Fig. 11-3 Energy spectra of neutrons emitted at several angles from a Au target bombarded by 68 MeV protons Circles represent the measured data. Black lines denote data calculated by the original cascade model, and red lines indicate data calculated by the improver ISOBAR model, respectively.

In a nucleus bombarded by high energy protons, the spallation reaction often occurs with the emission of many particles. To upgrade the accuracy of data calculated in the code which simulates spallation reaction mechanisms, the calculational model has been improved and checked by measuring data for energy spectra of neutrons emitted at several angles. It is well known that the ability to predict cascade behavior using the cascade-nucleon/meson transport code (NMTC/JAERI) is insufficient for reactions in a nucleus bombarded by a proton with energy less than 200 MeV because it approximates the high-energy intranuclear reaction as a two-body collision process between free nucleons. Now, two improvements on the intranuclear cascade model were made, 1) addition of the pre-equilibrium process between the cascade process and the successively-occurring equilibrium process (3STEP), and 2) adoption of the nucleon's nuclear medium effect, including reflection and refraction to take into account the wave motion of a nucleon acting in a nucleus (ISOBAR). After improvements, the calculated data have been compared with the measured data from proton irradiation experiments on carbon, copper and gold targets. It appears that the prediction accuracy was greatly improved for nuclear reactions induced by lower-energy protons (20 ~ 100 MeV). Fig. 11-3 shows a comparison of measured data and calculated data before and after improvement, for the energy spectrum of neutrons emitted at each angle from a gold target injected by 68 MeV protons. For an energy larger than 20 MeV, the data calculated by the original model (black solid line) reproduces relatively well the measured data in the direction of 60 degrees, overestimates data at 15 degrees ~ 30 degrees, and underestimates data at 120 degrees. However, data obtained by using the improved model (red solid line, ISOBAR) agreed well with the measured data in the entire energy region and at all angles.

Reference H. Takada, Analysis of Proton-injected Target Experiment with the Intranuclear Cascade Model, Genshikaku Kenkyu, 41(3), 39 (1996).

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