We present the first measurements of the survival time of ultracold neutrons (UCNs) in solid deuterium (SD2). This critical parameter provides a fundamental limitation to the effectiveness of superthermal UCN sources that utilize solid ortho-deuterium as the source material. Superthermal UCN sources offer orders of magnitude improvement in the available densities of UCNs, and are of great importance to fundamental particle-physics experiments such as searches for a static electric dipole moment and lifetime measurements of the free neutron. These measurements are performed utilizing a SD2 source coupled to a spallation source of neutrons, providing a demonstration of UCN production in this geometry and permitting systematic studies of the influence of thermal up-scatter and contamination with para-deuterium on the UCN survival time.PACS numbers: 29.25. Dz, 28.20.Gd, 32.80.Pj Neutrons with kinetic energies less than 340 neV (corresponding to a temperature T< 5mK) can be trapped in material bottles and are referred to as ultracold neutrons (UCNs) [1,2,3]. UCN densities at reactor sources have gradually increased with reactor power and improved techniques for extracting the UCN flux. The highest bottled densities reported in the literature, 41/cm 3 , have been obtained at the Institut Laue-Langevin (ILL) reactor in Grenoble [4].Measurements of the neutron electric dipole moment [5,6] and the neutron lifetime [7,8,9] attest to the utility of bottled UCNs for fundamental experiments with neutrons. UCNs may prove useful in improved measurements of angular correlations in neutron beta-decay [10,11], although experiments of this kind utilizing UCNs have not yet been performed. All of these experimental programs have been limited by the available densities of UCNs.Superthermal UCN production, where the production rate of UCNs due to down-scattering in energy is larger than the combined up-scatter and nuclear-absorption rates in the material, was first proposed in 1975 by Golub and Pendlebury [12] in superfluid 4 He and experimentally investigated shortly thereafter [13,14]. In this process phonon creation in the liquid is used to down-scatter cold neutrons to the UCN regime, while up-scattering is suppressed by maintaining the superfluid at sufficiently low temperature. Because 4 He has no nuclear absorption, the only limitations to the density of UCNs accumulated are wall losses and neutron beta decay. The production of UCNs by this process has been observed and agrees with theoretical expectations [14,15,16,17,18].While superfluid 4 He is an excellent superthermal converter, a few other materials, such as solid deuterium (SD 2 ), satisfy the criteria for superthermal production. The limiting UCN density, ρ UCN , one can obtain using a SD 2 source is given by the product of the rate of UCNs production in the solid, R, and the lifetime of UCNs in the solid, τ SD : ρ UCN = Rτ SD . A storage bottle opened to such a source will come into density equilibrium with the density in the solid. This led to the proposal of a thi...

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We report the first measurement of an angular correlation parameter in neutron beta decay using polarized ultracold neutrons (UCN). We utilize UCN with energies below about 200 neV, which we guide and store for approximately 30 s in a Cu decay volume. The interaction of the neutron magnetic dipole moment with a static 7 T field external to the decay volume provides a 420 neV potential energy barrier to the spin state parallel to the field, polarizing the UCN before they pass through an adiabatic fast passage spin flipper and enter a decay volume, situated within a 1 T field in a 2x2pi solenoidal spectrometer. We determine a value for the beta-asymmetry parameter A_{0}=-0.1138+/-0.0046+/-0.0021.

The total scattering cross sections for slow neutrons with energies E in the range 300 neV to 3 meV for gaseous and liquid ortho-2H2 have been measured. The cross sections for 2H2 gas are found to be in excellent agreement with both the Hamermesh and Schwinger and the Young and Koppel models. For liquid 2H(2), we confirm the existing experimental data in the cold neutron range and the discrepancy with the gas models. We find a clear 1 / square root[E'] dependence at low energies for both states. A simple explanation for the liquid 2H2 cross section is offered.

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