1.Ion Beam and Optical Physical Laboratory of Xianyang Normal University and Institute of Modern Physics, Chinese Academy of Sciences, Xianyang 712000, China 2.School of Science, Xi’an Jiaotong University, Xi’an 710049, China
Abstract:The vacancy can be produced through impact ionization of target atom by energetic particles. It is of significant importance to study the vacancy state by the measurement of angular distribution of typical X-rays. At present, accurate ionization cross-section data of the atomic inner shell are urgently required in many areas. However, the precise measurement of ionization cross-section of the atomic inner shell is largely dependent on the fact that whether the characteristic radiation (e.g., X-ray) is isotropic. In this experiment, the characteristic Lι, Lα, Lβ and Lγ1 X-rays for Au target are measured by a silicon drift detector in an emission angle range from 130° to 170° in steps of 10°. A mini-X ray source is utilized to produce bremsstrahlung with the center energy of 13.1 keV.Considering detection efficiency of the detector and the absorption of the target, relative intensity ratios, I(Lα)/I(Lγ1) and I(Lι)/I(Lγ1), are obtained at different detection angles based on the experimental energy spectrum results. Moreover, the angular dependence of X-ray intensity ratio is investigated and it is found that the X-rays Lι and Lα exhibit anisotropic emission.According to the X-ray intensity ratio I(Lι)/I(Lγ1) and the P2(cosθ), and using the least square method, the anisotropic parameter β of characteristic X-ray Lι is derived to be 0.25. Due to the relation β = ακA20, the value of the alignment degree A20 for L3 sub-shell is determined to be 0.577 ± 0.08. Alignment degree A20 for L3 sub-shell is dependent on its intrinsic physical properties, while the anisotropy parameter β of typical X-rays can be affected by Coster-Kronig transition process.The behavior of the alignment for inner-shell vacancy states calls for more research results both in theory and in experiment. Therefore, it is quite relevant and meaningful to perform more experiments to further study the angular distribution of vacancy states by electrons, photons and ions impacting a target. Keywords:photoionization/ characteristic X-rays/ angular distribution/ anisotropy parameter
其中μinc表示入射光子(入射中心能量为13.1 keV的轫致辐射光子)的质量吸收系数, μemt表示出射光子(出射特征X射线Li, Lj)的质量吸收系数, 其数值由Storm和Israel[24]报道的靶材对不同能量光子的吸收系数得到. 入射束和出射束与靶平面法向的夹角由θ1和θ2分别表示, Au靶的质量厚度t为77.2 mg·cm–2. 图 2 探测角度为140°时、中心能量为13.1 keV轫致辐射入射Au靶产生L系特征X射线能谱图 Figure2. Fitted L X-ray spectrum of Au induced by impact with bremsstrahlung with central energy of 13.1 keV and measured at the emission angle of 140°.
经过(2)式和(3)式的计算, 在图3中完整展示了Au的L壳层X射线Lι与Lγ1及Lα与Lγ1的强度比和P2(cosθ)之间的关系. 采用最小二乘法对不同探测角度下得到的强度比I(Lι)/I(Lγ1)进行拟合, 拟合曲线的斜率, 即为特征X射线Lι的各向异性参数. 本文数据统计过程中, X射线强度比的误差主要来自于5%的高斯拟合误差、3%的统计误差、6%的背景误差和6%的探测器立体角误差, 系统总体误差大约为14%. 图 3 Au 靶特征X射线强度比I(Lα)/I(Lγ1)和I(Lι)/I(Lγ1)与P2(cosθ)的关系 Figure3. Intensity ratios of I(Lα)/I(Lγ1) and I(Lι)/I(Lγ1) as a function of P2(cosθ) for Au.