The Physics of Spallation Processes

Chapter 1IntroductionNeutron production in spallation reactions induced by energetic particles in heavy targetshas been observed already in the late 40’s. As a result of a continuous progress in acceleratortechnology, the construction of powerful intense spallation sources became possible,providing new opportunities for solid state physics, life and material science. In fact, avariety of projects have been initiated recently, including the construction at the PaulScherer Institute (SINQ) [Bau96] of an accelerator-based, continuous neutron source andincluding several pulsed, high-intensity neutron sources, planned or under construction.Among the latter projects are the ambitious 10 MW European Spallation Neutron SourceESS [ess02-III, Gol02] 1 , the 2 MW Spallation Neutron Source SNS [App95, sns02] in theUS, and the Japanese facility J-PARC at KEK/JAERI [Nag99]. As genesis or originatorof all spallation sources the Intense Pulsed Neutron System (IPNS) [Car78] as a nationalfacility for condensed matter research realized at the Argonne National Laboratory maybe considered. Studies for intense neutron generators (ING) based on Pb-Bi targets and atremendous beam power of 65 MW (1 GeV proton beam) have been reported already 1966by G.A. Bartholomew and P.R. Tunnicliffe [Bar66] - however the project was terminatedin 1968. A very good review of early work before 1978 can be found in ref. [Bar78].Intense, short-pulse neutron beams from accelerator-based sources make it possible tostudy a wide range of scientific problems via neutron scattering, exploiting time-of-flighttechniques and allowing kinetic studies of various processes. In addition, powerful neutronsources, such as the sub-critical spallation/fission hybrid reactors [Nif99, Rub95, Rub97],provide a basis for various, potentially important applications often entitled as ADS—“Accelerator Driven Systems”. For example, such facilities may be used to effectivelyproduce tritium[Bro96] or to achieve the incineration or transmutation of radioactive nuclearwaste [Bow92, AIP94, Bow96b, Ven96, Fil97, ENEA01]. It is also important thatthe accelerator-based neutron sources are much more acceptable from the environmentalpoint of view than nuclear reactors and that they show greater promise for future improvementsin peak neutron intensities. However the notion to operate the planned highintense European neutron facility ESS also as a “multi-purpose-facility” like the Japaneseproject has meanwhile been disapproved due to consolidated findings of the CONCERTstudy [Con01].1 In the original ESS feasibility study of 1996 [ess96-III] the spallation source was planned to have twoshort pulse target stations and 5 MW power. The new concept favors both a so called long pulse and ashort pulse target station with 5 MW each.1