吸收法在强激光与固体靶所致脉冲X射线能谱测量中的研究进展北大核心CSCD

脉冲X射线能谱测量,对于强激光装置中的物理诊断以及辐射防护具有重要意义。脉冲X射线具有脉冲时间短、注量大、能谱范围宽等特点,常规脉冲测量技术往往受到探测器死时间、堆积效应的限制而无法适用。目前多个国家都建立了强激光装置的研究平台,并开展X射线能谱测量相关研究。本文首先介绍了基于吸收法原理且适用于中低能脉冲X射线的测量方法:Ross Pair法和衰减法。然后针对这两种方法从5个方面(探测器结构、滤片材料、探测介质选择、散射控制以及解谱方法)综述了脉冲X射线吸收谱仪的研究进展,并分析了各自的适用性。目前激光装置中脉冲X射线能谱的测量还面临着能量分辨率不理想、结果不确定度无法量化和被动式能谱测量操作不便等问题。随着激光装置的不断升级,脉冲X射线注量以及打靶频次将不断增加,对探测器的耐辐照性能以及响应速度提出了更高的要求。     

Photon and positron production by ultrahigh-intensity laser interaction with various plasma foils

The generation of γ photons and positrons using an ultrahigh-intensity laser pulse interacting with various plasma solid foils is investigated with a series of quantum electrodynamic particlein-cell(PIC) simulations. When ultrahigh-intensity lasers interact with plasma foils, a large amount of the laser energy is converted into γ photon energy. The simulation results indicate that for a fixed laser intensity with different foil densities, the conversion efficiency of the laser to γphotons and the number of produced photons are highly related to the foil density. We determine the optimal foil density by PIC simulations for high conversion efficiencies as approximately 250 times the critical plasma density, and this result agrees very well with our theoretical assumptions. Four different foil thicknesses are simulated and the effects of foil thickness on γ photon emission and positron production are discussed. The results indicate that optimal foil thickness plays an important role in obtaining the desired γ photon and positron production according to the foil density and laser intensity. Further, a relation between the laser intensity and conversion efficiency is present for the optimal foil density and thickness.     

Nanosecond laser preheating effect on ablation morphology and plasma emission in collinear dual-pulse laser-induced breakdown spectroscopy

Focus-offset collinear dual-pulse laser-induced breakdown spectroscopy is designed and used to investigate the laser ablation and spectral intensity with an aluminum alloy sample. The laser crater morphologies and ablation volumes were measured. An inter-pulse time delay dependent ablation efficiency on a nanosecond laser-heated sample was observed, which was similar to the trend of spectral intensity versus inter-pulse time delay in the delay time less than 3 μs. Based on the observation, the nanosecond pulse laser preheating effect on subsequent second laser ablation and signal enhancement is discussed, which will be helpful for understanding the ablation and signal enhancement mechanism in the standard collinear DP-LIBS technique.     

Development of Three-Dimensional Numerical Model for 222Rn and Its Decay Products Coupled with a Mesoscale Meteorological Model II. Numerical Analysis on the Increase in Gamma Dose Rate Observed in the Coastal Area of Fukushima Prefecture

A three-dimensional Eulerian numerical model for ²²²Rn and its decay products coupled with a mesoscale meteorological model, MM5-TMNR, is applied to investigate the mechanism of the naturally induced increase in gamma dose rate observed at the coastal area of Fukushima prefecture in October 2002. The results obtained by MM5-TMNR are verified by comparisons with observed wind, precipitation and gamma dose rate to be adequate for examining the mechanism. The unusual increase in gamma dose rate is observed by the combination of the synoptic-scale (about 2000 to 5000 km horizontally) transport of natural radionuclides due to the inflow of cold air mass from the Asian Continent and the meso-_-scale (about 20 to 200 km horizontally) precipitation process within the coastal area of Fukushima prefecture. The contribution rate of natural radionuclides from the Asian Continent to the increase in gamma dose rateis estimated to be more than 60%.     

Cesium (Cs) particle formation based on a laser photochemical reaction with a self-injection-seeded Ti:sapphire laser for Cs isotope separation

We studied laser isotope separation of cesium (Cs) on the basis of a laser photochemical reaction and two-photon excitation scheme using a narrow line width Ti:sapphire laser to reduce long-term radioactive toxicity of a long-lived fission product. Using resonant laser irradiation to Cs atoms in hydrogen gas, we observed cesium hydride fine particles and confirmed the formation by calculations using rate equations. Our results show that the process seems promising for efficient Cs isotope separation.     

Investigation on plasma characteristics in a laser ablation pulsed plasma thruster by optical emission spectroscopy

In order to further improve the propulsion performance of pulsed plasma thrusters for space micro propulsion, a novel laser ablation pulsed plasma thruster is proposed, which separated the laser ablation and electromagnetic acceleration. Optical emission spectroscopy is utilized to investigate the plasma characteristics in the thruster. The spectral lines at different times,positions and discharge intensities are experimentally recorded, and the plasma characteristics in the discharge channel are concluded through analyzing the variation of spectral lines. With the discharge energy of 24 J, laser energy of 0.6 J and the use of aluminum propellant, the specific impulse and thrust efficiency reach 6808 s and 70.6%, respectively.     

Spectroscopic studies of homogeneous thin carbon erosion films on mirrors and flakes with a high deuterium content formed in tokamak T-10

Redeposited hydrocarbon films on plasma facing elements in tokamaks accumulate hydrogen isotopes. In the present study such films were made to redeposit on stainless steel mirror substrates as thin films and without any substrate as bare flakes with high deuterium content, under deuterium-plasma discharges inside T-10 tokamak vacuum chamber. These films were subjected to spectral characterizations through Fourier-transform infrared (FT-IR), electron paramagnetic resonance (EPR), and photoluminescence techniques. IR spectra showed the presence of two main deuterium states as observed by the CD_2,3 sp³ stretching modes at 2100–2200 cm⁻¹ and the CD₂ sp³ bending modes at 600–1100 cm⁻¹. Among these, CD₃ stretching mode at 2217 cm⁻¹ may serve as a control for deuterium desorption during the cleanup process of the reactor. As a comparative measure, C60 films were also studied, the luminescence excitation spectrum of which showed similarity in peak positions with tokamak bare flakes pertained to sp² luminescence centers. The observed spectral differences are mainly due to more localized sp² states for C60 and sp³ states for tokamak flakes. EPR spectra of the bare flakes showed the defective states with a high spin density, ∼10¹⁹ cm⁻³ which serve as luminescence quenching centers, and provide a path for hydrogen isotopes adsorption.     

Spin polarization and production rate studies of surface muons in a novel solenoid capture system based on CSNS

A novel surface muon capture system with a large acceptance was proposed based on the China spallation neutron source(CSNS).This system was designed using a superconducting solenoid where a long graphite target was put inside it.Firstly,the spin polarization evolution was studied in a constant uniform magnetic field.As the magnetic field can interact with the spin of the surface muon,both the spin polarization and production rate of the surface muons collected by the new capture system were calculated by the G4beamline.Simulation results showed that the surface muons could still keep a high spin polarization([90%)with different magnetic fields(0–10 T),and the larger magnetic field is,the more surface muons can be captured.Finally,the proton phase space,Courant–Snyder parameters,and intensities of surface muons of different beam fractions were given with magnetic fields of 0 and 5T.The solenoid capture system can focus proton and surface muon beams and collect p?and l?particles.It can also provide an intense energetic positron source.