The Physics of Spallation Processes

2.3. THE EUROPEAN SPALLATION NEUTRON SOURCE ESS 132.3 The European Spallation Neutron Source ESSThermal neutron flux [n/(cm 2. s)]10 1810 1510 1210 910 610 3Berkeley 37-inchcyclotronNRXX-10CP-10.35mCi Ra-Be sourceChadwickMTRCP-2NRUHFIRHFBR10 01920 1930 1940 1950 1960 1970 1980 1990 2000 2010 2020ILLKENSTohoku LinacYearSNSESSISISIPNS LANSCESINQ-IISINQFission reactorsParticle driven (steady state)Particle driven (pulsed)trendline reactorstrendline pulsed sourcesFigure 2.2: Evolution of the performance of reactors and pulsed spallation sources.Fig. 2.2 provides insight to trends in the developments in neutron reactor and acceleratorfacilities over time, starting with the discovery of the neutron by Sir James Chadwickin 1932. The figure compares the peak thermal neutron flux versus the year the facilitybegan or is scheduled for operation. Note that the figure compares peak fluxes forboth steady-state and pulsed sources, and even though the pulsed sources have high peakfluxes, they lack the integrated neutrons of steady-state sources; and while peak flux isan important parameter for neutron scattering, total flux is needed for such objectives astransmutation or isotope production. While reactor sources have leveled off, acceleratorbasedsources show considerable promise for even higher intensities in the future.Advances in accelerator design and technology, new approaches to instrumentationdesign and measurement techniques, and increasing difficulties in gaining environmentalacceptance for nuclear research reactors (especially in Germany) have all contributed tothe trend toward using accelerators for the new sources being planned. As for examplewith a beam power more than 30 times that of ISIS and improved instrumentation forpulsed sources, effective intensity gains up to three orders of magnitude over ISIS canbe anticipated in some cases through combination of source brightness and instrumentdevelopment. The European Spallation Source ESS [ess02-III, Gol02] being discussed inmore detail in the following is found to be the most ambitious project besides the SNSis the US. However a high power spallation source like the ESS, injecting a 1.334 GeV/ 62.5 A peak current pulsed proton beam into liquid Hg at a 50 Hz repetition rate hasnever before been realized. Therefore technical boundaries need to be pushed beyondpresent limits. Furthermore only a limited amount of both theoretical and experimentaldata on the nuclear spallation process, cross sections and reaction products are availablefor such target stations. One of the goals of this work is to provide such nuclear data forspallation sources.