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Sources of Radiation
Published in Douglas S. McGregor, J. Kenneth Shultis, Radiation Detection, 2020
Douglas S. McGregor, J. Kenneth Shultis
In a spallation neutron source, pulses of very energetic protons (up to 1 GeV), produced by an accelerator, strike a heavy metal target such as mercury or liquid bismuth. Such an energetic proton when it strikes a target nucleus “spalls” or knocks out neutrons. Additional neutrons boil off as the struck nucleus heats up. Typically, 20 to 30 neutrons are produced per spallation reaction. These pulses of neutrons are then slowed down or thermalized by passing them through cells filled with water, or even liquid hydrogen if very slow neutrons are needed.
Light Scattering by Polymer Solutions
Published in Timothy P. Lodge, Paul C. Hiemenz, Polymer Chemistry, 2020
Timothy P. Lodge, Paul C. Hiemenz
A detailed description of a SANS instrument is beyond our scope, but a generic configuration consists of a source, a wavelength selector, beam guides, sample environment, and a two-dimensional detector (see Figure 8.14). The neutrons can be created in a reactor (such as at the National Institute of Standards and Technology National Center for Neutron Research, the High Flux Isotope Reactor at Oak Ridge National Laboratory, or the Institute Laué-Langevin in Grenoble) or at a spallation source (Spallation Neutron Source at Oak Ridge or ISIS at Rutherford Laboratory, UK); we will focus on the former. The wavelength can be selected by Bragg diffraction from a crystal such as silicon, or by a velocity selector; the latter has the virtue of tunability and higher flux, but at the cost of larger wavelength spread (Δλ/λ ≈ 0.1 − 0.2). The beam guides shape the beam over a distance of several meters, but it is important to emphasize that the instrument components we take for granted in optical experiments (lenses, mirrors, fiber optics, polarizers, etc.) are much less refined for neutrons. The beam dimension at the sample is defined by an aperture, and is typically ≈ 1 cm in diameter. The area of the detector is about 1 m2, with either 64 × 64 or 128 × 128 pixels. Each neutron is detected after it collides with a 3He atom; this sets off an exothermic ionization cascade that is detected as a spatially and temporally resolved pulse of electrical current. Thus the detector records a very high fraction ε of all incident neutrons. The detector is mounted on a rail in a large evacuated chamber, and can be moved automatically from 1 − 15 m from the sample, in order to tune the accessible q range (see Problem 8.33). The largest accessible q is determined by the outermost pixels of the detector; the lowest usable q is set by the diameter of the beamstop, which prevents the unscattered beam from damaging the detector.
Preliminary Neutronics Study of an Accelerator-Driven Molten Spallation Target–Molten Lithium Source of Tritium
Published in Fusion Science and Technology, 2023
Michal Cihlář, Slavomír Entler, Tomáš Czakoj, Václav Dostál, Jan Prehradný, Pavel Zácha
The Spallation Neutron Source (SNS), part of Oak Ridge National Laboratory, is a neutron source that provides the most intense neutron beam in the world. The spallation neutrons are produced by proton bombardment of a liquid mercury target. The spallations are then used for material research, fundamental physics research, and many other applications. An upgrade of the current SNS accelerator is planned in the coming years.27