N. Rowley

On the “distribution of barriers” interpretation of heavy-ion fusion

Abstract The significance of the interpretation of heavy-ion fusion cross sections in terms of a distribution of potential barriers is discussed. The smoothing due to the quantal barrier penetration is shown to replace a set of discrete barriers by an effective continuous distribution. It is shown how this smoothed distribution may be obtained rather directly from the measured cross sections at near-and sub-barrier energies.

Strong dependence of sub-barrier fusion on the nuclear hexadecapole deformation

Abstract Fusion cross-sections σ for the reaction 186 W + 16 O have been measured with high precision at small energy intervals, for energies E spanning the barrier region. The distribution of fusion barriers, derived from the curvature of Eσ , has the shape expected classically from a target nucleus with a negative hexadecapole deformation. Comparison with the previously measured distribution for 154 Sm + 16 O demonstrates the strong sensitivity of fusion to the hexadecapole deformation. The magnitudes of the deformation parameters for 186 W, extracted using a purely geometrical description, are different from those determined from non-fusion measurements. This is interpreted as reflecting the sensitivity of the excitation function to small additional couplings.

Fusion cross sections at deep sub-barrier energies

A recent publication reports that heavy-ion fusion cross sections at extreme subbarrier energies show a continuous change of their logarithmic slope with decreasing energy, resulting in a much steeper excitation function compared with theoretical predictions. We show that the energy dependence of this slope is partly due to the asymmetric shape of the Coulomb barrier, that is its deviation from a harmonic shape. We also point out that the large low-energy slope is consistent with the surprisingly large surface diffusenesses required to fit recent high-precision fusion data.

Cranked-shell-model calculations in a truncated space

Abstract Cranking calculations are presented for a deformed single-j shell containing particles which interact through short-range two-body forces. Truncation schemes which might pave a way for carrying out realistic multi-j shell calculations are investigated.

Fusion excitation function measurements for the 16O+58Ni and 16O+62Ni systems

Abstract High precision fusion excitation functions have been measured for the 16 O+ 58 Ni and 16 O+ 62 Ni systems from which fusion barrier distributions have been evaluated. Coupled-reaction-channels (CRC) calculations, which describe elastic and quasi-elastic scattering, also satisfactorily reproduce the fusion cross sections and barrier distributions. The small value of Z 1 Z 2 in this case leads to barrier distributions with relatively little structure. However, in conjunction with the detailed elastic scattering data for these systems, this allows us to elucidate the role of previously ignored states in 16 O in pushing the entire distribution to lower energies. These shifts are consistent with derived magnitudes of polarization potentials for both systems.

Neutron flow and necking in heavy-ion fusion reactions

Abstract The “barrier distributions” arising from neutron transfer channels are discussed. It is shown that sequential transfers can lead to the broad distributions characteristic of many experimental fusion cross sections. Finite Q-value effects can lead to neutron flow and a build up of a neck between the colliding nuclei. “Anti-necking” may also occur.

Effect of deformation on the elastic and quasielastic scattering of heavy ions near the Coulomb barrier

Abstract It is shown that elastic and quasielastic cross sections at large angles provide the same information on static deformation properties as does sub-barrier fusion. The results have their “classical” form even for coupling to a small number of excited states.

Fusion oscillations for symmetric light heavy-ion systems

Abstract The low-energy fusion cross section for 20 Ne+ 20 Ne has been measured. The data extend the systematics on fusion oscillations for such light symmetric systems. A simple extension of the usual barrier penetration model displays the decisive role of the symmetrisation of the system. It is also capable of qualitatively explaining the trends in this effect, in particular its decreasing importance as the mass of the system increases. The structure is essentially due to the sharpness of the cut-off of transmission coefficients as a function of angular momentum. The curvature of the Coulomb barrier is important in determining these coefficients and it is shown that the required values of this quantity are consistent with reasonable values of the surface diffuseness of the nuclear potential.

Effect of multiphonon coupling on heavy-ion fusion

Abstract The effect of the strong coupling of multiphonon states on the fusion of heavy ions is discussed. The model (CCFUS) of Dasso and Landowne is reviewed and it is shown that it is possible to generalise this to multiphonon states comprised of identical phonons without greatly complicating the calculations. The effect of the muliphonon channels on the barrier distribution for fusion is discussed in detail. In particular, it is shown that for such a system, one barrier may be pulsed up to rather high energies. This high-energy barrier will be difficult to see in the fusion data but we show that it may have a significant effect on the large-angle elastic (or quasielastic) excitation functions.

Obtaining average angular momenta from fusion excitation functions near the Coulomb barrier

Abstract Strong coupling to direct reaction channels causes the fusion excitation function close to the Coulomb barrier to behave very differently from that due to a simple potential barrier. The average compound nucleus angular momentum 〈itl > 〉 is also strongly affected. It is shown how to obtain 〈 l 〉 from a fusion excitation function using a model whose only parameter is the diffuseness of the nuclear potential.