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.     

The reaction19F(n,α)16N atE n =13.9 MeV and 15.6 MeV

Differential cross sections of the reaction¹⁹F(n, α)¹⁶N have been measured atE _n = 13.9 MeVd and 15.6 MeV. The results can be interpreted in terms of compound nucleus theory. Statistical model calculations were undertaken to clear up the different reaction behaviour of the reactions¹⁹F(n, α)¹⁶N and¹⁹F(p, α)¹⁶O at bombarding energies near 14 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.     

Resonance strengths of some light nuclei

The strengths of theE _c.m.=324 and 829 keV resonances in¹⁹F(ρ, αγ)¹⁶O have been determined relative to the Rutherford elastic scattering cross section. With these values as standards, the strengths of theE _c.m.=259, {pp143} (also 1,106) and 1,259 keV resonances in¹⁴N(ρ, γ)¹⁵ O,¹⁸O(ρp, γy)¹⁹F and¹⁴N(α, γ)¹⁸F were measured, respectively. In addition, the cross section of the direct capture process in¹⁶O(ρ, γ)¹⁷F atE _c.m.=853 keV has been obtained. All measurements were carried out by interchanging the role of projectiles and target nuclei, i.e.¹⁴N,¹⁶O,¹⁸O and¹⁹F ion beams of appropriate energies were guided into windowless gas target systems filled with hydrogen or helium gas. The results are compared with previous work.     

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.     

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.     

The parity of the 3134 keV,J=1 State in18F

The¹⁶O(³He,?γ)¹⁸F reaction atE(³He)=2.95 MeV was used to study the low-lying states in¹⁸F belowE _x =3.2 MeV. A combination of γ-ray angular distribution and linear polarization measurements using a single planar Ge(Li) polarimeter resulted in aJ ^π =1^? assignment to the 3134 keV state. In addition, the positive parity assignments to the 937(3⁺) and 1702(1⁺) keV states have been verified. Information on branching and mixing ratios as well as on lifetimes are reported. The results are discussed in terms of the weak-coupling and strong-coupling models.     

Direct α particle emission in incomplete fusion reaction on a spherical target124Sn

Direct α particles cross sections and angular distributions have been measured when bombarding a¹²⁴Sn target with various projectiles:¹⁶O (90, 105, 125 MeV)¹⁹F (104 MeV)²⁰Ne (112, 156 MeV). Results have been found independant of the projectile structure. In the same reactions, gamma multiplicity measurements have shown that direct α particles are emitted in collisions with intermediate impact parameters. The angular momentum is found to be about half of the valuel _cr for complete fusion.     

On the reaction mechanism of the two-nucleon transfer reaction 16O(6Li, α)18F

The two-nucleon transfer reaction ¹⁶(⁶Li, α)¹⁸F at E_L = 34 MeV is analyzed on the basis of the one-step (simultaneous) and two-step (successive) transfer processes. Exact finite-range calculations have been performed for both processes. The ground and first-excited states of ¹⁷F and ¹⁷O are taken into account in the two-step calculations. It is found that the calculated cross section due to the two-step process is comparable to that of the one-step process and is responsible for the backward rise of the angular distribution.     

Selectivity of the 16O(7Li, α) reaction and states with (sd)3 and (sd)2(fp)1 configurations in 19F

The ¹⁶O(⁷Li, α)¹⁹F reaction has been studied at 35 MeV up to 11 MeV excitation energy. Comparison with the ¹⁶O(⁶Li, ³He)¹⁹F and ¹⁶O(⁶Li, t)¹⁹Ne reactions shows very good agreement up to 5.5 MeV excitation energy and large differences at higher excitation energies which can be explained by different angular momentum matching conditions in the different reactions. The identification of mirror states in A = 19 and of states with (sd)³ and (sd)² (fp)¹ configurations in ¹⁹F are discussed.     

Infrared-microwave two-photon spectra of the ν3 bands of 12CH3F and 13CH3F

Infrared-microwave two-photon spectra have been obtained for the ν₃ bands of ¹²CH₃F and ¹³CH₃F with a two-photon spectrometer employing a CO₂ laser and a computer-coupled microwave source operating in the 8–18 GHz region. Even though the intensities of the spectra for the double parity levels in these molecules are inversely proportional to the square of the microwave frequency, transitions have been observed with microwave frequencies of up to 16 GHz. Comparison of these observed two-photon frequencies to frequencies predicted from infrared laser Stark spectroscopy, and to frequencies calculated from vibration-rotation parameters obtained by fitting these and other frequencies, shows agreement to within a few MHz. Spectroscopic parameters for the ground and ν₃ excited states of the two species are reported.     

9Be + 12C, 9Be + 16O,9Be + 19F reactions at energies below the barrier

The ¹⁶O + ⁹Be reactions have been studied from E_c.m. = 2.0 MeV to 5.1 MeV, an energy near the top of the Coulomb barrier. The cross section for the neutron transfer reaction ⁹Be(¹⁶O,¹⁷O¹ (0.87 MeV))⁸Be has been measured over this range by detecting the prompt 0.87 MeV γ-rays. The total fusion cross section has been determined from E_c.m. = 2.8 to 5.1 MeV by observing individual γ-ray transitions in the evaporation residues with a Ge(Li) detector, and then summing the separate yields. Direct processes are found to dominate the reaction yield below E_c.m. = 4 MeV. A comparison of the energy dependence of the fusion cross section for this reaction and the ¹²C + ¹³C reaction, which proceeds via the formation of the same compound nucleus, ²⁵Mg, reveals differences at sub-barrier energies. Optical model and incoming-wave boundary condition calculations are presented. Data have also been obtained for the near optimum Q-value neutron-transfer reactions ⁹Be(¹²C, ¹³C¹)⁸Be and ⁹Be(¹⁹F, ²⁰F)⁸Be, and these are discussed in terms of a simple model of sub-barrier direct reactions.     

The 16O + p → 17F (T = 32) resonances at Ep = 11.26,12.71 and 14.57 MeV

Excitation functions for ¹⁶O+p reactions have been measured with high energy resolution in the region of the first, second and seventh $\text{T =}\text{3}\text{2}$ resonances in ¹⁷F at extreme backward angles. The observed resonance shapes have been analyzed with a single-level resonance formula taking the off-resonance spin-flip amplitude into account. The resonance parameters of the ¹⁷F first $\text{T =}\text{3}\text{2}$ state studied with special emphasis are E_x = 11193.3 ± 2.3 keV, Γ = 200 ± 40 eV and Γ_p0 = 19 ± 3 eV. This result and other results are compared with previous studies and theoretical predictions. The comparison with data of the mirror nucleus ¹⁷O is discussed with respect to the observed charge asymmetry of the isospin-forbidden particle decay widths.     

High-precision laser and RF spectroscopy of the spin-rotation and HFS interactions in the X2Σ+ and B2Σ+ states of LaO

The spin-rotation and hyperfine interactions in the X²Σ⁺ and B²Σ⁺ electronic states of ¹³⁹La¹⁶O have been studied using Doppler-free laser-induced fluorescence and molecular-beam laser-rf double resonance. Observations were made for several values of v and many values of N, allowing evaluation of the principal interaction strengths and their N and v dependences for both the X and B states. The results are compared with earlier results for the isoelectronic system ¹³⁷Ba¹⁹F.     

Rotation-vibration spectrum of CH3F in the range 2000–2100 cm−1

The spectrum of CH₃F between 2000 and 2100 cm^?1 has been investigated under high resolution (0.025 cm^?1). Three parallel bands have been analyzed: 2ν₃ of ¹²CH₃F for which the rotational K structure has been studied, 3ν₃-ν₃ of ¹²CH₃F, and 2ν₃ of ¹³CH₃F. The band center of the main band 2ν₃ of ¹²CH₃F has been found at 2081.383 cm^?1.     

The decay of78Rb and the level scheme of78Kr

The level scheme of⁷⁸Kr has been studied with the techniques of γ ray spectroscopy: decay spectra of⁷⁸Rb produced by the reactions⁶⁴Zn(¹⁶O,pn) and⁶⁵Cu(¹⁶O,3n) have been measured; in-beam γγ coincidences have been taken during the irradiation of⁶⁴Zn with¹⁶O ions; and an angular distribution measurement of γ rays from the reaction⁶⁴Ni(¹⁶O, 2n)⁷⁸Kr has been performed. Fifteen excited states in⁷⁸Kr have been observed and spins have been proposed for most of these levels. A⁷⁸Rb decay with a half-life of 17.5 ± 2 min has been found in addition to the previously known 6.0 ±0.2 min decay.     

Thermal neutron scattering parameters for light nuclei (A=12 to 27)

Neutron scattering lengths and free scattering cross sections were measured for isolated isotopes and natural elements in the mass region fromA=12 toA=27. By means of the Christiansen filter technique values for the coherent scattering lengths were obtained for the bound atoms of^12C,^13C,^14N,^15N,^16O,^17O,^18O, Mg and¹⁹F. Transmission experiments with neutrons of 0.51 meV, 1.26 eV and 5.19 eV yielded data for the free scattering cross sections of^19F,²³Na, Mg and¹⁹F. From all these results and with previous data the fundamental spin state scattering lengths for the concerned light nuclei have been deduced. The comparison of these values with resonance parameters has shown that the resonance data are complete. Therefore, values for the potential scattering radii could be obtained and discussed.     

Weak bands in the far-infrared spectrum of HCNO

Several weak transitions in the far-infrared spectrum of H¹²C¹⁴N¹⁶O between the (0000_v) levels with v = 2, 3, and 4 have been analyzed. Vibrational levels of improved accuracy have been calculated for both the (0000_v) series (v ≦ 5) and the (0001_v) series (v ≦ 3). For the latter, some inconsistencies in previous assignments have been removed. Two transitions from the (00020)² level have been assigned tentatively. The two lowest bands of the (0000_v) series of H¹³C¹⁴N¹⁶O have been found. Two new types of graphical plots have been used in the assignment.     

High spin states in120Xe

High spin states in ¹²⁰Xe, populated via¹⁰⁷Ag(¹⁶O, p2n) and¹⁰⁸Pd(¹⁶O, 4n)reactions have been studied using γ-ray spectroscopic techniques. 21 new transitions have been added to extend the level scheme. Two S-bands are extended upto 26⁺ and 22⁺;these bands are found to cross the g. s band at hΩ=0. 39 and 0. 47 MeV. The band structures have been discussed within the framework of CSM.     

A comparative spectroscopic investigation of21Ne and21F by pickup reactions

The reactions²²Ne(d, t)²¹Ne and²²Ne(d, τ)²¹F have been investigated in a parallel experiment atE _d=52MeV. Energy spectra of tritons andτ particles have been measured up to excitation energies of 18MeV in both²¹Ne and²¹F nuclei. Thel values could be determined, and spectroscopic factors have been obtained by a DWBA analysis of the measured angular distributions. Together with the results from a preceding \({}^{22}Ne(\vec d, \tau ){}^{21}F\) experiment, several spins could be determined in²¹F. From a comparison of the triton andτ particle spectra,T=3/2 states in²¹Ne could be identified and on this basis the new information on spins of²¹F states could be transferred to their analog states in²¹Ne. The completeT _< andT _> parts of the (2s, 1d) strengths could be observed. The 1p strength is highly fragmented. Of theT _< part only 60% of the 1p1/2 and 40% of the lp3/2 strength could be found, but the totalT _> part of the 1p strength could be localized. The spectroscopic results for positive parity states are in qualitatively good agreement with shell model predictions.