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).
The high power (density) in the target resulting from the heat deposition by the proton beam and the space limitations for optimal neutronic performance, asks for forced convection heat removal by liquid metal. The high proton current density leads to a windowless target design: the separation between the beam vacuum and the target material is the liquid metal free surface itself.
The Pb-Bi eutectic is chosen for MYRRHA because of its low melting temperature (123 °C).
More information: Engineering > MYRRHA spallation target