The HERMES gas target: 10 years on

Over the past decade, the HERMES experiment at HERA, DESY, has successfully explored the spin structure of the nucleon. Unlike the H1 and ZEUS experiments, which detect collisions between electron and protons travelling in opposite directions in beams stored in HERA, HERMES has scattered HERA’s 27.5 GeV polarized electron beam off polarized nucleons at rest in a sophisticated target cell of polarized hydrogen or deuterium gas. This target, which has run throughout the decade, has been a key to the experiment’s success.

To achieve its goals, the design of the target had to overcome three major challenges. These were to develop a gas target of high polarization with unequalled areal density; to measure its electron and nuclear polarization online to a precision of 3%; and to operate a target over long periods in the environment of a high-energy storage ring, without affecting the operation of the collider experiments too much.

Meeting the challenges

The first challenge dates back to the design achieved while preparing a proposal for the FILTEX experiment, which was submitted to CERN in 1985. The idea was that antiprotons circulating in the Low Energy Antiproton Ring at CERN were to be polarized by spin-dependent attenuation of the beam, a process known as spin-filtering. To achieve a reasonable build-up time of around 10 hours, this required a hydrogen filter target with high polarization, P ∼ 1, and an areal density, t = 10¹⁴ atoms/cm². These figures represent a benchmark that still holds today. For a deep inelastic scattering (DIS) experiment in a high-energy electron ring, luminosity in the order of 10³¹/cm² is needed; for a 30 mA electron current, this requires target figures comparable to those in the FILTEX proposal. However, the densities of gas-jet targets available in the 1980s were a few 10¹¹/cm², and the most dense thermal atomic-beam target recently developed has been for the Relativistic Heavy Ion Collider at Brookhaven with a density of (1.3±0.2) × 10¹²/cm².

The areal density of a polarized jet can be boosted by a factor of around 100 by using a storage cell or vessel, as Willy Haeberli of the University of Wisconsin proposed in 1965. Figure 1 illustrates this principle. Polarized atoms enter the T-shaped storage cell ballistically, without hitting the walls, via a narrow feed tube. On their way out, they perform many collisions with the walls (∼300) resulting in an increase of the gas density.

In the 1980s and early 1990s, high-intensity atomic-beam sources and radiation-resistant coatings for the cell walls were developed, and the first test of a high-density storage-cell target in a storage ring was performed in 1992 in the Heidelberg Heavy-Ion Test Storage Ring. This had a target density t = (0.96±0.04) × 10¹⁴ atoms/cm² and a measured polarization P = 0.46±0.01 in a low magnetic field, which was expected to double in a strong guide field (Zapfe et al. 1996).

During the same period, the use of a polarized storage cell target for DIS experiments was being discussed. A first letter of intent to DESY dates back to 1988 and in 1990 the HERMES collaboration submitted a proposal for the study of the nucleon’s spin structure. After the successful target tests and encouraging results on the electron polarization, HERMES was approved in 1992 and constructed during the following two years. For the commissioning run in 1995, an optically pumped ³He target was operated to study the neutron-spin structure (de Schepper et al. 1998). Then in 1996, the hydrogen target set-up was installed. The elliptical, 40 cm long storage cell operating at 100 K was protected by a narrow tungsten collimator – the “bottleneck” of the HERA electron ring.

The challenge of a precise polarization measurement independent of the stored beam was met by using a polarimeter that measured the complete substate population of a sample beam extracted from the centre of the cell. This made possible the precise online determination of the target parameters, i.e. the polarization of protons and electrons, P_z and P_e, respectively, and the fraction of molecules, which for a high-quality surface was at most a few percent. One of the potentially harmful effects is RF depolarization caused by harmonics of the HERA bunch frequency of 10.4 MHz, so a strong guide field was carefully chosen to avoid resonances. With all these precautions, stable operation with longitudinally polarized hydrogen was obtained, leading to high-quality data on the proton-spin structure.

In 1998, the target was converted to one of longitudinally polarized deuterium with nuclear spin one. This allowed not only vector polarization P_z but also second-rank tensor polarization P_zz to be produced. The latter is related to the structure function b₁ of the deuteron, which HERMES measured for the first time. Owing to the low magnetic moment of the deuteron, decoupling of nuclear and electron spin at the guide field of 0.33 T was nearly complete, resulting in close to ideal performance, i.e. no detectable depolarization by the cell walls. In addition, recombination to molecules was also negligible. The experiment collected a large data sample of high-quality deuterium data, in particular during the successful run in 2000. The extremely stable target performance during this run is shown in figure 2.

For the next phase, from 2001 to 2005, a transversely polarized hydrogen target was required to study transversity, the last missing leading-twist structure function of the nucleon. For this purpose, a dipole magnet with a large gap and high uniformity was developed with a field limited to 300 mT. This resulted in acceptable synchrotron radiation power levels and high target polarization.

All the running with the polarized target at HERMES was performed in parallel with the operation of the collider detectors H1 and ZEUS. The areal density achieved with the storage-cell target was only about an eighth of the density allowed before it would adversely affect the stored electron beam. There were also special studies using unpolarized gas such as H₂, D₂, He, N₂, Ne, Ar, Kr and Xe in the target cell. In this case the density was chosen according to the maximum allowed reduction of the electron-beam lifetime, yielding higher statistics relative to running with the polarized target. In additional special end-of-fill runs, when the collider experiments were switched off, the remaining beam of 12-15 mA was consumed within about an hour at extra-high densities, yielding high statistics data samples with little extra time.

End of an era

In the course of running the HERMES experiment, the physics interest has moved from semi-inclusive to exclusive measurements, such as deeply virtual Compton scattering). Clean exclusive measurements require the detection of recoil particles, e.g. the proton. However, a recoil detector turned out to be incompatible with the polarized storage cell. Therefore, the HERMES collaboration decided to run during the final phase from 2006 to 2007 with a recoil detector in addition to the standard forward spectrometer and an unpolarized high-density, very compact storage-cell target. On 13 November 2005 polarized running ended and the target was removed. Commissioning of the recoil detector to replace the target set-up began in February 2006.

The removal of the target marks the end of a very fruitful era extending over 20 years. Groups from Beijing, Erlangen, Ferrara, Heidelberg, Liverpool, Marburg, Munich, Wisconsin, Yerevan and elsewhere have contributed to the target’s outstanding performance and stability. In this way, the original idea for FILTEX, which led to the HERMES target, has enabled 20 years later a wealth of new results on nucleon-spin structure. Fortunately, after ten years of operation in HERA, there is a good chance that the present target may serve future experiments. The project for the Facility for Antiproton and Ion Research (FAIR) at GSI, Darmstadt, with its planned antiproton source, has again stimulated interest in using spin filtering to produce polarized stored beams of antiprotons, this time for measurements by the Polarized Antiproton Experiments (PAX) at the facility’s High Energy Storage Ring. Tests with protons and antiprotons in preparation for PAX are foreseen at Forschungszentrum Jülich and CERN. The HERMES target may thus play a key role in paving the way for a new experiment at FAIR, aimed at studying hadron structure in the interaction of polarized protons with polarized antiprotons.

• The contribution of numerous students, postdocs, senior scientists and technicians to the unprecedented performance of the HERMES target is gratefully acknowledged. Special thanks are owed to my colleagues G Court, P Dalpiaz-Ferretti, D Fick, G Graw, W Haeberli, B Povh, and K Rith; to P Lenisa, the target coordinator from 2000 to 2005; to the funding agencies, in particular the Bundesministerium für Bildung und Forschung in Germany and INFN in Italy; and to the HERMES and DESY management.

The operation and performance of the hydrogen and deuterium target over the full running period are summarized in a paper (Airapetian et al. 2005) in which many additional references can be found.