Development of experimental and analytical methods for low-rate (α,n) neutron source characterization

Ney, Adam J.
Thumbnail Image
Other Contributors
Ji, Wei
Liu, Li (Emily)
Trumbull, Timothy
Zerkle, Michael
Blain, Ezekiel
Danon, Yaron
Issue Date
Nuclear engineering and science
Terms of Use
This electronic version is a licensed copy owned by Rensselaer Polytechnic Institute (RPI), Troy, NY. Copyright of original work retained by author.
Full Citation
The production of (α,n) neutrons occurs intrinsically for many important nuclear materials which contain actinide species and low-mass nuclides. 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. 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. One neutron is emitted in each (α,n) reaction, and the (α,n) neutrons are emitted with a distribution of kinetic energies. 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. 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. 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. 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. Because the production mechanism of (α,n) neutrons is complex, the neutron yield [n/s] of (α,n) neutron sources is difficult to determine computationally. 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. 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. 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. The latter consideration is especially important for (α,n) neutron sources manufactured by mixing actinide ceramic and light nuclide powders, such as AmBe sources. 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. 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. 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. 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. 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 [n/s] of a source to an accuracy better than 5%, and perform low-resolution neutron spectroscopy to estimate the (α,n) neutron energy spectrum. The detection system and data analysis methods were validated using a set of four production measurements involving a calibrated AmBe (α,n) neutron source and two uncalibrated Cf-252 spontaneous fission neutron sources. Analysis of the measured data found that the total neutron counting and novel coincidence methods were able to deduce (α,n) neutron yield values which agreed to within 1% with the vendor calibrated neutron yield of the AmBe source for all measurements. Additionally, low-resolution unfolding capability was demonstrated to estimate the AmBe (α,n) neutron energy spectrum in both pure (α,n) and mixed source conditions.
August 2022
School of Engineering
Dept. of Mechanical, Aerospace, and Nuclear Engineering
Rensselaer Polytechnic Institute, Troy, NY
Rensselaer Theses and Dissertations Online Collection
Restricted to current Rensselaer faculty, staff and students in accordance with the Rensselaer Standard license. Access inquiries may be directed to the Rensselaer Libraries.