The reaction 16O(6Li,d)20Ne at 75 MeV

The angular distributions of the reaction ¹⁶O(⁶Li, d)²⁰Ne at 75 MeV for transitions to members of the ground state band are well fitted by exact finite-range DWBA calculations assuming a direct α-cluster transfer and yield spectroscopic factors in good agreement with shell-model predictions.     

(6Li,d) reactions on 24Mg and 26Mg at 73 MeV

The ^{24, 26}Mg(⁶Li, d)^{28, 30}Si reactions have been studied at 73 MeV bombarding energy. The angular distributions were analyzed with exact finite-range distorted wave Born approximation calculations assuming a direct α-cluster transfer. Extracted spectroscopic strengths leading to low-lying levels of ²⁸Si and relative spectroscopic strengths between transitions to ²⁸Si and ³⁰Si ground states are consistent with those previously obtained by several α-transfer reactions. Many strongly populated levels have been observed at E_x ? 10 MeV for ²⁸Si. A marked similarity was found between the deuteron spectrum and the ²⁴Mg(α, α)²⁴Mg excitation function in this excitation energy region. A brief comparison of the present α-transfer results with previous two-nucleon transfer data leading to ^28,30Si is also presented.     

The (d, 6Li) reaction on 24Mg, 26Mg and 28Si at 35 MeV

Data for the (d, ⁶Li) reaction on targets of ²⁴Mg, ²⁶Mg and ²⁸Si have been obtained at 35 MeV bombarding energy. Angular distributions were measured for low-lying states in the residual nuclei. Zero-range distorted-wave Born approximation (DWBA) calculations have been used to analyze the data. The DWBA calculations account for the shapes of the experimental distributions reasonably well. The observation of significant population of unnatural parity states implies, however, that other transfer mechanisms may be important. The experimental spectroscopic factors are in qualitative agreement with those obtained from SU(3) theory.     

Quasi-free (p,pα) scattering on 24Mg, 28Si, 40Ca and 58Ni at 157 MeV

Sixfold energy spectra have been measured for the (p, pα) reaction at 157 MeV on ²⁴Mg, ²⁸Si, ⁴⁰Ca and ⁵⁸Ni around quasi-free kinematic conditions. For the three s-d shell nuclei the experiment covered a map ranging from 0 to 220 MeV/c in recoil momentum and from 0 to 20 MeV in excitation energy of the final nucleus. Recoil momentum distributions have been obtained for the 0⁺ ground state and the 2⁺ first excited state of ²⁰Ne, ²⁴Mg and ³⁶Ar, and also for the states around 4.4 MeV (mainly 4⁺) of ³⁶Ar. The a spectroscopic factors extracted by a DWIA analysis are about three times larger than those predicted by the SU(3) model; however, they agree quite well in relative magnitude for a number of cases. The disagreement in shape between experiment and theory observed at low recoil momentum for the 2⁺ states might result from another reaction mechanism. The cross sections for ⁵⁸Ni are about a factor often smaller than those for ⁴⁰Ca. The ⁵⁸Ni(p, pα)⁵⁴Fe reaction seems to lead mainly to excited states of the final nucleus.     

12C(7Li,t)16O和20Ne(d,6Li)16O反应的研究

本文把用于处理重离子引起的单粒子转移反应的Goldfarb-Buttle方法推广到多粒子转移反应。计算了¹²C(⁷Li,t)¹⁶O和²⁰Ne(d,⁶Li)¹⁶O反应,用了¹⁶O的全相干波函数,考虑了某些反冲因素,并将结果与实验作了比较。     

Energy dependence of the 24Mg(16O, 12C)28Si reaction

Excitation functions for the reaction ²⁴Mg(¹⁶O, ¹²C)²⁸Si(g.s., 2⁺₁) were measured at 5°(lab) in the energy range 32 < E_c.m. < 49 MeV. Although the resonant structure, previously observed at lower energies, becomes progressively weaker, three new correlated maxima have been observed near E_c.m. = 37.5, 40.2 and 43.5 MeV. Angular distribution measurements at these energies yield spin assignments, from P²_j(cos θ) comparisons, of 27, 29 and 31, respectively. Attempts to find a consistent optical-model fit to the elastic scattering in the entrance channel and an exact finite-range DWBA fit to the four-nucleon transfer reaction in this energy range were unsuccessful. Such a failure is to be expected if strong couplings between the elastic channel and inelastic channels of either the initial or final system are important. The features of the resonance phenomena in the transfer reaction are discussed within a band crossing model framework.     

Statistical fluctuations and compound elastic scattering for 12C(12C, 12C)12C and 16O(16O, 16O)16O

The difference between the observed fluctuations in the elastic excitation functions for carbon-carbon and oxygen-oxygen scattering, just above the Coulomb barrier, is interpreted in terms of the compound nucleus model.     

40Ca+58Fe,58Ni反应中的径向膨胀流研究

利用同位旋相关的Boltzmann-Langevin方程研究了⁴⁰Ca+⁵⁸Fe和⁴⁰Ca+⁵⁸Ni两个反应系统在53,100,150和200MeV/u入射能量下对心碰撞的径向膨胀流.发现对于丰中子系统⁴⁰Ca+⁵⁸Fe的径向膨胀流系统性地小于稳定系统⁴⁰Ca+⁵⁸Ni的径向膨胀流.在假定轰击能量与反应体系的压缩密度呈抛物线关系时,能够解释入射能量和径向膨胀流之间呈现的直线关系.提取了出现径向膨胀流的轰击能量阈值,发现对丰中子系统⁴⁰Ca+⁵⁸Fe得到的能量阈值小于稳定系统⁴⁰Ca+⁵⁸Ni所得到的能量阈值.     

The 64Ni(6Li,d)68Zn reaction

The ⁶⁴Ni(⁶Li, d)⁶⁸Zn reaction has been studied at 28 MeV bombarding energy. For several low-lying states in ⁶⁸Zn, S-values extracted via DWBA analysis are compared with shell model and collective model (IBA) predictions.     

16O, 24, 26Mg, 28Si, 32S, 40Ca(α, 12C) and multi-α-cluster transfer reactions

The (α, ¹²C) reaction has been studied on a variety of nuclei, A = 16 to 40, at E_α = 90.3 MeV. The data indicate a rapid fall-off of cross sections with increasing target mass, approximately as A_t^{?5 ± 1}. This and other systematics are used to estimate cross sections for multi-α-cluster transfer reactions in heavy nuclei and suggest σ_T < 10^?34 cm² consistent with present experimental limits. The data for ²⁴Mg(α, ¹²C)¹⁶O has been studied in more detail and indicates a selective population of final states including ¹⁶O g.s., with oscillatory angular distributions in some instances. Finite-range distorted-wave Born approximation calculations for direct ⁸Be pickup have been performed utilizing cluster overlap amplitudes obtained with zero-order SU(3) wave functions. The calculations are in qualitative, and often quantitative, agreement with shapes and absolute magnitudes of the measured angular distributions although the cross sections for certain α-cluster states (2⁺, E_x ≈ 7 MeV; 4⁺, E_x ≈ 10.3 MeV) are greatly overestimated with this model. Other more complicated mechanisms, such as successive α-transfer, cannot be excluded. The systematics of the calculated ⁸Be cluster overlaps and the calculated and measured (α, ¹²C) cross sections are investigated, and implications for multi-α-cluster transfer reactions are discussed.     

Comparative study of the selectivity displayed by (6Li,d) and (16O, 12C) reactions

Four-particle-transfer processes induced by the (⁶Li, d) and (¹⁶O, ¹²C) reactions to strongly excited 0⁺ states are compared. Although the selectivity displayed by these reactions is similar, the corresponding relative cross sections are rather different. These results emerge naturally from a simple microscopic model.     

The 12C(18O, α)26Mg and 12C(18O, 8Be)22Ne reaction

Excitation functions for α-emission leading to the ground and first excited states of ²⁶Mg and ⁸Be emission leading to the ground and first and second excited states of ²²Ne have been measured at several forward angles for E_c.m. = 15 to 22.4 MeV. There is little evidence for correlated structure. The angular distribution at 16.5 MeV for the α + ²⁶Mg(g.s.) channel is rather structureless while that for the ⁸Be+²²Ne(g.s.) channel appears to be dominated by a J = 13 contribution. Statistical model calculations indicate that much of the yield for both the α and ⁸Be exit channel is compound nuclear in origin, with some indication of a larger direct contribution for the ⁸Be channel at the lower end of the bombarding energy range.     

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.     

Value of the absolute cross section for the reaction 40Ca(16O, 12C)44Ti

The reaction ⁴⁰Ca(¹⁶O, ¹²C)⁴⁴Ti(g.s.) is calculated making use of a microscopic finite-range form factor and full recoil. The states connected by the reaction are described in terms of shellmodel wave functions. They were obtained making use of Kuo-Brown matrix elements in the (p, sd) and (f, p) subspaces for the case of ¹⁶O and ⁴⁴Ti, respectively. The predicted value of the cross section is a factor of the order of 100 smaller than the experimental value.     

Fusion and nonfusion phenomena in the 6Li + 40Ca reaction at 156 MeV

Reaction products from ⁶Li-induced reactions on ⁴⁰Ca at 156 MeV have been studied using the dE × E identification as well as the inclusive γ-ray method. The complete fusion cross section has been found to be σ_F = 67 ± 20 mb. The Z-distribution of fusion evaporation residues is compared with statistical model predictions. The Z-spectrum of reaction products shows a maximum at 15 ? Z ? 20, probably due to transfer and to incomplete fusion. It is suggested that the small value of the fusion cross section is due to the strong competition of ⁶Li break-up processes.     

Alpha-particle strengths from the 16O(6Li,d)20Ne reaction

The ¹⁶O(⁶Li, d)²⁰Ne reaction has been studied at bombarding energies of 20, 32 and 38 MeV. The α-particle spectroscopic strengths have been extracted for levels up to 12.15 MeV in excitation. Nondirect processes appear to contribute significantly to all levels at 20 MeV and to high spin levels (6⁺ and 8⁺) at 32 MeV. Strengths extracted for members of the ground state band assuming (sd)⁴ transfer are unequal at both 32 and 38 MeV, in marked contrast to theoretical predictions. To explain this, particle-hole correlations in ¹⁶O(g.s.), inelastic channel coupling in the reaction and perhaps other effects as well, have to be considered. Strengths extracted for members of excited bands and α-decay reduced widths compare poorly with each other and with simple SU(3) predictions.     

12C(7Li,t)16O and stellar helium fusion

The reaction ¹²C(⁷Li, t)¹⁶O has been studied at $\text{E(}{}^7\text{Li}\text{) = 34}\text{MeV}$ with the LASL tandem accelerator and QDDD magnetic spectrometer. Angular distributions to levels with E_x < 11 MeV have been obtained from 0° to 90°, including 0°. The results have been analyzed with finite-range distorted-wave Born approximation theory. The α-particle spectroscopic factors and reduced widths obtained are compared with those calculated with group theory (SU(3)) and other models. The analysis of data for the 7.1 and 9.6 MeV J^π = 1^? levels, which are of great importance in stellar helium buring, yields a ratio, R, of dimensionless reduced α-widths $\text{θ}{}^2{}_{\mathrm{<mtext>a</mtext>}}\text{(7.1}\text{MeV}\text{)}\text{θ}{}^2{}_{\mathrm{<mtext>a</mtext>}}\text{(9.6}\text{MeV}\text{)}\text{= 0.35b ± 0.13}$. The observed line width of the 9.6 MeV level (Γ_c.m. = 390 ± 60 keV) is less than the accepted value (Γ_c.m. = 510 ± 60 keV) and implies θ²_a(9.6 MeV) ≈ 0.6. These results as well as data for the 6.92 MeV J^π = 2⁺ and 10.35 MeV J^π = 4⁺ “α-cluster” states indicate 0.09 < θ²_a(7.1 MeV) < 0.33 with a mean value θ²_a(7.1 MeV) = 0.14 ± 0.04. The implication for stellar helium burning is discussed.     

Evidence for inelastic processes in the 24Mg(13C, 12C)25Mg reaction at elab = 30 MeV

The ²⁴Mg(¹³C, ¹²C)²⁵Mg reaction has been studied at 30 MeV using a magnetic spectrometer. Differential cross sections for transitions to several final states in ²⁵Mg have been measured and analysed using an exact finite range DWBA code. The DWBA predictions have fitted the bell-shaped distributions satisfactorily, yielding spectroscopic factors which are in reasonable agreement with those obtained using (d, p) reactions. The exceptions are the $\text{3}\text{2}$⁺ state at 0.97 MeV which displays a marked departure from the bell-shaped angular distribution obtained for the other $\text{3}\text{2}$⁺ state at 2.80 MeV, and the $\text{7}\text{2}$⁺ state at 1.61 MeV whose angular distribution has an unusual shape, displaying a deep minimum located at the grazing angle. A semiquantitative model has been used to suggest that the angular distribution for the 0.97 MeV state is evidence for the coupling of inelastic processes in this transition. In the case of the 1.61 MeV state it is suggested that the angular distribution shows the influence of indirect Coulomb excitation on the transfer cross sections.     

The (3He,d) reaction on 24Mg and 28Si at 38.5 MeV

The (³He, d) reaction at a sufficiently high energy is considered to be a reliable single-proton stripping direct reaction. The data for the low-lying states produced by this reaction on ²⁴Mg and ²⁸Si are compared to the predictions of the strong-coupling rotational model. The agreement with the model predictions for ²⁵Al is quite good, both for the single-proton spectroscopic factors in that nucleus and most particularly for the excitation of $\text{7}\text{2}{}^{\mathrm{+}}$ states in ²⁵Al and ²⁹P by two-step processes.