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The use of neutrons from a nuclear explosion for physics research such as the measurement of fission cross sections of various isotopes by the time of flight method has a number of advantages over other pulsed neutron sources [1].In measurements reported in [2] fission fragments were recorded by semiconductor detectors. The processing and recording of the current sigalal from the detector requires complex electronic equipment. The system requires careful shielding against scattered neutrons and gamma rays and electromagnetic induction.It is therefore expedient in making measurements with neutrons from a nuclear explosion to use polymeric film as a fission fragment detector since it is insensitive to other kinds of radiations.In our experimental arrangement for measuring fission cross sections a film of polyearbonate with a molecular weight of 90,000 developed by the Scientific-Research Institute of Plastics was fastened to the rim of a eylinder (Fig. 1) rotating ~104 rpm at the instant of the neutron pulse.Layers of fissionable isotopes were placed in the neutron flux close to the polycarbonate film, and fission fragments passing through a narrow collimator fell on the film forming tracks. The number of tracks on the part of the film corresponding to the time interval between t and t + At is where af(E n) is the fission cross section, Nnu is the number of fissionable nuclei in the layer, ~2 is the efficiency of counting fragments determined mainly by the sizes and relative position of the layer and the collimator, {I, is the local neutron flux at time t, and t is the neutron time of flight.The time and energy resolution in the procedure described are determined by the angular velocity of the cylinder, the flight path, and the width of the fission fragment collimator slit:1 Fig. 1. Geometry of experimental arrangement: 1) collimators with layers of fissionable isotopes; 2) collimator with layers for measuring the background of scattered neutrons; 3, 4) polymeric films.

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