A high count rate alpha particle detection method for accompanying particle imaging in fusion neutron generators

Abstract

The present invention relates to a high counting rate alpha particle detection method for accompanying particle imaging of a fusion neutron generator, which can effectively improve the measurement counting rate and timing accuracy of the accompanying particle imaging system of a fusion neutron source. The method comprises: by setting a barrier layer of a semiconductor detector, the thickness of a detection sensitive area, and thinning the thickness of an electrode and a substrate, the energy deposition of interference particles in a deuterium-tritium neutron source in the sensitive area of ​​the detector is reduced, and the proportion of energy deposition of accompanying alpha particles in the sensitive area of ​​the detector is increased, so that the amplitude of the current pulse signal output by the detector is proportional to the particle deposition energy; a current sensitive amplifier is used to amplify and reduce noise on the pulse signal output by the detector, and a narrow width voltage signal proportional to the current signal of the detector is output; a leading edge timing discriminator is used to time the pulse leading edge of the amplified voltage signal, and a reasonable trigger threshold is set to eliminate interference particle signals, so that only accompanying alpha particle timing signals are obtained.

Description

Technical Field

[0001] The present invention relates to a high counting rate alpha particle detection method for companion particle imaging of a fusion neutron generator, which belongs to the field of radiation detection and nuclear analysis, and can effectively improve the measurement counting rate and timing accuracy of a companion particle imaging system of a fusion neutron source, and improve the working efficiency and measurement accuracy of the companion particle imaging system. Background Art

[0002] Associated Particle Imaging (API) is a new type of nondestructive imaging detection technology using neutron rays. It has the characteristics of strong penetration, high detection sensitivity, three-dimensional spatial recognition and multi-element recognition. It has broad application prospects in the fields of explosive detection, industrial component detection, biomedicine, etc. API technology uses the characteristics of the quasi-monoenergetic α particles and neutrons produced by deuterium-tritium fusion reaction, which are correlated in time and direction of movement. By detecting the emission time and direction of the accompanying α particles, the flight time and direction of the neutrons are marked. The marked neutrons react with the nuclides in the sample to produce prompt γ rays. The time information of the prompt γ emitted by the sample is associated with the time information of the accompanying α particles. At the same time, the energy and intensity of the characteristic peaks of the γ energy spectrum are analyzed to determine the type and content of the nuclides in the γ emission area. Compared with the traditional material analysis method based on neutron technology, API technology greatly reduces the background in the γ energy spectrum due to the use of time coincidence measurement technology, improves the speed and accuracy of sample analysis, and especially provides three-dimensional spatial distribution information of material elements, making it have great application prospects in the fields of explosive detection and mineral screening.

[0003] The core of API technology is the timing measurement of α-particles. The maximum counting rate and timing accuracy of α-particles are key parameters that affect the performance of API systems. The maximum counting rate of α-particles determines the upper limit of the α-γ coincidence counting rate. The higher the counting rate of α-particles, the more neutrons are labeled per unit time, the greater the probability of α-γ coincidence, and the higher the working efficiency of the API system. The higher the timing accuracy of α-particles, the better the accuracy of α-γ coincidence measurement, which can narrow the time coincidence window of γ spectrum detection and thus improve the signal-to-noise ratio of γ spectrum data; at the same time, high timing accuracy can make the neutron flight time spectrum more accurate, and the spatial information of the elements where neutrons interact with samples can be calculated more accurately.

[0004] Under current technical conditions, the α-particle detector can obtain higher timing accuracy by using semiconductor detectors. In order to reduce the influence of interfering particles and noise signals, it needs to be matched with a charge-sensitive preamplifier. The α-particle signal is amplified and input into the back-end filter shaping circuit, and then input into the constant ratio timing circuit to reduce the timing error. However, due to the charge integration effect of the charge-sensitive preamplifier, the amplified signal has a long tail, resulting in a signal width of the order of microseconds. Therefore, the counting rate of the α-particle measurement method using this scheme can only reach 10 5 Due to the low count rate of the accompanying alpha particle measurement method, the neutron source intensity of the currently operating API system is 10 9 However, the source strength of neutron sources can now reach 10 12 Therefore, it is necessary to provide a companion α particle detection method with a higher counting rate to increase the proportion of labeled neutrons and improve the detection efficiency of the system. Summary of the invention

[0005] The technology of the present invention solves the problem and provides a high counting rate alpha particle detection method for companion particle imaging of a fusion neutron generator, which can effectively improve the maximum counting rate and timing accuracy of the companion alpha detector in a deuterium-tritium fusion neutron source, improve the alpha-gamma coincidence counting rate and the resolution of the coincidence time spectrum, and enhance the working efficiency and measurement accuracy of the companion particle imaging system.

[0006] The technical solution of the present invention is as follows:

[0007] A high counting rate alpha particle detection method for companion particle imaging of a fusion neutron generator, the method comprising the following steps:

[0008] Step 1): using a semiconductor detector with particle selection characteristics to output a current pulse signal; the semiconductor detector includes a barrier layer, an insulating layer, a front electrode, a detection sensitive area, a substrate and a rear electrode; by setting the thickness of the barrier layer, optimizing the thickness of the detection sensitive area and thinning the sum of the thickness of the front electrode, the rear electrode and the substrate, the energy deposition of the interfering particles in the deuterium-tritium neutron source in the detection sensitive area is reduced, and the proportion of the energy deposited by the accompanying alpha particles in the detection sensitive area is increased, so that the amplitude of the current pulse signal output by the semiconductor detector is proportional to the particle deposition energy;

[0009] Step 2): using a current sensitive amplifier to amplify and reduce noise of the current pulse signal output by the semiconductor detector, and outputting a narrow width voltage signal proportional to the current pulse signal, reducing the pile-up distortion of the signal, so that the amplitude of the amplified signal is proportional to the deposition energy of the particle in the detection sensitive area;

[0010] Step 3): Use a leading edge timing discriminator to time the pulse leading edge of the voltage signal, and eliminate the interfering particle signal by setting a reasonable trigger threshold to obtain only the accompanying α particle timing signal.

[0011] Furthermore, the step 1) includes: according to the density of the conductive material, using a conductive material of suitable thickness as a barrier layer to prevent low-energy interference particles (mainly deuterium particles and helium-3 particles) from entering the front-end electrode and the detection sensitive area, and the thickness enables the barrier layer to completely block the low-energy interference particles; according to the density and insulation performance of the insulating material, using an insulating material of suitable thickness as an insulating layer to isolate the barrier layer from the front-end electrode, and the thickness is the minimum thickness at which the insulating layer will not be broken down after a voltage is applied to the front-end electrode of the semiconductor detector; after completing the front-end electrode preparation process, reducing the thickness of the front-end electrode without affecting the electrical performance of the semiconductor detector The energy loss of the incident accompanying α particles is reduced under the condition of reducing the thickness of the electrode to the minimum value without affecting the electrical performance of the semiconductor detector; the thickness of the detection sensitive area is set to be slightly larger than the range of the α particles in the detection sensitive area, so as to just completely block the accompanying α particles and reduce the energy deposition of high-energy interfering particles; after completing the preparation of the detection sensitive area and the front-end electrode, the substrate of the semiconductor detector is thinned without affecting the performance of the semiconductor detector, so that the thickness of the substrate is the minimum thickness without affecting the performance of the semiconductor detector, thereby reducing the energy loss of high-energy interfering particles at the rear-end electrode and reducing the impact on the signal output.

[0012] Furthermore, the gain and bandwidth of the current sensitive amplifier are designed according to the pulse waveform characteristics of the current pulse signal caused by the α particles in the detector, wherein the gain of the current sensitive amplifier is designed according to the amplitude of the current pulse signal accompanying the α particles and the amplitude requirement of the leading edge timing discriminator, and the bandwidth of the current sensitive amplifier is designed according to the width of the current pulse signal of the α particles. The current sensitive amplifier is composed of an integrated operational amplifier or a transistor matched with suitable resistors and capacitors, and the integrated operational amplifier can be a current feedback or voltage feedback operational amplifier.

[0013] Furthermore, the trigger threshold of the leading edge timing discriminator is selected according to the signal waveform output by the current sensitive amplifier, and the trigger threshold is greater than the maximum amplitude of the noise and the non-associated α-particle signal, less than the maximum amplitude of the associated α-particle signal, and is located at the point where the leading edge slope of the associated α-particle signal is maximum.

[0014] Furthermore, the thickness of the blocking layer and the insulating layer is just enough to completely block the deuterium particles emitted from the deuterium-tritium neutron source target and the helium-III particles produced by the deuterium-tritium reaction, so that the deuterium particles and the helium-III particles deposit all their energy in the blocking layer; the blocking layer and the insulating layer are attached to the front of the front electrode of the detector as a thin film, the preparation process of the thin film adopts evaporation or sputtering, and the material of the blocking layer is one or more of aluminum, nickel, copper, gold, silver, platinum, tantalum, and titanium; the blocking layer is connected to the ground wire of the external circuit to release the accumulated charge of the interfering particles.

[0015] Furthermore, the thickness of the detector sensitive area can just completely block the accompanying alpha particles after penetrating the blocking layer, the insulating layer and the front electrode, so that the alpha particles incident on the detection sensitive area can deposit all the energy in the detection sensitive area, that is, the thickness is the minimum thickness when the alpha particles incident on the detector sensitive area can deposit all the energy in the sensitive area. The semiconductor material used in the detector is silicon, germanium, silicon carbide, gallium nitride, boron nitride or diamond, and the detection sensitive area is a space charge area formed by a PN junction, a PIN junction or a Schottky junction.

[0016] In the above steps, after the preparation method of the front-end electrode is optimized, in order to ensure that the electrical performance of the detector does not degrade, according to the different electrode materials, the thickness of the front-end electrode can be reduced to the minimum value that the process can achieve through the process to reduce the energy loss of the incident accompanying α particles. The thickness of the sensitive area of ​​the detector is set to just completely block the accompanying α particles and reduce the energy deposition of high-energy interference particles (such as protons). The thickness of the sensitive area of ​​the detector should just completely block the accompanying α particles after penetrating the blocking layer, the insulating layer and the front-end electrode. The semiconductor material used in the detector can be silicon, germanium, silicon carbide, gallium nitride, boron nitride, diamond, etc. The semiconductor detector substrate and the back-end electrode are thinned to reduce the energy loss of high-energy interference particles near the electrode and reduce the impact on the output of the accompanying α particle signal. The gain, bandwidth and frequency response characteristics of the current sensitive amplifier in step 2) are designed according to the pulse waveform characteristics of the current signal induced by the accompanying α particles in the detector. The current pulse signal directly output by the detector has a very small rising edge and width, so the width of the output signal of the current sensitive amplifier is very small, and signal accumulation will not occur at a high counting rate, which helps to increase the counting rate of accompanying alpha particles.

[0017] The advantages of the present invention are:

[0018] (1) The present invention uses a semiconductor detector with particle selection characteristics, which helps to reduce the impact of interfering particles generated by the nuclear reaction of the deuterium-tritium neutron source on the accompanying alpha particle signal. By designing a blocking layer, thinning the front-end electrode, selecting a detector with a specific sensitive area thickness, and thinning the substrate and the back-end electrode, low-energy charged particles can be blocked outside the detector, reducing the energy deposition of high-energy charged particles in the detector sensitive area and the electrode, while ensuring that most of the energy of the accompanying alpha particles is deposited in the detection sensitive area, so that the signal output by the detector is mostly derived from the quasi-monoenergetic accompanying alpha particles, and the particles are selected at the physical level, reducing the difficulty of identifying the accompanying alpha signal. The optimized semiconductor detector has a smaller electrode and substrate thickness, which can effectively reduce parasitic inductance and capacitance, and reduce interference with the detector output signal.

[0019] (2) The present invention adopts a current sensitive amplifier matched with a semiconductor detector. Since the energy of the accompanying alpha particles is relatively single, the amplitude and waveform fluctuation of the current pulse signal generated in the detector are very small, so the current sensitive amplifier only needs to amplify specific signals, which can have better effects in terms of low noise and high rate. The amplified voltage signal output by the current sensitive amplifier is proportional to the input current pulse signal, which can effectively reduce the width and tailing of the amplified signal, avoid signal accumulation distortion at high counting rates, and help to achieve higher counting rate accompanying alpha particle measurement.

[0020] (3) The present invention uses a leading-edge timing discriminator to time the accompanying α-particle signal. Since the energy of the accompanying α-particle is concentrated, the amplitude and waveform of the pulse signal excited by it are consistent. The time shift caused by the leading-edge timing method is small, and it has higher timing accuracy. By setting a reasonable trigger threshold, the interference signal and noise can be filtered out by using the signal amplitude difference, and only the timing signal of the accompanying α-particle is output. In addition, the leading-edge timing circuit has the characteristics of simple structure and easy integration. When a multi-channel accompanying α-particle detector is used, it is easy to make an integrated multi-channel timing circuit, reducing the integration and development difficulty of the electronic circuit.

[0021] (4) The present invention aims at the core problems and technical bottlenecks of the companion particle imaging technology. Based on the types of interfering particles in the deuterium-tritium neutron source and the energy characteristics of the accompanying α particles, the present invention proposes a companion α particle timing method by designing the detector, the amplifier circuit, and the timing circuit separately and using them in coupling. This method helps to improve the counting rate and timing accuracy of the accompanying α particles, and improve the working efficiency and measurement accuracy of the companion particle imaging system. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is the overall flow chart of the present invention;

[0023] Figure 2A structural diagram of a semiconductor detector with particle selection characteristics designed for the present invention;

[0024] Figure 3 Functional schematic diagram of a semiconductor detector with particle selection characteristics designed for the present invention;

[0025] Figure 4 It is a schematic diagram of using a leading-edge timing discriminator in the present invention to perform timing discrimination on an accompanying alpha particle signal;

[0026] Figure 5 This is a flow chart of processing the accompanying alpha particle signal output by the detector in the present invention;

[0027] Figure 6 is the track of α, T and p in the detector in the embodiment of the present invention;

[0028] Figure 7 The energy distribution of alpha particles after passing through the barrier layer, the insulating layer and the front electrode in the embodiment of the present invention;

[0029] Figure 8 The energy distribution of T particles after passing through the barrier layer, the insulating layer and the front electrode in the embodiment of the present invention;

[0030] Fig. 9 The deposition energy of α particles and T particles in the sensitive area of ​​the detector varies with depth in the embodiment of the present invention;

[0031] Fig.10 The current pulse waveform generated by the energy deposited by alpha particles at different depths of the detector in the embodiment of the present invention;

[0032] Fig.11 The current pulse waveform generated by the energy deposited by T particles at different depths of the detector in the embodiment of the present invention;

[0033] Fig.12 : is the average current pulse waveform generated by α particles and T particles in the embodiment of the present invention. DETAILED DESCRIPTION

[0034] The present invention is described in detail below in conjunction with the accompanying drawings and specific embodiments. However, the following embodiments are limited to explaining the present invention, and the protection scope of the present invention should include the entire contents of the claims, and is not limited to the present embodiments.

[0035] See also Figure 1 , a high counting rate alpha particle detection method for companion particle imaging of a fusion neutron generator, comprising the following steps:

[0036] Step 101: A semiconductor detector with particle selection characteristics reduces the energy deposition of interfering particles in the deuterium-tritium neutron source in the sensitive area of ​​the detector by setting a blocking layer, optimizing the thickness of the detection sensitive area, and thinning the thickness of the electrode and the substrate, reduces the parasitic resistance and capacitance of the detector, and increases the proportion of the energy deposited by the accompanying alpha particles in the sensitive area of ​​the detector, so that the amplitude of the detector output current pulse signal is proportional to the particle deposition energy.

[0037] Specifically, the semiconductor detector structure with particle selection characteristics described in step 101 is as follows: Figure 2 As shown, it includes a blocking layer 201, an insulating layer 202, a front electrode 203, a detection sensitive area 204, a substrate 205 and a back electrode 206. The actual function of the semiconductor detector with particle selection characteristics is as follows Figure 3 As shown. The barrier layer 201 is composed of a conductive material of suitable thickness, which can prevent D (deuterium) and 3H (helium-3) particles are incident on the front electrode 203 and the detection sensitive area 204 of the detector to prevent the discharge effect of the accumulated charge of the interfering particles and the energy deposition in the sensitive area from interfering with the output signal of the detector. The insulating layer 202 is made of an insulating material of suitable thickness and is used to isolate the blocking layer 201 from the front electrode 203 of the detector. The blocking layer and the insulating layer are attached to the front of the front electrode of the detector as a thin film and become a whole with the detector. The film preparation process adopts evaporation or sputtering, and the material of the blocking layer is one or more of aluminum, nickel, copper, gold, silver, platinum, tantalum, and titanium. The blocking layer is connected to the ground wire of the external circuit to release the accumulated charge of the interfering particles. The function of the front electrode 203 is signal extraction. After the preparation method is optimized, the thickness of the front electrode is minimized without affecting the electrical performance of the detector, thereby reducing the energy loss of the incident accompanying α particles. The function of the detection sensitive area 204 is to completely prevent the accompanying α particles after penetrating the blocking layer, the insulating layer and the front electrode, and reduce the energy deposition of p (protons). At the same time, T (tritium) particles will also deposit energy in the detection sensitive area, but due to the large energy gap with α particles, the location of their energy deposition is different from that of α particles, and the amplitude of the pulse signal generated is much smaller than the amplitude of the signal excited by α particles. Since the energy of the detected accompanying α particles received by the detector at a fixed position is relatively single, after proper design, the energy of the accompanying α particles is just completely deposited near the bottom of the detector sensitive area, and the output current pulse waveform has a similar shape and amplitude. The function of the substrate 205 is to support the detection sensitive area and to be connected to the rear electrode 206. The function of the rear electrode 206 is to lead out the signal. Among them, the semiconductor material used in the detector can be silicon, germanium, silicon carbide, gallium nitride, boron nitride, diamond, etc. Thinning the semiconductor detector substrate and the rear electrode can reduce the energy loss of high-energy interfering particles near the electrode, reduce the impact on the output of the accompanying α particle signal, and also help to reduce parasitic resistance and capacitance, and reduce the impact on the output pulse signal waveform.

[0038] Step 102: A current-sensitive amplifier is used to amplify and reduce noise on the pulse signal output by the detector, and an output narrow-width voltage signal whose amplitude is proportional to the detector current signal is produced, thereby reducing the pile-up distortion of the signal and making the amplified signal amplitude proportional to the deposition energy of the particle in the sensitive area of ​​the detector.

[0039] Specifically, the gain, bandwidth and frequency response characteristics of the current sensitive amplifier described in step 102 are designed according to the pulse waveform characteristics of the current signal induced by the accompanying α particles in the detector. Since the energy of the measured accompanying α particles is relatively single, about 3.5 MeV, after appropriate design, the accompanying α particle energy is just completely deposited near the bottom of the sensitive area of ​​the detector, and its output current pulse waveform shape is similar and the amplitude is similar. Therefore, the current sensitive amplifier only needs to optimize the signal characteristics of the specific accompanying α particles to obtain a signal with a small rise time and pulse width as much as possible and a large signal amplitude that is proportional to the input current pulse amplitude. Since the requirements for other performance aspects of the current sensitive amplifier are reduced, the low noise characteristics of the amplifier can be well improved, and the waveform of its output voltage signal can be highly consistent with the input current pulse signal, retaining the waveform characteristics of the accompanying α particle excitation pulse. The current pulse signal directly output by the detector has an extremely small rising edge and width, so the width of the output signal of the current sensitive amplifier is also very small, and there will be no signal accumulation phenomenon at a high counting rate, which is helpful to improve the counting rate of the accompanying α particles. After using the current sensitive amplifier, the width of the amplified signal can be kept at the nanosecond level, and the maximum counting rate can be increased to 10 8 cps, which is three orders of magnitude higher than the traditional measurement scheme using charge sensitive circuits. The current sensitive amplifier can be composed of an integrated operational amplifier or a transistor matched with a suitable resistor and capacitor, and the integrated operational amplifier can be a current feedback or voltage feedback operational amplifier.

[0040] Step 103: A leading edge timing discriminator is used to time the pulse leading edge of the amplified voltage signal. By setting a reasonable trigger threshold, the interfering particle signal can be eliminated and only the accompanying α particle timing signal can be obtained. By utilizing the single energy characteristic of the accompanying α particle, a higher timing accuracy can be obtained.

[0041] Specifically, the leading edge timing discriminator in step 103 uses the leading edge of the input signal for signal timing, and the trigger threshold is selected according to the amplitude of the signal waveform output by the current sensitive amplifier. Figure 4The schematic diagram of the timing discrimination of α-particle signals by the leading-edge timing discriminator in the present invention is shown. Since the energy of the detected accompanying α-particles is relatively single, and the energy broadening after the detector is optimized and designed is very small, the deposited energy of the accompanying α-particles in the sensitive area of ​​the detector is relatively concentrated, and the pulse signal amplitude of the detector output is proportional to the deposited energy. Therefore, the pulse amplitude of the signal excited by the accompanying α-particles is relatively consistent, and the difference between the particle signals and noise such as T and p is large. Selecting a suitable trigger threshold to trigger a signal of a specific amplitude can effectively screen out the accompanying α-particle signal. The trigger threshold should be greater than the maximum amplitude of the noise and non-accompanying α-particle signals, less than the maximum amplitude of the accompanying α-particle signals, and located at the maximum slope of the leading edge of the accompanying α-particle signal. The leading-edge timing circuit has a smaller timing error for input signals with smaller rise time and amplitude changes, so it can obtain higher timing accuracy, and its circuit structure is simple and easy to integrate, which has greater advantages in the realization of multi-channel timing circuits matching multi-pixel α detectors.

[0042] Specifically, Figure 5 The processing flow chart of the accompanying α-particle signal output by the detector in the present invention is shown. Among them, 501 is the current pulse signal excited by the α-particle output by the detector, 502 is the current pulse signal excited by the T-particle output by the detector, and 503 is the current pulse signal excited by the p-particle output by the detector; 504 is the α-particle signal amplified by the current sensitive amplifier, 505 is the T-particle signal amplified by the current sensitive amplifier, and 506 is the p-particle signal amplified by the current sensitive amplifier; 507 is the α-particle signal in the leading edge timing discriminator, 508 is the T-particle signal in the leading edge timing discriminator, and 509 is the p-particle signal in the leading edge timing discriminator; the pulse leading edge of 507 exceeds the trigger threshold V T , the amplitudes of 508 and 509 are both smaller than the trigger threshold, and the leading-edge timing discriminator output signal 510 is only the signal accompanying the α-particle excitation.

[0043] The present invention is further described below by specific embodiments:

[0044] In this embodiment, according to step 101, a semiconductor detector with particle selection characteristics is designed, and the energy deposition of interfering particles in the deuterium-tritium neutron source in the sensitive area of ​​the detector is reduced by setting the thickness of the blocking layer, optimizing the thickness of the detection sensitive area, and thinning the thickness of the electrode and the substrate, and the like, and the parasitic resistance and capacitance of the detector are reduced, and the proportion of the energy deposited by the accompanying alpha particles in the sensitive area of ​​the detector is increased, so that the amplitude of the detector output current pulse signal is proportional to the particle deposition energy.

[0045] In this embodiment, the barrier layer is 3μm thick aluminum, the insulating layer is 100nm thick aluminum nitride, the front electrode is 50nm gold, the detection sensitive area is 10μm silicon, the substrate is 100μm silicon, and the rear electrode is 100nm gold. The barrier layer and the insulating layer are prepared by magnetron sputtering. The barrier layer is directly sputtered and deposited by an aluminum target, and the insulating layer is sputtered and deposited by an aluminum target in nitrogen; a P-type silicon thin layer is prepared by ion implantation on the surface of N-type silicon to form a PN junction, and a fully depleted detection sensitive area can be formed inside the silicon semiconductor after applying an external bias voltage, and the silicon without a space charge area becomes the substrate; the front electrode and the rear electrode are prepared by magnetron sputtering.

[0046] In this embodiment, the possible nuclear reactions in the deuterium-tritium fusion neutron source include D(d,n) 3 He,D(d,p)T,T(d,n) 4 He, when the detector is located at 90° to the deuterium (D) beam, the accompanying α particle ( 4 The energy of He is about 3.5MeV, and the other interfering particles include neutrons with energies of about 2.5MeV and 14.1MeV, 3 He particles, 1MeV tritium (T) particles, 3MeV protons (p), and scattered D particles with energy of about 300keV. When these particles are incident on the detector surface, 3μm aluminum can completely block D particles and 3 He particles, T, α and p will pass through the barrier layer, insulating layer and front electrode to reach the detection sensitive area, while T and α will completely deposit their energy in the detector sensitive area, and p will continue to penetrate until it exits the detector. Among them, due to the thin thickness of the barrier layer, insulating layer and front electrode, the energy loss of T and α after penetration is very small, the energy broadening is not obvious, and most of the remaining energy is deposited in the detector sensitive area. However, p has higher energy, smaller charge and mass, and the energy deposition in each layer is small. The deposited energies of α, T and p in the detector sensitive area are 2.8MeV, 0.72MeV and 0.2MeV respectively. Figure 6 The tracks of α, T and p in the detector in the embodiment of the present invention are shown. The average range of α in the detector is 9.7 μm, the average range of T in the detector is 7.3 μm, and p completely penetrates the detector.

[0047] Taking into account that the amplitude and waveform of the detector output current pulse signal are related to the position and size of the particle deposition energy, the energy deposition and position distribution of α and T are calculated in the embodiment of the present invention. Since the deposition energy of protons is very small, only one tenth of α, the amplitude of the proton output pulse signal is much lower than the signal excited by α particles, and its interference with the α particle signal can be ignored. Figure 7The energy distribution of alpha particles after passing through the barrier layer, the insulating layer and the front electrode in the embodiment of the present invention is shown. The energy broadening is very small and concentrated around 2.8MeV. Figure 8 The energy distribution of T particles after passing through the barrier layer, the insulating layer and the front electrode in the embodiment of the present invention is shown. The energy broadening is very small and concentrated around 0.72MeV. Fig. 9 α and T are the deposited energies at different depths in the sensitive area of ​​the detector in the embodiment of the present invention. The energy of α particles is mainly deposited within 9 μm, and the energy of T particles is mainly deposited within 6 μm.

[0048] According to the calculation formula of the output current waveform of the depletion type PN junction semiconductor detector, the electron current at different ionization positions is i n =(1-x / d)*N*e*exp(t / τ n ) / τ n , 0≤t≤T n , where x is the ionization position, d is the thickness of the detector sensitive area, N is the number of electrons excited by the particle, and e is the electron charge; T n is the maximum electron collection time, which is a function related to the electron migration speed and action depth, usually τ n *ln(d / dx); where τ n =d 2 / 2μ n V, μ n is the electron mobility, V is the bias voltage, τ n and τ p Generally only depends on the semiconductor material. At the same time, the induced current generated by the hole is i p =(1-x / d)*N*e*exp(-t / τ p ) / τ p , 0≤t≤T p , T p =τ p *ln(d / x). In silicon detectors, τ p is about nanoseconds, when τ p When it is 1ns, due to the μ n is μ p Three times, τ n 1 / 3ns. Knowing that the ionization energy of silicon is 3.6eV, the number of electron-hole pairs generated when α and T deposit energy in the detector sensitive area can be calculated. Considering the continuous slowing process of α and T in the detector sensitive area, based on Fig. 9 The calculated energy deposition size and position distribution of α and T are used to calculate the signals generated by ionization at different depths to obtain Fig.10 and Fig.11The current pulse waveforms generated by the energy deposition of α and T particles at different depths in the detector in the embodiment of the present invention shown. It can be seen that even considering the continuous energy deposition process of α and T in the sensitive area of ​​the detector, the current signal pulse amplitude excited by the α particles is greater than the signal amplitude generated by T. The pulse leading edge is mainly generated by the drift of electrons, and the pulse trailing edge is generated by the drift of holes. Since the drift speed of electrons is faster, the leading edge of the current pulse is very short, only in the sub-nanosecond order. Averaging the energy and action depth position of the particles can obtain an approximate output current pulse waveform after the energy of each particle is completely deposited at a certain point between the front and rear end electrodes of the detector. The pulse waveform characteristics are between the complete energy deposition near the front and rear end electrodes, and can be equivalent to the output pulse signal when the charged particles undergo continuous energy deposition in the detection sensitive area, such as Fig.12 As shown, it can be seen that the amplitude of the output current pulse waveform caused by α and T particles is very different.

[0049] The detector output current waveform is input to the current sensitive amplifier. After amplification, the output voltage signal amplitude is proportional to the input current, so the α particle signal amplitude is much larger than the T particle signal amplitude. The current sensitive amplifier uses an ideal operational amplifier. The feedback resistor of the first stage amplification is selected as 300Ω. The amplitude of the α particle signal output by the first stage is about 120mV. After the subsequent amplification, considering the rise time of the common current sensitive amplifier itself, the final output α particle amplified signal rise time is within 3ns, the α particle signal amplitude is about a few volts, and the pulse width is within 10ns.

[0050] In the leading edge timing discriminator, since the fluctuation of the energy deposited by 3.5MeV α particles in the sensitive area of ​​the detector is very small, the fluctuation of the amplitude and rise time of the amplified signal is very small. Compared with other timing methods, the time shift of the leading edge timing method is the smallest, which can reach less than 100ps. By setting the trigger threshold to 80% of the amplitude of the accompanying α particle signal, the accompanying α particle signal can be screened out. The output timing signals are all caused by the accompanying α particles and can be directly matched with the prompt γ signal.

[0051] In summary, the present invention relates to a high counting rate alpha particle detection method for accompanying particle imaging of a fusion neutron generator, and the main contents are: a semiconductor detector with particle selection characteristics, which reduces the energy deposition of interference particles in a deuterium-tritium neutron source in the sensitive area of ​​the detector by setting a barrier layer, optimizing the thickness of the detection sensitive area, and thinning the thickness of electrodes and substrates, and increases the proportion of energy deposition of accompanying alpha particles in the sensitive area of ​​the detector, so that the amplitude of the current pulse signal output by the detector is proportional to the particle deposition energy; a current sensitive amplifier, which is used to amplify and reduce noise on the pulse signal output by the detector, output a narrow width voltage signal proportional to the detector current signal, reduce the pile-up distortion of the signal, and make the amplitude of the amplified signal proportional to the deposition energy of the particle in the sensitive area of ​​the detector; a leading edge timing discriminator, which is used to time the pulse leading edge of the amplified voltage signal, and can eliminate interference particle signals by setting a reasonable trigger threshold, and only obtain the accompanying alpha particle timing signal, and obtain higher timing accuracy by utilizing the single energy characteristic of the accompanying alpha particles. Through the method described in the present invention, according to the energy deposition calculation results of the particles in the detector and the common performance parameters of the current sensitive amplifier and the leading edge timing discriminator, it can be seen that the timing counting rate of the accompanying α particles can reach 10 8 The timing accuracy is about 100ps, which is higher than the traditional timing measurement method of accompanying α particles, and can significantly improve the working efficiency and measurement accuracy of the accompanying particle imaging system.

[0052] It should be noted that, according to the above-mentioned embodiments of the present invention, those skilled in the art can fully implement the full scope of the independent claims and dependent rights of the present invention, and the implementation process and method are the same as the above-mentioned embodiments; and the parts not elaborated in detail in the present invention belong to the well-known technology in the field.

[0053] The above descriptions are only some specific implementation methods of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by any person familiar with the art within the technical scope disclosed in the present invention should be covered within the protection scope of the present invention.

Claims

1. A high count rate alpha particle detection method for companion particle imaging of a fusion neutron generator, characterized in that: The method comprises the following steps: Step 1): A semiconductor detector with particle selection characteristics is used to output a current pulse signal; the semiconductor detector includes a blocking layer, an insulating layer, a front electrode, a detection sensitive area, a substrate and a rear electrode; by setting the thickness of the blocking layer, optimizing the thickness of the detection sensitive area and thinning the sum of the thickness of the front electrode, the rear electrode and the substrate, the energy deposition of the interfering particles in the deuterium-tritium neutron source in the detection sensitive area is reduced, and the proportion of the energy deposited by the accompanying alpha particles in the detection sensitive area is increased, so that the amplitude of the current pulse signal output by the semiconductor detector is proportional to the particle deposition energy; Step 2): A current-sensitive amplifier is used to amplify and reduce noise of the current pulse signal output by the semiconductor detector, and a narrow-width voltage signal proportional to the current pulse signal is output to reduce the pile-up distortion of the signal, so that the amplitude of the amplified signal is proportional to the deposition energy of the particle in the detection sensitive area; Step 3): Use a leading edge timing discriminator to time the pulse leading edge of the voltage signal, and eliminate the interfering particle signal by setting a reasonable trigger threshold to obtain only the accompanying α particle timing signal.

2. A high counting rate alpha particle detection method for companion particle imaging of a fusion neutron generator according to claim 1, characterized in that: The step 1) includes: according to the density of the conductive material, using a conductive material of appropriate thickness as a blocking layer to prevent low-energy interference particles from entering the front-end electrode and the detection sensitive area; according to the density and insulation performance of the insulating material, using an insulating material of appropriate thickness as an insulating layer to isolate the blocking layer from the front-end electrode; after completing the front-end electrode preparation process, reducing the thickness of the front-end electrode to reduce the energy loss of the incident accompanying α particles without affecting the electrical performance of the semiconductor detector; setting the thickness of the detection sensitive area to be slightly larger than the range of the α particles in the detection sensitive area so as to just completely block the accompanying α particles and reduce the energy deposition of high-energy interference particles; after completing the preparation of the detection sensitive area and the front-end electrode, thinning the substrate of the semiconductor detector without affecting the performance of the semiconductor detector to reduce the energy loss of high-energy interference particles at the rear-end electrode and reduce the impact on the signal output.

3. A high counting rate alpha particle detection method for companion particle imaging of a fusion neutron generator according to claim 1, characterized in that: The gain and bandwidth of the current sensitive amplifier are designed according to the pulse waveform characteristics of the current pulse signal caused by the α particles in the detector, wherein the gain of the current sensitive amplifier is designed according to the amplitude of the current pulse signal accompanying the α particles and the amplitude requirement of the leading edge timing discriminator, and the bandwidth of the current sensitive amplifier is designed according to the width of the current pulse signal of the α particles. The current sensitive amplifier is composed of an integrated operational amplifier or a transistor matched with suitable resistors and capacitors, and the integrated operational amplifier is a current feedback or voltage feedback operational amplifier.

4. A high counting rate alpha particle detection method for companion particle imaging of a fusion neutron generator according to claim 1, characterized in that: The trigger threshold of the leading edge timing discriminator is selected according to the signal waveform output by the current sensitive amplifier. The trigger threshold is greater than the maximum amplitude of the noise and the non-associated α-particle signal, less than the maximum amplitude of the associated α-particle signal, and is located at the maximum slope of the leading edge of the associated α-particle signal.

5. A high counting rate alpha particle detection method for companion particle imaging of a fusion neutron generator according to claim 2, characterized in that: The thickness of the blocking layer and the insulating layer can just completely block the deuterium particles emitted from the deuterium-tritium neutron source target and the helium-III particles produced by the deuterium-tritium reaction, so that the deuterium particles and the helium-III particles deposit all their energy in the blocking layer; the blocking layer and the insulating layer are attached to the front part of the front electrode of the detector as a thin film, the preparation process of the thin film adopts evaporation or sputtering, and the material of the blocking layer is one or more of aluminum, nickel, copper, gold, silver, platinum, tantalum, and titanium; the blocking layer is connected to the ground wire of the external circuit to release the accumulated charge of the interfering particles.

6. A high counting rate alpha particle detection method for companion particle imaging of a fusion neutron generator according to claim 2, characterized in that: The thickness of the detector sensitive area can just completely block the accompanying alpha particles after penetrating the blocking layer, the insulating layer and the front-end electrode, so that the alpha particles incident on the detection sensitive area can deposit all the energy in the detection sensitive area. The semiconductor material used in the detector is silicon, germanium, silicon carbide, gallium nitride, boron nitride or diamond, and the detection sensitive area is a space charge area formed by a PN junction, a PIN junction or a Schottky junction.

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