Johannes Beijers

Proton-decaying states in Mg-22 and the nucleosynthesis of Na-22 in novae

Populating states in Mg-22 via the (p,t) reaction in inverse kinematics with a 55-MeV /nucleon Mg-24 beam, we have measured the proton-decay branching ratios of the levels at 5.96 MeV and 6.05 MeV and obtained an experimental upper limit on the branching ratio of the 5.71-MeV state. On the basis of the present and previous measurements, we assign spins and parities to the 5.96-MeV and 6.05-MeV states. We combine our branching ratios with independent measurements of the lifetimes of these states or their Ne-22 analogs to compute the resonance strengths and thereby the astrophysical rate of the Na-21(p,gamma)Mg-22 reaction. We perform hydrodynamic calculations of nova outbursts with this new rate and analyze its impact on Na-22 yields.

Alignment and orientation in He 21P and 31P excitation by electron impact

The authors have studied the orientation and alignment of the 21P and 31P states of helium caused by electron impact at incident energies between 50 and 80 eV both experimentally and theoretically as a function of the electron scattering angle. The angular correlations between inelastically scattered electrons and VUV de-excitation photons were measured, particularly in the region where the orientation changes sign. The experimental results were compared with several distorted-wave models. The best of these models gave reasonably good agreement with the experimental data for excitation of the 21P and 31P states for scattering angles of less than 60 degrees and qualitative agreement for larger angles.

A pepper-pot emittance meter for low-energy heavy-ion beams

A novel emittance meter has been developed to measure the four-dimensional, transverse phase-space distribution of a low-energy ion beam using the pepper-pot technique. A characteristic feature of this instrument is that the pepper-pot plate, which has a linear array of holes in the vertical direction, is scanned horizontally through the ion beam. This has the advantage that the emittance can also be measured at locations along the beam line where the beam has a large horizontal divergence. A set of multi-channel plates, scintillation screen, and ccd camera is used as a position-sensitive ion detector allowing a large range of beam intensities that can be handled. This paper describes the design, construction, and operation of the instrument as well as the data analysis used to reconstruct the four-dimensional phase-space distribution of an ion beam. Measurements on a 15 keV He(+) beam are used as an example.

HE-2+-H2 COLLISIONS - NONDISSOCIATIVE AND DISSOCIATIVE ONE-ELECTRON CAPTURE

Electron-redistribution processes in collisions of He2+ ions on H2 are studied for energies from 1 to 25 keV amu-1. One-electron capture and target excitation cross sections are determined by photon-emission spectroscopy. At energies exceeding approximately 5 keV amu-1 capture into excited states is the dominant charge-transfer process while at lower energies one-electron capture into the ground state dominates. The latter process is found to be accompanied by dissociation of the target and excitation of the resulting hydrogen atom. This can be explained qualitatively with the classical overbarrier model under the assumption that during the collision binding energy is exchanged between the electrons, which is a kind of auto-excitation process.

STATE-SELECTIVE ELECTRON-CAPTURE AND CORE EXCITATION IN SLOW NE6+-HE COLLISIONS

The authors have studies in this paper the competition between (projectile) core-conserving and core-varying single-electron capture processes in Ne6+-He collisions for impact energies between 0.07 and 1.2 keV amu-1. By deconvoluting the spectra of the VUV radiation emitted by the decaying product ions, absolute state-selective cross sections for these processes were determined. The authors also obtained capture cross sections for metastable Ne6+ ions colliding on helium. Core-conserving capture channels dominate the charge transfer process at the higher impact energies investigated; with decreasing impact energies, however, the core-varying channels gain strength at the expense of the core-conserving channels. The results show that the capture cross sections for the core-conserving processes involving metastable primary ions are comparable with those for the ground-state primary ions. At lower impact energies, however, the capture cross sections for the metastables are almost three times larger than the cross sections for ground-state primary ions.

State-selective charge transfer in slow collisions of O3+ with H and H-2

We have used photon emission spectroscopy to determine absolute, state-selective cross sections for single-electron capture by O3+ ions colliding on atomic and molecular hydrogen. The impact energy was varied between 45 and 752 eV amu(-1). For O3+ + H collisions we find that the cross section for capture into the 2p3s state of O2+ strongly increases with impact energy up to an energy of approximate to 100 eV amu(-1). At higher impact energies the 2p3s cross section becomes comparable to the more or less constant 2p3p cross section. For the O3+ + H-2 system these state-selective cross sections both stay approximately constant over the entire energy range investigated. Our results will be compared with existing experimental and theoretical data.

Characteristics of the KVI electron cyclotron resonance ion source

In this article, an update will be given of last year’s activities to modify the CAPRICE-type ECR ion source into an AECR-type ECR ion source as developed in Berkely, Argonne and Jyvaskyla. Here we will present measurements on the radial and the axial magnetic field, the vacuum system, report on the ECR plasma produced by the KVI-AECR ion source.

Electron capture into He+ (41) states in collisions of He2+ on Li

Absolute cross sections for He II (n=4 to n=3) visible light emission subsequent to charge changing collisions (0.8-9.75 keV amu-1) of He2+ with Li have been measured. The respective l state selective He+ (41) electron capture cross sections have been determined from the intensity of the He II (n=4 to n=3) transition along the ion beam path by means of a deconvolution technique based on the lifetimes of the He+ (41) states.

Optimization of a charge-state analyzer for electron cyclotron resonance ion source beams

A detailed experimental and simulation study of the extraction of a 24 keV He-ion beam from an ECR ion source and the subsequent beam transport through an analyzing magnet is presented. We find that such a slow ion beam is very sensitive to space-charge forces, but also that the neutralization of the beams space charge by secondary electrons is virtually complete for beam currents up to at least 0.5 mA. The beam emittance directly behind the extraction system is 65 pi mm mrad and is determined by the fact that the ion beam is extracted in the strong magnetic fringe field of the ion source. The relatively large emittance of the beam and its non-paraxiality lead, in combination with a relatively small magnet gap, to significant beam losses and a five-fold increase of the effective beam emittance during its transport through the analyzing magnet. The calculated beam profile and phase-space distributions in the image plane of the analyzing magnet agree well with measurements. The kinematic and magnet aberrations have been studied using the calculated second-order transfer map of the analyzing magnet, with which we can reproduce the phase-space distributions of the ion beam behind the analyzing magnet. Using the transfer map and trajectory calculations we have worked out an aberration compensation scheme based on the addition of compensating hexapole components to the main dipole field by modifying the shape of the poles. The simulations predict that by compensating the kinematic and geometric aberrations in this way and enlarging the pole gap the overall beam transport efficiency can be increased from 16 to 45%.A detailed experimental and simulation study of the extraction of a 24 keV He(+) beam from an ECR ion source and the subsequent beam transport through an analyzing magnet is presented. We find that such a slow ion beam is very sensitive to space-charge forces, but also that the neutralization of the beams space charge by secondary electrons is virtually complete for beam currents up to at least 0.5 mA. The beam emittance directly behind the extraction system is 65 π mm mrad and is determined by the fact that the ion beam is extracted in the strong magnetic fringe field of the ion source. The relatively large emittance of the beam and its non-paraxiality lead, in combination with a relatively small magnet gap, to significant beam losses and a five-fold increase of the effective beam emittance during its transport through the analyzing magnet. The calculated beam profile and phase-space distributions in the image plane of the analyzing magnet agree well with measurements. The kinematic and magnet aberrations have been studied using the calculated second-order transfer map of the analyzing magnet, with which we can reproduce the phase-space distributions of the ion beam behind the analyzing magnet. Using the transfer map and trajectory calculations we have worked out an aberration compensation scheme based on the addition of compensating hexapole components to the main dipole field by modifying the shape of the poles. The simulations predict that by compensating the kinematic and geometric aberrations in this way and enlarging the pole gap the overall beam transport efficiency can be increased from 16% to 45%.

STATE-SELECTIVE ELECTRON-CAPTURE IN SLOW COLLISIONS OF C6+ AND O6+ WITH HE

State-selective capture and emission cross sections for single-electron capture in collisions of bare C6+ ions and He-like O6+ ions with helium are presented for impact velocities between 0.05 and 0.27 au (62<or=E(eV amu-1)<or=1852). Experimental data have been obtained using VUV photon emission spectroscopy. They join previous higher energy measurements smoothly. Detailed comparisons are made with existing AO and MO close-coupling calculations. The entire data set is consistent with results concerning the influence of the projectile core on the capture process found in a theoretical study on collisions between multiply-charged ions and atomic hydrogen.