Heiko Timmers

A case study of collectivity, transfer and fusion enhancement

Abstract Precise fusion and quasi-elastic scattering excitation functions have been measured for the systems 40 Ca + 90,96 Zr at energies spanning the Coulomb barrier. From both the fusion and the quasi-elastic scattering data representations of the barrier distributions have been extracted. The barrier distributions of the two reactions are qualitatively very different. While the lighter system is well described by coupled-channels calculations including multi-phonon excitations, this coupling scheme fails completely for the heavier system, possibly due to multi-neutron transfer.

Nitrogen-rich indium nitride

K.S.A.B. would like to acknowledge the support of an Australian Research Council Fellowship. We would also like to acknowledge the support of the Australian Research Council through a Large grant and a Discovery grant; the support of a Macquarie University Research Development Grant, and the Australian Institute of Nuclear Science and Engineering for SIMS access.

Lattice damage produced in GaN by swift heavy ions

Wurtzite GaN epilayers bombarded at 300 K with 200 MeV Au-197(16+) ions are studied by a combination of transmission electron microscopy (TEM) and Rutherford backscattering/channeling spectrometry (RBS/C). Results reveal the formation of near-continuous tracks propagating throughout the entire similar to1.5-mum-thick GaN film. These tracks, similar to100 Angstrom in diameter, exhibit a large degree of structural disordering but do not appear to be amorphous. Throughout the bombarded epilayer, high-resolution TEM reveals planar defects which are parallel to the basal plane of the GaN film. The gross level of lattice disorder, as measured by RBS/C, gradually increases with increasing ion fluence up to similar to10(13) cm(-2). For larger fluences, delamination of the nitride film from the sapphire substrate occurs. Based on these results, physical mechanisms of the formation of lattice disorder in GaN in such a high electronic stopping power regime are discussed

Probing fusion barrier distributions with quasi-elastic scattering

Excitation functions have been measured for quasi-elastic back-scattering of 16O on 92Zr, 144Sm, 154Sm and 186W targets. A new procedure to extract representations of fusion barrier distributions is suggested and applied. The results are compared with barrier distributions obtained from fusion data and with coupled-channel calculations. The representations from quasi-elastic scattering are consistent with those from fusion below the average barrier energy. However, the effect of transfer smooths the quasi-elastic representations at higher energies, so that the sensitivity to the barrier distribution is reduced.

The nature of nitrogen related point defects in common forms of InN

The role of point defects related to the presence of excess nitrogen is elucidated for InN thin films grown by different techniques. Elastic recoil detection analysis has shown the presence of excess nitrogen in state-of-the-art InN films. Using x-ray photoelectron spectroscopy and x-ray diffraction it is shown that two distinct forms of point defects can be distinguished; one of these appears to be an interstitial form of nitrogen, common in some forms of polycrystalline InN. The other is associated with a combined biaxial and hydrostatic strain observed for molecular beam epitaxy (MBE) and chemical vapor deposition (CVD) grown films, and may be a mixture of the nitrogen-on-metal antisite defect and lower densities of indium vacancies and interstitial nitrogen. The high density of defects present in all the InN samples examined suggests that stoichiometry related point defects dominate the electrical and optical properties of the material. The difference in the type of point defect observed for polycryst...

Strong isotopic dependence of the fusion of 40Ca + 90,96Zr

Abstract Precise fusion excitation functions measured for the reactions 40Ca + 90,96Zr yield barrier distributions which are qualitatively very different. While the lighter system is well described by coupled-channels calculations including multi-phonon excitations, this coupling scheme fails completely for the heavier system, possibly due to the role of transfer channels.

Crystal size and oxygen segregation for polycrystalline GaN

The authors would like to acknowledge the support of a U. S. NICOP Contract, No. N00014-99-1-GO17 sponsored through the U. S. Office of Naval Research. One of the authors (K.S.A.B.) would like to further acknowledge the support of a Macquarie University Research Fellowship.

Simplifying position-sensitive gas-ionization detectors for heavy ion elastic recoil detection

Abstract Heavy ion elastic recoil detection is an effective tool for materials analysis. This has been facilitated by the development of novel detection systems, including large solid-angle gas-ionization detectors with position-sensitivity. With such detectors the exposure of the sample to the beam is reduced, however, data analysis can be complex. This paper reports on a detailed investigation of a detector design commonly used. The results of this work have led to suggestions for design-modifications, which considerably simplify detector operation and data analysis. The modifications have been implemented and tested. With the modified detector, the total energy information can be obtained by adding the signals from the anode electrodes, but also directly from a grid-electrode, obviating the need for the relative calibration of the anode electrodes. The subdivision of the energy loss electrode, together with a carefully chosen position of the entrance window to maintain optimum resolution for two ΔE signals, enable light and heavy ions to be resolved at the same gas pressure. The placement of a sawtooth electrode within the anode gives position information, which is linear and independent of atomic number and ion energy. Protons can be identified simultaneously with heavy ions by combining the information from the grid and residual energy signals, both amplified with high gain.

Absence of fusion suppression due to breakup in the 12C + 7Li reaction

Abstract The fusion cross-section for 12C + 7Li has been determined, using a 12 C beam, at an energy near the Coulomb barrier. Previous measurements showed a large discrepancy in cross-sections, leading to conflicting conclusions regarding fusion suppression due to breakup. The present work resolves this discrepancy, and shows that there is no significant inhibition of fusion near the barrier.

The development of a facility for heavy-ion elastic recoil detection analysis at the Australian National University

Abstract The design of a facility at the Australian National University for heavy-ion elastic recoil detection analysis with minimal beam exposure is presented. The system is based on an established technique using a position-sensitive gas-ionisation detector with a large acceptance solid angle. The kinematic energy spread of the detected recoil ions is corrected. The accuracy of this correction is demonstrated. The capabilities of the new facility are discussed.