Fission-fragment mass separator and the nuclear charge distribution of fission fragments of a single mass

Abstract

An energetic fission fragment passing through a gas changes its charge frequently by electron capture and loss collisions with gas molecules. Its average charge e is found to be very nearly proportional to velocity, so that magnetic deflection depends only on the mass; thus, passing fission fragments through a gas-filled magnetic spectrograph gives mass separation. At low pressure the resolution is limited by the statistics of the electron capture and loss process; increasing the pressure to reduce this effect is eventually limited by energy loss and the resulting multiple scattering. However, before this occurs there is a pressure range where the resolution is limited by the nuclear charge distribution of fragments of a single mass. All of these effects were studied experimentally. The best resolution obtained for mass-97 fragments was 3.5 % full width at half maximum (with 20 mm-Hg of helium), of which 2.2 % is due to the width of the nuclear charge distribution. Combining this with the results of a study of the dependence of e on nuclear charge, gives the width of the nuclear charge distribution for mass-97 fragments as 2.4±0.5 units. This is in agreement with determinations from yields of shielded isotopes, but is in disagreement with the predictions of most theories. The best obtainable resolution from the mass separator is 4.5 mass units for light fragments. Several applications of the separator are described and others are suggested.