Reconstruction Of Intrinsic Radioactivity Background Of LaBr₃（Ce） Gamma Spectrometer

The energy resolution of LaBr₃（Ce）scintillation detector to gamma ray at room temperature can reach 2.7%（FWHM/ENEGRY,0.662 Me V）.As a gamma spectrometer,it has remarkable excellent characteristics and shows good application prospect in ground gamma spectrum measurement and airborne gamma spectrum measurement.It has a trend to gradually replace the traditional Na I（TI）scintillation detector in the field of uranium exploration and environmental radiation monitoring.However,LaBr₃（Ce）crystal has inherent radioactivity,which mainly comes from the radioactive isotope ¹³⁸La of lanthanum and the impurity radioactive element actinide²²⁷Ac and its decay daughter in the crystalα,βandγDecay.Taking the inherent radioactivity background of LaBr₃（Ce）crystal as the object,this paper studies the problems ofγ+X,γ+βcoincidence summing effect in the inherent radioactive background of LaBr₃（Ce）crystal and reconstruction of intrinsic radioactivity background spectrum of LaBr₃（Ce）scintillation gamma spectrometer.The selected topics come from the National Key Research and Development Program"High-Resolution Airborne Gamma Spectrum Measurement and Airborne Imaging Spectrum Measurement Technology"（Project Number:2017YFC060210）,the National Natural Science Foundation of China"Research on Group Energy Spectrum Response of High-Resolution Airborne Gamma Array Detector"（Project Number:41774147）and the Sichuan Science and Technology Program"Research on Airborne Gamma Spectrum Detection Technology"（Project Number:2020YJ0334）.The research results can significantly improve the accuracy and sensitivity of LaBr₃（Ce）scintillation gamma spectrometer,which have important practical value and scientific significance for China’s mineral resources（especially uranium resources）exploration,environmental nuclear radiation monitoring equipment and its technology.The main research contents and results are as follows:（1）Study on the physical mechanism of inherent radioactivity background of LaBr₃（Ce）crystal.The inherent radioactivity background of LaBr₃（Ce）crystal mainly comes fromα,β,γand X-rays were produced by the radioactive isotope ¹³⁸La of lanthanum and the decay of the impurity radionuclide actinide ²²⁷Ac and its daughters.The energy ofα,γand X-rays are characteristic.The energy ofβrays is continuous.The energy ofα,βand X-rays are mainly deposited in LaBr₃（Ce）crystal（except at the edge of the crystal）,forming the full energy peak on the instrument spectrum.Theγray energy may escape from LaBr₃（Ce）crystal,forming compton electron flat.¹³⁸La releases 0-255.3 ke Vβray and 788.7 ke Vγray throughβdecay andγdecay,formingγ+βcoincidence spectrum;¹³⁸La emits 1435.7 kevγray and 33.3 ke V or 4.3 ke V X-ray through electron capture,formingγ+X coincidence spectrum.²²⁷Ac and its daughters in LaBr₃（Ce）crystal releaseα、βandγray through cascade decay.Theβandγemissivity of ²²⁷Ac and its daughters relative toβandγof ¹³⁸La is about three orders of magnitude smaller.In LaBr₃（Ce）crystalsαray light yield is only 30-40%ofγray light yield with the same enegy.4.86-8.01 Me Vαray formed 1.6-2.8 Me V equivalent gamma energy inγ-ray instrument spectrum.（2）Study on the reconstruction ofγspectrum andγ+X coincidence spectrum.The mathematical model of 788.7 ke V and 1435.7 ke Vγ-ray reconstruction instrument spectrum adopts subsection representation,that is,the reflection peak,single escape peak,double escape peak,full-energy peak and other instrument spectrum characteristic peaks are characterized by gaussian function,compton flat is characterized by univariate linear subsection function,and compton edge is characterized by arctangent function.Comparison of 788.7 ke Vγ-ray reconstructed spectrum and its montecarlo numerical simulation spectrum,in the energy range of1-850 ke V,the maximum relative error of 788.7 ke Vγ-ray simulation spectrum and reconstructed spectrum in each energy segment（with an interval of 50 ke V）is 7.14%and the minimum relative error is 0.28%.Comparison of 1435.7 ke Vγ-ray reconstructed spectrum and its montecarlo numerical simulation spectrum,the maximum relative error between the reconstructed spectrum and its simulated spectrum in each energy segment（with an interval of 100 ke V）is 3.29%and the minimum relative error is 0.18%.Due to all the X-ray energy is deposited in the crystal,the mathematical characterizationγ+X coincidence reconstructed spectrum andγ-ray reconstructed spectrum is the same in the instrument spectrum,and only the energy of 4.3 ke V or 33.3 ke V is shifted to the right on the independent variable axis.The comparison betweenγ（1435.7 ke V）+X（4.3 ke V and 33.3 ke V）coincidence reconstructed spectrum and its montecarlo numerical simulation spectrum shows that the maximum relative error between them in each energy segment（at an interval of100 ke V）is 5.63%and the minimum relative error is 0.18%in the energy range of1-1600 ke V.（3）Study on the reconstruction ofγ+βcoincidence spectrum.The mathematical characterization of 255.3 ke Vβray continuous reconstructed instrument spectrum is based on the latest version of the ¹³⁸Laβexperimental data in LNHB laboratory.The multivariate polynomials are used for least square fitting to obtain the inherent mathematical expression ofβ-ray reconstructed instrument spectrum.Maximum energy 255.3 ke Vβ-ray and 788.7 ke Vγ-ray formedγ+βcoincidence specrtrum.The mathematical characterization of theγ+βreconstructed instrument spectrum is sum of convolution ofβ-ray instrument spectrum andγ-ray instrument spectrum.In accordance withγ+βmontecarlo numerical simulation spectrum,theγ+βsimulated spectrum and its reconstructed spectrum are in good agreement at the energy range of150-900 ke V,the average relative error is 8.70%.The large deviation occurs in the initial energy section（0-100 ke V）and the end energy section（1051-1100 ke V）,and the average relative deviation is 21.69%and 98.2%respectively.It is mainly caused by the increase of statistical fluctuation error caused by the relatively small number of particles ofγandβ-ray at the beginning and end energy segments.（4）Study on the reconstruction ofαequivalent gamma instrument spectrum.Theαequivalent gamma instrument spectrum produced by single nuclide in ²²⁷Ac decay chain is mathematically characterized by double gaussian model.The mathematical model ofαequivalent gamma instrumental spectrum produced by ²²⁷Ac,²¹⁵Po,²¹⁹Rn,²¹¹Bi,²²⁷Th,²²³Ra and other nuclides in ²²⁷Ac decay chain is characterized by the cumulative sum of double Gaussian models of each single nuclide.In order to solve the standard deviation,peak position energy and peak position count value in each gaussian model,this paper uses the combination of genetic algorithm（GA）and particle swarm optimization（PSO）to form particle swarm optimization with unknown quantities such as standard deviation、peak position energy and peak position count value,taking theαequivalent gamma instrument spectrum as the fitting object which are measured in a low background lead chamber with LaBr₃（Ce）gamma spectrometer,the genetic particle swarm optimization（GA-PSO）hybrid algorithm is used to theαequivalent gamma instrument spectrum is solved bidirectionally and point by point,so as to avoid the problem of violent oscillation in the solution space in the solution process,and finally obtain the inherentαequivalent gamma reconstructed instrument spectrum of the LaBr₃（Ce）.In the energy range of 1500-3000 ke V,theαequivalent gamma reconstructed spectrum is in good agreement with its measured instrument spectrum.The relative error of full spectrum segment counting is 0.02%.The minimum relative error of sub spectrum segment counting is 0.64%and the maximum relative error sub spectrum segment counting is 3.45%.（5）Study on the reconstruction and evaluation of LaBr₃（Ce）intrinsic background radioactivity instrument spectrum.On the basis of determining the radioactive activity and unit activity detection efficiency of ¹³⁸La,²²⁷Ac and its daughters in LaBr₃（Ce）crystal,the reconstructed instrument spectrum of LaBr₃（Ce）inherent radioactive background is the cumulative sum ofγ+X,γ+βcoincidence instrument spectra andα,β,γand X-ray spectra.The relative errors between the measured instrument spectrum and the reconstructed spectrum of LaBr₃（Ce）gamma spectrometer in 50-3000 ke V,50-500 ke V,501-1000 ke V,1001-1500 ke V,1501-2000ke V,2001-2500 ke V and 2501-3000 ke V are 0.71%,0.49%,0.85%,0.82%,11.42%,0.42%and 0.29%respectively.The relative errors between the measured spectrum and its reconstructed instrument spectrum in 1382-1538 ke V(⁴⁰K energy window),1673-1857 ke V(²³⁸U enegry window),2522-2706 ke V(²²⁷Th enegry window),634-690 ke V(¹³⁷Cs enegry window),1142-1208 ke V(⁶⁰Co enegry window 1)and1269-1378 ke V(⁶⁰Co enegry window 2),are 0.35%,16.35%,0.49%,1.3%,1.72%and 6.84%respectively.（6）Study on the application effect of the LaBr₃（Ce）inherent background radioactive instrument spectrum.In this paper,an gamma spectrometer composed of seven LaBr₃（Ce）detectors is built,taking ¹³⁷Cs and ⁶⁰Co radioactivity sources,environmental radioactivity and radioactivity standard model in the experimental area as the detection objects,the application effect of the inherent background radioactivity reconstructed instrument spectrum of LaBr₃（Ce）detector array is evaluated.For ¹³⁷Cs and ⁶⁰Co radioactive sources,on the difference spectrum between the measured instrument spectrum and its reconstructed instrument spectrum of the LaBr₃（Ce）detector array,the morphological characteristics of the inherent radioactive background instrument spectrum of LaBr₃（Ce）basically disappear.The peak to back ratio in ¹³⁷Cs energy window and ⁶⁰Co energy window increased by 26.67%（662 ke V）,44.55%（1173 ke V）and 24.56%（1332 ke V）respectively.The environmental radioactivity measurement of the experimental site（West Hall of the nuclear engineering building of Chengdu University of Technology）shows that the difference spectrum of LaBr₃（Ce）detector array is similar to the measured instrument spectrum of Ce Br₃detector array,and the count value of characteristic peak energy window of⁴⁰K（1.46 Me V）,²¹⁴Bi（1.76 Me V）and ²⁰⁸TI（2.62 Me V）in environmental radioactivity measured by LaBr₃（Ce）detector array is close to the peak-to-back ratio of the measured instrument spectrum of Ce Br₃detector array with similar volume,it truly reflects the environmental radioactivity of the experimental site.The calibration experiment of uranium,thorium and potassium contents of radioactive elements on the National Airborne radioactivity measurement standard model in Shi Jia Zhuang shows that the contents of potassium,uranium and thorium in the mixed model calculated by the measured instrument spectrum of LaBr₃（Ce）detector array are significantly higher,and the relative errors between the mixed model and the recommended values of the mixed model are 79.82%,-32.90%and 86.53%,reflecting the serious interference caused by the inherent radioactivity background of LaBr₃（Ce）scintillation detector.After the calibration（difference spectrum）of the instrument spectrum reconstructed from the inherent radioactivity background of LaBr₃（Ce）gamma spectrometer,the relative errors between the calculated contents of potassium,uranium and thorium in the mixed model and the recommended values are 5.69%,-2.62%and 3.18%respectively,which fully meet the error requirements specified in the"Specification for Airborne Gamma Ray Spectrum Measurement".