Fast, Radiation Tolerant Detector for Thermal Neutrons

Neutron reflectometry is a powerful technique to study depth profiles in thin film samples. The spallation neutron source at ORNL, for example, has dedicated beam-lines for magnetism reflectometry and liquids reflectometry, which use specular and off-specular neutron reflection to study magnetism and solid/liquid interfaces for a broad range of applications. Given that interfaces are literally everywhere (our body, food and drinks, plants, soil and more) the benefits of improved reflectometry instrumentation will have a broad societal impact. The signal of interest is generally orders of magnitude smaller in intensity than that from diffuse scatter, and a dynamic range of 10-6 is currently state-of-the-art. In this context, insensitivity to gamma-rays is of paramount importance. Future reflectometry detectors will have a goal of 10-7 dynamic range, while simultaneously being capable of higher absolute rates, bringing new types of dynamic study to fruition. In pursuit of 10-7 dynamic range, a diamond/boron sensor proposed here is a thin-absorbing layer technique that captures nearly all the ionization from the neutron interaction and which will improve gamma and other background rejection. Advances in the growth of high quality Chemical Vapor Deposition (CVD) diamond have created high purity diamond and an opportunity for the application of this material in practical detectors. Diamond is a semiconductor with a large band gap (5.45 eV) which allows production of detectors with very low leakage currents. The high electron and hole mobility in the diamond material provides very fast signal response with very short rise times and total pulse widths. The large lattice displacement energy for atoms and small cross section give diamond excellent radiation tolerance. In Phase 1 we will develop diamond thermal neutron detectors while investigating novel ways of applying converter layers to maximize signal and minimize gamma radiation background. Phase II will enable us to develop a large area detector with suitable packaging for neutron reflection studies of magnetism and liquid-solid interfaces. The use of this solid-state diamond detector-based technology paves the way to light-weight hand-held systems for homeland security applications and small sensors for nuclear power generation monitoring.