@article{anghel_bailey_bison_blau_broussard_clayton_cude-woods_daum_hawari_hild_et al._2018, title={Solid deuterium surface degradation at ultracold neutron sources}, volume={54}, ISSN={1434-6001 1434-601X}, url={http://dx.doi.org/10.1140/epja/i2018-12594-2}, DOI={10.1140/epja/i2018-12594-2}, abstractNote={Solid deuterium (sD_2) is used as an efficient converter to produce ultracold neutrons (UCN). It is known that the sD_2 must be sufficiently cold, of high purity and mostly in its ortho-state in order to guarantee long lifetimes of UCN in the solid from which they are extracted into vacuum. Also the UCN transparency of the bulk sD_2 material must be high because crystal inhomogeneities limit the mean free path for elastic scattering and reduce the extraction efficiency. Observations at the UCN sources at Paul Scherrer Institute and at Los Alamos National Laboratory consistently show a decrease of the UCN yield with time of operation after initial preparation or later treatment (`conditioning') of the sD_2. We show that, in addition to the quality of the bulk sD_2, the quality of its surface is essential. Our observations and simulations support the view that the surface is deteriorating due to a build-up of D_2 frost-layers under pulsed operation which leads to strong albedo reflections of UCN and subsequent loss. We report results of UCN yield measurements, temperature and pressure behavior of deuterium during source operation and conditioning, and UCN transport simulations. This, together with optical observations of sD_2 frost formation on initially transparent sD_2 in offline studies with pulsed heat input at the North Carolina State University UCN source results in a consistent description of the UCN yield decrease.}, number={9}, journal={The European Physical Journal A}, publisher={Springer Nature}, author={Anghel, A. and Bailey, T. L. and Bison, G. and Blau, B. and Broussard, L. J. and Clayton, S. M. and Cude-Woods, C. and Daum, M. and Hawari, A. and Hild, N. and et al.}, year={2018}, month={Sep} }
 @article{korobkina_medlin_wehring_hawari_huffman_young_beaumont_palmquist_2014, title={Ultracold neutron source at the PULSTAR reactor: Engineering design and cryogenic testing}, volume={767}, ISSN={1872-9576}, DOI={10.1016/j.nima.2014.08.016}, abstractNote={Construction is completed and commissioning is in progress for an ultracold neutron (UCN) source at the PULSTAR reactor on the campus of North Carolina State University. The source utilizes two stages of neutron moderation, one in heavy water at room temperature and the other in solid methane at ~40K, followed by a converter stage, solid deuterium at 5 K, that allows a single down scattering of cold neutrons to provide UCN. The UCN source rolls into the thermal column enclosure of the PULSTAR reactor, where neutrons will be delivered from a bare face of the reactor core by streaming through a graphite-lined assembly. The source infrastructure, i.e., graphite-lined assembly, heavy-water system, gas handling system, and helium liquefier cooling system, has been tested and all systems operate as predicted. The research program being considered for the PULSTAR UCN source includes the physics of UCN production, fundamental particle physics, and material surface studies of nanolayers containing hydrogen. In the present paper we report details of the engineering and cryogenic design of the facility as well as results of critical commissioning tests without neutrons.}, journal={NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT}, author={Korobkina, E. and Medlin, G. and Wehring, B. and Hawari, A. I. and Huffman, P. R. and Young, A. R. and Beaumont, B. and Palmquist, G.}, year={2014}, month={Dec}, pages={169–175} }
 @article{pattie_anaya_back_boissevain_bowles_broussard_carr_clark_currie_du_et al._2009, title={First measurement of the neutron beta asymmetry with ultracold neutrons}, volume={102}, number={1}, journal={Physical Review Letters}, author={Pattie, R. W. and Anaya, J. and Back, H. O. and Boissevain, J. G. and Bowles, T. J. and Broussard, L. J. and Carr, R. and Clark, D. J. and Currie, S. and Du, S. and et al.}, year={2009} }
 @article{korobkna_wehring_hawari_young_huffman_golub_xu_palmquist_2007, title={An ultracold neutron source at the NC state university PULSTAR reactor}, volume={579}, ISSN={0168-9002}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34547682155&partnerID=MN8TOARS}, DOI={10.1016/j.nima.2007.04.116}, abstractNote={Research and development is being completed for an ultracold neutron (UCN) source to be installed at the PULSTAR reactor on the campus of North Carolina State University (NCSU). The objective is to establish a university-based UCN facility with sufficient UCN intensity to allow world-class fundamental and applied research with UCN. To maximize the UCN yield, a solid ortho-D2 converter will be implemented coupled to two moderators, D2O at room temperature, to thermalize reactor neutrons, and solid CH4, to moderate the thermal neutrons to cold-neutron energies. The source assembly will be located in a tank of D2O in the space previously occupied by the thermal column of the PULSTAR reactor. Neutrons leaving a bare face of the reactor core enter the D2O tank through a 45×45 cm cross-sectional area void between the reactor core and the D2O tank. Liquid He will cool the disk-shaped UCN converter to below 5 K. Independently, He gas will cool the cup-shaped CH4 cold-neutron moderator to an optimum temperature between 20 and 40 K. The UCN will be transported from the converter to experiments by a guide with an inside diameter of 16 cm. Research areas being considered for the PULSTAR UCN source include time-reversal violation in neutron beta decay, neutron lifetime determination, support measurements for a neutron electric-dipole-moment search, and nanoscience applications.}, number={1}, journal={NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT}, author={Korobkna, E. and Wehring, B. W. and Hawari, A. I. and Young, A. R. and Huffman, P. R. and Golub, R. and Xu, Y. and Palmquist, G.}, year={2007}, month={Aug}, pages={530–533} }
 @article{korobkina_golub_wehring_young_2002, title={Production of UCN by downscattering in superfluid He-4}, volume={301}, ISSN={0375-9601}, DOI={10.1016/S0375-9601(02)01052-6}, abstractNote={Ultra-Cold Neutrons (UCN) are neutrons with energies so low they can be stored in material bottles and magnetic traps. They have been used to provide the currently most accurate experiments on the neutron life time and electric dipole moment. UCN can be produced in superfluid Helium at significantly higher densities than by other methods. The predominant production process is usually by one phonon emission which can only occur at a single incident neutron energy because of momentum and energy conservation. However UCN can also be produced by multiphonon processes. It is the purpose of this work to examine this multiphonon production of UCN. We look at several different incident neutron spectra, including cases where the multiphonon production is significant, and see how the relative importance of multiphonon production is influenced by the incident spectrum.}, number={5-6}, journal={PHYSICS LETTERS A}, author={Korobkina, E and Golub, R and Wehring, BW and Young, AR}, year={2002}, month={Sep}, pages={462–469} }