Measurements of ultracold neutron upscattering and absorption in polyethylene and vanadium

Abstract

The study of neutron cross sections for elements used as efficient "absorbers" of ultracold neutrons (UCN) is crucial for many precision experiments in nuclear and particle physics, cosmology and gravity. In this context, "absorption" includes both the capture and upscattering of neutrons to the energies above the UCN energy region. The available data, especially for hydrogen, do not agree between themselves or with the theory. In this report we describe measurements performed at the Los Alamos National Laboratory UCN facility of the UCN upscattering cross sections for vanadium and for hydrogen in CH2 using simultaneous measurements of the radiative capture cross sections for these elements. We measured σup = 1972±130 b for hydrogen in CH2, which is below theoretical expectations, and σup < 25 ± 9 b for vanadium, in agreement with the expectation for the neutron heating by thermal excitations in solids. PACS numbers: 25.40.Fq, 25.40.Kv, 28.20.Ka Several recent reviews highlight the application of ultracold neutrons in past and ongoing experiments for searching for the neutron electric dipole moment [1], for precision measurement of the neutron lifetime [2], for neutron β-decay correlations [3] and for gravitational quantum states [4], as well as for solving new issues in cosmology [5]. Ultracold neutrons greatly increase the sensitivity of experiments because they can be confined in the apparatus for times comparable with the neutron lifetime owing to the action of the 'optical' Fermi potential, U F ,where m is the neutron mass, N is the number density of the material, and b is the neutron scattering length. Materials with high positive potential, U F 250 neV, are used for trapping UCN because neutrons with kinetic energy lower than that value are reflected from walls at all angles of incidence. The velocity of such neutrons is below 7 m/s. On the other hand, materials made from elements with a small negative optical potential, like hydrogen, vanadium, titanium serve for removal of UCN through absorption and/or upscattering to higher energy. The efficiency of this removal is important for many measurements and this continues to motivate the study of such materials. Vanadium and hydrogen are perfect neutron incoherent scatterers, and both have small negative, practically non-reflective potential U F −7.1 neV. For these elements, the theory of upscattering in one-phonon incoherent approximation is usually applied in the theory of upscattering, and the UCN isotropic differential upscattering cross section for cubic lattices can be calculated by a simple formula [6]:where σ b = 4πb 2 is the cross section for bound nuclei with the mass number A, E i is the initial UCN energy, E is the energy after upscattering, g(E) is the phonon density of states and the last exponent is the Debye-Waller factor * Corresponding author; Electronic address: morris@lanl.gov

Storage of ultracold neutrons in the magneto-gravitational trap of theUCNτexperiment

“…Quasi-bound UCN loaded into the trap could escape on a time scale similar to τ n , which could systematically affect the exponential decay curve and introduce an additional time constant. There are ongoing experimental and theoretical studies of this effect [38][39][40], which has been observed in material and magnetic traps under certain experimental conditions [22,37]. In the current experiment, we remove the population of quasi-bound UCN during and after filling the trap by using a 35.5 × 66.0 cm 2 polyethylene sheet ("cleaner") suspended at the top of the trap (C in figure).…”

Time-of-flight Fourier UCN spectrometer

“…Historically the first method of the UCN spectroscopy was the time-of-flight technique [1]. As a direct and universal method of measuring the neutron speed, it has been successfully used in numerous experiments with UCN (see for example [2][3][4][5][6]). …”

Beta decay measurements with ultracold neutrons: a review of recent measurements and the research program at Los Alamos National Laboratory

“…The UCNτ experiment is investigating an 'internal' UCN detection system that uses a vanadium foil lowered into the trap to absorb all surviving UCNs at the end of the storage interval. The activated foil is then raised into a shielded counting enclosure, where the activation is measured by detecting a subsequent coincidence of β and γ particles emitted from the decay of UCN-activated 52 V [206,207]. Such a detector has the advantage of a high detection efficiency and a fast UCN collection time.…”

“…Vanadium is an excellent material for this application, in that 51 V has an isotopic abundance of 99.75%, the absorption cross-section for 4 − m s 1 UCNs is 2700 barn, the UCN upscatter cross-section is two orders of magnitude smaller than the absorption cross-section, and the Fermi potential for natural V is = − V 7 neV F [206]. This last fact ensures that the reflection probability for UCNs on natural V is effectively negligible (below about 5%) for UCN energies greater than 5 neV.…”