[en] The neutron-induced low temperature (below 5 K) luminescence of neutron shielding and scintillation detection materials is studied. Strong luminescence is observed for the neutron absorbing materials boron nitride (BN) and lithium fluoride (LiF). A measurable, but substantially smaller luminescence is observed from boron oxide (B₂O₃). An upper bound of 10^-3 was determined for the fraction of the luminescence due to time-correlated multiphoton events in the BN. Other materials tested - boron carbide (B₄C), polymethyl methacrylate (PMMA or acrylic), expanded polytetrafluoroethylene (PTFE) with an evaporated coating of the downconverting fluor tetraphenyl butadiene (TPB) and a boron/lithium loaded glass - displayed no detectable luminescence. The boron/lithium loaded glass was determined to activate, by the secondary reaction ¹⁶O(T,n)¹⁸F, with the triton produced in the neutron capture reaction ⁶Li(n,T)⁴He

[en] We investigate the performance of a large-area (13 mmx13 mm) avalanche photodiode at temperatures ranging from 4.2 to 77 K. We find that the gain, at a given bias voltage, increases with decreasing temperature down to 40 K, below which a premature breakdown phenomenon occurs. The quantum efficiency of the device decreases with decreasing temperature until approximately 40 K, at which point it drops abruptly to <15% of its room temperature value. The sensitivity of the device above 40 K makes it a good candidate for detection of scintillation light in low-temperature systems

[en] Detectors for counting low energy (less than 1 MeV) ionizing events in liquid helium are developed and characterized. These devices employ wavelength shifting fluors to convert extreme ultraviolet (EUV) helium scintillation light to the visible, allowing transport of signal light to room temperature. Three technological approaches are developed and tested: wavelength shifting fiber, composite acrylic tube, and diffuse reflecting tube of expanded teflon. The tube-based detectors have been used to detect magnetically trapped neutrons. All of the technological approaches have utility in other experiments, such as a more sensitive measurement of the neutron electric dipole moment and the monitoring of the low-energy solar neutrino flux

[en] Production of ultracold neutrons (UCN) by single-phonon downscattering of cold neutrons from superfluid helium (the 'superthermal process') utilizes input neutrons only in a narrow wavelength band around 0.89 nm. Delivering a monochromatic 0.89 nm neutron beam to a superfluid helium target reduces backgrounds in the UCN production region with minimal loss in the UCN production rate. The design, construction, and testing of a 0.89 nm neutron monochromator is reported herein. This monochromator is constructed from nine tiled pieces of stage 2 potassium-intercalated graphite with mosaics between 1.1 deg. and 2.1 deg. and reflectivities of (73-91)% at 0.89 nm. In addition to stage 2 potassium-intercalated graphite, fluorophlogopite and stage 1 potassium-intercalated graphite are also characterized

[en] The ¹⁰B(α,n)¹³N reaction is studied as an activation process in a variety of solid boron-containing neutron shielding materials. The source of α-particles is the neutron capture reaction ¹⁰B(n,α)⁷Li. Samples of boron carbide, boron oxide, and boron nitride are irradiated with thermal neutrons and the rate of ¹³N production is determined. ¹³N promptly decays, emitting a positron. This positron efficiently annihilates with electrons in the material and the resultant 511 keV gamma ray is detected. For each of the above-mentioned materials, the rate of ¹³N production is (1-2) x 10^-10 per captured neutron

[en] We report progress towards magnetic trapping of ultra-cold neutrons (UCN) in preparation for a neutron lifetime measurement. UCN will be produced by inelastic scattering of cold (0.89 nm) neutrons in a reservoir of superfluid ⁴He and confined in a three-dimensional magnetic trap. As the trapped neutrons decay, recoil electrons will generate scintillations in the liquid He, which should be detectable with nearly 100% efficiency. This direct measure of the number of UCN decays vs. time can be used to determine the neutron beta-decay lifetime

[en] The time dependence of extreme ultraviolet (EUV) fluorescence following an ionizing radiation event in liquid helium is observed and studied in the temperature range from 250 mK to 1.8 K. The fluorescence exhibits significant structure including a short (∼10 ns) strong initial pulse followed by single photons whose emission rate decays exponentially with a 1.6-μs time constant. At an even longer time scale, the emission rate varies as '1/time' (inversely proportional to the time after the initial pulse). The intensity of the '1/time' component from β particles is significantly weaker than those from α particles or neutron capture on ³He. It is also found that for α particles, the intensity of this component depends on the temperature of the superfluid helium. Proposed models describing the observed fluorescence are discussed

[en] The neutron beta-decay lifetime plays an important role both in understanding weak interactions within the framework of the Standard Model and in theoretical predictions of the primordial abundance of ⁴He in Big Bang Nucleosynthesis. In previous work, we successfully demonstrated the trapping of ultracold neutrons in a conservative potential magnetic trap. A major upgrade of the apparatus is nearing completion at the National Institute of Standards and Technology Center for Neutron Research (NCNR). In our approach, a beam of 0.89 nm neutrons is incident on a superfluid ⁴He target within the minimum field region of an Ioffe-type magnetic trap. A fraction of the neutrons is downscattered in the helium to energies <200neV, and those in the appropriate spin state become trapped. The inverse process is suppressed by the low phonon density of helium at temperatures less than 200 mK, allowing the neutron to travel undisturbed. When the neutron decays the energetic electron ionizes the helium, producing scintillation light that is detected using photomultiplier tubes. Statistical limitations of the previous apparatus will be alleviated by significant increases in field strength and trap volume resulting in twenty times more trapped neutrons.