Radiometric measuring device and method for operating a radiometric measuring device

Abstract

The present invention relates to a radiometric measuring device with a scintillation counter, comprising at least one semiconductor photomultiplier (10) optically coupled to a scintillator (3) for generating electrical signals based on the light signals, a signal evaluation electrically coupled to the semiconductor photomultiplier (10) for generating a measurement signal based on the electrical signals, and measuring electronics for controlling the measuring device, wherein the measuring electronics comprise an activation device (17) and a timing device, wherein the semiconductor photomultiplier (10) can be activated and deactivated by means of the activation device (17), and wherein the timing device measures the time interval between activation of the semiconductor photomultiplier (10) and a first event, wherein the measuring electronics are configured such thatthat the semiconductor photomultiplier (10) is deactivated immediately after detection of the event for a predetermined period of time and a measurement result is determined from the duration of this time.

Description

[0001] The invention relates to a radiometric measuring device, in particular for level measurement, limit level measurement, density measurement or flow measurement according to the preamble of claim 1, and to a method for operating such a radiometric measuring device according to the preamble of claim 7.

[0002] A variety of radiometric measuring instruments with so-called scintillation counters are known from the state of the art. Radiometric measuring instruments with scintillators have been used in radiation physics and industry for decades. They utilize the property of scintillating materials to convert incident gamma radiation into light. These are pulses of a few nanoseconds for plastic scintillators and several tens of nanoseconds to microseconds for sodium iodide scintillators. These light pulses are evaluated by means of scintillation counters, i.e., light detectors that convert the optical pulses into an electrical signal. A measured value can be determined by simply counting pulses per unit of time. Photomultipliers or avalanche photodiodes, for example, can serve as light detectors. A further development of avalanche photodiodes are so-called semiconductor photomultipliers, e.g.Silicon photomultipliers, which are arrays of avalanche photodiodes.

[0003] The basic structure and operation of such a radiometric measuring device are related to Fig. 4 are shown and explained below.

[0004] Fig. Figure 4 shows the schematic setup of a scintillation counter 1, as 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 gamma radiation, whereby the excitation energy from the collisions is released again in the form of light. By measuring the amount of light, the energy deposited in the scintillator 3 by the radiation can be determined, and by measuring the number of light flashes per unit of time (count rate), the intensity of the radiation can be deduced.

[0005] For this purpose, the scintillation counter 1 includes a photomultiplier tube 20, or photomultiplier for short, connected downstream of the scintillator 3. In the photomultiplier 20, photons arriving from the scintillator 3 release electrons from the surface of a photocathode. These electrons are then repeatedly accelerated and multiplied in an electron multiplier within an electric field between several electrodes until a measurable electrical signal is present at the end of the electron multiplier. In a counter 13 connected downstream of the photomultiplier 20, the number and optionally also the amplitude of the electrical signals thus generated are recorded and can be further processed in electronics not shown in detail here.

[0006] Measuring instruments in industrial environments, and especially radiometric instruments for level measurement, limit level measurement, density measurement, or flow measurement, can be exposed to widely fluctuating temperatures. To prevent damage to the instrument, robust and well-insulated housings can be used. However, these are complex to manufacture and have a higher weight.

[0007] In DE 10 2017 205 738 A1, a radiometric level measuring device with a scintillator and a photodetector, which is cooled or heated as needed via an active cooling element within the measuring device, is therefore proposed. This allows the application range of the radiometric measuring device to be expanded and the effort required for the manufacture of the housing to be reduced.

[0008] Reference is also made to DE 10 2019 122 115 A1, US 2011 / 0 248 175 A1 and EP 3 413 091 A1.

[0009] Silicon photomultipliers are also known in the prior art, which are intended to serve as a replacement for the photomultiplier tubes that have been used until now.

[0010] A silicon photomultiplier is an array of multiple avalanche photodiodes (APDs). They typically measure a few square millimeters, housing several hundred APDs, which are also referred to as pixels.

[0011] In the present application, the term semiconductor photomultipliers is used generally. Within the scope of this application, the term semiconductor photomultiplier is to be understood as photodetectors with avalanche photodiodes as sensitive elements, which may be fabricated on the basis of various substrates, in particular silicon, germanium, or other suitable substrates.

[0012] The avalanche photodiodes used in a semiconductor photomultiplier operate 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 produce several million charge carriers due to the acceleration in the multiplication zone caused by the high electric field strength.

[0013] Since an avalanche photodiode operating in the Geiger range delivers pulses of a fixed amplitude, the signal amplitude of a semiconductor photomultiplier is proportional to the number of triggered pixels. The signal amplitude can be expressed in pe (pixel equivalent). One pe corresponds to the signal amplitude of a single pixel. Compared to conventional photomultiplier tubes, semiconductor photomultipliers offer several advantages.

[0014] Semiconductor photomultipliers are inexpensive, compact, and relatively robust mechanically. They also require a significantly lower supply voltage than photomultiplier tubes. Furthermore, semiconductor photomultipliers are unaffected by magnetic fields, which is another advantage for use in large detectors.

[0015] A major disadvantage is the strong temperature dependence of semiconductor photomultipliers. The photodiodes used in a semiconductor photomultiplier often have a relatively narrow temperature range in which they can deliver good measurement results and tend to generate dark current at excessively high temperatures, which can negatively affect the measurement accuracy of the radiometric instrument. Due to the strong temperature dependence of the thermal noise of semiconductor photodiodes, and thus semiconductor photomultipliers, their use in combination with a scintillator in a radiometric instrument at higher temperatures, e.g., 60 °C, is difficult. The dark noise of a silicon photomultiplier doubles approximately every 10 K. Thus, the dark noise at 60 °C is about 16 times higher than at 20 °C.

[0016] The resulting dark current can be so large that its use with a two-wire field device, i.e., a measuring device that is operated in a two-wire current loop, is not possible due to the high additional power consumption.

[0017] For example, a current draw of 0.5 mA was measured at an ambient temperature of 80°C and a supply voltage of 27 V. This results in an additional power loss of 13.5 mW due to dark current. This power loss corresponds to the total power consumption of a state-of-the-art measuring device with a classic photomultiplier, including the necessary high-voltage generation, an analog amplifier unit, and data processing by a microcontroller.

[0018] As the temperature increases, the breakdown voltage of avalanche photodiodes also rises, and the amplification factor therefore decreases. Simultaneously, the dark current increases, i.e., the number of pulses per unit time that occur without optical excitation. In this context, the spontaneous formation of free charge carriers due to heat in a photosensitive semiconductor, such as a photodiode, is called dark current.

[0019] Semiconductor photomultipliers consist of a multitude of microcells (pixels). The signal amplitude generated by a microcell due to the absorption of a photon is defined as 1 pe. A sum signal that the silicon photomultiplier provides due to the parallel-connected microcells is therefore proportional to the number of excited cells. Statistically, it is possible that m microcells simultaneously provide a first signal S1 as dark current due to thermal noise, where S1=m×pe

[0020] Another characteristic property of semiconductor photomultipliers is so-called crosstalk. In this process, electron-hole pairs recombine in an avalanche photodiode, and the resulting photons can generate a signal in neighboring pixels. This optical crosstalk from one microcell to n neighboring cells can cause a second signal S2, where S2=(n+1)×pe

[0021] Thus, a semiconductor photomultiplier can generate pulses with amplitudes of several pe without optical excitation. As the dark current increases with temperature, the number of dark pulses per unit time, as well as their amplitude, also increases. Since dark pulses with large amplitude cannot be distinguished from a counted event based solely on their amplitude, the count rate of such a photodetector can change significantly with temperature.

[0022] By combining the measurement signals from at least two semiconductor photomultipliers and using suitable signal acquisition / evaluation, dark noise can be significantly reduced or eliminated, thus minimizing the distortion (increase) of the count rate due to the increase in dark pulses at higher temperatures. This can be achieved by capturing signals occurring within a definable time interval, which are simultaneously (i.e., coincidentally) detected by both semiconductor photodiodes. Consequently, statistically distributed dark pulses contribute only minimally to the measured count rate.

[0023] The semiconductor photomultipliers are ideally arranged directly adjacent to each other, for example, in an array at one end of a scintillator. This ensures that the light pulses generated by gamma rays or particles in the scintillator material produce electrical signals of comparable amplitude, whose amplitude or waveform is not distorted by absorption in the scintillator material due to differing signal propagation paths. Because of the close proximity of the at least two semiconductor photodetectors, they also have approximately the same temperature, so that their dark noise changes during operation to a comparable extent.

[0024] However, these measures do not eliminate all dark pulses. And the power loss caused by dark current increases even further.

[0025] To save power, the prior art also proposes the possibility of cyclically switching the semiconductor photomultiplier and the associated signal processing on and off. However, cyclically switching the aforementioned components on and off has the disadvantage that measurement accuracy and / or measurement speed decrease, since no pulses are registered when the components are switched off.

[0026] Semiconductor photomultipliers operate in so-called "Geiger mode." In this operating mode, the semiconductor photomultipliers exhibit an avalanche effect, which, once triggered, must be interrupted by an external influence. The current flow caused by the avalanche effect can only be interrupted by reducing the voltage applied to the semiconductor photomultiplier below the breakdown voltage, thus returning the semiconductor photomultiplier to the blocking state.

[0027] In avalanche diodes and semiconductor photomultipliers, this can be achieved by a so-called quenching resistor, which is connected in series with the avalanche diode or semiconductor photomultiplier. The current flowing through the avalanche diode or semiconductor photomultiplier causes a voltage drop across the quenching resistor, so that the voltage across the diode drops below the pinch-off voltage and the avalanche diode or semiconductor photomultiplier is switched off again.

[0028] To detect the next pulse, the avalanche diode must return to reverse bias, meaning the junction capacitance must be discharged. This is also achieved via the quenching resistor. The time it takes for the avalanche photodiode, or semiconductor photomultiplier, to be ready for the next radiating event determines the maximum possible measurement speed. This time is also known as the decay time. The larger the quenching resistor, the faster the avalanche effect is terminated, but the longer it also takes for the diode capacitance to discharge and a new radiating event to be detected.

[0029] The object of the present invention is to improve the known devices and methods in such a way as to achieve a higher measurement speed and / or reduced energy consumption.

[0030] This problem is solved by a radiometric measuring device having the features of claim 1 and a method having the features of claim 7.

[0031] Preferred embodiments, features and characteristics of the proposed field device correspond to those of the proposed method and vice versa.

[0032] Advantageous embodiments and variants of the invention are described in the dependent claims and the following description. The features listed individually in the dependent claims can be combined with each other and with the features explained in more detail in the following description in any technically sensible manner, and represent other advantageous embodiments of the invention. It should be noted that the features listed individually in the claims can be combined with each other in any technically sensible manner (even across category boundaries, for example, between method and apparatus) and demonstrate further embodiments of the invention. The description further characterizes and specifies the invention, particularly in conjunction with the figures.

[0033] A radiometric measuring device according to the invention, comprising a scintillation counter, has at least one semiconductor photomultiplier optically coupled to a scintillator for generating electrical signals based on light signals, a signal evaluation electrically coupled to the semiconductor photomultiplier for generating a measurement signal based on the electrical signals, and measuring electronics for controlling the measuring device, and is characterized in that the measuring electronics have an activation device and a timing device, wherein the semiconductor photomultiplier can be activated and deactivated by means of the activation device, and a time interval between activation of the semiconductor photomultiplier and a first event can be determined by means of the timing device, wherein the measuring electronics are configured such thatthat the semiconductor photomultiplier is deactivated immediately after detection of the event for a predetermined period, and a measurement result is determined from this duration.

[0034] In the present application, activation and deactivation of the semiconductor photomultiplier are understood to mean the following: When activated, the semiconductor photomultiplier is active, meaning it is ready to detect photons and output an electrical signal using the avalanche effect. In this context, activation means operating the semiconductor photomultiplier in the Geiger region, i.e., at a voltage higher than the breakdown voltage of the avalanche photodiodes used. In the Geiger region, even a single photon can trigger a complete avalanche breakdown, which propagates across the entire activated photodiode and generates a current pulse, regardless of the number of incident photons.

[0035] When deactivated, the semiconductor photomultiplier is inactive, meaning that incoming photons are not detected and, unlike the previously described operating mode, do not trigger an avalanche breakdown. Preferably, the semiconductor photomultiplier is operated with a voltage below its breakdown voltage. Lowering the voltage below the breakdown voltage eliminates the avalanche effect. Preferably, the semiconductor photomultiplier is disconnected from the supply voltage, or the resistance of a connection to the supply voltage is significantly increased, thus reducing the voltage applied to the semiconductor photomultiplier.

[0036] The radiometric measuring device may also include a scintillator for generating light signals due to radiative events. Examples of scintillators used include plastic scintillators or sodium iodide scintillators.

[0037] In a further training course, the radiometric measuring device is designed in such a way that the signal evaluation can be activated and deactivated by means of the activation device, or put into a standby mode and woken up again.

[0038] Activating and deactivating, in the context of signal processing, also means switching the signal processing on or off. Alternatively, the signal processing unit can be disconnected from and reconnected to its power supply, or its supply voltage can be reduced below its operating voltage. Another option is to put the signal processing unit into a standby mode with reduced power consumption and then wake it up again.

[0039] When the signal processing unit is disconnected from the power supply, it may take some time before it is operational again. This can be due, for example, to the fact that resonant circuits require a settling time to reach a stable state. Therefore, in these situations, it is often preferable to put the signal processing unit into a standby or sleep mode with reduced power consumption, as this significantly reduces the time it takes for the signal processing unit to become operational again.

[0040] In a further embodiment of the radiometric measuring device, the activation device can include a switching device for changing the supply voltage of the semiconductor photomultiplier. This switching device allows two different supply voltages to be applied to the semiconductor photomultiplier, so that in one switching position it is operated with a first voltage above the breakdown voltage of the semiconductor photomultiplier, particularly in Geiger mode, and in another switching position with a second voltage below the breakdown voltage of the semiconductor photomultiplier.

[0041] In one embodiment of the radiometric measuring device, the switching element is designed as a MOSFET, which allows the semiconductor photomultiplier to be disconnected from a supply voltage. Disconnecting the semiconductor photomultiplier from the supply voltage immediately stops the current flow and extinguishes any avalanche breakdown. In this way, an avalanche breakdown caused by a detected event in a semiconductor photomultiplier operating in Geiger mode can be stopped very quickly. Once the avalanche breakdown is stopped, charge carriers can dissipate, so that the semiconductor photomultiplier is then ready for use again, i.e., ready to detect photons.

[0042] Additionally or alternatively, the switching device can include a changeover switch by means of which the supply voltage of the silicon photomultiplier can be switched between a first supply voltage above a breakdown voltage and a second supply voltage below a breakdown voltage of the silicon photomultiplier. As already described, an avalanche breakdown can also be extinguished in this way. Furthermore, in a voltage range below the breakdown voltage, charge carriers can flow out of the semiconductor photomultiplier, so that it is subsequently ready for use again.

[0043] In a further development, the radiometric measuring device is designed such that the period for which the semiconductor photomultiplier and / or the signal evaluation is deactivated or put into a standby mode is at least 50 µs, preferably at least 75 µs, particularly preferably at least 100 µs.

[0044] The longer the duration, the greater the energy savings that can be achieved. For example, if 10,000 pulses per second are expected, this means that there is an average of 100 µs between two pulses. If the signal processing is deactivated or put into standby mode for 100 µs after an event is detected, the average waiting time for the next event is 100 µs. However, only half of the pulses are detected.

[0045] The method of shortening the dead time of a detector, during which no further pulses can be detected, by lowering the supply voltage below the breakdown voltage, was originally developed for Geiger-Müller counter tubes and is called the "time-to-first-count method." By shortening the dead time, more pulses can be measured per unit of time, thus improving measurement accuracy and / or speed. However, since the detector's dead time remains unknown, a defined waiting period is introduced between deactivation and activation of the detector, which must be longer than the actual detector dead time. This method can be used to determine a measured value because a measurement of a time interval until the next event can be converted into the usual measurement of pulses per time interval by taking the reciprocal.

[0046] No pulses can be detected during the waiting period; therefore, parts of the signal processing can be switched off during this time, thus saving energy. This option can be particularly advantageous with two-wire devices.

[0047] Using the numerical example above, we get: Pulse rate = 10,000 pulses per second Average active time until the next heartbeat = 100µs Waiting time (passive time) = 100µs Ratio of passive time to active time = 100µs / 100µs Proportional active time 100µs / (100µs+100µs) = 50% Proportional passive time 100µs / 100µs+100µs) = 50% = Energy saving

[0048] The following calculation results for a pulse rate 10 times higher: Pulse rate = 100,000 pulses per second Average active time until the next heartbeat = 10µs Waiting time (passive time) = 100µs Ratio of passive time to active time = 100µs / 10µs Proportional active time 10µs / (10µs+100µs) = 9,1% Proportional passive time 100µs / 10µs+100µs) = 90.9% = Energy saving

[0049] A method according to the invention for operating a radiometric measuring device as described above is characterized by the following method steps: - Activation of the semiconductor photomultiplier, - Determining the time interval between the activation of the semiconductor photomultiplier and the detection of a first event, - Deactivating at least the semiconductor photomultiplier immediately after detection for a predetermined period and - Determining a measurement result based on the duration of time.

[0050] In a further development of the method, a signal evaluation of the semiconductor photomultiplier is activated and deactivated or put into a standby mode and then woken up again.

[0051] By activating and deactivating the signal evaluation, or putting it into standby mode and waking it up again, the energy consumption of the radiometric measuring arrangement can be significantly reduced.

[0052] To activate and deactivate the semiconductor photomultiplier, for example, the supply voltage of the semiconductor photomultiplier can be changed. By changing the supply voltage of the semiconductor photomultiplier, it can be achieved that, when activated, it operates with a supply voltage above the breakdown voltage, and when deactivated, with a supply voltage below the breakdown voltage. This has the effect that, in the active state, operation in Geiger mode is achieved, and in the inactive state, any avalanche breakdown that has occurred is canceled and the semiconductor photomultiplier is cleared of generated charge carriers.

[0053] Additionally or alternatively, the semiconductor photomultiplier can be disconnected from the supply voltage to deactivate it. Disconnecting the semiconductor photomultiplier from the supply voltage allows an avalanche breakdown to be extinguished very quickly, thus minimizing the dead time of the semiconductor photomultiplier.

[0054] Changing the supply voltage of the semiconductor photomultiplier can be achieved, for example, by switching between a first voltage above the breakdown voltage and a second voltage below the breakdown voltage of the semiconductor photomultiplier.

[0055] The radiometric measuring device according to the present application is preferably designed as a two-wire field device. The aforementioned energy-saving measures make it possible to operate a semiconductor photomultiplier in a two-wire field device using the limited energy available there.

[0056] A two-wire field device according to the present invention is understood to be a field device which is connected to a higher-level unit via two wires, whereby both a power supply and a measurement transmission take place via these two wires.

[0057] The energy and / or signal transmission between the two-wire field device and the higher-level unit is carried out according to the well-known 4 mA to 20 mA standard, in which a 4 mA to 20 mA current loop, i.e., a two-wire connection, is established between the field device and the higher-level unit. In addition to the analog transmission of signals, the measuring devices can transmit or receive further information to or from the higher-level unit according to various other protocols, particularly digital protocols. Examples include the HART protocol and the Profibus PA protocol.

[0058] These field devices are also powered via the 4 mA to 20 mA current signal, so no additional power supply line is required besides the two-wire connection. To minimize wiring and installation effort, as well as safety measures, for example when used in explosion-proof areas, it is also undesirable to provide additional power supply lines.

[0059] With two-wire field devices, the available input power is significantly limited. The electronics in the field device must be designed to operate reliably even with a minimum signal current of 4 mA.

[0060] Two-wire field devices, for example, offer significantly reduced installation and wiring requirements compared to four-wire field devices. With two-wire field devices, the additional installation and wiring of a supply voltage is completely eliminated, as this is provided via the two-wire cable, as described above. This offers considerable advantages, particularly in applications where explosion protection regulations must be observed, since the separate cables for the supply voltage and the necessary additional components do not need to be considered during the planning phase.

[0061] Two-wire field devices can also be designed to be intrinsically safe, thus extending their range of application in explosion-proof (Ex) areas. Maintenance work on field devices in Ex areas is significantly easier and safer with two-wire devices than, for example, with four-wire devices, as it can be carried out safely even during ongoing measurement. With four-wire devices, however, the power supply must first be interrupted and secured against reconnection. This is generally done in the connection rooms, which are often located a considerable distance from the measuring point.

[0062] The present invention is explained in detail below with reference to exemplary embodiments and the accompanying figures. These show: Fig. 1 an embodiment of a radiometric measuring instrument according to the present application, Fig. 2 the course of the voltage at the output of the discriminator Fig. 1, Fig. 3 an embodiment of a method for operating a radiometric measuring device according to Fig. 1 and Fig. 4 the construction of a radiometric measuring device according to the state of the art (already covered).

[0063] In the figures, unless otherwise indicated, identical reference symbols denote identical or corresponding components with the same function.

[0064] It should also be noted that the conjunction “and / or” used herein, which stands between two features and links them together, is always to be interpreted in such a way that in a first embodiment of the object according to the invention only the first feature may be present, in a second embodiment only the second feature may be present, and in a third embodiment both the first and the second feature may be present.

[0065] Fig.Figure 1 shows an embodiment of a radiometric measuring device 100 according to the present application.

[0066] The radiometric measuring device 100 features a semiconductor photomultiplier 10 as its photosensitive element. The semiconductor photomultiplier is optically coupled to a scintillator (not shown) that converts incoming gamma quanta into an optical signal in the form of photons. The semiconductor photomultiplier 10 is formed by an array of parallel-connected avalanche photodiodes and is located in the Fig. For the sake of simplicity, the circuit diagram shown in section 1 is represented as a photodiode, since its basic electrical properties are the same as those of a photodiode.

[0067] The semiconductor photomultiplier 10 is operated in reverse bias and is connected to ground via a measuring resistor Rmess on the anode side. On the cathode side, the semiconductor photomultiplier 10 is connected to a supply voltage UB via an element 11, which is represented as a switch. Depending on the design, the element 11 can be implemented as a variable resistor or a switching element, e.g., in the form of a field-effect transistor or a changeover switch.

[0068] The anode of the semiconductor photomultiplier 10 is connected to the non-inverting input of a discriminator 13, which compares the voltage applied to the node with a reference voltage Uref and outputs a signal if the measured voltage Umess exceeds the reference voltage Uref, and not if the measured voltage Umess is below the reference voltage Uref. In the simplest case, the discriminator 13 is a comparator built with an operational amplifier that compares the measured voltage Umess with the reference voltage Uref, but it can also be a more complex circuit.

[0069] The output signal of the discriminator 13 is fed to an evaluation circuit 15, which includes a timing device that measures the time tmess from the activation of the evaluation circuit 15 until the detection of the first event after activation. The evaluation circuit is connected to an activation device 17 of the measuring instrument electronics, which is configured to deactivate the evaluation circuit 15 immediately after the detection of an event and the output of the measured time tmess for a predetermined or predefinable standby time Ts. Simultaneously with the deactivation of the evaluation circuit 15, the element 11 is actuated, thereby deactivating the semiconductor photomultiplier 10.

[0070] In the context of the semiconductor photomultiplier, this means that an avalanche breakdown triggered by a photon is quenched. As explained above, this can be achieved by reducing the supply voltage UB of the semiconductor photomultiplier 10 below its breakdown voltage. This can be accomplished, for example, by disconnecting the connection between the cathode and the supply voltage Ub, by increasing a variable resistance between the supply voltage Ub and the cathode, or by connecting the cathode to a second supply voltage U2 that is below the breakdown voltage of the semiconductor photomultiplier 10.

[0071] From the measured time Tmess, a measurement processing unit 19 downstream of the evaluation circuit 15 can determine a count rate, i.e., the number of radiative events per unit of time, using statistical methods. From this, for example, a fill level, density, or density profile can be determined. The final measured value is output via a two-wire interface 21 and transmitted to a higher-level unit, e.g., a control room.

[0072] After the standby time Ts has elapsed, the semiconductor photomultiplier 10 and the evaluation circuit 15 are reactivated and a new measurement can be carried out.

[0073] Fig. 2 shows the measured voltage Umess across the measuring resistor Rmess. Fig. 1 in three different configurations.

[0074] The output signal labeled 101 shows the measured voltage Umess when a 100 kΩ resistor is used as element 11. In this configuration, the decay time is approximately 100 ns until the measured voltage Umess drops back to 0 V due to the voltage drop across the resistor.

[0075] In the curves labeled 102 and 103, the output signal 101 is shown when an avalanche breakdown of the semiconductor photomultiplier is actively quenched, e.g., by interrupting the supply voltage UB. In this configuration, element 13 is designed as a field-effect transistor, which is opened by the activation device 17 after an avalanche breakdown, so that the semiconductor photomultiplier 10 exhibits a shortened current flow.

[0076] The signal designated 104 would require a discriminator 13 for detection, which has a response time of less than 5 ns. The signal designated 103 could be detected with a discriminator 13 with a response time of approximately 10 ns. However, due to the high processing speed, such discriminators require a power consumption of 10-15 mW or more, which represents an unacceptable power draw and thus energy consumption in a two-wire field device and is therefore not feasible.

[0077] The signal designated 102 can be detected with a discriminator 13 with a response time of approximately 40 ns, which is achievable with currently available components with a power consumption of less than 1 mW.

[0078] Fig. Figure 3 shows an embodiment of a method for operating a radiometric measuring device 100 according to Fig. 1.

[0079] The procedure starts in step 301 with the successful commissioning of the field device, which includes the radiometric measuring device 100. The measuring device electronics are activated at this point, and in step 302 the semiconductor photomultiplier 10 and the evaluation circuit 15 are activated.

[0080] With the activation of the evaluation circuit 15, the timing device is activated and begins in step 303 to measure the time until the detection of a first radiative event or the resulting measurement signal.

[0081] The output signal of the discriminator 13 is processed in the evaluation circuit 15 in step 304 and output to the measurement processing unit 19. Simultaneously, a signal is sent to the activation circuit 17, which then detects a radiative event in step 304, so that in step 305 the evaluation circuit 15 and the semiconductor photomultiplier 10 are deactivated.

[0082] Step 306 represents a delay element that delays the process for the duration of the standby time and then passes it back to step 302. Reference sign 3 Scintillator 10 Semiconductor photomultipliers 11 Element 13 counters / discriminator 15 Evaluation circuit 17 Activation device 19 Measured value processing 20 photomultiplier tubes 21 Two-wire interface 100 radiometric measuring devices 101 Output signal 102 Signal 103 Signal 104 Signal 301-306 Procedural steps

Claims

[1] Radiometric measuring device with a scintillation counter, comprising at least a semiconductor photomultiplier (10) which can be optically coupled to a scintillator (3) for generating electrical signals based on the light signals, a signal evaluation unit electrically coupled to the semiconductor photomultiplier (10) for generating a measurement signal based on the electrical signals and, a measuring device electronics for controlling the measuring device characterized by , that the measuring device electronics has an activation device (17) and a time measuring device, wherein the semiconductor photomultiplier (10) can be activated and deactivated by means of the activation device (17), and has a time period between an activation of the semiconductor photomultiplier (10) and a first event by means of the time measuring device, wherein the measuring device electronics is designed such that the semiconductor photomultiplier (10) is deactivated for a predetermined time period immediately after detection of the event and a measurement result is determined from the time period. [2] Radiometric measuring device according to claim 1, characterized by that the signal evaluation can be activated and deactivated by means of the activation device (17) or can be put into a standby mode and woken up again. [3] Radiometric measuring device according to one of claims 1 or 2, characterized bythat the activation device (17) has a switching device for changing a supply voltage of the semiconductor photomultiplier (10). [4] Radiometric measuring device according to one of the preceding claims, characterized by that the switching device is designed as a MOS-FET, by means of which the semiconductor photomultiplier (10) can be separated from a supply voltage. [5] Radiometric measuring device according to one of the preceding claims, characterized by that the switching device is designed as a changeover switch, by means of which a supply voltage of the semiconductor photomultiplier (10) can be switched between a first supply voltage above a breakdown voltage and a second reference voltage below the breakdown voltage of the semiconductor photomultiplier (10). [6] Radiometric measuring device according to one of the preceding claims, characterized bythat the time duration is at least 50 µs, preferably at least 75 µs, particularly preferably at least 100 µs. [7] Method for operating a radiometric measuring device with a scintillation counter, comprising at least a scintillator (3) for generating light signals due to radiant events, a semiconductor photomultiplier (10) which can be optically coupled to the scintillator (3) for generating electrical signals based on the light signals, a signal evaluation unit electrically coupled to the semiconductor photomultiplier (10) for generating a measurement signal based on the electrical signals and, a measuring device electronics for controlling the measuring device characterized by the steps: - Activating the semiconductor photomultiplier (10), - determining a period of time between the activation of the semiconductor photomultiplier (10) and the detection of a first event, - deactivating at least the semiconductor photomultiplier (10) immediately after the detection for a predetermined period of time and - Determine a measurement result based on the period. [8] Method according to claim 7, characterized by that the signal evaluation of the semiconductor photomultiplier (10) is activated and deactivated or put into a standby mode and woken up again. [9] Method according to one of claims 7 and 8, characterized by that a supply voltage of the semiconductor photomultiplier (10) is changed to activate and deactivate the semiconductor photomultiplier (10). [10] Method according to one of claims 7 to 9, characterized by that the semiconductor photomultiplier (10) is disconnected from a supply voltage to deactivate it. [11] Method according to one of claims 7 to 10, characterized bythat a supply voltage of the semiconductor photomultiplier (10) is switchable between a first voltage above a breakdown voltage and a second voltage below the breakdown voltage of the semiconductor photomultiplier (10).

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