This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5). This project will develop a facility to use nuclear fusion of deuterium atoms and ions to create a large concentration of neutrons with uniform energy of 2.45 million electron volts. The fusion reaction has been successfully induced in previous experiments, but this project will use a novel radially inward-focussing deuterium ion beam geometry to pursue a neutron concentration 1000 or more times higher than previously achieved. This neutron generator will be used to irradiate geological samples for the argon-40/argon-39 dating method, in which potassium-39 must be converted to argon-39 through neutron capture. The source of neutrons for this widely used dating method has previously been conventional nuclear reactors powered by fission of uranium, producing a broad range of neutron energies with several suboptimal effects. In contrast, the device to be developed by this project will reduce atomic recoil and unwanted nuclear reactions, thus producing more accurate age information with less radiological risk to analysts, and without generating hazardous radioactive waste. These improvements will enhance the ability to accurately date fine-grained minerals such as clays, whose ages are often useful as a guide for petroleum and ore deposit exploration. Other likely applications include the chronology of human evolution and of the early solar system. The device will be useful for applications beyond dating, specifically including trace element geochemistry, nuclear medicine, materials science, and condensed matter physics. The ability to provide rapid neutron activation data will be applicable to the design of a next generation of fission reactors for nuclear power. The project will provide training to students and postdoctoral researchers and thus contribute to replenishing the depleted pipeline of early career scientists with expertise in nuclear sciences, thereby addressing growing concerns arising from national security and energy needs. The project will also enhance cross-disciplinary collaboration between nuclear scientists and earth scientists.
This project aimed to produce a device to generate neutrons of a specific energy to irradiate samples for the 40Ar/39Ar dating method. This method is used to date materials as young as a few thousand years, and as old as our Solar System (4.5 billion years old), but has some limitations caused by irradiating samples with the broad range of energies produced in conventional fission reactors. The two most important problems associated with a broad neutron energy spectrum are: (1) recoil displacement of 39Ar atoms, which limits the method to relatively coarse-grained materials; and (2) the production of unwanted Argon isotopes, requiring accuracy-limiting corrections. The project is expected to provide substantial mitigation of both of these limitations. This project uses a nuclear fusion process, combining heavy hydrogen (deuterium) to make neutrons with a highly restricted energy range. Deuterium gas is converted into plasma by radio frequency antennae, and the charged atoms (deuterium ions) are then accelerated at high energy to a titanium target, which absorbs the deuterium atoms for fusion with subsequent incoming ions. Neutrons are then created at the target, along with benign helium-3, and geologic samples are placed between two targets for maximum neutron dose. The critical challenge for this project was to design a target assembly that would produce the maximum neutron dose. This required striking an optimal balance between the maximum number of deuterium ions produced, and dissipating the heat produced on the target to prevent the deuterium implanted on the target from degassing. In addition to the basic neutron generator unit, a computer-control system based on the LabView platform was constructed to regulate plasma generation, gas dosing, accelerating voltage, coolant water flow, and to record radiation levels. Failsafe features provide for shutdown of the generator if external radiation levels exceed safe thresholds, or if the target temperature exceeds crital levels, or if the door to the enclosing vault is opened. A sophisticated coolant water system controls water flow to the target, radiofrequency plasma generators, and turbomolecular pump. The generator is housed in a vault with concrete walls 1-3 meters thick, in the Nuclear Engineering Dept. at the University of California, Berkeley, under a memorandum of understanding between the Berkeley Geochronology Center and UC Berkeley. UC Berkeley has provided substantial support for the project in the form of personnel, materials, space, site preparation, electrical power, and coolant water. Additional personnel support and materials for shielding have been provided by Lawrence Berkeley and Lawrence Livermore National Laboratories. Interest in the project by nuclear scientists has provided substantial benefit at no cost to the NSF grant. Ongoing involvement of these institutions, who will also be users, will be beneficial to the maintenance and support of the facility. Intellectual Merit The project will deliver the highest flux of 2.5 MeV energetically uniform neutrons ever achieved. This will reduce or eliminate many of the existing limitations of the 40Ar/39Ar dating method. The results will enhance the ability to date fine-grained materials such as clay minerals, and reduce the production of unwanted isotopes that reduce the accuracy of the method. The innovations in target design and radiofrequence plasma generation will likely be of interest to researchers elsewhere. Broader impacts The design and construction of the neutron generator has provided training for seven graduate students and two postdoctoral researchers. In addition, two engineering consultants and four senior researchers gained highly specialized, but broadly applicable experience in instrument design and system integration. The project has provided a focus of interdisciplinary learning for geoscientists, nuclear engineers and physicists. The impact of the project extends beyond geoscience, and the generator will be used by nuclear scientists to make fundamental measurements of nuclear processes, including some related to nuclear forensics applied to national security. The facility will be incorporated into curriculum for students in geosciences and nuclear engineering at UC Berkeley. The neutron generator greatly reduces the production of radioactive waste relative to Uranium-based fission reactors, thus is likely to stimulate reduction of the need for handling and storage of such waste in the long run.
