Solid deuterium surface degradation at ultracold neutron sources

Abstract

Solid deuterium (sD2) is used as an efficient converter to produce ultracold neutrons (UCN). It is known that the sD2 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 sD2 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 sD2. We show that, in addition to the quality of the bulk sD2, 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 D2 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 sD2 frost formation on initially transparent sD2 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.

Improved Neutron Lifetime Measurement with UCNτ

“…Detectors at different heights relative to the beam line have differing sensitivity to the energy spectrum, and analyzing the full ensemble of monitor detectors captures changes to the spectrum. For example, heat load on the UCN source during operation gradually reduces the solid deuterium crystal quality, hardening the spectrum [36]. This changes the relationship between the monitor counts and the number of initially trapped UCNs in a given run.…”

Improved neutron lifetime measurement with UCN$τ$

“…Detectors at different heights relative to the beamline have differing sensitivity to the energy spectrum, and analyzing the full ensemble of monitor detectors captures changes to the spectrum. For example, heat load on the UCN source during operation gradually reduces the solid deuterium crystal quality, hardening the spectrum [35]. This changes the relationship between the monitor counts and the number of initially-trapped UCN in a given run.…”

Improved Search for Neutron to Mirror-Neutron Oscillations in the Presence of Mirror Magnetic Fields with a Dedicated Apparatus at the PSI UCN Source

“…The storage time was scanned repeatedly between 10 s and 360 s over a continuous 48 h period while the remaining parameters remained unchanged. Following this, we aimed to demonstrate the monitoring methodology proposed to compensate for the slow decrease in the UCN source output [79]. We present one example measurement of the drift in monitoring counts C M and counts after storage C S over around a day and a half.…”

“…We present one example measurement of the drift in monitoring counts C M and counts after storage C S over around a day and a half. The slow decrease in count rate occurs due to degradation of the surface of the solid deuterium in the source [79] and other variations occur due to fluctuations in the accelerator and source operation. As part of the normal operation of the UCN source, a "conditioning" process where the surface is repaired, is performed several times per week, recovering the initial UCN output.…”