New possibilities to prepare enriched isotope targets

Isotope targets are very important in nuclear research. But to use separated isotopes as evaporating sources may be very expensive and usually needs special evaporating equipment. In principle, circularly shaped isotope targets can be made in a single preparation starting with a feed of low natural abundance by a quadrupole mass filter (QMF). A normal quadrupole can be realized by using hyperbolic electrodes, but abnormal quadrupoles constructed by circular rods are often used for the ease of machining. In the preparation of targets using such a QMF, elements other than a given mass number may deposit on the rod surfaces, and if a relatively large amount of evaporants is processed, the geometric tolerances due to deposits may affect the original performance and cause difficulties in operation. In order to attain quasi-hyperbolic fields by improved structures other than circular rods, potential distributions of five categories of abnormal quadrupoles (with nine different designs) are studied by the finite element method, which can give the best fit to the curvilinear boundaries and through which denser nodes can be arranged in the region of interest. Computer analyses have been carried out to compare the respective working regions of the different designs, in which the potential deviations to the exact hyperbolic field do not exceed +- 1%. Finally the proposed QMF and its predicted performances are calculated for the preparation of 144Sm targets. (orig.)