The Study On Anti-coincidence Phoswich Detection Technique For γ-energy Spectrum Measurement

As one of the methods to detect nuclear radiation,γspectrum measurement has been widely used in many fields of nuclear science and technology.The Compton continuums and other background seriously affect the accuracy and measurement limit of the detector.The idea of Compton suppression is applied to the process ofγ-ray spectroscopy measurement,and has a good suppression effect.With the development of phoswich detector,a compact phoswichγspectrometer for Compton suppression has been developed.Its advantages are small size and small cost.This paper focuses on the development,performance testing and algorithm improvement of compactγ-ray anti-coincidence phoswich detector.Theγspectrometer has a phoswich structure with a cylindrical primary crystal and a well-type secondary crystal surrounding the primary crystal.The out window of the phoswich is coupled to a photomultiplier tube.The difference of luminescence decay time of the phoswich was used to judge the energy deposition ofγrays in the crystals,and then the count of Compton platform,environmental background and cosmic ray were identified.By subtracting these counts from the spectrum,the Compton and background suppressedγspectra are obtained.This paper focuses on the construction of the compactγ-ray anti-coincidence phoswich detector to achieve the anti-coincidence phoswich detection technology ofγspectrum,and mainly carries out the following specific work:（1）The optical properties of scintillation crystals were investigated and summarized,and the combination of LaBr₃:Ce/NaI:Tl and GAGG:Ce/CsI:Tl were selected as the primary and secondary crystals of the compactγ-ray anti-coincidence phoswich detector,respectively.The luminescent structure of the scintillator is studied based on the ground state and excited state.By analyzing and constructing all possible excited states and considering the bands,density of states and transition dipole moment of excited states,a method for calculating the central emission wavelength of CsI:Tl crystal is proposed.The central emission wavelength of CsI:Tl crystal is predicted to be 517 nm（2.40 e V）to 538 nm（2.31 e V）,while the intrinsic luminescence wavelength is predicted to be 375 nm（3.31 e V）to 405 nm（3.06 e V）.It is in good agreement with the central emission wavelength（514.2 nm,2.41 e V）and the shoulder peak（410.6 nm,3.02 e V）in the experimental spectra.（2）GEANT4 simulation code was used to design the geometry of LaBr₃:Ce/NaI:Tl compactγ-ray anti-coincidence phoswich detector.LaBr₃:Ce main crystal is cylindrical,35 mm in diameter and 70 mm in height.NaI:Tl secondary crystal is well-type,well diameter is 36 mm,well depth is 71 mm,cylindrical diameter is 75 mm,height is 90mm.A photomultiplier tube is coupled to the out window of the phoswich.The voltage pulse generated by the anode of the photomultiplier tube is read by digital waveform acquisition technology,and the method of pulse shape identification is run.Combining theoretical calculation and experimental data analysis,the voltage pulses with rise time of 40-60 ns correspond to the event thatγray deposits energy only in LaBr₃:Ce main crystal,the recognition accuracy is 92.5%.By counting this part into the energy spectrum,the Compton and background suppressedγspectra are obtained.For LaBr₃:Ce/NaI:Tl detector,the peak-to-total ratios are 0.545(¹³⁷Cs)and 0.367(⁶⁰Co),which are 48%and 54%higher than that of a single LaBr₃:Ce detector with the same volume,respectively.Background is reduced from 129/s for a single LaBr₃:Ce detector to 50/s.The energy resolution of LaBr₃:Ce/NaI:Tl detector is significantly worse than that of a single LaBr₃:Ce detector with the same volume,and the effect of small-angle Compton scattered photons is high,but the effect of large-angle Compton scattered photons is poor.（3）The GAGG:Ce/CsI:Tl compactγ-ray anti-coincidence phoswich detector is designed with the phoswich design that the incident window of the main crystal is surrounded by the secondary crystal,which avoids the poor energy resolution for phoswich detector with the single crystal detector,and improves the suppression efficiency of large angle Compton scattered photons.The GAGG:Ce main crystal is cylindrical,with a diameter of 25.4mm and a height of 60mm.CsI:Tl s secondary crystal is a well-type with a well diameter of 27.4 mm,a well depth of 60 mm,a cylindrical diameter of 67.4 mm and a height of 65 mm.A photomultiplier tube is coupled to the out window of the phoswich.The voltage pulse generated by the anode of the photomultiplier tube is read by digital waveform acquisition technology,and the pulse shape identification method is run.The voltage pulse with rise time of 150-450ns corresponds to the event thatγrays deposit only in GAGG:Ce main crystal,and the recognition accuracy is 93.3%.By counting this part into the energy spectrum,the Compton and background suppressedγspectra are obtained.For GAGG:Ce/CsI:Tl detector,the total peaking ratios are 0.769(¹³⁷Cs)and 0.561(⁶⁰Co),which are 40%and47%higher than that of single GAGG:Ce detector with the same volume,respectively.Background is reduced from 86/s for a single GAGG:Ce detector to 22/s.GAGG:Ce/CsI:Tl detector has almost the same energy resolution as a single GAGG:Ce detector with the same volume,and has high efficiency of suppression on large-angle and small-angle Compton scattered photons.（4）The U-Net network combined with LSTM layer is used to separate and identify the height of pile-up pulses,which will improve the count ofγ-spectrum and the detection efficiency of the detector.The counting rate ofγ-spectrum treated by U-Net network is 28%(¹³⁷Cs)and 14%(⁶⁰Co)higher than that of the pile-up discard strategy.The innovations of this paper are as follows:（1）The first principles method was used to investigate the luminescence dynamics process of CsI:Tl crystal.The central emission wavelength of CsI:Tl crystal is predicted theoretically,which is in good agreement with the experiment.（2）The LaBr₃:Ce/NaI:Tl compactγ-ray anti-coincidence phoswich detector is designed and implemented.It is a small size and low cost anti-Comptonγspectrometer.The peak-to-total ratio of the detector reaches 0.545(¹³⁷Cs)and 0.367(⁶⁰Co).Peak-to-total ratio increases of 48%and 54%,respectively,compared with a single LaBr₃:Ce detector with the same volume.The background rate of 50/s is 61%lower than 129/s for a single LaBr₃:Ce detector of the same volume.（3）The GAGG:Ce/CsI:Tl compactγ-ray anti-coincidence phoswich detector,that the GAGG:Ce crystal incident window is surrounded,maintains the same energy resolution as a single GAGG:Ce detector,and achieves high Compton suppression efficiency of large-angle scattered photons.The peak-to-total ratio of the detector reaches 0.769(¹³⁷Cs)and 0.561(⁶⁰Co),which are 40%and 47%higher than that of the single GAGG:Ce detector with the same volume,respectively.The background rate of22/s is 74%lower than 86/s for a single GAGG:Ce detector of the same volume.（4）The U-Net network combined with LSTM layer is used to separate and identify the height of pile-up pulses,and the counting rate is improved by 28%(¹³⁷Cs)and 14%(⁶⁰Co)compared with theγspectrum of pile-up discard strategy.