![]() Total neutron counting, neutron spectroscopy, and novel coincidence algorithms were developed to analyze measured data and accomplish two objectives - determine the (α,n) neutron yield of a source to an accuracy better than 5%, and perform low-resolution neutron spectroscopy to estimate the (α,n) neutron energy spectrum. The detector system, a moderated He-3 detector array influenced by classic neutron multiplicity counter designs, utilizes digital data acquisition electronics to collect high fidelity neutron detection event data with both temporal and spatial dimensionality. The neutron sources of interest are considered to have unknown material composition, and multiple actinide and light nuclide species may be present such that a variety of (α,n), spontaneous fission, and neutron-induced fission reactions contribute to the total (α,n) and fission neutron yields of a given source. In this work, a neutron detection system was developed at Rensselaer Polytechnic Institute (RPI) for measurements of (α,n) neutron yield of low-rate (nominal ~ 100 n/s) neutron sources emitting a mixed field of (α,n) and fission neutrons. Such complexity lends itself to the use of experimental methods to characterize (α,n) neutron sources of interest, and experimental methods supplement computational development by providing benchmark data for computational model validation. The latter consideration is especially important for (α,n) neutron sources manufactured by mixing actinide ceramic and light nuclide powders, such as AmBe sources. This level of detail is difficult and expensive to obtain for a given source, and also leads to the consequence that every (α,n) neutron source is unique- for two (α,n) neutron sources of the same type, the sources may have different neutron yields because of slight differences in size, density, nuclide composition, and nuclide spatial distribution in the material. As a result, computational models of a given (α,n) neutron source must not only have accurate (α,n) reaction nuclear data but also be detailed to the level of source microscopic composition and physical properties to accurately model alpha particle transport. The root of the complexity is that the alpha particles emitted in radioactive decay which drive (α,n) neutron production have an extremely short range in matter. Because the production mechanism of (α,n) neutrons is complex, the neutron yield of (α,n) neutron sources is difficult to determine computationally. The prevalence of (α,n) neutrons, often overshadowed by their fission neutron counterparts, make characterization of (α,n) neutron sources of significant interest in many fields. The relevance of these materials spans many nuclear engineering disciplines across the lifetime of a nuclear reactor, from next-generation reactor research and development, reactor operation and engineering, spent fuel and waste management, and nuclear safeguards and nonproliferation. Important nuclear materials in which intrinsic (α,n) neutron production is possible include U3O8 and UF6 used in the nuclear fuel cycle, UO2, PuO2, and (U,Pu)O2 ceramic nuclear fuels, and AmBe or PuBe manufactured (α,n) neutron sources used for calibration and testing of neutron detection systems or as startup sources for nuclear reactors. Given the fact that many actinides, especially those of interest for nuclear reactor engineering, readily undergo some amount of neutron-induced and spontaneous fission, the intrinsic emission of (α,n) neutrons is often accompanied by fission neutrons produced intrinsically via spontaneous fission and extrinsically via neutron-induced fission. One neutron is emitted in each (α,n) reaction, and the (α,n) neutrons are emitted with a distribution of kinetic energies. ![]() In materials which produce (α,n) neutrons, alpha particles emitted in the radioactive decay of an actinide in the material slow down and may induce an (α,n) reaction with a light nuclide, such as oxygen, beryllium, or fluorine, during the slowing-down process. Actinides are fundamentally radioactive due to instability caused by their high mass numbers and generally undergo the competing radioactive decay processes of alpha decay and spontaneous fission. AbstractThe production of (α,n) neutrons occurs intrinsically for many important nuclear materials which contain actinide species and low-mass nuclides.
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