Research On The Characteristics Of ?-ray Radiation Driven By Picosecond Laser

In the process of interaction between ultra-intense laser and matter,electrons will be accelerated to relativistic energy by means of resonance absorption,vacuum heating,and ponderomotive acceleration.Then the relativistic electrons generate gamma rays through bremsstrahlung radiation.Because of the harm to the human body and the wide application prospects of gamma radiation,it has attracted much attention.The current research focuses on the optimization of gamma radiation sources,including reducing the gamma radiation divergence angle,improving the high energy segment(near MeV)gamma radiation,and exploring new mechanisms for gamma radiation generation.This article first explores the gamma radiation characteristics from double-layer targets based on the Shenguang ? upgrade laser facility.The ns laser is used to interact with the thin film target to generate a large-scale near-critical density plasma,and then a ps laser interacts with the plasma to generate high-energy electrons.The high-energy electrons pass through a 2-mm-thick Au target to generate ? rays by bremsstrahlung.The experiments measured the gamma spectrum in different directions and the gamma doses outside the target chamber.It was found that the gamma radiation is concentrated in the ps laser propagating direction with a small divergence angle,and the high-energy parts of gamma rays are enhanced in this direction.The design of the double-layer target can improve the energy coupling efficiency of the ps beam and the plasma,increase the number of high-energy electrons,and facilitate the concentration of the high-energy gamma radiationin the propagating direction of the ps laser beam.In addition,it was 277 ?Gy that the maximum gamma-radiation dose measured at 1.25m from the targets outside the target chamber in the experiments,and necessary shielding measures were required.In the past experiments on the interaction between ultra-intense laser and solid targets,people pay more attention to the gamma radiation of thick targets,because gamma radiation is generally thought to be lainly generated by bremsstrahlung,and the effect of thick targets is stronger.However,the thin target is completely different from the thick target.There is a strong sheath field on the target rear of the thin target.How does the dtfferenl point aftect the generation of gamma radiation?Based on the Shenguang ? upgraded laser device,this article continued to study the gamma radiation of thin targets of three different materials,high Z.medim Z and low Z,and obtained gamma-radiation energy spectra in dilterent directions.It was found that for thin targets,the gamma-radiation energy spectra in different directions of the target back are similar.It is possible to cause a deflection of the electron motion due to the strong sheath field on the back of tlie target of the thin target.Moreover,it is found that under the condition that the laser conditions are basically the same,the gamma-radiation energy spectrum of the three different materials has little difference in the same direction.It is different from the bremsstrahlung of thick targets.In this paper,the existence of the target rear sheath field may make the gamma radiation of different material targets approach,and it is speculated that the strength of the target rear sheath field produced by different materials is different,so the influence on ? radiation will be different.During the experiment,the gamma-radiation doses outside the target were measured,but it was impossible to distinguish the proportion of gamma radiation generated by the interaction between the laser and the target in the same direction and the interaction between the escape electrons and the target chamber wall.Therefore,the last part of this article designed an electron-gamma spectrometer.In summary,by generating a near-critical density plasma,the near-MeV gamma ray intensity is increased and the gamma ray divergence angle is reduced.By measuring the thin target gamma radiation characteristics,it is found that the radiation is different from the thick target.The research results provide an important reference for the protection design of the target chamber and the mrther optimization and application of the laser-driven gamma radiation source.