CN114518589A - Method for realizing energy calibration of gas proportional detector based on thick radioactive source - Google Patents
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Abstract
The invention provides a method for realizing energy calibration of a gas proportional detector based on a thick radioactive source, which mainly solves the problem that the energy calibration of the gas proportional detector cannot be normally developed due to obvious self-absorption when the radioactive source is thick in a coincidence measurement system. The method of the invention limits the irradiation area of the radioactive source on the semiconductor detector, so that only the rays with larger deposition energy in the gas proportional detector can reach the semiconductor detector to generate coincidence events. Meanwhile, only partial events with higher energy of the semiconductor detector, namely the events with the minimum self-absorption, are selected from all the coincidence events, so that the influence of the self-absorption on energy scales is avoided, and the purpose of improving the energy measurement accuracy is achieved.
Description
Technical Field
The invention relates to the field of nuclear tests and nuclear instruments, in particular to a method for realizing energy calibration of a gas proportional detector based on a thick radioactive source.
Background
The nuclear detector generally needs to pass through an energy scale to convert the output voltage or charge signal into correct energy data. Generally adopts a nuclear probeThe single energy ray depositing all energy in the detector is subjected to energy calibration. As shown in figure 1 of the drawings, in which,3the core body of the He sandwich spectrometer is composed of two opposite semiconductor detectors 2 and a charge device between the two semiconductor detectors 23He gas is proportional to the detector 1 composition. A small cathode box of the gas proportional detector is arranged outside the cathode box241Am radioactive source, which uses the emitted 5.486MeV alpha ray to develop energy calibration. The semiconductor detector is calibrated by direct irradiation with 5.486MeV alpha rays under vacuum. Since the size of the gas proportional detector is only 2cm × 2cm × 1cm, and alpha rays cannot deposit all energy in the gas, the alpha rays can only be measured by a coincidence method, namely, the alpha rays penetrate through the gas proportional detector and then enter the semiconductor detector and deposit residual energy. As in formula (2), wherein EGFor gas proportional detector deposition energy, Ec is the coincidence energy (when not self-absorbed, the coincidence energy equals the radiation source energy (5.486MeV)), EsDepositing energy for the semiconductor detector.
EG＝Ec-Es (2)
The radioactive source is placed outside the opening of the side wall of the cathode box of the gas proportional detector, and one radioactive source can irradiate two semiconductor detectors. In order to ensure that the radioactive source has a larger incident angle to the gas proportional detector, the size of the radioactive source is generally smaller, and the actual diameter is about 1 mm. The non-standard radioactive source is generally prepared manually in a laboratory, is relatively thick on the premise of ensuring basic activity, and has larger self-absorption, and the energy is deposited as formula (3), wherein EaIs self-absorbing energy of a radioactive source.
EG＝Ec-Es-Ea (3)
The semiconductor detector can perform energy calibration in a vacuum state, an alpha peak is widened and a peak position moves to a low energy end due to self-absorption of a radioactive source, as shown in fig. 2, energy spectrums of the radioactive source and a film source (which can be considered as no self-absorption) are measured by using an alpha spectrometer under a vacuum condition, wherein a wider spectral line is a used radioactive source energy spectrum, a narrower spectral line is a film source spectral line, the energy spectrums of the radioactive source move to low energy obviously seen from the figure, and an actual peak position is at a half position (a dotted line position) of the peak high energy end. The deposition energy of alpha rays about 5MeV in the gas proportional detector is less, generally only 500 keV-800 keV, the self-absorption of the radioactive source can cause serious influence on the energy scale of the gas proportional detector, and even normal scale cannot be developed at all.
In summary, since the gas proportional detector cannot deposit the whole energy of the alpha ray, the calibration can be performed only by using a coincidence method, and meanwhile, the energy calibration is difficult to increase due to the self-absorption of the alpha ray by a thick source. At present, the number of the current day,3the energy scale of the gas proportional detector of the He sandwich spectrometer is mostly as follows241The calculated deposition Energy of the direct distance of alpha rays emitted by the Am Source in the detector is used as a calibration, for example, Marsh JW, High Resolution Measurements of Neutron Energy Spectra from Am-Be and Am-B Neutron Source [ D [ ]]Kingston Polytechnic, 1990. Due to the fact that the energy self-absorption of a large radioactive source exists and the track difference (namely the deposited energy difference is large) of alpha rays in the gas proportional detector, the energy calibration method brings large errors to the energy calibration of the gas proportional detector.
Disclosure of Invention
The invention provides a method for realizing energy scale of a gas proportional detector based on a thick radioactive source, which mainly solves the problem that the energy scale of the gas proportional detector cannot be normally developed due to obvious self-absorption when the radioactive source is thick in a coincidence measurement system. The method is developed by using a thick source3The method for measuring the energy scale of the gas proportional detector in the He coincidence measurement system can improve3He conforms to neutron energy measurement accuracy in a measurement system.
In order to achieve the purpose, the technical scheme of the invention is as follows:
a method for realizing energy calibration of a gas proportional detector based on a thick radioactive source comprises the following steps:
step one, a coincidence measurement system is built, and the coincidence measurement system can simultaneously measure the energy spectrums of a gas proportional detector and a semiconductor detector and judge coincidence events;
step two, limiting the irradiation area of the radioactive source on the semiconductor detector, so that the alpha rays which can generate coincident events have the same range in the gas proportional detector;
1.1) arranging a radiation source hole on a cathode box of the gas proportional detector, wherein the radiation source hole is arranged at the center of the side wall of the cathode box;
1.2) arranging a radioactive source outside a cathode box of the gas proportional detector, wherein the head of the radioactive source is convexly arranged in a radioactive source hole, meanwhile, a shielding layer with a small hole is arranged inside the cathode box, the distance between the center of the small hole of the shielding layer and the center of a radiation source hole is H, and H is 1.2-1.5 times of the sum of the radius of the radioactive source and the radius of the small hole, so that the radioactive source can only irradiate the area on one side of the central line of the semiconductor detector, and the irradiation area of the radioactive source is limited;
step three, energy calibration of the semiconductor detector;
3.1) acquiring an alpha ray energy spectrum emitted by a semiconductor detector measuring radioactive source under a vacuum condition;
3.2) generating 8-10 single energy pulses by using a pulse generator, measuring an ADC zero point by linear fitting of the amplitude of the single energy pulses and an ADC channel address, and performing linear fitting again by adopting the ADC zero point and the energy in the alpha-ray energy spectrum to complete the energy scale of the semiconductor detector to obtain a fitting constant a1And b1As shown in formula (1);
Es＝a1+b1ch (1)
wherein E issDepositing energy for the semiconductor detector, wherein ch is an ADC channel address;
step four, filling the gas proportional detector3He gas is measured by adopting the coincidence measurement system in the step one, and a coincidence event detected by the gas proportional detector and the semiconductor detector is obtained;
step five, selecting n coincidence events with the deposition energy larger than half of the peak high-energy end in the semiconductor detector from the coincidence events obtained in the step four, averaging ADC (analog to digital converter) channel addresses of the semiconductor detector in the n coincidence events, substituting the average value of the ADC channel addresses of the semiconductor detector into a formula (1), and obtaining the average value E of the deposition energy of the semiconductor detectors′；
Step six, in n coincidence eventsAveraging the addresses of the gas proportional detectors to obtain the average value of the addresses of the gas proportional detectors, wherein the average value of the addresses corresponds to the deposition energy E of the gas proportional detectorsGThen finishing the energy calibration of the gas proportional detector;
EG＝Ec-Es′
where Ec is the coincidence energy.
Further, in the second step, the value of H is 0.6-1.0 mm.
Further, in the second step, the diameter of the radioactive source hole is 2-3 mm.
Furthermore, in the second step, the diameter of the small hole of the shielding layer is 0.1 mm-0.5 mm, and the thickness of the shielding layer is 0.1-0.2 mm.
Further, in the second step, a gold foil is specifically used as the shielding layer.
Compared with the prior art, the invention has the following beneficial effects:
1. the method can improve the accuracy of quantity measurement. The method of the invention limits the irradiation area of the radioactive source on the semiconductor detector, so that only the rays with larger deposition energy in the gas proportional detector can reach the semiconductor detector to generate coincidence events. Meanwhile, only part of the events with higher energy of the semiconductor detector are selected from all coincidence events. Therefore, the method can select the partial event with the largest deposited energy of the gas proportional detector and the semiconductor detector, namely the event with the smallest self-absorption, thereby avoiding the influence of the self-absorption on the energy scale and achieving the purpose of improving the energy measurement accuracy.
2. Because the thick source intensity is larger and the irradiation damage of the semiconductor detector is stronger, the method can obviously weaken the irradiation damage of the semiconductor detector and prolong the service life of the semiconductor detector.
Drawings
FIG. 1 prior art3A core structure diagram of a He sandwich spectrometer;
FIG. 2 is a comparison of measured spectra of a radioactive source and a thin film source in an alpha spectrometer;
FIG. 3 is a drawing showing3He sandwich spectrometer3After He gasA semiconductor detector measures an energy spectrogram of a radioactive source;
FIG. 4a is a schematic view showing an irradiation range of a radiation source;
FIG. 4b is a schematic view of the irradiation range of the radioactive source II;
FIG. 4c is a schematic view of the irradiation range of the radioactive source;
FIG. 5 is a schematic illustration of the position of the radiation source and the shield in the method of the present invention;
FIG. 6a is a measured energy spectrum of a gas proportional detector when the irradiation area is not limited;
FIG. 6b is a measured energy spectrum of the gas proportional detector after the irradiation area is limited;
FIG. 7 is a block diagram of a prior art thermal neutron measurement electronics system;
FIG. 8 is a drawing showing3A schematic diagram of a pulse stack thermal column thermal neutron energy spectrum is actually measured by a He sandwich spectrometer.
Reference numerals: 1-gas proportional detector; 2-semiconductor detector, 11-cathode box, 12-anode wire, 13-radioactive source, 14-small hole.
Detailed Description
The invention is described in detail below with reference to the drawings and the detailed description. It should be understood by those skilled in the art that these embodiments are only for explaining the technical principle of the present invention and are not intended to limit the scope of the present invention.
The detector generally needs a radioactive source with known energy to perform energy calibration for practical use, and since the gas proportional detector in the coincidence measurement system cannot deposit all the energy of the calibration ray (namely alpha ray), the coincidence measurement method needs to be adopted to perform energy calibration. The coincidence event measured by the coincidence measurement system is formed by that calibration rays penetrate through the gas proportional detector and finally enter the semiconductor detector, the energy of the calibration rays is simultaneously deposited in the gas proportional detector and the semiconductor detector, and the sum of the measurement energy of the semiconductor detector and the gas proportional detector is the energy of the calibration rays theoretically. The semiconductor detector finishes energy calibration in vacuum, the deposition energy can be accurately measured, and the calibrated ray energy minus the deposition energy in the semiconductor detector is the deposition energy in the gas proportional detector, as shown in formula 2.
Because the self-absorption of radiation source, the partial deposit of the real ray energy of maring is unable to be measured inside the radiation source, and every coincidence event energy self-absorption is all inequality moreover, leads to unable through formula (2) calculation gaseous proportional detector internal deposition energy, and gaseous proportional detector is unable normal scale at this moment. Meanwhile, when the radioactive source is thicker, self-absorption is more obvious, and the gas proportional detector deposits energy EGSemiconductor detector deposition energy EsSelf-absorption energy E with radioactive sourceaRelated, as in formula (3), due to EaIs a random number resulting in a gas proportional detector deposition energy EGThe energy scale cannot be calculated and normally developed.
The deposition energy of the gas proportional detector is related to the track length of the calibration ray in the gas, and the deposition energy in the gas is basically the same when the track length is fixed. The invention mainly researches the track and energy deposition characteristics of alpha rays in a gas proportional detector, and provides a method for utilizing a shielding layer to enable a radioactive source to only irradiate a small area far away from the radioactive source, so that only a part of alpha rays with larger deposition energy in the gas proportional detector can generate a coincidence event. Semiconductor detector deposition energy EsAt a certain time, if the alpha ray generates obvious self absorption, the track of the alpha ray in the gas is shortened, and the alpha ray can only irradiate an area close to the source, and can be shielded by the shielding layer to form a coincidence event. Equivalent to the deposition energy E of a gas proportional detectorGAt a certain time, the semiconductor detector deposits energy EsThe more the radiation source absorbs energy E by itselfaThe less. Thus only events where the semiconductor detector energy is above the dashed line position in fig. 3 are selected among all coincidence events. Then substituting the average address of the semiconductor detector conforming to the event into formula (1) to calculate the average deposition energy of the semiconductor detector, and substituting the average address into formula (2) to calculate the deposition energy E of the gas proportional detectorG。
Firstly, in order to realize the energy deposition limitation of the gas proportional detector, the structure improvement of the coincidence measurement system is carried out, a radioactive source is fixed on the outer side of a cathode box of the gas proportional detector, alpha rays are injected into the gas proportional detector firstlyPart of the energy is deposited in the gas and finally part of the alpha rays can be emitted into the semiconductor detector to form a coincidence event. To deposit approximately the same energy and a larger energy value in a gas proportional detector for particles that cause coincidence, the area of the radiation source on the semiconductor detector must be small enough and as far away from the radiation source as possible. Therefore, the irradiation area of the radioactive source can be limited by adding a shielding layer with small holes in front of the radioactive source, so that the trajectories of the rays in the gas proportional detector are very close. In the embodiment of the present invention, it is,241am material is plated on the convex surface with the diameter of 1mm, and a hole with the diameter of 3mm is arranged on one side of the cathode box of the gas proportional detector. The alpha rays emitted by the radioactive source can respectively irradiate two semiconductor detectors, and most areas of the semiconductor detectors can be irradiated when the radioactive source is not limited as shown in fig. 4 a. For this purpose, a 0.2mm thick gold foil with a 0.2mm diameter hole is added inside the cathode box of the gas proportional detector, and the irradiation area of the radioactive source will become smaller due to the limitation of the gold foil, as shown in fig. 4 b. The irradiation area is still larger, and the irradiation area of the radioactive source can be changed by adopting a method of properly adjusting the position of the small hole in order to further reduce the irradiation area of the radioactive source. Because the radiation source needs to irradiate the upper semiconductor detector and the lower semiconductor detector at the same time, the irradiation area of the radiation source cannot be adjusted by adopting a method of moving the small hole up and down (because the irradiation area of one semiconductor detector is reduced and the irradiation area of the other semiconductor detector is increased due to the up and down movement). As shown in fig. 5, the small hole 14 can be moved horizontally, and the small hole 14 is moved to the right or left by H0.6-1 mm (larger than the sum of the radius of the radiation source and the small hole), and then the radiation beam spot of the radiation source 13 on the semiconductor detector moves to the small hole 14 side. Since the semiconductor detector is circular and the edge height is significantly reduced, the particle beam spot will be concentrated only in a small area at the edge, and the trajectories of the alpha rays which have coincident events occur in the gas proportional detector are approximately the same, as shown in fig. 4 c.
Secondly, coincidence judgment is carried out, when the irradiation area of the radioactive source is limited to a small area, the deposition energy in the gas proportional detector and the coincidence event occurrence area are obviously changed, the deposition energy spectrum low-energy area in the gas proportional detector is basically not counted after the shielding layer is added, and the coincidence event occurrence area is obviously narrowed as shown in fig. 6a and 6 b. If a certain alpha ray is originally absorbed with partial energy, the energy in the semiconductor detector is reduced, and at the moment, only coincidence events with the energy of the semiconductor detector above a dotted line in the graph 3 are selected (namely coincidence events at the position of more than half of the peak high-energy end are selected), and coincidence events generated by the alpha ray with obvious energy self-absorption can be eliminated.
Finally, the deposition energy in the gas, through statistics of coincidence events selected in the previous step, is about 4.755MeV on average in the semiconductor detector, and 731keV on average in the gas proportional detector according to equation (2), but consisting of "3The output amplitude compensation method of He sandwich spectrometer proportional chamber detector (chinese patent CN 107340533A)' known that the actual output amplitude has a certain attenuation due to the different distances between each point on the alpha particle trajectory and the anode wire of the gas proportional detector, and by using the method for calculating the distance between the original distance and the anode wire and the actual output, the actual output energy of the alpha particle corresponding to the actual measurement amplitude can be calculated to be 647 keV. The gas proportional detectors for all coincidence events are averaged over a channel site corresponding to an energy of 647 keV. Similarly, a pulse generator can be adopted to generate the energy zero point of the corresponding ADC of the series of single-energy signal scale gas proportional detectors, and the zero point and the calculated E are utilizedGAnd (4) linearly fitting an energy calculation formula (4) of the gas proportional detector to finish the energy calibration of the gas proportional detector. EGDeposition energy for gas proportional detector, ch is gas proportional detector ADC channel address, a2，b2Is a fitting constant.
EG＝a2+b2ch (4)
Based on the above description, the method for realizing the energy scale of the gas proportional detector based on the thick radioactive source provided by the invention specifically comprises the following steps:
step one, a coincidence measurement system is set up, the coincidence measurement system can simultaneously measure the energy spectrums of a gas proportional detector and a semiconductor detector and judge coincidence events, one coincidence event comprises a semiconductor detector signal and a gas proportional detector signal, and the coincidence measurement system can ensure that the two signals come from the same scale alpha ray;
step two, limiting the irradiation area of the radioactive source on the semiconductor detector, so that the alpha rays which can generate coincident events have approximately the same range in the gas proportional detector;
1.1) a radiation source hole is arranged on a cathode box 11 of the gas proportional detector and used for placing a radiation source 13, and the radiation source hole is arranged at the center of the side wall of the cathode box 11, namely the distance between the radiation source and the two ends of an anode wire 12 is the same;
1.2) sticking a radioactive source on the outer side of a cathode box, placing the head of the radioactive source in a radioactive source hole in a protruding manner, meanwhile, placing a shielding layer with a small hole on the inner side of the cathode box, wherein the distance between the center of the small hole of the shielding layer and the center of the radioactive source hole is H, and H is about 1.2-1.5 times of the sum of the radius of the radioactive source and the radius of the small hole, so that the radioactive source can only irradiate the area on one side of the central line of a semiconductor detector, and the irradiation area of the radioactive source is limited;
step three, energy calibration of the semiconductor detector;
3.1) acquiring an alpha ray energy spectrum emitted by a semiconductor detector measuring radioactive source under a vacuum condition;
because the radioactive source is thicker and self-absorption is stronger, the peak position can move to the low energy end, and the alpha spectrometer is used for simultaneously measuring the energy spectrums of the radioactive source and the thin film surface under the vacuum condition. As shown in fig. 2, the wider peak is the measured data of the radioactive source, and the narrow peak at the dotted line cursor is the measured spectrum of the thin film source, so that it can be determined that the peak position of the radioactive source moves down due to self-absorption, and the actual peak position should be at the position of the dotted line cursor half of the peak energy end, and the position corresponds to the alpha ray energy of 5.486 MeV;
3.2) generating 8-10 single energy pulses by using a pulse generator, measuring an ADC zero point by linear fitting of the amplitude of the single energy pulses and an ADC channel address, and performing linear fitting by adopting the ADC zero point and the energy in the alpha-ray energy spectrum to complete energy calibration of the semiconductor detector to obtain a fitting constant a1And b1As in formula (1);
Es＝a1+b1ch (1)
wherein E issIs a semiconductorThe body detector deposits energy, and ch is an ADC channel address;
step four, filling the gas proportional detector3He gas is measured by adopting the coincidence measurement system in the step one, and a coincidence event of the gas proportional detector and the semiconductor detector is obtained;
step five, selecting n coincidence events (namely n coincidence events of a dotted line in fig. 3) with the deposition energy larger than half of the peak high-energy end in the semiconductor detector from the coincidence events obtained in the step four, averaging the ADC (analog to digital converter) channel addresses of the n coincidence events, substituting the average value of the ADC channel addresses into a formula (1), and obtaining the average value E of the deposition energy of the semiconductor detectors′；
Step six, averaging the addresses of the gas proportional detectors in the n coincidence events to obtain an average value of the addresses of the gas proportional detectors, wherein the average value of the addresses corresponds to the deposition energy E of the gas proportional detectorsG；
EG＝Ec-Es′
Wherein Ec is the energy of241Am α -ray 5.486 MeV).
Energy scale verification is performed below, and in order to prove the reliability of the method for realizing the energy scale of the gas proportional detector based on the thick radioactive source, thermal neutron energy spectrum measurement work is performed by using a Siemans pulse reactor thermal column. A path of semiconductor detector measuring equipment is added on the basis of the prior energy calibration equipment, and fig. 7 is an electronic schematic diagram of the equipment. The Siemens pulse reactor thermal column provides well-moderated thermal neutron beam current, the diameter of an outlet neutron beam spot is 35mm, the neutron intensity in the beam spot is uniform, and the whole gas proportional detector sensitive area can be covered. Thermal neutron and3he reaction energy is 765keV, utilizing3The final energy of thermal neutrons actually measured by a He proportional detector is between 760keV and 770keV, and the energy of the thermal neutrons is equal to that of the neutrons3He reaction energy 765keV coincidence, proving3The energy scales of two semiconductor detectors and a gas proportional detector in the He sandwich spectrometer are correct, so that the reliability of the method is proved, as shown in figure 8, a distance weight correction curve in the figure is utilized "3Output amplitude compensation method for proportional chamber detector of He sandwich spectrometer (Chinese patent CN 10)7340533A) "patent method modified final data with peak position representing thermal neutron and3he reaction energy.
Claims (5)
1. A method for realizing energy calibration of a gas proportional detector based on a thick radioactive source is characterized by comprising the following steps:
the method comprises the following steps of firstly, building a coincidence measurement system, wherein the coincidence measurement system can simultaneously measure the energy spectrums of a gas proportional detector and a semiconductor detector and judge coincidence events;
step two, limiting the irradiation area of the radioactive source on the semiconductor detector, so that the alpha rays which can generate coincident events have the same range in the gas proportional detector;
1.1) arranging a radiation source hole on a cathode box of the gas proportional detector, wherein the radiation source hole is arranged at the center of the side wall of the cathode box;
1.2) arranging a radioactive source outside a cathode box of the gas proportional detector, wherein the head of the radioactive source is convexly arranged in a radioactive source hole, meanwhile, a shielding layer with a small hole is arranged inside the cathode box, the distance between the center of the small hole of the shielding layer and the center of a radiation source hole is H, and H is 1.2-1.5 times of the sum of the radius of the radioactive source and the radius of the small hole, so that the radioactive source can only irradiate the area on one side of the central line of the semiconductor detector, and the irradiation area of the radioactive source is limited;
step three, energy calibration of the semiconductor detector;
3.1) acquiring an alpha ray energy spectrum emitted by a semiconductor detector measuring radioactive source under a vacuum condition;
3.2) generating 8-10 single energy pulses by using a pulse generator, measuring an ADC zero point by linear fitting of the amplitude of the single energy pulses and an ADC channel address, and performing linear fitting again by adopting the ADC zero point and the energy in the alpha-ray energy spectrum to complete the energy scale of the semiconductor detector to obtain a fitting constant a1And b1As shown in formula (1);
Es＝a1+b1ch (1)
wherein E issDepositing energy for the semiconductor detector, wherein ch is an ADC channel address;
step four, gasProportional volume detector3He gas is measured by adopting the coincidence measurement system in the step one, and a coincidence event detected by the gas proportional detector and the semiconductor detector is obtained;
step five, selecting n coincidence events with the deposition energy larger than half of the peak high-energy end in the semiconductor detector from the coincidence events obtained in the step four, averaging ADC (analog to digital converter) channel addresses of the semiconductor detector in the n coincidence events, substituting the average value of the ADC channel addresses of the semiconductor detector into a formula (1), and obtaining the average value E of the deposition energy of the semiconductor detectors′；
Step six, averaging the addresses of the gas proportional detectors in the n coincidence events to obtain an average value of the addresses of the gas proportional detectors, wherein the average value of the addresses corresponds to the deposition energy E of the gas proportional detectorsGThen finishing the energy calibration of the gas proportional detector;
EG＝Ec-Es′
where Ec is the coincidence energy.
2. The method for realizing the energy calibration of the gas proportional detector based on the thick radioactive source as claimed in claim 1, wherein: in the second step, the value of H is 0.6-1.0 mm.
3. The method for realizing the energy calibration of the gas proportional detector based on the thick radioactive source as claimed in claim 1, wherein: in the second step, the shielding layer is made of gold foil.
4. The method for realizing the energy calibration of the gas proportional detector based on the thick radioactive source as claimed in claim 1, wherein: in the second step, the diameter of the small hole of the shielding layer is 0.1 mm-0.5 mm, and the thickness of the shielding layer is 0.1-0.2 mm.
5. The method for realizing the energy calibration of the gas proportional detector based on the thick radioactive source as claimed in claim 1, wherein: in the second step, the diameter of the radioactive source hole is 2-3 mm.
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