微型X射线管出射谱特征研究及Be窗厚度确定

微型X射线管已广泛应用于现场元素分析、放射性医疗等领域，对于微型X射线管铍窗，普遍认为除保证管内真空外，应越薄越好。采用蒙特卡洛方法，模拟了从50～500 μm范围内共13个Be窗厚度的微型X射线管出射X射线谱。按照在应用中的作用，将出射X射线划分为不同能量段进行分析。通过分析谱线特征，发现Be窗厚度应依据其应用要求合理选择。因此，提出了K系特征X射线与轫致辐射强度的比值和低能射线与激发射线计数比值等参量作为评价Be窗厚度最优化的判断依据。除上述评判指标外，铍窗的厚度最优化选择还应考虑Be窗对不同能量X射线的屏蔽效果。依据模拟结果分析，原位(现场)X射线能量色散荧光分析应用中，Be窗厚度约250 μm的微型X射线管最为合适。与50 μm铍窗厚度出射射线相比，71.66%低能原级X射线被屏蔽，5~50 keV能量原级X射线仅有21.31%被屏蔽，低能射线强度占总X射线比值小于10%，且K系X射线占激发射线的比例仍保持较高的水平。因此，采用250 μm铍窗厚度的微型X射线管作为能量色散激发源，能保证探测器探测的有效信号比值较高，低能X射线对探测器的能量分辨率的影响最小，而且能量色散分析谱线的散射本底相对强度处于较低的水平，从而保证元素分析结果精准度。对于放射性治疗的应用中，则铍窗厚度越薄越好，此时，低能X射线具有较高的通量，能保证辐射剂量在治疗组织中剂量的集中。

Research on Spectral Characteristic of Miniature X-Ray Tube and Determination of Beryllium Window Thickness

Applying Monte Carlo method, the present paper simulates the emitted X-ray spectrum of miniature X-ray tube with thirteen thickness of beryllium window in the range from 50 to 500 μm. By analyzing the characteristic of the spectrums, the reasonable choice of thickness of beryllium window relies on the application and for the beryllium window it is not the thinner the better. Taking in-situ EDXRF as an example, though the emission X-ray intensity is higher as the thickness of the beryllium window becomes thinner, the proportion of useless low-energy X-ray (<5 keV) intensity to all energy X-ray intensity also is higher (>20%). The accuracy of in-situ EDXRF will be reduced when the high-throughput low-energy X-ray enters the detector. Therefore, this paper puts forward several parameters as judgment index for beryllium window thickness, which is described as follows: ①The intensity ratios of the K-series X-ray to middle-energy (5～25 keV) bremsstrahlung and middle-high-energy (5～50 keV) bremsstrahlung (F₁ and F₃); ②The intensity ratios of useless low-energy X-ray (<5 keV) to middle-energy (5～25 keV) X-ray and middle-high-energy (5～50 keV) X-ray (F₂ and F₄), it can reflect the relative intensity of useless low-energy X-ray. The simulation results demonstrate that with the increase in the beryllium window thickness, the value of F₁ (F₃) improves slowly, and the value of F₂ (F₄) decreases rapidly. In addition to the judgment index discussed above, and considering the X-ray shielded by beryllium window, the beryllium window of miniature X-ray tube can be determined. Based on simulation analysis, the thickness of around 250μm is appropriate to miniature X-ray tube applied in the in-situ EDXRF. Comparing the emitted spectrum with 50 μm-thick beryllium window, 71.66% of low-energy X-rays are shielded, only 21.31% of X-rays with energy from 5 to 50 keV is shielded, the intensity ratio of low-energy X-ray to total energy X-ray is less than 10%, and the intensity proportion of K-series X-ray to middle-high energy X-ray maintains a high level. In other words, when the mobile X-ray source with 250 μm beryllium window is used in the in-situ EDXRF, proportion of effective signal is higher, and effect of energy resolution of the detection is least; Moreover, the relative intensity of the excitation spectral scattering background, which is obtained by detection for specimen excitation analysis, will remain at low level, thus to ensure the precision of the result of element analysis. For the beryllium window in the application of radiation therapy, the thicker the better. At this time, low-energy X-ray flux maintains a high level, and it can ensure that radiation dose is concentrated on treatment tissue.