Positron beam production study at Shanghai Laser Electron Gamma Source

BackgroundPositron beams have extensive applications in positron emission tomography (PET) and non-destructive testing (NDT), with positron annihilation techniques providing valuable insights into submicroscopic defects. Among various generation methods, pair production through gamma-matter interaction is one of the most effective approaches.PurposeThis study aims to develop a high-quality positron beam with adjustable energy at MeV levels using the continuously adjustable gamma beam at the Shanghai Laser Electron Gamma Source (SLEGS) of the Shanghai Synchrotron Radiation Facility (SSRF).MethodsFirstly, two positron beam generation and separation methods at SLEGS were investigated using continuously adjustable gamma rays (0.66~21.7 MeV) with intensity of ~10 7 photons·s -1 to interact with selected target materials. Secondly, comprehensive Monte Carlo simulations were conducted using Geant4 software to optimize target materials, target thickness, deflecting magnet configurations, and focusing systems for both single-target magnetic field separation and multi-target lateral extraction modes. Finally, experimental verification was performed by directly injecting gamma beams into lead targets, measuring the angular distribution of 511 keV annihilation gamma rays using LaBr3(Ce) detectors, and conducting preliminary positron annihilation lifetime spectroscopy measurements.ResultsVerification results show that the single-target magnetic field separation mode achieves positron beams with energy ranging from 1.0 MeV to 12.9 MeV and intensity of 10 2~10 3 e +·s -1·cm -2. The optimized multi-target lateral extraction mode significantly improves beam quality, yielding positrons with energy range of 1.0~9.1 MeV and intensity of approximately 10 3 e +·s -1·cm -2 in the low-energy region and 10 5 e +·s -1·cm -2 in the high-energy region. The scattered gamma background is successfully reduced by approximately one order of magnitude in this configuration. Experimental measurements yield 2.5×10 8 annihilation photons in full space, showing excellent agreement with the simulation result of 2.47×10 8 photons within error margins.ConclusionsMonte Carlo simulations successfully optimize SLEGS positron beam parameters, with experimental verification confirming the simulation's reliability. The optimized solenoid multi-target lateral extraction mode enhances positron beam intensity while effectively eliminating scattered gamma background interference. Under current experimental conditions, accurate measurement of positron start time remains challenging for positron annihilation lifetime spectroscopy. Future improvements will focus on obtaining more precise start time signals from the SSRF storage ring high-frequency system or implementing short-pulse laser triggering methods to achieve reasonable positron annihilation lifetime spectra.