It has been suggested 1 that the fission-neutron multiplicity (v) might be a function of incident neutron energy in the resolvable resonance regions of the common fissile nuclides. We have measured the energy dependence of V, for a 239p u sam pi e) f or incident neutrons in the range from 20 to 100 eV. The results show that the multiplicity values do vary from resonance to resonance, and fall into two distinct groups that appear to be correlated with the known spins of these levels.For the experiment reported here the neutrons required for the time-of-flight measurements were produced by the Rensselaer 90-MeV electron linac, and the fission measurements carried out at the end of a 25-m flight path. Fission events were detected by the occurrence of a coincidence between the signals from a fission ionization chamber 2 located in the neutron beam and signals from a gadolinium-loaded liquid scintillation tank that surrounded the chamber. The fission neutrons were detected, after thermalization in the scintillant, by means of the gamma radiation emitted following neutron capture in the gadolinium. The detected neutron multiplicity for each of 256 time-of-flight channels were first stored in a buffer and then transferred to a 256x12 array in the memory of an on-line computer. Thus, if a fission event were recorded at time of flight i with v neutrons detected, the event would have been stored in the zth time-of flight channel of the i4h section of the computer memory. others. The principal differences between this experiment and the others can be attributed to the necessity of working with reaction rates associated with a very broad spectrum of neutron energies rather than a monoenergetic beam. The methods used to correct our multiplicity data for time-dependent (a) background events, (b) accidental coincidences, and (c) scaler dead-time effects are briefly described below.(a) In general, the background multiplicity per fission gate is correlated with the scattering, capture, and fission cross sections of the material present in the fission chamber. A comparison of the upper graph in Fig. 1 (background events per gate) with the lower (fission rate) indicates the character of the background correlations that we observed in this experiment. In order to determine the number of background events per gate, background sampling pulses from a free-running pulse generator were used to initiate simulated fission coincidences. The multiplicity data associated with the sampling pulses were stored in a temporary buffer. If no fission coincidence was detected during the 240-/xsec interval centered on the opening of the gate, then the number of the time-of-flight channel and the value of the multiplicity were stored in an additional 256x12 array in the computer.(b) The fission chamber and associated electronics were designed to minimize the likelihood of detecting alpha particle pile-up events. Nevertheless, there was a reasonable probability that Measurements were made of the fission-neutron multiplicity associated with eleven 239p u...
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