Preliminary measurements of the performance of the source of ultra-cold and very cold neutrons at ILL

The ultra-cold (UCN) and very cold neutron (VCN) source, which has been put into operation in April 1977 at the high-flux reactor of the Institut Laue-Langevin (ILL), provides a beam of neutrons with velocities lower than 50ms-1 (wavelength longer than 80A) which do not exist in the other beams. A description is given of the different parts of the source: the in-pile guide, in an inclined beam tube, with a room-temperature water converter in a flux of 6x1014ncm-2s-1, the curved guide inside an external shield, and the distribution system to three experimental sites. Theoretical neutron spectra of such a source are indicated. The measurements of the flux and of the spectra are made by means of four different methods conventional time-of-flight over the whole range, time-of-flight with a pre-selection of UCN, gravity spectrometer, and UCN storage in a 'bottle', for the UCN range. The total flux in the beam is 2x105ncm-2s-1 with the maximum of the spectra at 60ms-1. The total UCN flux (velocity lower than 6.2ms-1) is, after correction of the TOF and 'bottle' measurements, evaluated at about 300ncm-2s-1. (author)