Loss and spinflip probabilities for ultracold neutrons interacting with diamondlike carbon and beryllium surfaces

Abstract

The storage of ultracold neutrons (UCN) in a combined magnetic, gravitational, and material trap is described. Wall materials investigated were diamondlike carbon (DLC) coatings on solid and flexible foil substrates as well as beryllium coatings on solid substrates. The loss coefficient per wall collision, η, and the depolarization probability β were measured simultaneously as a function of temperature (from 70 to 400 K) and energy (from 30 to 80 neV). The results at 70 K are η = (0.7 ± 0.1) × 10 −4 , β = (15.4 ± 1.0) × 10 −6 for DLC on polyethyleneterephtalate (PET) foil and η = (1.7 ± 0.1) × 10 −4 , β = (0.7 ± 0.3) × 10 −6 for DLC on aluminum foil. At room temperature the loss coefficients are larger by a factor of about 2 whereas the depolarization probabilities are found to be independent of temperature. The corresponding values for Be at room temperature are η ∼ 5 × 10 −4 , β ∼ 10 × 10 −6 . The DLC results for β and for the temperature-dependent part of the loss coefficient, η T , are interpreted in terms of incoherent scattering by hydrogen. The hydrogen admixture was measured independently by elastic recoil detection analysis to be about 1 × 10 16 atoms/cm 2 . The data do not support the hypothesis of hydrogen being chemically bound within the top layers of the DLC. Using two different models with a thin waterlike film on top of the substrate we obtain consistency between the temperature-dependent loss contribution and the measured hydrogen contamination.

Limits on a nucleon–nucleon monopole–dipole coupling from spin relaxation of polarized ultra-cold neutrons in traps

“…Probability of the neutron depolarization was not measured directly in these experiments, but from the reported very good magnetic resonance curves it can be concluded that the UCN depolarization probability at a single collision with the walls was not higher than in [16][17][18]. According to [24] the neutron depolarization time in the EDM experiment [21][22][23] can be estimated to be not less than τ dep ∼ 800 s. From this figure we can estimate the depolarization probability per single collision with the walls: β = l/(τ dep v) = 18/(800 × 500) ∼ 4 × 10 −5 , where l is the mean free path between two consecutive collisions, which can be found from dimensions of the neutron bottle: in the experiment [21][22][23] it was a cylinder of the internal diameter about 44 cm and height 15 cm.…”

“…There are two published experimental data on the ultracold neutron depolarization: [16,17] and [18], in which special experiments are described to measure this value. In both publications, for a variety of materials, the measured values of the neutron depolarization probability per one neutron collision with the walls of storage cavity were around β ∼ 10 −5 .…”

“…In both publications, for a variety of materials, the measured values of the neutron depolarization probability per one neutron collision with the walls of storage cavity were around β ∼ 10 −5 . The β ∼ 10 −6 was measured in [18] for the diamond like carbon foils (DLC). The magnetic fields in the storage chambers were partly due to stray fields from strong magnets used for the polarization of the incident neutrons [16][17][18] and partly were formed by special magnets [18].…”

“…The β ∼ 10 −6 was measured in [18] for the diamond like carbon foils (DLC). The magnetic fields in the storage chambers were partly due to stray fields from strong magnets used for the polarization of the incident neutrons [16][17][18] and partly were formed by special magnets [18]. In [16,17] magnetic field was estimated [19] to be near 50 G, and in the experiment [18] it was reported to be ∼55 G. In both cases, this rather large external magnetic field suppressed effect of the additional hypothetical spin-dependent interaction on depolarization of the ultra-cold neutrons in traps, and therefore decreased sensitivity of these measurements to establishing constraints on the axion-like interaction.…”

“…According to [24] the neutron depolarization time in the EDM experiment [21][22][23] can be estimated to be not less than τ dep ∼ 800 s. From this figure we can estimate the depolarization probability per single collision with the walls: β = l/(τ dep v) = 18/(800 × 500) ∼ 4 × 10 −5 , where l is the mean free path between two consecutive collisions, which can be found from dimensions of the neutron bottle: in the experiment [21][22][23] it was a cylinder of the internal diameter about 44 cm and height 15 cm. We constrain the monopole-dipole interaction in two assumptions for the depolarization probability in [21][22][23]: one-β = 4 × 10 −5 , and another one-corresponding to the best value obtained in the experiments of the PSI group [18]-β = 10 −6 .…”