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

为了检测空气中痕量的碘分子，采用LaB_6ds包覆铼线作为离子发射器，将负表面电离型离子源连接到带有电子倍增器的商用质谱仪上。在离子源周围的表面温度约为1400 K，样品气体压力约为3 × 10 <-5> Torr的实验条件下，电离表面的功函数约为2.7 eV，由于碘分子的离解度和碘分子的离解度基本一致，因此碘分子的电离效率实际上是由其粘附在表面的概率决定的。正如我们的负表面电离理论所预测的那样。为了减少系统内的碘污染，对易于控制的空气中碘浓度的样品气体入口系统进行了多次改进，并通过初步实验找到了系统的最佳操作条件，然后测量了碘离子负电流与碘浓度的函数关系，从而得出了这样一个校准图，即电流与浓度在50到0.1 ppm之间成正比。这一结果表明，该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.