JP5126739B2 - Gas detector for neutron measurement - Google Patents

Description

  The present invention relates to a gas detector for neutron measurement that measures the amount of neutrons jumping into a sealed space of helium 3 gas, and in particular, it is generated by collision between neutrons trapped in the gas detector and helium 3 gas. The present invention relates to a processing technique for increasing the counting rate and stabilizing the signal processing means for detecting the charge amount.

  Currently, neutrons are widely used in various experiments in physical physics and high energy physics. For example, the material structure can be analyzed by observing various phenomena when a high-speed neutron collides with a certain material. In such neutron experiments, while the neutron source has been increasingly strengthened in recent years, it has become important to measure neutrons quickly and accurately, and there has been a strong demand for higher neutron detectors.

  Currently, the most commonly used gas detector for neutron measurement is the isotope of normal helium 4 (proton: 2, neutron: 2, electron: 2) because of its high detection efficiency and low gamma-ray sensitivity. The amount of charge generated when neutrons have entered the gas detector sealed with helium 3 gas (proton: 2, neutron: 1, helium consisting of electrons: 2, hereinafter referred to as “He3”). This is the type of detector to be measured.

  FIG. 13 is a schematic diagram for explaining the structure of the gas detector 11. As shown in FIG. 13, the gas detector 11 usually has a cylindrical body 21 made of an extremely thin stainless steel material, and the inside of the cylindrical body 21 is filled with He 3 gas of 10 to 20 atmospheres. Various structures and sizes of the cylindrical body 21 exist, and as an example, there are those having a diameter of about 2 to 3 cm and a length of about 50 to 60 cm. Here, a conductive core wire 22 is stretched at the center of the cylindrical body 21, and a voltage of about 1500 to 2,000 V is applied between the cylindrical body 21 and the core wire 22 via a resistor.

  When such a gas detector 11 is placed in a neutron measurement target field and neutrons jump into the cylindrical body 21 filled with He3 gas, charges corresponding to the number of neutrons that have jumped in are generated. The amount of charge can be detected by the core wire 22. That is, when one neutron collides with the He3 gas in the cylindrical body 21, a unit charge is generated. This unit charge collides with the He3 gas in a chain in the electric field in the cylindrical body 21 and the amount of the charge. Is amplified at a constant rate, and the amplified electric charge reaches the core wire 22 in the cylindrical body 21.

  Here, the electric charges detected in the core wire 22 are amplified by colliding with each other in two directions in the cylindrical body 21. Therefore, the time required for the electric charge to reach the core wire 22 varies depending on the two flying directions, and is detected by spreading for a predetermined time according to the shape of the cylindrical body 21 and the electric field strength.

  FIG. 14 shows an example of a conventional gas detection device. As shown in FIG. 14, in the conventional gas detection device, the charge detected by the gas detector is integrated by the first integration circuit 32 having a large time constant, and the analog value obtained by the integration processing is included. Noise component is removed by a waveform shaping circuit (band filter) 35 comprising a differentiating circuit 33 and a second integrating circuit 34, the charge integrated output value is A / D converted by an A / D converting circuit 36, and then the digital wave height is obtained. The charge is measured by processing the wave height with the processing device 37.

  As described above, in the conventional gas detection device, the time until the charge generated in the gas detector 11 reaches the core wire 22 is different in conditions such as the location where the neutron is captured and the direction in which the generated charge flies. However, in the integration process, the latest charge generated in order to obtain the integrated charge corresponding to the reaction energy between the neutron and the He3 gas is used. It was necessary to adjust the time constant. Further, since the integrated value of the charge obtained in this way is waveform-shaped by the waveform shaping circuit (band filter) 35 and then the digital wave height processing device 37 performs the wave height processing, the conventional gas detection device has a processing speed. Is very slow and is usually used in accordance with the charge output speed of the gas detector 11 from a value of about 0.5 μs to 2 μs.

  Thus, in the conventional gas detection apparatus, since the maximum speed of the gas detector 11 cannot be extracted, it is difficult to increase the counting rate of neutron measurement.

  The present invention solves various problems of such a conventional neutron measurement gas detection device, and in the neutron measurement gas detection device, the neutron to be measured is accurately measured at high speed. The purpose is to realize a high counting rate.

For this reason, the present invention provides a gas detection means for measuring a neutron and outputting a charge corresponding to the detected amount, a first integration amplifier means for integrating and amplifying an output from the gas detection means, and the first A waveform shaping circuit comprising a differentiating circuit for differentiating the output from the integrating amplifier means, and a second integrating amplifier means for integrating and amplifying the output from the differentiating circuit, and the output of the second integrating amplifier means as A / D conversion, a digital integrator means for integrating the digital output value, a, integration time constant of the first integrating amplifier means, Ri pole-zero cancellation time constant equivalent der of the differentiating circuit, the waveform shaping It is another object of the present invention to provide a gas detector for neutron measurement, characterized in that a time constant of a circuit is set in accordance with an initial detection time of the electric charge .

  The first integrating amplifier means includes a first analog amplifier, and a capacitor C1 and a resistor R1 aligned and connected between the input terminal and the output terminal of the first analog amplifier.

  Further, the differentiating circuit includes a capacitor C2 connected to the output of the first analog amplifier and a resistor R2 connected in parallel thereto, one terminal of which is connected in series to the capacitor C2, and the other terminal connected to the capacitor C2. And a resistor R3 connected to the input of the second integrating amplifier means.

  The second integrating amplifier means includes a second analog amplifier, and a capacitor C3 and a resistor R4 connected in parallel between the input terminal and the output terminal of the second analog amplifier.

  The product of the capacitor C1 and the resistor R1 is equal to the product of the capacitor C2 and the resistor R2.

  Further, the product of the capacitor C2 and the resistor R3 is equal to the product of the capacitor C3 and the resistor R4.

  In the present invention, the integration is performed by the digital processing circuit without performing the integration by the analog circuit, which is the cause of the decrease in the processing speed in the conventional apparatus. The analog circuit simply amplifies the generated charge, and integration is performed by the digital processing circuit, so that the maximum speed of the gas detector can be achieved. It has already been confirmed that the present invention can obtain data having a counting rate that is twice or more that of the conventional method.

  In addition, since integration is conventionally performed by an analog circuit, there is a problem that even a fast signal is delayed. However, according to the present invention, the source signal is reproduced by digital integration to prevent a decrease in detection efficiency. it can.

  Furthermore, since the signal after integration can be determined whether it is a neutron, stable and accurate measurement is possible.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a schematic diagram showing a configuration of a gas detector for neutron measurement according to the present invention.
In the present invention, as shown in FIG. 1, the analog circuit is configured to faithfully amplify the generated charges. That is, the time constant is conventionally adjusted to the slowest signal and the integration amplifier (34 in FIG. 14) is used for all signals. The time constant is adjusted to the fastest signal and the normal amplifier 12 is used for all signals. The electric charge generated in the gas detector 11 is amplified and outputted as it is. For this reason, the time constant of the analog portion, which has been a bottleneck for increasing the counting rate so far, can be set from 0.5 μs to 0.1 μs. Since the pulse width is the same as the charge generation time, the pulse width is longer than this time constant, but the speed can be increased twice or more as compared with the conventional gas detector for neutron measurement. The charge integration is digitized by the high-speed A / D conversion circuit 17 and digitally processed by the digital integration means 18. In the present invention, since waveform processing is performed in real time and digital integration is completed for each pulse, the maximum speed of the gas detector can be obtained. In addition, since the signal after integration can be determined as to whether it is a neutron, stable measurement is possible.

  Hereinafter, the details of the present invention will be described more specifically.

  FIG. 2 shows a partial circuit of a charge-type amplifier that is normally used in a gas detector for neutron measurement.

  As described above, the gas detector 11 generates an electric charge when it detects neutrons. However, since the charge is obtained by ionizing the surrounding gas with the reaction energy of neutron and He3 gas, the charge generation time can vary considerably depending on the conditions. However, since the reaction energy is constant, a signal having a constant intensity can be obtained by integrating these charges.

  In the conventional method shown in FIG. 14, the time constant of the waveform shaping circuit 35 including the differentiation circuit 33 and the second integration circuit 34 is set to the slowest signal so that the pulse value is equal to the charge integration value. I was sending it out. Here, the differentiation circuit 33 and the second integration circuit 34 form a band filter. Thereby, a pulse having a height of the output of the first integrating circuit 32 is obtained.

  In the present invention, the integration function of the integration circuit 34 (second integration circuit 15 in FIG. 1) in the waveform shaping circuit 35 is weakened, and the waveform shaping circuit 16 is operated as a differentiation circuit. As a result, when the waveform shaping time constant is adjusted to the fastest charge generation time, the waveform shaping circuit 16 acts as a differentiating circuit for most slow signals, so that the integration effect of the first integrating circuit 13 is canceled and gas detection is performed. The electric charge generated by the vessel 11 is amplified as it is. Since the generated charge itself is output, according to the present invention, the limit count rate of the gas detector 11 can be obtained. Further, this signal is digitized by the A / D conversion circuit 17 and digitally integrated once again by the digital integration means 18, so that the same charge amount can be obtained for any signal while maintaining high speed.

  Thereby, in the conventional gas detector for neutron measurement, for example, the time constant is 0.5 μs in accordance with the slowest signal of the gas detector 11, C1 = 5 pF, R1 = 5 MΩ, C2 = 220 pF, R2 = 220 KΩ, R3 = 2KΩ, C3 = 10pF, R4 = 48KΩ, but in the embodiment using the present invention, the fastest signal is assumed to be 0.1 μs, C1 = 5pF, R1 = 1MΩ, C2 = 220pF, R2 = 24 KΩ, R3 = 390Ω, C3 = 10 pF, R4 = 10 KΩ.

  FIG. 3 is a diagram showing a calculation result of SPICE (Simulation Program with Integrated Circuit Emphasis) when a step signal is input to the neutron measurement gas detector according to the present invention through a capacitor. FIG. 3A shows the case where the rise of the step is 0.1 μs, and FIG. 3B shows the case where the rise of the step is 1 μs. The waveform shaping time constant of the amplifier 12 was set to 0.1 μs, and a step signal was input through a capacitor. In these figures, the lower graph gives a slope with a step signal, and the upper graph shows an output pulse. A signal faster than the time constant becomes a signal having the same height, but an input longer than the time constant has a differential effect by the differentiating circuit 14 and is a short but wide signal indicating the input amount of charge. Therefore, the area indicating the integration amount is the same.

  FIG. 4 shows a waveform obtained when the output of the gas detector 11 is passed through an amplifier having a fast waveform shaping time constant. Here, the waveform shaping time constant is set to 0.1 μs. It clearly shows that the output signal of the gas detector 11 is output in a mixed state of a fast signal and a slow signal. The fast signal catches up quite a bit, but the slow signal is short and wide, and is a slow component from the SPICE results. In this gas detector, signals of about 0.2 μs to 1.0 μs are mixed.

  FIG. 5 shows the waveform of an actual gas detection device. FIG. 5A shows a waveform using a conventional gas detector for neutron measurement (0.5 μs time constant), and FIG. 5B shows a waveform using a neutron measurement gas detector of the present invention (at 0.1 μs). Constant). In the conventional gas detector for neutron measurement shown in FIG. 5 (a), the integration is sufficiently performed, but the pulse width is widened, and a high counting rate cannot be achieved. In the neutron measurement gas detector of the present invention shown in FIG. 5B, the fast signal catches up, but the slow signal is considerably short and wide. This gas detector shows that signals of about 0.2 μs to 1.0 μs are mixed.

  In the signal of FIG. 5 (a) using a conventional gas detector for neutron measurement, the pulse width spreads to about 3.6 μs, and when measuring 10% loss, an average count rate of 36 μs is 28K count / Seconds (cps) are the limit. On the other hand, FIG. 5 (b) using the gas detector for neutron measurement according to the present invention has the worst spread of 1.6 μs, so it can be used for measurement of 63 Kcps with measurement of 10% loss. Therefore, according to the present invention, the counting rate can be increased by a factor of 2 or more at worst. Further, even if the second signal enters as in the example of FIG.

  Next, a specific method for obtaining signal integration in real time in the present invention will be described.

  From the results of FIG. 4, typical waveforms of a fast signal and a slow signal are assumed, and both can be dealt with. Actually, the waveforms of FIGS. 6A and 6B are assumed. This is an algorithm for digitizing and processing the output of the amplifier 12 by a high-speed A / D conversion circuit 17. Here, an A / D conversion circuit 17 having a sampling rate of 40 MHz and a conversion bit number of 12 bits is used.

The order of processing is as follows.
(1) Measurement of Baseline Here, the signal of the A / D conversion circuit 17 is always divided into fixed intervals and compared, and the minimum value is taken as the baseline. The reason for setting the minimum value is that the best result can be obtained as a result of trial and error. FIG. 7 shows a flowchart for measuring the baseline. Here, the minimum value is obtained every 64 steps (1.6 μs). First, as shown in FIG. 7A, the base line counter STCT is counted up from 0 to 63, and is returned to 0 when the base line counter STCT reaches 63 (steps S1 to S3). Here, in obtaining the minimum output value of the A / D conversion circuit 17, in order to further increase the accuracy, the measurement is performed twice with a shift of 0.8 μs. That is, as shown in FIGS. 7B and 7C, the minimum value of 64 clocks (1.6 μs) is measured twice by shifting 32 clocks (0.8 μs). In FIG. 7B, when the baseline counter STCT becomes 31, the output signal value DADC of the A / D conversion circuit 17 at that time is input to MINI0 (steps S4 and S5), and the subsequent A / D conversion circuit 17 The output signal DADC and MINI0 are compared, and the smaller signal value is updated and held as MINI0 (steps S6 and S7). At the same time, as shown in FIG. 7C, the minimum value MINI1 is updated and held every 64 clocks shifted by 32 clocks (steps S8 to S11). As shown in FIG. 7D, when the baseline counter STCT is 0 to 31, the latest baseline BBASE is set to MINI0 (steps S12 and S13), and the baseline counter STCT is 32 to 63. Sets the latest baseline BBASE to MINI1 (steps S12, S14). It is assumed that the clock is synchronized with the basic clock at the clock synchronization position in the figure and operates once every clock rise.

(2) Detection of rising edge FIG. 8 shows a flowchart for measuring pulse integration.
If the signal continues to increase (step S30) at a preset number of steps or more, it is considered that the pulse has risen.
Specifically, first, the time limit counter ADDCT checks whether the time is within the time limit (step S21), and the output signal value DADC of the A / D conversion circuit 17 is determined from the baseline data recorded in the baseline buffer memory BASE. Is compared (step S22), and it is further checked whether the rising counter RISECT exceeds a predetermined number of steps (here, 4 steps (100 ns)) (step S23). If any of these conditions is satisfied, Let rise. The integration is performed by accumulating values obtained by subtracting the baseline from the data of the A / D conversion circuit 17. At this time, if the data of the A / D conversion circuit 17 is below the baseline, the data is discarded. For discarding data (step S32), the latest baseline data is copied and each counter is cleared.

(3) Detection of Maximum Value (MAX) Next, the maximum value MAX of the output signal value DADC of the A / D conversion circuit 17 is detected and stored (step S24).

(4) Detection of MAX / 2 When the output signal value DADC of the A / D conversion circuit 17 detects a value lower than MAX / 2 (step S25), it is determined that the pulse has ended, and the integral value is equal to or greater than the set threshold value. If it is (step S27), it is stored as data (step S28), and the process proceeds to discarding data (step S32). If it is less than the threshold, nothing is done and the process proceeds to discarding data (step S32). That is, if the output signal value DADC of the A / D conversion circuit 17 is equal to or less than MAX / 2, the maximum value has already been detected, and the integration value of the integration counter INTEG exceeds the threshold value LLD, the integration is performed. The value is stored as an output peak value PEAK (steps S25 to S28).

(5) Discard at time limit If the above (1) to (4) do not end within the time limit set, the data is discarded (step S32), and the process waits for the next signal. In addition to this, if the output signal value DADC of the A / D conversion circuit 17 does not exceed the baseline data, the data is discarded at any time to prevent a false signal from being mixed.

  FIG. 9 shows an amplifier output waveform when a step signal is input through a capacitor. 9A shows a waveform with a step signal slope of 200 ns, FIG. 9B shows a waveform of 500 ns, and FIG. 9C shows a waveform of 1.3 μs. The lower waveform is an input step signal with a slope, and the upper graph is an amplifier output waveform. Since step signals are input through a 0.5 pF capacitor, all of them are outputs when receiving a charge amount of 0.5 pC. The waveform shaping time constant is set to 0.1 μs. This figure shows that the same output as the SPICE calculation result is obtained.

  FIG. 10 shows data obtained by obtaining a pulse height distribution map from a pulse peak value with a conventional gas detector for neutron measurement. FIG. 10A shows a waveform with a step signal slope of 200 ns, FIG. 10B shows a waveform of 500 ns, and FIG. 10C shows a waveform of 1.3 μs. The horizontal axis represents the pulse peak value, and the vertical axis represents the count number. It can be seen that the signals are distributed at positions that match the peak values of the amplifier output waveform of FIG.

  FIG. 11 is data obtained by obtaining a pulse height distribution map from the pulse integral value in the gas detector for neutron measurement according to the present invention. 11A shows a waveform with a step signal slope of 200 ns, FIG. 11B shows a waveform of 500 ns, and FIG. 11C shows a waveform of 1.3 μs. The horizontal axis is the pulse integration value, and the vertical axis is the count number. It can be seen that the signal is distributed at a position that matches the integrated value (area) of the amplifier output waveform of FIG.

  FIG. 12 shows comparison data between the pulse peak value measured by the conventional gas detector for neutron measurement and the pulse integral value measured by the gas detector for neutron measurement of the present invention in actual neutron measurement. FIG. 12A shows the pulse peak value measured by the conventional gas detector for neutron measurement, and FIG. 12B shows the pulse integral value measured by the gas detector for neutron measurement of the present invention. As is clear from these figures, it can be seen that in FIG. 12 (b), the high wave height data increases, indicating an ideal distribution, and it is proved that the present invention is sufficiently effective. .

  As mentioned above, although embodiment of this invention was described, this invention is not limited to the said embodiment, Based on the meaning of this invention, various deformation | transformation are possible, These are excluded from the scope of the present invention. is not.

  The present invention relates to a neutron measurement gas detector for measuring the amount of neutrons jumping into a sealed space of He3 gas, and in particular, a signal for detecting the amount of charge generated when neutrons jump into the gas detector. The present invention relates to a processing technique for increasing the counting rate and stabilizing the processing means, and has industrial applicability.

It is a schematic diagram which shows the structure of the gas detection apparatus for neutron measurement which concerns on this invention. The partial circuit of the charge-type amplifier normally used with the gas detector for neutron measurement is shown. It is a figure which shows the calculation result of SPICE (SimulationProgram with Integrated Circuit Emphasis) at the time of inputting a step signal through a capacitor | condenser. It is a figure which shows the waveform acquired when the output of a gas detector is passed through the amplifier of a quick waveform shaping time constant. It is a figure which shows the waveform of the conventional and actual gas detection apparatus of this invention. It is a figure which shows the typical waveform assumed of the fast signal and the slow signal. It is a flowchart for the measurement of the baseline in the gas detection apparatus for neutron measurement which concerns on this invention. It is a flowchart for the measurement of the pulse integration in the gas detector for neutron measurement concerning the present invention. It is a figure which shows the amplifier output waveform at the time of inputting a step signal through a capacitor | condenser. It is a figure which shows the data which obtained the wave height distribution map from the pulse wave height value with the conventional gas detection apparatus for neutron measurement. It is a figure which shows the data which obtained the wave height distribution map from the pulse integral value with the gas detector for neutron measurement of this invention. It is a figure which shows the comparison data of the pulse peak value measured with the conventional gas detection apparatus for neutron measurement in the actual neutron measurement, and the pulse integral value measured with the gas detection apparatus for neutron measurement of this invention. It is a schematic diagram for demonstrating the structure of a gas detector. It is a figure which shows the example of the conventional gas detection apparatus.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 Gas detector 12 Amplifier 13 1st integration circuit 14 Differentiation circuit 15 2nd integration circuit 16 Waveform shaping circuit 17 A / D conversion circuit 18 Digital integration means

Claims (6)

Gas detection means for measuring neutrons and outputting a charge corresponding to the detected amount;
First integrating amplifier means for integrating and amplifying the output from the gas detecting means;
A waveform shaping circuit comprising: a differentiation circuit for differentiating an output from the first integration amplifier means; and a second integration amplifier means for integrating and amplifying the output from the differentiation circuit;
Digital integration means for A / D converting the output of the second integration amplifier means and integrating the digital output value;
The integration time constant of the first integrating amplifier means is equivalent to the pole zero cancellation time constant of the differentiating circuit;
A gas detection apparatus for neutron measurement, wherein a time constant of the waveform shaping circuit is set in accordance with an initial detection time of the charge.   The first integrating amplifier means includes a first analog amplifier, and a capacitor C1 and a resistor R1 aligned and connected between an input terminal and an output terminal of the first analog amplifier. The gas detector for neutron measurement as described.   The differentiating circuit includes a capacitor C2 connected to the output of the first analog amplifier and a resistor R2 connected in parallel thereto, one terminal of which is connected in series to the capacitor C2, and the other terminal connected to the second terminal. The gas detector for neutron measurement according to claim 2, comprising a resistor R3 connected to an input of said integrating amplifier means. Said second integrating amplifier means comprises a second analog amplifier, a capacitor C3 and a resistor R4 connected in parallel between the input and the output of the second analog amplifier, to claim 3, which is constituted by The gas detector for neutron measurement as described.   The gas detector for neutron measurement according to claim 4, wherein a product of the capacitor C1 and the resistor R1 is equal to a product of the capacitor C2 and the resistor R2.   The neutron measurement gas detection apparatus according to claim 4, wherein a product of the capacitor C2 and the resistor R3 is equal to a product of the capacitor C3 and the resistor R4.

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