A study of three-nucleon transfer reactions on 17O

The reactions ¹⁷O(¹¹B, ⁸Li)²⁰Ne, ¹⁷O(¹²C, ⁹Be)²⁰Ne and ¹⁷O(¹³C, ¹⁰B)²⁰F have been studied using beams of 115 MeV ¹¹B and ¹²C and 105 MeV ¹³C incident on a gas target. Shell-model calculations have been performed for ²⁰Ne and ²⁰F, for comparison with the experimental data. It is found that the data can be interpreted using the shell model spectroscopic factors and a semi-classical reaction theory. We justify the use of the latter by applying it to the cases of three-nucleon transfer on ¹⁶O. Spin assignments are suggested for previously unidentified states in ²⁰F.     

Cluster states in the neutron excess nucleus 22Ne

High-spin states of the ²²Ne nucleus in the excitation energy range of 15–30 MeV have been studied. The angular correlation method has been used to determine the spins of excited states. A number of new states with high angular momenta—20.0 MeV (9^?), 20.7 MeV (11^?), 21.6 MeV (9^?), 22.2 MeV (12⁺), and 25.0 MeV (9^?)—have been revealed. They are intensely populated in the reaction ¹⁴C(¹²C, α₁)²²Ne* → α₂ + ¹⁸O and correspond to the rotational bands of various structures.     

Measurement and folding-potential analysis of the elastic α-scattering on light nuclei

Differential cross sections ofα-elastic scattering have been measured for the target nuclei¹¹B,¹²C,¹³C,¹⁴N,¹⁵N, and¹⁶O atE=48.7 and 54.1 MeV and for the nuclei¹⁷O,¹⁸O, and²⁰Ne atE=54.1 MeV. The experimental results were analysed in terms of the optical model using different complex potentials. Special emphasis is given to the application of the double-folding approach for the real part of the potential. The imaginary part is expressed in terms of Fourier-Bessel functions. Differential cross sections for theα-¹⁶O scattering over a wide energy range and for the elasticα-scattering for nuclei in the mass rangeA=11 up toA=24 atE=54.1 MeV are analysed by this method. A close correlation between the absorptive part of the potential and nuclear deformation is observed.     

Spectroscopy of 13B, 14B, 15B and 16B using multi-nucleon transfer reactions

New information has been obtained on excited states of the neutron-rich boron isotopes ¹⁴B, ¹⁵B and ¹⁶B, using the reactions ¹²C(¹⁴C,¹²N)¹⁴B, ¹³C(¹⁴C,¹²N)¹⁵B and ¹⁴C(¹⁴C,¹²N)¹⁶B at about 24 MeV/A. The mass excess of ¹⁶B has been measured for the first time, it is 37.08(6) MeV. This means that ¹⁶B is unbound by only 0.04(6) MeV. Furthermore, the nucleus ¹³B has been investigated with the four reactions ¹⁶O(¹⁴C,¹⁷F), ¹²C(¹⁴C,¹³N), ¹²C(¹³C,¹²N) and ¹²C(¹⁵N,¹⁴O). Choosing different target-projectile combinations, it was possible to populate states with different selectivity. New states are observed in ¹³B at excitation energies above the threshold for two-neutron decay. Received: 27 December 1999 / Revised version: 11 February 2000     

Fusion and neutron-transfer cross sections for 13C + 10B and 13C + 11B reactions at subbarrier energies

The cross sections for the ¹⁰B(¹³C, ¹²C)¹¹B neutron-transfer reaction, leading to the ¹¹B 4.45 and 6.74 MeV and ¹²C 4.44 MeV excited states, and for ¹³C + ¹⁰B fusion have been measured by the characteristic and total γ-ray yield methods, respectively, over the energy (c.m.) interval 2.4–5.8 MeV. For ¹³C + ¹¹B, with no transfer reactions present, the fusion cross sections have been measured between E_c.m. = 2.3 and 6.4 MeV. The fusion cross sections for ¹³C + ¹⁰B and ¹³C + ¹¹B are found to be almost equal and slightly enhanced with respect to those for ¹²C + ¹⁰B and ¹²C + ¹¹B.     

Compound transfer and direct transfer reactions induced by 17O on 13C

The elastic scattering, inelastic scattering, single-neutron transfer reactions ¹³C(¹⁷O, ¹⁶O) ¹⁴C, ¹³C(¹⁷O, ¹⁸O)¹²C and ¹³C(¹⁷O, ¹⁸O_{2+, 1.98})¹²C, and seven other exit channels which involve ⁷Li, ⁹Be, ¹¹B and ¹⁵N have been measured for the system ¹⁷O+¹³C at 12.9 and 14 MeV c.m. It is shown that all reactions mentioned above have significant contributions from compound nuclear decay, following fusion of projectile and target.     

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.     

Optical-model analysis of Li elastic scattering

The elastic scattering cross sections of 28 MeV ⁶Li ions from the nuclei ¹¹B, ¹²C and ¹³C were analyzed using the optical model. The analysis has been extended to other ⁶Li elastic scattering data on ¹⁶O (29.8 MeV) and on ¹²C (24.5 and 30.6 MeV) previously measured elsewhere. The fits obtained with the usual six-parameter Woods-Saxon potential are good. Parameter ambiguities were studied and the results of the analysis were compared with the predictions of the folding model.     

Cross sections for 6Li production in the reactions 10B + 16O and 12C + 14N at low energies

Absolute cross sections have been measured for the reactions ¹⁰B(¹⁶O, ⁶Li)²⁰Ne and ¹²C(¹⁴N, ⁶Li)²⁰Ne at several energies in the range E_c.m. = 7.5–16.2 MeV, and 13.8–16.6 MeV, respectively. The predictions of Hauser-Feshbach calculations show generally good agreement with the experimental data without parameter variation. The consequences of an angular momentum cutoff in the entrance channel and in the compound nucleus are discussed.     

Yrast and high-spin states in 22Ne

High-spin states in ²²Ne have been investigated by the reactions ¹¹B(¹³C, d)²²Ne and ¹³(¹¹B, d)²²Ne up to $\text{E}{}^1\text{～- 19}\text{MeV}$. Yrast states were observed at 11.02 MeV (8⁺) and 15.46 MeV (10⁺) excitation energy. A backbending in ²²Ne is observed around spin 8⁺. The location of high-spin states I ≦ 10 is discussed in terms of the rotational band structure, Strutinsky-type calculations, and pure shell-model predictions.     

Helium burning of 18O

The ¹⁸O(α, γ)²²Ne capture reaction has been studied at E_α = 0.6?2.3 MeV. The known resonance at E_α = 2.20 MeV has been established and fourteen new resonances have been found in the energy range covered. The E_α = 1.16, 1.32, 1.45, 1.53, 1.87, 1.96 and 2.15 MeV resonances correspond to resonances observed previously in the ¹⁸O(α, n)²¹Ne reaction. The E_α = 0.77, 1.25 and 1.27 MeV resonances represent new compound states in ²²Ne. Information on branching ratios, ωγ values and total widths is reported. Transition strength arguments and analyses of γ-ray angular distribution data together with results from previous work resulted in the most likely J^π assignments for the resonances.The E_α = 1.66 and 1.78 MeV resonances are good candidates for the two J^π = 8⁺ states predicted at E_x ≈ 11–12 MeV and are probably members of the K^π = 0⁺ (ground state) and K^π = 2⁺ rotational bands in ²²Ne.The investigated energy range of E_α together with that of previous work corresponds to stellar temperatures of T = (0.3?4.3) × 10⁹ K. The astrophysical reaction rate determined from these data is compared with predictions based in part on the nuclear optical model. The rate is also compared with that of the competing ¹⁸O(α, n)²¹Ne reaction.     

Potentials for heavy ions scattering from 208Pb

Data for the elastic scattering of ¹²C(96 MeV), ¹⁶O(129.5 and 192 MeV) and ²⁰Ne (161.2 MeV) from ²⁰⁸Pb have been used to deduce properties of the ion-ion potential. In particular, the strength of the absorptive potential in the surface region was found to be comparable to that of the real potential. The importance of small-angle data is emphasized.     

Statistical giant resonance effects in heavy-ion radiative capture

Heavy-ion radiative capture has been observed in the two reactions ⁹Be(¹²C, γ)²¹Ne and ¹⁶O(⁷Li, γ)²³Na for c.m. energies between 3 and 9 MeV. High-energy γ-rays populating levels up to 4.7 MeV in the final nuclei were detected with a large volume, anticoincidence-shielded NaI spectrometer. Peak cross sections for individual transitions in both reactions were found near 60 nb/sr. For ⁹Be + ¹²C all excitation functions were rather smooth with a broad bump around E_c.m = 5–6 MeV. Statistical model calculations were in good agreement with the data suggesting that perhaps as much as half of the GDR strength in ²¹Ne is statistical. More structure was found in ¹⁶O + ⁷Li superimposed on still sizeable statistical yield. Previous measurements of the same reaction were found to be partially in error.     

Observation of nuclear molecular resonances through the (16O, α) reaction on light nuclei

Spectra of the (¹⁶O, α) reaction induced by a 145 MeV beam at forward angles are measured on ¹²C, ¹³C, ¹⁴N, ¹⁶O, and ²⁰Ne targets. Prominent broad peaks are seen for ¹²C, ¹⁶O, and ²⁰Ne targets, but not for ¹³C and ¹⁴N. A direct ¹²C transfer to molecular states is suggested.     

Nonstatistical structures in the excitation functions of20Ne(16O,12C)24Mg and20Ne(16O,20Ne)16O reactions

The data on the excitation functions of²⁰Ne(¹⁶O,¹²C)²⁴Mg,²⁰Ne(¹⁶O,¹²C)²⁴Mg^*(1.37, 2⁺),²⁰Ne(¹⁶O,¹²C)²⁴Mg^*(4.12, 4⁺+4.24, 2⁺) +²⁰Ne(¹⁶O,¹²C^*(4.44, 2⁺))²⁴Mg,²⁰Ne(¹⁶O,¹²C)²⁴Mg^*(6.01, 4⁺+6.43, 0⁺),²⁰Ne(¹⁶O,²⁰Ne)¹⁶O,²⁰Ne(¹⁶O,²⁰Ne^*(1.63, 2⁺))¹⁶O, and²⁰Ne(¹⁶O,²⁰Ne^*(4.25, 4⁺))¹⁶O reactions atθ _lab=13° fromE _c.m.=22.8 to 38.6 MeV have been subjected to a statistical analysis comprising of the calculations of the distribution of cross sections, deviation functions, cross-correlation functions, summed excitation functions, cross-channel correlation coefficients and coherence widths. The analysis confirms the existence of nonstatistical structures atE _c.m.=24.6, 27.8, 31.7 and 35.5 MeV, and identifies a new structure of the same nature atE _c.m. =25.6 MeV.     

π-π andK-¯K channel coupling and scalar mesons

Total nuclear reaction cross-sections are determined by means of a 4π-γ method. The results cover a wide span of targets for various stable beams. The validity of the method is shown in a combined systematics including also the results of transmission-type experiments. The data are very well described by the formula developed by Kox et al. The same method is applied to secondary fragment beams produced from a 44 MeV/u²²Ne beam on a 332mg/cm^{2 181}Ta target. Using the LISE spectrometer the fragments^{4, 6}He,^{6–9, 11}Li,^{7, 9–12, 14}Be,^{10–15, 17}B^11–19C,^13–19N,^15–21O,^{18– 21}F and^20,21 Ne are analyzed and transported to interact with a 199.4 mg/cm² Cu target surrounded by a 4π-γ counter. The measured total reaction cross-sectionsσ _R are discussed in terms of the reduced strong absorption radiusr ₀ and compared with other experimental results.     

Total reaction and transfer cross sections for 11B + 9Be and 13C + 9Be reactions at low energies

The total reaction cross sections for ¹¹B + ⁹Be and ¹³C + ⁹Be have been measured by the total γ-ray yield method over the energy intervals E_c.m. = 1.4–4.4 MeV and E_c.m. = 2.0–5.2 MeV, respectively. The cross sections for the neutron transfer reactions ¹¹B(⁹Be, ⁸Be)¹²B, leading to the ¹²B 0.953 and 1.674 MeV states, and ¹³C(⁹Be, ⁸Be)¹⁴C, leading to the ¹⁴C 6.094, 6.728 and 6.902 MeV states, have been obtained from the yields of the characteristic γ-rays. The α-transfer reaction ¹¹B(⁹Be, ⁵He)¹⁵N, leading to many unresolved ¹⁵N states, has been observed with large cross section. There is, however, no evidence for the ¹³C(⁹Be, ⁵He)¹⁷O transfer process in the ¹⁷O + nα channels. This different behaviour of the ¹¹B + ⁹Be and ¹³C + ⁹Be systems seems to indicate that the α-transfer reaction at sub-barrier energies is not a direct transfer process, and that it probably occurs via molecular state formation.     

Radiative decays of unbound levels in 20Ne

Nine levels in the range 8.7 to 12.5 MeV in ²⁰Ne have been investigated with the ¹⁶O(α, γ)²⁰Ne and ¹⁶O(α, α')¹⁶O¹(6.13 MeV) reactions using a differentially pumped windowless gas target. Three of the levels have not been observed previously in these reactions, and new information has been obtained for most of the others. In particular, the 11.27 MeV 1^? level is shown to have T = 1, a result of relevance to a proposed parity violation experiment, and the analogue of the 1.97 MeV (3^?T = 1) level in ²⁰F is shown to lie at 12.25 MeV in ²⁰Ne rather than at 12.39 MeV as proposed previously. In addition, the 12.25 MeV level has a width Γ < 1 keV, in contrast to the value Γ ～ 5 keV reported in other work. The electromagnetic transition rates for positive parity T = 1 states in ²⁰Ne are compared with shell-model calculations.