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.     

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.     

A study of the 26Mg(12C, 12B)26Al charge-exchange reaction

The reaction ²⁶Mg(¹²C, ¹²B)²⁶A1(5⁺, 3⁺) has been studied using a beam of 102 MeV of ¹²C. Shell-model, microscopic direct model and finite-range coupled reaction channel (CRC) calculations including recoil effects, have been performed, for comparison with the experimental data. DWBA calculations were performed for the intermediate states of interest in the ¹¹B + ²⁷Al and in the ¹³C + ²⁵Mg channels and these results were also compared with the experimental ones. The dominant reaction mechanism for ²⁶Mg(¹²C, ¹²B)²⁶Al(5⁺, 3⁺) appears to be the sequential mode.     

The isoscalar strength distribution in 24, 26Mg, 28Si and 40Ca obtained from inelastic alpha scattering at 120 Mev

The inelastic α-scattering reaction at E_α = 120 MeV with an energy resolution of 90–150 keV has been used to investigate isoscalar strength distributions in ^{24, 26}Mg, ²⁸Si and ⁴⁰Ca. For ^{24, 26}Mg and ²⁸Si the E2 strength between E_x = 14 and 27 MeV is strongly fragmented. In ⁴⁰Ca the E2 strength is mainly concentrated near $\text{E}{}_{\mathrm{<mtext>x</mtext>}}\text{~ 65 A}{}^{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}$ MeV, although here the onset of fragmentation can be observed. The sum rule strength for the different multipolarities was obtained by applying for each nucleus an L-dependent normalization procedure. In this way we observed in total in ^{24, 26}Mg, ²⁸Si and ⁴⁰Ca for excitation energies up to 27 MeV an amount of (61⁺⁸_?6), (50⁺⁹_?8), (38⁺⁸_?6) and (94 ± 14)%, respectively, of the isoscalar E2 energy weighted sum rule (EWSR) of which (36⁺⁷_?5), (28⁺⁸_?7), (24⁺⁷_?5) and (74 ± 12)% was found between E_x = 14 and 27 MeV. In addition isoscalar E0, E3 and E4 strength was observed in this excitation energy region. A detailed comparison has been made between the isoscalar quadrupole strength distribution observed in the ^{24, 26}Mg(α, α') reaction and the E2 strength excitation function obtained from radiative α-capture measurements. In the low excitation energy region coupled channel effects have been observed, especially for the excitation of the 3⁺ states. Moreover, a considerable percentage of the 1?ω isoscalar dipole and octupole strength has been observed for excitations below 14 MeV.     

The 26Mg(14N, 10B)30Si and 26Mg(14N, 11B)29Si reactions

Reactions induced by ¹⁴N on ²⁶Mg at bombarding energies of 60–95 MeV have been studied. Angular distributions for states populated in ²⁹Si by the (¹⁴N, ¹¹B) reaction and in ³⁰Si by the (¹⁴N, ¹⁰B) reaction have been compared with Hauser-Feshbach and DWBA calculations to determine the reaction mechanism and to deduce spectroscopic information. The cross sections for the states populated in ²⁹Si and ³⁰Si are in poor agreement with statistical model calculations, indicating a non-compound nucleus mechanism.     

The 12C(9Be,n)20Ne reaction

A study of the ¹²C(⁹Be, n)²⁰Ne reaction has been carried out at bombarding energies of 16 and 24 MeV. The spectra at both incident energies are dominated by a consistent set of levels between an excitation energy of 7.3 ± 0.4 and 15.7 ± 0.3 MeV. The rotational band based on the K^π = 0₃⁺ state appears to be strongly populated. Based on this selectivity, additional evidence is provided in favor of identification of the 8⁺ state at 15.9 MeV with this 0₃⁺ band. Angular distributions measured at the higher bombarding energy are compared with statistical compound-nuclear calculations. It appears that a non-statistical mechanism is responsible for the reaction's selective population of states with 8p-4h configuration. Such a mechanism, involving the preferential breakup of the ⁹Be into ⁸Be plus a neutron, is discussed.     

An experimental study of the 9Be(p, π+)10Be and 9Be(p, π−)10C reactions at 185 MeV

A magnetic spectrometer was used for the momentum analysis of pions produced by 185 MeV protons on a ⁹Be target. The obtained energy resolution (0.55 MeV FWHM) made it possible to resolve a number of discrete final states in ¹⁰Be and ¹⁰C. Energy spectra were measured in the angular region 15°–130° (lab). Angular distributions for six peaks in ¹⁰Be and four peaks in ¹⁰C are presented in tables and graphs. A peak corresponding to a not previously reported level in ¹⁰Be was observed at 11.75 ±0.11 MeV excitation energy. The measured $\text{π}{}^{\mathrm{+}}\text{π}{}^{\mathrm{?}}$ ratio for the ground state analogues was found to be angular dependent and varied from 30 at 35° (lab) to 2 at 125° (lab). The results are compared with theoretical predictions and discussed in terms of one- and two-nucleon models.     

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.     

Study of 11B at high excitations with the 9Be(3He,p)11B and 9Be(α, d)11B reactions

The nucleus ¹¹B has been studied over the excitation energy range from 8.5 MeV to 21.5 MeV with the ⁹Be(³He, p)¹¹B / reaction at / E_{³He = 38 MeV}. The analogs of the parent states in ¹¹Be have been located at 12.56, 12.92, 14.40, 16.44, 17.69, 18.0, 19.15 and 21.27 MeV. A complementary measurement with the ⁹Be(α, d)¹¹B reaction at E_α = 48 MeV demonstrates that the 16.44, 17.69, 18.0 and 19.15 MeV resonances have rather pure isospin $\text{T}{}_{\mathrm{f}}\text{=}\text{3}\text{2}$. The 14.40 MeV state is a strongly isospin-mixed analog of the $\text{5}\text{2}{}^{\mathrm{+}}\text{1.78}\text{MeV}$ state in ¹¹Be. It is argued that spin S = 1 transfer is involved in the excitation of the 16.44 MeV state and its 3.887 MeV parent in ¹¹Be in a two-step stripping process. The $\text{T}{}_{\mathrm{f}}\text{=}\text{1}\text{2}$ states and the lowest three $\text{T}{}_{\mathrm{f}}\text{=}\text{3}\text{2}$ states are compared with the predictions of DWBA utilizing shell-model form factors. It is concluded that the $\text{T}{}_{\mathrm{f}}\text{=}\text{1}\text{2}$ strength is more strongly fragmented than is implied by the calculations of Teeters and Kurath.     

(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.     

Désintégration β− de 26Na

Gamma-rays following the β^? decay of ²⁶Na have been measured with a Ge(Li) detector. The ²⁶Na sources were produced by fast neutron bombardment of an enriched ²⁶Mg sample. Evidence for eight previously unreported β^? branches in the disintegration of ²⁶Na has been deduced and their log ft values determined. The half-life of ²⁶Na was measured to be 1.05 ± 0.02 s. Allowed β^? transitions (log ft = 4.7 and 6.1 respectively) to the 1.81 MeV, 2⁺, and to the 4.90 MeV, 4⁺, levels of ²⁶Mg determine J^π = 3⁺ for the ²⁶Na ground state.     

The reaction 26Mg(d, τ)25Na and the structure of 25Na

The nuclear structure of the nucleus ²⁵Na has been studied with the (d, τ) proton pick-up reaction on ²⁶Mg at a bombarding energy of 29 MeV with an energy resolution of 25 to 30 keV FWHM. Excited states in ²⁵Na have been measured up to excitation energies of 8 MeV. The experimental angular distributions show good agreement with the predictions from the standard distorted-wave Born-approximation theory (code DWUCK; non-local and finite range). However, the agreement is improved considerably if the procedure of Kunz, Rost and Johnson is applied which accounts approximately for strong couplings to inelastic channels in the initial and final (strongly deformed) nuclei. The influence of this treatment on the evaluation of spectroscopic factors has been investigated and was found to be particularly pronounced for l = 0 transitions. The measured spectroscopic factors are compared to those from other experimental work and from shell-model and Nilsson-model calculations.     

Cluster decay of the high-lying excited states in 14C

A cluster-transfer experiment of ⁹Be(⁹Be,¹⁴C→α+¹⁰Be)α at an incident energy of 45 MeV was carried out in order to investigate the molecular structure in high-lying resonant states in ¹⁴C. This reaction is of extremely large Q-value, making it an excellent case to select the reaction mechanism and the final states in outgoing nuclei. The high-lying resonances in ¹⁴C are reconstructed for three sets of well discriminated final states in ¹⁰Be. The results confirm the previous decay measurements with clearly improved decay-channel selections and also show a new state at 23.5(1) MeV. The resonant states at 22.4(3) and 24.0(3) MeV decay primarily into the typical molecular states at about 6 MeV in ¹⁰Be, indicating a well developed cluster structure in these high-lying states in ¹⁴C. Further measurements of more states of this kind are suggested.     

Elastic scattering of 9Be on light target nuclei

The elastic scattering of ⁹Be on the target nuclei ⁹Be, ¹²C, ¹³C and ¹⁶O was measured at projectile energies of 14, 20 and 26 MeV. All angular distributions show a diffraction-like structure. In addition, excitation curves were measured for the scattering of ⁹Be on ⁹Be at three different angles in the energy range of 9–22 MeV. An optical potential having parameters independent of mass number and energy and reproducing all the measured angular distributions in the forward angular region was found and compared with that for the scattering of ⁹Be on medium and heavy nuclei.     

The (d, 6Li) reaction on 12C, 16O, 24Mg, 40Ca and 58Ni at 54.25 MeV

The (d, ⁶Li) reaction was studied at E_d = 54.25 MeV on the target nuclei ¹²C, ¹⁶O, ²⁴Mg, ⁴⁰Ca and ⁵⁸Ni. The data were analyzed with finite-range DWBA calculations. The absolute values of the α-cluster spectroscopic factors and the target mass dependence of the relative S_α were in agreement with those in the (p, pα) reaction at E_p = 100 and 157 MeV. The theoretical calculations of the relative S_α were in better agreement with the experimental data at higher energy than at the lower energies.     

Evidence for critical angular momenta in the formation of 26Al via the 14N + 12C channel

States in ²⁰Ne have been studied through the ¹²C(¹⁴N, ⁶Li)²⁰Ne reaction. Excitation functions have been measured from 20 MeV to 60 MeV in steps of 5 MeV at different angles for ²⁰Ne states up to 10 MeV excitation energy. States of ²⁴Mg have been also populated using the ¹²C(¹⁴N, d)²⁴Mg reaction; excitation functions of ²⁴Mg states up to 9 MeV excitation energies as well as angular distributions at 35 MeV bombarding energy have been obtained. Comparisons of data with Hauser-Feshbach calculations show clearly that the compound nucleus mechanism is the main process for both ¹²C(¹⁴N, ⁶Li)²⁰Ne and ¹²C(¹⁴N, d)²⁴Mg reactions. Strong evidence has been provided for inhibition of the ²⁶Al compound nucleus formation for angular momenta higher than critical values. The location of the yrast line in the ²⁶Al nucleus is discussed.     

A study of 14N using the 13C(d,n)14N reaction

Absolute differential cross sections for the ¹³C(d, n)¹⁴N reaction were measured at deuteron bombarding energies of 4.5, 5.0 and 5.5 MeV. Spectroscopic factors and statistical compound-nucleus contributions are obtained by treating the observed cross sections as an incoherent sum of distorted-wave Born approximation and compound-nucleus contributions. Energy-averaged spectroscopic factors are derived. An anomaly is observed in the yield for the 2.313 MeV T= 1 state.