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Mitigation of Background in Gamma-Ray Spectroscopy
Published in Douglas S. McGregor, J. Kenneth Shultis, Radiation Detection, 2020
Douglas S. McGregor, J. Kenneth Shultis
Because neutrons are absorbed mainly when they have near thermal energies, a neutron shield for fast neutrons first attempts to slow the neutrons through scattering interactions (or moderate them) and then absorb them with materials such as cadmium, boron, or lithium, that have large thermal-neutron cross sections. To moderate neutrons rapidly, materials such as water or polyethylene with a high hydrogen content are normally used because the lighter the scattering nucleus, the more energy a neutron loses on a scatter. However, for cosmic ray neutrons with energies above 10 MeV, the cross section for scattering from hydrogen decreases rapidly and becomes ineffective at slowing the neutrons. Consequently,concrete used for neutron shielding against fast neutrons usually contains high Z materials, such as iron, which slow fast neutrons through inelastic scattering (and also produce unwanted inelastic scattering gamma rays). Once the neutrons are slowed below 1 MeV, hydrogen can then quickly moderate the neutrons towards thermal energies where they are absorbed by impurities or by materials with large neutron absorption cross section that are added to the moderating material.
Radioisotopes: their characterization and interaction with matter
Published in R.J. Pentreath, Nuclear Power, Man and the Environment, 2019
Neutrons, as we have seen, are present in the atomic nucleus and have no charge. Free neutrons are produced when alpha or gamma radiations interact with light elements. A typical reaction is that of an alpha particle and beryllium (9Be) which results in a compound nucleus of carbon being formed, 13C, in an excited state. The compound nucleus immediately rids itself of the excitation energy by releasing one neutron of high energy, becoming 12C. Another method of neutron formation is nuclear fission. A number of very heavy nuclei decay by spontaneous fission in which the parent nucleus breaks very approximately in half and releases a number of neutrons: the fission can also be induced, as will be discussed in chapter 4. Neutrons with high energies, in excess of 0.1 MeV, are termed fast neutrons, and those with very low energies (~ 0.025 eV) are termed thermal neutrons because they are in approximate thermal equilibrium with their surroundings. Thermal neutrons are obtained from fast neutrons by deliberately slowing them down, a process called moderation.
Nuclear Energy
Published in Efstathios E. Michaelides, Energy, the Environment, and Sustainability, 2018
While it is rather easy to visualize the chain reaction, its practical realization with fuels other than pure 92U235 poses several difficulties. The main problem is that neutrons produced from a fission reaction are fast neutrons with energies approximately 2 MeV, while fissions are primarily caused by slower, thermal neutrons with energies 0.025 eV or lower. The retardation of neutrons in a reactor is caused by a large number of collisions of the neutrons with the nuclei of lighter atoms, such as hydrogen (1H1), deuterium (1H2), or carbon (6C12). The first two types of atoms are common in water and heavy water and the last in graphite. The materials that slow down the neutrons—and their conversion from fast neutrons to thermal neutrons—are called moderators.
Nuclear Science for the Manhattan Project and Comparison to Today’s ENDF Data
Published in Nuclear Technology, 2021
Existing accelerator equipment was brought to Los Alamos from various universities across the country. Wisconsin provided two Van de Graaff electrostatic voltage generators (2.4 and 4 million volts), allowing the creation of quasi-monoenergetic neutrons from a few tenths of an mega electron-volt up to 1.8 MeV using an Li(p,n) reaction and up to 6 MeV with D + D reactions. The “short-tank” accelerator had been built mainly by Joseph McKibben, and most of the group there came to Los Alamos. Dick Taschek got his PhD under Breit at the University of Wisconsin in 1941, and both Benedict and Hanson finished their PhD theses in 1943 under Ray Herb, the architect of the best Van de Graaffs.72 These Van de Graaff machines, referred to as the “Short Tank” and “Long Tank” proved to be the most useful ones at Los Alamos for precision fission experiments in William’s group. Illinois provided a 600-keV Cockroft Walton generator, creating 2.5- to 3-MeV neutrons with D + D reactions. Manley had worked with this machine at Illinois and proved to be Oppenheimer’s “right-hand man” in assembling all the accelerator equipment; having moved to Chicago, he put Harold Agnew in charge of moving the Illinois machine to Los Alamos and putting it in the basement of the Z building for his group. For Robert Wilson’s group, a cyclotron was provided by Harvard that could produce protons up to 7 MeV and deuterons up to 11 MeV. Thermal neutrons were produced following a D + Be reaction with subsequent moderation in graphite, as well as neutrons with energies in the 0.001 to 100 eV range using time-of-flight methods. Manley describes73 the process by which “I was the one in charge of getting all those damned machines up to Los Alamos,” and the impressive feat of getting them all operational by July 1943.