UCN (under construction)

UCN scheme Top part: Cut-view (schematic) through the FRM II moderator vessel and the tangential end-to-end beam tube SR-6. The cryogenic supply lines to the UCN converter are inserted from the left-hand side. The converter vessel is positioned at a distance of 60 cm from the central fuel element (not shown) and 25 cm from the middle of the beam tube. The produced UCN are extracted to the right (SR-6 beam port exit) and fed into connected experiments.

Bottom part: Cut-view of the UCN converter vessel (zoomed out, schematic). Supercritical helium at 5 K flows between the two walls of an aluminium converter cap, cooling the hydrogen moderator inside the vessel and the deuterium converter frozen to the outer wall of the cap. The converter head is connected to cryogenic supply lines coming from the left-hand side.

Top part: Cut-view (schematic) through the FRM II moderator vessel and the tangential end-to-end beam tube SR-6. The cryogenic supply lines to the UCN converter are inserted from the left-hand side. The converter vessel is positioned at a distance of 60 cm from the central fuel element (not shown) and 25 cm from the middle of the beam tube. The produced UCN are extracted to the right (SR-6 beam port exit) and fed into connected experiments.

Bottom part: Cut-view of the UCN converter vessel (zoomed out, schematic). Supercritical helium at 5 K flows between the two walls of an aluminium converter cap, cooling the hydrogen moderator inside the vessel and the deuterium converter frozen to the outer wall of the cap. The converter head is connected to cryogenic supply lines coming from the left-hand side.

The ultra-cold neutron (UCN) source is installed in the tangential end-to-end beam tube SR-6. From one side (SR-6b). the necessary cryogenic lines are inserted, and to the other side (SR-6a beam port exit), the UCN are extracted into the connected experiments (see fig.).

The central part of the UCN source is the converter vessel, a double-walled toroidal-shaped aluminium container. It contains 12.5 mol of solid hydrogen (sH₂) to moderate the incoming thermal neutron flux (1·10¹⁵ cm^-2s^-1) to an effective neutron temperature of 40K. The converter assembly is cooled down to 5K by a continuous flux of a closed supercritical helium loop in between the outer and inner shell. The necessary cooling power is supplied by two conventional cold boxes to the separated closed supercritical He-loop. The sD₂ UCN converter (maximum amount 12.5 mol) is frozen onto the outer surface of the vessel by re-sublimation of D₂ gas to the solid phase. The pre-moderated incoming neutrons can enter the sD₂ converter, where they excite solid state excitations (mainly phonons) of the crystal lattice. By populating one single excited state by neutron scattering in the crystal lattice, the incoming neutron loses nearly its total initial energy and is converted into the energy regime of ultra-cold neutrons (<300 neV). The UCN generated by this process can leave the sD₂, and are guided to the SR-6a beam tube exit.