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The spallation reaction is provoked by bombarding target nuclei with high energy (few hundres MeV) nucleons, such as accelerated protons. During a first stage, called the intranuclear cascade, the incident particles "see" and interact individually with the constituent nucleons (neutrons and protons) of the target. This leads to the emission of very energetic secondary particles (mainly neutrons and protons, some alpha-particles, pions, etc.). In a second stage, the target nucleus is left in a very high excitation state and will de-excite by evaporating mainly a large amount of "low" (few MeV) energy neutrons. High energy fission also produces extra secondary particles. |
These phenomena can be complemented by the classical nuclear fission process at lower energy and in presence of fissile nuclei as target material. As a result of these processes, one can obtain a large amount of spallation neutrons depending on the initial energy of the incident particle and on the atomic number of the target nuclei. For example, a lead target bombarded with 1 GeV protons can yield about 25 neutrons per incident proton, at 600 MeV one expects about 13 neutrons per incident proton (from the spallation reaction alone). Due to the very high power density one can reach in the target, the most appropriate solution would be to use a liquid metal target. The Pb-Bi eutectic is chosen for MYRRHA because of its low melting temperature (123 °C).