Search for 8+ strength in 16O via the 12C(α, α2)12C1(7.66 MeV) reaction

Closely spaced angular distributions have been measured for the ${}^{12}\text{C}\text{(α, α}{}_2\text{)}{}^{12}\text{C}\text{1(7.66}\text{MeV}\text{)}$ reaction between E_α = 17.39 and 20.5 MeV in a search for 8⁺ strength in ¹⁶O. No evidence of 8⁺ strength is found, but evidence is found for a narrow 7^? resonance at 21.52 MeV excitation.     

High-spin states of 26Mg populated in the 12C(18O, α) reaction

The structure of ²⁶Mg has been investigated by means of the ¹²C(¹⁸O, α) reaction. Several previously unknown states were populated between excitation energies of 0 to 16 MeV. Excitation functions were measured for 126 states for bombarding energies between 43.2 and 45.9 MeV in 300 keV steps at a lab angle of 7°. The experimental energy averaged differential cross sections were compared with statistical model calculations and good agreement was obtained for the states whose spins and parities were previously established. The statistical model calculations were used to suggest the spins and parities for the rest of the states. In particular, candidates for 6⁺ and 8⁺ states were interpreted as members of three rotational bands in ²⁶Mg: the ground-state band, the K = 2⁺ band based on the 2.938 MeV 2⁺ state, and a K = 0⁺ band based on the 3.588 MeV 0⁺ state. Back bending of the yrast band is observed and it is suggested that it may be due to band crossing of the ground-state and first excited K = 0⁺ bands.     

High resolution study of the reactions 54, 56, 58Fe(16O, 12C)58, 60, 62Ni and a comparison with (6Li,d) α-transfer spectroscopy

Spectra and angular distributions for the reactions ^{54, 56, 58}Fe(¹⁶O, ¹²C)^{58, 60, 62}Ni (E_x = 0.0–4.5 MeV) have been measured at 50 MeV with an energy resolution of 45–80 keV using a Q3D spectrograph. The selectivity of the (¹⁶O, ¹²C) reaction is found to be very similar to the (⁶Li, d) reaction. The close correspondence recently noted between the (⁶Li, d) spectra and levels strongly excited in (t, p) and (³He, n) two-nucleon transfer reactions is also observed to be present for the (¹⁶O, ¹²C) reaction. Relative α spectroscopic factors for (¹⁶O, ¹²C) and (⁶Li, d) obtained in a DWBA analysis assuming direct one-step α-cluster transfer are in very good quantitative agreement. Unnatural parity states, whose excitation is forbidden in the DWBA α-cluster approximation, are observed to be very weakly populated. These results, together with previous work on s-d shell and Ni targets, strongly suggest that the spectroscopic information provided by the (¹⁶O, ¹²C) and (⁶Li, d) reactions is essentially the same and that this information may be reliably extracted by DWBA analysis.     

Core-excited states of Fe from the Fe(p, t)Fe reaction at 52 MeV

The perturbation theory for projected states is applied to the two-level pairing force model. Both approximate and exact forms of number projection are considered. The results are compared with the exact ones and with ordinary perturbation theory based on BCS wave functions without projection.     

6Li elastic scattering ON 12C, 16O, 40Ca, 58Ni, 74Ge, 124Sn, 166Er and 208Pb at E(6Li) = 50.6 MeV

The elastic scattering of ⁶Li ions from a variety of targets, A = 12 to 208, has been measured at a bombarding energy of 50.6 MeV. The angular distributions are characteristic of strongly absorbed particles, such as ³He and heavy ions, and less diffractive than for ⁴He. A simple optical model with Woods-Saxon real and imaginary volume potentials is adequate to fit the data. Spin-orbit effects are not apparent in the data.     

Two-proton transfer reaction48Ca(18O,16C)50Ti at 102 MeV

Differential cross sections for¹⁸O elastic scattering and the (¹⁸O,¹⁶C) and (¹⁸O,¹⁷N) reactions on⁴⁸Ca were measured at 102 MeV using a Q3D magnetic spectrograph. The transitions to the 7/2^? ground state (g.s.) of⁴⁹Sc and the 0⁺ (g.s.), 2⁺ (1.554 MeV), 4⁺ (2.675 MeV), and 6⁺ (3.198 MeV) states of⁵⁰Ti were analyzed by DWBA calculations which include finite-range and recoil effects. Simple cluster-transfer calculations were done for all two-proton transfer transitions. For the 0⁺ (g.s.) transition a two-nucleon transfer code employing microscopic wave functions was also used. It was found that absolute cross sections for this kinematically well-matched transition were underrated by a factor of about 7 for a reasonable amount of configuration mixing in the nuclear states involved in this transition. This factor is very close to the value 5 derived for the similarly well-matched⁴⁸Ca(¹⁸O,¹⁶O)⁵⁰Ca reaction.     

12C(12C, 8Be)16O angular distributions and excitation functions between 35 and 69 MeV bombarding energy

An array of eight detectors has been developed for identifying the particle unstable ⁸Be nucleus from nuclear reactions with high detection efficiency. Absolute cross sections have been measured for the reaction ¹²C(¹²C, ⁸Be_g.s.)¹⁶O to the ground state and to several excited states in ¹⁶O. Excitation functions at seven angles from 15° to 45° (lab) in 5° steps have been measured for bombarding energies between E¹²_C(lab) = 35 and 69 MeV. Excitation functions were obtained for the following states in the residual nucleus ¹⁶O which were found to be strongly populated: g.s.(0⁺); 6.1 MeV (0⁺, 3^?); 6.9 MeV (2⁺); 10.4 MeV (4⁺); 11.1 MeV (4⁺); 14.7 MeV (6⁺,…) and 16.3 MeV (6⁺,…). The energy range is covered in 250 keV (c.m.) steps; at certain energy ranges in ¹²⁵ keV or 50keV steps. All excitation functions exhibit a strong energy dependence of the cross section; pronounced gross structures with superimposed fine structures, similar to those observed for ¹²C+¹²C elastic and inelastic scattering at these energies, are observed. At 19.3 MeV, where resonant structures were observed in the reactions ¹²C(¹²C, p)²³Na, ¹²C(¹²C, n)²³Mg and ¹²C(¹²C, d)²²Na, no resonance is found for the reaction studied here. At 60, 61 and 63 MeV angular distributions have been measured in 1° and 2.5°(lab) angular steps. The excitation functions have been analyzed in terms of Ericson fluctuations and cross-correlation functions.     

The 18O(18O, 16O)20O reaction at 52 MeV

The structure of the ²⁰O nucleus was studied by the ¹⁸O(¹⁸O, ¹⁶O)²⁰O reaction at E_1ab = 52 MeV. Angular distributions for the transitions to the lowest four states in ²⁰O were obtained and analyzed with finite-range DWBA calculations. Optical potential sets were used which fit the experimental elastic scattering differential cross sections over almost the whole angular range. The two L = 0 transitions to the ground state and the 4.45 MeV state of ²⁰O populated by the ¹⁸O(¹⁸O, ¹⁶O) reaction were analyzed with exact finite-range DWBA calculations using microscopic form factors. These calculations underestimate the absolute cross sections by a factor of 11. The relative strength of the two L = 0 transitions is well reproduced in the ¹⁸O(¹⁸O, ¹⁶O) reaction. However, DWBA calculations for the ¹⁸O(t, p)²⁰O reaction overestimated the relative cross sections for the excited 0⁺ state by a factor of 6. Several model wave functions were tested for the ground-state transition. It was found that the absolute cross sections of the (¹⁸O, ¹⁶O) reaction are very sensitive to the mixing of shell-model configurations. The angular distribution shapes are also slightly dependent on the mixing.     

The 12C(α, γ)16O reaction and stellar helium burning

The cross section for the reaction ¹²C(α, γ)¹⁶O has been measured for a range of c.m. energies extending from 1.41 MeV to 2.94 MeV, by using ¹²C targets of high isotopic purity, large NaI(T1) crystals, and the time-of-flight technique for the suppression of prompt neutron background and time-independent background. Gamma-ray angular distributions were measured at c.m. energies of 2.18, 2.42, 2.56 and 2.83 MeV. By means of theoretical fits, which include the coherent effects of the 1^? states of ¹⁶O at 7.12 MeV, 9.60 MeV, and those at higher energies, the electric-dipole portion of the cross section at astrophysically relevant energies has been determined. A three-level R-matrix parametrization of the data yields an S-factor at E_c.m. = 0.3 MeV, S(0.3 MeV) = 0.14^+0.14_?0.12 MeV · b. A “hybrid” R-matrix optical-m parameterization yields S(0.3 MeV) = 0.08^+0.05_?0.04 MeV · b. This S-factor is of crucial importance in determining the abundances of ¹²C and ¹⁶O at the end of helium burning in stars.     

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.     

The (p,t) reaction on 12C, 54Fe and 208Pb at 80 MeV

Angular distributions have been measured for the low-lying levels of the residual nuclei for the ¹²C, ⁵⁴Fe and ²⁰⁸Pb(p, t) reactions at E_p = 80 MeV. The shapes of these angular distributions are generally well reproduced by the zero-range distorted-wave Born approximation (DWBA). Enhancement factors extracted from the data show that the DWBA predicts relative strengths consistent with those observed at lower bombarding energies. However, the overall empirical DWBA normalization at E_p = 80 MeV is observed to be $\text{1}\text{12}$ ($\text{1}\text{4}$) of that required at 40 MeV for ²⁰⁸Pb (⁵⁴Fe).     

Tensor polarization of 12C[2+ 1] in the 16O(13C,12C)17O reaction at 50 MeV

The ¹⁶O(¹³C,¹²C)¹⁷O reaction at 50 MeV has been investigated using the kinematical coincidence method. Polarization tensors t ₂₀ and t ₄₀ of ¹²C[2⁺ ₁] for the quantization axis taken along the direction of propagation have been measured by analyzing the energy spectrum of ¹²C[2⁺ ₁], modulated by the effect of γ ray emission. The deduced t ₄₀ values significantly deviate from zero, contrary to the prediction of the distorted-wave Born approximation theory based on one-step p shell neutron stripping without spin-dependent interactions. The phenomenological spin–orbit interaction necessary to reproduce the magnitude of measured t ₄₀ is found to be much larger than the folding model prediction. It is shown that the experimental polarization tensors as well as the cross sections can be reproduced by introducing multi-step processes involving excitations in ¹²C and ¹³C without introducing spin-dependent interactions. Received: 2 August 1999 / Revised version: 3 February 2000     

Resonant effects in the12C+12C→8Be gs +16O gs reaction aroundE cm=32.5 MeV

The¹²C+¹²C→⁸Be _gs +1¹⁶O _gs reaction has been studied at c.m. energies of 27.9, 32.5 and 35.0 MeV. The energy dependence of the cross section is consistent with a resonant behaviour like the one observed for two¹²C nuclei excited in a very deformed state in the final channel. The angular distributions for the two reactions show also a similar oscillatory behaviour. The analysis suggests that the reaction proceeds through the formation of an intermediate state with a complex nature, involving largely deformed configurations as well as almost spherical ones.     

The58Ni(12C,γ)70Se reaction

The⁵⁸Ni(¹²C,γ)⁷⁰Se capture reaction was studied with beam energies ofE _12C=30 to 42 MeV. The capture events were identified by means of the residual activity produced in the reaction. At a beam energy of 38 MeV the capture cross section has been determined to 1.5±0.7 μb. AtE _12C=30, 34 und 42 MeV we established upper limits of 0.3, 0.5 and 0.6 μb, respectively. The experimental results are compared with a statistical model calculation.     

Investigation of 16O excited states via the 14N(3He,p) reaction

The reaction ¹⁴N(³He, p)¹⁶O has been investigated at a bombarding energy of 15.0 MeV, using a differentially pumped gas target. Angular distributions were measured for 30 levels below 16.5 MeV in excitation. Data were compared with shell-model calculations of Zuker, Buck and McGrory for states whose correspondence with theory is established. Many states are found to possess a large compound-nucleus reaction component. Several previously unreported levels are observed at high excitation. Angular distributions for all except the weakest levels have been compared with DWBA calculations.     

Phenomenological analysis of the 24Mg(12C, α)32S resonant reaction studied at low incident energy

The ²⁴Mg(¹²C, α)³²S reaction has been studied in the region of the Coulomb barrier. Three angular distributions for the α-particle groups populating the ground and first-excited 2⁺ state of ³²S have been measured spanning the apparent resonant structure observed at E_c.m = 14.20 MeV. Regge-pole and phase-shift analyses suggest a spin assignment of 8 for the corresponding resonance at E_x = 30.51 MeV in ³⁶Ar.     

First study of the (14C, 12C) reaction: Selectivity of the reaction and the energy levels of 28Mg and 30Si

The first measurements are reported for (¹⁴C, ¹²C) two-neutron stripping reactions. Energy spectra up to an excitation energy of 12 MeV have been measured at 69 MeV for the ²⁶Mg(¹⁴C, ¹²C) and ²⁸Si(¹⁴C, ¹²C) reactions. A strong selectivity of this reaction is observed. Using this selectivity, the comparison of the spectra suggests J^π assignments for several ³⁰Si and ²⁸Mg states. The results from the other two-neutron stripping reactions, (t, p) and (¹⁸O, ¹⁶O) are compared with those of the (¹⁴C, ¹²C) reaction.     

Large-angle enhancements in the 16O(6Li, α)18F reaction at 48 MeV

The elastic scattering of ⁶Li + ¹⁶O at 48 MeV has been measured and fitted with an optical model calculation. Measurements have been made of the ¹⁶O(⁶Li, α)F reaction at 48 MeV populating the 1⁺ g.s., 3⁺ 0.927 MeV and 5⁺ 1.122 MeV states in ¹⁸F. The data exhibit cross sections at large angles comparable to those at forward angles, and have been compared with exact finite-range DWBA calculations. Exchange contributions were included for the 1⁺ g.s. and were unable to account for the large-angle data. Calculated statistical compound nucleus cross sections were approximately a factor of 100 below the data. The same conclusions are reached for previously published data at 34 MeV.     

A study of the 11B(3He,p)13C reaction at 12, 10, 8, 6 and 4 MeV

Angular distributions for eight proton groups corresponding to states in ¹³C have been measured at incident ³He energies of 4, 6, 8, 10 and 12 MeV. These states are p₀(ground state), P₁(3.09), p₁(3.68), p₃(3.85), p₄(6.86), p₇(7.68), pio(9.50) and p₁₁ (9.90). The angular distributions have been fitted with DWBA analyses using a zero-range approximation. No radial cut-offs were used. In general the data are reasonably well fit at incident energies of 10 and 12 MeV.     

The 15N(p, α)12C reaction with polarized protons from 0.34 to 1.21 MeV

A polarized beam was used to measure angular distributions of the analyzing power of the ¹⁵N(p, α)C reaction at 0.34 MeV and at five energies from 0.92 to 1.21 MeV. The analyzing power can be fitted with associated Legendre polynomials, P₁¹ and P¹₂ sufficing to describe the results except near 1.2 MeV where P₃¹ is also required. Polarization excitation functions were measured throughout the entire energy range at angles where the polynomials P¹₂ and P¹₃ are zero. A polarization contour map is given.