Spectroscopy of intermediate 16F states with the reaction 14N(3He,np)15O

Excited states of ¹⁶F have been investigated with the reaction ¹⁴N(³He, np)¹⁵O at E = 10.5 and 12 MeV in kinematically complete experiments. Proton groups corresponding to the decays of intermediate ¹⁶F states were observed at various angles with counter telescopes in time coincidence with the associated neutrons detected at θ_n^lab = 0° with a time-of-flight spectrometer. Excitation energies and decay widths Γ_p0 of these states have been extracted from the proton spectra. Lower limits for the orbital angular momentum in the decay channel and for the spin of the states have been deduced from the obtained angular correlations. By comparison with the reaction ¹⁴N(³He, pp)¹⁵N measured at E = 13 MeV, pairs of $\text{T = 1}{}^{16}\text{F}$ parent/¹⁶O analog states have been identified. J^π assignments and shell-model configurations are discussed on the basis of the selectivity of the reactions measured.     

Resonance structure in 15N(p,n0)15O in the region Ep = 8.5–19.0 MeV

Excitation functions at seven angles, covering the energy range E_p = 8.5–19.0 MeV, have been measured for ¹⁵N(p, n_o)¹⁵O using time-of-flight methods. Angular distributions were also measured at five energies. Activation methods were used to determine the total cross section below the excited-state threshold. Ten resonances were observed, four of which have also been found in ¹⁵N(p, γ)¹⁶O data. The energies, widths and nucleon widths have been extracted and are used, together with proton capture data, to derive radiative widths for certain levels.     

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.     

The reaction 14C(3He,n)16O and the coexistence model of 16O

The reaction ¹⁴C(³He, n)¹⁶O has been measured at a ³He bombarding energy of 25.4 MeV. The zero-degree differential cross section for the excitation of the three low lying 0⁺T = 0 states, at energies 0.0, 6.05 and 12.05 MeV are, respectively, 1.33 ± 0.10, 0.49 ± 0.10, and 0.50 ± 0.10 mb/sr These measured cross sections are in rough agreement with single-step zero-range DWBA calculations using an empirically determined ¹⁴C ground state wave function and in which the Brown and Green coexistence-model wave functions are used to describe the ¹⁶O 0⁺ states. The angular distribution of the transition to the ground state is measured between 0° and 32°.     

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.     

Elastic scattering of tritons by 16O

Angular distributions and excitation functions have been measured for the ¹⁶O(t, t) reaction from 1.4 to 3.7 MeV triton energy using a differentially pumped gas target. The observed resonances have been analysed with the Humblet-Rosenfeld theory. It appears that the 13–15 MeV excitation range of the compound system of ¹⁹F is dominated by several levels of large triton partial width. Levels populated in the three-particle transfer reaction (⁷Li, α) on ¹⁶O are compared with the resonances.     

Phase-shift analysis of 16O + 16O elastic scattering data

A phase-shift analysis of precisely measured elastic ¹⁶O + ¹⁶O data has been carried out in order to establish the existence of molecular resonances between E_c.m = 15.5 and 18 MeV and to investigate the possible structure of these resonances.     

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.     

The heavy-ion reaction channels of the system 16O + 16O

The reaction channels of the system ¹⁶O + ¹⁶O with outgoing heavy particles from lithium to magnesium have been measured using a ΔE-E telescope. Excitation functions from 49 to 65 MeV at θ_Lab = 30° and angular distributions from θ_Lab = 10° (20°) to 50° at E_Lab = 51.5 MeV are presented for the strong transitions. The excitation function of the ¹²C-²⁰Ne (4.25 MeV) channel shows a pronounced regular cross structure with peaks at 52 and 60 MeV. A selective excitation of certain states in the inelastic scattering and the ¹²C-²⁰Ne channel is observed; the yields of the other heavy-ion channels being weaker by at least one order of magnitude. An explanation of this phenomenon is given by considering the angular momentum matching between entrance and exit channels. Furthermore it is shown that no strong dependence of the cross sections on the transferred angular momentum or on the nuclear structure of the final states is observed. Possible implications of these results on the reaction mechanism are discussed.     

Full recoil analysis of the elastic scattering of 16O on 19F by the core-exchange model

An analysis of the scattering ¹⁶O-¹⁹F measured at the energies E_lab = 21.4, 25.8 and 32.0 MeV has been carried out by using the core-exchange model. The value of cfp 〈¹⁹F|}¹⁶O, triton〉 obtained in the present analysis is consistent with that obtained from the analysis of ¹⁹F (³He, ⁶Li)¹⁶O when the recoil effects are included.     

Alpha cluster states in 16O

A very successful cluster-core model for the rotational bands of light nuclei has been extended to treat excited core states. The resulting coupled-channels problem has been solved for the ${}^{16}\text{O }{}^{12}\text{C + α}$ system. As well as the known + ve and ? ve parity bands many new levels are predicted and compared with experiment. The agreement between theory and experiment is very good both for the excitation energies and the decay properties. We predict the positions of two low-spin non-natural parity levels with E_x < 20 MeV and the positions of many high-spin states. In particular, we explain why the 8⁺ level seen in α-transfer at 22.5 MeV could not be found in the α-scattering cross sections and predict a second 8⁺ at 24.4 MeV. We discuss how the levels may be regarded as members of rotational bands and determine the terminating J-values for these bands. Finally we show that the usual rotational model would be a poor approximation for the cluster bands of ¹⁶O.     

Microscopic study of elastic 12C+16O scattering

The L=0 to 17 phase shifts are calculated microscopically for elastic ¹²C+¹⁶O scattering with a generator coordinate method. The experimental resonances are shown to belong to four different bands. Interpretations and spin assignments are suggested for several recently observed anomalies. A spin J=15 seems likely for the 22.8 MeV resonance. A missing J=13 resonance should be searched for between 17 and 18 MeV.     

Levels of 16O near 13 MeV excitation from 15N + P reactions

Angular distributions, a 0° excitation function and Doppler-broadened γ-ray profiles for the reaction ¹⁵N(p, α₁γ), and angular distributions for the ¹⁵N(p, α₀) reaction, have been measured for proton energies from about 900 to 1250 keV. These data, together with analysing powers from the ${}^{15}\text{N}\text{(}\text{p}$, α₀) reaction, have been satisfactorily fitted by means of R-matrix theory in terms of the known levels of ¹⁶O in the 13 MeV region together with background contributions.     

Spectroscopy of A = 16 from 13C(6Li,t)16O

The ¹³C(⁶Li, t)¹⁶O reaction has been studied at 34 MeV. Selective population of narrow states is observed up to 21 MeV excitation in ¹⁶O. This reaction populates strongly both unnatural-and natural-parity states that have little or no ¹²C + α₀ width. The measured angular distributions are compared with Hauser-Feshbach and finite-range DWBA calculations. Reasonable agreement with the DWBA calculations is found for most of the states strongly populated. The widths of the narrow states populated in the 16–20 MeV excitation region are presented. Comparison of the present data with that from medium-energy inelastic scattering and other multiparticle transfer reactions is made.     

On the fusion cross section for 16O + 12C

The total fusion cross section σ_f for ¹⁶O + ¹²c.m. has been measured in 250 keV steps between 17 and 28 MeV c.m. energy via γ-ray techniques. Individual yields for 10 nuclides in the evaporation chain were determined. The oscillations in σ_f, which agree in gross structure but not in detail with previous work, appear to be most closely associated with reactions leading to ²⁰Ne. It is observed that the reaction yield is >80% dominated by nuclides having at least one α particle in the evaporation chain.     

Mass and excited levels of the neutron-rich nuclei 73Zn and 74Zn studied with the 76Ge(14C, 17O) and (14C, 16O) reactions

The ⁷⁶Ge(¹⁴C, ^{16, 17}O)^{74, 73}Zn reactions have been studied at 72 MeV bombarding energy. The mass excesses of ⁷³Zn and ⁷⁴Zn were determined to be ?65.41 ± 0.04 and ?65.62 ± 0.04 MeV, respectively. In addition, previously unknown excited levels were identified in both nuclei. The structure of ⁷³Zn is discussed in terms of HFB calculations.     

Unnatural parity levels populated in the 16O(6Li,d)20Ne reaction

The ¹⁶O(⁶Li, d)²⁰Ne reaction to the 2^?, 4.97 MeV, 3^?, 5.63 MeV, and 4^?, 7.01 MeV members of the K^π = 2^? band has been studied. Angular distributions were measured at 32 MeV from 7.5° to 145° (lab). Excitation functions were measured at 15° (lab) and 145° (lab) from 31 to 33 MeV and 31.75 to 32.5 MeV, respectively. Results of multi-step and compound nuclear calculations are compared to the data. At this incident energy, both mechanisms appear to contribute to the population of the unnatural parity levels.     

Excitation of the 6.41 MeV, T=14− state in 16F by the 16O(p,n)16F reaction

The angular distribution for the ¹⁶O(p, n) ¹⁶F (6.41 MeV) reaction at 35 MeV has been measured and analyzed using a distored-wave approximation. With analyses of 99 and 135 MeV data fixing the characteristics of the nuclear structure and direct reaction models required, analysis of the 35 MeV data reveals evidence of higher-order reaction processes, which we suggest involve virtual excitation of giant multipole resonances.