Spectrum Mutual Response Of Array Scintillant Detectors For Airborne Gamma Spectrometry And Energy Compensation

Airborne gamma spectrum measurement is an important technical means in many fields such as geological survey,radioactive mineral exploration,deep exploration,environmental radiation monitoring and nuclear emergency measurement.Because its advantages of low cost,high efficiency and high precision,it has been the focus of radioactive measurement research in China and abroad.The development trend of airborne gamma spectrum is improving measurement accuracy and sensitivity and one of effective methodology is increasing sensitive volume and resolution of detector crystal.Traditional airborne gamma spectrometer is composed of NaI(Tl)scintillation detectors with a crystal volume of 4L,and the energy resolution is between 7%?9%(relative full width at half maximum@0.661Me V).CeBr₃ scintillation detectors has higher energy resolution(about 4%@0.661Me V).But,limited by process and cost,CeBr₃ crystal is smaller than NaI(Tl)crystal,generally only 0.1L.Therefore,the high-resolution airborne gamma spectrometer adopts a multi-crystals array structure to achieve the purpose of simultaneously improving energy resolution and sensitive volume of detectors.However,for small-volume crystal detectors,after gamma photons enter any detector crystal(primary detector),there is a certain probability that the secondary photons generated by compton scattering and electron pair effects,which are more likely to escape form primary detector and deposited in adjacent detector(secondary detector),that resulting in a single gamma photon producing pulses in multiple detectors.This phenomenon is the spectrum mutual response of array detectors.Affected by the spectrum mutual response,the performance of array detector is lower than one single crystal detector with same volume.Such as the spectrum mutual response between multiple crystals will cause the Compton counts increased in the spectrum of primary detector.The spectrum mutual response is a technical problem that cannot be ignored in high-resolution airborne gamma spectrum measurement.At present,there are few researches and solutions about spectrum mutual response of array detector.In this paper,based on theoretical derivation,Monte Carlo numerical simulation and physical experiments,we research the formation mechanism and influence degree of the spectrum mutual response in CeBr₃ scintillation crystal array detector,and propose an airborne gamma spectrometer full energy peak counts and energy correction method.The research results have important scientific significance and practical value for the design of new generation high-resolution airborne gamma array detector and improvement of ground detection sensitivity of airborne gamma spectrum measurement technology.The main research contents and results as follows:1.This paper analyzed the transport and energy loss process of gamma-rays inside array detector,and established a mathematical model of deposition photon flux based on the formula of once Compton scattering theory.According to the mathematical model,calculated the photon flux with energy deposition in the primary detector and the secondary detector when the single-energy narrow beam gamma-rays incidence array detector,and researched the influencing factors of spectrum mutual response,such as detector radius,incident gamma-rays energy,gamma-rays incident position and isolation material.Theoretical calculation results indicate:(1)On same size of single detector((?)50(?)50mm),the spectrum mutual response of NaI(Tl)array detector(0.51%)is higher than that of CeBr₃ array detector(0.42%);(2)On the same detector type,the degree of spectrum mutual response is negatively related to the radius of detector;(3)As the primary gamma-ray energy increases,the degree of mutual response gradually is increased in the 0?1Me V energy range,and gradually decreased in the 1Me V?3Me V energy range;(4)As the distance from the incident point to the secondary detector decreases,the degree of spectrum mutual response is increased,and as the distance from the incident point to the secondary detector increases,the degree of mutual response decays exponentially;(5)For the three isolation materials of air,polyethylene and quartz sand,Air has the highest degree of mutual response(0.51%),followed by polyethylene(0.44%),and quartz sand is lowest(0.37%).2.This paper carry out the numerical simulation of the energy spectrum mutual response of the array CeBr3 scintillation detector based on Monte Carlo numerical simulation method,and further explore the transport and energy loss process of the gamma-rays inside the array detector and the isolation material.The energies of the incident gamma-rays are set to be 0.661 Me V,0.609 Me V,1.112 Me V,1.173 Me V,1.332 Me V,1.764 Me V and 2.446 Me V respectively;the incident position are distributed in three directions on the bottom surface of the primary detector;the types of isolation materials are set to air and polyethylene and quartz sand respectively.The simulation results indicate:(1)The normalized full-energy peak count of the primary detector decreases as the linear distance between the incident position and the origin increases,and as the distance between the incident point and the secondary detector decreases,the normalized full-spectrum count of the secondary detector increases exponentially.The type of isolation material has no effect on the full-energy-peak count of the main detector,but has a significant effect on the normalized full-spectrum count of the secondary detector.The comparison of the ⁶⁰Co dual-energy gamma-rays incident simulation results with the single-energy 1.173 Me V and 1.332 Me V gamma-rays incident simulation results shows that the multi-energy incident mutual response result can be explained by the single-energy incident result,and the maximum relative error between the both is 0.16%.(2)The comparison between the numerical simulation value and the theoretical calculation value shows that when the primary gamma-ray energy is low,the theoretical value and the simulated value are in good agreement.When the primary ray energy is 0.661 Me V,the relative error between the two is 1.87%,and when the energy is high When the energy is 2.446 Me V,the relative error is 25.28%.It proves that the higher the primary gamma-ray energy,the higher the proportion of secondary scattering and electron pair effects in the mutual response.(3)Based on the particle trajectory simulation method,this paper simulates the spatial collision probability distribution of scattered rays in the array detector.The simulation results show:for a2×1 array detector,the probability of energy loss of scattered rays in the secondary detector is approximately is about 20%,the probability of all energy deposited is about6%;the probability of energy loss of scattered rays in the aluminum housing and insulating material is about 0.8%.(4)This paper simulates the energy deposition process of different energy gamma rays in the detector aluminum housing and isolation material,and obtains the energy deposition probability of different primary gamma-rays in air,polyethylene and quartz sand of different thicknesses,and establishes the energy deposition share expectation,the isolation material energy compensation function of array detector.3.In this paper,we set up a physical experiment platform to carry out single-energy incidence(¹³⁷Cs radiation source),dual-energy incidence(⁶⁰Co radiation source)and multi-energy incidence(Radium radiation source)CeBr₃ and NaI(Tl)array detector mutual response experiments.The distribution of experimental values compared with the simulation value is basically same,the maximum value of the relative error of the normalized full spectrum count is 33%,and the minimum relative error is 0.21%.The experimental results prove that the correctness of simulation results.The comparison between the theoretical value,simulated value and experimental value shows that when the isolation material is quartz sand,the consistency between the theoretical value and the experimental value is the highest,with a relative error of 17.38%.4.We set up a measurement system for the energy spectrum mutual response and energy compensation of the airborne gamma array spectrometer,carry out physical experiment verification of the energy spectrum mutual response correction and energy compensation of the array detector.The experiment uses 4×5 array CeBr₃ scintillation detector(single crystal size is(?)45(?)50mm)and uses the particle mode gamma spectrometer to record particle event information such as the time series,signal amplitude and detector number to analysis the mutual response phenomenon of the energy spectrum of the array detector.Using the energy compensation function to amend the count and energy of the full-energy-peaks of the array detector's synthetic instrument spectrum.The results indicate:for the full-energy-peak with energy0.661Me V(the peak site is 14120 channels and the total number of channels is 65536).After mutual response correction,the peak site of full-energy peak conforming on the reduction spectrum of coincident events is 14026,and the total number of full-energy peak of the main detector increases by 4.51%.After energy compensation,the peak site of full-energy peak on the reduction spectrum of coincident events is 14144.The energy deviation of energy compensation is 70 channels lower than the mutual response correction result,and the full-energy peak count of the main detector increases by4.97%.The mutual response correction and energy compensation technology proposed in this paper can effectively correct the mutual response phenomenon of array detectors.