A solid-deuterium source of ultra-cold neutrons at the FRM-II

Ultra-cold neutrons (UCN) will be produced at the new high-flux reactor FRM-II at Garching near Munich in a solid-deuterium storage source (Mini-D2). The UCN density in this source is expected to be nearly three orders of magnitude larger than that reached in the up-to-now strongest UCN source (at Institut Laue-Langevin in Grenoble). The scheme of Mini-D2 is simple: a small cylindrical volume (diameter about 6 cm, length 9 cm) with solid deuterium, the converter, is coupled to a large cylindrical evacuated storage tube with Be-coated walls (diameter also 6 cm, length about 7 m). The source will be installed with the converter very close to the cold source of FRM-II and the core-far end of the storage tube already outside the moderator tank. Solid deuterium at temperatures around 5 K is a good converter. The UCN production rate from cold neutrons is large, the absorption cross section small (in the mbarn/atom region for 40-K neutrons), and the up-scattering cross-section at 5 K even smaller. Most of the UCN produced in the converter leave it into the storage tube before being up-scattered or absorbed. The losses in the tube, mainly by up-scattering or absorption at the Be-coated walls, are small, as its temperature is kept at about 25 K. The energy band of the neutrons in the tube reaches from 100 neV, the energy a neutron gains when it leaves the deuterium, to about 250 neV, the maximum energy for total reflection at the Be-coated walls. With a cold-neutron flux of 3x1013 cm-2s-1 at the place of Mini-D2 we expect UCN densities of about 5x104 cm-3 and a corresponding UCN flux of about 3x106 cm-2s-1, when the end of the storage tube is opened to an experimental set-up. The solid-deuterium in the converter will be produced by deposition of deuterium gas on the 5-K converter walls. As the storage tube is kept at 25 K, the amount of solid deuterium deposited on its walls is small. The necessary cooling power for the source, about 160 W at the 5-K level and 400 W at 25 K, is provided by two separate cooling systems. Another UCN source is investigated at the moment at Garching. An existing set-up for superfluid helium at 0.7 K, MARK 3000, has been transferred from Japan and successfully cooled. It shall be tested in autumn at FRM-I. (author)