阅读说明：本技术 基于脉冲形状甄别的中子-γ周围剂量当量率仪 (Neutron-gamma ambient dose equivalent rate instrument based on pulse shape discrimination ) 是由 夏文明 节帅 龚军军 于 2021-01-12 设计创作，主要内容包括：本发明公开了基于脉冲形状甄别的中子-γ周围剂量当量率仪,包括有机闪烁体探测器、光电转换器件、信号处理电路、脉冲形状甄别电路和仪器控制电路；有机闪烁体探测器在中子或γ射线的照射下发出荧光,光电转换器件将荧光转换成电流信号,信号预处理电路将光电转换器件产生的电流信号转换成时间常数有差异的电压脉冲信号,脉冲形状甄别电路可根据电压信号的脉冲形状差异进行中子和γ甄别,并分别记录中子和γ产生的电荷积分谱,仪器控制电路将单位时间内中子和γ产生的脉冲的电荷积分分别换算成中子和γ周围剂量当量率。本发明可以用同一个探测器实现中子和γ周围剂量当量率的测量,可以大大减轻中子周围剂量当量率仪的重量。(The invention discloses a neutron-gamma ambient dose equivalent rate instrument based on pulse shape discrimination, which comprises an organic scintillator detector, a photoelectric conversion device, a signal processing circuit, a pulse shape discrimination circuit and an instrument control circuit, wherein the organic scintillator detector is connected with the photoelectric conversion device through a photoelectric conversion circuit; the organic scintillator detector emits fluorescence under the irradiation of neutrons or gamma rays, the photoelectric conversion device converts the fluorescence into current signals, the signal preprocessing circuit converts the current signals generated by the photoelectric conversion device into voltage pulse signals with different time constants, the pulse shape discrimination circuit can discriminate the neutrons and the gamma according to the pulse shape difference of the voltage signals and respectively record charge integration spectrums generated by the neutrons and the gamma, and the instrument control circuit converts the charge integration of the pulses generated by the neutrons and the gamma in unit time into the neutron and gamma surrounding dose equivalent rates. The invention can use the same detector to realize the measurement of the neutron and gamma ambient dose equivalent rate, and can greatly reduce the weight of the neutron ambient dose equivalent rate instrument.)

1. Neutron-gamma ambient dose equivalent rate appearance based on pulse shape is discriminated which characterized in that: the dose equivalent rate meter comprises:

organic scintillator detector: for emitting a fluorescent signal differing in temporal characteristics with respect to the action of neutrons and gamma;

a photoelectric conversion device: the device is used for converting a fluorescence signal emitted by the organic scintillator detector into a voltage or current pulse signal;

a signal processing circuit: the device is used for processing voltage or current pulse signals output by the photoelectric conversion device, converting the voltage or current pulse signals into voltage pulse signals with signal amplitude and impedance matched with the pulse shape discrimination circuit, and keeping the difference of time characteristics of neutron and gamma signals;

the pulse shape discrimination circuit: the voltage pulse signal processing circuit is used for carrying out digital conversion on the voltage pulse signal output by the signal processing circuit, discriminating the voltage pulse signal according to the difference of the time characteristics of the neutron and the gamma signal, completing charge integration calculation of the signal and respectively obtaining charge integration of the neutron and the gamma pulse signal;

the instrument control circuit: the neutron and gamma pulse integrating circuit is used for converting charge integration of neutron and gamma pulse signals obtained by processing of the pulse shape discrimination circuit into neutron and gamma ambient dose equivalent results.

2. The pulse shape discrimination based neutron-gamma ambient dose equivalent rate apparatus of claim 1, wherein: the organic scintillator detector adopts doping¹⁰B element scintillator detector.

3. The pulse shape discrimination based neutron-gamma ambient dose equivalent rate apparatus of claim 1, wherein: the pulse shape discrimination circuit comprises a high-speed AD conversion circuit and an FPGA system, the high-speed AD conversion circuit is used for carrying out digital processing on a voltage pulse signal output by the signal processing circuit, the FPGA system is used for discriminating the difference of time characteristics of neutron and gamma signals by adopting a pulse shape discrimination PSD algorithm, and the pulse shape discrimination PSD algorithm is one or more of a rise time method, a charge comparison method, a pulse gradient method and a frequency gradient method.

4. The pulse shape discrimination based neutron-gamma ambient dose equivalent rate apparatus of claim 3, wherein: the FPGA system judges the signal amplitude of the pulse signal after digital processing, determines the pulse event as a valid particle pulse event when the amplitude exceeds a set trigger threshold, caches the data of the event in an FIFO memory inside the FPGA, starts operation at the same time, automatically searches the position of a pulse starting point and the position of an end point according to a set pulse starting point determining method, and then automatically searches the position of the pulse starting point and the position of the end point according to a set long-gate time window T_longShort door time window T_shortIntegrating the charges in the time windows to respectively obtain the charge integrals Q in the two time windows_longAnd Q_short(ii) a Meanwhile, integrating the charges in the time period from the starting point position to the end point position of each pulse signal to obtain the charge integral Q of each pulse signal; according to Q of each pulse_longAnd Q_shortQ, calculating a pulse shape and discriminating a PSD value, counting neutrons and gamma through a value range of the PSD value, and sending a calculation result to an instrument control circuit by the FPGA.

5. The pulse shape discrimination based neutron-gamma ambient dose equivalent rate apparatus of claim 4, wherein: the instrument control circuit:

determining gamma counting distribution of different charge integrals Q in each second, and integrating the gamma charge integral corresponding to the different charge integrals Q according to the distribution and the gamma charge integral-dose conversion function G_γ(Q) calculating a dose equivalent rate of γ;

determining different charges per secondThe neutron count distribution of the integral Q, and the neutron charge integral-dose conversion function G corresponding to the integral Q of different charges according to the distribution_nAnd (Q) calculating the dose equivalent rate of neutrons.

6. The measurement method of the neutron-gamma ambient dose equivalent rate meter based on pulse shape discrimination according to claim 4, characterized in that: the calculation method of the pulse shape discrimination PSD value comprises the following steps: PSD ═ Q_long--Q_short)/Q_long(ii) a Wherein Q is_longSelecting the length of the pulse signal for the charge integration of the pulse signal long gate; q_shortThe leading edge of the pulse signal is time-selected for charge integration of the short gate of the pulse signal.

7. The measurement method of the neutron-gamma ambient dose equivalent rate meter based on pulse shape discrimination according to claim 5, characterized in that: the formula for calculating the dose equivalent rate of γ is:

wherein i is 1,2,3, …, n; n is the channel number of the gamma energy spectrum detected by the organic scintillator detector; q_iAn integrated value of charge detected for the organic scintillator;represents that the integral value of the charge detected by gamma in the organic scintillator detector is Q_iCorresponding count rate, G_γ(Q_i) Represents the gamma charge integration-dose transfer function G corresponding to that trace_γ(Q) weight value.

8. The measurement method of the neutron-gamma ambient dose equivalent rate meter based on pulse shape discrimination according to claim 5, characterized in that: the formula for calculating the dose equivalent rate of neutrons is:

whereinThe light output detected for the organic scintillator detector is Q_i-1To Q_iCount rate of (G)_n(Q_i) Is the neutron charge integration-dose transfer function G_nWeight value of (Q), Q_i-1<Q_i,i＝1,2,3,…,n。

9. The measurement method of the neutron-gamma ambient dose equivalent rate meter based on pulse shape discrimination according to claim 7, characterized in that: gamma charge integration-dose transfer function G_γThe functional expression of (Q) is:

where k is the number of terms of the exponential functional relation, A_kAre weight coefficients.

10. The measurement method of the neutron-gamma ambient dose equivalent rate meter based on pulse shape discrimination according to claim 8, characterized in that: neutron charge integration-dose transfer function G_nThe functional expression of (Q) is:

G_n(Q)＝aQ³+bQ²+cQ+d

wherein a, b, c and d are constant coefficients.

Technical Field

The invention relates to the technical field of nuclear radiation detection, in particular to a neutron-gamma ambient dose equivalent rate meter based on pulse shape discrimination.

Background

Neutron and gamma ambient dose equivalent rates are typically measured by two different instruments.

The neutron peripheral dose equivalent rate instrument generally comprises a probe and a signal acquisition and processing system. The probe is generally moderated by the probeThe composition is shown in the specification. General use of the detector³He proportional counter tube, BF₃Proportional counter tube or⁶LiI scintillator detectors, and the like. The neutron and the sensitive material in the detector produce pulse signals after the action, the signal acquisition and processing system counts the pulse signals after proper processing, and the counting rate is converted into the neutron surrounding dose equivalent rate. The moderating body is used for moderating neutrons with high energy so that the neutrons can be detected by the detector, and the purpose of optimizing energy response of the dose equivalent rate instrument around the neutrons is achieved. Most of the probes of the conventional neutron peripheral dose equivalent measuring device have the structure, and have the defects of unsatisfactory energy response performance, heavy weight and large volume.

The gamma ambient dose equivalent rate meter also typically consists of a detector, typically one of a GM-tube, a scintillator detector or a semiconductor detector, and a signal processing system. When the GM counting tube is used as a detector, an energy compensation layer is generally required to be added outside the GM counting tube, and a signal processing system counts pulses output by the GM counting tube or performs time-counting processing on the pulses to obtain a dosage rate result. When a scintillator detector or a semiconductor detector is used as a detector, generally, pulse amplitude analysis is performed on signals output by the detector, and then a G (E) function is used for converting the signals into a dosage rate result.

Disclosure of Invention

Aiming at the problems that the measurement of neutron and gamma ambient dose equivalent needs two different instruments to be completed and a neutron ambient dose equivalent measuring device is large in size and heavy in weight at present, the invention provides a neutron-gamma ambient dose equivalent rate instrument based on pulse shape discrimination.

In order to achieve the above object, the present invention provides a neutron-gamma ambient dose equivalent meter based on pulse shape discrimination, which is characterized in that the dose equivalent meter comprises:

organic scintillator detector: for emitting a fluorescent signal differing in temporal characteristics with respect to the action of neutrons and gamma;

a photoelectric conversion device: the device is used for converting a fluorescence signal emitted by the organic scintillator detector into a voltage or current pulse signal;

a signal processing circuit: the device is used for processing voltage or current pulse signals output by the photoelectric conversion device, converting the voltage or current pulse signals into voltage pulse signals with signal amplitude and impedance matched with the pulse shape discrimination circuit, and keeping the difference of time characteristics of neutron and gamma signals;

the pulse shape discrimination circuit: the voltage pulse signal processing circuit is used for carrying out digital conversion on the voltage pulse signal output by the signal processing circuit, discriminating the voltage pulse signal according to the difference of the time characteristics of the neutron and the gamma signal, completing charge integration calculation of the signal and respectively obtaining charge integration of the neutron and the gamma pulse signal;

the instrument control circuit: the neutron and gamma pulse integrating circuit is used for converting charge integration of neutron and gamma pulse signals obtained by processing of the pulse shape discrimination circuit into neutron and gamma ambient dose equivalent results.

Further, the organic scintillator detector adopts doping¹⁰B element scintillator detector.

Furthermore, the pulse shape discrimination circuit comprises a high-speed AD conversion circuit and an FPGA system, the high-speed AD conversion circuit is used for performing digital processing on the voltage pulse signal output by the signal processing circuit, the FPGA system is used for discriminating the difference of the time characteristics of the neutron and the gamma signal by adopting a pulse shape discrimination PSD algorithm, and the pulse shape discrimination PSD algorithm is one or more of a rise time method, a charge comparison method, a pulse gradient method and a frequency gradient method.

Furthermore, the FPGA system judges the signal amplitude of the pulse signal after digital processing, determines the pulse event as a valid particle pulse event when the amplitude exceeds a set trigger threshold, caches the data of the event in an FIFO memory inside the FPGA, starts operation at the same time, automatically searches the position of a pulse starting point and the position of an end point according to a set pulse starting point determining method, and then automatically searches the position of the pulse starting point and the position of the end point according to a set long-gate time window T_longShort door time window T_shortIntegrating the charges in the time windows to respectively obtain the charge integrals Q in the two time windows_longAnd Q_short(ii) a Meanwhile, the charges in the time period from the starting point position to the end point position of each pulse signal are integrated to obtain the charge integration of each pulse signalQ; according to Q of each pulse_longAnd Q_shortQ, calculating a pulse shape and discriminating a PSD value, counting neutrons and gamma through a value range of the PSD value, and sending a calculation result to an instrument control circuit by the FPGA.

Still further, the instrument control circuitry: determining gamma counting distribution of different charge integrals Q in each second, and integrating the gamma charge integrals corresponding to the different charge integrals Q according to the distribution and the gamma charge integral-dose conversion function G_γ(Q) calculating a dose equivalent rate of γ; determining neutron counting distribution of different charge integrals Q in each second, and integrating the neutron charge to the dose conversion function G corresponding to the distribution and the different charge integrals Q_nAnd (Q) calculating the dose equivalent rate of neutrons.

Furthermore, the method for calculating the pulse shape discrimination PSD value is as follows: PSD ═ Q_long--Q_short)/Q_long(ii) a Wherein Q is_longSelecting the length of the pulse signal for the charge integration of the pulse signal long gate; q_shortThe leading edge of the pulse signal is time-selected for charge integration of the short gate of the pulse signal.

Further, the formula for calculating the dose equivalent rate of γ is:

wherein i is 1,2,3, …, n; n is the channel number of the gamma energy spectrum detected by the organic scintillator detector; q_iIs a charge integral value detected by the organic scintillator detector;represents that the integral value of the charge detected by gamma in the organic scintillator detector is Q_iCorresponding count rate, G_γ(Q_i) Represents the gamma charge integral-dose transfer function G corresponding to the trace_γ(Q) weight value.

Further, the formula for calculating the dose equivalent rate of neutrons is:

whereinThe light output detected for the organic scintillator detector is Q_i-1To Q_iCount rate of (G)_n(Q_i) Is the neutron charge integration-dose transfer function G_nWeight value of (Q), Q_i-1<Q_i, i＝1,2,3,…,n。

Further, the gamma charge integration-dose conversion function G_γThe functional expression of (Q) is:

where k is the number of terms of the exponential functional relation, A_kAre weight coefficients.

Further, the neutron charge integration-dose transfer function G_nThe functional expression of (Q) is:

G_n(Q)＝aQ³+bQ²+cQ+d

wherein a, b, c and d are constant coefficients.

Compared with the prior art, the invention has the beneficial effects that:

(1) the invention can simultaneously measure the neutron and gamma ambient dose equivalent, discriminate the neutron and gamma according to the pulse shape difference of the voltage signal, and calculate the neutron and gamma ambient dose equivalent rate;

(2) the charge integration-dose conversion function G provided by the invention_n(Q) and G_γ(Q) converting the charge integrals of the pulses generated by neutrons and gamma in a unit time to the neutron and gamma ambient dose equivalent rates, respectively;

(3) the scintillator material of the organic scintillator detector adopted by the invention is doped¹⁰B element to improve the response of low-energy neutrons;

(4) the invention can greatly reduce the volume and the weight under the condition of ensuring that the measured energy range is larger.

Drawings

FIG. 1 is a schematic view of the overall structure of the present invention;

FIG. 2 is a schematic diagram of an exponentially decaying pulse signal sampled by an ADC;

FIG. 3 is a schematic diagram of neutron-gamma light pulse signal shape discrimination;

FIG. 4 is a schematic of a charge comparison method;

FIG. 5 shows the discrimination effect of the instrument on an Am-Be neutron source;

FIG. 6 is a schematic diagram of the pulse distribution of MCNP monoenergetic neutrons;

FIG. 7 is a graph illustrating dose conversion coefficients.

Detailed Description

In order to make the technical scheme and the beneficial effects of the invention more clearly understood, the invention is further described in detail below with reference to the accompanying drawings and the embodiments.

As shown in fig. 1, the neutron- γ ambient dose equivalent ratio apparatus based on pulse shape discrimination provided by the present invention includes an organic scintillator detector, a photoelectric conversion device, a signal processing circuit, a pulse shape discrimination circuit, and an apparatus control circuit. Wherein

Organic scintillator detector: for emitting a fluorescent signal differing in temporal characteristics with respect to the action of neutrons and gamma;

a photoelectric conversion device: the device is used for converting a fluorescence signal emitted by the organic scintillator detector into a voltage or current pulse signal;

a signal processing circuit: the device is used for processing voltage or current pulse signals output by the photoelectric conversion device, converting the voltage or current pulse signals into voltage pulse signals with signal amplitude and impedance matched with the pulse shape discrimination circuit, and keeping the difference of time characteristics of neutron and gamma signals;

the pulse shape discrimination circuit: the voltage pulse signal processing circuit is used for carrying out digital conversion on the voltage pulse signal output by the signal processing circuit, discriminating the voltage pulse signal according to the difference of the time characteristics of the neutron and the gamma signal, completing charge integration calculation of the signal and respectively obtaining charge integration of the neutron and the gamma pulse signal;

the instrument control circuit: the neutron and gamma pulse integrating circuit is used for converting charge integration of neutron and gamma pulse signals obtained by processing of the pulse shape discrimination circuit into neutron and gamma ambient dose equivalent results.

The organic scintillator detector is a chemical composite material, can be in a liquid state or a solid state, and can emit fluorescence signals with different time characteristics for the action of neutrons and gamma. The content of different components of the organic scintillator detector has certain influence on neutron dose measurement performance, and mainly comprises the energy response performance and n-gamma discrimination performance of the detector. Common organic scintillators for n-gamma pulse shape discrimination are BC series, NE series, EJ series and the like, organic scintillator detectors of different models have different performance but are similar in chemical composition, and the EJ339A type scintillator detector is preferably selected in the embodiment, and is doped in organic scintillator materials¹⁰B element to improve the response of low-energy neutrons.

The working principle of the scintillation detector is as follows: (1) the radioactive particles strike the detector and produce a flash of light within the scintillator; (2) the generated scintillation photons are transmitted to a photocathode of a photomultiplier through a light guide and converted into photoelectrons; (3) the electrons generated on the photocathode are multiplied for multiple times by a photomultiplier system to generate output signals with enough size; (4) the electric signal output by the photomultiplier is amplified and processed by a matched electronic instrument to provide the required information.

In an embodiment of the present invention, the photoelectric conversion device may be a photomultiplier or a SiPIN diode, in this embodiment, the photomultiplier is used as the photoelectric conversion device, the photomultiplier and the EJ339A type scintillator are coupled by silicone grease and then packaged in an aluminum alloy housing, and a magnetic shielding sleeve, a built-in high voltage power supply module and a voltage divider circuit are configured in the housing.

The signal processing circuit is used for carrying out primary processing on the voltage or current pulse signal output by the photoelectric conversion device to enable the voltage or current pulse signal to become a voltage pulse signal matched with the pulse shape discrimination circuit in terms of signal amplitude and impedance, and the difference of the time characteristics of the neutron and the gamma signal is reserved. The signal processing circuit of the embodiment is a program-controlled amplifying circuit, and can properly amplify the signal output by the photomultiplier tube without changing the time characteristic difference of the neutron and the gamma signal, so that the amplitude of the voltage pulse signal is 0-1V, and the requirement of AD conversion of a subsequent circuit is met.

The pulse shape discrimination circuit is composed of a high-speed AD conversion part and a digital pulse information processing part. The high-speed AD conversion section digitally converts the voltage pulse processed by the signal processing circuit into a digital signal. The digital pulse information processing part is a digital signal processing system, can discriminate the neutron and the gamma according to the difference of time characteristics of voltage pulse signals of the pulse generated by the neutron and the gamma, can complete the charge integration calculation of the signals, and respectively obtains the charge integration of the neutron and the gamma pulse generated signals.

The pulse shape discrimination circuit of the embodiment is composed of a high-speed AD conversion circuit and an FPGA system. The high-speed AD conversion circuit adopts an AD9680 chip, the sampling rate is 1GSps, the sampling precision is 14 bits, and a JESD204B high-speed digital interface is adopted to communicate with the FPGA system. The FPGA system adopts an XC7K325T chip for processing the digital signals after AD conversion, and the maximum data rate is 12.5 Gb/s. In this embodiment, pulse shape discrimination is performed on neutron and γ pulse signals by using an algorithm in the FPGA, different PSD algorithms such as a rise time method, a charge comparison method, a pulse gradient method, and a frequency gradient method may be used, and the charge comparison method is preferably used as the PSD algorithm running in the FPGA in this embodiment. The signal output by the photomultiplier tube passes through a signal processing circuit to generate a rapid exponential decay pulse signal, the signal is digitized by a high-speed ADC and acquired by an FPGA in real time, and the exponential decay pulse signal sampled by the ADC is shown in FIG. 2 (the horizontal axis unit ns and the vertical axis unit mV are shown in the figure).

After the FPGA acquires ADC real-time data, firstly judging the signal amplitude, when the amplitude exceeds a set trigger threshold value, the system considers that the event is a valid particle pulse event, caching the data of the event into an FIFO (first in first out) memory inside the FPGA, starting operation by a pulse shape discrimination module inside the FPGA at the same time, and automatically searching a pulse starting point position and an end point position by the pulse shape discrimination module according to a set pulse starting point determining methodThen according to the set long door time window T_longTime window T of short door_shortIntegrating the charges in the time windows to respectively obtain the charge integrals Q in the two time windows_longAnd Q_short. Meanwhile, integrating the charges in the time period from the starting point position to the ending point position of the pulse signal to obtain the charge integral Q of each pulse signal; according to Q of each pulse_longAnd Q_shortQ, calculating a pulse shape and discriminating a PSD value, counting neutrons and gamma through a PSD value interval, and sending a calculation result to an instrument control circuit by the FPGA.

The instrument control circuit is a microprocessor system running embedded software and can convert the charge integration of neutron and gamma pulse obtained by the processing of the pulse shape discrimination circuit into neutron and gamma ambient dose equivalent results. The instrument control circuit of the embodiment is a single chip microcomputer system, after receiving a calculation result of a pulse, the instrument control circuit calculates the PSD value of the pulse by using an embedded program in the single chip microcomputer, and determines whether the pulse is generated by gamma or neutron action according to the PSD value of the pulse.

Determining the gamma counting distribution of different charge integrals Q in each second(s), and integrating the gamma charge corresponding to the different charge integrals Q according to the distribution and the gamma charge integral-dose conversion function G_γAnd (Q), the single chip microcomputer calculates the dose equivalent rate of gamma.

Neutron counting distribution of different charge integrals Q determined in each second(s), and neutron charge integral-dose conversion function G corresponding to the distribution and different charge integrals Q_nAnd (Q) calculating the dose equivalent rate of neutrons by the single chip microcomputer.

The instrument control circuit distinguishes neutrons and gamma rays by a pulse shape discrimination method. The pulse shape discrimination is to discriminate neutrons from gamma rays by using the characteristic that the scintillation light pulse signal shapes generated by n-gamma rays in a scintillator are different. For the same scintillator, the fluorescence pulse excited by incident particles in the scintillator is divided into a fast component and a slow component, the intensity ratio of the fast component to the slow component is different for different incident particles, the fast component fraction of neutrons is small, the slow component fraction is large, the fast component fraction of gamma rays is large, and the slow component fraction is small, so that the shapes of optical pulse signals detected by the scintillator detector of n-gamma rays are different, as shown in fig. 3, the discrimination of the n-gamma rays can be realized by processing and analyzing the shape difference of the optical pulse signals.

The present embodiment adopts a pulse shape discrimination method of a charge comparison method. In the present method, the charge integration is referred to as the integration of the amplitude of the single pulse signal over time.

The charge comparison method defines the integral value of the leading edge generated by the persistence of the transient photon in the pulse signal as the fast component, and the integral value of the trailing edge generated by the persistence of the slow photon in the pulse signal as the slow component, and the sum of the two is called the total charge integration. As shown in fig. 4, the charge comparison method uses a long gate and a short gate to determine the position of signal integration, the pulse shape discrimination PSD value in this embodiment is used as the discrimination basis, and the PSD is defined as follows:

PSD＝(Q_long--Q_short)/Q_long

wherein Q_longSelecting the length of a complete pulse signal for the charge integration of a pulse signal long gate; q_shortThe leading edge of the signal waveform is selected for the charge integration of the short gate of the pulse signal, the integration time of the short gate. Obviously, Q_longI.e. the total charge integral, Q_long-Q_shortI.e. the slow component charge integration. Since neutrons have a slower decay rate than the pulse signal generated by gamma rays, the pulse shape discrimination PSD value of neutrons is larger than that of gamma rays. The n-gamma pulse signal discrimination can be realized through the quantitative discrimination.

In the charge comparison method, a time window T is set according to the position of a pulse signal starting point_longAnd T_shortThen, the charges in the time windows are integrated to respectively obtain the charge integrals Q in the two time windows_longAnd Q_short. Wherein, T_longAnd T_shortRespectively, the time domain of the long gate and the short gate.

T_longAnd T_shortThe selection process is as follows: when the integral value of the pulse signal is larger than a smaller value for the first time (the instrument is found out through experiments)Defined as 30), the position is defined as the starting position time t₀Then, the door time end position t is lengthened_long＝t₀+Δt_longShort door time end position t_short＝t₀+Δt_short；Δt_longAnd Δ t_shortT can be obtained by setting and adjusting the value with the best discrimination effect during operation_longTime window of t₀，t_long]、T_shortTime window of t₀，t_short]. Q is obtained by integrating the pulse signal over time in time windows_long、Q_short. The PSD value can be discriminated according to the pulse shape of each pulse signal by a PSD calculation formula, and neutrons and gamma are distinguished according to the different size distributions of the PSD values.

Matlab programming language for charge comparison. The screening effect of the instrument on an Am-Be neutron source in an experiment is shown in FIG. 5.

After distinguishing neutron and gamma particle, the instrument control circuit generates the charge integral value Q of the pulse according to different particles and the charge integral-dose conversion function G_n(Q) and G_γThe (Q) function calculates the dose equivalent rate produced by neutrons and gamma as follows:

pulsed charge integration-dose transfer function G_γ(Q)

After a certain gamma standard point source irradiates a scintillator detector at a certain point in space, the detector at the certain point uses G_γ(Q) calculated air absorption dose rateCan be expressed as:

wherein i is 1,2,3, …, n; n is the channel number of the gamma energy spectrum detected by the detector; q_iIs a charge integral value in the detector; n (Q) is the gamma energy spectrum distribution measured by a detector;indicating that the integral value of the charge detected by gamma at the detector is Q_iThe corresponding counting rate; g_γ(Q_i) Indicates G corresponding to the track_γ(Q) function weight values. Because the number of standard point sources is limited in the experiment, the least square method is adopted to solve G_γ(Q) function. Let G_γ(Q) is expressed as an exponential function, as follows:

k is the term number of the exponential function relation, and k is 1,2,3, …; a. the_kIs a weight coefficient; then the detector is according to G_γThe dose rate calculated by the (Q) function should be:

for the jth standard gamma source (j ═ 1,2,3, …) it is according to G_γThe (Q) function calculates the corresponding dose rate as:

wherein the content of the first and second substances,

if it isThe dose rate standard value of the jth standard gamma source at the same point is obtainedAndrelative deviation S_jSum deviation squared sum S²Are respectively as：

According to the principle of least square method, ifAndthe relative deviation is minimal, and the following equation holds:

according to the equation, a weight coefficient A is obtained_kValues of which the charge integral-dose conversion function G of the gamma pulse is derived_γ(Q). The gamma absorption dose rate at a certain point in space detected by the detector is as follows:

in the embodiment, the maximum value of j is generally 6, and the maximum value of k is 3-5. The charge integral distribution N (Q) of the gamma pulse detected by the detector and eachAs is known, the measured gamma absorption dose rate can be obtained by performing accumulation operation through an electronic instrument according to the formula.

Neutron charge integration-dose transfer function G_n(Q)

When the dose equivalent rate around the neutron is measured, the neutron is measured by using a recoil proton method. Typically, neutrons enter the scintillator and are flashedHydrogen atoms in the scintillator undergo elastic collision reaction, and the energy of neutrons is E_nAnd the energy of recoil protons is E_pAnd the recoil angle is theta, the following relation exists:

E_p＝E_ncosθ

since the energy of the recoil proton is related to the recoil angle, even if a single-energy neutron is incident, the energy of the recoil proton is continuously distributed and the maximum value is equal to E_n. The recoil proton method is that elastic scattering of neutrons and hydrogen nuclei is utilized to generate recoil protons, under the condition that a hydrogen-containing substance target is quite thin, the fluence rate of incident neutrons with certain energy is in direct proportion to the generated recoil proton number, and the neutron fluence rate is determined by measuring the recoil proton number and other related quantities.

In the EJ339A liquid scintillator detector, generated recoil protons cause the scintillator to emit light, and if the amount of pulse signal charge detected by an electronic circuit after the light emission is Q, the Q and the energy E of incident neutrons_nIs in direct proportion. The pulse height distribution obtained after the neutron is detected by the scintillator detector is P (Q), and the neutron ambient dose equivalent rate measured by the detectorComprises the following steps:

where p (q) is the neutron pulse height distribution measured by the detector.The light output detected by the detector is Q_i-1To Q_iThe count rate of. G_n(Q_i) Is a light output of magnitude Q_i-1To Q_iSpectral weight value of (Q)_i-1<Q_i,i＝1,2,3,…,n。

In practice, if multiple monoenergetic neutron sources are used for G_n(Q_i) And the difficulty of calculation is high. By means of a circuit based on Monte CarloMCNP program F8 card of the method simulates a plurality of single-energy neutron sources to irradiate a scintillator detector at a certain point in space to obtain different energies E_n(n＝1,2,…,n；E_n-1<E_n) Pulse height distribution matrix P (E) of monoenergetic neutron source_p) (actually also the pulse height distribution matrix of the recoil protons) is:

P(E_pi，E_n) Is a monoenergetic neutron with an energy of E_nThe recoil proton energy of the time detector is E_p(i-1)To E_piThe probability of (c). In this embodiment, when n and m are equal to each other, P (E)_p) Is a square matrix. The pulse height distribution of EJ339A after incidence of different monoenergetic neutrons was calculated by MCNP as shown in FIG. 6.

Let the dose transfer function corresponding to the pulse height distribution calculated by the MCNP single energy neutron source be G (E)_p). Matrix G (E)_p)：

[G(E_p1) G(E_p2) G(E_p3) ...... G(E_pm)]^T

G(E_pi) Is a pulse of height E_p(i-1)To E_piThe spectral weight value of (c).

The standard value of the dose equivalent of the single-energy neutron source at the periphery of the neutrons of the detector position is H through MCNP simulation calculation, and the standard value is obtained by multiplying the single-energy neutron fluence by a fluence-dose conversion coefficient given by ICRP74 data. The neutron surrounding dose equivalent standard value matrix H of different monoenergetic neutron sources is as follows:

then the detector is according to G (E)_p) The calculated ambient neutron dose equivalent is equal to the standard value of the ambient neutron dose equivalent calculated by MCNP simulation, then the following equation is given:

expressed as a matrix equation: h ═ P (E)_p)·G(E_p) Namely: g (E)_p)＝P^-1(E_p)·H

From this equation, G (E) can be found_p) The value of each element. The dose conversion factor curve is shown in figure 7.

To G (E)_p) And E_pFitting to a quartic curve, G (E) can be obtained_p) The function is:

G(E_p)＝xE_p ³+yE_p ²+zE_p+t

wherein x, y, z and t are constant coefficients. In this embodiment, the values of x, y, z, and t of the best-fit curve are: 4.253,10.9593, -1.273,0.0731.

Naturally, G (E) is obtained_p) For actual measurement, experimental calibration is also required. Because the pulse signal charge Q actually detected by the detector is in a direct proportion relation with the energy of the recoil proton and the energy of the incident neutron, and the proportionality coefficient can be set to be K, then the following conditions exist: q & K & E_pIn such a relation, K is a undetermined coefficient. Obviously, the dose equivalent value of the neutron also has such a proportional relationship with the two results measured by the MCNP and the experiment. In practice, a certain standard neutron radiation field is measured through experiments, and the undetermined coefficient is determined according to different dosages at different positions. After determining the coefficients, according to G (E)_p) Formula for function fitting curve, E_pValue is replaced by Q to obtain G_nThe (Q) function is:

G(Q)＝aQ³+bQ²+cQ+d

wherein a, b, c and d are constant coefficients. Therefore, the detector can obtain the ambient dose equivalent rate of neutrons when actually measuring neutrons:

although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that changes, modifications, substitutions and alterations can be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents.

Details not described in this specification are within the skill of the art that are well known to those skilled in the art.