Positron beam production study at Shanghai Laser Electron Gamma Source

[Background]: Positron has a wide range of applications, the most important of which are PET CT studies and Non-destructive Testing (NDT) with positron beam. Positron NDT is utilized to study submicroscopic defects, including dislocations and vacancies. Positron annihilation techniques provide insight into the kinetic energy distribution and density distribution of matter. There are several ways to obtain positron beam current, and the pair production of gamma-matter interaction is one of the most effective methods. [Purpose]: To obtain a high-quality positron beam with adjustable energy, adjustable beam spot and low signal-to-noise ratio at MeV energy level based on the gamma beam of the Shanghai Laser Electron Gamma Source (SLEGS) of the Shanghai Synchrotron Radiation Facility (SSRF). In order to provide conditions for carrying out positron beam experiments in the future. [Methods]: Positrons were generated and separated by adding SLEGS gamma rays with continuously adjustable energy, positron generating target, deflecting magnet and focusing system to Geant4 code, and the parameters were optimised at each gamma energy point to obtain a better quality positron beam. The experimental study of gamma direct injection into the target was carried out to analyse the gamma angular distribution of positron annihilation and the lifetime spectrum of positron annihilation, and the experimental results and simulation results were checked against each other. In the experiment, 13.0 MeV SLEGS gamma rays were injected into a 1 cm lead target, and the angular distribution of gamma rays from positron annihilation was measured by multiple LaBr3(Ce) detectors. The same experimental conditions were added to the Geant4 code to verify the reliability of the procedure and the experimental results. [Results]: Combining with the continuously adjustable energy of SLEGS gamma rays, the positron beams with continuously adjustable energies are obtained. The energy range of the positron beam in single-target & magnetic deflection mode is from 1.0 MeV to 12.9 MeV, and the beam intensity is 102~103 e+/s/cm2 . The multi-target & lateral elicitation mode are increased and optimised to achieve a beam energy 1.0 MeV- 9.1 MeV, and beam intensity of ~103 e+/s/cm2 in the low-energy region and ~105 e+/s/cm2 in the high-energy region, and the scattering gamma background is reduced by one order of magnitude. The optimised SLEGS positron beam has the advantages of wide energy range, high beam intensity, tunable spot size and low background. In the positron annihilation gamma angular distribution measurement experiment, the 511 keV gamma ray counts measured by six LaBr3(Ce) detectors are summed up to 2.8×106 photons, and combined with the solid angle covered by the detectors of 0.028 and the efficiency of 40%, the full-space positron annihilation gamma rays are calculated to be 2.5×108 photons. In the simulation with the addition of the same gamma rays and lead target as in the experiment, and use of a 4πcollection in place of the LaBr3(Ce) detector, a 511 keV gamma ray count of 2.47×108 photons was obtained, which is in good agreement with the experimental measurements. [Conclusions]: SLEGS-based positron beam studies have been optimised in simulation. It was found that the optimisation of the number of targets and the inclusion of solenoids increased the intensity of the positron beam and eliminated the effect of the scattered gamma background, resulting in a SLEGS positron beam with a wide energy range, high beam intensity, tunable spot size and low background. The angular distribution of positron annihilation gamma rays measured by SLEGS has very good results and agrees well with the simulation results. At the same time, the detector electronics and experimental layout of SLEGS positron annihilation life spectrum, as well as the preliminary experimental research, are also presented