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Experimental Investigation of Fast Neutron Decays in Lead Assemblies

L. E. Beghain, F. Hofmann, S. Wilensky

Nuclear Science and Engineering

Volume 27 / Number 1 / January 1967 / Pages 80-84


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A pulse of monoenergetic fast neutrons of several nanoseconds duration is used to excite a lead assembly. The neutron decay is measured as a function of assembly size with conventional nanosecond time-of-flight equipment. The neutron detector is biased to eliminate all nonelasticly scattered neutrons. A theoretical relationship has been developed by Paik which relates the assembly size to the measured decay constant. The nonelastic cross section appears as a parameter in Paik's theory and can be chosen to give the best fit to the experimental data. Decay constants were measured at 2.1 and 1.7 MeV for lead assemblies 20-in. wide x 20-in. high and thicknesses from 1 to 8 in. Paik's theory assumes the establishment of an asymptotic spacial decay mode. This assumption was verified by measuring the neutron decay at various positions of the assembly. The results show that it takes the order of 10 to 15 nsec to establish a spacial mode. This method has been used to measure the total nonelastic cross section for lead at 2.1 and 1.7 MeV.

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