Concepts of UCN sources for the FRM-II

Three concepts for sources of ultra-cold neutrons (UCN) for the reactor FRM-II at Garching near Munich are studied: one, Mini-D₂, is a source with 170 cm³ of solid deuterium in the beam tube SR4 and the second one a large solid-deuterium source (volume about 30 dm³), mounted in the beam tube SR5 as an advanced cold source with a number of neutron guides. The third one, Mark 3000, uses superfluid ⁴He at a cold-neutron guide. A UCN density of up to 7×10⁴ cm^?3 may possibly be achieved in the storage volumes of Mini-D₂ yielding more than 10⁹ UCN for extraction to an attached experimental setup. The usable UCN flux at the periphery of the large deuterium source is predicted to be 2×10⁷ cm^?2 s^?1. Mark 3000, finally, is expected to yield a UCN density of about 10⁵ cm^?3.