Solid oxygen as converter for the production of ultra-cold neutrons

We have investigated solid oxygen as a converter material for the production of ultra-cold neutrons. In a first series of experiments the crystal preparation was examined. An optically semi-transparent solid $\alpha$-oxygen crystal has been prepared. In a second series of experiments such a crystal prepared indentically as in the first series of experiments has been exposed to the cold neutron flux of the MEPHISTO beam line of the FRM II. Ultra-cold neutrons produced inside the oxygen crystal have been extracted and the count rates have been measured at different converter temperatures. The results of these measurements give a clear signal of the superthermal UCN production mechanism in $\alpha$-oxygen. The mean free loss length of UCN inside the crystal at a temperature of 5\,K was determined to be in the order of $20\,\mathrm{cm}$.