ANKE is a new experimental facility for the spectroscopy of products from proton-induced reactions on internal targets. It has recently been implemented in the accelerator ring of the cooler synchrotron COSY of the Forschungszentrum Julich (FZ-Julich), Germany. The device consists of three dipole magnets, various target installations and dedicated detection systems. It will enable a variety of hadron-physics experiments like meson production in elementary proton–nucleon processes and studies of medium modifications in proton–nucleus interactions.

ANKE, a new facility for medium energy hadron physics at COSY-Jülich

The quasifree $pn\ensuremath{\rightarrow}d\ensuremath{\phi}$ reaction has been studied at the Cooler Synchrotron COSY---J\"ulich, using the internal proton beam incident on a deuterium cluster-jet target and detecting a fast deuteron in coincidence with the ${K}^{+}{K}^{\ensuremath{-}}$ decay of the $\ensuremath{\phi}$ meson. The energy dependence of the total and differential cross sections are extracted for excess energies up to 80 MeV by determining the Fermi momentum of the target neutron on an event-by-event basis. Though these cross sections are consistent with $s$-wave production, the kaon angular distributions show the presence of $p$ waves at quite a low energy. Production on the neutron is found to be stronger than on the proton but not by as much as for the $\ensuremath{\eta}$ meson.

Precision Measurement of the Quasifree pn→dφ Reaction Close to Threshold

Polarizing a stored proton beam by spin flip

We propose a method for polarising antiprotons in a storage ring by means of a polarised positron beam moving parallel to the antiprotons. If the relative velocity is adjusted to v/c ≈ 0.002 the cross-section for spin-flip is as large as about 2 . 1013 barn as shown by new QED calculations of the triple spin cross-sections. Two possibilities for providing a positron source with sufficient flux density are presented. A polarised positron beam with a polarisation of 0.70 and a flux density of approximately 1.5 . 1010 /(mm2 s) appears to be feasible by means of a radioactive 11C dc-source. A more involved proposal is the production of polarised positrons by pair production with circularly polarised photons. It yields a polarisation of 0.76 and requires the injection into a small storage ring. Such polariser sources can be used at low (100MeV) as well as at high (1GeV) energy storage rings providing a time of about one hour for polarisation build-up of about 1010 antiprotons to a polarisation of about 0.18. A comparison with other proposals show a gain in the figure of merit by a factor of about ten.

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A surprising method for polarising antiprotons

A source of highly dissociated, intense, cold hydrogen and deuterium atomic beams to be used as a dissociator in polarized gas targets for storage ring experiments has been developed. This dissociator is based on a plasma source which couples a 2.45 GHz surface wave to the discharge. At the end of the discharge tube a cold atomic beam is formed by means of a cooled nozzle. The entire system has been optimized to obtain a beam with a high atomic fraction and long-term stable reproducible operation. We measured a high degree of dissociation of more than 70% at gas throughputs up to 3 mbar l/s for both hydrogen and deuterium, corresponding to an atomic flow rate of 1.2×1020 s−1. Excellent long-term stability has been achieved. We also studied quantitatively the effect of recombination of atoms on the nozzle surface for various materials.

High intensity source for cold atomic hydrogen and deuterium beams

A high-density target of polarized atomic hydrogen gas for applications in storage rings was produced by injecting atoms from an atomic beam source into a T-shaped storage cell. The influence of the internal gas target on electron-cooled beams of 27 MeV α-particles and 23 MeV protons in the Heidelberg Test Storage Ring has been studied in detail. Target polarization and target thickness were measured by means of 27 MeV α-particles. For hyperfine states 1 + 2 a target thickness of n = (0.96±0.04) × 1014H→/cm2 was achieved with the cell walls cooled to 100 K. Working with a weak magnetic holding field (≈5 G) the maximum target polarization was PT = 0.84±0.02 when state 1 and PT = 0.46±0.01 when states 1 + 2 were injected. The target polarization was found to be constant over a period of 3 months with a net charge of Q ≈ 100C passing the storage cell.

Detailed studies of a high-density polarized hydrogen gas target for storage rings

The nuclear polarization of spin-polarized hydrogen gas targets can be measured using the spin coupling of electron and nucleus in the hydrogen atom. The polarized target atoms are excited by electron impact at low energy. Due to fine and hyperfine interaction the spin polarization of electron and proton is partially transferred to circular polarization of the emitted Hα-light. In a weak magnetic field the light polarization is proportional to the initial nuclear polarization with a measured analyzing power of 0.33 ± 0.03. The simple principle allows a compact polarimeter setup. It can be used as target polarization monitor, e.g., for targets at low energy storage rings where weak guide fields at the target are required.

J. Stenger

Papers

Detailed studies of a high-density polarized hydrogen gas target for storage rings

A compact polarization monitor for thermal beams of polarized hydrogen