Chi-Nu experiment reveals new insights into actinide fission

The Chi-Nu physics experiment conducted at Los Alamos National Laboratory has yielded invaluable, previously unobserved data that significantly contributes to nuclear security applications, enhances our understanding of criticality safety, and aids in the design of fast-neutron energy reactors. This extensive project, spanning several years, was dedicated to measuring the energy spectrum of neutrons emitted during neutron-induced fission. Notably, it recently concluded an exhaustive uncertainty analysis for three crucial actinide elements: uranium-238, uranium-235, and plutonium-239.

Keegan Kelly, a physicist at Los Alamos National Laboratory, emphasized the significance of these findings, stating, “Nuclear fission and the associated chain reactions were only discovered slightly over 80 years ago, and researchers are still piecing together the complete puzzle of fission processes for major actinides. Throughout this project, we have detected distinct fission process signatures, many of which had never been observed in prior experiments.”

The Los Alamos team’s latest study, focusing on the isotope uranium-238, was recently published in the journal Physical Review C. This particular experiment centered on measuring the prompt fission neutron spectrum of uranium-238, which includes both the energy of the neutron causing the fission event and the potentially diverse energy distribution of the resultant neutrons. Chi-Nu specialized in “fast-neutron-induced” fission, involving incident neutron energies in the millions of electron volts, an area where measurements have historically been scarce.

Essential data for fission-related work

The Chi-Nu experiments, in conjunction with similar measurements on uranium-235 and plutonium-239, now stand as a primary source of experimental data that significantly influences contemporary endeavors to assess the prompt-fission-neutron spectrum. These invaluable findings play a crucial role in informing nuclear models, Monte Carlo simulations, reactor performance calculations, and more.

Actinide elements, a group of 15 radioactive elements with atomic numbers ranging from 89 to 103, are of paramount importance in the realms of nuclear weaponry and energy production. When a nucleus undergoes fission or splits, it emits multiple neutrons, potentially triggering fission in neighboring nuclei, thus initiating a chain reaction. The likelihood of subsequent reactions in this chain hinges on the energy of the fission neutrons.

The Chi-Nu experiment took place at the Weapons Neutron Research facility within the Los Alamos Neutron Science Center (LANSCE). It relied on a sophisticated apparatus designed to test various energy ranges. In this setup, a proton beam from LANSCE collided with a tungsten target, producing neutrons that followed a designated flight path towards the Chi-Nu apparatus. Upon striking the uranium-238 isotope, a fission event could occur, resulting in the splitting of the uranium-238 nucleus, and these events were meticulously recorded.

Subsequently, the neutrons emitted during the fission event were measured using either a liquid scintillator or lithium-glass detector array, chosen depending on the experiment’s specific energy range. Both types of detectors recorded bursts of light induced within them by the incoming neutrons.

Jaime Gomez (left) and Keegan Kelly work to set up the Chi-Nu experiment, calibrating detector distances and installing gas lines for the fission-counting target (center). Credit: Los Alamos National Laboratory

Future applications of Chi-Nu skills

The ongoing quest to unravel the intricate details of actinide isotopes persists. In parallel endeavors, the Chi-Nu experimental team is presently engaged in the collection and analysis of data concerning plutonium-240 and uranium-233.

As the Office of Experimental Sciences measurements phase comes to a close, the team is now gearing up to apply their honed skills and methodologies, originally developed for fission neutron measurements, to a series of other isotopes. Furthermore, their focus is shifting towards conducting measurements of neutrons emitted during neutron scattering reactions.

In these unique reactions, neutrons traverse through a material while imparting their energy. Researchers meticulously capture data on the energy and angular spectra of both the emitted neutrons and gamma rays, as well as the probability of the reaction taking place—commonly referred to as the neutron scattering cross section.

Source: Los Alamos National Laboratory

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