Measurement of Activation Reaction Rate Distributions in a Lead Assembly Bombarded with 500-MeV Protons

Reaction rate distributions of various activation detectors such as the ^natNi(n,x)⁵⁸Co, ¹⁹⁷Au(n,2n)¹⁹⁶Au, and ¹⁹⁷Au(n,4n)¹⁹⁴Au reactions were measured to study the production and the transport of spallation neutrons in a lead assembly bombarded with protons of 500 MeV. The measured data were analyzed with the nucleon-meson transport code NMTC/JAERI combined with the MCNP4A code using the nuclide production cross sections based on the JENDL Dosimetry File and those calculated with the ALICE-F code. It was found that the NMTC/JAERI-MCNP4A calculations agreed well with the experiments for the low-energy-threshold reaction of ^natNi(n,x)⁵⁸Co. With the increase of threshold energy, however, the calculation underestimated the experiments, especially above 20 MeV. The reason for the disagreement can be attributed to the underestimation of the neutron yield in the tens of mega-electron-volt regions by the NMTC/JAERI code.