Yuanzineng kexue jishu (Mar 2023)
Study on Photon Dose of Ultra-intense and Ultra-short Laser Facility Based on Fully Monte-Carlo Simulation
Abstract
The possibility of using lasers to achieve previously unobtainable states of matter in the laboratory gained much attention following the demonstration of the first pulsed laser in 1960. In the following few years there was plenty research activity as attempts were made to increase the peak power and focused intensity in order to reach extreme conditions within the laboratory. Laser-acceleration technology is rapidly developing. Petawatt class lasers have been constructed for specific research activities, including particle acceleration, inertial confinement fusion and radiation therapy, and for secondary source generation (X rays, electrons, protons, neutrons and ions). As a new source of ionizing radiation, dose measurement in ultra-intense and ultra-short laser facilities is of great value for radiation protection of the environment, occupational staff and shielding design in laser device design stage. Owing to the pulsed and mixed characteristics of radiation field around the laser facility, it is really challenging to accurately measure the dose equivalent induced by bremsstrahlung photon in the target chamber of the laser facility. Moreover, the rationality of dose assessment model needs more experimental data and to be further verified. To reach this point, the Monte-Carlo program FLUKA was employed to characterize the radiation filed distribution. Firstly, a complete and thorough target chamber module was established according to the XG-Ⅲ geometry parameter and material composition. The in-chamber and out-chamber dose distribution were then calculated and analyzed based on the existing theoretical model of electron source term and typical experimental conditions. The electron energy distribution, characteristic electron temperature, scaling laws and laser-to-energy conversion efficiency were introduced. Secondly, the typical electron spectrum was divided into four intervals. The portion of different energy intervals to the total equivalent dose was determined. Thirdly, a bremsstrahlung photon dose equivalent measuring module was designed and optimized for ultra-intense and ultra-short laser facilities. The simulation results indicate that: 1) The electron dose equivalent is higher than the bremsstrahlung photons both inside and outside the target chamber; 2) The dose equivalent contribution of 0.15 MeV electron energy interval to the total energy range is dominant, and the module was design based on the conclusion; 3) The designed module mainly consists of PMMA and metallic shielding material. With this approach, the module can reduce the electron dose equivalent to 0.3% and photon dose equivalent to 69.1%. The dose ratio of electron to photon is about 128%. In addition, 5% of the total photon dose is contributed by the occurrence of bremsstrahlung of highenergy electron in PMMA.
