Thick Bi 2 Sr 2 CaCu2O 8 films have been produced by melting Bi 2 Sr 2 CaCu2O 8 powder on MgO substrates at temperatures just above the melting temperature of the powder. X-ray diffraction measurements indicate enhanced c-axis alignment throughout the thickness of the films. Films melted at 900 °C show greater alignment and contain less Bi 2 Sr 2 Cu06 compared to those processed at 950 °C. Both the degree of alignment and the Bi 2 Sr 2 Cu0 6 content increase as the cooling rate is decreased. Transport measurements show that films quickly cooled from low melt-processing temperatures have the highest critical current densities (J c -13 kA/cm 2 at 30 K). The temperature dependence of J c is best described by a flux creep model. Finally, J c at 4.2 K (in both parallel and perpendicular magnetic fields) undergoes a sudden drop at low fields, but levels off above 0.5 T to about 30% of the zero field value.
EXECUTIVE SUMMARY
OBJECTIVEThis research and development effort, Film Implementation of a Neutron Detector (FIND), serves as proof of concept for a new type of solid-state neutron detector technology, potentially providing improvements in sensitivity, size, weight, power consumption, operator safety, transportability, and cost compared to current neutron detector technology. The aim of the FIND project is to demonstrate the detection of thermal neutrons with a solid-state device based on boron nitride (BN) films.
RESULTSA proof-of-concept FIND device, with a thin bulk BN detecting element, has been successfully demonstrated to be sensitive to irradiation by thermal neutrons. The required material properties for a BN detecting element and its electrical contacts have been defined and investigated in thin film and bulk samples of hexagonal-phase boron nitride. Various characterization methods have been examined for measuring the critical properties of a number of BN samples, including: "* Microwave cavity perturbation (carrier lifetime) "* Time-resolved reflectivity (carrier lifetime) * Constant-current corona-discharge-induced surface potential build-up (carrier mobility) "• Transmission line method (contact resistance)
RECOMMENDATIONSThe successful operation of a bulk BN thermal neutron detector affirms the basic concept of the FIND device. Further work is needed to make a reproducible and reliable detector, and to incorporate and optimize a high-quality BN film detection element that would have the following advantages: "* Thinner than typical bulk material --+ low bias voltage (not kilovolts) "• Fewer structural defects -4 higher charge carrier lifetime --higher detection efficiency "* Lower porosity level -+ more nuclear reactions in given volume -* higher detection efficiency An alternative to a BN film detection element is very thin, robust, highly oriented bulk BN (including single crystal BN, when BN crystal growth is at an advanced enough stage).In any case, successful implementation of a FIND device will require the continued refinement and use of methods for measuring the carrier mobility V' and lifetime r (or the lifetime-mobility product PT) of the BN detecting element. Measurement of the specific resistivity and chemical/ structural properties of the device electrical contacts is also crucial. Characterization of these BN and contact properties will be essential for screening and improving FIND devices, and ultimately for quality control.iii
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