通过传统能量色散X射线荧光分析(EDXRF)对Cd元素进行痕量分析时, X光管发出的原级轫致辐射连续谱对Cd元素的分析具有严重的影响。 为了减弱原级X射线对测量结果的影响, 通过Geant4程序包模拟不同几何尺寸下偏振激发X射线荧光分析(P-EDXRF)中荧光靶的结构, 研究其在减弱测量过程中原级X射线轫致辐射连续谱的影响。 为了提高模拟效率, 本文分三步进行模拟。 第一步模拟不同管电压下电子打靶过程, 得到不同管压下的X光管原级能谱。 第二步针对Cd元素的痕量分析设置不同种类、 不同几何条件的Te及BaSO₄作为荧光靶材料进行模拟。 结果表明在使用K_α1能量(27.468 keV)与Cd元素吸收限26.711 keV最为接近的Te作为荧光靶材料时, 随着Te靶厚度增加Te元素的特征峰强度在100 μm前快速增长, 在150 μm后趋于稳定。 而其信噪比(SNR)在80 μm到达最大值21.434。 继续增加厚度由于荧光靶材料的自吸收作用SNR开始有些许下降, 达到饱和吸收厚度后稳定。 在不同应用场景时荧光靶选材应有不同, 对于测量时间没有限制的条件下, 应选择荧光强度更大的荧光靶厚度。 而对于测量时间相对较短的条件下, 则应该选择SNR更大的荧光靶厚度。 第三步模拟通过荧光靶后的出射能谱激发含0.01%Cd的样品, 经过Te靶后的出射能谱激发样品, 得到Cd元素K_α1峰背比为8.28。 相较于直接通过原级谱激发样品, Cd元素K_α1峰背比为2.29, 其峰背比明显提高但Te元素的散射峰对Cd元素K_α1峰仍然有所影响。 选用特征X射线能量与Cd元素K_α1相距更远的BaSO₄作为荧光靶材料, 可减弱由于样品中的基体轻元素的散射作用而引起目标元素的峰背比降低情况, 可将Cd元素K_α1峰背比提高至14.179。 通过增大X光管管电压可进一步提高激发效果, 在管电压为70 kV时对于Cd元素有最佳激发效果, 峰背比为21.431。

Primary bremsstrahlung spectrum of X-ray tube has serious influence for trace Cadmium analysis in traditional EDXRF. Secondary targets with different geometry sizes were studied by Geant4 code. To enhance the efficiency of Geant4 simulations, the simulation processes were divided into three stages. In the first stage, primary spectra at different tube voltages were acquired using Geant4 code to simulate electrons of different voltages hitting anode target. In the second stage, Te and BaSO₄ of different kinds and geometry as secondary target materials, simulated. The simulation results show that when Te, whose K_α1 energy (27.468 keV) closes to the absorption limit of Cd (26.711 keV) is used as the fluorescence target material, the characteristic peak intensity of Te increases rapidly before 100 μm with the increase of target thickness, and tends to be stable after 150 μm. However, the signal to noise ratio (SNR) reaches the maximum value of 21.434 at 80 μm. Due to the self-absorption effect of the secondary target material, SNR declines slightly and becomes stable after reaching the saturation absorption thickness. In different application scenarios, the materials of the secondary target should be various. When there is no limit to the measurement time, the secondary target with greater fluorescence intensity should be selected. But, when the measurement time is relatively short, the secondary target of greater SNR should be selected. In the third stage, output spectra of the secondary target were used to activate sample containing 0.01% cadmium element. The output spectra of the Te element secondary target were used to activate samples, and the peak-to-background ratio of the K_α1 peak of Cd element was 8.28. The primary spectra were used to activate samples, and the peak-to-background ratio is 2.29. Although it has a great increase, the scattering peak of the Te element always influences the K_α1 peak of the Cd element. The BaSO₄ was selected as secondary target material because the characteristic X-ray energy is farther away from the K_α1 peak of the Cd element. The decrease of the peak-to-background ratio of target element could be weakened caused by the matrix elements of the sample. The peak-to-background ratio is increased to 14.179. The activation effect can be further improved by increasing the tube voltage of the X-ray tube. The optimal peak-to-background ratio of 21.431 could be obtained at the 70 kV tube voltage.