Negative-parity Excited States Research Articles

A continuum calculation of the neutron polarisation in the 3H+p reaction and the resonant states of 4He

The energy dependence of the neutron polarisation P n in the 3H(p, n) 3He reaction is calculated using the 1p−1h coupled-channel model of nuclear reactions proposed earlier by Ramavataram et al. to describe the formation and decay of the negative parity excited states of A = 4. It is shown that the polarisation data provides tangible evidence for the T = 0, 1 − resonance predicted by them. Two positive parity T = 0 resonances proposed experimentally are also confirmed in this calculation. Inclusion of a 1 +, T = 0 resonance around 25 MeV and a 2 +, T = 0 resonance around 29 MeV is necessary to obtain agreement with experiment. It is shown that among these resonances the 1 +, T = 0 resonance can be generated at about the right energy in the coupled-channel calculation but that the 2 +, T = 0 resonance is too high in energy and it may be necessary to include 2p−2h configurations in this case. The proposed level scheme also provides a consistent description of the angular dependence of the differential cross section and polarisation P p in the elastic channel as well as the P n angular distribution.

A microscopic description of the negative parity resonances in the mass-4 system by the coupled-channels method (II)

The coupled-channels method for the treatment of the continuum is used to study the negative parity excited states of the 4He nucleus in the Tamm-Damcoff approximation. It is shown that the splittings within the SU(4) supermultiplet are well reproduced in this calculation when the spin-dependent forces responsible for the removal of the degeneracy are taken appropriately into account. The location of the T = 0, 1 − resonance and the admixture of spurious c.m. excitation in the intrinsic excitation are discussed. In addition to interpreting the observed spectrum, the available experimental data for the particle channels are well described by the theory.

Electron scattering from excited states inN14andBe9

Electron scattering form factors have been measured for the first six excited states in $^{14}\mathrm{N}$ and for the 2.429-MeV (${\frac{5}{2}}^{\ensuremath{-}}$) level in $^{9}\mathrm{Be}$. The form factors for the lowest $T=1$ level in $^{14}\mathrm{N}$ along with the previously measured ground-state magnetic moment are used to specify the $T=0$ and $T=1$ wave functions for the mass-14 system assuming two $p$-shell valence particles in an $\mathrm{LS}$-coupling basis. The amplitudes of the various configurations so derived are generally in poor agreement with previous determinations, although the present wave functions yield values of the lifetime of the first $T=0$ state in $^{14}\mathrm{N}$ and of the $^{14}\mathrm{N}$ ground-state quadrupole moment that are in excellent agreement with previous measurements. The present data are not sufficiently precise to allow a direct separation of the $T=0$ and $T=1$ components for the first four negative-parity excited states. However, for two of these states which are excited primarily by $C1$ transitions, a comparison of the radiative strengths determined in this experiment with previous lifetime measurements sets a lower limit of 2% for the $T=1$ admixture.[NUCLEAR REACTIONS $^{14}\mathrm{N} (e,{e}^{\ensuremath{'}})$; measured form factors for first six excited levels; deduced wave functions for first $T=0$ and $T=1$ levels in mass 14; $^{9}\mathrm{Be} (e,{e}^{\ensuremath{'}})$; measured form factors for 2.429-MeV level.]

Negative-parity excited states of the alpha-particle

This paper presents the results of a study of the alpha-particle structure using a realistic nonlocal potential. The spectrum of the negative-parity states appears in excellent agreement with the results of a recent experimental analysis. A detailed examination of various level splittings, which depend in a simple manner on different types of forces, allows us to draw some interesting conclusions on the choice of alternative sets of potential parameters.

Higher-order corrections to collective particle-hole excitations using green function techniques

Using Green function techniques, we determine higher-order corrections to the Tamm-Dancoff and Random Phase Approximation equations. In particular, we find that the second-order corrections to the Tamm-Dancoff equations consist of a t-matrix term and a core-polarization term. If we truncate our configuration space, we obtain an additional second-order correction from the particle-hole ladder term. We carry out numerical calculations of these second-order corrections for the negative-parity excited states of 4He using the Tabakin separable potential.

Properties of the [15]-DimensionalSU(4)Supermultiplets of Negative-Parity Excitations ofO16Using Realistic Nuclear Forces

Using particle-hole theory in the Tamm-Dancoff approximation and Wigner's $\mathrm{SU}(4)$ supermultiplet theory, we determine the negative-parity excited states of $^{16}\mathrm{O}$ in terms of [15]-dimensional supermultiplets. Only the supermultiplets with total $L=1$ are studied in detail, since they are the ones of importance with regard to the giant dipole resonance and to muon capture and electron scattering. Working in terms of nonmixing supermultiplets, we find that each supermultiplet can be theoretically explained in terms of five parameters, while there are eight pieces of experimental data regarding each supermultiplet. Thus, our theory is overdetermined, and we can eliminate the parameters to obtain empirical sum rules, which can then be used to predict unobserved levels or to check the consistency of our calculation, independent of the radial dependence of the nuclear force. Since there are two $L=1$ supermultiplets, we also determine the mixing between them. Even after mixing, we obtain a sum rule for the $T=0$ states. We then use the realistic nuclear potentials of Tabakin and Brueckner, Gammel and Thaler to calculate the $L=1$ spectrum, so that we can compare it with the experimental spectrum and our sum-rule predictions. Theoretically, we find that some of the parameters are small enough to be neglected with respect to the other parameters and that there is very little mixing between the two $L=1$ supermultiplets. Both of these results allow us to obtain approximate sum rules. We get a total of five sum rules, of which four can be used for making empirical predictions of so-far unobserved experimental quantities. We also calculate the squared matrix elements for muon capture to our theoretically determined $T=1$ levels and obtain results similar to previous calculations.

Some observations concerning the supermultiplet of negative-parity excited states in the α particle

Seven theoretical equations for the supermultiplet of negative-parity excited states of 4He can be expressed in terms of four parameters. Using four empirically determined energy splitting the following quantities can be predicted: the 0- and 1-, T = 0 states lie near 22 and 28 MeV excitation energy respectively; the ratio of gamma-decay widths of the 1-, T = 1 states at 30 and 26 MeV is approximately 0.4. These predictions are compared with experimental evidence.

Nuclear Spectroscopy in theαParticle

We discuss the properties of the 15-dimensional $\mathrm{SU}(4)$ supermultiplet of negative-parity excited states of the $A=4$ system. This multiplet consists of the electric-dipole resonance and its spin-isospin analogs. The position of the "center of gravity" of this supermultiplet is determined by the Wigner and Majorana components of the nucleon-nucleon force, while the splittings within the supermultiplet are due to the spin-dependent parts of the force. We calculate the energies and state vectors of these levels within the framework of the shell model after isolating the center-of-mass motion and assuring ourselves that we are dealing with proper intrinsic excitations of the system. We compute the position of the "center of gravity" of the supermultiplet and the splittings within it,u sing both a Kurath force and a force taken from low-energy nucleon-nucleon scattering. The results are compared with some recent experimental findings concerning these levels.