ANSTO LUCAS HEIGHTS, AUSTRALIA 15 NOVEMBER 2012
History of Spallation Neutron Sources John M. Carpenter IPNS, Argonne National Laboratory and SNS, Oak Ridge National Laboratory
“Spallation”: the word W. H. Sullivan and Glenn T. Seaborg, at Lawrence Radiation Laboratory, Berkeley, California, coined the term “spallation” on 20 August 1947*. They intended the word to designate the process, already fairly well-known, in which a nucleus struck by a high-energy particle emits a rather large number of nucleons (mostly neutrons) or fragments. The products include practically all the nuclei of smaller mass number than the target nucleus that lie on the neutron-poor side of the line of stability, and most of the lighter nuclei. * B. G. Harvey, Ch. 3 “Spallation” in Progress in Nuclear Physics, Editor O. R.
Outline Early Knowledge Early Spallation Sources The 10-GeV Question Spallation Source Development Target Developments Operational Experience Present Day Sources
The Spallation-Fission Process Schematic illustration of our modern understanding of the spallation-fission (when fission is possible) process. (Courtesy L. Waters, LANL.)
π First stage: intranuclear cascade
α Intermediate stage: preequilibrium d
Second stage: evaporation and/or fission
t Final stage: residual deexcitation
Frisch, Vol. 7, Pergamon Press pp 90-120 (1959).
Cosmic Ray Protons Cosmic-ray protons (of extra-solar origin) impinge isotropically and steadily on the Earth. Consequently, there is no daily or annual variation in the incident cosmic ray proton flux.
Discovery of Cosmic Rays Victor Hess, 1912 International Herald Tribune 8 August, 2012
The energy spectrum of the incident cosmic-ray protons extends up to tens of GeV, higher than that of solar protons, and can penetrate the Earth’s magnetic field and the atmosphere. The average energy of cosmic-ray protons is higher at lower latitudes than at high latitudes near the poles. The Earth’s field deflects lowestenergy protons, (about 4. GeV at Chicago), depending on the observer’s magnetic latitude. Solar protons cannot penetrate the Earth’s magnetic field because of their lower energies, except near the magnetic poles. The intensity of solar protons varies according to the level of solar activity.
Victor Hess received the Nobel Prize in Physics in 1932
Harold Agnew’s 1944 Flying Neutron Detector (B-29)
Atmospheric Spallation Neutrons: Fermi notes The figure, from Enrico Fermi’s University of Chicago lectures of 1948*, illustrates the cosmic-rayproton-induced neutron flux as a function of atmo-spheric depth. There are always neutrons around us; the thermal neutron flux at the Earth’s surface is on the order of 10-4—10-3 n/cm2sec.
Agnew’s result, exp(-0.083 Pcm Hg), corresponds to that in Fermi’s book for low magnetic latitudes, exp(-xgm/cm2/160), when related according to the density of mercury, 0.083 Pcm Hg/13.55gm/cm3= xgm/cm2/163. Earth’s atmosphere is equivalent to a layer of about 10 meters of water. The cosmic-ray-induced neutron flux varies considerably according to the barometric pressure (daily weather-dependent variations in the thickness of the atmosphere). Heavy shielding around detectors can increase the neutron flux nearby. This is important in detector testing activities and in measurements of low counting rate phenomena.
*J. Orear, A. H. Rosenfeld, and R. A. Schluter, “Nuclear Physics,” revised ed., The University of Chicago Press, Chicago (1949).
Tidal Effects The rising tide covers and the receding tide exposes heavy material (rocks) that produce more spallation neutrons than the water. The local cosmic-rayproduced neutron background varies with the tides.
Harold Agnew, APS Mtg. 1946
The neutron flux is lower but less strongly attenuated by