Lifetimes of low-lying states in 19F

Lifetimes of low-lying states in ¹⁹F were measured using the Doppler-shift attenuation method through the ¹⁵N(α, γ)¹⁹F reaction. Values of τ_m = 3700 ± 700 fs (1.35 MeV), 140 ± 15 (1.46), 19 ± 7 (4.00) and 63 ± 19 (4.03) were obtained for the lowest $\text{5}\text{2}{}^{\mathrm{?}}$, $\text{3}\text{2}{}^{\mathrm{?}}$, $\text{7}\text{2}{}^{\mathrm{?}}$ and $\text{9}\text{2}{}^{\mathrm{?}}$ members, respectively, of the $\text{K}{}^{\mathrm{\pi }}\text{=}\text{1}\text{2}{}^{\mathrm{?}}$ rotational band and 5 ± 3 fs (1.55 MeV) and 370 ± 25 (2.78) for the $\text{3}\text{2}{}^{\mathrm{+}}$ and $\text{9}\text{2}{}^{\mathrm{+}}$ members of the $\text{K}{}^{\mathrm{\pi }}\text{=}\text{1}\text{2}{}^{\mathrm{+}}$ ground-state band. For the Doppler-shift attenuation analysis correction factors of the nuclear and electronic stopping powers were determined by measuring the Doppler-shift attenuation and γ-ray line shape of the 2.78 → 0.20 MeV transition and range values of 100, 200. 300 and 370 keV ¹⁹F nuclei in tantalum. All calculations were done with Monte Carlo methods. The transition strengths are discussed in terms of different theoretical predictions.     

High-spin negative parity states in 19F

Levels at 7.17, 8.29, 8.96 and 9.88 MeV in ¹⁹F have been assigned spin and parity $\text{11}\text{2}{}^{\mathrm{?}}$, $\text{13}\text{2}{}^{\mathrm{?}}$, $\text{11}\text{2}{}^{\mathrm{?}}$ and $\text{11}\text{2}{}^{\mathrm{?}}$, respectively, from resonance strength and γ-ray angular distribution measurements employing the ¹⁵N(α,γ) ¹⁹F reaction. An earlier assignment of $\text{11}\text{2}{}^{\mathrm{+}}$ to the 8.96 MeV level is incorrect. The measured properties of the $\text{11}\text{2}{}^{\mathrm{?}}$ states are compared with the results of both SU (3) shell model and cluster model calculations.     

New states in 16F

Ten new states in ¹⁶F have been identified through an investigation of the reaction ¹⁴N(τ, n)¹⁶F. For several levels J^π and/or transition L-values have been deduced.     

A re-investigation of 19O(β−)19F

Gamma rays observed in¹⁹O(β^?)¹⁹F have been studied with Ge(Li) spectroscopy utilizing activity created via the ¹⁸O(d, p) ¹⁹O reaction. Precision measurements of γ-ray energies and intensities, in conjunction with previous work, define the following excitation energies (in keV) and β-branching ratios (in %) for states of ¹⁹F: 109.894(5), 0.055(13/38); 197.143(4), 45.4(1.5); 1345.67(13), 0.017(2); 1554.038(9), 54.4(1.2); 2779.849(34), < 0.002; 3908.17(20), 0.0081(5); 4377.700(42), 0.0984(30). The corresponding values of logft are 8.34(30/10); 5.384(14); 8.25(5); 4.625(10); > 8.17; 6.133(27); and 3.859(17), respectively. The β-branches to the 1346 and 3908 keV states had not been observed previously. The β-branch for the 4378 keV state is in significantly better accord with theoretical expectations than the previous value of 0.160(12) %.     

Low-lying states of the 16F nucleus

Low-lying proton-emitting states of ¹⁶F have been investigated through the sequential particle decay reactions ¹⁴N(³He, n)¹⁶F(p)¹⁵O. Excitation energies were determined by measuring outgoing proton energies. Estimations of proton decay widths and spin limitations were made from proton spectra and angular correlation data which were obtained by detecting the protons in time coincidence with the associated neutrons at θ_n = 0°. To date, the ground-state spin of ¹⁶F has been considered to be J = 0; however, the present work suggests J = 1 to be preferable.     

Photoproton and photo-α cross sections of 19F to excited residual states

Cross sections and integrated cross sections for photodisintegration of ¹⁹F to excited residual states following emission of protons, neutrons and α-particles have been measured. The size of the photo-α cross sections appear to be too large to be explained by a semidirect reaction mechanism.     

Mass distributions of fusion products in reactions between 19F and 12C

Nuclear reactions between ¹⁹F and ¹²C target nuclei were studied at beam energies of 50, 63.2 and 76 MeV. Reaction products in the mass range from A = 11 to 31 emitted in forward direction (4°–20°) were identified with a time-of-flight telescope. Mass distributions of the fusion products were obtained. Their characteristic structure is interpreted as a consequence of the superposition of nucleon and α-particle emission. Qualitative conclusions are drawn on the relative intensities of the different decay modes of the compound nucleus. Fusion cross sections and estimates of the contribution of direct channels to the total reaction cross section are discussed.     

NMR study of both 19F and 205Tl motions in TlZrF5

Spin-lattice relaxation times in the laboratory T_l and rotating frames T_l? for both ¹⁹F and ²⁰⁵Tl nuclei were measured as a function of temperature. Comparison of the temperature dependence of longitudinal NMR relaxation rates of both ¹⁹F and ²⁰⁵Tl suggests that Tl⁺ ions are more mobile than F^? ones at high temperatures. Furthermore Fourier transform experiments show that two kinds of Tl⁺ ion can be distinguished at high temperatures, one of them being more mobile than the other one and probably responsible for the ionic conductivity of this material.     

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.     

Inelastic processes in the 19F(3He,d)20Ne reaction

We have examined the role of inelastic processes in the ¹⁹F(³He, d)²⁰Ne reaction. By coupling three levels in both the entrance and exit channels it was found that the inelastic processes were able to account for both the magnitude and rather flat shape of the angular distribution for the 4.25 MeV 4⁺ level observed in the ¹⁹F(³He, d)²⁰Ne reaction at 16 MeV bombarding energy. In contrast the DWBA could not account for the data. The magnitude of the inelastic processes was found to be quite sensitive to some of the optical model parameters involved. The DWBA predictions for the 0⁺ and 2⁺ cross sections were modified by the inelastic processes requiring some adjustment of the spectroscopic amplitudes to account for the data.     

The 19F(p,d)18F reaction studied at Ep = 19.3 MeV

The ¹⁹F(p, d)¹⁸F reaction was used at E_p = 19.3 MeV to study the level properties of ¹⁸F up to 6 MeV excitation energy. Angular distributions were measured over the angular range θ_lab = 10°–70° with an overall resolution of 35 keV. The experimental data were analyzed by DWBA theory to obtain l_n and C²S_l values, which were compared with existent theoretical nuclear structure calculations.     

The transfer reactions induced by 38 MeV 19F ions on 27Al

The reactions induced by 38 MeV ¹⁹F on ²⁷Al have been studied using the Tandem Van de Graaff accelerator at Harwell together with a mass identification system based on a combination of a Buechner magnetic spectrograph and position sensitive silicon detectors in the focal plane of the magnet. Comparison of the elastic scattering measurements with Coulomb cross sections provided an absolute normalisation to the measured reaction yields. Transfer reactions were measured as a function of emitted particle energy and angle for single nucleon pick-up and stripping and for 2, 3 and 4 particle stripping. Essential features of the experimental technique are outlined and the experimental results are presented. The significance of the energy spectra and angular distributions are discussed. The results of DWBA analyses of one-nucleon transfer reactions yield spectroscopic factors for levels in ²⁸Si, ²⁸Al and ²⁶Al which are in fair agreement with those obtained in other studies.     

Fine-structure splittings of 2F states of singly ionized strontium

We have studied the 4d?nf and 5d?nf (n = 4?8) transitions of singly ionized strontium using high-resolution optical spectroscopy. The ²F fine structure is inverted in all the states observed and decreases from ?1.06cm^?1 in 4f to ?0.26 cm^?1 in 8f.     

Alpha-particle scattering from 9Be

The elastic and inelastic scatterings of 35.5 MeV ⁴He ions from ⁹Be are analyzed in the strong coupling limit by the coupled channel method. A good account of the ground-state ($\text{K =}\text{3}\text{2}{}^{\mathrm{?}}$) band is obtained, in agreement with the electromagnetic properties of this band. Data for three states of the $\text{K =}\text{1}\text{2}{}^{\mathrm{+}}$ band are analyzed, yielding much larger isoscalar dipole scattering amplitudes for the $\text{3}\text{2}{}^{\mathrm{+}}$ state than are obtained by electron scattering for electric dipole transitions.     

Energy levels in 23Na from the 19F(α, n)22Na reaction

The 0° differential cross sections for the production of four neutron groups, populating the ground state and the four lowest excited states of the residual ²²Na nucleus were measured. For incident α-energies of up to 4.7 MeV more than sixty resonances, corresponding to levels in ²³Na were seen; most of them had not been reported previously. Angular distributions were measured at eight α-energies. The excitation curve for the ¹⁹F(α, P₁γ)²²Ne reaction was also obtained and eighteen additional levels were identified in ²³Na.     

The lifetime of the 3907 keV state of 19F

The mean lifetime of the 3907 keV state of ¹⁹F has been measured by the Doppler shift attenuation method (DSAM). The value obtained was 9 ± 5 fs. Transition strengths derived from this lifetime and previously measured branching ratios are compared with the results obtained from shell model calculations. The importance of including particle-hole configurations is discussed and it is shown for example that the energy and lifetime of the state are consistent with a predominantly p^?2 sd⁵ structure.     

Evidence of E1 excited states in 19F from the (γ, t) reaction2

The ¹⁹F(γ, t)¹⁶O reaction was studied over an energy region of 18 to 23 MeV. Two resonances were observed at 18.8 MeV and 20.1 MeV. The angular distribution of tritons from these resonances indicated that the tritons are emitted predominantly in a p-wave, implying $\text{j}{}^{\mathrm{\pi }}\text{=}\text{1}\text{2}$^?, or $\text{3}\text{2}$^? with T = $\text{1}\text{2}$ for both levels. A comparison with other experiments i the states could be attributed to ¹⁶O+t cluster configurations. In addition, from the measured ratio σ(γ, α)/σ(γ, t) ? 0.01 it is concluded that the formation of ¹⁵N+α cluster states is small in this region of excitation.     

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.