Recent research on a type of reaction called the neutron-proton (np) reaction could help us understand the age of the Earth and build less expensive nuclear power plants. Scientists can determine the age of some of oldest rocks on earth by using an np reaction. This reaction converts a radioactive form of potassium, an element that occurs naturally in rock, into argon. Next, scientists measure the relative abundance of potassium and argon. This method is based on the natural radioactive decay of potassium into argon, which takes place over scales of billions of years. Fast neutron-induced proton knockout np reactions are hard to measure on nuclei of light elements. However, measuring these reactions is key to many applications. Not all of these applications are in geology. For example, the np reaction on chlorine is important for some nuclear reactors because the reaction removes neutrons from the reactor core. This causes the core to shut down. In recent studies, nuclear scientists used a new neutron source to show that these important np reaction rates occur in ways very different from scientists’ initial expectations.
The results on potassium and argon will enable scientists to estimate the age of rocks with approximately 50 percent better accuracy than current methods. This will improve our understanding of the earth’s history. For example, it will help us understand the formation of our Moon, the timing and causes of mass extinctions, and the pace of human evolution and climate variability. The results on chlorine indicate that molten chloride salt in nuclear reactors could possibly be made with natural salt rather than much more expensive isotopically enriched variants. This could potentially save millions of dollars per reactor. Nuclear scientists are now conducting additional tests of this application.
The measurements were performed at the Berkeley High Flux Neutron Generator (HFNG). Samples of volcanic rock or sodium chloride (depending on the experiment) were mounted in the target area of the HFNG where they were bombarded by neutrons with an energy from 2.2-2.7 megaelectronvolts from deuterium fusion reactions depending on their orientation relative to the deuteron beam. The neutrons converted the potassium-39 in the rock to argon-39 and the chlorine-35 in the salt sample to radioactive sulfur-35. Researchers then removed the samples and determined the amounts of argon-39 and sulfur-35 in the samples.
This work was performed under the auspices of the Department of Energy (DOE) by Lawrence Berkeley National Laboratory. This research was also supported by the DOE Isotope Program and the Office of Nuclear Physics, both within DOE’s Office of Science; DOE’s Office of Nuclear Energy; and the Berkeley Geochronology Center, under a National Science Foundation grant.