WO2013186239A2

WO2013186239A2 - Method for locating at least one photon emission source

Info

Publication number: WO2013186239A2
Application number: PCT/EP2013/062088
Authority: WO
Inventors: Fabien PANZA
Original assignee: Institut De Radioprotection Et De Surete Nucleaire
Priority date: 2012-06-12
Filing date: 2013-06-12
Publication date: 2013-12-19
Also published as: WO2013186239A3; FR2991782A1; FR2991782B1

Abstract

The invention concerns a method for locating at least one photon source in a site, comprising the steps consisting of measuring (210) spectrometric data at at least three separate points on the site and, for each measurement, calculating (220), on the basis of the measured spectrometric data, a range of possible photon emission levels from a hypothetical radiation source at the origin of said data, on the basis of the possible distance between said hypothetical source and the point of measurement, and correlating (230) the ranges of photon emission levels from a hypothetical source on the basis of the possible distance between the hypothetical source and the current point of measurement, with the ranges of emission levels from a hypothetical source on the basis of the possible distances between the source and the previous points of measurement, in order to locate the source or sources and determine the photon emission level or levels of same.

Description

METHOD FOR LOCALIZATION OF THE SAME EMISSION SOURCE

PHOTONS

FIELD OF THE INVENTION

The invention relates to the field of the acquisition and processing of spectrometric data, for the real-time location of photon emission sources and the mapping of sites contaminated by such sources.

The invention is applicable and noticeable to the use of radioactive substances.

STATE OF THE ART

Radioactive source location techniques are already known in real time by acquisition of spectrometric or dosimetric data.

These techniques are implemented using a photon detector, for example X-ray, infrared, or gamma, which indicates in real time a count rate of photons detected for a given energy. Spectrometry makes it possible to know the identity of the atom, for example the radionuclide that emits the photons.

As the user moves and the count rate of the detector changes, the user infers that it is likely to move closer to or away from the photon source.

This approach makes it possible to locate a source by trial and error. However, it does not allow to measure the photon emission rate of the source, ie for example the activity when the source is radioactive.

It also does not allow to distinguish several sources if these sources are adjacent, and if they are of similar compositions, that is to say if they emit photons of the same energy.

In addition, this approach involves moving around the source in order to locate it, and even reaching the location of the source to obtain the maximum counting rate and thus locate the source. This can cause exposure to intense radiation, and cause danger to the operator's health. Another known technique consists in carrying out a series of measurements of the count rate by systematically squaring the area where the source is thought to be. Using this series of measurements it is possible to map the active area (if the response function of the detector is known) or the counting rate and thus find the location of the source.

This technique makes it possible to precisely locate a radioactive source but requires reprocessing of the data after taking measurements. It is therefore not adapted to a context where one seeks to locate one or more sources in real time.

PRESENTATION OF THE INVENTION

The invention aims to overcome at least one of the aforementioned problems. In particular, one of the aims of the invention is to propose a method of locating one or more sources of photon emission in real time, making it possible to determine the photon emission rate of the source, while limiting the risk exposure of the operator implementing the method.

In this regard, the invention provides a method for locating at least one photon source in a site, comprising the steps of measuring spectrometric data at at least three distinct points of the site, and at each measurement,

calculating, based on the measured spectrometric data, a range of possible photon emission rates of a hypothetical source of radiation at the origin of said data, as a function of the possible distance between said hypothetical source and the measurement point, said measure comprising:

o at each measurement point, the modeling of a surface centered on said point,

o at each point of the surface, the attribution of a photon emission rate of a hypothetical source situated at said point, as a function of the distance between the point and the measuring point, and

correlate the photon emission rate ranges of a hypothetical source as a function of the possible distance between the hypothetical source and the current measurement point, with the emission rate ranges of a source hypothetical according to the possible distances between the source and the previous measurement points to locate the source (s) and determine their photon emission rate,

the method being characterized in that the correlation step comprises:

identifying commonalities between

o the modeled surface at the current measurement point, and

o a modeled surface of at least one previous measurement point, and, for each common point, the calculation of a weighted average of the photon emission rate of a hypothetical source located at said point.

Advantageously, but optionally, the invention may further comprise at least one of the following features:

the spectrometric data measured includes a photon count rate in a given energy band.

the surface is modeled by a matrix, each element of which corresponds to a point of determined size of said surface.

the measuring step further comprises measuring, at each measuring point, the geographical coordinates of said point, and the step of calculating a range of possible photon emission rates of a hypothetical source comprises the projection on a geographical map of the site of the surface modeled from the geographic coordinates of the measuring points.

the weighted average at a point in common to several modeled surfaces takes into account the uncertainty in calculating the emission rate at the point and the distance of the point from the measurement point on which each surface is centered.

at the end of the correlation of the photon emission rates, the maximum levels correspond to the presence of a real source of photon emission. the source is a radioactive source and a photon emission rate corresponds to the activity of the source.

the phase of calculating a range of possible photon emission rates from a hypothetical source comprises calculating the activity according to a function which is written as follows: A (r, e, v) = N _f ¹

^ (r, e) - ^ - (e) _T (r, e, v)

where r is the distance between the measuring point and a source, e is the energy of the photons detected, v a model of distribution of radionuclides in the soil, Ν0 / Φ the intrinsic efficiency of the detector, N _f / N ₀ l geometric efficiency of the detector, and Φ / Α a correlation function between an activity in the ground and a flux of photons arriving on the detector.

The invention also relates to a system for locating at least one source of photons, the system comprising:

a mobile carriage, adapted to be moved in a site,

a radiation detector, mounted on said carriage, adapted to measure at a plurality of measurement points spectrometric data, a computing and processing unit, and

a memory in communication with the computing and processing unit, the system being characterized in that:

the radiation detector is adapted to transmit the spectrometric data to the computing and processing unit, and

said unit is adapted for, from the data,

calculating, based on the measured spectrometric data, a range of possible photon emission rates from a hypothetical source as a function of possible distances between the source and a measurement point, said ranges being stored in the memory, said calculation comprising modeling, at each measurement point, a surface centered on said point, and at each point of the surface, the attribution of a photon emission rate of a hypothetical source located at said point, as a function of the distance between the point and the point of measurement, and for

correlate the photon emission rate ranges of a hypothetical source as a function of the possible distances between the source and the current measurement point with the emission rate ranges of a hypothetical source as a function of the possible distances between the source and previous measurement points to localize the source (s) and determine their rate photon emission, said correlation comprising the identification of common points between

^■ the modeled surface at the current measurement point, and

^■ a surface modeled from the inside of a previous measurement point, and,

for each common point, the calculation of a weighted average of the photon emission rate of a hypothetical source located at said point.

The invention has many advantages.

It allows the development of a wide range of natural resources that are necessarily based on a systematic grid of the studied site. The source can further be located remotely. These two aspects reduce the exposure of the operator to radiation.

The sources thus determined are of known composition because the location is performed for a photon energy band selected by the operator.

In addition, the proposed method provides information on the position of the source in real time, which provides information quickly on the position of the source and its photon emission rate.

DESCRIPTION OF THE FIGURES

Other features, objects and advantages of the present invention will appear on reading the detailed description which follows, with reference to the appended figures, given by way of non-limiting examples and in which:

FIG. 1 represents a detection system adapted for implementing the method according to the invention.

FIG. 2 represents the main steps of the method according to the invention.

Figures 3a and 3b show the relationships between photon emission rate and distance to the detector simulated from the spectrometric measurements during the implementation of the method. FIG. 4 represents a pixel seen from two successive measurement points. Figure 5 shows the emission spectrum of an atomic species. Figures 6a and 6b respectively show a context of implementation of the method according to the invention, and the results obtained in this context. DETAILED DESCRIPTION OF AT LEAST ONE MODE OF IMPLEMENTATION

The method according to the invention makes it possible to locate at least one source of photon emission. The source may be a radioactive source that emits gamma photons as a result of atomic de-excitation. It may also be an infrared source or X-rays, the latter may be artificially excited for fluorescence detection of metal-type materials, for example.

In addition, the source may be of a diffuse or punctual nature, it may come from a surface deposit, a diffuse pollution, be a buried source, etc. The invention also applies where a site has undergone decontamination to verify whether there is a source of radiation.

Detection system

During the method according to the invention, an operator moves a detection system in a site where there is supposed to be at least one photon source.

With reference to FIG. 1, such a detection system 1 comprises a mobile carriage 10 on which a radiation detector 11 is mounted. The detector is adapted to the type of radiation studied; for example, in the case of the location of a radioactive source, the detector is advantageously a germanium or GeHP semiconductor spectrometer because it has a very fine resolution, of the order of 0.25% to the photon energy 600 keV.

In the case of a germanium spectrometer, the system is also provided with a cooling system 14, for example a liquid nitrogen tank or a cryo-electric system for cooling the detection crystal.

This detector is not collimated, so that it can detect photons coming from the surface with a detection opening of 180 °, corresponding to a solid angle of 2π steradians.

The system further comprises a computing and processing unit 12, and a memory 13 connected to said unit. The detector is provided with an interface enabling it to transmit data to the computing and processing unit. The system may further comprise a positioning device 16, for example a satellite navigation system (GPS), connected to the computing and processing unit, for associating with the spectrometric data acquired at a measurement point the geographic coordinates of said point.

Location method

Referring to Figure 2, there is shown the main steps of the locating method.

initialization

This method comprises an initialization step 100, during which an operator selects, for example by means of a man-machine interface integrated in the computing and processing unit, a band of energy lines to be detected. Indeed, whatever the nature of the source, the energies of the emitted photons are characteristic of the composition of said source.

For example, if an operator searches for a source of radioactive cesium, the decay energy of cesium 137 is 667.6 keV. An operator therefore selects an energy band centered on the 661.6 keV line.

The operator also selects other measurement parameters such as the depth distribution of the source in the ground (for example surface or buried), information on the nature of the ground (composition, flatness ...) and parameters. related to the detector (elevation in relation to the ground, type of detector ...).

The importance of selecting these parameters during an initialization step is explained below. Measurement and simulation

Following this initialization step, the operator positions the system on the site at a first measurement point. At this measurement point, during a step 210, it measures spectrometric data, as well as the geographical coordinates of the measurement point.

The spectrometric data in particular comprise a photon counting rate, that is to say a number of events per unit of time, in the previously selected energy band. In the case of a radioactive source, this photon count corresponds to the number of gamma decays of the source, which depends on its activity. Then, from the spectrometric data measured by the detector, and which are transmitted to the computing and processing unit, the latter implements a photon emission rate calculation step 220 of a hypothetical source. function of the distance between the source and the measurement point that will be presented with reference to Figures 3a and 3b.

By way of non-limiting example, FIG. 3a shows curves representing the relationship between the distance between a hypothetical source at the origin of detected photons and the measurement point, and the actual photon emission rate of this source (that is, the activity in the case of a radioactive source).

Each curve is made for a photon count rate set at 1 per second, for a detector positioned at 0.3 m height from the ground surface, for a given detector whose response function is known, and for a type of distribution of radionuclides in the soil.

The curves of Figure 3a have been modeled for three types of photon emission source distributions in the soil:

Surfacic, corresponding to a deposit on the surface,

Exponential, corresponding to an initial deposit on the surface followed by an infiltration into the ground, and

Homogeneous, corresponding to a natural presence.

These curves were further modeled for different extents of a source, namely 1 m ² , 5 m ² , and 10 m ² . For the case of an exponential distribution, the surface activity also depends on a coefficient b or β called "coefficient of relaxation mass", the activity with: A (z) = A ₀ e ^~ (b ) ^z where A ₀ is the surface activity, and p is the density of the soil in g. cm ^"3. The curves of Figure 3a have been modeled with b = 1 g / cm ^2.

These curves are given, in the case of a radioactive source, by the following equation:

A (r, e, v) = N _f ¹

^ - (r, e) - ^ - (e) _T (r, e, v)

OR

- A is the activity of the source, in Bq or Bq, - R is the distance between the detector, ie the measuring point, and the source, in meters,

Nf is the count rate recorded by the detector in counts / s, v is the distribution of radionuclides in the soil,

e is the energy of the radiation, in keV,

- is the angular distribution of the photon flux, calculated analytically, which depends solely on the distribution of radionuclides in the soil,

corresponds to the intrinsic efficiency of the detector, that is to say to the detection efficiency of a photon flux reaching the detector parallel to its axis, and

Nr

- - corresponds to the geometric efficiency of the detector. It translates the

N ₀

variation of the term ¾n as a function of the angle of incidence of the photon flux with respect to the axial incidence. It depends on the geometry of the detector. The function is advantageously tabulated prior to

the use of the detection system, depending on the parameters of the detector used, and for different types of radionuclide distributions in the soil, as well as for different photon energies to be detected.

The functions thus tabulated are stored in the memory 13.

In this way, during the initialization step 100 by the operator, the input of the various parameters enables the computing and processing unit 12 to load the corresponding function. Then, at each measurement, the detector supplies the calculation unit with the counting rate N _f, and the calculation unit can obtain, by a simple calculation 221, the activity A of a source as a function of its distance as illustrated. in Figure 3a.

During a step 222, the computing unit generates a matrix of pixels centered on the measurement point, each pixel representing a determined diameter of a surface centered on the measurement point.

With reference to FIG. 3b, the calculation unit uses the equation of the above activity to associate each pixel with a given activity, as a function of the distance between the zone represented by the pixel and the measurement point. to obtain the count rate measured at the detector. This matrix is thus constructed as if a hypothetical source was present at each point of the map, each source being at the origin of the counting rate measured.

Finally, during a step 223, the computing unit projects, on a map of the area studied, the m a m i nsi m odelized according to the geographic coordinates of the measurement point that were acquired in the same time as the spectrometric data.

Correlation

Returning to FIG. 2, once a measurement has been acquired and the calculation step 220 completed, the operator moves the detection system 1 during a step 300 to arrive at a new measurement point.

At the new measurement point, measurement steps 210 and calculation 220 are repeated. Then, a correlation step 230 is implemented by the computing and processing unit 12.

This step consists of correlating the values of photon emission rates calculated from a point of the site appearing in several matrices modeled during the simulation step 220. Thus, each measurement at a new measurement point provides information. additional and refines the estimate of the photon emission rate at each point.

To do this, pixels of the surface generated at the current measurement point are identified, which correspond to points of the studied zone represented by pixels of other surfaces modeled in one or more of the preceding measurements. We therefore identify points of the studied geography zone that are common to several matrices modeled around each measurement point.

This identification is enabled by the knowledge of the respective positions of the different measurement points, the size of the modeled surface represented by a matrix, and the dimension represented by each pixel of each surface, the latter elements being parameters chosen according to the calculation we want to conduct.

With reference to FIG. 4, there is shown such a pixel seen from two successive measurement points i and i + 1. The coordinates of this pixel are I and m respectively in abscissa and ordinate. For each pixel, the activity of the pixel A (l, m) is calculated by the average r party calculated activities of all measurement points weighted by the uncertainty of each activity: A (m) = Σ _{J Î} _"Î _Î ^ -

Now, to allow the real-time calculation of the photon emission rates (activities in the case of radioactive emission) for the different points of the studied site, the previous calculation is simplified by keeping to each measurement only the weighted average of the measures of the previous points.

Thus, in a current measure, the weighted average of the activity at the current measurement point and the average activity for the previous measurement points are only plotted. We thus obtain the following equation:

Garlic; m) _k = + fik-i-A (l; m) _fc _ ₁

k is the index corresponding to the current measurement point and k-1 is the index corresponding to the average measurement point of the previous measurement points,

- May-m ^. ^ - ^ -. - ^. Or. ^ -)

ε is the efficiency of the detector, which depends on the distance r between the pixel and the detector,

N is the count rate of a given measure,

And μ is the weight of a given measure.

The weighting is calculated according to the uncertainty of the count rate of the detector and the distance between the pixel and the detector,

μ, is expressed as follows:

1

μ ^{ΐ, 'ηΐ =} (o _Ai ; m). d ^ l; m) f

Where σ _Α . (1; m) is the error on the photon emission rate (the activity in the case of radioactive radiation) calculated for the coordinate pixel (I; m), and d is the distance between the detector and the pixel (I; m).

The coefficient σ _Α (1; m) is explained with reference to FIG. 5, which represents a peak of photon emission for a given atomic species. The surface H corresponds to the number of photons recorded by the spectrometer during the acquisition time for the emission line characteristic of the species in question.

The area I on the right of the peak corresponds to the background noise and the surface G corresponds to the additional background noise produced by the Compton background of the higher energy photons. The count rate measured in the calculation of the activity of a point of an area corresponds to the surface area of the surface H.

The uncertainty on the rate of discharge is as follows:

With a _s . the uncertainty on the surface S, of the peak, and (^ j) the relative uncertainty of the calibration of the spectrometer, of the order of 10%.

Back to Figure 5, we have:

j = Xc

Where h _j is the total number of events in channel j (see Figure 5) and ¾ the number of events of channel j belonging to the Compton background of higher energy lines.

The uncertainty on the area of the ic is thus calculated as: a _s . =

= σ _Η . + a _G. = JIÏI + JGI

The area G under the peak is calculated by a linear adjustment of the Compton bottom to the right and to the left of the peak. Per channel the number of events ¾ is calculated as follows:

3j = ( ^x j - ^x c) "_ _x ) + y _t

x _c and x _e being the abscissas of the extreme terminals of the peak in energy, and y _ap and y _av the respective ordinates of extreme bo rn es in co u ps pa rs eco nde, which correspond to the background noises before and after the peak.

The background noise before and after the peak is calculated as follows:

bin V

Where bin is the width of a channel in keV, x _b and x _has the abscissa of points in the area before the peak, and x _f and x _g the abscissa of points in the area after the peak.

The data, once correlated, make it possible to identify the absolute position of a source, in the site on which the measurements were made, as well as the photon emission rate of the source.

For this, the measurement steps 210, simulation 220, and correlation steps 230 are repeated in at least three measurement points distinct from the site, to "triangulate" the position of one or more sources. Of course, the more the number of measurement points is important, and the more accurately the source is located.

Application of the invention

Referring to Figure 6a and Figure 6b, there is shown an example of application of the invention. FIG. 6a shows a mapped site in which a ¹³⁷ Cs radioactive source S remains in an old building. This source has an extension of 3 * 3 m ² , and an activity of 1000 Bq / kg.

A path from an operator between a starting point A and an arrival point B is shown, during which the operator moves a detection system 1 and implements the method illustrated in FIG. 2, the measurements being acquired all the meters.

Figure 6b shows the result of the implementation of the method. The darkest areas have an activity less than 100 Bq / kg. The lightest areas correspond to an activity of between 100 and 1000 Bq / kg. The intermediate zones correspond to an activity exceeding 1000 Bq / kg.

It is therefore found that it is possible for a source S with activity greater than 1000 Bq / kg located by the method to exist in the intermediate zone.

Claims

1. A method of locating at least one photon source in a site, comprising the steps of measuring (210), at at least three distinct locations of the site, spectrometric data, and at each measurement,

calculating (220), based on the measured spectrometric data, a range of possible photon emission rates of a hypothetical source of radiation at the origin of said data, as a function of the possible distance between said hypothetical source and the measuring, said step comprising:

o at each measurement point, the modeling (222) of a surface centered on said point,

correlating (230) the photon emission rate ranges of a hypothetical source as a function of the possible distance between the hypothetical source and the current measurement point, with the emission rate ranges of a hypothetical source based on possible distances between the source and the previous measurement points to locate the source (s) and determine their photon emission rate

the method being characterized in that the correlation step comprises:

identifying commonalities between

o the modeled surface at the current measurement point, and

o a modeled surface of at least one previous measurement point, and for each common point, the calculation of a weighted average of the photon emission rate of a hypothetical source located at said point.

2. Method of location according to the preceding claim, wherein the measured spectrometric data comprises a photon count rate in a given energy band.

3. Location method according to one of claims 1 or 2, wherein the surface is modeled by a matrix of which each element corresponds to a predetermined size point of said surface.

4. The locating method according to claim 3, wherein:

the measuring step further comprises measuring, at each measurement point, the geographical coordinates of said point, and

the step of calculating (220) a range of possible photon emission rates from a hypothetical source comprises projecting (223) on a geographical map of the site of the modeled surface from the geographic coordinates of the measurement points .

5. A method of locating according to one of the preceding claims, wherein the weighted average at a point common to several modeled surfaces takes into account the uncertainty on the calculation of the emission rate at the point and the distance from the point relative at the measuring point on which each surface is centered.

6. Location method according to one of the preceding claims wherein, at the end of the correlation of photon emission rates, the maximum levels correspond to the presence of a real source of photon emission.

7. The method of location according to one of the preceding claims wherein the source is a radioactive source and a photon emission rate corresponds to the activity of the source.

The locating method according to claim 7, wherein the computing step (220) of a possible photon emission rate range of a hypothetical source comprises calculating the activity (221) according to a function that is written as follows:

Mr, e, v) = N _r ¹

flj (e) -j- (e) -j (r, e, v) where r is the distance between the measuring point and a source, e is the energy of the photons detected, v a model of distribution of radionuclides in the soil, Ν0 / Φ the intrinsic efficiency of the detector, N _f / N ₀ l geometric efficiency of the detector, and Φ / Α a correlation function between an activity in the ground and a flux of photons arriving on the detector.

9. System (1) for locating at least one photon source, the system comprising:

a mobile carriage (10), adapted to be moved in a site,

a radiation detector (1 1), mounted on said carriage, adapted to m e u u r e n u e p u l l e d o n e d e d e m e d e d e d e s s o n e s spectrometric,

a computing and processing unit (12), and

a memory (13) in communication with the computing and processing unit, the system being characterized in that:

the radiation detector (1 1) is adapted to transmit the spectrometric data to the computing and processing unit, and

said unit (12) is adapted for, from the data,

calculating, based on the measured spectrometric data, a range of possible photon emission rates from a hypothetical source as a function of possible distances between the source and a measurement point, said ranges being stored in the memory, said calculation comprising modeling (222), at each measurement point, a surface centered on said point, and at each point on the surface, assigning a photon emission rate from a hypothetical source located at said point, depending on the distance between the point and the point of measurement, and for

o correlate the photon emission rate ranges of a hypothetical source as a function of the possible distances between the source and the current measurement point with the emission rate ranges of a hypothetical source as a function of the possible distances between the source and the preceding measurement points to locate the source or sources and determine their photon emission rate, said correlation comprising the identification of common points between ^■ the modeled surface at the current measurement point, and

^■ a modeled surface of at least one previous measurement point, and,