class. They are grouped together and all treated in the same way. The system matrix is defined as a group of lookup tables. Each lookup table describes, for one symmetry class, the response behavior of the detector pairs of this symmetry class to an infinitely distant, punctiform radiation source. The lookup tables can be obtained using a simulation calculation or from measurements with point-shaped reference radiators. For practical purposes, it is sufficient if the reference radiators are set up at a finite distance from the measuring device, provided that the distance is large compared to the device dimensions.

All emission points of the radiation field should be far enough away from the measuring device that the radiation falls on the device in parallel (far field).

According to the experimental situation, a distinction is made between two cases. In the 3-dimensional directional measurement, the directional distribution is modeled on the celestial sphere as a function Y (ω, h) that is dependent on two variables. The celestial sphere is a sphere used in astronomy and navigation to determine positions in the sky. It is an apparent sphere surrounding the measuring device on all sides with a radius of any size, onto which the radiation sources are projected. The function value X (ω, h) describes the radiation intensity in a solid angle segment dΩ on the celestial sphere in the azimuth direction ω at the elevation angle h.

In the 2-dimensional direction measurement, a function X (ω) dependent on a variable is considered along the horizon circle. The function value X (ω) describes the radiation intensity in the direction ω. It is assumed that the sources are in the same plane as the detectors of the measuring device.

The task of the algorithm is to determine the function X (ω, h) or X (ω).

There is a lookup table for each symmetry class and each radionuclide. The totality of all lookup tables forms the system matrix of the measuring device. The lookup tables represent the mathematical relationships between the radiation sources and the measurement data.

The elements of a lookup table are the mapping functions LUT _j [ϑ]. They represent the probabilities that a radiation source at an angle ϑ to the detector pair axis will cause coincidence events in Bin j of the measured value distribution f (E1, E2). The index j indicates the jth class of the measured value distribution f (E1, E2). The options for f (E1, E2) are e.g. B. the in Eq. (2) defined energy asymmetry A or the one in Eq. (3) Defined quantity CCW, the cosine of the Compton scattering angle, is available.

In the following, an embodiment of the invention is described how the lookup tables are generated experimentally with the aid of point-shaped reference radiators.

It is advantageous to carry out the measurements with several reference sources of different nuclides. The sources should be selected so that they represent the energy range in which the measuring device is used. The distance between the sources should be large enough to ensure parallel incidence of radiation. Ideally, the distance is chosen as large as possible, as the strength of the source allows, in order to obtain measurement data in reasonable acquisition times.

A series of several measurements at different angular positions is carried out for each source. It must be ensured that the source always has the same radial distance from the center of the device in all measurements. All measurements are carried out with the same measuring time t _k . The data acquisition should take place under the same conditions as they are intended for operational operation.

If the conditions of use of the measuring device provide for not only providing relative intensities but also absolute radiation intensities in units of measurement of e.g. pGy sr ^-1 s ^-1 or nSv sr ^-1 h ^-1 , it must be ensured that only calibrated reference sources are used.

In all other cases it is sufficient if the data of the lookup table are acquired with constant relative radiation intensity; an absolute calibration of the radiation intensity is then not necessary. Sources of any strength can then be used, provided that their activity is sufficient for good counter statistics.

After completing the measurements with the reference sources, the natural radiation background should be recorded in a further measurement without a source. The background measurement should be carried out with the same _{measurement duration t K} as the measurements with the reference sources before.

The procedure for obtaining the lookup tables from the measurement data is the same for all nuclides and is described here for one nuclide as an example.

It is initially assumed that all detector pairs are divided into symmetry classes. Each symmetry class has an identification number SK (i), which must be defined for each detector pair i.

A separate lookup table LUT ^{SK is} created for each symmetry class SK, which spans the angular range from 0 ° to 180 ° and is divided into equidistant angular steps.

The data recorded at a specific angular position are to be evaluated for each detector pair. Accordingly, the current position of the reference source ω at an azimuth angle and an elevation angle is an angular distance h _i θ i is calculated for each detector pair: $${\vartheta }_i=\text{arccos}\left(\text{cos}{\beta }_i\text{cos}H\text{cos}\left(\omega -{\phi }_i\right)+\text{sin}{\beta }_i\text{sin}H\right)$$

berechnet werden.In the above expression, φ _i denotes the azimuth angle and β _i the elevation angle of detector pair i. In the two-dimensional case, the angular distance ϑ _{i can be expressed} as $${\vartheta }_i=\text{arccos}\left(\text{cos}\left(\omega -{\phi }_i\right)\right)$$

be calculated.

The process is repeated for all angular positions of the reference source.

The measurement data are now sorted according to the affiliation of the detector pairs to symmetry classes in order to generate the lookup tables from them. For all pairs i of a symmetry class SK (i), the measured values are available at the angular distances ϑ _i . The aim is to fill the associated lookup table LUT ^SK with data for each symmetry class SK in the entire value range from ϑ = 0 ° to 180 °.

For each value ϑ from the range from ϑ = 0 ° to 180 °, the data record ϑ _{i of} a symmetry class SK that is closest to the current value ϑ is selected. This data record is copied to position ϑ in the lookup table. Proceed in this way until all positions ϑ from 0 ° to 180 ° in a lookup table are filled with data. The process must be carried out for all symmetry classes. The lookup tables of a measuring device are generated according to this procedure.

bezeichnet und gilt für alle Detektorpaare einer Symmetrieklasse SK.The measurement without a reference source is used to measure the natural radiation background. The natural radiation background can be assumed to be isotropic. All underground data sets belonging to the same symmetry class can be averaged. The averaged data set is with $b_j^{\mathrm{S.}K}$

denotes and applies to all detector pairs of a symmetry class SK.

der Untergrund $b_j^{SK}$

im Bin j der Messwertverteilung f(E1, E2) subtrahiert wird.The radiation background contained in the lookup tables can be deducted from each element of the lookup table $\mathrm{L.}U{T}_j^{\mathrm{S.}K}\left(\vartheta \right)$

the underground $b_j^{\mathrm{S.}K}$

in bin j of the measured value distribution f (E1, E2) is subtracted.

The device according to this invention is suitable for measuring nuclide-specific dose rates. To measure nuclide-specific dose rates, count rates of interaction events are used, the total energy of which corresponds to the respective nuclide energy. There are many options. On the one hand, the nuclide-specific count rates of all detectors of medium to high atomic number contained in the device can be used; on the other hand, count rates of coincidence events can also be used whose total energy E1 + E2 lies in a certain interval around the nuclide energy. These count rates can be calibrated in units of the equivalent dose rate or in units of the absorbed dose rate (in air). The procedure is briefly outlined here.

und die Äquivalenz-Dosisleistung Ḣ *(10) aus: $$\dot{H}*\left(10\right)=\frac{dH*\left(10\right)}{dt}=\frac{\wedge {\Gamma }_H}{r^2}$$

The calibration is carried out with a reference source of calibrated activity Λ. The dose rate Ḋ or the equivalent dose rate Ḣ * (10) at the center of the device can be calculated from the activity Λ of the source, its distance r and the nuclide-specific dose rate constants Γ or Γ _H. The (physical) dose rate Ḋ results from: $$\dot{\mathrm{D.}}=\frac{d\mathrm{D.}}{dT}=\frac{\wedge \mathrm{<figure-callout\; id="2"\; label="\Gamma \; r"\; filenames="DE102019131696A1\_0045.png,DE102019131696A1\_0046.png"\; state="\{\{state\}\}">\Gamma </figure-callout>}}{r^{}}$$

and the equivalent dose rate Ḣ * (10) from: $$\dot{H}*\left(10\right)=\frac{dH*\left(10\right)}{dt}=\frac{\wedge {\mathrm{<figure-callout\; id="2"\; label="\Gamma \; H\; r"\; filenames="DE102019131696A1\_0045.png,DE102019131696A1\_0046.png"\; state="\{\{state\}\}">\Gamma </figure-callout>}}_H}{r^{}}$$

The values of the dose rate constant Γ and the equivalent dose rate constant Γ _H for the various nuclides can be found in tables in the relevant specialist literature.

From the available nuclide-specific count rates of the device, a count rate or a combination of count rates must now be selected which is suitable for representing the dose rate while avoiding systematic errors. A possible cause of systematic errors when measuring the dose rate is an uneven response behavior of the device in relation to the direction of the incident radiation. The count rates can be impaired, for example, by shading effects. In particular, the coincidence counting rates are characterized by their strong directional dependence. The directional sensitivity of the coincidence events, which is advantageous for direction measurement, can be a significant cause of errors when measuring the dose rate.

Two embodiments of the invention are presented below, showing how a measurement of nuclide-specific dose rates with small systematic errors is possible.

kann ein gutes Maß für die Dosisleistung sein, wenn es in jeder Richtung mindestens einen Detektor hoher Ordnungszahl gibt, der der einfallenden Strahlung ungehindert ausgesetzt ist. _{For devices in which shading effects are negligible, it makes sense to consider the nuclide-specific counters N m} detected in the detectors m = 1, ..., M high atomic number in the photo peak. The count rate Z, defined as: $$\dot{Z}=\frac{d\left(\text{Max}\left({\mathrm{<figure-callout\; id="1"\; label="N"\; filenames="DE102019131696A1\_0045.png,DE102019131696A1\_0046.png"\; state="\{\{state\}\}">N</figure-callout>}}_1,...,N_{\mathrm{M.}}\right)\right)}{dt}$$

can be a good measure of the dose rate if there is at least one high atomic number detector in each direction that is exposed to the incident radiation unhindered.

richtungsunabhängig ist. Bei Vorhandensein vieler Detektorpaare mittelt sich die Richtungsabhängigkeit individueller Detektorpaare heraus; die Gesamtzählrate ist dann näherungsweise richtungsunabhängig.An alternative approach is available for devices that consist of many detectors. The sum of all coincidence counters in the frequency distribution Y can serve as a measure for the absorbed dose, provided that the total count rate $$\dot{Z}=\frac{d\left({\displaystyle \sum {}_{i,j}{Y}_{ij}}\right)}{dt}$$

is directional. If there are many pairs of detectors, the directional dependency of individual pairs of detectors is averaged out; the total counting rate is then approximately independent of direction.

The suitability of a certain variable Z for calibrating the dose rate can be checked by moving a reference source around the device at a constant distance r. If the count rate Z remains constant while the source is moved around the device, the condition is fulfilled. The variable Z can then be used to calibrate the dose rate.

und die Äquivalenz-Dosisleistung Ḣ *(10) $$\dot{H}*\left(10\right)=C_H\left(\dot{Z}-{\dot{Z}}_b\right)$$

aus der untergrundkorrigierten Zählrate Ż - Ż_b berechnet werden können.For the calibration of the dose rate with the counting rate Z, a background measurement without a source is recorded. The counting rate determined in this way is referred to as Ż _b . Device-specific calibration constants C and C _H can now be defined with which the dose rate Ḋ $$\dot{\mathrm{D.}}=\mathrm{C.}\left(\dot{Z}-{\dot{Z}}_b\right)$$

and the equivalent dose rate Ḣ * (10) $$\dot{H}*\left(10\right)={\mathrm{C.}}_H\left(\dot{Z}-{\dot{Z}}_b\right)$$

can be calculated from the background-corrected counting rate Ż - Ż _b.

The spatial directional distribution X (ω, h) of the radiation field (or X (ω) in the 2-dimensional case) can now be calculated using any statistical image reconstruction method known from emission tomography. The statistical reconstruction methods allow uniform processing of the measurement data of the entire measuring device, regardless of whether a certain detector pair is unidirectional or bidirectional.

• - Maximum Likelihood Expectation Maximization (MLEM) Algorithmus
• - Ordered Subset Expectation Maximization (OSEM) Algorithmus
• - List Mode - Maximum Likelihood Expectation Maximization (LM-MLEM) Algorithmus
• - List Mode - Ordered Subset Expectation Maximization (LM-OSEM) Algorithmus

Common statistical image reconstruction methods include, among others

• - Maximum Likelihood Expectation Maximization (MLEM) algorithm
• - Ordered Subset Expectation Maximization (OSEM) algorithm
• - List Mode - Maximum Likelihood Expectation Maximization (LM-MLEM) algorithm
• - List Mode - Ordered Subset Expectation Maximization (LM-OSEM) algorithm

All the methods mentioned are examples of algorithms, the scope of which is not restricted to medicine. With the procedure described above according to this invention, they can also be used without further ado in NBC and radiation protection.

In the following, two embodiments of the invention are described how a spatial directional distribution X (ω, h) or a planar directional distribution X (ω) can be calculated using the Maximum Likelihood Expectation Maximization (MLEM) algorithm. The explanations apply to devices in which the sum Σ _{i, j} Y _{ij of} all entries in the frequency distribution Y is not directional.

An algorithm for reconstructing a 3-dimensional directional distribution is described below. The surface of the celestial sphere is first divided into degrees of latitude and longitude. Each of the surface elements of the same size on the celestial sphere corresponds to an image pixel (k, l) for which the irradiance X _kl = X (ω _k , h _l ) at the azimuth angle ω _k and elevation angle h _{l is to} be calculated. Make sure that the classes for the elevation angle h _l are chosen in such a way that the sine of the elevation angle is uniformly distributed in the interval (-1,1). There is a total of K · L pixels X _kl for K classes ω _k and L classes h _l .

Maximum Likelihood Expectation Maximization (MLEM) is an iterative method that requires initial values for the directional distribution X: $$\begin{array}{cc}\forall k=\mathrm{1,}...,K,l=\mathrm{1,}...,\mathrm{L.}& X_{kl}^{\left[0\right]}=1\end{array}$$

aus den Vorgängerwerten $X_{kl}^{\left[n\right]}$

lautet: $$X_{kl}^{\left[n+1\right]}=X_{kl}^{\left[n\right]}=\frac{KL}{{\displaystyle \sum {}_{i,j}{Y}_{ij}}}{\displaystyle \sum _{i,j}\frac{LU{T}_j^{SK\left(i\right)}\left[\vartheta \left({\omega }_k,h_l,{\phi }_i,{\beta }_i\right)\right]Y_{ij}}{{\displaystyle {\sum }_{k\text{'}=1}^K{\displaystyle {\sum }_{l\text{'}=1}^{L}LU{T}_j^{SK\left(i\right)}\left[\vartheta \left({\omega }_{k\text{'}},h_{l\text{'}},{\phi }_i,{\beta }_i\right)\right]X_{k\text{'}l\text{'}}^{\left[i\right]}}}}}$$

The MLEM iteration rule for calculating a new representation $X_{kl}^{\left[n+1\right]}$

from the previous values $X_{kl}^{\left[n\right]}$

reads: $$X_{kl}^{\left[n+1\right]}=X_{kl}^{\left[n\right]}=\frac{K\mathrm{L.}}{{\displaystyle \sum {}_{i,j}{Y}_{ij}}}{\displaystyle \sum _{i,j}\frac{\mathrm{L.}U{T}_j^{\mathrm{S.}K\left(i\right)}\left[\vartheta \left({\omega }_k,H_l,{\phi }_i,{\beta }_i\right)\right]Y_{ij}}{{\displaystyle {\sum }_{k\text{'}=1}^K{\displaystyle {\sum }_{l\text{'}=1}^{\mathrm{L.}}\mathrm{L.}U{T}_j^{\mathrm{S.}K\left(i\right)}\left[\vartheta \left({\omega }_{k\text{'}},H_{l\text{'}},{\phi }_i,{\beta }_i\right)\right]X_{k\text{'}l\text{'}}^{\left[i\right]}}}}}$$

berechnet werden, wobei φ_i der Azimutwinkel und β_i der Höhenwinkel von Detektorpaar i sind.Here, Y _ij denotes the counters (the measured values) registered in the frequency table Y for a detector pair i in bin j of the measured value distribution f (E1, E2). When the lookup tables LUT ^{SK (i) are called} , the angular distance ϑ between a pixel (k, l) on the celestial sphere and a detector pair i must be calculated. In the 3-dimensional case, the angular distance ϑ can be according to $$\vartheta \left({\omega }_k,H_l,{\phi }_i,{\beta }_i\right)=\text{arccos}\left(\text{cos}{\beta }_i\text{cos}{H}_l\text{cos}\left({\omega }_k-{\phi }_i\right)+\text{sin}{\beta }_i\text{sin}{H}_l\right)$$

can be calculated, where φ _{i is} the azimuth angle and β _{i is} the elevation angle of detector pair i.

The MLEM algorithm is a method that uses a frequency distribution, namely the measured frequency distribution Y, which indicates how often coincidences have occurred in certain detector pairs for certain measured values, and calculates a new frequency distribution from this, namely the sought directional distribution X, which indicates with which _{Frequency of radiation arriving} at the detection location from certain angles of incidence ω _k and h l.

Equation (13) is formulated in such a way that the iteration rule can be executed in parallel with the data acquisition, while the coincidence counters Y _ij are continuously accumulated in the frequency distribution Y.

und eine Iterationsschleife mit der Iterationsvorschrift: $$X_k^{\left[n+1\right]}=X_k^{\left[n\right]}\frac{K}{{\displaystyle \sum {}_{i,j}}{Y}_{ij}}{\displaystyle \sum _{i,j}\frac{LU{T}_j^{SK\left(i\right)}\left[\vartheta \left({\omega }_k,{\phi }_i\right)\right]Y_{ij}}{{\displaystyle {\sum }_{k\text{'}=1}^{K}LU{T}_j^{SK\left(i\right)}\left[\vartheta \left({\omega }_{k\text{'}},{\phi }_i\right)\right]X_{k\text{'}}^{\left[n\right]}}}}$$

durchlaufen. Der Winkelabstand ϑ wird im 2-dimensionalen Fall mit $$\vartheta \left({\omega }_k,{\phi }_i\right)=\text{arccos}\left(\text{cos}\left({\omega }_k-{\phi }_i\right)\right)$$

berechnet.In a further embodiment of the invention, a planar directional distribution X _k = X (ω _k ) can be calculated using the MLEM algorithm. An initial distribution is initialized $$\begin{array}{cc}\forall k=\mathrm{1,}...,K& X_k^{\left[0\right]}\end{array}=1$$

and an iteration loop with the iteration rule: $$X_k^{\left[n+1\right]}=X_k^{\left[n\right]}\frac{K}{{\displaystyle \sum {}_{i,j}}{Y}_{ij}}{\displaystyle \sum _{i,j}\frac{\mathrm{L.}U{T}_j^{\mathrm{S.}K\left(i\right)}\left[\vartheta \left({\omega }_k,{\phi }_i\right)\right]Y_{ij}}{{\displaystyle {\sum }_{k\text{'}=1}^K\mathrm{L.}U{T}_j^{\mathrm{S.}K\left(i\right)}\left[\vartheta \left({\omega }_{k\text{'}},{\phi }_i\right)\right]X_{k\text{'}}^{\left[n\right]}}}}$$

run through. The angular distance ϑ is in the 2-dimensional case with $$\vartheta \left({\omega }_k,{\phi }_i\right)=\text{arccos}\left(\text{cos}\left({\omega }_k-{\phi }_i\right)\right)$$

calculated.

The directional distribution X of the radiation field is initially unitless. The values X _kl represent the relative radiation intensities in the various directions of incidence ω _k and h _l . With the calibration of the radiation intensity described above, however, X can also be specified as a distribution with the unit of measurement of a dose rate per solid angle. For this purpose, the intensity values X _{kl are} multiplied by a scaling factor.

die Äquivalenz-Dosisleistung Ḣ *(10) pro Raumwinkel dΩ mit: $$\frac{dH*\left(10\right)\left({\omega }_k,h_l\right)}{dtd\mathrm{\Omega }}=\frac{C_H\left(\dot{Z}-{\dot{Z}}_b\right)}{4\pi }{X}_{kl}$$

berechnet.The dose rate Ḋ per solid angle dΩ is given by $$\frac{d\mathrm{D.}\left({\omega }_k,H_l\right)}{dtd\Omega }=\frac{\mathrm{C.}\left(\dot{Z}-{\dot{Z}}_b\right)}{\mathrm{4th}\pi }{X}_{kl}$$

the equivalent dose rate Ḣ * (10) per solid angle dΩ with: $$\frac{dH*\left(10\right)\left({\omega }_k,H_l\right)}{dtd\mathrm{\Omega }}=\frac{{\mathrm{C.}}_H\left(\dot{Z}-{\dot{Z}}_b\right)}{\mathrm{4th}\pi }{X}_{kl}$$

calculated.

und: $$\frac{dH*\left({\omega }_k\right)}{dtd\mathrm{\omega }}=\frac{C_H\left(\dot{Z}-{\dot{Z}}_b\right)}{2\pi }{X}_k$$

In the 2-dimensional case, the following applies: $$\frac{d\mathrm{D.}\left({\omega }_k\right)}{dtd\mathrm{\omega }}=\frac{\mathrm{C.}\left(\dot{Z}-{\dot{Z}}_b\right)}{2\pi }{X}_k$$

and: $$\frac{dH*\left({\omega }_k\right)}{dtd\mathrm{\omega }}=\frac{{\mathrm{C.}}_H\left(\dot{Z}-{\dot{Z}}_b\right)}{2\pi }{X}_k$$

For the dose rate range up to 1 µSv / h, the (physical) dose rate per solid angle can be specified, ^{for example, with the unit of measurement pGy sr -1} s ^-1. The equivalent dose rate per solid angle can be measured, ^{for example, in units of nSv sr −1} h ^−1.

If the dose rate per solid angle is averaged over all directions of incidence, the result is the dose rate Ḋ, which is received from all directions, divided by 4π. The same applies to the equivalent dose rate: The equivalent dose rate averaged over all directions of incidence per solid angle is equal to the equivalent dose rate Ḣ * (10), divided by 4π.

The recorded lookup tables are a central component of the method according to the invention. Lookup tables are energy-specific and apply to the characteristic radiation energy of the radiation source. Various options are available for setting up a measuring device for specific operating conditions. If the determination of use is linked to a manageable number of nuclides, it can make sense to create a set of lookup tables for each nuclide. If, on the other hand, the measuring device is to be used in a wide energy range with very different nuclides, it is advantageous to to carry out a further stage of data preparation. The frequency distributions of the lookup tables can be fitted with functions. The fit parameters are recorded for different gamma energies and stored as energy-dependent functions. During a measurement, look-up tables for any gamma energies can be generated from the stored energy-dependent functions. The device can thus be dynamically adapted to the specific situation. Radiation sources can be mapped over a wide energy range.

The method according to the invention is described in more detail below with reference to exemplary embodiments and in drawings. The explanations are merely exemplary and do not restrict the general inventive concept.

• - Kalibrieren der Signale von allen Detektoren als absorbierte Strahlungsenergie E;
• - Auswählen eines geeigneten Koordinatensystems;
• - Identifizieren der für die Richtungsmessung geeigneten Detektorpaare, wobei die definierten Detektorpaare mindestens einen Detektor aus einem Material mit einer Ordnungszahl von Z_eff > 30 enthalten und Kennzeichnen dieser mit einer ID-Nummer i;
• - Verschalten aller Detektoren in einer Koinzidenzschaltung derart, dass Koinzidenzen aller identifizierten Detektorpaare erfasst werden;
• - Kennzeichnen der beiden Detektoren eines jeden definierten Detektorpaares i mit den Nummern 1 bzw. 2, der Detektor mit der niedrigeren Ordnungszahl erhält die Nummer 1, derjenige mit der höheren Ordnungszahl die Nummer 2, bestehen beide Detektoren aus dem gleichen Material kann die Kennzeichnung als 1 bzw. 2 willkürlich getroffen werden;
• - Erfassen der Richtungen der definierten Detektorpaare, wobei in der 2-dimensionalen Richtungsmessung die Richtung von Detektorpaar i mit dem Azimutwinkel φ_i erfasst wird und wobei in der 3-dimensionalen Richtungsmessung die Richtung von Detektorpaar i mit dem Azimutwinkel φ_i und dem Höhenwinkel h_i erfasst wird;
• - Definieren eines Funktionswerts f(E1,E2) welcher aus zwei koinzidenten Energiewerten E1 und E2 in den beiden Detektoren eines Detektorpaares i berechnet wird, wobei der Messbereich für f(E1,E2) festgelegt und in äquidistante Messwertkanäle j = 1, ...,J eingeteilt wird;
• - Anlegen eines oder mehrerer 2-dimensionaler Arrays mit I · J Feldern als Datenstrukturen zur Speicherung der Häufigkeitsverteilungen Y, in welchen Koinzidenzereignisse entsprechend ihrer ID-Nummer i und ihres Messwertkanals j registriert werden; für jedes Radionuklid wird ein separates 2-dimensionales Array Y angelegt;
• - Einteilen der Detektorpaare i in Symmetrieklassen SK, wobei diejenigen Detektorpaare i, die bei Drehung oder Verschiebung auf andere baugleiche Detektorpaare abgebildet werden, zu jeweils einer Symmetrieklasse SK(i) zusammengefasst werden;
• - Anlegen einer oder mehrerer Lookup-Tabellen LUT^SK für jede Symmetrieklasse SK und jedes Radionuklid, wobei diese einen Winkelbereich ϑ von 0° bis 180° umspannen und in äquidistante Winkelschritte eingeteilt werden;
• - Erstellen einer Vorschrift zur Erzeugung und Validierung der Lookup-Tabellen LUT^SK aus Referenzmessungen, welche die Referenzquellen auswählt, den Nuklidtyp berücksichtigt, sowie die Messbedingungen definiert, unter denen die Messungen durchzuführen sind;
• - Ausführen von Referenzmessungen für alle in der Vorschrift zuvor definierten Schritte;
• - Erstellen und Validieren der Lookup-Tabellen LUT^SK gemäß der zuvor definierten Vorschrift zur Auswertung der Messdaten aus den Referenzmessungen;
• - Erfassen des natürlichen Strahlungshintergrundes b^SK(i) für alle Symmetrieklassen SK;
• - Exkludieren des natürlichen Strahlungshintergrundes b^SK aus den Lookup-Tabellen LUT^SK;
• - Übergeben sämtlicher Lookup-Tabellen LUT^SK für alle Symmetrieklassen und alle Radionuklide an einen Algorithmus der Bildrekonstruktion, welcher die Messdaten verarbeitet und die Richtungsverteilungen X berechnet.

According to an advantageous exemplary embodiment of the method according to the invention, before step a) it additionally comprises, in part or in whole, the following sequence of steps:

• - calibrating the signals from all detectors as absorbed radiant energy E;
• - Selecting a suitable coordinate system;
• - Identifying the detector pairs suitable for the direction measurement, the defined detector _pairs containing at least one detector made of a material with an ordinal number of Z eff> 30 and identifying this with an ID number i;
• - Interconnection of all detectors in a coincidence circuit in such a way that coincidences of all identified detector pairs are detected;
• - Identify the two detectors of each defined pair of detectors i with the numbers 1 and 2, the detector with the lower ordinal number receives the number 1, the one with the higher ordinal number the number 2, if both detectors are made of the same material, the identification as 1 or 2 are hit at random;
• - Detecting the directions of the defined detector pairs, whereby in the 2-dimensional direction _{measurement the direction of detector pair i is} detected with the azimuth angle φ i and where in the 3-dimensional direction measurement the direction of detector pair i with the azimuth angle φ _i and the elevation angle h _i is captured;
• - Define a function value f (E1, E2) which is calculated from two coincident energy values E1 and E2 in the two detectors of a detector pair i, whereby the measuring range for f (E1, E2) is defined and divided into equidistant measuring value channels j = 1, ... , J is classified;
• Creation of one or more 2-dimensional arrays with I · J fields as data structures for storing the frequency distributions Y, in which coincidence events are registered according to their ID number i and their measured value channel j; a separate 2-dimensional array Y is created for each radionuclide;
• - Division of the detector pairs i into symmetry classes SK, with those detector pairs i that are mapped onto other structurally identical detector pairs when rotated or shifted, each being combined into a symmetry class SK (i);
• Creation of one or more lookup tables LUT ^SK for each symmetry class SK and each radionuclide, these spanning an angular range ϑ from 0 ° to 180 ° and being divided into equidistant angular steps;
• - Creation of a rule for generating and validating the lookup tables LUT ^SK from reference measurements, which selects the reference sources, takes the nuclide type into account, and defines the measurement conditions under which the measurements are to be carried out;
• - Execution of reference measurements for all steps previously defined in the regulation;
• - Creation and validation of the lookup tables LUT ^{SK in} accordance with the previously defined rule for evaluating the measurement data from the reference measurements;
• - Detecting the natural radiation background b ^{SK (i)} for all symmetry classes SK;
• - Excluding the natural radiation background b ^SK from the lookup tables LUT ^SK ;
• Transfer of all lookup tables LUT ^SK for all symmetry classes and all radionuclides to an image reconstruction algorithm which processes the measurement data and calculates the direction distributions X.

According to a further advantageous embodiment of the method according to the invention, a scintillation detector is used as the detector and / or the scintillator is designed as a monolithic block or as a pixelated scintillator module and / or the scintillator is made from pure or doped materials from the group of PVT, anthracene, stilbene, p-terphenyl , CaF ₂ , BaF ₂ , Nal, CeBr ₃ , LaBr ₃ , LaCl ₃ , La (Br _x Cl _1-x ) ₃ , Csl, SrI ₂ , CLYC, CLBC, CLCB, BGO, LSO, LYSO, GAGG, YAP and / or YAG is formed.

According to a further advantageous embodiment of the method according to the invention, a semiconductor detector is used as the detector and / or the semiconductor is designed as a segmented or unsegmented semiconductor and / or which has a planar or coaxial geometry and / or which is made of materials from the group of Ge, GaAs, CdTe and / or CdZnTe is formed.

According to a further advantageous exemplary embodiment of the method according to the invention, at least two detectors are used.

According to a further advantageous exemplary embodiment of the method according to the invention, all detectors used are essentially structurally identical, or at least two of the detectors used are different from one another.

According to a further advantageous exemplary embodiment of the method according to the invention, the radiation sources emit a discrete and / or continuous distribution of radiation.

According to a further advantageous exemplary embodiment of the method according to the invention, the radiation sources emit gamma, electron, positron, proton and / or neutron radiation. The radiation can come from the radioactive decay of one or more radionuclides.

According to a further advantageous embodiment of the method according to the invention, the radiation has a lower intensity than z. B. is the case in astronomy.

According to a further advantageous exemplary embodiment of the method according to the invention, detector and system electronics are used that use analog and / or digital electronic components. The analog electronic components can include a combination of different modules, including a high-voltage supply, a preamplifier, an amplifier, a pulse shaper, a charge integrator, a pulse height analyzer, a multi-channel analyzer (MCA) and / or a coincidence circuit.

According to a further advantageous embodiment of the method according to the invention, the digital electronic components comprise a combination of various hardware and software components, including a high-voltage supply, an A / D converter per detector, a field programmable gate array (FPGA), a storage medium, a digital signal processor and / or an evaluation software count.

According to a further advantageous embodiment of the method according to the invention, the function value f (E1, E2) calculated from the two energy values E1, E2 is the energy asymmetry f (E1, F2) = (E2-E1) / (E1 + E2) or the cosine of Compton -The scattering angle f (E1, E2) = 1 - mc ² / E2 + mc ² / (E1 + E2) with mc ² = 511 keV.

According to a further advantageous exemplary embodiment of the method according to the invention, a selection condition with regard to the energy sum E1 + E2 of the energies detected in both detectors of a pair is applied in the selection of coincidence events.

According to a further advantageous exemplary embodiment of the method according to the invention, a separate frequency distribution Y is created for each radionuclide and / or a separate directional distribution X is calculated for each radionuclide.

According to a further advantageous embodiment of the method according to the invention, several selection conditions with regard to the energy sum E1 + E2 are applied to radionuclides with several gamma energies and / or one or more frequency distributions Y are applied for such radionuclides with several gamma energies.

According to a further advantageous exemplary embodiment of the method according to the invention, the lookup tables LUT ^{SK are created} by measurements with the detector system or by Monte Carlo simulations or by means of a theoretical model.

According to a further advantageous embodiment of the method according to the invention, the statistical image reconstruction method is part or in whole of the Maximum Likelihood Expectation Maximization (MLEM) method, the Ordered Subset Expectation Maximization (OSEM) method, the List Mode - Maximum Likelihood Expectation Maximization (LM-MLEM) ) Procedure and / or the List Mode - Ordered Subset Expectation Maximization (LM-OSEM) procedure.

aus einer Anfangsverteilung $$\begin{array}{cc}\forall k=\mathrm{1,}\dots ,K& X_k^{\left[0\right]}=1\end{array}$$

unter Nutzung von Lookup-Tabellen LUT^SK und einer Winkelabstandsfunktion gemäß $$\vartheta \left({\omega }_k,{\phi }_i\right)=\text{arccos}\left(\text{cos}\left({\omega }_k-{\phi }_i\right)\right)$$

berechnet.According to a further advantageous embodiment of the method according to the invention, a 2-dimensional directional distribution X _k = X (ω _k ) according to is for each detected radionuclide $$X_k^{\left[n+1\right]}=X_k^{\left[n\right]}\frac{K}{{\displaystyle {\sum }_{i,j}{Y}_{ij}}}{\displaystyle \sum _{i,j}\frac{\mathrm{L.}U{T}_j^{\mathrm{S.}K\left(i\right)}\left[\vartheta \left({\omega }_k,{\phi }_i\right)\right]Y_{ij}}{\mathrm{L.}U{T}_j^{\mathrm{S.}K\left(i\right)}\left[\vartheta \left({\omega }_{k\text{'}},{\phi }_i\right)\right]X_{k\text{'}}^{\left[n\right]}}}$$

from an initial distribution $$\begin{array}{cc}\forall k=\mathrm{1,}...,K& X_k^{\left[0\right]}=1\end{array}$$

using lookup tables LUT ^SK and an angular distance function according to $$\vartheta \left({\omega }_k,{\phi }_i\right)=\text{arccos}\left(\text{cos}\left({\omega }_k-{\phi }_i\right)\right)$$

calculated.

aus einer Anfangsverteilung $$\begin{array}{cc}\forall k=\mathrm{1,}\dots ,K,l=\mathrm{1,}\dots ,L& X_{kl}^{\left[0\right]}=1\end{array}$$

unter Nutzung von Lookup-Tabellen LUT^SK und einer Winkelabstandsfunktion gemäß $$\vartheta \left({\omega }_k,h_l,{\phi }_i,{\beta }_i\right)=\text{arccos}\left(\text{cos}{\beta }_i\text{cos}{h}_l\text{cos}\left({\omega }_k-{\phi }_i\right)+\text{sin}{\beta }_i\text{sin}{h}_l\right)$$

berechnet.According to a further advantageous embodiment of the method according to the invention, a 3-dimensional directional distribution X _kl = X (ω _k , h _l ) according to is for each detected radionuclide $$X_{kl}^{\left[n+1\right]}=X_{kl}^{\left[n\right]}\frac{K\mathrm{L.}}{{\displaystyle {\sum }_{i,j}{Y}_{ij}}}{\displaystyle \sum _{i,j}\frac{\mathrm{L.}U{T}_j^{\mathrm{S.}K\left(i\right)}\left[\vartheta \left({\omega }_k,H_l,{\phi }_i,{\beta }_i\right)\right]Y_{ij}}{{\displaystyle {\sum }_{k\text{'}=1}^K{\displaystyle {\sum }_{l\text{'}=1}^{\mathrm{L.}}\mathrm{L.}U{T}_j^{\mathrm{S.}K\left(i\right)}\left[\vartheta \left({\omega }_{k\text{'}},H_{l\text{'}},{\phi }_i,{\beta }_i\right)\right]X_{k\text{'}l\text{'}}^{\left[n\right]}}}}}$$

from an initial distribution $$\begin{array}{cc}\forall k=\mathrm{1,}...,K,l=\mathrm{1,}...,\mathrm{L.}& X_{kl}^{\left[0\right]}=1\end{array}$$

using lookup tables LUT ^SK and an angular distance function according to $$\vartheta \left({\omega }_k,H_l,{\phi }_i,{\beta }_i\right)=\text{arccos}\left(\text{cos}{\beta }_i\text{cos}{H}_l\text{cos}\left({\omega }_k-{\phi }_i\right)+\text{sin}{\beta }_i\text{sin}{H}_l\right)$$

calculated.

The invention further relates to a device for carrying out the method according to the invention, the device according to the invention comprising a group of several synchronized detectors for detecting radiation, at least one detector material having an ordinal number of Z _eff > 30 and all detectors measuring the energies E, which in Interactions of the radiation with the detector materials occur.

The device according to the invention also has system electronics which register coincidence events when interactions take place simultaneously in two detectors from a list of defined detector pairs i, the two detectors of all defined detector pairs i being uniquely identified with the numbers 1 and 2;

The device according to the invention also has a data acquisition system which arranges the energies (E1, E2) measured in coincidence events according to their identification and saves them in a chronological list with the attributes {i, E1, E2} and the detection time t.

In addition, the device according to the invention has an analysis unit which creates one or more frequency distributions Y from the data stored by the data acquisition system and reconstructs one or more direction distributions X of the radiation field.

According to an advantageous embodiment of the device according to the invention, this is partially or completely designed as a Compton camera, a Compton telescope, a single plane Compton camera, a neutron camera and / or a dual gamma / neutron camera.

In the context of the invention, a device is also understood to mean a measuring device, or device for short, or a detector system.

According to a further advantageous embodiment of the device according to the invention, the entirety of all detectors of the device has an annular shape in which the detectors are arranged in a ring, with no, one or more central detectors being present in the interior of the ring. Furthermore, the ring preferably contains four or five and particularly preferably six plastic scintillation detectors. Furthermore, one or two scintillation detectors made of Nal, Csl, CeBr ₃ and / or LaBr _{3 are} preferably located inside the ring.

According to a further advantageous exemplary embodiment of the device according to the invention, it is preferably constructed from scintillation detectors of size 1 "x1", 1.5 "x 1.5", 2 "x2" and / or 3 "x 3".

• eine nicht-maßstabsgetreue, schematische Darstellung eines unidirektionalen Detektorpaars in einer zwei-dimensionalen Messsituation, gemäß einer Ausführungsform der Erfindung;
• eine nicht-maßstabsgetreue, schematische Darstellung eines bidirektionalen Detektorpaars in einer zwei-dimensionalen Messsituation gemäß einer weiteren Ausführungsform der Erfindung;
• eine nicht-maßstabsgetreue, schematische Darstellung des Strahlungseinfalls von einer Quelle auf ein Detektorpaar 1,2 in einer zwei-dimensionalen Messsituation, gemäß einer weiteren Ausführungsform der Erfindung;
• eine nicht-maßstabsgetreue, schematische Darstellung eines Vorrichtungsmodells mit einem zentralen Cerbromid-Detektor, der von sechs ringförmig angeordneten Plastik-Detektoren umgeben ist, gemäß einer weiteren Ausführungsform der Erfindung;
• eine nicht-maßstabsgetreue, schematische Darstellung eines Vorrichtungsmodelles gemäß , in dem ein weiterer Cerbromid-Detektor eingebracht ist, gemäß einer weiteren Ausführungsform der Erfindung;
• eine schematische Darstellung einer Häufigkeitsverteilung Y des Vorrichtungsmodells aus in einer 2-dimensionalen Richtungsmessung, gemäß einer weiteren Ausführungsform der Erfindung;
• eine nicht-maßstabsgetreue, schematische Darstellung einer ersten Symmetrieklasse von insgesamt vier zu einem Quadrat angeordneten, würfelförmigen Detektoren, wobei zwei horizontale und zwei vertikale Detektorpaare vorliegen, gemäß einer weiteren Ausführungsform der Erfindung;
• eine nicht-maßstabsgetreue, schematische Darstellung einer weiteren Symmetrieklasse der vier zu einem Quadrat angeordneten, würfelförmigen Detektoren aus , gemäß einer weiteren Ausführungsform der Erfindung;
• eine schematische Darstellung einer Häufigkeitsverteilung Y des Vorrichtungsmodells aus in einer 3-dimensionalen Richtungsmessung, gemäß einer weiteren Ausführungsform der Erfindung;
• eine nicht-maßstabsgetreue, schematische Darstellung einer Richtungsverteilung für ein Strahlungsfernfeld, dessen Emissionspunkte alle in einer Ebene liegen. Die Richtungsverteilung wird als vom Azimutwinkel ω abhängige Funktion X(ω) entlang des Horizontkreises modelliert, gemäß einer weiteren Ausführungsform der Erfindung;
• eine nicht-maßstabsgetreue, schematische Darstellung einer Richtungsverteilung für ein Strahlungsfernfeld, dessen Emissionspunkte beliebig im Raum verteilt sind. Die Richtungsverteilung wird als vom Azimutwinkel ω und vom Höhenwinkel h abhängige Funktion X(ω, h) auf der Himmelskugel modelliert, gemäß einer weiteren Ausführungsform der Erfindung;
• eine schematische Darstellung einer Lookup-Tabelle, gemäß einer weiteren Ausführungsform der Erfindung;
• ein Diagramm mit gemessenen Abbildungsfunktionen aus einer Lookup-Tabelle des Vorrichtungsmodells aus für CeBr₃ - Plastik Detektorpaare bei ausgewählten Asymmetriewerten A, gemäß einer weiteren Ausführungsform der Erfindung;
• vier Diagramme von berechneten Asymmetriekurven in den Lookup-Tabellen von uni- und bidirektionalen Detektorpaaren für die Radionuklide Co-60 und Cs-137, gemäß einer weiteren Ausführungsform der Erfindung;
• eine nicht-maßstabsgetreue, schematische Darstellung eines Messplans mit Winkelpositionen einer Referenzquelle zur Erstellung einer Lookup-Tabelle für das Vorrichtungsmodell aus , gemäß einer weiteren Ausführungsform der Erfindung;
• eine schematische Darstellung eines Rechenschemas für ein MLEM Verfahren zur Rekonstruktion der Richtungsverteilung eines Strahlungsfeldes, gemäß einer weiteren Ausführungsform der Erfindung;
• neun Diagramme von gemessenen Richtungsverteilungen für Strahlungsfelder mit einer, zwei und drei punktförmigen Strahlungsquellen. Die Darstellungen sind Beispiele für 2-dimensionale Richtungsmessungen mit dem Vorrichtungsmodell aus , gemäß einer weiteren Ausführungsform der Erfindung;
• eine nicht-maßstabsgetreue, schematische Darstellung eines Vorrichtungsmodells samt der Verschattungsverhältnisse, gemäß einer weiteren Ausführungsform der Erfindung;

Show it:

• a schematic representation, not true to scale, of a unidirectional pair of detectors in a two-dimensional measurement situation, according to an embodiment of the invention;
• a schematic representation, not true to scale, of a bidirectional pair of detectors in a two-dimensional measurement situation according to a further embodiment of the invention;
• a schematic representation, not true to scale, of the incidence of radiation from a source on a pair of detectors 1, 2 in a two-dimensional measurement situation, according to a further embodiment of the invention;
• a not to scale, schematic representation of a device model with a central cerium bromide detector, which is surrounded by six ring-shaped plastic detectors, according to a further embodiment of the invention;
• a not to scale, schematic representation of a device model according to FIG , in which a further cerium bromide detector is introduced, according to a further embodiment of the invention;
• a schematic representation of a frequency distribution Y of the device model in a 2-dimensional direction measurement, according to a further embodiment of the invention;
• a schematic representation, not true to scale, of a first symmetry class of a total of four cube-shaped detectors arranged in a square, two horizontal and two vertical detector pairs being present, according to a further embodiment of the invention;
• a schematic representation, not true to scale, of a further symmetry class of the four cube-shaped detectors arranged in a square , according to a further embodiment of the invention;
• a schematic representation of a frequency distribution Y of the device model in a 3-dimensional direction measurement, according to a further embodiment of the invention;
• a schematic representation, not true to scale, of a directional distribution for a radiation far field, the emission points of which are all in one plane. The direction distribution is modeled as a function X (ω), which is dependent on the azimuth angle ω, along the horizon circle, according to a further embodiment of the invention;
• a schematic representation, not true to scale, of a directional distribution for a radiation far field, the emission points of which are arbitrarily distributed in space. The directional distribution is modeled on the celestial sphere as a function X (ω, h) dependent on the azimuth angle ω and the elevation angle h, according to a further embodiment of the invention;
• a schematic representation of a lookup table, according to a further embodiment of the invention;
• a diagram with measured mapping functions from a lookup table of the device model for CeBr ₃ plastic detector pairs with selected asymmetry values A, according to a further embodiment of the invention;
• four diagrams of calculated asymmetry curves in the lookup tables of unidirectional and bidirectional detector pairs for the radionuclides Co-60 and Cs-137, according to a further embodiment of the invention;
• a not to scale, schematic representation of a measurement plan with angular positions of a reference source for creating a lookup table for the device model , according to a further embodiment of the invention;
• a schematic representation of a calculation scheme for an MLEM method for reconstructing the directional distribution of a radiation field, according to a further embodiment of the invention;
• nine diagrams of measured directional distributions for radiation fields with one, two and three point radiation sources. The representations are examples of 2-dimensional direction measurements with the device model , according to a further embodiment of the invention;
• a not to scale, schematic representation of a device model including the shading conditions, according to a further embodiment of the invention;

In the is a not true to scale, schematic representation of a unidirectional detector pair in a two-dimensional measurement situation. The detector with the lower atomic number is marked with the number 1, the detector with the higher atomic number with the number 2. The reconstruction of the direction of incidence remains ambiguous. It is true that the angle of the incident radiation to the connecting line of the two detectors can be measured, but not the side on which the source lies.

shows a schematic representation of a bidirectional pair of detectors in a two-dimensional measurement situation. Both detectors are made of materials with the same or similar atomic number. The identification of the two detectors of a bidirectional pair in detector 1 and detector 2 is defined once and must then be used consistently in all calculations. As with the unidirectional pair of detectors in the reconstruction of the direction of incidence remains ambiguous. The angle of the incident radiation to the connecting line of the two detectors can be measured, but not the side on which the source lies.

shows the incidence of radiation from a source on a pair of detectors 1, 2 in a two-dimensional measurement situation.

shows two device models according to the invention. In the model of a central cerium bromide detector is surrounded by six ring-shaped plastic detectors. This model is suitable for 2-dimensional direction measurement. If another cerium bromide detector is placed under the central cerium bromide detector, the model of is created , which allows 3-dimensional direction measurements.

Fig. 12 shows an example of a frequency distribution Y (the measurement data) for the device model . The system electronics determine which detectors were coincident and calculate the asymmetry value A = {E2 - E1) / (E1 + E2) from the coincident energy inputs E1 and E2. The coincidence events are registered in the frequency distribution Y according to their detector pair identification number and their asymmetry value A.

illustrates how detector pairs are divided into symmetry classes. Four cube-shaped detectors are arranged in a square. This detector arrangement has two classes of symmetry. The first symmetry class, in shown comprises two horizontal and two vertical pairs of detectors. The two diagonal pairs of detectors, in to see form another symmetry class.

Fig. 12 shows an example of a frequency distribution Y (the measurement data) for the device model . The measurement data Y were selected according to the three symmetry classes of the device model grouped into three areas. Detector pairs I to VI belong to symmetry class 1, detector pairs VII to XII to symmetry class 2. The thirteenth detector pair forms its own symmetry class 3.

shows two schematic representations of directional distributions for far radiation fields. A radiation far field is fully characterized by its directional dependence. There are two types of distributions, depending on the measurement situation. If all radiation sources and the measuring device lie in one plane, the distribution X (ω) along the horizon circle, which is dependent on the azimuth angle ω, is used, in shown. The general directional distribution is that in The distribution X (ω, h) on the celestial sphere shown is dependent on the azimuth angle ω and the elevation angle h. The size X represents the radiation intensity. It can either be normalized to the largest value that occurs or specified with a unit of measurement.

shows schematically a lookup table. A lookup table is a 2-dimensional frequency distribution that records coincidence counters as a function of the angular distance ϑ of a radiation source from the detector pair axis and a measured variable. The measurand here is the energy asymmetry A = {E2 - E1) / (E1 + E2) of the two energy inputs in a coincidence event.

shows various measured imaging functions from a lookup table for CeBr ₃ plastic detector pairs for selected asymmetry values A. The imaging functions represent the frequencies with which radiation incident at angle ϑ is registered in certain measurement value intervals of A. A lookup table can be understood as a group of mapping functions. The example shows three of the functions recorded with a 10 µCi Co-60 source. Coincidence events of both Co-60 emission lines at 1173 keV and 1332 keV were stored together.

shows asymmetry curves for lookup tables calculated for the radionuclides Co-60 and Cs-137, according to an embodiment of the invention. The asymmetry curves show the areas in a lookup table with the highest frequencies. If there were no measurement errors, all frequencies other than zero would lie on the curves shown. A distinction is made here between unidirectional and bidirectional detector pairs according to lookup tables. Lookup tables of unidirectional detector pairs ( and ) are characterized by the fact that there is only one prominent asymmetry curve. On the other hand, lookup tables of bidirectional detector pairs ( and ) two distinctive asymmetry curves. For Co-60, the asymmetry curves are shown for both emission lines at 1173 keV and 1332 keV; Cs-137 has only one emission line at 662 keV.

shows an example of the angular positions ω of a measurement plan for creating the lookup table for the in shown device model according to an embodiment of the invention. According to this plan, measurements are carried out in seven angular positions. The source always has the same distance to the center of the device. A lookup table for the device model can be generated from the seven measurements. In this example the lookup table has a step size of 5 °. For every angle ϑ in the value range from 0 ° to 180 ° there is at least one detector pair that has the correct angular distance to the radiation source and whose data record can be copied to the respective location in the lookup table. The measurement plan defines the assignments between the angular positions ω of the reference source, the detector pairs and the angular values ϑ of the lookup table.

shows a calculation scheme for an MLEM method for reconstructing the directional distribution of a radiation field according to an embodiment of the invention. With each iteration step n, all in Performed calculation steps shown. As the number n of iterations progresses, the MLEM method leads to the directional distribution X which has the highest probability of having caused the observed frequency distribution Y. Equations (13) and (16) all include in illustrated calculation steps in compact form, including the forward and back projection.

shows various examples of 2-dimensional direction measurements with the device model of . All measurements were carried out with 10 µCi Co-60 sources at a distance of 105 cm. The radial measured value scale is the same in all representations and extends from 0 to 300 nSv rad ^-1 h ^-1 . , and show measurement results for a Co-60 source at an angle of incidence of 125 °. A second Co-60 source at 50 ° and finally a third Co-60 source at 90 ° were then added successively. , and show the measured directional distributions for the 2-source radiation field, , and for the 3-source radiation field.

shows the shading situation for a device model made up of four detectors, according to an embodiment of the invention. The four detectors are arranged in the corner points of a square. It makes sense here to work with two variants of the lookup table in order to correctly record the frequency distributions on the shaded and unshaded side of the detector pair axis. The example of the pair of detectors P1 shows how the other detectors of the device attenuate the incidence of radiation on P1. However, the attenuation only affects one side of the detector pair axis, the other side does not suffer any losses.

The method according to the invention and the device use unidirectional and bidirectional detection processes for the multi-dimensional direction measurement of gamma radiation. The direction measurement, which partially or completely makes use of bidirectional detection processes, was described here for the first time. This is in the bidirectional direction measurement This is based on the functional principle shown here, which will be briefly explained here.

In contrast to unidirectional detection processes, which use a functional relationship between a measured value (here the asymmetry A) and the scattering angle ϑ for the direction measurement, bidirectional detection processes do not have such a calibration curve A (ϑ). The lookup tables of bidirectional detector pairs show two distinctive structures along the in and asymmetry curves shown. For a certain scattering angle ϑ there are two associated measured values A1 and A2, which are observed with certain frequencies. Nevertheless, a certain scattering angle, can be clearly assigned to the value pair (A1, A2). Each pair of values (A1, A2) exists exactly once over the entire angular range from 0 ° to 180 °. It cannot be confused with any other pair of values (A1, A2) at a different angle ϑ. It is therefore also possible to measure the scattering angle ϑ with a bidirectional pair of detectors. Statistical reconstruction methods already have the functionality required for direction measurement with bidirectional detector pairs. Methods such as MLEM are therefore suitable for processing the measurement data from the device models according to the invention.

Measurements with the device model are intended here which is made up of six 3''x3 '' plastic scintillation detectors and one 3''x3 '' cerium bromide scintillation detector. The six plastic detectors form a ring around the central cerium bromide detector. The diameter of the detector ring is 25 cm.

Each of the six plastic detectors can be combined with the central cerium bromide detector to form a unidirectional detector pair. In such a detector pair, the radiation is primarily scattered in the plastic detector and absorbed in the cerium bromide detector.

shows measured directional distributions for different arrangements from several point radiation sources. All measurements were carried out with 10 µCi Co-60 sources at a distance of 105 cm. First, a first 10 µCi Co-60 source was placed at an angle of 125 °. The dose rate generated by a 10 µCi Co-60 source at a distance of 105 cm is approx. 100 nSv h ^-1 . ) and c) show the results of the direction measurement after 10 s, 20 s and 60 s measuring time, respectively. The number of MLEM iterations was dynamically linked to the measurement time. As the measurement period progresses, the directional distribution of the point source becomes steeper and higher. The intensity averaged over all directions is unchanged at 16 nSv rad ^-1 h ^-1 . Integrated over the full angle of 2π rad results in a dose rate of 2π rad · 16 nSv rad ^-1 h ^-1 = 100 nSv h ^-1 . A second Co-60 source was then added at 50 °. The results are in ) and f) to see. The presence of two sources can be clearly identified from a measurement time of 10 s. The intensity averaged over all directions is now 32 nSv rad ^-1 h ^-1 . Finally, a third Co-60 source was added to the arrangement at 90 °. The results in ) and i) show that from a measurement time of 20 s, three point sources can also be resolved separately.

A method was developed with which the quality of the direction measurement can be formulated as a time specification from which a reliable direction measurement is available. The aim of the procedure was to quantitatively evaluate the quality of the direction measurement of a point source depending on the nuclide type, the dose rate and the angular position. A Co-60, a Cs-137 and a Co-57 source of activity of 10 µCi each were available as sources.

Two quality classes have been defined: ± 5 ° and ± 10 ° angular accuracy. At each point in time t during a measurement, the MLEM method supplies an estimated value for the direction of the radiation source. The measurements each ran over a specific measurement period and were repeated 20 times. At each point in time t it can now be determined how many of the measurements fall into the ± 5 ° or ± 10 ° quality class. A confidence level of 90% was chosen. t ₉₀ is defined as the time from which at least 18 of the 20 measurements are correct, ie lie within a ± 5 ° or ± 10 ° interval around the true value. In 90% of all measurements, the specified accuracy is reached or exceeded after the minimum measuring time.

The device model of has a characteristic symmetry. First of all, it was investigated what influence the device-specific detector arrangement has on the minimum measurement times, which are measured at different angles of incidence. The sensitivity of the direction measurement depends on the position of the source relative to the device. If the source is in the direction of one of the six ring detectors, longer measurement times are required than if the source is in the middle between two ring detectors. In this study, all sources were kept at a constant distance of 1 m from the center of the device. The minimum measuring time t ₉₀ for ± 10 ° accuracy of the 10 µCi Co-60 source was 2.7 s in the direction of the ring detectors and was shortened to 0.5 s in the middle between two ring detectors. Similar behavior was observed for Cs-137. Here the minimum _{measuring time t 90} for ± 10 ° accuracy was 5.8 s in the direction of the ring detectors and was shortened to 0.7 s in the middle between two ring detectors.

Next, it was investigated how the t ₉₀ minimum measurement times depend on the dose rate. These measurements were carried out in the direction of the ring detectors. They represent the lower limit of the device's performance. The distance between the sources was varied from 1 to 5 meters. The dose rate was expressed as the local dose rate Ḣ * (10) of the source at the measuring point. It was found that above 0.05 µSv / h the product of the local dose rate Ḣ * (10) and t ₉₀ minimum measurement time is constant.

parametrisiert werden. Die Parameter T(Nuklid, σ) sind gerätespezifische Parameter, die in Abhängigkeit vom Nuklidtyp und der Güteklasse σ definiert werden. Für das Vorrichtungsmodell mit sechs 3''x3'' Plastik-Detektoren und einem 3''x3'' Cerbromid-Detektor wurden die Parameter folgendermaßen bestimmt: T(⁶⁰Co, ±10°) = 0,32 s, T(⁶⁰Co, ±5°) = 0,48 s, T(¹³⁷Cs, ±10°) = 0,16s und T(¹³⁷Cs, ±5°) = 0,24 s.Under this condition, the t ₉₀ minimum measurement times can be achieved with the simple analytical expression $$t_{90}=T\left(\text{nuclide}\text{,}\sigma \right)\cdot {\left(\frac{\dot{H}*\left(10\right)}{\mathrm{\mu }\text{Sv}/H}\right)}^{-1}$$

be parameterized. The parameters T (nuclide, σ) are device-specific parameters that are defined depending on the nuclide type and the quality class σ. For the device model with six 3 "x 3" plastic detectors and one 3 "x 3" cerium bromide detector, the parameters were determined as follows: T ( ⁶⁰ Co, ± 10 °) = 0.32 s, T ( ⁶⁰ Co , ± 5 °) = 0.48 s, T ( ¹³⁷ Cs, ± 10 °) = 0.16s and T ( ¹³⁷ Cs, ± 5 °) = 0.24 s.

berechneten Werten. Mit dem zur Verfügung stehenden Vorrichtungsmodell war es ohne Schwierigkeiten möglich, auch bei wenigen nSv/h die Richtung einer Quelle zuverlässig mit ±5° Genauigkeit zu messen. Die Messzeiten liegen dann im Bereich von einigen Minuten. Beispielsweise erreichte die Richtungsmessung für eine Co-60 Quelle mit 3 nSv/h Dosisleistung nach 140 s eine Genauigkeit von ±10° und nach 220 s eine Genauigkeit von ±5°. Eine Cs-137 Quelle mit 4 nSv/h Dosisleistung am Messort konnte nach 55 s mit ±10° und nach 90 s mit ±5° geortet werden. Die Richtungsmessung einer Co-57 Quelle mit 5 nSv/h Dosisleistung hatte nach 22 s eine Genauigkeit von ±10° und nach 35 s eine Genauigkeit von ±5°.It was also investigated how the direction measurement behaves at very low local dose rates. High-quality directional measurements are also possible below 0.05 µSv / h, but the required measurement times are then longer and may be longer than $t_{90}=T\left(\text{nuclide}\text{,}\sigma \right)\cdot {\left(\frac{\dot{H}*\left(10\right)}{\mathrm{\mu }\text{Sv}/H}\right)}^{-1}$

calculated values. With the device model available, it was possible without difficulty to measure the direction of a source reliably with an accuracy of ± 5 °, even with a few nSv / h. The measurement times are then in the range of a few minutes. For example, the direction measurement for a Co-60 source with a dose rate of 3 nSv / h achieved an accuracy of ± 10 ° after 140 s and an accuracy of ± 5 ° after 220 s. A Cs-137 source with a dose rate of 4 nSv / h at the measuring location could be located after 55 s with ± 10 ° and after 90 s with ± 5 °. The direction measurement of a Co-57 source with a dose rate of 5 nSv / h had an accuracy of ± 10 ° after 22 s and an accuracy of ± 5 ° after 35 s.

If quasi-stationary measurement conditions exist, the direction measurement can be carried out in a clocked manner. It is then z. B. possible to track the movement of radiation sources. The dose rate Ḣ * (10) is in turn a suitable variable to regulate the clock frequency. For example, if the movement of a point source is to be tracked, the clock frequency can be set to the t ₉₀ time.

For an experiment under quasi-stationary measurement conditions, the device model from mounted on a turntable. The clock frequency of the measuring program was set to the t ₉₀ time. At a rotation speed of 0.2 revolutions per minute, the current position of a 10 µCi Co-60 source could be followed well at a distance of 1 meter.

It should be pointed out at this point that with certain device models it is advantageous to take the shading situation into account when defining the lookup tables. As an example, consider this in Device model shown. This model contains two high atomic number detectors and two low atomic number detectors. The total of four detectors can become four unidirectional and a bidirectional pair of detectors. In the case of the unidirectional detector pairs, it is advantageous if two different lookup tables are used on both sides of their respective connecting axis, which take account of the different shading conditions. Each of the four unidirectional pairs has one shaded and one unshaded side. On each of the shaded sides there are two detectors that shade certain angular areas. In contrast, the unshaded sides are free of detectors.

geeignet, der einen Parameter V_i benutzt, der für jedes Detektorpaar i als V_i = 1 oder V_i = -1 definiert werden muss. λ(ω_k, φ_i) kann die Werte -1 und 1 annehmen, womit die Auswahl nach verschattet/unverschattet getroffen wird.For the mathematical description, interpret for the device model three lookup tables, one for the bidirectional pair and two for the unidirectional pairs, the latter differentiated according to shaded / unshaded. The selection of a lookup table for the detector pair i depends on the azimuth angle ω _k and on the angle φ _{i of} the detector pair. To select the type of shading, z. B. the expression $$\lambda \left({\omega }_k,{\phi }_i\right)=\{\begin{array}{cc}-{\mathrm{V.}}_i,& \text{sin}\left({\omega }_k-{\phi }_i\right)<0\\ {\mathrm{V.}}_i,& \text{sin}\left({\omega }_k-{\phi }_i\right)\ge 0\end{array}$$

suitable, which _{uses a parameter V i} , which must be defined for each detector pair i as V _i = 1 or V _{i = -1.} λ (ω _k , φ _i ) can have the values -1 and 1, whereby the selection is made according to shaded / unshaded.

The present invention has been described on the basis of a few specific embodiments of the devices and methods. Of course, those skilled in the art of radiation detection will be able to make changes to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.

QUOTES INCLUDED IN THE DESCRIPTION

This list of the documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent literature cited

• US 2012/0043467 [0009]

Claims (15)

Method for the multi-dimensional direction measurement of gamma radiation in the far field by means of a group of several energy-discriminating, mutually synchronized detectors for detecting radiation, the method being lookup tables LUT ^SK , a defined function value f (E1, E2), a list of defined detector pairs with an identification number ID for defined pairs of detectors and one or more frequency distributions Y are used to record the measured values and include the following steps: a) Acquisition of measured values in list mode, the measured values originating from a radiation distribution in the far field, the radiation emanating from one or more radionuclides and the measured values are the interaction energies of the radiation occurring in the detectors; b) selecting the measured values according to coincidence events in defined detector pairs from two detectors each; c) associating selected coincidence events with an identification number ID; d) calculating the defined function value f (E1, E2) from two coincident energy values E1, E2 per selected coincidence event; e) recording the selected coincidence events according to their identification number ID and their function values f (E1, E2) in one or more frequency distributions Y, with a separate frequency distribution Y being available for each radionuclide, f) calculating one or more direction distributions X from the frequency distributions Y with a statistical image reconstruction method of emission tomography using lookup tables LUT ^SK , with a separate directional distribution X being available for each radionuclide. Procedure according to Claim 1 , the method prior to step a) additionally comprising, in part or in whole, the following sequence of steps: calibrating the signals from all detectors as absorbed radiant energy E; - Selecting a suitable coordinate system; - Identifying the detector pairs suitable for the direction measurement, the defined detector _pairs containing at least one detector made of a material with an ordinal number of Z eff> 30 and identifying this with an ID number i; - Interconnection of all detectors in a coincidence circuit in such a way that coincidences of all identified detector pairs are detected; - Identify the two detectors of each defined pair of detectors i with the numbers 1 and 2, whereby the detector with the lower ordinal number receives the number 1 and the one with the higher ordinal number receives the number 2, whereby the designation as 1 or 2 is chosen arbitrarily can if both detectors are made of the same material; - Detecting the directions of the defined detector pairs, whereby in the 2-dimensional direction _{measurement the direction of detector pair i is} detected with the azimuth angle φ i and where in the 3-dimensional direction measurement the direction of detector pair i with the azimuth angle φ _i and the elevation angle h _i is captured; - Define a function value f (E1, E2) which is calculated from two coincident energy values E1 and E2 in the two detectors of a detector pair i, the measuring range for f (E1, E2) being defined and divided into equidistant measuring value channels j = 1, ..., J is classified; Creation of one or more 2-dimensional arrays with I · J fields as data structures for storing the frequency distributions Y, in which coincidence events are registered according to their ID number i and their measured value channel j; a separate 2-dimensional array Y is created for each radionuclide; - Division of the detector pairs i into symmetry classes SK, with those detector pairs i that are mapped onto other structurally identical detector pairs when rotated or shifted, each being combined into a symmetry class SK (i); Creation of one or more lookup tables LUT ^SK for each symmetry class SK and each radionuclide, these spanning an angular range ϑ from 0 ° to 180 ° and being divided into equidistant angular steps; - Creation of a rule for generating and validating the lookup tables LUT ^SK from reference measurements, which selects the reference sources, takes the nuclide type into account, and defines the measurement conditions under which the measurements are to be carried out; - Execution of reference measurements for all steps previously defined in the regulation; - Creation and validation of the lookup tables LUT ^{SK in} accordance with the previously defined rule for evaluating the measurement data from the reference measurements; - Detecting the natural radiation background b ^{SK (i)} for all symmetry classes SK; - Excluding the natural radiation background b ^SK from the lookup tables LUT ^SK ; - Transfer of all lookup tables LUT ^SK for all symmetry classes and all radionuclides to an image reconstruction algorithm which processes the measurement data and calculates the direction distributions X. Method according to one of the preceding claims, characterized in that - a scintillation detector is used as the detector and / or the scintillator is designed as a monolithic block or as a pixelated scintillator module and / or the scintillator is made of pure or doped materials from the group of PVT, anthracene, stilbene , p-terphenyl, CaF ₂ , BaF ₂ , Nal, CeBr ₃ , LaBr ₃ , LaCl ₃ , La (Br _x Cl _1-x ) ₃ , Csl, SrI ₂ , CLYC, CLBC, CLCB, BGO, LSO, LYSO, GAGG, YAP and / or YAG is formed; and / or that a semiconductor detector is used as the detector and / or the semiconductor is designed as a segmented or unsegmented semiconductor and / or the semiconductor has a planar or coaxial geometry and / or made of materials from the group of Ge, GaAs, CdTe and / or CdZnTe is formed. Method according to one of the preceding claims, characterized in that at least two detectors are used; and / or that all detectors used are essentially structurally identical or that at least two of the detectors used are different from one another. Method according to one of the preceding claims, characterized in that the radiation sources emit a discrete and / or continuous distribution of radiation; and / or that the radiation sources emit gamma, electron, positron, proton and / or neutron radiation; and / or that the radiation originates from the radioactive decay of one or more radionuclides; and / or that the radiation is of low intensity, such as in astronomy. Method according to one of the preceding claims, characterized in that detector and system electronics are used which use analog and / or digital electronic components; and / or that the analog electronic components comprise a combination of different modules, which include a high-voltage supply, a preamplifier, an amplifier, a pulse shaper, a charge integrator, a pulse height analyzer, a multi-channel analyzer (MCA) and / or a coincidence circuit; and / or that the digital electronic components comprise a combination of different hardware and software components, including a high-voltage supply, an A / D converter per detector, a field programmable gate array (FPGA), a storage medium, a digital signal processor and / or evaluation software counting. Method according to one of the preceding claims, characterized in that the function value f (E1, E2) calculated from the two energy values E1, E2 represents the energy asymmetry $$f\left(\mathrm{E.}\mathrm{1,}\mathrm{E.}2\right)=\left(\mathrm{E.}2-\mathrm{E.}1\right)/\left(\mathrm{E.}1+\mathrm{E.}2\right)$$

or the cosine of the Compton scattering angle $f\left(\mathrm{E.}\mathrm{1,}\mathrm{E.}2\right)=1-m{c}^2/\mathrm{E.}2+m{c}^2/\left(\mathrm{E.}1+\mathrm{E.}2\right)$

is $\left(m{c}^2=511\text{keV}\right).$

Method according to one of the preceding claims, characterized in that in the selection of coincidence events, a selection condition with regard to the energy sum E1 + E2 of the energies detected in both detectors of a pair is applied; and / or that a separate frequency distribution Y is created for each radionuclide; and / or that a separate directional distribution X is calculated for each radionuclide. Method according to one of the preceding claims, characterized in that, in the case of radionuclides with several gamma energies, several selection conditions with regard to the energy sum E1 + E2 are applied; and / or that one or more frequency distributions Y are applied for such radionuclides with several gamma energies. Method according to one of the preceding claims, characterized in that the lookup tables LUT ^{SK are created} by measurements with the detector system or by Monte Carlo simulations or by means of a theoretical model. Method according to one of the preceding claims, characterized in that the statistical image reconstruction method is part or in whole of the Maximum Likelihood Expectation Maximization (MLEM) method, the Ordered Subset Expectation Maximization (OSEM) method, the List Mode - Maximum Likelihood Expectation Maximization (LM -MLEM) procedure and / or the List Mode - Ordered Subset Expectation Maximization (LM-OSEM) procedure. Method according to one of the preceding claims, characterized in that for each detected radionuclide a 2-dimensional directional distribution X _k = X (ω _k ) according to $$X_k^{\left[n+1\right]}=X_k^{\left[n\right]}\frac{K}{{\displaystyle \sum {}_{i,j}{Y}_{ij}}}{\displaystyle \sum _{i,j}\frac{\mathrm{L.}U{T}_j^{\mathrm{S.}K\left(i\right)}\left[\vartheta \left({\omega }_k,{\phi }_i\right)\right]Y_{ij}}{{\displaystyle {\sum }_{k\text{'}=1}^K\mathrm{L.}U{T}_j^{\mathrm{S.}K\left(i\right)}\left[\vartheta \left({\omega }_{k\text{'}},{\phi }_i\right)\right]X_{k\text{'}}^{\left[n\right]}}}}$$

from an initial distribution $$\begin{array}{cc}\forall k=\mathrm{1,}...,K& X_k^{\left[0\right]}=1\end{array}$$

using lookup tables LUT ^SK and an angular distance function according to $$\vartheta \left({\omega }_k,{\phi }_i\right)=\text{arccos}\left(\text{cos}\left({\omega }_k-{\phi }_i\right)\right)$$

is calculated. Method according to one of the preceding claims, characterized in that for each detected radionuclide a 3-dimensional directional distribution X _kl = X (ω _k , h _l ) according to $$X_{kl}^{\left[n+1\right]}=X_{kl}^{\left[n\right]}\frac{K\mathrm{L.}}{{\displaystyle \sum {}_{ij}{Y}_{ij}}}{\displaystyle \sum _{i,j}\frac{\mathrm{L.}U{T}_j^{\mathrm{S.}K\left(i\right)}\left[\vartheta \left({\omega }_k,H_l,{\phi }_i,{\beta }_i\right)\right]Y_{ij}}{{\displaystyle {\sum }_{k\text{'}=1}^K{\displaystyle {\sum }_{l\text{'}=1}^{\mathrm{L.}}\mathrm{L.}U{T}_j^{\mathrm{S.}K\left(i\right)}\left[\vartheta \left({\omega }_{k\text{'}},H_{l\text{'}},{\phi }_i,{\beta }_i\right)\right]X_{k\text{'}l\text{'}}^{\left[n\right]}}}}}$$

from an initial distribution $$\begin{array}{cc}\forall k=\mathrm{1,}...,K,l=\mathrm{1,}...,\mathrm{L.}& X_{kl}^{\left[0\right]}=1\end{array}$$

using lookup tables LUT ^SK and an angular distance function according to $$\vartheta \left({\omega }_k,H_l,{\phi }_i,{\beta }_i\right)=\text{arccos}\left(\text{cos}{\beta }_i\text{cos}{H}_l\text{cos}\left({\omega }_k-{\phi }_i\right)+\text{sin}{\beta }_i\text{sin}{H}_l\right)$$

is calculated. Device for carrying out the method for multidimensional direction measurement of gamma radiation in the far field according to one of the Claims 1 - 13th comprising: a group of several synchronized detectors for detecting radiation, at least one detector material having an ordinal number of Z _eff > 30 and all detectors measuring the energies E which occur in interactions of the radiation with the detector materials; System electronics which register coincidence events when interactions take place simultaneously in two detectors from a list of defined detector pairs i, the two detectors of all defined detector pairs i being uniquely identified with the numbers 1 and 2; - A data acquisition system that arranges the energies (E1, E2) measured in coincidence events according to their identification and saves them in a chronological list with the attributes {i, E1, E2} and the detection time t; and an analysis unit which creates one or more frequency distributions Y from the data stored by the data acquisition system and reconstructs one or more direction distributions X of the radiation field. Device according to Claim 14 , characterized in that the device is partially or completely designed as a Compton camera, a Compton telescope, a single plane Compton camera, a neutron camera and / or a dual gamma / neutron camera; and / or wherein the totality of all detectors has an annular shape, in which the detectors are arranged in a ring and / or wherein no, one or more central detectors are present in the interior of the ring; and / or the ring preferably contains four or five and particularly preferably six plastic scintillation detectors and / or there are preferably one or two scintillation detectors made of Nal, Csl, CeBr ₃ and / or LaBr ₃ inside the ring; and / or the scintillation detectors are preferably designed in the sizes 1 "x 1", 1.5 "x 1.5", 2 "x 2" and / or 3 "x 3".

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