Development of negative-ion mass spectrometry for monitoring trace amounts of radioactive iodine molecules included in off-gasses of a nuclear fuel reprocessing plant

为了检测空气中微量的碘分子，将一个以LaB_6ds涂层的金属丝为离子发射体的负表面电离型离子源与一个带电子倍增器的商用质谱仪相连。在表面温度约为1400 K，样品气压约为3 × 10 ~（-4）Torr的实验条件下<-5>，电离表面的功函数约为2.7 eV，碘分子的电离效率实际上取决于其粘附在表面上的几率，因为I_2的离解度和I的电离度基本上是1，这与我们的负表面电离理论一致。在对空气中碘浓度易于控制的样气入口系统进行多次改进以减少系统内的碘污染后，并且在通过初步实验找到操作系统的最佳条件后，测量了负碘离子电流作为碘浓度的函数，可以产生这样的校准曲线，即电流在50和0.1 ppm之间的范围内与浓度成比例。这一结果表明，本方法可用于监测核燃料后处理设施废气中的放射性碘分子。

In order to detect the trace amount of iodine molecules included in air, a negative surface ionization type ion source employing an LaB_6ds -coated rhenium wire as an ion emitter was connected to a commercial mass spectrometer with an electron multiplier. Under the experimental condition that the surface temperature and the sample gas pres- sure around the ion source were about 1400 K and 3 x 10^<-5> Torr, respectively, the work function of the ionizing surface was about 2.7 eV, and the ionization efficiency of the iodine molecule was governed practically by its sticking probability to the surface because the degrees of dissociation of I_2 and of ionization of I were essentially unity, just as prodicted by our theory on negative surface ionization. After a sample gas inlet system readily controllable of iodine concentration in air was improved many times in order to decrease iodine contamination inside the system and also after the best condition for operating the system was found by preliminary experiments, the negative iodide ion current was measured as a function of the iodine concentration, theraby yielding such a calibration plot that the current was proportional to the concentration in the range between 50 and 0.1 ppm. This result suggests that the present method may be employed to monitor radioactive iodine molecules included in the off-gases of nuclear fuel reprocessing facilities.