基于神光Ⅱ升级装置,研究了纳秒/皮秒双束激光联合驱动双层靶的伽马(γ)辐射特征。利用ns束激光与CH薄膜靶相互作用,产生大尺度近临界密度等离子体,然后将ps束激光作用在该等离子体上,产生高能电子,高能电子穿过2 mm厚的Au靶,通过轫致辐射产生γ射线。对不同方向的γ辐射能谱和靶室外的γ辐射剂量分布进行实验测量,发现γ辐射集中在激光前冲方向,具有较小的发散角,而且在该方向上高能段的γ辐射较强。这说明双层靶的设计可以提高ps束激光与等离子体的能量耦合效率,提高高能电子温度,增加高能电子数目,有利于高能段γ辐射在ps束激光的前冲方向集中。另外,在靶室外距离靶点1.25 m处测到的50 keV以上γ辐射的单发次最大剂量为277 μGy。本研究结果对γ辐射的防护和应用具有参考价值。

We explore the gamma radiation characteristics of double-layer targets driven by nanosecond/picosecond two-beam lasers based on the Shenguang II upgrade laser facility. A nanosecond laser is used to interact with a thin film target to generate a large-scale near-critical density plasma. Further, a picosecond laser interacts with the plasma to generate high-energy electrons. The high-energy electrons pass through a 2 mm thick Au target to generate gamma rays via bremsstrahlung. Subsequently, the experiments measure the gamma energy spectra in different directions and gamma doses outside the target chamber. It is found that the gamma radiation is concentrated in the picosecond laser propagating direction with a small divergence angle, and the high-energy parts of the gamma rays are enhanced in this direction as well. The design of the double-layer target can improve the energy coupling efficiency of the picosecond beam and plasma, increase the temperature and number of high-energy electrons, and facilitate the concentration of high-energy gamma radiation in the propagating direction of the picosecond laser beam. Furthermore, the single-shot maximum dose of gamma radiation, which has energy higher than 50 keV and is measured at 1.25 m from the target outside the target chamber, is 277 μGy. Results of the present study could be considered as references for the shielding and application of gamma radiation.