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.     

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.     

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.     

Alpha-particle and triton cluster states in 19F

We calculate the energies and other properties of three- and four-particle cluster states in the nuclei ¹⁹F and ¹⁹Ne, treated as eigenstat es of a local cluster-core potential. The potential we consider is a symmetrized Saxon-Woods well which has the advantage that a single value of the potential depth can generate rotational spectra of levels with different values of orbital angular momentum L; in this respect, it is very similar to the folding potentials which have been used in previous cluster calculations. We discuss the cluster states in the light of recent three- and four-particle heavy ion transfer reactions and studies of the electromagnetic decay properties of bound and continuum states. Most of the states observed to be strongly populated in the transfer experiments can be unambiguously assigned to cluster bands based on ¹⁶O + t and ¹⁵N + α configurations with various angular momenta and node numbers. Some evidence for mixing between triton and alpha configurations exists and is discussed. We also calculate the electromagnetic properties of the cluster states and find that, with few exceptions, they are in good agreement with the experimental data. Some El transitions are predicted to be very large, contrary to existing experiments, and new experiments are proposed to investigate this discrepancy.     

Fine-structure splittings of excited P-and D-wave states in charmonium

It is shown that, in the relativistic case, the fine-structure splittings of the excited 2³ P _J and 3³ P _J states in charmonium are as large as those of the 1³ P _J state if the same value of α_s(μ)≈0.36 is used. The predicted mass of M(2³ P ₀)=3.84 GeV appears to be 120 MeV lower than the center of gravity of the 2³ P _J multiplet and lies below the D $\bar D$ ^* threshold. Our value of M(2³ P ₀) is nearly 80 MeV lower than that from the Godfrey and Isgur article [Phys. Rev. D 32, 189 (1985)], while the differences in other masses are not greater than 20 MeV.     

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.     

Stark broadening of singly ionized strontium and calcium lines

Profiles of five calcium II and six strontium II lines have been measured in an argon plasma behind the reflected shock wave. The plasma was produced in an electromagnetically drivenT tube and calcium and strontium were present as an impurities. Electron densities were in the range 1.91–3.40·10¹⁷ cm^?3 and the electron temperatures between 10300 and 14200 °K. The half halfwidths of the measured profiles of some Ca II and Sr II multiplets show large discrepancies with theoretical predictions.     

A new level of 18F

A new $\text{T = 0 level of}{}^{18}\text{F, E}{}_{\mathrm{x}}\text{= 4848.3 ± 0.5}\text{keV}$, has been studied using the ⁶Li(¹⁶O, αγ)¹⁸F reaction. The γ-decay by a 65 ± 4 % branch to the 1121 keV (5⁺) level and 35 ± 4 % to the 3791 keV (3^?) level has been observed and a lifetime limit τ > 2 ps has been given. A tentative assignment J^π = 5^? is proposed which is supported by the α-γ angular correlation measurements. The possible structure of the new level and reasons why it had not been observed in past experiments are discussed.     

Exotic Magnetic Rotation in 22F

The rotation structure in exotic neutron-rich nucleus 22F is investigated based on the configuration πd5/2（×）vd5/2s^-1 with the newly developed tilted axis cranking relativistic mean field theory in a fully microscopic and seff- consistent way. The possible existence of magnetic rotation is suggested for ^22F via investigating the spectra, the relation between the rotational frequency and the angular momentum, the electromagnetic transition probabilities B（ M1） and /3（E2） together with the shears mechanism characteristic of magnetic rotation. The effect of the nuclear current is also discussed by comparing the calculation results with and without currents.     

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.     

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) %.     

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.     

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.     

Lifetimes of low-lying levels in 18F

Mean lifetimes of levels in ¹⁸F have been measured using the Doppler-shift attenuation method and the inverse reaction ³He(¹⁶O, p)¹⁸F. Targets of ³He implanted into Al, Nb, and Au foils were employed in the measurements. The Doppler-broadened lineshapes observed at 0° to the beam were analyzed to obtain the following lifetime values: 0.971 ± 0.030, 0.605 ± 0.029 and 0.435 ± 0.041 ps for the 1.70(1⁺), 2.52(2⁺) and 3.36(3⁺)MeV members of the K^π = 1⁺ rotational band, 5.12 ± 0.56, 0.403 ± 0.018 and 1.91 ± 0.17 ps for the 2.10(2^?), 3.13(1^?) and 3.79(3^?) MeV members of the K^π = 0^? bands, and 〈1.2, 2.7^+4.1_?2.7 and 20 ± 2 fs for the 3.06(2⁺, T = 1), 3.72(1⁺) and 3.84(2⁺) MeV states, respectively.     

Some studies of T = 32 states in 17F

Measurements have been made of some parameters of the second and sixth $\text{T =}\text{3}\text{2}$ states in ¹⁷F. For the second state, the resonance energy was found to be E_p = 12.707 ± 0.001 MeV (E_n = 12.550±0.001 MeV), which agrees with and improves on the accuracy of earlier work. For the sixth $\text{T =}\text{3}\text{2}$ state, at E_p = 14.435 MeV, the γ-decay was determined to be predominantly γ₀ with a branch to the first excited state of Γ(γ₁)/Γ(γ₀) ≦ 0.14. Together with other work, this determines J^π to be $\text{3}\text{2}{}^{\mathrm{?}}$. The capture strength is found to be (2J + 1)Γ_pΓ_γ/Γ = 11.4 ± 2.6 eV.     

Characterization of the photochromic centre in CaF2 using 19F ENDOR

Measurements of ¹⁹F ENDOR for the Ce³⁺ unswitched photochromic centre in CaF₂ confirm that the centre comprises a Ce³⁺ ion associated with a nearest neighbour F^- vacancy containing two electrons (an F′ centre).     

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.     

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.