A sensitive technique is described to search for a dependence of delayed neutron spectra on fast vs. thermal fission of U-235.The results of such a search are given.
EXPERIMENTAL METHODOur general method1 for measuring composite delayed-neutron spectra following U-235 fission has been optimized to perform a search for any dependence which the spectra may have on the energy of neutrons which induce fission. In this method, fission products produced in a fission chamber are transferred by helium jet and tape transport to a beta-neutron time-of-flight (TOF) spectrometer located in a low-background counting room. Neutron TOF spectra are measured for successive delay-time intervals following fission.In seeking differences in spectral structure due to energy dependence, it was clearly desirable to perform the thermal/fast measurements successively for each delay time interval under as nearly identical experimental conditions as possible. This required that both sets of measurements be made using our 5 . 5 MV Van de Graaff accelerator and the Li-7(p,n) reaction as a neutron source, whereas our earlier thermal measurements were performed with the fission chamber in the thermal column of the U. Lowell 1-MW reactor.To make thermal neutrons the dominant mode for inducing fission using the accelerator, the fission chamber and thick-lithium target assembly were encased in a paraffin block, approximately 0.3 m3 in volume. For the fast neutron measurements a nearly bare geometry was used, with the fission chamber wrapped in cadmium to shield against any residual thermal neutrons. Indeed, our concern in this case was not so much with a thermal neutron component, which can be very effectively removed with cadmium, but with a possible epithermal component for which no equally good shield exists. A truly bare geometry was undesirable for the fast neutron measurements since this would have created an unacceptable neutron * Supported by the U.S. National Science Foundation 751 Downloaded by [University of Calgary] at 00:30 05 February 2015 78 W. SCHIER ET ,4L.background in neighboring areas, including the counting and control rooms. To provide effective shielding it was, therefore, necessary to construct a neutron cavity to house the target and fission chamber and several different cavity geometries were examined.Our final geometry consisted of a closed, vertical, and approximately cylindrical cavity, 1 m in diameter and 2.2 m high, generated by an inside corner of the concrete shield walls and large water containers. The fission chamber and target assembly were positioned at its center. In arriving at this geometry a study of the transferred fission-fragment rate, as indicated by the beta count rate at the spectrometer, enabled changes in the fission-fragment production rate to be monitored as the shield geometry was changed.Measurements made with and without the cadmium shield enabled the ratio of fast-plus-epithermal to thermal fission rates to be determined for various geometries.For the small cavity provided by the paraffin encl...
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