States Of 16N Research Articles

Complete set of bound negative-parity states in the neutron-rich nucleus N18

High-resolution gamma-ray spectroscopy of 18N is performed with the Advanced GAmma Tracking Array AGATA, following deep-inelastic processes induced by an 18O beam on a 181Ta target. Six states are newly identified, which together with the three known excitations exhaust all negative-parity excited states expected in 18N below the neutron threshold. Spin and parities are proposed for all located states on the basis of decay branchings and comparison with large-scale shell-model calculations performed in the p-sd space, with the YSOX interaction. Of particular interest is the location of the 0^-_1 and 1^-_2 excitations, which provide strong constrains for cross-shell p-sd matrix elements based on realistic interactions, and help to simultaneously reproduce the ground and first-excited states in 16N and 18N, for the first time. Understanding the 18N structure may also have significant impact on neutron-capture cross-section calculations in r-process modeling including light neutron-rich nuclei.

Radii of the bound states in 16N from the asymptotic normalization coefficients**Supported by National Natural Science Foundation of China (11505117, 11490560, 11475264, 11321064, 11375269), Natural Science Foundation of Guangdong Province (2015A030310012), 973 program of China (2013CB834406) National key Research and Development Province (2016YFA0400502)

The asymptotic normalization coefficients (ANCs) of the virtual decay 16N → 15N + n are extracted from the 15N(7Li, 6Li)16N reaction populating the ground and first three excited states in 16N. The root-mean-square (rms) radii of the valence neutron in these four low-lying 16N states are then derived by using the ANCs. The probabilities of the valence neutron staying out of the core potentials are found to be 31% ± 8%, 58% ± 12%, 32% ± 8%, and 60% ± 12%. The present results support the conclusion that a one-neutron halo may be formed in the 16N first and third excited states, while the ground and second excited states do not have a one-neutron halo structure. However, the core excitation effect has a strong influence on the one-neutron halo structure of the ground and first excited states in 16N.

Proton widths of the low-lying16Fstates from the15N(7Li,6Li)16Nreaction

All the 16F levels are unbound by proton emission. To date the four low-lying 16F levels below 1 MeV have been experimentally identified with well established spin-parity values and excitation energies with an accuracy of 4 - 6 keV. However, there are still considerable discrepancies for their level widths. The present work aims to explore these level widths through an independent method. The angular distributions of the 15N(7Li, 6Li)16N reaction leading to the first four states in 16N were measured using a high-precision Q3D magnetic spectrograph. The neutron spectroscopic factors and the asymptotic normalization coefficients for these states in 16N were then derived based on distorted wave Born approximation analysis. The proton widths of the four low-lying resonant states in 16F were obtained according to charge symmetry of strong interaction.

Nuclear pion photoproduction: a theory and the16O(γ,π+)16N(bound) example

A momentum-space expression for the nuclear pion photoproduction amplitude has been obtained and suitable approximations for it have been developed. Such a formulation naturally includes all pion-momentum-dependent terms of the amplitude and allows for an easy simultaneous treatment of several effects such as nucleon Fermi motion, relativistic and gauge invariance of the pion-nucleus photoproduction amplitude and multiple rescattering of pions. They all make important contributions to the photoproduction cross section. The 16O( gamma , pi +) reaction leading to the bound states of 16N is analysed as an example.

Structure of 1+states in16N

Two-nucleon transfer data on 14C and 14N, and beta decay of 16C, to the lowest 1+ states of 16N are used to derive empirical wavefunctions.

Towards a better understanding of 16O nuclear structure: Muon capture and radiative pion capture reactions

Muon capture and radiative pion capture reactions on 16O have been analysed with the aim to extract more information on the importance of higher components of the A = 16 nuclear ground and excited states. Good agreement with the experimental data for both reactions may be achieved if at least 2p2h components of the nuclear wave functions are taken into account. The positive parity excited states in 16N are shown to contribute 43 % to the radiative π − capture but only ≈ 10 % to the muon capture rate.

Effect of Many Particle, Many Hole States in 16O on Positive Pion Photoproduction

The 1p–1h odd parity T = 1 states of 16N (2−, 0−, 3−, 1−) are amended to include the 3p–3h configuration admixtures consistent with the choice of a 16O ground state containing 2p–2h correlations. Estimates indicate that these 3p–3h admixtures result in a nonnegligible increase or decrease of the cross sections depending upon the amount of 2p–2h correlations in the ground state of 16O.

Muon capture rates on 16O leading to bound states of 16N

The partial muon capture rates on 16O leading to all four particle-stable states in 16N have been measured. The results obtained are compared to previous measurements and are also used to extract a value for the induced pseudoscalar coupling constant in muon capture.

A study of photoproduction of charged pions from 16O

Photoproduction of π + from 16O leading to the four bound states in 16N has been studied using different dynamical amplitudes and considering particle-hole correlations in the nuclear wave functions. It is shown that the total cross section is strongly affected by the choice of dynamical amplitudes whereas the effect of 3p - 3h correlations in 16N is small.

Corrélations dans l'Etat Fondamental de l'16O et Absorption Muonucléaire

The influence of 2 particle – 2 hole (2p–2h) configurations of the 16O ground state on the excitation probabilities of odd states in 16N by muon absorption has been studied.

16O analogue states in the 15N(p, n) 15O reaction

The excitation function for the total cross section of the 15N(p, n) 15O reaction has been measured in small energy steps from the threshold at 3.78 MeV to 12 MeV (E x ( 16 O) = 15.7−23.2 MeV) using protons from the tandem Van de Graaff at AERE Harwell. The target thickness was varied from 8–50 keV depending on the width of the structure. An activation technique was used, the positons from the decay of 15O being detected by a plastic scintillator. The absolute total cross section was measured at 5.52, 5.68 and 5.75 MeV by counting in coincidence the annihilation quanta from the positon decay, and these values were used to normalize the relative total cross-section curve. About 18 resonances were observed ranging in width from 20 keV to 650 keV; half of them could be identified in energy and width as T = 1 analogues of known states in 16N. Matching these analogues resulted in an isobaric mass difference 16N- 16O of 12.83±0.02 MeV. A summary of the currently available information on spin-parity values of these T = 1 states is given; rather few states have better than a probable assignment.

T = 2 states in 16N

T = 2 levels have been observed in 16N at excitations of 9.928 and 11.701 MeV in the 14C( 3He, p) 16N reaction. Analysis of the proton angular distributions indicates 0 + for the 9.928 MeV level and 1 − or 2 + for the 11.701 MeV level.

Neutron emission following muon capture in 16O

Using a generalized Goldhaber-Teller model for the nuclear giant dipole resonances, and its extension to giant multipole resonances by the authors, we evaluated the muon capture rates in 16O leading to 0 + and 1 + monopole, 0 − 1 − and 2 − dipole, and 1 + 2 + 3 + quadrupole states in 16N. The subsequent neutron emission was considered on the basis of Wigner's R matrix theory. The neutron spectrum is dominated by the dipole resonances, especially by the 2 − state.

59. The excited States of 16N

59. The excited States of 16N