Solid deuterium surface degradation at ultracold neutron sources (Vol. 49 No.5-6)

As shown in the figure for the PSI UCN source, the intensity decrease can be fully recovered by a conditioning process, removing the frost from the sD2 and restoring good surface quality.

Highest intensities of ultracold neutrons (UCN) are in worldwide demand for fundamental physics experiments. Tests of the Standard Model of particle physics and searches for physics beyond it are performed with UCN.

Two of the leading UCN sources, at Paul Scherrer Institute (PSI) and at Los Alamos National Laboratory (LANL), are based on solid deuterium (sD2) at temperatures around 5 K. Here, together with NCSU they joined forces to understand UCN intensity decreases observed during pulsed neutron production. The study shows that the decrease can be completely explained by the build-up of frost on the sD2 surface during operation. Pulsed proton beams hitting the spallation targets generate heat pulses causing cycles of D2 sublimation and subsequent resublimation on the sD2 surface. Even very small frost flakes can act as total reflectors for UCN and cause an intensity decrease. Optical observation of the sD2 surface at NCSU – not possible at the operating spallation neutron sources – confirmed a severe surface degradation due to heat pulsing with an external heater in strong support of the frost model.

A. Anghel and 29 co-authors, Solid deuterium surface degradation at ultracold neutron sources, Eur. Phys. J. A 54, 148 (2018)