Octupole states in ¹⁵N and ¹⁶O☆

The reaction ¹⁶O(d, ³He)¹⁵N has been investigated using 29 MeV deuterons, and angular distributions were obtained for levels in ¹⁵N up to 10 MeV excitation energy. The measured distributions were subjected to distorted-wave (DWBA), compound nucleus (Hauser-Feshbach) and coupled-channel (CCBA) analyses. Only the strong transitions to the $\text{1}\text{2}$⁻ ground state and the $\text{3}\text{2}$⁻ state at 6324 keV exhibit distributions which are well described by DWBA. The spectroscopic factors are in agreement with shell-model estimates. The weak transitions generally show little structure and the spectroscopic factors extracted for these transitions tend to be unreasonably large. Contributions from compound nucleus formation were estimated and found to vary between about 10 % and 100 % of the observed cross sections with an average of the order of 30 %. The CCBA analysis for the transitions to the $\text{5}\text{21}{}^{\mathrm{+}}$, $\text{5}\text{22}$⁺ and $\text{7}\text{2}$⁺ states at 5271, 7155 and 7566 keV, respectively, was performed using the spectroscopic amplitudes from weak coupling shell-model wave functions. Inelastic excitations to one-phonon states in the target and residual nuclei were included. The agreement between calculated and experimental distributions is good for both shape and magnitude, a conclusion which is not disturbed by the addition of small compound nucleus contributions. It is evident that spectroscopic factors extracted for the weak transitions on the basis of a direct one-step reaction mechanism alone are unreliable.

We have measured the reactions ¹⁴C(p, d), (d, d′) and (d, p) for E_p = 27 MeV and E_p = 17 MeV. The (d, d′) and (d, p) reactions were studied between θ_lab = 15° and 85°; the (p, d) reaction between θ_lab = 5° and 40°. The ¹⁴C deformation parameters were deduced from the deuteron inelastic scattering and found to agree with deformations measured in nearby doubly even nuclei. The spectroscopic factors deduced from the (p, d) reaction allowed a ¹⁴C ground-state wave function to be deduced which compares faborably with a theoretically deduced wave function. The (p, d) and (d, p) spectroscopic factors are consistent. The implications of our ¹⁴C ground-state wave function regarding the problem of the ¹⁴C hindered beta decay are discussed.

The inelastic electron scattering form factors for the $\text{5.27}\text{MeV}\text{(}\text{5}\text{2}{}^{\mathrm{+}}\text{), 7.15}\text{MeV}\text{(}\text{5}\text{2}{}^{\mathrm{+}}\text{), 7.30}\text{MeV}\text{(}\text{3}\text{2}{}^{\mathrm{+}}\text{)}\text{and}\text{7.56}\text{MeV}\text{(}\text{7}\text{2}{}^{\mathrm{+}}\text{)}$ levels have been measured in the momentum transfer range between 0.6 and 1.1 fm⁻¹. The resulting B(E3) values are discussed in terms of weak coupling models and the spherical shell model of Zuker, Buck and McGrory.

The 2p-3h character of negative-parity states of ¹⁵N was studied using the two-particle transfer ¹³C(³He, p)¹⁵N reaction. Absolute differential cross sections were measured at 5 MeV bombarding energy for all excited states below 8 MeV and for the 9.05, 9.152+9.155 and 9.22 MeV states. According to a two-particle DWBA analysis of these angular distributions, the 9.22 MeV state is assigned negative parity. From γ-γ angular correlation measurements of the 9.22–6.32–0 MeV cascade decay, a preferred spin value of $\text{1}\text{2}$ is found for this state. This is the first excited state in ¹⁵N of the same J^π as the ground state. The Doppler-shift attenuation method was applied to Ge(Li)γ-ray spectra to obtain lifetimes of ≦ 100 fs for states from 7 to 10 MeV excitation in ¹⁵N. Analog states in ¹⁵N and ¹⁵O are identified with corresponding theoretical negative-parity states for the A = 15 system. The DWBA analysis indicates that the 9.22 MeV state has the configuration $\text{(}\text{lp}\text{)}{}^{\mathrm{-3}}\text{(2}\text{s}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{)}{}^2$ and that the 9.152 MeV state is a mixture of the configurations $\text{(}\text{lp}\text{)}{}^{\mathrm{-3}}\text{(2}\text{s}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{)}{}^2$ and $\text{(}\text{lp}\text{)}{}^{\mathrm{-3}}\text{(}\text{ld}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}\text{)}{}^2$. Small but significant admixtures of 2p-3h configurations are required to account for the measured angular distributions for the ground and 6.32 MeV states.

The level structure of ¹⁶O from 20 to 23 MeV has been studied by the measurement of excitation functions for the reactions ¹⁵N(p, p')¹⁵N^∗ in energy increments of 20 keV. An enriched ¹⁵N gas target was used and the 5.271–5.299 MeV doublet of positive-parity states in ¹⁵N was resolved by means of a magnetic spectrometer. Decay to the first excited state at 5.271 MeV was found to be the most strongly resonant channel. Nine states in ¹⁶O, five of them new, have been identified. Their widths have been measured and limits have been placed on some of the spins. Elastic scattering data were also taken over selected energy ranges. Comparisons are made with other reactions over the giant El resonance region of ¹⁶O. Alternative interpretations of the low-lying positive-parity states of ¹⁵N are discussed in light of the data.

A particle-gamma angular correlation coincidence technique enabling the measurement of 0° coincidences was used to study levels in ¹⁵O at 5.24 MeV $\text{(J}{}^{\mathrm{\pi }}\text{=}\text{5}\text{2}{}^{\mathrm{+}}\text{)}$, 6.18 MeV $\text{(J}{}^{\mathrm{\pi }}\text{=}\text{3}\text{2}{}^{\mathrm{-}}\text{)}$, 6.79 MeV $\text{(J}{}^{\mathrm{\pi }}\text{=}\text{3}\text{2}{}^{\mathrm{+}}\text{)}$ and 6.86 MeV $\text{(J}{}^{\mathrm{\pi }}\text{=}\text{5}\text{2}{}^{\mathrm{+}}\text{)}$. The multipole mixing ratios of the de-excitation γ-rays were determined as the following: x(5.24→ 0) = +0.08±0.08 an E3/M2 mixture; x(6.86 → 5.24) = +0.04±0.03, an E2/M1 mixture; x(6.79 → 0) = −0.02±0.02, an M2/E1 mixture and x(6.18 → 0) = −0.121 ±0.008 an E2/M1 mixture. The latter value is to be compared with x((E2/M1) = +0.13 ±0.02 for the mirror 6.323 MeV transition in ¹⁵N indicating $\text{|x(}{}^{15}\text{O}\text{)| ≦ |x(}{}^{15}\text{N}\text{)|}$ contrary to previous findings (10). The results obtained, together with those corresponding to mirror transitions in ¹⁵N are compared with recent IPM and core polarization theoretical investigations.