A Solid-State, High-Energy Neutron Detector Based on Neutron-Induced Fission of Uranium-238

Heavy ion therapy is being applied as an alternative to photon radiotherapy for cancer treatment. Improving on traditional X-ray and γ-ray techniques, ion therapy enables precise radiation dose delivery to tumors with minimal harm to the surrounding healthy tissue. Dose targeting with ions is possible since the energy deposited by a heavy charged particle sharply increases as it comes to rest, a phenomenon referred to as the Bragg peak. As a result, radiation oncologists are able to position the Bragg peak to more effectively treat certain cancers. Of the ions types employed for radiotherapy, the most prevalent is carbon12 with more than 8000 patients having received treatment at The National Institute of Radiological Sciences in Chiba, Japan alone [1]. While the therapy beam spares healthy tissue, high-energy neutrons generated by the interactions of the primary ions in matter contribute to a secondary patient-absorbed dose that is difficult to quantify with existing detectors. Innovation in solid-state neutron detection has begun to offer alternatives to historically proven technologies allowing low-voltage operation, compact form factor and imaging capability; however, sensitivity to energetic neutrons has remained uncompetitive. The most prevalent solid-state neutron detectors are based on p-n junction diodes paired with a conversion material that absorbs incoming neutrons and produces charged particle reactants for sensing. The conversion materials are traditionally based on the isotopes 6Li and 10B that effectively capture thermal neutrons but become exponentially less likely to interact as neutron energy increases. Fissionable actinides exhibit the reverse trend and instead are more likely to react with high-energy neutrons. Therefore, neutron induced fission of actinides was studied as the basis to improve the sensitivity of solid-state neutron detectors with the specific goal of identifying a potential technology to enhance neutron monitoring capability during heavy ion radiotherapy. Uranium-238, comprising more than 99 % the elemental natural abundance, was identified as the best candidate to develop a neutron conversion layer to improve the sensitivity of solid-state detectors beyond the range of thermal neutrons. Leveraging the technology used in the leading-efficiency commercial solid-state thermal neutron detectors, a uranium oxide based conversion material was incorporated into microstructured silicon diodes and experimentally evaluated with fast neutrons generated from the 7Li(p, n) 7Be reaction.