Experimental results on X-rays in antiprotonic Te, Sm, and Pt isotopes are presented. Attenuation of some lines due to the dynamic E2 mixing with nuclear rotational excitations was observed. The results are analyzed in terms of the p optical potential. Some effects of deformation are discussed. Previous findings such as a strong ls potential and the density dependence of the optical potential are confirmed.

Experiments with relativistic ions at the test storage ring TSR [P. Baumann et al., Nucl. Instr. and Meth. A268 (1988) 531] demonstrate the potential of the interaction of laser light with energetic stored ions for spectroscopic purposes as well as for manipulation of the ion velocity. Latest results for Li+ ions are reported. At the ion energies available at ESR [B. Franzke, Nucl. Instr. and Meth. B24B25 (1987) 19] it will become possible to prepare and store bare ions up to U92+. Experiments using these exotic beams are discussed and an outlook to the situation at even higher energies is given.

As a consequence of Liouville's theorem, the momentum spread and the emittance in charged-particle beams cannot be reduced by means of ion optics. This limits the number of ions that can be accelerated or decelerated in a circular machine and, in turn, the intensity and luminosity availab le for experiments. Electron cooling can overcome this obstacle. The mechanism underlying electron cooling is equivalent to that of temperature relaxation in a plasma consisting of a hot and a cold component. Initially electron cooling was proposed for the accumulation of antiprotons, but today its application is found mainly in the improvement of light- and heavy-ion beams to be used for precision experiments in atomic and nuclear physics.

Recent measurements of fine-structure splitting in\(\bar p\) atoms of174Yb are analysed. Effects of nuclear deformation are calculated. The strength of nuclear spin-orbit coupling is determined and its implications on theN\(\bar N\) potential are discussed.

First electron cooling experiments were performed with 107 to 2×109 stored antiprotons of 50, 21 and 6 MeV at the Low Energy Antiproton Ring (LEAR) at CERN. Most effort was put into the study of the longitudinal cooling. Schottky pick-up signals were used to measure the equilibrium momentum spread and the longitudinal cooling time. From the equilibrium between stochastic heating and electron cooling the longitudinal friction force in the low 103 m/s relative velocity range could be deduced. This method was used also to increase the cooling force by improving the alignment between the antiproton and the electron beam. Some of the experimental data are compared with results of a simulation program for electron cooling (SPEC).

The introduction of electron cooling has opened up new possibilities in atomic, nuclear and particle physics. Now many newly constructed ion storage rings are making use of this technique.

The production of antihydrogen by merging beams of antiprotons and positrons is described. Both beams, kept in storage devices, are continuously recirculated. Antihydrogen is formed by radiative recombination of positrons and antiprotons. Production rates of a few thousand per second are expected. The semi-relativistic atomic beam of antihydrogen would have a divergence of less than 1 mrad and a beam diameter of a few millimeter. The possibilities to increase these rates by induced recomtination are discussed. The scheme of antihydrogen production in overlapping beams is compared to other approaches.

The first results are presented of electron cooling experiments in the Low-Energy Antiproton Ring (LEAR) at CERN, performed with a proton beam of about 50 and 21 MeV. The number of stored protons ranged from 107 to 3 × 109. Cooling times of the order 1 s and proton drag rates of up to 0.7 MeV/s were obtained. The capture of cooling electrons by protons producing hydrogen atoms was used to derive an effective electron temperature (0.25 eV). From the angular profile of the neutral hydrogen beam an upper limit of 3π mm.mrad could be deduced for the horizontal equilibrium proton-beam emittance. The lowest equilibrium momentum spread was 2 × 105 (FWHM), as derived from the analysis of the longitudinal Schottky signal. This Schottky signal exhibited an unusual behaviour with beam intensity and under certain conditions showed a doublepeak structure which was associated with collective beam noise. For very cold beams transverse instabilities were observed, which resulted in a rapid spill-off of protons and a stabilization at lower intensities. The threshold of these instabilities was raised by heating the proton or the electron beam. The cooling of a bunched proton beam was investigated. The reduction of the proton momentum spread led to bunch lengths of about 2 m, containing 3 × 108 protons.

We have measured the X-rays emitted in the energy region from 2 to 30 keV from the electron beam in the electron cooler built for the CERN Low Energy Antiproton Ring (LEAR). Besides an intense continuum of bremsstrahlung photons, a high rate of characteristic X-rays is also observed. A sensitive dependence of the intensity of these lines on the parameters of the electron cooler, such as beam energy, loss current, and the residual pressure is found. This gives information about the parts of the cooler which are hit by electrons, in particular by those in the loss current, and therefore demonstrates that X-rays could be a sensitive diagnostics for electron beam losses. The results give indications of the background which can be expected in X-ray measurements of electron-ion recombination in the cooler. A beneficial use of this X-ray background could be for wavelength calibration.

We propose a scheme to stack and accumulate positrons, emitted randomly from a radioactive source. The positrons are moderated and accumulated at low energy.

The motivation of using electron cooling in low-energy antiproton storage rings and the expected cooling performance are discussed. Results obtained recently, during the first operation of electron cooling in LEAR at CERN with a 50 MeV proton beam, are summarized, concerning in particular the equilibrium beam properties, the recombination between cooling electrons and cooled protons, and the deceleartion of acceleration of protons by friction in the electron beam. Conclusions are drawn for the formation of antihydrogen with the cooled antiproton beam, and for the deceleration of antiprotons to energies close to or below 1 MeV.

Recent work on the electron cooling device at the CERN Low Energy Antiproton Ring (LEAR) is summarized. This included an improvement of the collector performance and of the vacuum. Relations between the different cooler parameters were measured with a view to simplifying the control system. Results of vacuum tests performed before the installation in LEAR are presented, and a short description of the status of the cooling device after its installation is given.

Based on our measurement of the ratio f(π−π0p)f(π+π−n)=2.07±0.05 for antiproton annihilation at rest in liquid deuterium, we find that S-wave annihilation of the antiproton on the proton or neutron into ππ is dominant. We quote a 95% confidence level upper limit of 8% for P-wave annihilation into ππ.

The strong ¯p-nucleus spin-orbit interaction was investigated in a measurement of the strong-interaction effects of the 9→8 transition in ¯p174Yb at the Low-Energy Antiproton Ring (LEAR) at CERN. This measurement was part of an experimental programme where, for the first time, the fine-structure components of the last observable X-ray transition in a ¯p atom, which carries information on the strong ¯p-nucleus interaction, were resolved and studied individually. The observed splitting ΔEexp=2408±26 eV consists of the electromagnetic fine-structure splitting ΔEFS=2350 eV and an additional splitting Δɛ=58±26 eV. In addition, one finds a significant difference in the level widths of Δ=195±59 eV with the larger value⇊=1216±41 eV for the lower fine-structure level. This experiment follows an earlier measurement on ¯p138Ba, where the transition 8→7 is influenced by the strong interaction. In this case, however, the fine-structure components could not be resolved. The results for174Yb may be attributed to a spin-orbit (LS) term in the complex strong-interaction potential.

We have observed a large yield (5.77±0.16/ann.) of neutrons from antiproton annihilation at rest in a thin uranium target. Significant components of fission, evaporation and direct neutrons comprise this total. The large fission yield (2.50±0.10/ann.), as well as the high temperature (94±10 MeV) of the direct component, have not been anticipated in the intranuclear cascade model.

The outcome of the measurements on antiprotonic atoms performed in recent years is summarized. The experiments, done at the low-energy antiproton ring (LEAR), mainly studied the strong interaction of antiprotons with nucleons. The principle goals of the investigations were the measurement of level energies and widths in antiprotonic atoms with the aim of getting information on the overall antiproton-nucleus and antiproton-nucleon interactions in general, and the study of the spin and isospin dependence of that force. Moreover, the magnetic moment of the antiproton was determined with a factor of 2 higher precision than previously known. Furthermore, the antiproton absorption was studied in detail. The analysis of the strong-interaction effects in terms of an antiproton-nucleon potential is in progress.

A new collector design for electron coolers was done in the Karlsruhe electron cooling group at LEAR (CERN). The performance of the collector was tested in a linear set-up with an electron beam of energies lower than 18 keV and a current of up to 1.25 A. In this range, current loss rates below 6 × 10-5 were achieved at a collector perveance of ⩽27 μA V-3/2. The maximum collector perveance in the present set-up is 37 μA V-3/2.

Thomson scattering of laser light was applied to a relativistic cooling electron beam. High-power laser pulses were necessary because of the very low scattering cross section. A spectrometer of high resolution and large background suppression factor in combination with time-gated photon counting served for signal detection. Analysis of Doppler shift and broadening of the backscattered light provided the determination of the electron beam energy with a precision of 2 × 10-3, and an upper limit of the longitudinal energy spread of 10-3 eV. A ratio of longitudinal to transverse electron temperature of 10-2 was found, indicating a flattened velocity distribution as is expected from the acceleration of the electrons.