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

The use ofstorage cells has become a standard technique f internal gas targets in storage rings. In case ofpolarized hydrogen or deuterium targets, recombination ofthe atoms occurs during the collisions ofthe atoms with the walls of the storage cell and may lead to a reduction ofthe target polarization. In this paper, we present measurements of recombination at the polarized internal hydrogen and deuterium gas target ofthe HERMES experiment in the years 1997–1999 within a temperature range of35–250 K : The underlying reaction mechanisms will be discussed with respect to the measured temperature and gas density dependence ofsurf ace recombination. Special attention is paid to the influence ofwater on recombination. These dependencies can be consistently described by a combination ofthree 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 ofnew 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,