Inclusive measurements of the break-up of 156 MeV6Li-ions at extreme forward angles

Inclusive alpha particle and deuteron spectra from collisions of 156 MeV⁶Li-ions with¹²C and²⁰⁸Pb were measured at extreme forward emission angles including zero degree. The measurements were performed with the Karlsruhe magnetic spectrograph ‘Little John’ and required an efficient reduction of the background from small-angle scattering. The observed double differential cross sections and angular distributions have been analysed on the basis of Serber's spectator break-up model. When going to angles smaller than grazing, where Coulomb effects are expected to be dominating, transitional features may appear. Corresponding effects probably associated with Coulomb break-up are observed with the²⁰⁸Pb-target and require a slight extension of the Serber approach. In the case of the¹²C-target the break-up cross sections in forward direction seem to reflect the shape of the internal momentum distribution of the alpha particle and deuteron cluster in the⁶Li-projectile and are in agreement with a 2S-type wave function. However, at larger angles the shape appears to be distorted, possibly by final state interactions.     

Angular correlations from 6Li break-up near the Coulomb barrier of 118Sn and 208Pb

Absolute coincidence cross sections for three-particle final states have been measured inplane in the ⁶Li + ¹¹⁸Sn and ⁶Li +²⁰⁸Pb reactions at incident energies of 22.2 and 23.0 MeV, respectively. Most strongly populated is the α + d branch, proceeding sequentially through the first excited state (E_x = 2.18 MeV, J^π = 3⁺) of ⁶Li. The angular distributions are fitted by DWBA calculations including Coulomb interaction in the excitation of the projectile. The main competing reaction channels above and at the Coulomb barrier (¹¹⁸Sn: E/E_c ≈ 1.3; ²⁰⁸Pb: E/E_c ≈ 0.9) are: neutron transfer (⁶Li, αp) and the non-sequential α + d break-up of ⁶Li. The latter spectra are consistent with a quasi-free break-up mechanism. No ³He or tritons have been found in the coincidence spectra, as well as no evidence for a three-particle dissociation of ⁶Li into α + p + n.     

Complete and incomplete fusion in reactions of 7Li + 56Fe at E(7Li) = 50 and 68 MeV from analysis of recoil range and light-particle measurements

Excitation functions for the production of eight radioactive products of the reactions of ⁷Li on ⁵⁶Fe have been measured up to E(⁷ Li) = 89 MeV. Recoil range distributions for these products, together with inclusive proton, deuteron, triton and alpha spectra, have been measured at energies of 50 and 68 MeV. The α, t and d spectra show characteristic “break-up” components at forward angles, while the recoil distributions show evidence of complete fusion and incomplete-fusion process ⁵⁶Fe( ⁷Li,α) ⁵⁹Co ^* . A parallel study on ⁵⁵Mn shows some evidences of the ( ⁷Li,t) incomplete-fusion process, but the cross-section for this process is significantly less than for the triton fusion process. The recoil distributions can be reproduced on the assumption that essentially all the observed break-up fragments are in fact associated with incomplete fusion, but uncertainties in normalisation leave open the possibility of a significant contribution of pure break-up. A diffraction model of the ( ⁷Li,α) transfer process reproduces the observed break-up α spectra with some success. Received: 29 March 2001 / Accepted: 12 November 2001     

Coulomb break-up and possible evidence for cluster effects in some light nuclei

The possibility of obtaining information about cluster structures in light nuclei from Coulomb break-up experiments is discussed. A few favorable cases are pointed out. A preliminary estimate of the break-up cross section of⁷Li into alpha particles and tritons and of⁷Be into alpha particles and³He in the field of a heavy target nucleus is given. The need for more experiments as well as more refined calculations is stressed.     

12C + 7Li Reaction measurements and the 12C(7Li,t)16O reaction

A measurement of the residues from the ¹²C + ⁷Li reaction has been obtained for ⁷Li energies from 10 to 38 MeV. From these measurements the fusion cross sections and critical angular momenta for the ¹²C + ⁷Li system have been deduced. Cross sections for the ⁷Li(¹²C, t)¹⁶O reaction have been obtained for ¹²C energies from 54 to 62 MeV at θ_lab = 2.7°. The critical angular momenta obtained from the fusion cross sections have been used to perform Hauser-Feshbach calculations for the ¹²C(⁷Li, t)¹⁶O reaction. These calculations have been compared to measured angular distributions over a wide energy range. By comparing the fusion cross sections required by the Hauser-Feshbach calculations to fit the ¹²C(⁷Li, t)¹⁶O(8.87 MeV) reaction and the measured residue cross section it is estimated that at least 80 % of the measured residues are fusion products. The calculations also indicate that direct processes dominate the population of many ¹⁶O levels at forward angles and the 10.35 MeV state at backward angles. The necessity for using a critical angular momentum in Hauser-Feshbach calculations is discussed.     

Direct processes in the6Li(α, d)8Be reaction

The angular distributions of deuterons from the ⁶Li(α, d)⁸Be reaction corresponding to the 0⁺ ground state and the 2.9 MeV (2⁺) state in⁸Be have been measured at E_α = 17.3, 23.3 and 25.1 MeV. The excitation functions for this reaction have been measured at seven emission angles in the region from 12 up to 25 MeV. The experimental angular distributions are compared with theoretical predictions obtained using DWBA stripping calculations and the plane wave approximation (PWA) for various direct processes. It is shown that the experimental angular distributions can be described throughout the angular region using the simple mechanisms associated with the break-up of the ⁶Li target nuc clusters and heavy-particle stripping. The interference of these processes is taken into account.     

Semiclassical theory of analysing powers for polarised heavy-ion elastic scattering

A unified semiclassical theory of analysing powers of all ranks for polarised heavy-ion elastic scattering from a spin-zero target at energies above the Coulomb barrier is presented. Illustration is provided for by specialising to the two interactions, spin-orbit and tensor. The present approach recovers the phenomenological result, obtained by taking the difference of the nuclear overlap between unpolarised and aligned projectiles (shape-effect model), which successfully explained the structureless pattern of rank-2 observables for aligned ⁷Li + ⁵⁸Ni at E_lab ～ 20 MeV. Deviations observed for ⁷Li + ¹²C (oscillations) are interpreted as due to interference between diffraction (positive-angle) and negative-angle scattering terms.     

Investigation of the sequential break-up mode6Li→6Li*(31 +)→α+d of 156 MeV6Li projectiles on208Pb in the very forward angular hemisphere

The sequential break-up of 156 MeV⁶Li projectiles colliding with²⁰⁸Pb has been investigated in the very forward angular hemisphere in order to explore the role of the Coulomb and nuclear interaction in inducing projectile break-up processes. The experiments use the Karlsruhe spectrograph Little John with a newly developed technique to detect efficiently break-up pairs of-particles and deuterons emitted with small relative energies and within a small opening angle. The observed differential cross section for the projectile excitation⁶Li⁶Li^* (I=3_i ⁺) has been analysed (together with forward elastic scattering) by a coupled channel approach, and various sensitivities to the nuclear potential and other ingredients have been studied. We conclude, that in the angular region below half the grazing angle the excitation is completely due to the Coulomb force. This feature is a basis for an application of the disintegration approach for studies of radiative capture cross section of astrophysical interest.On leave from Institute of Atomic Physics Bucharest, Romania     

Further study and analysis of the <Superscript>7</Superscript>Li on <Superscript>56</Superscript>Fe reaction at 50 MeV incident energy

We present deuteron and triton spectra measured at 12 laboratory angles from the reaction of 50 MeV ⁷Li on ⁵⁶Fe together with evaporative components as modeled by CASCADE and fitted to the backward angle data. The deuteron and triton “break-up” spectra obtained by subtracting the evaporative components from the measured spectra are also presented. The break-up of the ⁷Li projectile near the surface occurs with high probability and the major fraction of the break-up cross-section is taken by (⁷Li,α) transfer process. A crude estimate of the fraction of the total cross-section is found to be of the same order of magnitude as the overall spectroscopic factor determined by the diffraction model. The value of the estimated fraction of total cross section at 50 MeV incident energy is compared to that at 68 MeV incident energy for the same reaction. However, these values of estimated fraction of total cross section are found very much consistent with the measured yields at both incident energies. The importance of the level density parameter in locating the maximum of excitation energy is indicated in the diffraction model.     

Orbital dispersion and wavefunction mapping in inclusive break-up experiments

Inclusive break-up cross sections of the reactions¹²C (⁶Li,a)X and²⁰⁸Pb (⁶Li,a)X at 26 MeV/amu, observed for the emission of break-up fragment into the forward hemisphere, are analysed in view of the information about the internal momentum distribution of the projectile. The longitudinal momentum distribution of the fragments in the projectile rest-frame is found to reflect the internal momentum distribution of the clusters whereas the transverse distribution is affected by the projectile-target interaction.     

Proton-neutron correlation in the deuteron breakup at 56 MeV and prior-form DWBA analysis

Proton-neutron angular correlations in the ¹²C, ⁵¹V and ¹¹⁸Sn(d, pn) reactions have been measured at 56 MeV to investigate the deuteron breakup process. The elastic breakup which leaves the target nucleus in its ground state dominates the coincident spectra. The elastic breakup cross sections are estimated to be 36–48% of the inclusive breakup yields at 15° or 17.5°. In the angular correlations the protons are emitted predominantly on the side of the beam opposite to the neutrons. The experimental data have been analyzed using the prior-form DWBA. For both nuclear and Coulomb breakup, sufficient convergence of the calculations is obtained by including the pn angular momenta up to l = 2. For the nuclear breakup calculations, the l = 0 and 2 contributions dominate the cross sections. For the Coulomb breakup the l = 1 contribution is predominant. In the calculations the effect of the Coulomb breakup is seen at forward angles of the angular correlation. The DWBA calculations reproduce fairly well the coincident energy spectra and the angular correlations in the angular region where the protons are emitted on the side of the beam opposite to the neutrons. On the other hand the calculations overestimate the break-up cross sections by a factor of 2 to 10 in the angular region where the protons are emitted on the same side of the beam as the neutrons. The distributions of deuteron c.m. angular momenta that contribute to the breakup amplitude are examined to obtain information on the region of space in which the breakup reactions takes place.     

Inclusive break-up reactions of 6Li at an incident energy of 26 MeV/nucleon

Inclusive charged particle spectra were measured from nuclear reactions induced by 156 MeV ⁶Li on ⁴⁰Ca. At forward angles the spectra exhibit broad break-up distributions centered around the energy corresponding to the beam velocity. The double differential cross sections together with previous results for a ²⁰⁸Pb target were analyzed in the framework of the DWBA approach to projectile break-up taking into account elastic and inelastic reactions of the break-up fragments. The high energy tails of the background due to preequilibrium emission of complex charged particles were estimated on the basis of the coalescence model.     

Electric moments of the first excited state of 18O

The static quadrupole moment Q_2⁺ and the B(E2; 0⁺ → 2⁺) value of the first excited state of ¹⁸O at e_x = 1.982 MeV have been determined using the reorientation effect in Coulomb excitation. Surface-barrier detectors at laboratory angles of 90° and 174° were used to detect ¹⁸O ions elastically and inelastically scattered from ²⁰⁸Pb. At both angles, we determined experimentally the maximum bombarding energies at which nuclear interference effects were negligible. It is found that Q_⁺ = ?0.023 ± 0.021 e · b (?0.052 ± 0.021 e · b) for destructive (constructive) interference from higher states. This result is in good agreement with theoretical expectation. For the transition moment we find B(E2; 0⁺ → 2⁺) = 0.00390 ± 0.00018 e² · b² (0.00371 ± 0.00018 e² · b²) for destructive (constructive) interference.     

Folding interactions in the scattering of soft heavy ions by light nuclei

Scattering of polarized ⁷Li by ¹²C and polarized ⁶Li by ¹⁶O at E_lab～20 MeV is studied by both cluster folding interactions and double folding ones, where projectile virtual excitation effects are included by the coupled-channel method. The results strongly depend on the choices of interactions and reaction mechanisms reflecting low Coulomb barrier effects.     

The nonelastic projectile break-up cross section from particle-gamma coincidence measurements for the 6li + 40ca reaction at 156 mev

The cross sections for nonelastic break-up modes have been studied for the ⁶Li + ⁴⁰Ca reaction at E_Li = 156 MeV. Gamma-ray spectra from target-like residual nuclei were measured in coinci- dence with beam-velocity projectile fragments and a value for the nonelastic break-up cross section σ^b.u_non = 582 ± 110 mb has been found. Together with results of inclusive charged-particle measure- ments we infer for the total break-up cross section σ^b.u_tot = 930 ± 115 mb comprising about 50% of the total reaction cross section. The nonelastic contribution of the break-up reaction appears to be less than predicted by the DWBA break-up theory. This result is directly evident from the differential cross sections by comparing inclusive and exclusive results.     

Isovector giant resonances in light nuclei observed by (7Li, 7Be) reaction

Isovector giant resonances were studied with the (⁷Li, ⁷Be) reaction on ¹²C, ²⁴Mg, ²⁷Al, ²⁸Si and ⁴⁸Ti at an incident energy of 150 MeV. Differential cross sections were measured at forward scattering angles including 0°. We observed the octupole resonance at E_x ∽ 16 MeV and the giant dipole resonance at E_x ∽ 20 MeV in all residual nuclei.