Reduction of surface hydrogen on ultra cold neutron container materials by ion bombardement

The hydrogen concentration near to the surface of some typical ultra cold neutron container materials has been measured with theH(¹¹ B, ) 2 reaction. The concentration of 2 – 3 × 10¹⁶ H atoms/cm² is in agreement with results obtained by other methods. Using theoretical neutron scattering cross sections we confirm that at room temperature the experimentally known high UCN reflection loss rate could be caused by the measured concentration of hydrogen. The original hydrogen content has been reduced by ion bombardment to about one tenth of its original value. The redeposition of hydrogen has been studied as a function of the residual pressure. We conclude that it is possible to produce by sputtering container surfaces with hydrogen (and other surface impurity) contents sufficiently low and stable to test directly with UCN whether their losses are mainly due to upscattering on impurities or not.