Measurements of atomic recombination in the HERMES polarized hydrogen and deuterium storage cell target

The use of storage cells has become a standard technique for internal gas targets in storage rings. In case of polarized hydrogen or deuterium targets, recombination of the atoms occurs during the collisions of the atoms with the walls of the storage cell and may lead to a reduction of the target polarization. In this paper, we present measurements of recombination at the polarized internal hydrogen and deuterium gas target of the HERMES experiment in the years 1997-1999 within a temperature range of 35-250 K. The underlying reaction mechanisms will be discussed with respect to the measured temperature and gas density dependence of surface recombination. Special attention is paid to the influence of water on recombination. These dependencies can be consistently described by a combination of three reaction mechanisms. The first one, dominating at temperatures above 120 K, is an activated Eley-Rideal reaction. A second process dominating below 100 K in case of new storage cells, is interpreted as a tunneling reaction between a physisorbed state and an atom chemically bound on the surface. When the storage cell coating is aged by the influence of the HERA positron beam, the second process is suppressed and a third (weaker) process starts to dominate. This third mechanism is described by Langmuir-Hinshelwood type reactions between physisorbed atoms. (C) 2002 Published by Elsevier Science B.V.