SiPM-based Positron Annihilation Detection Technology

Positron annihilation spectroscopy is a non-destructive technique that enables to characterize atomic-scaled microstructure of materials,especially the vacancy-type defects.With the development of detector and computational physics,positron annihilation spectroscopy has been widely used in material science,medical diagnosis,etc.Positron annihilation detection technology has also been a research hotspot in the field of positron annihilation spectroscopy.Positron annihilation spectroscopy usually uses photomultiplier tubes(PMT)to construct spectrometers.Due to high gain,high photon detection efficiency,high single-photon resolution,and compact size,silicon photomultiplier(SiPM)has great potential to replace and surpass PMT in the positron annihilation spectroscopy.Based on γ detection technology and experimental methods,a series of researches on SiPM detector,readout electronics,SiPM array detector,and measurement system was carried out.The time,energy,and position information of the γ rays deposited in the detector were obtained and the accurate measurement of the positron annihilation lifetime(PAL)parameter in the material was realized.The main works are listed as follows:1.The first SiPM-based PAL spectrometer was developed.A high time resolution(TR)of 136 ps is achieved,which is much better than conventional spectrometer using PMT(～210 ps).Under the guidance of scintillation luminescence,photon transmission,photoelectric conversion,and readout process,the selection of scintillator and photodetector was completed.The effects of SiPM bias voltage(30-34 V),leading edge timing threshold(4-12 mV)and crystal length(5.0,9.5,13.8,and 20.0 mm)on the TR were experimentally studied.PAL spectra of GaN measured by SiPM-PAL spectrometer with different scintillators(including material and length)give results in good agreement with previous measurements.The coincidence count rates ranged from 15 to 45 cps(source activity:1.85 MBq).The feasibility of using SiPM to develop a PAL spectrometer is also verified.2.A pulse area estimation method based on multi-ToT(Time over Threshold)was designed and the multi-ToT readout electronics for SiPM detector was developed.A multi-ToT readout circuit timing path jitter is 22.7±0.1 ps(FWHM),which is much smaller than the coincidence time resolution of the SiPM detector.Compared to amplitude,pulse area expresses energy information more accurately.A pulse area estimation method based on triple-ToT and dual-ToT was designed.The energy performance and pulse area integral nonlinearity were investigated with a SiPM-based scintillation detector.Using triple-ToT,the pulse area linearity in INL in the energy range of 511～1333 keV,is superior than 4.21%,which is at least 25%better than that with dual-ToT.The energy resolution and energy linearity of triple-ToT is also better than dual-ToT.The multi-ToT is effective and convenient for improving energy linearity and dynamic range.The quantitative energy and timing results show that the circuit can greatly simplify the readout electronics of the PAL equipment,replacing complex timing and multi-channel devices,and also provide a multi-threshold discrimination method for the measurement system.3.A SiPM-based multi-channel detection system was developed.The system realizes 64 annihilation coincidence cases and 128 PAL cases by using 4 SiPM array detector modules(16 SiPMs and 16 crystals).The acquisition of the position,time,and energy information of the 1275 keV and 511 keV γ rays deposited in the detector was achieved.A high-performance SiPM array detector based on resistor chain readout and a high-efficiency trigger logic module were developed.The SiPM array detector covers the emission solid angle of the positron in a large range by using multiple arrays of small crystals,and the trigger logic module uses multiple discriminators to trigger cases efficiently.The position of the scintillation crystal array to γ rays is clearly visible.The average energy resolution is 11.2%,and the average TR can reach 211 ps.A multi-channel lifetime spectrum synthesis method was developed.It shifted lifetime spectrum by the time difference offset of the coincidence time spectrum,effectively synthesized multiple lifetime spectra into one lifetime spectrum.The count rate can reach～223 cps(source activity:0.9 MBq),which improves the data utilization.The accuracy of synthesis method and the reliability of measurement system were verified by the PAL parameters obtained by Si samples.The system provides a technical solution for positron angular correlation of annihilation radiation and slow positron beam measurement.