Fig. 6-2 Variation of particle distributions as a function of time when a 5-GeV proton impinges on iron In this figure, the full red, full blue, hollow cyan, and hollow red circles denote protons, neutrons, delta- or N*-particles, and pions, respectively. The time unit is 3 x 10-24 second. The center-of-gravity is fixed on the origin. In this reaction many particles are emitted from the nucleus. The reaction is called a "spallation" reaction.

We cannot observe nuclear reactions with our eyes because the nuclei are too small, typically on the order of 10-12cm. Usually the shapes and reaction mechanisms of nuclei are investigated by producing collisions between various particles (such as neutrons and protons) and nuclei. The distributions of emerging particles are then detected. Such reactions, however, can be observed with the aid of three-dimensional computer graphics by simulating the nuclear reactions on high-speed computers. A method called Quantum Molecular Dynamics (QMD), which is a version of molecular dynamics modified for the nuclear reactions, has been employed. Figure 6-2 shows a nuclear reaction induced by a 5-GeV proton incident on iron. The time origin is at the top left; time evolves toward the bottom right. In this reaction, the incident proton completely penetrates the iron nucleus at T = 10 and produces several delta- and N*-particles. After that, a considerable number of pions and nucleons are emitted. Such a complicated reaction is known as a spallation reaction. In this way, the processes occurring in the very middle of a nuclear reaction can be identified. This provides a better understanding of nuclear reactions.

Reference S. Chiba et al., Nucleon Induced Preequilibrium Reactions in Terms of the Quantum Molecular Dynamics, Phys. Rev. C53, 1824 (1996).

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