Ultracold neutrons (UCN), i.e. neutrons with an energy below around 250neV, have a series of characteristic features: they can be stored in vessels with specially coated walls (such as beryllium or Fomblin oil) or be contained in magnetic bottles. They lose a great part of their energy by passing a moderate distance through the earth's gravitational field (approx. 100neV for each metre of elevation). The resultant storability of the UCN makes them particularly suitable for experiments related to the fundamental properties of neutrons.

The ultracold neutron (UCN) source at FRM II will be located in beam tube SR4. Expected densities and fluxes from this superthermal source with solid deuterium are greater than those previously achieved anywhere in the world: in storage operation, a density of approx. 10⁴cm^-3, and in continuous operation, which is also possible, a flux of at most 10⁶cm^-2s^-1. It should thus be possible to perform experiments on the fundamental properties of the neutron, such as lifetime and electric dipole moment (EDM), the accuracy of which will greatly exceed anything previously achieved. Other experiments are, however, conceivable which were out of the question at previously available UCN densities, for example rare final states in neutron decay and material investigations.

Neutrons from FRM II's cold source impinge upon a relatively small volume of solid deuterium, the converter. The solid deuterium is kept at a temperature at which the (temperature-dependent) cross-section for "upscattering" to higher neutron energies by interaction with the deuterium atoms is below the (temperature-independent) activation cross-section for absorption in deuterium. Due to the small volume, the UCN produced by interaction with the deuterium leave the converter before they are absorbed or upscat-tered back to higher energies and pass into a very much larger, evacuated volume, the UCN storage vessel. This has walls which are coated with beryllium and thus reflect UCN. The losses of ultracold neutrons are much lower in the storage vessel than in the converter and, over a period of several hundred seconds, an increased UCN density builds up in the storage vessel. This has been proven by simulations and direct integration of the corresponding diffusion equation. Every 3 to 5 minutes, the UCN may then be transferred to the experimental vessel attached to the port flange. If the UCN losses in the experimental vessel itself can be kept low enough, it is also conceivable to keep the connection between the storage vessel and experiment open while the storage vessel is being filled.

170cm³ of solid deuterium in the form of a hollow cylinder with an external radius of 7cm are arranged close to the reactor core in an approximately 8m long evacuated tube in SR4, in the immediate vicinity of the cold source. Supercritical helium is caused to flow around this cylinder, so keeping it a temperature of below 5K. The tube is made from an aluminium alloy (Al-6061-T6) and sputter-coated with beryllium. The converter and storage sections are thermally insulated from each other. The storage section is cooled to approx. 25K and the end remote from the reactor extends into the experimental hall, where various experimental instruments may be attached to the flange.