态选择电荷交换实验测量以及对天体物理软X射线发射模型的检验

Experimental measurement of state-selective charge exchange and test of astrophysics soft X-ray emission model

在高温天体等离子体环境中,低能高电荷态离子与中性原子和分子之间的电荷交换是天体物理环境中软X射线发射的重要机制之一.电荷交换软X射线发射相关的天体物理建模需要大量的主量子数n和角量子数l分辨的态选择俘获截面数据,目前这类数据主要来自于经典或者半经典的原子碰撞理论模型.本文利用反应显微成像谱仪,系统测量了炮弹能量为1.6—20.0 keV/u Ne~(8+)与He的单电子俘获n分辨的态选择俘获截面.将测得的相对态选择截面与多通道Landau-Zener方法以及分子库仑过垒模型计算的结果进行比较,发现理论模型计算结果与实验测量结果在弱反应通道上存在显著差异.进一步结合天体物理中常用的l分布模型,计算了1.6和2.4 keV/u Ne~(8+)与He之间电荷交换中的软X射线发射谱,通过与近期实验测量的X射线谱比较,发现计算的软X射线谱强度明显偏离已有的测量值.这些研究表明,多通道Landau-Zener方法、分子库仑过垒模型以及l分布模型在定量描述电荷交换态选择截面时存在一定的不足,如果将这些理论模型应用于天体物理的X射线背景研究中,可能导致对天体等离子体参数的描述不够准确.

Charge exchange,or electron capture,between highly charged ions and atoms and molecules has been considered as one of important mechanisms controlling soft X-ray emissions in many astrophysical objects and environments.However,to model charge exchange soft X-ray emission,astrophysicists commonly use principal quantum number n and angular momentum quantum numberl resolved state-selective capture cross section data,which are usually obtained by empirical and semi-classical theory calculations.The accuracy of the theoretical model is the key to constructing an accurate X-ray spectrum.With a newly-built cold target recoil ion momentum spectroscopy apparatus,we perform a series of precise state-selective cross section measurements on Ne8+ions’single electron capture with He targets,with the projectile energy ranging from 1.4 to 20 keV/u.The experimentally measured Q value spectrum shows that the process of electron captured to state of Ne7+with n=4 is the main reaction channel,and that with n=3 and 5 are the small reaction channels.Using Gaussian curve to fit the area of each channel on the Q value spectrum and normalizing the area of all channels,we obtain the n-resolved relative state-selective cross section.By comparing the measured relative cross sections with the results calculated by the multichannel Landau-Zener method and molecular Coulomb over-barrier model,significant difference among the strengths of small reaction channels is found.Specifically,the multichannel Landau-Zener method overestimates the contribution of n=2 channel and n=3 channel,and underestimates the contribution of n=5 channel.The molecular Coulomb over-barrier model overestimates the contribution of n=5 channel and underestimates the contribution of n=3 channel.The significant difference between the theoretical model calculation and experimental measurement is due to the limitations of semiclassical theoretical method and classical theoretical method.Furthermore,with l distribution models commonly used in the astrophysical literature,including the statistical model,separable model,Landau-Zener-I model,Landau-Zener-II model and even model,we calculate the soft X-ray emissions in the charge exchange between 1.6 and 2.4 keV/u Ne8+and He.It is found that the calculated intensities of X-ray spectra significantly deviate from the existing measurements,and only the separable model can partly match the laboratory simulated solar wind charge exchange X-ray measurement.Furthermore,we find that the intensity of the charge exchange X-ray emission spectrum measured experimentally is dependent on the collision energy,while the emission spectrum calculated based on the model seems to be unchanged with the increase of the collision energy.These results indicate that if the classical and semi-classical models are applied to the astrophysical plasma for studying diffusive soft X-ray background,the obtained parameters of the astrophysical plasma will be inaccurate.