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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.

The 10 B(α,n) 13 N 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 10 B(n,α) 7 Li. Samples of boron carbide, boron oxide, and boron nitride are irradiated with thermal neutrons and the rate of 13 N production is determined. 13 N 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 13 N production is (1–2) × 10 −10 per captured neutron.

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° and 2.1° 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.

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.

We investigate the performance of a large-area ð13 mm � 13 mmÞ 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 decreasingtemperature until approximately 40 K ; at which point it drops abruptly to o15% of its room temperature value. The sensitivity of the device above 40 K makes it a good candidate for detection of scintillation light in lowtemperature systems. r 2003 Elsevier B.V. All rights reserved. PACS: 07.60.Dq; 29.40.Mc; 29.40.Wk

Magnetically stabilized luminescence is observed in hexagonal boron nitride. The luminescence is induced by absorption of cold neutrons and is in the visible region. In the absence of a magnetic field, the photon emission level is observed to decay over several hundred seconds. A fraction of this luminescence can be suppressed if the temperature is T ~ 1.0 T. Subsequent to irradiation and suppression, luminescence can be induced by an increase in T or lowering of B. Possible explanations include stabilization of triplet states or the localization and stabilization of excitons., Comment: 11 pages, 7 figures, to appear in the Journal of Luminescence

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 super#uid 4He and con"ned 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% e$ciency. This direct measure of the number of UCN decays vs. time can be used to determine the neutron beta-decay lifetime. ( 2000 Elsevier Science B.V. All rights reserved.

Long-lived phosphorescence has been observed emanating from superfluid helium following excitation by ionizing radiation. This extreme ultraviolet light is predominantly from the radiative decay of metastable ${\mathrm{He}}_{2}(a{}^{3}{\ensuremath{\Sigma}}_{u}^{+})$ molecules and is detected through frequency down-conversion and photon counting techniques. This allows measurement of the phosphorescence lifetime in a low-molecular-density regime that is not dominated by two-body reactions. A phosphorescence lifetime of $13\ifmmode\pm\else\textpm\fi{}2 \mathrm{s}$ is observed.

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 (UCN) 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, 5 pages, 5 figures

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 $(\ensuremath{\sim}10\mathrm{ns})$ strong initial pulse followed by single photons whose emission rate decays exponentially with a $1.6\ensuremath{-}\ensuremath{\mu}\mathrm{s}$ 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 $\ensuremath{\beta}$ particles is significantly weaker than those from $\ensuremath{\alpha}$ particles or neutron capture on ${}^{3}\mathrm{He}.$ It is also found that for $\ensuremath{\alpha}$ particles, the intensity of this component depends on the temperature of the superfluid helium. Proposed models describing the observed fluorescence are discussed.

Three-dimensional magnetic confinement of neutrons is reported. Neutrons are loaded into an Ioffe-type superconducting magnetic trap through inelastic scattering of cold neutrons with 4He. Scattered neutrons with sufficiently low energy and in the appropriate spin state are confined by the magnetic field until they decay. The electron resulting from neutron decay produces scintillations in the liquid helium bath that results in a pulse of extreme ultraviolet light. This light is frequency downconverted to the visible and detected. Results are presented in which 500 +/- 155 neutrons are magnetically trapped in each loading cycle, consistent with theoretical predictions. The lifetime of the observed signal, 660 s +290/-170 s, is consistent with the neutron beta-decay lifetime., Comment: 17 pages, 18 figures, accepted for publication in Physical Review C

Accurate measurement of the lifetime of the neutron (which is unstable to beta decay) is important for understanding the weak nuclear force and the creation of matter during the Big Bang. Previous measurements of the neutron lifetime have mainly been limited by certain systematic errors; however, these could in principle be avoided by performing measurements on neutrons stored in a magnetic trap. Neutral-particle and charged-particle traps are widely used for studying both composite and elementary particles, because they allow long interaction times and isolation of particles from perturbing environments. Here we report the magnetic trapping of neutrons. The trapping region is filled with superfluid 4He, which is used to load neutrons into the trap and as a scintillator to detect their decay. Neutrons in the trap have a lifetime of 750(+330)(-200) seconds, mainly limited by their beta decay rather than trap losses. Our experiment verifies theoretical predictions regarding the loading process and magnetic trapping of neutrons. Further refinement of this method should lead to improved precision in the neutron lifetime measurement.

We describe the design, construction, and performance of three generations of superconducting Ioffe magnetic traps. The first two are low current traps, built from four racetrack shaped quadrupole coils and two solenoid assemblies. Coils are wet wound with multifilament NbTi superconducting wires embedded in epoxy matrices. The magnet bore diameters are 51 and 105 mm with identical trap depths of 1.0 T at their operating currents and at 4.2 K. A third trap uses a high current accelerator-type quadrupole magnet and two low current solenoids. This trap has a bore diameter of 140 mm and tested trap depth of 2.8 T. Both low current traps show signs of excessive training. The high current hybrid trap, on the other hand, exhibits good training behavior and is amenable to quench protection.

A demountable, superfluid-tight window for the transmission of neutrons and light has been developed. A fluorocarbon polymer (Teflon) film is both the window and sealing gasket. Transmission of blue light and bursting pressure are measured for films of varying thicknesses.

A superfluid-helium-tight optical fiber feedthrough has been developed. Heat shrinkable Kynar provides a removable seal around a plastic optical fiber. The seal preserves the continuity of the fiber and is reliable after repeated thermal cycles.

Fluorescence efficiencies of the organic scintillators tetraphenyl butadiene (TPB), p-terphenyl (TPH), and diphenyl stilbene (DPS) are measured relative to sodium salicylate at incident wavelengths of 58.4 and 74.0 nm. Optimum thickness and dopant concentration are determined for maximum fluorescence yield in evaporated, sprayed, and doped plastic films. Measurements made with alpha (α) particle induced scintillations in gaseous argon (a broad band vacuum ultraviolet light source) were in good agreement with those made using the line source. Transparent scintillator doped plastic films have been developed which yield fluorescence efficiencies comparable to that of sodium salicylate. Evaporated films show the highest fluorescent yields, reaching almost four times the efficiency of sprayed sodium salicylate. On the other hand, doped plastic films offer some advantages.