The results of the Chi-Nu physics experiment at the U.S. Department of Energy's Los Alamos National Laboratory provide never-before-seen important data for enhancing nuclear safety applications, understanding criticality safety, and designing fast neutron energy reactors. The Chi-Nu Project, a years-long experiment that measures the energy spectrum of neutrons emitted by neutron-induced fission, recently completed the most detailed and extensive uncertainty analysis of the three major actinide elements, uranium-238, uranium-235 and plutonium-239.
Jaime Gomez (left) and Keegan Kelly set up the Chi-Nu experiment, calibrating the detector's distance and installing gas pipes for the fission counting target (center). Source: Los Alamos National Laboratory
Keegan Kelly, a physicist at Los Alamos National Laboratory, said: "Nuclear fission and related nuclear chain reactions were discovered more than 80 years ago, and experimentalists are still working to provide a complete picture of the fission process of the major actinides. Throughout the project, we observed distinct features of the fission process that, in many cases, had never been observed in any previous experiment."
The Los Alamos team's final Chi-Nu study of the isotope uranium-238 was recently published in the journal Physical Review C. The experiment measured the instantaneous fission neutron spectrum of uranium-238: the energy of the fission-inducing neutrons—the ones that hit the nucleus and split it apart—and the potentially broad energy distribution (spectrum) of the neutrons thus released. The focus of the Chi-Nu experiment is "fast neutron-induced" fission, where incident neutrons have energies up to millions of electron volts, and for which measurements are generally sparse.
Physicist Keegan Kelly installed a fission-counting target for the Chi-Nu experiment, which contains about 100 milligrams of the related actinides. The device includes 54 liquid scintillation neutron detectors and 22 lithium glass detectors to measure neutrons in different energy ranges. Source: Los Alamos National Laboratory
Important data for fission-related work
Along with similar measurements made on uranium-235 and plutonium-239, the results of the Chi-Nu experiment are now in many cases the primary source of experimental data guiding modern assessments of transient fission neutron spectra. These data provide the basis for nuclear models, Monte Carlo calculations, reactor performance calculations, etc.
Actinides and their possible chain reactions are important for nuclear weapons and energy reactors. (The actinides refer to the 15 elements with atomic numbers from 89 to 103. They are all radioactive.) When a nucleus fission or split, several neutrons are released, which may cause the fission of neighboring nuclei, resulting in a chain reaction. The probability of subsequent reactions in a chain reaction depends on the energy of the fission neutron.
LANSCE experimental process
The Chi-Nu experiment, conducted at the Weapons Neutron Research Facility at the Los Alamos Neutron Science Center (LANSCE), relies on precision instruments that test multiple energy ranges. The LANSCE proton beam hits the tungsten target, and the neutrons produced fly along the flight path toward the Chi-Nu device. When these neutrons hit the uranium-238 isotope, a fission event occurs, in which the uranium-238 nucleus splits apart, and is recorded. Depending on the energy range of the experiment, the neutrons emitted by the fission event are then measured in an array of liquid scintillator or lithium glass detectors, both of which record the flash of light the neutrons cause within the detector.
future applications
Researchers continue to outline the full picture of actinide isotopes. In adjacent work funded by the Nuclear Criticality and Safety Program, the Chi-Nu Experiment team is currently collecting and analyzing data on plutonium-240 and uranium-233.
With the Office of Experimental Science measurement work now complete, the team is looking to apply the skills and methods gained from fission neutron measurements to measurements of a range of other isotopes. They are also moving toward measuring the neutrons emitted by the neutron scattering reaction. In these reactions, neutrons pass through the material, depositing energy as they do so. Along with measuring the energy and angular spectra of the emitted neutrons and gamma rays, the probability of a reaction occurring is also measured, often called the neutron scattering cross section.