Hydrogen surface contamination in the storage of ultra-cold neutrons

We have measured the hydrogen surface concentration on several materials used in ultra-cold neutrons UCN containers and have found sufficient amounts of hydrogen (1-10 x 1016 atoms/cm2) to account for the anomalously short confinement times which have been observed. In light of this result, we have studied the desorption and adsorption properties of hydrogen on surfaces in the context of extending the storage time of these neutrons. The technique we use to measure the hydrogen concentration employs a resonance in the nuclear cross section of 1H(15N, α gamma)12C. In the course of this work, we have measured the width of the first resonance of this reaction to be 0.3 +- 0.1 keV in the rest frame of the 15N. We have also measured the re-adsorption of hydrogen onto surfaces in ultra-high vacuum (5 x 10-9Torr). We have found that with moderate heating (5000C), it is possible to clean surfaces which then take several days to accumulate a monlayer. We propose a design for an ultra-cold neutron container which is continuously pumped to the ultra-high vacuum pressure region. The container should, when cleaned of hydrogen surface contamination (by heating or sputtering), remain clean for long periods of time