负热电离质谱法测定核燃料裂变产物中钼同位素比值

钼同位素⁹⁵Mo、⁹⁷Mo、⁹⁸Mo、¹⁰⁰Mo是核燃料的重要裂变产物，为了测定辐照后核燃料元件裂变产物中钼同位素比值，以SrCl₂为发射剂，采用负热电离质谱法测定MoO₃^-。在单铼带模式下，钼涂样量为0.1 μg时可获得约2 h稳定离子流，同位素比值测定相对标准偏差（RSD）小于0.5‰。实验使用5个法拉第杯一次跳峰完成7个钼同位素测定，分别采集跳峰前后的m/z 143信号，并选作同位素比值分母，避免信号随时间波动引入误差。通过评价天然钼本底水平，表明质谱测定过程中铼带和试剂本底的干扰可忽略；通过求解六元一次方程组，可扣除MoO₃^-基团中氧同位素干扰；通过测定锆、钌、钼混合样品，表明该方法可减小锆、钌同量异位素干扰。实验对天然钼及裂变产物钼同位素比值进行了测定，通过数据修正扣除燃料元件中天然钼干扰，可获得燃料元件裂变产物中钼同位素比值。

Determination of Molybdenum Isotope Ratios in Fission Products of Nuclear Fuel by Negative Thermal Ionization Mass Spectrometry

Molybdenum isotopes of ⁹⁵Mo,⁹⁷Mo, ⁹⁸Mo and ¹⁰⁰Mo are the important fission products in nuclear fuel. In order to determine the molybdenum isotope ratios of irradiated nuclear fuel element, MoO₃^- was detected by negative thermal ionization mass spectrometry (NTIMS) using SrCl₂ as an emitter. As a result, 0.1 μg Mo sample loaded onto single rhenium filament can obtain stable ion beam for about 2 h, and relative standard deviation (RSD) of molybdenum isotope ratio determination was less than 0.5‰. 7 molybdenum isotopes were measured using 5 Faraday cups with 1 peak jumping. The m/z 143 signal was collected before and after the peak jumping, respectively, and selected as the denominator of isotope ratio, which avoided the signal fluctuation error. Background level of natural molybdenum was evaluated, which indicated that background of rhenium filaments and reagents can be ignored in the process of mass spectrometry determination. Oxygen isotope interference in MoO₃^- ions was deducted by means of solving matrix equation group. The determination results of ruthenium, molybdenum and zirconium mixed samples show that the method can reduce the isobaric interferences of zirconium and ruthenium. The natural molybdenum interference in spent fuel element was deducted by data correction, and the molybdenum isotope ratios of fission product were obtained.