Development of Conventional Quadrupole Mass Analyzer for RI Target Analysis and Its Performance

A Knudsen-cell-based quadrupole mass analyzer has been developed for the analysis of a small amount of RI sample. To achieve a high signal/noise ratio, a continuous dynode electron multiplier was installed at the end of the mass filter and was operated in a pulse counting mode. Furthermore, the cylindrical shape molecular trap was set around the mass analyzer to reach high degree of vacuum and to reduce the RI contamination area in the apparatus. The isotope ratio of 135Cs/137Cs samples has been measured to demonstrate the performance of the mass analyzer. Taking the advantage of the Knudsen-cell's precise controllability over temperature, the contribution of the isobar 137Ba was successfully removed in the mass spectrum of cesium. With samples containing about 0.3μg 137Cs and almost the same amount of 135Cs, the isotope ratio measurement with the total error of 3% has been achieved. The molecular trap was also shown to be effective to reduce the contamination area in the apparatus.