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

An atomic beam polarimeter to measure the nuclear polarization in the HERMES gaseous polarized hydrogen and deuterium target

An atomic beam polarimeter for measurement of the hyperfine population numbers and the absolute polarization of thermal atomic hydrogen (H) and deuterium (D) beams is described. The principle of measurement and the method of calibration are given. The polarimeter measures the 4 (6) relative hyperfine population numbers of a thermal H (D) beam in the region of 1013 atomss−1 to an absolute error of less than 0.01. The polarimeter has been in continuous operation with the internal polarized hydrogen and deuterium gas target used in the HERMES experiment at DESY since 1996

Molecular flow and wall collision age distributions

The use of storage cells has become a standard technique for internal gas targets in conjunction with high energy storage rings. In case of spin-polarized hydrogen and deuterium gas targets the interaction of the injected atoms with the walls of the storage cell can lead to depolarization and recombination. Thus the number of wall collisions of the atoms in the target gas is important for modeling the processes of spin relaxation and recombination. It is shown in this article that the diffusion process of rarefied gases in long tubes or storage cells can be described with the help of the one-dimensional diffusion equation. Mathematical methods are presented that allow one to calculate collision age distributions (CAD) and their moments analytically. These methods provide a better understanding of the different aspects of diffusion than Monte Carlo calculations. Additionally it is shown that measurements of the atomic density or polarization of a gas sample taken from the center of the tube allow one to determine the possible range of the corresponding density weighted average values along the tube. The calculations are applied to the storage cell geometry of the HERMES internal polarized hydrogen and deuterium gas targe

Time-of-flight measurements in atomic beam devices using adiabatic high frequency transitions and sextupole magnets

Atomic beam devices are frequently equipped with sextupole magnets to focus the beam or to act as spin filters in combination with RF-transitions for manipulating the hyperfine population within the atomic beam. A useful tool for the analysis of sextupole systems, the application of time-of-flight (TOF) measurements is presented. TOF measurements are enabled without mechanical beam chopper by utilizing adiabatic radio frequency transitions to select atoms within a certain time interval. This method is especially interesting for the use in atomic beam devices that are already equipped with RF-transitions and sextupole magnets and where space limitations or the required quality of the vacuum do not allow the installation of a mechanical chopper. The measurements presented here were performed with the atomic beam polarimeter of the HERMES polarized deuterium target and the results have been used to optimize the sextupole system of the polarimeter