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A compact and highly segmented detector array consisting of 44 gas–silicon–caesium iodide, position sensitive, particle identification detector telescopes and up to 10 position-sensitive, silicon strip detectors has been constructed for the study of light-ion–heavy-ion reactions including cluster break–up in the energy range 5–15 MeV/nucleon. The detectors are housed in a purpose built vacuum chamber. The telescopes are placed in fixed positions, covering the forward hemisphere from 3 to 30° in the laboratory with the target placed at 535 mm from the front of the telescopes or 6–52° with the target placed at 215 mm. The strip detectors are placed in any of 30 fixed positions in the forward hemisphere. For 85 MeV 12 C ions the telescope energy resolution (gas plus silicon) is 345 keV with an angular resolution of 0.03°. Using the gas–silicon section ions with Z up to 21 can be identified. For ions that pass through the silicon isotopic identification is achieved using the silicon–CsI combination. The strip detector energy resolution is 200 keV, with an angular resolution of 0.1°.

Angular distributions of the differential cross section and the second-rank tensor analysing power T 20 have been measured for the one-proton transfer reaction induced by polarized 7 Li on 208 Pb at 33 MeV leading to several states in 209 Bi. The experimental data were analysed simultaneously with data for elastic and inelastic scattering of 7 Li by means of CCBA and CRC calculations. The spectroscopic factors for 209 Bi states are obtained.

An extensive set of coupled-reaction-channels (CRC) calculations has been performed for the 16 O+ 58 Ni and 16 O+ 62 Ni systems. Derived, effective local polarization potentials for the elastic channels of the two systems show energy-dependent behaviour near threshold similar to that of empirical optical-model potentials. In particular, a peak in the empirical imaginary term in the surface, which was found at approximately 10 MeV above the Coulomb barrier, also appears in the calculated potential close to the same energy. The existence of this peak is shown to be due largely to coupling to the first 3 − state of 16 O. Calculations generally underpredict the magnitudes of the empirical potentials. However, agreement is greatly improved if a real polarization potential is included in the non-elastic channels in the coupling scheme. This suggests that multi-step processes are important and must be taken into account even at energies very close to threshold.

A Q -value of −8.240 ± 0.041 MeV has been determined for the exotic transfer reaction: 198 Pt( 18 O, 17 F) 199 Ir. This result yields a value of −24.425 ± 0.041 MeV for the mass excess of the previously-unmeasured nucleus, 199 Ir. Excited states in this nucleus are indicated in the region of 160 and 240 keV. The new result for the mass excess, together with existing ones, has been used to investigate systematic trends in the n-p interaction strength across the Z = 82 and N = 126 shell closures.

Accurate angular distributions of 16O ions scattered by 58,60,62,64Ni have been measured at five energies between 60 and 120 MeV. An optical model analysis of these, together with existing data, indicates that, whereas there is a significant increase with A in the surface absorption, there is little, if any, isotopic dependence in the energy variation of the optical potentials, V(E) and W(E), near threshold (the threshold anomaly). Clear evidence is found for a hitherto unreported peak in W(E) just above threshold. CRC calculations, including inelastic excitation and transfer, reproduce the data but show no clear connection between the peak and a particular reaction channel.

Angular distributions of the differential cross section and the second-rank tensor analysing power T 20 have been measured for the one-neutron transfer reaction induced by polarized 7 Li ions on 208 Pb at 33 MeV leading to five states in 209 Pb. The experimental data were analysed simultaneously with data for elastic and inelastic scattering of 7 Li by means of CCBA and CRC calculations. A strong effect of the transfer channels on the elastic scattering was found. Coupling to the transfer channels cancelled out the effect of projectile excitation on the elastic-scattering cross section but was unable to account fully for the renormalization factor of the double-folding potential found previously in an optical-model analysis of 7 Li+ 208 Pb elastic scattering. The second-rank tensor analysing power for the elastic scattering was strongly affected by the transfer channels.

The elastic scattering of 6Li and 7Li by 208Pb has been measured at several energies near the Coulomb barrier and analysed in terms of the optical model. Previous reports have suggested that the normalization factor, NR, for the double-folded real potential is approximately 0.6 for both isotopes at bombarding energies well above the Coulomb barrier. In this paper we show that, whereas this low value for NR persists for 6Li at energies close to the barrier, for 7Li it is somewhat greater than unity, and indicates the existence of a threshold anomaly for this projectile.

Some experimental techniques used in the measurement and analysis of inverse kinematic reactions are described and applied to a kinematically complete

In several recent measurements the symmetric fission of 24 Mg has been reported. In this paper the results of a search for similar processes in 28 Si and 32 S are presented. In contrast to the results for 24 Mg, no evidence is seen for the fission of 28 Si to 16 O+ 12 C or 32 S to 16 O+ 16 O following inelastic excitation. However, these processes are observed to occur following pickup onto 24 Mg. The results are consistent with a simple model of the fission process developed by Harvey and suggest the existence of large scale cluster structure in all three nuclei.

The effect of nuclear reorientation coupling on the magnetic substate population in heavy ion inelastic scattering is discussed. Coupled channels calculation of the inelastic scattering of 56 MeV 16O projectiles by 92Zr leading to its first excited 2+ state is used as an illustration for the discussion. It is shown that at energies near the Coulomb barrier, the information on reorientation coupling can be extracted from the magnetic substrate population only by a careful analysis in the Coulomb-nucleus interference region.

A new high resolution measurement of the 12 C( 24 Mg, 12 C 12 C) 12 C breakup reaction has been performed. Sequential breakup is observed from specific states in 24 Mg at excitation energies ranging from 20 to 26 MeV. Angular correlation measurements indicate that states with spins ranging from J =4 to 8 are populated. These states are consistent with superdeformed shape isomeric bands predicted by cranked cluster model and Nilsson-Strutinsky calculations.

New measurements of the elastic scattering of 16O by 208Ph and inelastic scattering to the 208Pb(3−1) state have been made at 70, 74, 78, 82 and 94 MeV. These, together with existing data, have been analyzed using a conventional collective model and DWBA formalism. The real and imaginary parts of the transition potential determined in the surface region exhibit a rapid energy variation in the threshold region reminiscent of the threshold anomaly observed in the elastic potential. At several energies above threshold, inelastic distributions could not be adequately reproduced within the confines of the model. These results cast doubt on the adequacy of the collective model to describe this inelastic excitation process in regions of a threshold anomaly.

An explanation is offered for the large fusion radii obtained in the optical model description of heavy-ion fusion given by Udagawa {ital et} {ital al}. An extension of the model is suggested, and possible effects on the spin distribution are discussed. It is contended that the large fusion radii {ital cannot} be deduced from the elastic scattering alone. The role of Coulomb excitation for these systems is also discussed.

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.

A new nucleus 199Ir has been produced using the exotic transfer reaction 198Pt(18O, 17F)199Ir at 140 MeV. The mass of 199Ir has been measured by the determination of the reaction Q value. Its mass excess is -24.424 ± 0.034 MeV.

The elastic scattering, inelastic scattering and single-neutron transfer cross sections in the collision of 78 MeV 17O on 208Pb are measured. It is shown that these cross sections can be understood in terms of a three-body model of the system comprising a 16O core, a valence neutron and a 208Pb target. The theoretical analysis points to the importance of multistep effects in all reaction processes. The inherent energy dependence of the 16O208Pb elastic optical potential at energies close to the Coulomb barrier is also seen in 17O208Pb elastic scattering. The effect of the valence-neutron channel coupling on the 17O208Pb interaction potential is deduced, and is shown to be almost entirely imaginary. Implications for a possible isotope dependence of the sub-barrier fusion cross section are considered.

A systematic analysis of 16 O + 60 Ni elastic scattering data over a range of bombarding energies reveals strong variations of both the real and imaginary parts of the optical potential near the Coulomb barrier. The results are consistent with a dispersion relation interpretation of the effect which predicts a peak in the real surface potential close to the barrier.

We study the formal structure of the influence of channel coupling on near- and sub-barrier fusion. The reduction to a one-channel description is studied, with emphasis on the channel-coupling effects being manifest primarily as an energy dependence (the “threshold anomaly”) of the real nuclear potential. The relation to the barrier penetration model is examined critically. The results of large-scale coupled-channel calculations are used as “data” to illustrate the discussion. Particular emphasis is placed on the importance of reproducing correctly the partial-wave (or “spin”) distributions. The simple barrier penetration model is found to be adequate to exhibit the strong enhancements due to channel couplings when the threshold anomaly is taken into account, although there may be important corrections due to the long-ranged peripheral absorption, especially from Coulomb excitation.

The reactions 48 Ca( 17 O, 16 O) 49 Ca, 44 Ca( 17 0, 16 O) 45 Ca and 40 Ca( 17 O, 16 O) 41 Ca have been observed at bombarding energies of 27,27 and 28.5 MeV, respectively. The cross sections for transitions to levels in the final nuclei with predominantly 2p 3 2 configurations were measured over the angular range 100–170°. The results have been analysed using a DWBA theory with Coulomb wave functions describing the entrance and exit channels. For a chosen wave function for the 17 O ground state, the analysis yields the rms radii of the 2p 3 2 single-particle orbits in the odd Ca isotopes. With the use of a simple potential model the rms radii of the 1f 7 2 orbits are determined within reasonable limits and the sizes obtained compared with the rms radii of the neutron excesses in the Ca isotopes as calculated from Coulomb energy differences. The latter estimates are significantly lower than the values suggested by the present experiments.

16O+8Be breakup of 24Mg has been identified following inelastic scattering of 24Mg projectiles. The spectrum of states observed is compared to that previously measured in the 12C+12C breakup channel to obtain information on the partial decay widths.

Differential cross sections for 12 C + 12 C inelastic scattering to the 0 2 + state have been measured at several energies. The data are analysed in terms of DWBA calculations with no resonance contribution. In this description structure in the excitation function arises from barrier matching effects and interference between barrier and internal waves.

The reaction 86Sr(18O, 17O)87Sr has been studied at near Coulomb energies in order to determine the radial size of the 1 g 9 2 neutron distribution in the ground state of 87Sr. The rms radius of 4.60 ± 0.11 fm measured in this way is in good agreement with the value determined from the electron scattering measurements.

Single-neutron transfers induced by 12, 13C and 16, 17, 18O projectiles on 208Pb and the 12C(17O, 16O)13C reaction have been studied at energies close to the Coulomb barrier. These processes are well described by the distorted-wave Born approximation. Coupled-channels effects are found to be small. Normalization factors have been determined for all projectile and target transitions, and also for the triton-deuteron overlap by comparison with previous measurements of the 208Pb(d, t)207Pb reaction. The root-mean-square (rms) radii of single-particle neutron wave functions in 208Pb and 209Pb were calculated using known spectroscopic factors. The distribution of the point neutron excess density in the surface region of 208Pb has been derived and its rms radius determined to be 5.93 ± 0.13 fm with a local potential model. This is in good agreement with theoretical predictions, but is considerably larger than estimates based on Coulomb energy differences. The phenomena of core polarisation by the odd particle or hole outside 208Pb is discussed using the single-particle orbitals determined in this work.

Elastic scattering of 23.5 MeV α-particles by 208 Pb and inelastic scattering to the 3 − state at 2.62 MeV have been measured. Good fits to the data using either the conventional deformed potential or folding-model descriptions of the interaction could be obtained, provided that the distorting potential and inelastic form factor were derived consistently. It was also found necessary to include an energy dependence in the imaginary potential. Both the EM transition rate and the tail of the nuclear form factor are well determined. They are consistent with equal proton and neutron deformation parameters for the 3 − state, although these latter quantities are model dependent and not strictly comparable. The isoscalar transition rate is poorly determined. In terms of the folding-model description, surface distributions of the nuclear matter and transition densities were derived which are in excellent agreement with theoretical calculation.

The empirically observed energy dependence of the optical potential at energies close to the Coulomb barrier (the threshold anomaly) in the collisions of 16O on 208Pb is investigated within the framework of coupled-reactions-channels (CRC) calculations. The motivation is to determine the extent to which the observed threshold anomaly can be accounted for by strong coupling of the elastic to the inelastic and transfer channels. CRC calculations have been performed at a range of energies above and below the Coulomb barrier, both for the complete set of reaction channels, and also, to see the effects more simply, for individual inelastic and transfer channels. An energy-dependent local polarisation potential has been extracted. It will be seen how this polarisation potential satisfies the dispersion relations, and how well it reproduces the elastic scattering and the fusion cross sections over a range of energies.

Inelastic scattering of 17O and 18O by 208Pb has been measured at 78 MeV, and compared with theoretical predictions. In both cases the nuclear amplitude is dominated by the valence nucleon contribution. Direct excitation is the principal mechanism in 18O scattering, while for 17O excitation, two-step transfer is more important. Recoil and non-orthogonality effects must be taken into account.

Elastic scattering of 16 O on 208 Pb has been measured at 78 MeV. Analysis of these and other data at higher energies shows evidence for a progressive increase of the real part of the nuclear optical potential in the surface region as the Coulomb barrier energy is approached. This variation is not predicted by the double folding model, but is confirmed by comparison with total reaction cross section measurements. Possible origins for this effect are discussed.

Symmetric fission into two 12 C nuclei has been observed from states at high excitation energy in 24 Mg excited in inelastic scattering. Fission to the 10 B+ 4 N channel has also been observed and indirect evidence for the 16 O+ 8 Be channel noted. The same experiment also reveals fission into 16 O+ 12 C from states in 28 Si populated by alpha-particle pickup onto the 24 Mg projectile. The correlation of the states in 24 Mg observed in this experiment with those previously observed in 12 C+ 12 C scattering is discussed.

Coupled reaction channels calculations (CRC) of the reactions 208 Pb( 17 O, 16 O) 209 Pb leading to different states of 209 Pb are compared with DWBA predictions at projectile energies of 78, 86 and 102 MeV. The calculations exhibit strong effects of multistep processes on Q -value and angular-momentum-mismatched transfer reactions. It is shown that the contribution to the transfer through the inelastic excitation of 17 O contains a major part of the multistep effect. A simple three-channel model comprising the elastic, inelastic and transfer channels is constructed which simulates the CRC effects on the transfer cross sections. The polarization effects of the eliminated channels give rise to effective potentials which are mainly imaginary.

Two aspects of heavy-ion collisions at near-barrier energies deviate considerably from the predictions of simple potential models. Fusion cross sections are generally greatly underpredicted, and in a recent analysis of 16 O elastic scattering on 208 Pb the optical potential had a marked energy dependence. We show that the inclusion of both neutron and proton transfers in addition to inelastic excitation processes within a coupled channels framework can give a consistent description of the data.

Angular distributions are presented of the polarization produced in the elastic scattering of 30 MeV protons in the angular range 20°–120° (lab). Results for Ca, Co59, Ni58, Ni60, Sn120 and Pb208 show the characteristic oscillations well known at lower energies but exhibit higher maximum polarizations in general. The technique used involved the single scattering of a 30 MeV polarized proton beam.

Absolute differential cross sections are presented for the elastic scattering of 29 MeV He2 by Al, V and Cu in the angular range 15°–80° (c.m.). The results are fitted by an optical model potential with a Saxon-Woods form factor and no spin orbit term.

Angular distributions are presented for the polarization of 17.8 MeV protons elastically scattered by C 12 , Si, Ca, Ni 58 , Ni 60 and Mo and inelastically scattered by C 12 and Ca 40 . An attempt has been made to fit the experimental results in terms of an optical model potential. To obtain information concerning the spin-orbit potential the remaining parameters were chosen consistent with values found for other energies and elements. The spin-orbit potentials required to fit the data were real with the exception of Ca where it was necessary to introduce an imaginary term in order to obtain a good fit. The variation in the depth of the real part of the central potential for the two Ni isotopes is described in terms of a symmetry potential of the form V 1 ( N − Z )/ A , with V 1 = 23±6 MeV. The experimental technique involved the single scattering of a totally polarized proton beam obtained by bombarding gaseous hydrogen with 40 MeV alpha particles.

The absolute calibration of the polarisation efficiency of a proton polarimeter has been made; the polarimeter uses the elastic scattering of 16 MeV protons by carbon at an angle of 45°. The efficiency was found to be 0.574±0.009 for a mean energy of 15.7 MeV.

The results of a study of neutron transfer reactions at energies below the Coulomb barrier using a recoil separator are reported. Excitation functions for single-neutron pick-up at an angle of 180° in the center-of-mass have been measured for the systems 58 Ni+ 116,118,120,122,124 Sn over an energy range of 40 MeV from near to well below the Coulomb barrier. The data are well described in terms of both DWBA and semi-classical models of the transfer process. The relevance of the direct nucleon transfer cross sections to the sub-barrier fusion of these systems is discussed.