Nov 15, 2012 - tens of GeV, higher than that of solar protons, and can penetrate the. Earth's magnetic .... launched the
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
high-energy proton
p n
α Intermediate stage: preequilibrium d
Second stage: evaporation and/or fission
t Final stage: residual deexcitation
e±
γ
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
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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 the atmosphere at low magnetic latitudes (nearer the magnetic equator: 190 gm/cm2 ) than at high latitudes (nearer the pole: 160 gm/cm2), indicating that near the pole, the average energy of incident protons is lower than far from the pole(s). The locations of the magnetic poles wander slowly, a few degrees per decade.
Measurements have recently extended to ocean depths
Rolando Granada’s 1989 Submarine Neutron Detector ARA SANTA CRUZ S-41
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Proton cyclotrons generated the first accelerator-produced neutrons, starting in early 1930s. Glenn T. Seaborg was among the first workers.
Evaporation Neutron Spectrum The function shown in the inset has a mean energy of 1.98 MeV. A more accurate form is f(E) = exp(-1.036E)sinh √(2.29E), where E is expressed in MeV. This is, strictly speaking, the spectrum of neutrons produced by fission in 235U, but it applies approximately and in form to most other evaporation neutron spectra. A. M. Weinberg and E. P. Wigner, The Physical Theory of Neutron Chain Reactors, The University of Chicago Press (1958). See 111-115.
Early Spallation Sources: MTA
The MTA
E. O. Lawrence conceived the Materials Testing Accelerator (MTA) project in the late 1940s. Despite its name, MTA was never intended for materials research, rather, bomb stuff. The required production rate set the parameters of the accelerator—particles, deuterons; beam energy, 500 MeV; CW operation; current, 320 mA; beam power, 160 MW. RF was 12 MHz. Work went on at the site of the present Lawrence Livermore Laboratory. Efforts continued until 1955 when exploration revealed large uranium ore reserves in the US and the project terminated. By that time the preaccelerator had delivered CW proton currents of 100 mA and deuteron currents of 30 mA. The work was declassified in 1957.
people
A. P. Armagnac, “The Most Fantastic Atom Smasher,” Popular Science, p. 115 (Nov. 1959).
MTA Linac The accelerating cavities were very large because available highpower klystrons operated at only 12 MHz. (Now, commonly, 800 MHz.)
ING In 1963, the Chalk River Laboratory of Atomic Energy of Canada launched the Intense Neutron Generator (ING) project. The goal was a “versatile machine” providing a high neutron flux for isotope production and neutron beam experiments. The effort continued until 1968 when the project was cancelled. The proposed installation was based on a 1.5-km-long proton linac delivering 1.0-GeV, 65. mA (65 MW). The target was to have been of flowing lead-bismuth eutectic (LBE), 20. cm in diameter, 60. cm long, with the proton beam incident vertically downward, surrounded by an annular beryllium “multiplier” 20 cm thick. Technical developments that resulted from the ING project were significant, even seminal.
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The ING Facility Canadian scientists conceived the Intense Neutron Generator project in the early 1960s. ING was never built because the proposed accelerator was not feasible at that time. The figure shows the facility layout.
ING Conceptual illustration of the ING target arrangement, a flow-through design in which the protons impinge on the free surface of the LBE. “ING Status Report, July 1967: The AECL Study for an Intense Neutron Generator,” T. G. Church, Ed., Atomic Energy of Canada, Ltd., report AECL-2750 (1967). Also, “The AECL Study for an Intense Neutron Generator, Technical Details,” AECL report AECL 2600 (1966).
Neutron Yields Fraser’s data permit a simple fit for energies up to 1.5 GeV, Y(E, A) =
{
0.1(E GeV – 0.120)( A + 20), except fissionable materials; 238U . 50.(E GeV – 0.120),
Global neutron yields fall off as Ep0.8 for Ep > 1.0 GeV. Also in the mid-1960s, Bertini, notably, and others developed codes to compute the details of the spallation process. The Fraser data stand essentially unchanged in the light of subsequent experience, and the current generation of codes, exemplified by the Los Alamos MCNPX code, are the basis for spallation source design as well as for design of high-energy particle physics detectors.
ING Machine Specifications Proton linac Energy, 1 GeV Length: Alvarez section, 110 m, Waveguide section, 1430 m Total RF power, 90 MW Current, 65 mA (CW) Proton beam power, 65 MW
Neutron Yields In 1965, in support of the ING project, John Fraser and his colleagues studied thick-target neutron yields as a function of proton energy. The figure shows their results. These were the first systematic neutron yield data, which enabled quantitative design of neutron sources and inspired the creation of modern highenergy particle transport simulation codes such as MCNP.
The 10,000,000,000 eV Question: What’s the best energy for spallation neutron production? The power density variation as a function of axial position in a tantalum metal target (no coolant), for various proton energies. Beyond a short buildup range, the neutron production and the power densities diminish exponentially as governed by the ~constant proton-nuclear collision cross section. (At the higher proton energies the end-of-range Bragg peak is insignificant.) The power density and radiation damage rate per unit of beam power at the entry (proton window) end is considerably smaller for the higher energies than for the lower energies. Answer—It doesn’t make much difference: it’s what’s convenient.
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Pulsed Spallation Neutron Sources
Pulsed Spallation Neutron Sources
Developments of proton-driven spallation sources for slowneutron (below about ~100 eV) scattering applications began in the early 1970s. Most of these are “short-pulsed” machines, but there is one MW-level CW proton accelerator (SINQ). And ESS is to be a “long-pulse” system. Pulsed operation provides for momentarily high power and long inter-pulse periods to dissipate heat from the target and components. Based on the spallation reaction, these sources benefit from the fact that the heat to be dissipated is only about 30-40 MeV per neutron, about 60% of the proton beam power. Dense hydrogenous moderators close by the source slow down the ~ 1.-MeV neutrons from the source to useful energies, < ~1. eV, and provide a well defined time origin for time-of-flight spectroscopy. Nearby reflectors (e.g., beryllium) substantially increase neutron beam intensities
Intense, high-emittance negative hydrogen (H-) ion sources developed by the Russian scientists G. I. Budker and G. I. Dimov in the 1960s made possible development of high-current synchrotrons using stripping injection. H- ions from the ion source are accelerated to modest energy (~50 MeV) in a linac, transported into the circular machine through a thin foil in a magnetic field. Passing through the foil, the ion loses its electrons and circulates as a proton, bending oppositely in the field than the H- ions. Protons already captured in the ring pass again through the foil, unaffected. The process allows loading the ring with many (~1000) turns of protons. The procedure does not violate Liouville’s theorem because stripping is an irreversible process. This discovery was adopted for the 500-MeV Booster-accelerator injector for the 12-GeV Zero Gradient Synchrotron (ZGS) at Argonne.
Emission-Time Distribution (pulse shape) 63.3 meV neutrons from a 300-K moderator at IPNS
Moderators Pulsed-source moderators have designable features that can be tailored to instrument needs: Spectral temperature— 10K < T
