J. Stewart

The HERMES polarized atomic beam source

Abstract The HERMES atomic beam source is used to provide nuclear polarized hydrogen or deuterium atoms for the HERMES target. The setup including the dissociator, the sextupole magnet system and the high-frequency transitions is described. Measurements of the hydrogen intensity of up to 6.4×10 16 atoms / s in two hyperfine substates and of the deuterium intensity of up to 5.8×10 16 atoms / s in three hyperfine substates are presented. A degree of dissociation of 92.8% for H (94.5% for D) at the entrance of the storage cell and a nuclear polarization of around 0.97 (H) and 0.92 (D) have been found constant within a couple of percent over the whole running period of the HERMES experiment.

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

Abstract 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 10 13 atoms s −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.

A gas analyzer for the internal polarized target of the HERMES experiment

A gas analyzer has been developed for the internal polarized target of the HERMES experiment at DESY in order to determine the relative amount of atomic and molecular hydrogen or deuterium in a gas sample. The precise quantitative knowledge of this ratio is crucial because the nucleons in atoms and molecules contribute differently to the average nuclear polarization of the target gas. A new calibration technique used to derive the relative sensitivity to atoms and molecules is presented. As an example, it is shown how the gas analyzer is used within the HERMES environment to divide the molecules in the gas sample into an unpolarized and a potentially polarized fraction.

THE HERMES POLARIZED 3HE INTERNAL GAS TARGET

Abstract The HERMES experiment is investigating the spin structure of the proton and neutron via deep-inelastic scattering of polarized positrons from polarized nuclear targets. The polarized positrons are provided by the HERA positron storage ring at DESY, Hamburg, Germany. The targets are pure internal gas targets. Data acquisition began in 1995, utilizing a polarized 3 He internal gas target to study the spin structure of the neutron. The target gas was polarized using the metastability-exchange optical-pumping technique and then injected into a cryogenically cooled target cell. The target was designed to operate with either longitudinal or transverse directions of polarization. Operating conditions included polarizations of up to 54% and target thicknesses of 1×10 15 nucleons/cm 2 . In this paper the HERMES polarized 3 He internal gas target is described in detail.

The storage cell of the polarized H/D internal gas target of the HERMES experiment at HERA

The storage cell of the internal, longitudinally polarized, atomic gas target of the HERMES experiment is presented. The polarized atoms of hydrogen or deuterium are accumulated in an open-ended thin walled storage cell through which the circulating electron or positron beam of the HERA accelerator passes. The target areal density is 10 14 atoms=cm 2 ; two orders of magnitude larger than without the cell. The construction details of the cell are described and the cell’s performance during HERMES run of 1997–2000 is discussed. r 2002 Published by Elsevier Science B.V.

A silicon strip recoil detector for momentum measurement and tracking at HERMES

A recoil detector will he installed surrounding the internal gas target of the HERMES experiment at DESY. The recoil detector will improve the selection of exclusive events by a direct measurement of the momentum and track direction of the recoiling particle. The innermost layer of this recoil detector is a new silicon strip detector (SSD). Since Monte Carlo simulations predict proton momenta as low as 100 MeV/c, the SSD will be placed inside the HERA vacuum. A new setup of the electronics enables a dynamic range from below 4 fC at a signal-to-noise ratio of 6.8 up to 270 fC. In this report, the assembly of the first module and the final setup within the HERMES experiment will be presented. Results from charge-injection tests of a prototype module are shown.

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,

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

Abstract 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.