Four-nucleon pickup reaction on 12 ⩽ A ⩽ 94 nuclei

The differential cross sections for the (d, ⁶Li) reaction on targets of ¹²C, ¹⁶O, ²⁴Mg, ⁴⁰Ca, ⁵⁸Ni and ⁹⁴Mo have been measured at E_d = 54.2 MeV. The measured angular distributions have been analyzed by finite-range DWBA calculations, and spectroscopic factors for an α-cluster transfer have been extracted. In the DWBA analysis using an optical model with a phenomenological Woods-Saxon potential, the spin-orbit part of the deuteron optical potential played a significant role in reproducing the measured angular distributions but the spin-orbit part of the ⁶Li optical potential had a minor effect. Relative spectroscopic factors extracted from the present data were compared with theoretical predictions together with the results from the (p, pα) reaction. The relative spectroscopic factors for 1p-shell nuclei were in quantitative agreement with theoretical predictions. However, there were certain discrepancies between the spectroscopic factors extracted from the (d, ⁶Li) reaction and those from the (p, pα) reaction, and the discrepancies increased with target mass number.     

Multistep processes in the (d,Li) reaction

At 28 MeV, the (d, ⁶Li) reaction on A = 4n targets in the 1p and 2s-1d shells systematically populates unnatural-parity states with large relative cross sections. This results suggests the presence of multistep reaction processes, making questionable the extraction of nuclear-structure information with DWBA.     

Multibody final states of the 6Li + 6Li reaction AT 97 MeV

Absolute coincidence cross sections were measured for the reactions ⁶Li + ⁶Li → 3α, ⁶Li(⁶Li, 2α), and ⁶Li (⁶Li, 2d), where the latter two represent $\text{N-}\text{body}\text{(N ≧ 4)}$ final states. Broad peaks from the ⁶Li (⁶Li, 2α) reaction are well described by a double spectator pole (DSP) model utilizing a Hulthén wave function, whereas near 40 MeV the DSP peaks are much narrower than predicted. A broad peak in the 3α final-state spectrum, attributed to a single-spectator pole (SSP) process, is well described with the same wave function. The SSP is the principal mechanism for the 3α reaction, in contrast to data near 40 MeV which show that sequential decay from ⁸Be levels is dominant.     

An eikonal treatment including two-step mechanisms for the (p,n) reaction on light nuclei

The reactions ⁷Li(p, n)⁷Be(g.s.), ⁷Li(p, n)⁷Be(0.43 MeV), and ¹³C(p, n)¹³N(g.s.) are treated in an eikonalized distorted-wave impulse approximation (DWIA), including the contribution of a two-step mechanism involving isovector correlations for the last reaction. Our calculation uses the free on-shell nucleon-nucleon t-matrix, with spin and isospin dependence, and incorporates antisymmetrization of it. We find agreement at roughly the 20% level with the forward experimental charge-exchange results in the energy range 120 to 200 MeV. The two-step contribution for the ¹³C case is rather small, being of the order of 10% at 0°; it interferes constructively, thus improving somewhat the agreement with experiment.     

Quasi-free scattering in the 6Li(d,dα)2H reaction at 52 MeV

In order to study the momentum distribution of an α-cluster in ⁶Li we have investigated the quasi-free scattering of deuterons in the ⁶Li(d, dα)²H reaction, using 52 MeV deuterons. The α-particle detector was kept at a fixed position of 30° and the second detector scanned over a range of coplanar angles. The α-cluster momentum distribution has been extracted by means of the plane-wave impulse approximation, using both the energy-sharing method and the angular correlation method. The momentum distributions so obtained are in good agreement with those obtained from other reactions such as (p, pα) and (α, 2α), and are compared with the spectator model. The reasons why the model works well are discussed. For the first time, to our knowledge, quasi-free scattering of deuterons on an α-cluster of a nucleus has been measured and analysed.     

The competition between cluster and constituent multiparticle emission as a means to identify the reaction mechanism

Since the competition between cluster and constituent multiparticle emission producing the same residual nucleus in a nuclear reaction depends on the mechanism of the reaction, thep n overd ratio in the¹⁶O(⁶Li,pn/d) reaction, measured between 10 and 19 MeV, is used to identify its mechanism as a function of energy.     

Strong reaction channels at barrier energies in the system 6Li + 208Pb

Large cross-section reaction channels were measured in the systems ⁶Li( ⁷Li) + ²⁰⁸Pb with high statistical accuracy at 5(3) energies around the Coulomb barrier from 29 to 39 MeV. These channels were assigned (mainly) to the breakup of ⁶Li, very loosely bound, into α + d and to the breakup of ⁵Li, produced by n-transfer to the target, into α + p and to similar processes with ⁷Li beam. The cross-sections with ⁶Li, S _α = 1.475 MeV, are systematically larger than the ⁷Li ones. This reflects, most likely, the higher binding energy of ⁷Li, S _α = 2.468 MeV. Theoretical predictions for the ⁶Li + ²⁰⁸Pb system which include for ⁶Li breakup to continuum states within a continuum discretized coupled-channels approach (CDCC) and resonant breakup plus n-transfer with DWBA reproduce the angular distribution shapes but still underestimate the cross-sections by a factor ∼ 3. Received: 15 January 2001 / Accepted: 3 March 2001     

Energy levels of 24Na from the 23Na(d,pγ)24Na reaction

Excited states in ²⁴Na have been investigated by means of the ²³Na(d, pγ)²⁴Na reaction at incident deuteron energies of 2.5 and 2.8 MeV. Excitation energies and γ-ray decay for levels up to 4.2 MeV have been determined from proton-gamma coincidence spectra obtained with a surface barrier detector and a Ge(Li) detector on-line with a computer. Two new levels at 3944 ± 2 and 4195 ± 3 keV excitation energies are reported.     

Measurement of the excitation function for the <Superscript>7</Superscript>Li(t,p)<Superscript>9</Superscript>Li reaction using detection of delayed neutrons

The excitation function for the ⁷Li(t, p)⁹Li reaction is measured at the EGP-10 electrostatic tandem accelerator (VNIIEF) at the incident triton energies E _t = 5–11.3 MeV. The measurements were based on detection of delayed neutrons resulting from the decay of ⁹Li nuclei. The neutrons were detected by the 4π detector consisting of ³He counters enclosed in a polyethylene moderator. A pulsed mode was used for irradiating the LiF target (210 μg cm^?2) of natural isotopic composition on the tantalum backing. Absolutization of the excitation function was performed against the ⁷Li(p, n) reaction cross sections.     

On the Li(p,pt)He reaction

Using a cluster model, the reaction ⁷Li(p, pt)⁴He has been analysed in the distorted-wave impulse approximation. All exchange terms arising from antisymmetrization of the target wave function have been calculated. When the correct asymptotic form for the intercluster wave function is used, the magnitude and shape of the measured angular correlation are reproduced both at 156 and 55 MeV. The analysis indicates that ⁷Li in its ground state is strongly clustered.     

First excited 0+ states in the germanium isotopes via the Se(d, 6Li) reaction

A systematic study of the even-A germanium isotopes with mass 70 ≦ A ≦ 78 via the Se(d, ⁶Li) reaction has been performed at E_d = 45 MeV. The reaction products were momentum analysed and mass identified with a QMG/2 magnetic spectrograph and the accompanying focal-plane detector system. The main emphasis of this investigation was on the nature of the first excited 0⁺ states. The striking variation in strength from one isotope to the other already observed in the (p, t) reactions to the same final nuclei is also seen for the α-pickup reaction. Previously derived wave functions that assume the excited 0⁺₂ states to be pure proton configuration states can also account for the present results.     

Neutron-induced charged-particle reactions in 6Li and 9Be

Angular distributions of tritons from the ⁶Li(n, t)⁴He reaction were measured at E_n = 7.25, 6.77, 6.57, 5.24 and 4.71 MeV. Angular distributions of douterons from respectively, the ⁶Li(n, d)⁵He two-body breakup reaction were measured at E_n = 6.77 and 6.57 MeV, and of protons from the ⁶Li(n, p)⁶He reaction at E_n = 6.77, 5.24 and 4.71 MeV. All these reactions in ⁶Li were analyzed as direct interaction in the formalism of the distorted wave Born approximation. The optical model for the nuclear interaction was found to apply reasonably well to nuclei as light as ⁴He, ⁵He, ⁶He and ⁶Li. In addition, ⁶Li as an alpha-deuteron cluster gives the best bound-state wave function to describe the experimental angular distribution of tritons. The excitation functions at forward angles of the ⁶Li(n, t)⁴He, ⁶Li(n, d)⁵He and ⁶Li(n, p)⁶He reactions were measured for incident neutron energies between 4.4 and 7.3 MeV. It is found that the ⁶Li(n, d)⁵He two-body breakup reaction has a threshold at about E_n = 5.3 MeV. Angular distributions at E_n = 18.3 MeV for tritons and protons from the ⁹Be(n, t)⁷Li and ⁹Be(n, p)⁹Li, respectively, were also measured.     

Quark-quark spin-isospin-interaction and6Li(p, δ+ +)6He reaction

The elementary reaction pp→nΔ ^{+ +}, through which the⁶Li(p, Δ ^{+ +})⁶He reaction proceeds, is considered as the flip of spins and isospins of two quarks, one in each of the two quark bags of the interacting nucleons. Using the experimental data on the⁶Li(p, Δ ^{+ +})⁶He reaction at 1.04 GeV and the distorted wave Born approximation (DWBA) for the reaction mechanism, the strength of the spin-isospin quark-quark interaction is determined. It is found to be 70 MeV fm³.     

Mechanism of the 6Li(6Li, 2α)α reaction from 36 to 46 MeV

The principal mechanism of the ⁶Li(⁶Li, 2α)α reaction for E₀ = 36 to 46 MeV is the formation and sequential decay of ⁸Be levels near E_x = 11, 17, and 20 MeV. In contrast to previous results obtained at lower bombarding energies, the cross section for the direct reaction involving a spectator α-particle is, under the most favorable conditions, only one third of that for excitation of these ⁸Be levels.     

Analysis of the normal optical,Michel and molecular potentials on the <Superscript>40</Superscript>Ca(<Superscript>6</Superscript>Li,d)<Superscript>44</Superscript> Ti reaction

Full finite-range (FFR) distorted-wave Born approximation (DWBA) method has been applied to analyse the angular distributions of cross-sections of the ⁴⁰Ca(⁶Li, d) ⁴⁴Ti reaction at 28 MeV incident energy for the 22 transitions involving both the bound and unbound states of ⁴⁴Ti by using the normal optical, Michel and molecular potentials. The extracted spectroscopic factors for the three optical potentials are compared with those of some previous studies of zero-range (ZR) calculations of the ⁴⁰Ca(⁶Li, d) ⁴⁴Ti reaction using the normal optical potential. The χ² values of all the levels are obtained for the three optical potentials to estimate the quality of the fits. Molecular and Michel potentials have been used for the first time to analyse the four-nucleon transfer reaction and it seems that the molecular potential fits the experimental data more satisfactorily for some of the states than the normal optical and Michel potentials.     

The 6Li(α, 2α)2H reaction at 700 MeV and the α-d momentum distribution

The results of an experimental study of the reaction ⁶Li(α, 2α)²H at 700 MeV are given. A comparison with the data from seven other reactions pertaining to the α⁺d cluster structure of ⁶Li is presented. The amplitude for the virtual transition ⁶Li→α+d obtained three of these experiments and the present one are comparable. The internal momentum distributions extracted with the plane-wave impulse approximation in four of the experiments are compatible with a pole form, the width parameter being given by the α-separation energy in ⁶Li. The remarkable consistency among these results is seen as evidence for the validity of the simple models used for both the reaction mechanism (PWIA) and to describe the α d-vertex (pole approximation) in ⁶Li.     

A study of the 18O(6Li,d)22Ne reaction at 32 MeV

Results from a study of the ¹⁸O(⁶Li, d)²²Ne reaction at a ⁶Li energy of 32 MeV are reported. The L-dependence of the shapes of the measured angular distributions provide a check on recent J^π assignments for some of the high-lying levels in ²²Ne. A finite range distorted wave analysis assuming a direct cluster transfer has been used to extract from the data α-particle spectroscopic strengths for most of the natural parity levels populated below 8 MeV of excitation. These strengths are compared with theoretical predictions for those few states for which a definite correspondence can be made between the calculated and experimental levels of ²²Ne. For transitions to the members of the ground-state band, the observed strengths disagree with the predictions. This disagreement has also been observed in the ¹⁶O(⁶Li, d) reaction and its cause is not understood. It is in marked contrast with the good agreement found for (⁶Li, d) reactions on targets of mass 20 ≦ A ≦ 24.     

Charge-exchange reaction on 12C with 217 MeV 3He particles

Differential cross sections for the (³He, t) charge-exchange reaction have been measured on a ¹²C target with 217 MeV ³He particles. High energy tritons were detected using a thick Ge(Li) crystal (43 mm) and a 4 mm Si(Li) detector in an E-ΔE telescope. Microscopic DWBA calculations have been carried out, using the rather well known wave functions of the ¹²C levels from Gillet and Vinh-Mau. The sensitivity of this reaction to the different terms of the two-body effective interaction is discussed. A comparison of the (³He, t) and (³He, ³He') reactions populating analog final states is also presented. The two-step mechanisms (³He,α)(α, t) and (³He, d)(d, t) are introduced for the first two levels 1⁺(g.s.) and 2⁺(0.969 MeV) and the results emphasize the importance of such processes at high energy.