An airborne gamma-ray spectrometer and a radioactive geophysical detection method

Abstract

The invention discloses an airborne gamma spectrum instrument and a radioactive geophysical detection method, which comprises a detector, a pulse signal amplifying circuit, an A / D digital acquisition circuit, an FPGA hardware circuit, a pulse signal analysis and processing module and a full spectrum data display module which are sequentially arranged along the transmission direction of a photoelectric signal; a current signal pulse is generated by the detector; the pulse signal amplifying circuit converts the generated current signal pulse into a voltage pulse signal; the A / D digital acquisition circuit collects the converted voltage pulse signal and outputs it alternately, and detects cosmic rays according to the overflow of the electric signal pulse; the FPGA hardware circuit provides a clock signal; the pulse signal analysis and processing module processes the received voltage pulse signal alternately output by the A / D digital acquisition circuit in real time according to the clock signal; and the full spectrum data display module displays and records the collected full spectrum data. The collected full spectrum data can be displayed and recorded in real time on a computer interface display, and the cosmic ray channel count can be accurately counted and identified.

Description

Technical Field

[0001] The invention relates to the technical field of aerial radioactive geophysical detection, and in particular to an aerial gamma spectrometer and a radioactive geophysical detection method. Background Art

[0002] In the field of airborne radioactive geophysical detection technology, my country's airborne gamma-ray spectrum measurement technology has undergone decades of technological development, from the initial 4-channel (potassium, uranium, thorium, total channel) analog spectrum window data measurement to the advanced level of 256-channel digital measurement technology for radioactive spectrum. At present, it has reached a new height of 1024-channel spectrum analysis and measurement of airborne radioactive spectrum with international advanced level. Airborne radioactive gamma spectrum measurement mainly uses crystal detectors to receive gamma-ray particles emitted by radioactive elements, extracts and analyzes the electrical signals generated by gamma spectrum rays, specifically: collects data of 0-3MeV energy gamma spectrum rays emitted by radioactive elements in nature and performs rapid digital analysis and processing.

[0003] When some models of airborne gamma spectrometers measure abnormal points of radioactive elements with relatively high gamma-ray energy, such as abnormal areas of radioactive element thorium, the cosmic ray channel will also experience varying degrees of count increases along with the increase in energy window count data, causing a large deviation in the cosmic ray channel counts. Summary of the invention

[0004] In view of this, the purpose of the present application is to provide an airborne gamma spectrometer to solve the technical problem of large deviation in cosmic ray channel counts existing in the prior art.

[0005] Based on the above purpose, in a first aspect of the present application, an airborne gamma spectrometer is provided, comprising:

[0006] Detectors, pulse signal amplifying circuits, A / D digital acquisition circuits, FPGA hardware circuits, pulse signal analysis and processing modules, and full spectrum data display modules are sequentially arranged along the transmission direction of the photoelectric signal;

[0007] The detector is used to receive incident gamma rays, generate excited electrons, and then generate photoelectrons, which are amplified multiple times to generate current signal pulses;

[0008] The pulse signal amplifying circuit is used to convert the generated current signal pulse into a voltage pulse signal;

[0009] The A / D digital acquisition circuit is used to collect the converted voltage pulse signal and output it alternately, and detect cosmic rays according to the electrical signal pulse overflow;

[0010] The FPGA hardware circuit is used to provide a clock signal;

[0011] The pulse signal analysis and processing module is used to process the received voltage pulse signal alternately output by the A / D digital acquisition circuit in real time according to the clock signal;

[0012] The full spectrum data display module is used to display and record the collected full spectrum data.

[0013] In some embodiments, the detector includes a NaI (Tl) crystal and a photomultiplier tube. When gamma rays are irradiated into the NaI (Tl) crystal, a large number of excited electrons are generated in the NaI (Tl) crystal. The photons generated by the excited electrons that emit fluorescence hit the cathode of the photomultiplier tube to generate photoelectrons, which are multiplied and amplified multiple times to generate charge current signal pulses at the anode.

[0014] In some embodiments, a high-voltage power supply module is also included to provide high-voltage power to the photomultiplier tube.

[0015] In some embodiments, the charge amplification relationship of a photon is expressed as: A = δ n , where δ is the photomultiplier tube dynode multiplication coefficient, n is the number of dynodes, and the photon charge is collected by the anode capacitor to generate a signal pulse. The pulse signal amplitude has a linear relationship with the incident gamma ray energy; the photomultiplier tube anode output signal is expressed by the following formula:

[0016]

[0017] In the formula, τ fl The luminescence time constant of NaI(Tl) crystal is about 0.25uS; τ α is the charging time constant; U 0 It is the maximum voltage amplitude after the charge is completely collected.

[0018] In some embodiments, the A / D digital acquisition circuit uses the AD9226 chip, which has 12-bit high-speed A / D conversion and parallel output. The maximum conversion rate can reach 65MSPS, and a 50MHz sampling frequency is selected. The clock of the A / D converter is provided by the FPGA phase-locked loop module, and the sampling period is 20ns. The AD9226 chip VREF is set to 2V, and the corresponding input pulse signal range is 1-3V and 2Vp-p value.

[0019] In some embodiments, the amplified voltage of the pulse signal amplifying circuit is expressed by the following formula:

[0020] Q=E γ *N phot *ε*G PMT

[0021] Where, EY is the energy of the gamma ray;

[0022] N phot is the number of photons generated when the gamma rays enter the scintillator / MeV,

[0023] ε is the efficiency of the photomultiplier tube cathode in collecting photons, and GPMT is the photomultiplier tube gain.

[0024] In some embodiments, it also includes:

[0025] The overflow time detection circuit is used to detect the overflow time of the pulse signal when it exceeds 2Vp-p.

[0026] In some embodiments, it also includes:

[0027] The cosmic ray identification module is used to identify cosmic rays according to the overflow time of the pulse signal when it exceeds 2Vp-p.

[0028] Based on the above purpose, in a second aspect of the present application, a radioactive geophysical detection method is also provided, comprising:

[0029] Gamma rays are detected using the airborne gamma spectrometer as described in any one of the first aspects.

[0030] In some embodiments, the process of debugging the airborne gamma spectrometer is also included, specifically including:

[0031] By placing radioactive element Cs source and Th source near the NaI (Tl) crystal, observe the received 1024-channel gamma-ray full spectrum graphic data, check the display position of the characteristic peak of each element's energy spectrum, adjust the amplifier static operating point and amplifier gain, so that the peak position of the gamma-ray energy spectrum produced by the radioactive element cesium is displayed and recorded at the 220-channel data storage position, and the peak position of the gamma-ray energy spectrum produced by the radioactive element thorium is displayed at the 872-channel data recording position.

[0032] The airborne gamma spectrometer and radioactive geophysical detection method of the embodiments of the present application can display and record the collected full spectrum data in real time on a computer interface display, and can accurately count and identify cosmic ray channel counts. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following is a brief introduction to the drawings required for the specific embodiments or the description of the prior art. In all the drawings, similar elements or parts are generally identified by similar reference numerals. In the drawings, the elements or parts are not necessarily drawn according to the actual scale.

[0034] Figure 1 This is a schematic diagram of the structure of an airborne gamma spectrometer according to an embodiment of the present application;

[0035] Figure 2 A schematic diagram of the structure of a pulse signal amplifying circuit according to an embodiment of the present application;

[0036] Figure 3 A schematic diagram of the structure of the FPGA hardware circuit of an embodiment of the present application;

[0037] Figure 4 The figure is a schematic diagram of the interface for detecting gamma rays using the airborne gamma spectrometer according to the embodiment of the present application. DETAILED DESCRIPTION

[0038] The following embodiments of the technical solution of the present invention are described in detail in conjunction with the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and are therefore only used as examples, and cannot be used to limit the protection scope of the present invention.

[0039] It should be noted that, unless otherwise specified, the technical terms or scientific terms used in this application should have the common meanings understood by those skilled in the art to which the present invention belongs.

[0040] like Figure 1 As shown, Figure 1 This is a schematic diagram of the structure of the airborne gamma spectrometer according to an embodiment of the present application. Figure 1 It can be seen that the aviation gamma spectrometer of this embodiment includes a detector, a pulse signal amplifying circuit, an A / D digital acquisition circuit, an FPGA hardware circuit, a pulse signal analysis and processing module and a full spectrum data display module arranged in sequence along the transmission direction of the photoelectric signal; the detector is used to receive the incident gamma rays, generate excited electrons, and then generate photoelectrons, which are amplified multiple times to generate current signal pulses; the pulse signal amplifying circuit is used to convert the generated current signal pulses into voltage pulse signals; the A / D digital acquisition circuit is used to collect the converted voltage pulse signals and output them alternately, and detect cosmic rays according to the overflow of the electrical signal pulses; the FPGA hardware circuit is used to provide a clock signal; the pulse signal analysis and processing module is used to process the received voltage pulse signals alternately output by the A / D digital acquisition circuit in real time according to the clock signal; the full spectrum data display module is used to display and record the collected full spectrum data.

[0041] In this embodiment, the measurement of airborne radioactive gamma spectrum mainly utilizes a crystal detector to receive gamma ray particles emitted by radioactive elements, extracts and analyzes the electrical signals generated by gamma spectrum rays. The design of the airborne radioactive 1024-channel gamma spectrum instrument mainly includes: gamma ray data acquisition, digital analysis and processing of spectrum pulse signals, graphic display and data storage of radioactive full spectrum data, specifically: data acquisition and rapid digital analysis and processing of gamma spectrum rays with energy of 0-3MeV emitted by radioactive elements in nature, gamma spectrum data are respectively accumulated and recorded in the data storage positions of 0-1023 channels, and the gamma ray spectrum of the main radioactive elements monitored and displayed is: potassium channel setting: 457-523 channels (1370-1570keV), uranium channel setting: 553-620 channels (1660-1860keV), thorium channel setting: 803-937 channels (2410-2810keV). The gamma ray energy emitted by the nuclei of natural radioactive substances in nature is below 3MeV, and gamma rays with energy exceeding 3MeV are high-energy particles from outer space. Therefore, the gamma ray energy corresponding to the 1024-channel energy spectrum data is 0-3.07MeV, 3KeV / channel, and gamma ray particles with energy exceeding 3.07MeV are counted as cosmic rays.

[0042] The aviation gamma spectrometer of this embodiment may use a scintillator detector composed of a 16-inch NaI (Tl) crystal and a photomultiplier tube, and the photomultiplier tube high voltage power supply uses an adjustable positive high voltage power supply module with a maximum voltage of 1400V.

[0043] When gamma rays irradiate into NaI(Tl) crystal, a large number of excited electrons will be generated in the crystal. The photons generated by the excited electrons that can emit fluorescence hit the cathode of the photomultiplier tube to generate photoelectrons. After multiple multiplication and amplification, a charge current signal pulse is generated at the anode. The charge amplification relationship can be expressed as: A = δ n (δ is the photomultiplier tube dynode multiplication coefficient, n is the number of dynodes), these charges are collected by the anode capacitor to generate signal pulses, and the pulse signal amplitude has a linear relationship with the incident gamma ray energy. The photomultiplier tube anode output signal is expressed by the following formula:

[0044]

[0045] In the formula, τ fl The luminescence time constant of NaI(Tl) crystal is about 0.25uS; τ α is the charging time constant; U 0 It is the maximum voltage amplitude after the charge is completely collected.

[0046] The A / D digital acquisition circuit uses the AD9226 chip, which is a 12-bit high-speed A / D conversion and parallel output. The maximum conversion rate can reach 65MSPS, and a 50MHz sampling frequency is selected. The clock of the A / D converter is provided by the FPGA phase-locked loop module, and the sampling period is 20ns. The AD9226 chip VREF is set to 2V, and the corresponding input pulse signal range is 1-3V and 2V pp value. The pulse signal amplifier circuit amplifies the current pulse signal output by the anode of the photomultiplier tube and converts it into a voltage pulse signal that meets the input characteristics of the AD9226 chip; the design requirements are: the output pulse signal generated by the gamma ray with an energy of 0-3.07MeV at the anode of the photomultiplier tube is amplified into a 1-3V voltage pulse signal. The amplifier circuit design uses the high-frequency operational amplifier AD8065 chip, and the amplifier circuit power supply design uses a low ripple coefficient, linear, +5V single power supply.

[0047] like Figure 2 The figure is a schematic diagram of the structure of the pulse signal amplifying circuit of the embodiment of the present application. The voltage amplitude of the signal at the output u1 point of AD8065 in the amplifying circuit can be expressed by the following formula:

[0048] Q=E γ *N phot *ε*G PMT

[0049] In the formula, E Y is the energy of the gamma ray;

[0050] N phot is the number of photons generated when the gamma rays enter the scintillator / MeV,

[0051] ε is the efficiency of collecting photons by the photomultiplier tube cathode, G PMT is the photomultiplier tube gain.

[0052] like Figure 3 As shown, it is a schematic diagram of the structure of the FPGA hardware circuit of the embodiment of the present application. The FPGA uses the Cyclone series EP4CE15F23 chip of ALTERA, and the external crystal oscillator is 50MHz. The crystal oscillator frequency signal provides a stable clock signal for each functional component through the PLL module circuit; the 12-bit digital output pin output by the parallel port of the AD9226 chip is connected to the FPGA interface pin, and the 50M clock frequency required by the AD9226 chip is provided by the FPGA. The data output chip uses the USB to UART chip CP2102. The 1024-channel full spectrum data analyzed and processed by the FPGA is sent to the external data acquisition computer in real time through CP2102, and the collected full spectrum data is displayed and recorded in real time on the computer interface display.

[0053] When some models of airborne gamma spectrometers measure abnormal points of radioactive elements with relatively high gamma-ray energy, such as abnormal areas of radioactive element thorium, the cosmic ray channel will also experience varying degrees of count increases along with the increase in energy window count data, causing a large deviation in the cosmic ray channel counts.

[0054] Cosmic gamma rays are high-energy gamma ray particles that enter the Earth from outer space, with energies exceeding 3MeV. In this design, the input voltage range of the A / D converter is 1-3V, and the corresponding received gamma ray energy is 0-3.07MeV, which generates an electrical pulse amplitude signal. The pulse signal generated by gamma rays with energies greater than 3MeV will overflow in the A / D converter. The signal pulses that cause overflow in the A / D converter include both pulse signals generated by cosmic gamma rays and non-cosmic gamma rays. At the same time, two or more particles enter the detector in a very short time and almost at the same time to generate a large signal pulse exceeding 2Vp-p, which will also cause A / D overflow; therefore, the pulse signal overflowed by the A / D converter cannot be judged as the count of cosmic rays with energies greater than 3MeV, otherwise this may cause the gamma ray energy spectrum detection instrument to measure at the abnormal point of natural radioactive elements with relatively high gamma ray energy. The cosmic ray channel count is significantly higher, and the cosmic ray record deviation is relatively large.

[0055] In order to reduce the counting deviation of cosmic rays, the GR-820 airborne gamma-ray spectrometer developed by Exploranium sets the cosmic ray counting threshold to 4MeV. The electrical pulse signals generated by gamma rays with energy exceeding 4MeV are counted as cosmic rays.

[0056] Compared with the prior art, the technical solution of the present application further adds an overflow time detection circuit and a cosmic ray identification module, wherein the overflow time detection circuit is used to detect the overflow time of the pulse signal when it exceeds 2Vp-p. The cosmic ray identification module is used to identify cosmic rays according to the overflow time of the pulse signal when it exceeds 2Vp-p.

[0057] Specifically, the cosmic ray extraction in this design adopts the method of monitoring pulse amplitude and width. The pulse signal output by the amplifier circuit: 2Vp-p corresponds to the received gamma ray quantum energy of 3.07MeV at full scale; by compiling software to measure the pulse overflow time of the signal when the pulse signal exceeds 2Vp-p, it is found that the time when cosmic rays are generated is mostly within the range of 3.5-4us in the design of the RC discharge time constant of this circuit. Most of the pulse overflow times of signals exceeding 2Vp-p generated by the superposition of electrical pulse signals are below 2us. The method adopted in this design is: detect the electrical pulse signal corresponding to the gamma ray, when the amplitude of the electrical pulse signal is greater than the 2Vp-p value, start to detect the overflow time of the pulse signal, and when the overflow time exceeds 1.8us, it is recorded as a cosmic ray count. This method greatly reduces the probability of gamma rays emitted by radioactive elements with relatively high energy entering the cosmic ray channel count.

[0058] FPGA program function design: PLL phase-locked loop module, dual-port memory module, 2 dual-port memories with 16-bit data width and 1024 storage units, CPU soft core module, UART serial port module, etc. are established in the project;

[0059] FPGA programming adopts Verilog HDL programming language. The hardware programming of 1024-channel gamma-ray spectrum analysis mainly includes: 50MHz / second high-speed receiving ADC circuit to collect data, design and write signal parallel analysis and processing module, use register array to track and record data, fast filtering, signal starting baseline judgment, pulse shape analysis method, extract pulse signal amplitude, pulse accumulation recognition, and discard or modify the processing method to process pulse accumulation data according to the overlap size of pulses, and judge and identify cosmic gamma rays; the collected energy spectrum data is recorded in real time into the 1024-channel memory through the dual-port memory A port, and the full spectrum data is counted and accumulated in the memory.

[0060] The 1024-channel data transmission of energy spectrum analysis adopts the embedded CPU control mode. The above-mentioned multi-channel energy spectrum analysis function module is designed and produced as an Avalon bus interface module as a peripheral of the CPU soft-core processor. The produced multi-channel energy spectrum analysis module is connected to the CPU processor through the Avalon slave bus interface. The CPU processor reads the collected 1024-channel full spectrum accumulation data through the dual-port memory B port. The two dual-port memories use the ping-pong working mode to record and read the full spectrum data in turn, that is, when the multi-channel energy spectrum analysis module records 1024 channels of energy spectrum data in one dual-port memory, the CPU reads 1024 channels of full spectrum data from another dual-port memory. The operation conversion of the two dual-port memories is controlled by the self-made multi-channel energy spectrum analysis module. The CPU transmits the read 1024-channel gamma-ray energy spectrum data to the external computer display interface through the UART to USB peripheral chip CP2102 in real time, and can receive external operation instructions from the serial port to change the working state of the multi-channel analyzer. In the design, more than 40,000 FPGA internal logic units are required to realize the 1024-channel energy spectrum analysis function without using dual-port memory; while only more than 10,000 FPGA internal logic units are required to design the 1024-channel energy spectrum analysis method with dual-port memory.

[0061] The airborne gamma spectrometer of the embodiment of the present application can display and record the collected full spectrum data in real time on the computer interface display, and can accurately count and identify the cosmic ray channel counts.

[0062] In addition, the present application also provides a radioactive geophysical detection method, comprising:

[0063] The gamma rays are detected by using any of the above-mentioned airborne gamma spectrometers. Before the detection, the process of debugging the airborne gamma spectrometer is also included, which specifically includes:

[0064] By placing radioactive element Cs source and Th source near the NaI (Tl) crystal, observe the received 1024-channel gamma-ray full spectrum graphic data, check the display position of the characteristic peak of each element's energy spectrum, adjust the amplifier static operating point and amplifier gain, so that the peak position of the gamma-ray energy spectrum produced by the radioactive element cesium is displayed and recorded at the 220-channel data storage position, and the peak position of the gamma-ray energy spectrum produced by the radioactive element thorium is displayed at the 872-channel data recording position.

[0065] like Figure 4The figure shows a schematic diagram of the interface for detecting gamma rays using the airborne gamma spectrometer of the embodiment of the present application. The display interface displays in real time the 1024-channel full spectrum data graph curves (channels 0-1022: gamma ray energy 0-3.06MeV, channel 1023 records cosmic ray data), K, U, Th, and total channel spectrum window data graph curves collected by the 1024-channel spectrum instrument, digitally displays K, U, Th, total channel spectrum window data counts and cosmic ray data, and records the received 1024-channel gamma ray full spectrum data in binary form into a data file in real time.

[0066] The data threshold settings of the 1024-channel energy spectrum window are as follows: total channel setting: 137-937 channels (480-2810 keV), K channel setting: 457-523 channels (1370-1570 keV), U channel setting: 553-620 channels (1660-1860 keV), Th channel setting: 803-937 channels (2410-2810 keV).

[0067] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein by equivalents. These modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention, and they should all be included in the scope of the claims and specification of the present invention.

Claims

1. An airborne gamma spectrometer, characterized in that: include: Detectors, pulse signal amplifying circuits, A / D digital acquisition circuits, FPGA hardware circuits, pulse signal analysis and processing modules, and full spectrum data display modules are sequentially arranged along the transmission direction of the photoelectric signal; The detector is used to receive incident gamma rays, generate excited electrons, and then generate photoelectrons, which are amplified multiple times to generate current signal pulses; The pulse signal amplifying circuit is used to convert the generated current signal pulse into a voltage pulse signal; The A / D digital acquisition circuit is used to collect the converted voltage pulse signal and output it alternately, and detect cosmic rays according to the electrical signal pulse overflow; The FPGA hardware circuit is used to provide a clock signal; The pulse signal analysis and processing module is used to process the received voltage pulse signal alternately output by the A / D digital acquisition circuit in real time according to the clock signal; The full spectrum data display module is used to display and record the collected full spectrum data; Also includes: Overflow time detection circuit, used to detect the overflow time of the pulse signal when it exceeds 2Vp-p; The cosmic ray identification module is used to identify cosmic rays according to the overflow time of the pulse signal when it exceeds 2Vp-p.

2. The airborne gamma spectrometer according to claim 1, characterized in that: The detector includes a NaI (Tl) crystal and a photomultiplier tube. When gamma rays irradiate into the NaI (Tl) crystal, a large number of excited electrons are generated in the NaI (Tl) crystal. The photons generated by the excited electrons that emit fluorescence hit the cathode of the photomultiplier tube to generate photoelectrons, which are multiplied and amplified multiple times to generate charge current signal pulses at the anode.

3. The airborne gamma spectrometer according to claim 2, characterized in that: It also includes a high-voltage power supply module, which is used to provide high-voltage power to the photomultiplier tube.

4. The airborne gamma spectrometer according to claim 3, characterized in that: The charge amplification relationship of photons is expressed as: A = δ n , where δ is the photomultiplier tube dynode multiplication coefficient, n is the number of dynodes, and the photon charge is collected by the anode capacitor to generate a signal pulse. The pulse signal amplitude has a linear relationship with the incident gamma ray energy; the photomultiplier tube anode output signal is expressed by the following formula: In the formula, τ fl The luminescence time constant of NaI(Tl) crystal is 0.25uS; τ α is the charging time constant; U0 is the maximum voltage amplitude after the charge is completely collected.

5. The airborne gamma spectrometer according to claim 4, characterized in that: The A / D digital acquisition circuit uses the AD9226 chip, which has 12-bit high-speed A / D conversion and parallel output. The maximum conversion rate can reach 65MSPS, and a 50MHz sampling frequency is selected. The clock of the A / D converter is provided by the FPGA phase-locked loop module, and the sampling period is 20ns. The AD9226 chip VREF is set to 2V, and the corresponding input pulse signal range is 1-3V and 2V pp value.

6. The airborne gamma spectrometer according to claim 5, characterized in that: The amplified voltage of the pulse signal amplifying circuit is expressed by the following formula: Q=E γ *N phot *e*G PMT In the formula, E Y is the energy of the gamma ray; N phot is the number of photons generated when the gamma rays enter the scintillator / MeV, ε is the efficiency of collecting photons by the photomultiplier tube cathode, G PMT is the photomultiplier tube gain.

7. A radioactive geophysical detection method, characterized in that: include: Gamma rays are detected using the airborne gamma spectrometer described in any one of claims 1 to 6.

8. The method according to claim 7, characterized in that The process of debugging the airborne gamma spectrometer is also included, including: By placing radioactive element Cs source and Th source near the NaI (Tl) crystal, observe the received 1024-channel gamma-ray full spectrum graphic data, check the display position of the characteristic peak of each element's energy spectrum, adjust the amplifier static operating point and amplifier gain, so that the peak position of the gamma-ray energy spectrum produced by the radioactive element cesium is displayed and recorded at the 220-channel data storage position, and the peak position of the gamma-ray energy spectrum produced by the radioactive element thorium is displayed at the 872-channel data recording position.

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