Use of gridded ionisation chamber for measuring fission fragment angle

Abstract

Measurement of the angle of fission fragments with the electric field direction of a gridded ionisation chamber is described and the angular resolutions which can be achieved by this method are experimentally deduced. The present measurements are carried out in thermal neutron fission of 235U and for each fission event the angle of emergence of the two fission fragments with respect to the electric field direction on either side of the ionisation chamber was determined by recording the coincident collector and grid pulses. The distribution of this difference (θ 1−θ 2) for different fragment mass windows has been obtained and the fwhm of the angular resolution function is estimated to be 8°–11°. Some results on the fragment mass and total kinetic energy distributions obtained when the fragments were electronically collimated into a cone perpendicular to the source foil are given to bring out the improvement that can be achieved by eliminating energy-degraded fragments emerging at large angles in a gridded ionisation chamber.