DE102019122115A1 - Detector unit for a scintillation counter, radiometric measuring device with such a detector unit, method for operating such a detector unit and computer program code - Google Patents

Abstract

The present invention relates to a detector unit for a scintillation counter, with at least two parallel signal paths, a first signal path having a first silicon photomultiplier and a second signal path with a second output signal having a second silicon photomultiplier, and the output signals of the signal paths being fed to a coincidence unit , characterized in that the detector unit has at least one unit for pulse shape analysis.

Description

The invention relates to a detector unit for a scintillation counter of a radiometric measuring device for level measurement, limit level measurement, density measurement or flow measurement according to the preamble of claim 1, a radiometric measuring device with such a detector, a method for operating such a detector according to the preamble of claim 9 and computer program code the preamble of claim 11.

A large number of radiometric measuring devices with so-called scintillation counters are known from the prior art. The basic structure and mode of operation of such a radiometric measuring device are related to 6th and are explained below.

6th shows the schematic structure of a scintillation counter 1 as is known from the prior art. The scintillation counter 1 has a scintillator 3 as a material sensitive to ionizing radiation. The scintillator 3 is excited, for example, by γ radiation, the excitation energy from the impact processes being given off again in the form of light. By measuring the amount of light you can see in the scintillator 3 Energy deposited by the radiation and by measuring the number of light flashes per unit of time (counting rate) can be deduced from the intensity of the radiation.

The scintillation counter indicates this 1 one to the scintillator 3 downstream photomultiplier tube 20th , or photomultiplier for short. In the photomultiplier 20th detach from the scintillator 3 Incoming photons Electrons from the surface of a photocathode, which are then accelerated and multiplied several times in an electron multiplier in an electric field between several electrodes until a measurable electrical signal is present at the end of the electron multiplier. In one of the photomultiplier 20th downstream counter 13 the number and optionally also the amplitude of the electrical signals generated in this way are recorded and can be further processed in electronics that are not shown in detail here.

Measuring devices in the industrial environment and in particular radiometric measuring devices for level measurement, limit level measurement, density measurement or flow measurement can be exposed to a strongly fluctuating temperature range. In order to avoid damage to the measuring device, robust and well-insulating housings can be used. However, these are complex to manufacture and have increased weight.

In the DE 10 2017 205 738 A1 Therefore, a radiometric level measuring device with a scintillator and a photodetector, which is cooled or heated as required via an active cooling element within the measuring device, is proposed. This allows the area of application of the radiometric measuring device to be expanded and the effort involved in manufacturing the housing to be reduced.

In the prior art, so-called silicon photomultipliers are also known, which are intended to serve as a replacement for the previously customary photomultiplier tubes.

A silicon photomultiplier is an arrangement of several avalanche photo diodes (APD). The typical size is a few square millimeters, on which several hundred avalanche photo diodes are housed. In this context, these are also referred to as pixels.

In the present application, a silicon photomultiplier should also be understood to mean photodetectors with avalanche photodiodes as sensitive elements that are produced on the basis of other substrates, for example on the basis of germanium or other suitable substrates.

The avalanche photodiodes used in a silicon photomultiplier are operated in the Geiger range. They achieve a short-term gain of up to 10 ⁸ , since an electron-hole pair generated by a single photon can generate several million charge carriers due to the acceleration in the multiplication zone caused by the high electric field strength.

Since an avalanche photodiode operated in the Geiger area supplies pulses with a fixed height, the signal height of a silicon photomultiplier is proportional to the number of pixels triggered. The signal level can be specified in pe (pixel equivalent). A pe corresponds to the signal amplitude of a single pixel. Compared to conventional photomultiplier tubes, silicon photomultipliers offer several advantages.

Silicon photomultipliers are inexpensive, space-saving and mechanically relatively insensitive. They also require a much lower supply voltage than photomultiplier tubes. Silicon photomultipliers are not affected by magnetic fields, which is another advantage for use in large detectors.

A major disadvantage is the strong temperature dependence of the silicon photomultiplier. The photodiodes used in a silicon photomultiplier often have a relatively small amount Temperature range in which they can deliver good measurement results, and tend to generate a dark current at excessively high temperatures, which can have a negative effect on the measurement accuracy of the radiometric measuring device. Due to the strong temperature dependence of the thermal noise of semiconductor photodiodes and thus silicon photomultipliers, their use in combination with a scintillator in a radiometric measuring device at higher temperatures, for example 60 ° C., is difficult. The dark noise of a silicon photomultiplier doubles approximately every 10 K. Thus, the dark noise at 60 ° C is approx. 16 times higher than at 20 ° C.

The breakdown voltage of the avalanche photodiodes also increases as the temperature increases, and the gain factor therefore decreases. At the same time, the dark rate increases, i. H. the number of pulses per unit of time that occur without optical excitation. In this context, dark current refers to the spontaneous formation of free charge carriers due to heat in a light-sensitive semiconductor, for example a photodiode.

Silicon photomultipliers consist of a large number of micro cells (pixels). The signal amplitude that a microcell generates due to the absorption of a photon is defined as 1 pe. A sum signal, which the silicon photomultiplier supplies due to the microcells connected in parallel, is thus proportional to the number of excited cells. From a statistical point of view, it is possible for m microcells to simultaneously supply a first signal S1 as a dark current due to the thermal noise, with $$\text{S.}1=\text{m}\times \text{pe}$$

Another characteristic property of silicon photomultipliers is what is known as cross-talk. Electron-hole pairs recombine in an avalanche photodiode and the resulting photons can generate a signal in neighboring pixels. This optical crosstalk from a microcell to n neighboring cells can result in a second signal S2, where $$\text{S2}=\left(\text{n}+1\right)\times \text{pe}$$

A silicon photomultiplier can thus deliver pulses with amplitudes of several pe without optical excitation. As the dark current increases with temperature, the number of dark pulses per unit of time and their amplitude increases. Since dark pulses with a large amplitude cannot be distinguished from an event to be counted only on the basis of their amplitude, the counting rate of such a photodetector can change significantly with temperature.

By combining the measurement signals of at least two silicon photomultipliers and a suitable signal acquisition / signal evaluation, the dark noise can be significantly reduced or eliminated, so that a falsification (increase) of the count rate due to the increase in dark pulses at higher temperatures is reduced. This can be done through the detection of signals occurring within a definable time interval, which are generated simultaneously by means of two semiconductor photodiodes, i.e. h coincident, to be detected, to be solved. Thus, statistically distributed dark pulses contribute only to a small extent to the count rate to be measured.

The silicon photomultipliers are ideally arranged directly adjacent, for example in an array at one end of a scintillator. This ensures that the light pulses generated by γ quanta or particles in the scintillator material generate electrical signals of comparable amplitude, the amplitude or signal shape of which is not falsified by absorption in the scintillator material due to different signal paths. Due to the local proximity of the at least two semiconductor photodetectors, they also have approximately the same temperature, so that their dark noise changes to a comparable extent with this during operation.

However, these measures cannot eliminate all dark pulses.

It is the object of the present invention to improve the known devices and methods in such a way that the measurement results are less falsified due to dark pulses.

A detector unit according to the invention for a scintillation counter, with at least two parallel signal paths, wherein a first signal path with a first output signal has a first silicon photomultiplier and a second signal path with a second output signal has a second silicon photomultiplier, and the output signals of the signal paths are fed to a coincidence unit is characterized in that the detector unit has at least one unit for pulse shape analysis.

Because two silicon photomultipliers connected in parallel are used in two parallel signal paths in the detector unit according to the invention, the pulses generated on the basis of a light flash generated in the scintillator can be distinguished from dark pulses in the downstream signal processing.

The output signals of both signal paths are fed to the coincidence unit. At the output of the coincidence unit, only the signal pulses that occur simultaneously on both signal paths are output. Statistically distributed pulses that cannot occur in both signal paths at the same time can be filtered out.

Because the light pulses generated by scintillation processes in the scintillator have a different time decay behavior than the dark pulses generated in the semiconductor photodiode, they can be distinguished from one another by a pulse shape analysis. In this way it is not only possible to exclude the randomly distributed dark pulses that do not occur coincidentally in the two silicon photomultipliers from pulse counting in the scintillation counter, but also those dark pulses that occur coincidentally can be excluded from the count due to the pulse shape analysis. This object is achieved by a detector unit with the features of claim 1, a radiometric measuring device with the features of claim 8, a method for signal evaluation with the features of claim 9 and computer program code according to claim 11.

In a preferred embodiment, the unit for pulse shape analysis is connected downstream of the coincidence unit; H. that the pulse shape is only analyzed for the pulses that actually occur on both signal paths at the same time.

The silicon photomultipliers can each be followed by a discriminator unit which only forwards those pulses at its output which exceed a predefined or predefinable threshold value. The dark pulses below a threshold value are filtered out by the discriminator unit. The discriminator is usually designed as an amplitude discriminator and thus filters pulses below a specific threshold specified in the discriminator. In this way, dark pulses of low amplitude are filtered out even before further signal processing and the coincidence unit and the pulse shape analysis only need to be applied to a reduced number of pulses, which means a saving in computing power and thus energy.

The unit for pulse shape analysis is advantageously designed to use a supplied signal to output an output signal to distinguish a dark pulse from a light pulse on the basis of a detection of a γ quant or the passage of a charged particle, for example a proton or muon. Due to a different decay behavior of pulses of a dark pulse, one by a γ-quantum, or more generally by a pulse triggered by a radiating event in the silicon photomultiplier, a distinction of the physical origin of the respective pulse can be recognized based on the pulse shape and a suitable output signal can be provided. The output signal can, for example, be such that only pulses generated due to radiative events are allowed to pass through, or that an output signal is given to a pass-through circuit to which the pulses are also fed, so that the desired pulses, i.e. H. in the present case, the pulses that are based on a radiating event let through.

The unit for pulse shape analysis can have at least one integrator for integrating supplied signal pulses, with a first integral formed over a first part of the supplied signal and a second integral formed over the entire supplied signal, and a decision criterion from a quotient of the difference of the second integral and the first integral and on the other hand the second integral is formed. On the basis of the decision criterion, it can then be decided which physical origin the examined pulse has, and the procedure can be carried out accordingly, i. H. For example, the pulse based on a radiating event can be let through.

The differentiation of the pulses of different physical origin can be done by means of signal integration in corresponding time windows relative to a time trigger mark of the pulse by means of a Psd Parameters. Of the Psd -Parameter is determined as follows: $$P\mathrm{S.}\mathrm{D.}=\frac{Q_{\text{long}}-Q_{\text{short}}}{Q_{\text{long}}}$$

The parameter Q is generated _briefly by signal integration in a time window around the signal edge of the pulse, which is positioned using the time stamp of the discriminator unit, and Q _long by an integration over the entire pulse. As a result, the parameter Q _{long is} proportional to the total energy of the event and Q _short determines the energy of the pulse in the time window around the rising edge. Are the time windows for integration to determine the Psd Parameter is selected accordingly, is the Psd -Parameters for a type of physical event, e.g. the detection of a γ quantum, largely independent of the energy.

The trigger mark for the pulse is, for example, when a predetermined threshold value is exceeded on the rising edge of the pulse. However, this simple threshold value discriminator is usually not usable for an exact determination of a time trigger mark, since the trigger time depends on the total pulse height (so-called time walk effect). Alternatively, the trigger mark can be derived from the signal from a constant fraction discriminator.

The input signal is processed in two parallel signal paths. In one path, it is delayed by a fixed amount of time that is less than the rise time of the signal. In the other path, the signal is inverted and multiplied by a fixed factor. The two signals treated in this way are added again. The first zero crossing of the sum signal can be used as a trigger mark.

The time windows for the integration Q _short and Q _long are ideally positioned relative to the temporal trigger mark of the event, which has been determined, for example, by means of a constant fraction discriminator.

Despite the dark noise, which is strongly dependent on the temperature, the use of semiconductor photodiodes or silicon photomultipliers is advantageous, as these require much lower operating voltages than conventional photomultiplier tubes, so that operation in a 4-20 mA current loop is possible. In a preferred embodiment, the radiometric measuring device and thus the detector unit are operated in a 4-20 mA current loop and are preferably completely supplied with energy via this.

Furthermore, due to the lower operating voltage, among other things, the structural measures required for operation in potentially explosive atmospheres are considerably simplified. Furthermore, in contrast to a photomultiplier tube, silicon photomultipliers are insensitive to strong, external magnetic fields and can therefore be used in a variety of ways.

Furthermore, the silicon photomultipliers are ideally operated by means of a temperature-compensated voltage supply so that their gain remains constant over temperature and thus comparable events deliver comparable signal amplitudes at different temperatures.

As shown above, the photodiodes used in a silicon photomultiplier often have a relatively low temperature range in which they can deliver good measurement results, and tend to generate a dark current at too high temperatures, which has a negative effect on the measurement accuracy of the radiometric measuring device can. The effects of the dark current can be reduced with the present invention. In addition, the silicon photomultiplier can be cooled so that the temperature moves in a range in which the dark current has no effect or at least no significant effect on the measurement.

It is therefore advantageous if the radiometric measuring device has an active cooling element for regulating the temperature of the detector unit. For example, the cooling element can be designed as a Peltier element and be able to actively cool or heat the detector unit and in particular the silicon photomultiplier, as required.

In particular, photodiodes or silicon photomultipliers can also be used, which are only permitted up to a temperature of, for example, minus 20 ° C., and which are heated by the Peltier element when the temperature falls below this. This is particularly advantageous when the area of application of the radiometric level measuring device is also below this temperature.

By reducing the dark current by means of temperature control, the sensitivity of the measuring device can be increased. This improves the accuracy and the stability of the measurement.

The radiometric measuring device can in particular be set up for connection to a two-wire loop (e.g. 4-20mA, fieldbus: HART, PA, FF), via which it is supplied with the power required for its operation and which is optionally also used for data exchange . In this case it can be provided that the active cooling element is supplied with energy separately, that is to say does not draw its energy via the two-wire loop. It can also be provided that the measuring device has an energy store, which stores energy from the two-wire loop and is responsible for supplying energy to the active cooling element.

• - Ermitteln von Signalen, die eine Amplitude aufweisen, die größer ist, als ein vorgebbarer Schwellenwert,
• - Ermitteln koinzidenter Signale oberhalb des Schwellenwerts in den Signalpfaden;

A method according to the invention for signal evaluation of a detector unit for a scintillation counter, with at least two parallel signal paths, a first signal path with a first output signal having a first silicon photomultiplier and a second signal path with a second output signal having a second silicon photomultiplier, the method using the signals from both signal paths has the following steps:

• - Determination of signals that have an amplitude that is greater than a predefinable threshold value,
• Determining coincident signals above the threshold value in the signal paths;

• - Analysieren der Signalform der Signale und Ausgeben der Signalen, die einem vorgebbaren Unterscheidungskriterium entsprechen,
• - Zählen der koinzidenten Signale oberhalb des Schwellenwertes, die das Unterscheidungskriterium erfüllen. Die einzelnen Verfahrensschritte können seriell oder ganz oder teilweise parallel ausgeführt werden.

The procedure is characterized by the following steps:

• - Analyzing the signal shape of the signals and outputting the signals that correspond to a predefinable differentiation criterion,
• - Counting the coincident signals above the threshold value which meet the distinguishing criterion. The individual process steps can be carried out in series or completely or partially in parallel.

In the two signal paths, the signals are first determined that have an amplitude or energy that is above a predeterminable threshold value. From these signals the signals are determined which are coincident, i.e. H. occur at the same time and are therefore very likely to have occurred due to a radiating event. The pulses due to a radiating event are coupled to the silicon photomultiplier directly from the scintillator material or via a light guide and therefore generate coincident electrical pulses.

Since there is a low probability that dark pulses with sufficient amplitude can occur coincidentally on both signal paths, a criterion for distinguishing between those pulses that can actually be traced back to a radiating event is then determined by analyzing the signal shape.

The pulses determined in this way are output and counted in a suitable manner in order to be able to make a statement about the radiation intensity at the location of the scintillator material and thus about the level, limit level, density or flow rate based on the number and, if necessary, intensity of the determined pulses.

The differentiation criterion can be calculated, for example, by forming a first integral over a first part of the supplied signal and a second integral over the entire supplied signal and forming a quotient from the difference between the second integral and the first integral on the one hand and the second integral on the other.

• - Ermitteln von Signalen, die eine Amplitude aufweisen, die größer ist, als ein vorgebbarer Schwellenwert,
• - Ermitteln koinzidenter Signale oberhalb des Schwellenwerts in den Signalpfaden, erfindungsgemäß wird die Recheneinheit dazu veranlasst, folgende Schritte auszuführen:
• - Analysieren der Signalform der Signale und Ausgeben der Signalen, die einem vorgebbaren Unterscheidungskriterium entsprechen,
• - Zählen der koinzidenten Signale oberhalb des Schwellenwertes, die das Unterscheidungskriterium erfüllen.

The present invention can also be implemented as computer program code which, when fed from a computing unit of a scintillation counter to the output signals of a detector unit with at least two parallel signal paths, wherein a first signal path with a first output signal a first silicon photomultiplier and a second signal path with a second signal path has a second silicon photomultiplier, is executed, this arithmetic unit causes the following steps to be carried out:

• - Determination of signals that have an amplitude that is greater than a predefinable threshold value,
• Determination of coincident signals above the threshold value in the signal paths; according to the invention, the computing unit is prompted to carry out the following steps:
• - Analyzing the signal shape of the signals and outputting the signals that correspond to a predefinable differentiation criterion,
• - Counting the coincident signals above the threshold value which meet the distinguishing criterion.

The computer program code can also contain components that cause the computing unit to form a first integral over a first part of the supplied signal and a second integral over the entire supplied signal in order to determine the differentiation criterion, and the differentiation criterion from a quotient of, on the one hand, the difference of the to form the second integral and the first integral and on the other hand the second integral.

According to the invention, there is thus a method for operating a radiometric level sensor at elevated operating temperatures with at least two semiconductor photodetectors, e.g. silicon photomultipliers, with temperature-compensated voltage supply, with only coincident, i.e. coincident, counting rates for the counting rate used to determine the level. H. simultaneous events of the two photodetectors contribute, which firstly have comparable amplitudes and / or signal shapes and / or energies and secondly are greater than a predetermined threshold value.

• 1 ein Blockschaltbild eines Szintillationszählers mit einer Detektoreinheit gemäß der vorliegenden Anmeldung,
• 2 Signalverläufe an den in 1 gekennzeichneten Stellen der Detektoreinheit,
• 3 den Ablauf eines Verfahrens zur Signalauswertung einer Detektoreinheit für einen Szintillationszähler gemäß der vorliegenden Anmeldung,
• 4a) ein Blockschaltbild eines Szintillationszählers mit einer Detektoreinheit mit digitaler Signalverarbeitung,
• 4b) eine Variante zur Analog-Digitalwandlung der Signale,
• 5 eine vergrößerte Darstellung eines Signalverlaufs eines strahlenden Ereignisses, die daraus ermittelte Triggermarke sowie die zur Auswertung verwendeten Integrationsfenster und
• 6 schematisch den Aufbau eines Szintillationszählers gemäß dem Stand der Technik (schon behandelt).

The present invention is explained in detail below on the basis of exemplary embodiments with reference to the accompanying figures. Show it:

• 1 a block diagram of a scintillation counter with a detector unit according to the present application,
• 2 Signal curves on the in 1 marked points of the detector unit,
• 3 the sequence of a method for signal evaluation of a detector unit for a scintillation counter according to the present application,
• 4a) a block diagram of a scintillation counter with a detector unit with digital signal processing,
• 4b) a variant for analog-digital conversion of the signals,
• 5 an enlarged representation of a signal curve of a radiant event, the trigger mark determined therefrom and the integration windows used for evaluation and
• 6th schematically the structure of a scintillation counter according to the prior art (already dealt with).

In the figures, unless otherwise stated, the same reference symbols denote the same components with the same function.

1 shows a block diagram of a scintillation counter 1 with a detector unit 10 according to the present application.

The scintillation counter 1 has a scintillator 3 as a sensitive element for ionizing radiation, in which light flashes are generated due to the incoming radiation. The scintillator 3 are in the present exemplary embodiment a first silicon photomultiplier 5.1 and a second silicon photomultiplier 5.2 downstream. The first silicon photomultiplier 5.1 and the second silicon photomultiplier 5.2 are arranged parallel to one another and in spatial proximity to one another in such a way that in the scintillator 3 generated flashes of light from both the first silicon photomultiplier 5.1 as well as from the second silicon photomultiplier 5.2 are recorded.

The first silicon photomultiplier 5.1 a first analog-digital converter is connected downstream after a first amplifier circuit and the second silicon photomultiplier 5.2 after a second amplifier circuit, a second analog-digital converter is connected downstream. The amplifier circuit can be, for example, an amplifier with programmable gain (PGA - Programmable Gain Amplifier) and / or an operational amplifier. The analog-to-digital converter can, for example, also be an analog-to-digital converter with several parallel inputs to which the signal from both silicon photomultipliers is received 5.1 / 5.2 is fed so that the data can be processed in parallel.

The first silicon photomultiplier 5.1 is a first discriminator unit 7.1 and the second silicon photomultiplier 5.2 a second discriminator unit 7.2 downstream. In this way there are two parallel signal branches, each consisting of a silicon photomultiplier 5.1 , 5.2 and a discriminator unit 7.1 , 7.2 exist, formed.

In the silicon photomultipliers 5.1 , 5.2 are the flashes of light from the scintillator 3 converted into electrical signals and the respective downstream discriminator unit 7.1 , 7.2 fed. The discriminator units 7.1 , 7.2 are designed in the present embodiment as an amplitude discriminator unit and filters out the signals from the supplied signal which have a predetermined threshold value, in the present case a predetermined amplitude Amin, 150 , 151 not exceed and thus with a high probability not to a flash of light due to a radiating event, ie due to the scintillator 3 relevant radiation. In other words, only the signals are sent to the discriminator units on the output side 7.1 , 7.2 provided that the minimum amplitude Amin, 150 , 151 exceed.

The output side of the discriminator units 7.1 , 7.2 signals that can be tapped off are a unit of coincidence 9 which coincidentally, ie simultaneously occurring pulses determined from the signals fed to it. So it is due to the discriminator units 7.1 , 7.2 at the same time the minimum amplitude Amin, 150 , 151 exceeding signal, this is done by the coincidence unit 9 made available on the output side. Signals that have the minimum amplitude A _min , 150 , 151 exceed, but not occur at the same time, that is, for example, only on one of the discriminator units 7.1 , 7.2 If a signal is present on the output side, the coincidence unit 9 filtered out.

The so determined, the minimum amplitude Amin, 150 , 151 Exceeding and coincident signals are one of the coincidence unit 9 downstream unit for pulse shape analysis 11 fed. The unit for pulse shape analysis 11 analyzes the signal course of the signals fed to it, using a parameter Psd a distinction can be made between the pulses of different physical origins. The parameter Psd is calculated in the present exemplary embodiment according to the following formula: $$P\mathrm{S.}\mathrm{D.}=\frac{Q_{\text{long}}-Q_{\text{short}}}{Q_{\text{long}}}$$

The parameter Q is generated _briefly by signal integration in a time window around the signal edge of the pulse and Q _long by integration over the entire pulse. As a result, the parameter Q _{lang is} proportional to the total energy of the event. Are the time windows for integration to determine the Psd Parameter is selected accordingly, is the Psd -Parameters for a type of physical event, e.g. the detection of a γ quantum, largely independent of the energy.

Depending on the parameters Psd a distinction can thus be made whether one of the unit for pulse shape analysis 11 supplied signal to a flash of light due to a radiant Event, for example a γ quant, or is based on a statistical event that has arisen due to thermal noise or optical crosstalk of individual cells of the silicon photomultiplier.

The pulses identified in this way, which can be traced back to radiating events, are sent on the output side of the unit for pulse shape analysis 11 made available and can for example via one of the unit for pulse shape analysis 11 downstream counter 13 are counted. In this way, by measuring the number and intensity of the light flashes per unit of time, conclusions can be drawn about the intensity of the radiation, with thermal noise from the silicon photomultiplier at the same time 5.1 , 5.2 as well as that in the silicon photomultipliers 5.1 , 5.2 occurring crosstalk can be suppressed as far as possible.

Both the discriminator units and the coincidence units and the units for pulse shape analysis can each be implemented as a discrete circuit, as an FPGA or as a software block. The software can run on a controller or microprocessor and implement the corresponding functionality.

2 shows examples of signal curves on the in 1 marked points at an exit 100 of the first silicon photomultiplier 5.1 , at an exit 101 of the second silicon photomultiplier 5.2 , at an exit 160 the coincidence unit 9 as well as at an exit 170 the unit for pulse shape analysis 11 .

In the 2 shown and with 100 and 101 marked signal curves show next to an event to be counted, the with 110 and 111 Numerous statistically distributed dark pulses of different amplitude are also marked. These dark pulses are usually anti-coincident. An example of this is the pulse 120 marked, from which it can be seen that between the two silicon photomultipliers 5.1 , 5.2 there is no correlated noise. From a statistical point of view, there are also simultaneous, i.e. coincident, dark pulses with a low probability 130 , 131 , 140 , 141 possible. Coincident dark pulses with a large amplitude are significantly rarer than pulses with a low amplitude. It is therefore very unlikely that dark pulses with a large and approximately the same amplitude are detected at the same time, but this can still occur and with 140 and 141 is marked.

By suppressing signals which are smaller than a predetermined minimum amplitude Amin as a threshold value 150 , 151 are in the discriminator units 7.1 , 7.2 , and of anti-coincidence signals that are not detected simultaneously on all measurement channels in the coincidence unit 9 will be with 160 reached, which, however, still contains the components of the dark noise that occur coincidentally with sufficient amplitude. By analyzing the in this waveform 160 Signals that are still contained with regard to their pulse shape and / or amplitude and / or energy can be almost completely eliminated due to the great differences therein, signals that are due to causes other than a radiating event. The differentiation of the different pulse shapes can be done, for example Psd Parameters. Different energies can be differentiated through signal integration over the complete pulse or alternatively over the pulse amplitude.

3 describes by way of example the sequence of a method for signal evaluation of a detector unit for a scintillation counter according to the present application.

The detector unit 10 has for this purpose at least two signal paths with a scintillator arranged in parallel and in spatial proximity to one another 3 downstream photodetectors, which are used as silicon photomultipliers 5.1 , 5.2 are trained on.

In a first step 301 are made by the silicon photomultiplier 5.1 , 5.2 signals made available on the output side are read out and in a second step 302 each determines signals that have a minimum amplitude Amin, 150 , 151 exceed. In a third step 303 coincident signals, ie signals that occur simultaneously, are determined from the two signal paths and in a fourth step 304 subjected to a pulse shape analysis. The pulse shape analysis provides the signals on the output side that can be clearly traced back to a flash of light due to a radiant event. In a fifth step 305, the signals made available are recorded and evaluated, which can mean, for example, a count of the signals determined and / or an analysis of the signal energy.

With the proposed method, low-energy radiation can thus also be recorded independently of the temperature, the count rate of which would otherwise be very strongly influenced by a corresponding dark noise at higher temperatures.

4a) shows a block diagram of a scintillation counter with a detector unit with digital signal processing. The scintillator 3 is according to, as well as in the design 1 a parallel connection of two silicon photomultipliers 5.1 , 5.2 downstream. The output of the first silicon photomultiplier 5.1 is a first amplifier 6.1 and the output of the second silicon photomultiplier 5.2 is a second amplifier 6.2 fed. The amplifiers 6.1 , 6.2 are designed as programmable gain amplifiers in the present exemplary embodiment, but can also be designed as operational amplifiers, for example as transimpedance amplifiers.

The first amplifier 6.1 is a first analog-to-digital converter 8.1 and the second amplifier 6.2 is a second analog-to-digital converter 8.2 downstream, both of which are operated with an identical cycle T. This ensures that the signals on the two signal paths are scanned with a synchronous clock signal and that the trigger marks can thus be compared directly with one another

The output signals of the analog-digital converter 8.1 , 8.2 are a digital processing unit 99 supplied, which performs the signal evaluation method described above. The processing unit 99 can, for example, be designed as an FPGA or, as in the present exemplary embodiment, as a microcontroller on which a computer program code is executed so that the microcontroller executes the corresponding method steps.

In particular, with digital signal processing it is possible that the process steps of determining signals that exceed a certain signal amplitude or signal energy, determining coincident signals, and the pulse shape analysis can run partially or completely in parallel, that is to say at the same time, and then due to the temporally parallel sampling the Results can be merged. In this way, significant time savings can be achieved in signal evaluation.

4b) shows a variant for the analog-digital conversion of the signals, as shown in 4a) is described. In 4b) is a multi-channel analog-to-digital converter 8th to which the output signals of the amplifier 6.1 , 6.2 are fed in parallel. With this refinement, sampling and analog-digital conversion of the train-routed signals with a synchronous clock T is achieved. If it is a multi-channel analog-digital converter whose inputs are scanned by means of a multiplexer, the known time offset between the individual channels can be taken into account in the evaluation. The signals scanned in this way are sent to the evaluation unit 99 serially fed. Due to the digitization of the signals, they can be used in the evaluation unit as previously described 99 processed in parallel.

5 shows an enlarged illustration of a signal profile of an event to be counted 110 , 111 , i.e. a signal pulse that goes back to a radiating event, the trigger mark Tr determined therefrom and the integration windows Q _short and Q _long used for evaluation. The trigger mark Tr is obtained, for example, by a constant fraction discriminator, ie by processing the input signal in two parallel signal paths. In one path, it is delayed by a fixed amount of time that is less than the rise time of the signal pulse. In the other path, the signal is inverted and multiplied by a fixed factor. The two signals treated in this way are added again. The first zero crossing of the sum signal is then used as a trigger mark.

The integration windows are positioned relative to the temporal trigger mark, which has been determined by means of a constant fraction discriminator with the same start of the integration windows.

List of reference symbols

1

Scintillation counter

3

Scintillator

5.1

first silicon photomultiplier

5.2

second silicon photomultiplier

6.1

first amplifier

6.2

second amplifier

7.1

first discriminator

7.2

second discriminator

8th

multi-channel analog-digital converter

8.1

first analog-to-digital converter

8.2

second analog-to-digital converter

9

Coincidence unit

10

Detector unit

11

Pulse shape analysis unit

13

counter

20th

Photomultiplier tube

Psd

parameter

S ₁

first signal

S ₂

second signal

301-305

Procedural steps

99

Evaluation unit

100

Waveform

101

Waveform

160

Waveform

170

Waveform

110

Pulse / signal pulse

111

Pulse / signal pulse

130

Pulse / signal pulse

131

Pulse / signal pulse

140

Pulse / signal pulse

141

Pulse / signal pulse

150

minimum amplitude

151

minimum amplitude

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

• DE 102017205738 A1 [0006]

Claims (12)

Detector unit (10) for a scintillation counter (1), with at least two parallel signal paths, a first signal path with a first output signal having a first silicon photomultiplier (5.1) and a second signal path with a second output signal having a second silicon photomultiplier (5.2) , and wherein the output signals of the signal paths are fed to a coincidence unit (9), characterized in that the detector unit (10) has at least one unit for pulse shape analysis (11). Detector unit (10) according to Claim 1 , characterized in that the unit for pulse shape analysis (11) is connected downstream of the coincidence unit (9). Detector unit (10) according to one of the preceding claims, characterized in that the silicon photomultipliers (5.1, 5.2) are each followed by a discriminator unit (7.1, 7.2). Detector unit (10) according to one of the Claims 1 or 2 , characterized in that the coincidence unit (9) is followed by a discriminator unit (7.1, 7.2). Detector unit (10) according to one of the preceding claims, characterized in that the unit for pulse shape analysis (11) is designed to use a supplied signal to output an output signal to distinguish a dark pulse from a detection of a γ quantum. Detector unit (10) according to Claim 5 , characterized in that the unit for pulse shape analysis (11) has at least one integrator for integrating the supplied signal, a first integral formed over a first part of a signal pulse and a second integral over the entire signal pulse, and a decision criterion from a quotient on the one hand the difference of the second integral and the first integral and on the other hand the second integral is formed. Detector unit (10) according to one of the preceding claims, characterized in that the energy supply of the detector unit takes place by means of a 4-20 mA interface. Radiometric measuring device with a scintillation counter (1), characterized in that the detector unit (10) is designed according to one of the preceding claims. Method for signal evaluation of a detector unit (10) for a scintillation counter (1), with at least two parallel signal paths, a first signal path with a first output signal a first silicon photomultiplier (5.1) and a second signal path with a second output signal a second silicon photomultiplier (5.2), wherein the signals from the signal paths are fed to the method and the method has the following steps: determining signals which have a signal property with a size that is greater than a predeterminable threshold value, determining coincident signal components above the Threshold value in the signal paths, characterized by the step: - Analyzing the signal shape of the coincident signal components and outputting the signal components that correspond to a predeterminable distinguishing criterion - Counting the coincident signal components above the threshold value that meet the distinguishing criterion. Procedure according to Claim 9 , characterized in that, to determine the differentiation criterion, a first integral is formed over a first part of a signal pulse in the supplied signal and a second integral over the entire signal pulse, and the output signal is formed from a quotient of the difference between the second integral and the first integral and on the other hand, the second integral is formed. Computer program code which, when fed from a computing unit of a scintillation counter (1) to the output signals of a detector unit (10) with at least two parallel signal paths, a first signal path with a first output signal, a first silicon photomultiplier and a second signal path with a second signal path comprises a second silicon photomultiplier, this causes the following steps to be carried out: Determination of signals which have a signal property with a size that is greater than a predeterminable threshold value, determination of coincident signal components above the threshold value in the signal paths, characterized by the step: - Analyzing the signal shape of the coincident signal components and outputting the signals which correspond to a predeterminable differentiation criterion - Counting the coincident signal components above the threshold value which meet the differentiation criterion. Computer program code according to Claim 11 , characterized in that a first integral is formed over a first part of a signal pulse in the supplied signal and a second integral over the entire signal pulse to determine the differentiation criterion, and the output signal is formed from a quotient of on the one hand the difference of the second integral and the first integral and on the other hand the second integral is formed.

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