基于LA-ICP-MS表面原位分析技术测定热电离质谱仪灯丝支架上铀的沉积分布

Determination of uranium deposition distribution on filament support of TIMS based on LA-ICP-MS surface in-situ analysis technology

热电离质谱法(Thermal ionization mass spectrometry,简称TIMS)是一种测量同位素丰度比的经典分析方法,在地质学和核工业领域得到广泛应用。热电离质谱法测铀同位素丰度比时一般采用三带结构,由于铀的第一电离能较高,需要较高的电离温度,长时间测试蒸发出铀化合物或电离出的铀离子在灯丝支架上沉积,影响灯丝支架上悬浮高压与源电压间的绝缘电阻,降低两者之间的电压差,导致绝大部分离子因动能不足无法进入离子透镜,最终影响离子流的稳定性,引起测试时信号产生波动。针对热电离质谱仪灯丝支架铀沉积导致的灯丝支架绝缘失效问题,采用激光剥蚀电感耦合等离子体质谱法(Laser ablation inductively coupled plasma mass spectrometry,简称LA-ICP-MS)对灯丝支架铀沉积分布进行了分析,取得以下结果:建立了LA-ICP-MS原位表面分析技术,其最佳激光频率为10 Hz,能量密度为6 J·cm-2,束斑直径为60μm。校准曲线相关系数R2=0.9992,表明建立的方法线性关系良好。应用建立的方法,测定了灯丝支架表面铀沉积的分布特征。结果显示:灯丝支架表面铀沉积随着灯丝支架使用时间的增加,对应的绝缘电阻变小;并沿着支架向两端延伸,铀沉积量逐渐降低。铀沉积主要集中在电离带灯丝支架中心上端,且蒸发带上铀沉积量明显低于电离带。同时绝缘挡片可以有效降低陶瓷绝缘体上铀的沉积,因此可以通过增大绝缘挡片来提高灯丝支架的绝缘效果。

Thermal ionization mass spectrometry(TIMS)is a classical analytical method for determination of isotope abundance ratios,which is widely used in geology and the nuclear industry.When measuring the uranium isotope abundance ratio using TIMS,a three-band structure is generally used.Due to the relatively high first ionization energy of uranium,a higher ionization temperature is required.During long-term testing,evaporated uranium compounds or ionized uranium ions deposit on the filament support,affecting the insulation resistance between the suspended high voltage and the source voltage on the filament support,reducing the voltage difference between the two,resulting in most ions not being able to enter the ion lens due to insufficient kinetic energy,ultimately affecting the stability of the ion current and causing fluctuations in the signal during testing.To address the problem of insulation failure caused by uranium deposition on the filament support of the thermal ionization mass spectrometer,laser ablation inductively coupled plasma mass spectrometry(LA-ICP-MS)was used to analyze the distribution of uranium deposition on the filament support,and the following results were obtained:LA-ICP-MS in-situ surface analysis technique was established,with the optimal laser frequency being 10 Hz,energy density of 6 J·cm-2,and beam diameter of 60μm.The calibration curve correlation coefficient R2=0.9992 indicates a good linear relationship.The established method was applied to determine the distribution characteristics of uranium deposition on the surface of the filament support.The results showed that as the usage time of the filament support increased,the corresponding insulation resistance decreased;and along the support towards both ends,the amount of uranium deposition gradually decreased.Uranium deposition was mainly concentrated at the center and upper end of the ionization zone filament support,and the amount of uranium deposition on the evaporation zone was significantly lower than that on the ionization zone.At the same time,the insulation baffle can effectively reduce the deposition of uranium on the ceramic insulator,so it is possible to improve the insulation effect of the filament support by increasing the insulation baffle.