The Physics of Spallation Processes

3.1. THE SPALLATION PROCESS 313.1 The Spallation ProcessThe definition found in Nuclear Physics Academic press:”Spallation—a type of nuclearreaction in which the high-energy level of incident particles causes the nucleus to ejectmore than three particles, thus changing both its mass number and its atomic number.Also, nuclear spallation” has to be slightly specified in the context of accelerator drivensystems or high intense neutron sources. Here spallation is the disintegration of a nucleusby means of high energetic proton induced reactions. Typically approximately 20 neutronsare created per incident GeV proton. This is 20 times as much as for a fission reaction ina conventional nuclear power plant with energy spectra of the neutrons similar up to theevaporation regime, but extending to higher kinetic energies up to the incident protonenergy in case of spallation reactions as shown in Fig. 3.1.Neutron Production10 -110 -210 -310 -410 -510 -610 -4 10 -3 10 -2 10 -1 1 10 10 2 10 3Figure 3.1: Neutron kineticenergy spectra from a fissionreactor and from spallation(800 MeV p, Los AlamosNeutron Scattering Center-LANSCE). In order to facilitatethe comparison the integralsof the spectra havebeen normalized to unity.Data adapted from [Lan91].Energy (MeV)When a high energy hadron (or lepton) interacts with a nucleus of the target materialcausing an intra-nuclear cascade (INC) inside the nucleus within a time scale of the orderof 10 −22 s, many secondary particles (n,p,π-mesons) are emitted which could themselveshave a high enough energy to produce further secondaries when they interact, thus creatingan inter-nuclear cascade, placing many individual nuclei into highly excited states asschematically shown in the upper panel of Fig. 3.2. The nuclei then release energy byevaporating nucleons (mainly neutrons), d, t, α’s and γ’s, some of which will leave thetarget. The process of evaporation taking place within a much longer time scale 1 of 10 −18to 10 −16 s may be characterized by a nuclear temperature T = 2 . . . 8 MeV, so that forthe spectrum of emitted neutrons Maxwellian distributionsdφ(E n ) = E ( )nT exp −EndE 2 n (3.1)Temerge, with E n being the kinetic energy of the emitted neutrons. Also unstable secondaryparticles can be created which may have a sufficiently long lifetime that a part of themwill have time to interact before they decay or, if they do decay, form particles whichthemselves have to be taken into account. Leptons however only rarely interact withnuclei but, if they are charged they will contribute to the radiation field by ionization1 depending strongly on the thermal excitation energy E ∗ of the hot nucleus