P-shell Nuclei Research Articles

(d,2He) reactions at Ed=125.2 MeV.

We have measured cross sections for (d${,}^{2}$He) induced reactions on the p-shell nuclei $^{6}\mathrm{Li}$, $^{12}\mathrm{C}$, and $^{13}\mathrm{C}$ and the sd-shell nucleus $^{24}\mathrm{Mg}$ at an energy of ${\mathit{E}}_{\mathit{d}}$=125.2 MeV. The measured excitation energy spectra are very similar to those from (p,n), (n,p), and (d${,}^{2}$He) reactions at higher energies. The measured 0\ifmmode^\circ\else\textdegree\fi{} (d${,}^{2}$He) cross sections show a remarkably well-defined linear relation with the Gamow-Teller strengths deduced either from \ensuremath{\beta} decay or from (p,n) reactions. Our results demonstrate that the (d${,}^{2}$He) reaction can be used as a powerful tool to study Gamow-Teller strengths in the ${\mathrm{\ensuremath{\beta}}}^{+}$ direction at energies as low as ${\mathit{E}}_{\mathit{d}}$=125.2 MeV.

(p,n) quasifree excitations in p-shell nuclei at 186 MeV.

Double differential cross sections for (p,n) quasifree scattering on p-shell nuclei, $^{6,7}\mathrm{Li}$, $^{10,11}\mathrm{B}$, and $^{12,13}\mathrm{C}$, have been measured at an incident proton energy of 186 MeV between ${\mathrm{\ensuremath{\theta}}}_{\mathrm{lab}}$=\ensuremath{\theta}\ifmmode^\circ\else\textdegree\fi{} and ${\mathrm{\ensuremath{\theta}}}_{\mathrm{lab}}$=50\ifmmode^\circ\else\textdegree\fi{} in steps of 5\ifmmode^\circ\else\textdegree\fi{}. The normal polarization transfer coefficient, analyzing power, and induced polarization were also measured at ${\mathrm{\ensuremath{\theta}}}_{\mathrm{lab}}$=0\ifmmode^\circ\else\textdegree\fi{}, 15\ifmmode^\circ\else\textdegree\fi{}, and 20\ifmmode^\circ\else\textdegree\fi{} using an incident proton beam of transverse polarization. Calculations by some competing quasifree models are presented and compared with the experimental results.

Microscopic derivation of local interactions between p-shell nuclei

Microscopic phase shifts are calculated in a single-channel two-centre generator-coordinate method for the elastic scattering of the 13C + 12C, 15N + 12C, 16O + 15C and 16O + 15N systems. For each partial wave, equivalent local potentials are derived with an accurate inversion technique. The properties of the parity-dependent and spin-orbit parts of these nucleus-nucleus potentials are analyzed and compared. The parity effect is stronger for 13C + 12C and 16O + 15N and its sign follows a rule deduced from a simplified microscopic model. The spin-orbit term is larger for even l than for odd l. In all cases, it is positive for even l at large distances.

Charge-exchange spin-dipole and quasifree excitations in p-shell nuclei

We have measured (p,n) differential cross sections and ( p, n) spin observables at E p = 186 MeV on p-shell nuclei, 6,7Li, 10,11B and 12,13C, using the neutron time-of-flight facilities at the Indiana University Cyclotron Facility. Cross section data were measured between θ lab = 0° and θ lab = 50° in 5° steps. Spin observables D NN, A y and P were measured at θ lab = 15° and 20°. Data for low-lying states and giant resonances in the nuclear sector and excitation of the quasifree have been obtained. Using a multipole decomposition analysis, results for the giant spin-dipole resonances are presented and compared with distorted-wave impulse approximation calculations. Contributions of the quasifree excitation in the giant resonance region are estimated. An analysis of the quasifree cross section is presented. Results of the location of the quasifree peak as a function of momentum transfer differ from similar (p,n) results obtained at E p = 495 MeV.

Measurements of quasi-elastic scattering and DNN(0°) with ( p, n) reactions

The quasi-elastic scattering (QES) process has been studied for the ( p, n) reaction at 300 and 400 MeV. Energy spectra for targets ranging from 2H to nat Pb have been obtained for a wide range of scattering angles corresponding to momentum transfers q = 1.0 – 2.4 fm −1. Energy shift of the QES peak from that of 2H to higher excitation energy is observed for all targets studied, with a gradual increase with target mass reaching almost a constant value of ∼26 MeV beyond 12C. The mass dependence oo the effective neutron number ( N eff ) for the ( p, n) QFS process is well represented by N eff = 0.85 N 0.5 over a wide mass range from 2H to nat Pb except for 6Li, 7Li and 9Be which deviate significantly. The analyzing powers of the QES peak region for the 12C and 40Ca targets are compared to those of the 2H( p, n) reaction at 300 and 400 MeV and found to be almost identical for q ≤ 1.5 fm −1. Transverse polarization transfer coefficients at 0 degree D NN (0°) have been measured for the 1p shell nuclei at 300 MeV. Large negative D NN (0°) values are observed for the continuum region up to an excitation energy of 50 MeV irrespective of targets. The D NN (0°) values observed for the GT transitions are around −0.28 which indicates the contribution from the tensor interactions. The DWIA calculations well reproduce the observed D NN (0°) values.

Nuclear structure of 10B studied with (e,e′), ( π, π′) and ( γ, π) reactions

We studied nuclear structure of 10B using ( e, e′), ( π, π′) and ( γ, π) reactions under the distorted wave impulse approximation(DWIA). For this purpose the off-shell dependence of the amplitude was taken into account in the momentum space DWIA. We used the off-shell elementary amplitude of ( π, π) and ( γ, π) by the model of Nozawa, Blankleider and Lee[1]. The first order core polarization effects were incorporated with the p-shell wave functions of Cohen and Kurath and Hauge and Maripuu. It was shown that the core-polarization effects greatly improve the agreement with the experimental data and that the simultaneous study of these reactions is useful to probe the spin structure of p-shell nuclei.

Description of light nuclei with realistic interactions.

In this work correlated basis functions theory is applied at the lowest order to analyze the ground state and low energy spectrum of light (p-shell) nuclei. The study is an extension of a previous work devoted to the $^{16}\mathrm{O}$ nucleus and includes state-dependent correlations, whose radial components are determined by solving a set of Euler-Lagrange equations. The matrix elements are computed by using a cluster expansion. Results of calculations performed using ${\mathit{v}}_{14}$ nucleon-nucleon potentials are presented and compared with the results obtained for ${\mathit{v}}_{8}$ potentials.

Spin-current contributions to M2 excitations of 1p-shell nuclei

Microscopic analysis of spin-current contributions to magnetic quadrupole resonances in (e, e′) cross sections has been performed using the particle-core coupling version of the shell model for 1p-shell nuclei. The contributions from spin-dipole and spin-octupole modes to M2 excitations of 1p-shell nuclei are traced up toq≈2.5 fm−1.

Magnetic electron scattering from p-shell nuclei

In this paper we report on shell-model calculations of transverse electron scattering form factors of states in several selected p-shell nuclei. Results will be presented for calculations in the p-shell model space and in the extended (0 + 2) h ̷ model space. The influence of meson-exchange currents on the calculated form factors will be examined. We find that in the beginning of the p-shell the description of the transverse form factors is improved by extending the model space. However, towards the end of the p-shell the situation deteriorates, indicating deficiencies in the wave functions.

Stretched proton-neutron configurations in fp-shell nuclei: (II). Systematics

The systematics of the binding energies of stretched proton-neutron configurations ( f 7 2 , g 9 2 ) 8 − , ( p 3 2 , g 9 2 ) 6 − , ( g 9 2 , p 3 2 ) 6− and ( g 9 2 ) 2 9 + are studied over a wide range of f p-shell nuclei. The effective proton-neutron interaction energies deduced from the data are nearly constant for ( p 3 2 , g 9 2 ) 6 − and ( g 9 2 ) 2 9 + states while the ( f 7 2 , g 9 2 ) 8 − configuration reveals an additional repulsive term proportional to the partial filling of the f 7 2 orbit in the target ground state. Two-body matrix elements are extracted. A crude shell model, which predicts that the excitation energy of a stretched state is equal to the sum of the single-particle energies, works well for the 6 − and 9 + states, but fails for the 8 − levels due to neglect of the additional interactions described above. The physics underlying the empirically introduced basic assumptions of the crude shell model is discussed. The binding energies are found to be linearly dependent on the mass number A and the isospin T z component and are well described by the weak-coupling model of Bansal and French. The derived parameters agree with averaged values of a similar analysis for the single-particle states in the corresponding odd-even neighbours. The data indicate a significant change of the particle-hole energies with closure of the proton f 7 2 shell.

Nuclear structure studies with intermediate energy probes

Nuclear structure studies with pions are reviewed. Results from a recent study of 1 p-shell nuclei using (e,e′), (π,π′) and (γ,π) reactions are reported. Future nuclear structure studies with GeV electrons at CEBAF are also briefly discussed.

Clustering of 1p-shell nuclei in the framework of the shell model

The two- and three-fragment clustering of the 1p-shell nuclei has been studied in the framework of the intermediate-coupling shell model. The absolute probabilities of different types of two- and three-fragment clustering in a given nucleus have been obtained by projecting its intermediate-coupling shell-model wavefunction onto the subspace of the orthonormal, completely antisymmetric two- and three-cluster states, respectively. With the aid of these data the selectivity in population of states of nuclei produced in multinucleon transfer reactions by transfer of a single multinucleon cluster or by the simultaneous tranfer of two lighter clusters has been analysed. The final states population predicted by means of the clustering probability distribution has been compared with the population predicted by the strength distribution, i.e. the sum of squared spectroscopic amplitudes, among the final states.

Reply to "Comment on 'p-shell nuclei in a (0+2)

In the preceding Comment, our shell-model calculations on p-shell nuclei are heavily criticized. In this Reply we argue that our calculations are relevant.Received 29 May 1991DOI:https://doi.org/10.1103/PhysRevC.45.477©1992 American Physical Society

Comment on "p-shell nuclei in a (0+2)

We point out that the shell-model calculations of Wolters, van Hees, and Glaudemans for the p-shell nuclei in a (0+2)\ensuremath{\Elzxh}\ensuremath{\omega} basis has several serious consistency problems, and we use the case of $^{16}\mathrm{O}$ to illustrate these problems. We discuss some limitations of the parametrization used for effective interaction, criticize the choice of experimental data used in the least-squares fit, and demonstrate that the resultant interaction exhibits some peculiar features which make it differ considerably from realistic interactions.

Charged Pion Photoproduction on the p-Shell Nuclei at Low Energy and Rescattering Effect of a Pion

0. Our results show that the rescattering processes give large effect on the negative pion photoproduc­ tions on 13C and l5N, while they give small effect on the positive pion photoproductions on l2C. This feature strongly depends upon the nuclear structure. ) or Blomqvist-Laget,ZZ) and pion wave function is described by the optical potential of the Kisslinger type Z3 ) or its improved ones. Z4 )-Z7) The sophisti­ cated DWIA calculation in the momentum space was also performed by several groups.Z8)-33) As results, most of the theoretical works succeeded in reproducing the experimental data of spin-flip type on the positive pion production, where the effects of the nuclear core polarization and the meson-exchange current were also investigat­ ed in connection with the magnetic dipole form factors in the electron scattering. 34 ) On the other hand, DWIA cannot explain the experimental data of the negative pion production such as 13C(y, 7(-) 13 N and 15N(y, 7(-) 15 0. These reactions take place between the spin and isospin doublet. The nuclear transition matrix element for these reactions consists of the EO and M1 types, both of which equally contribute to the reactions. By comparing the experimental data with DWIA results, Shoda et al. lO )

Charged Pion Photoproduction on the p-Shell Nuclei at Low Energy and Rescattering Effect of a Pion

We study the pion rescattering effect on the charged pion photoproduction reaction at low energy, We investigate two types of the pion rescattering processes, i.e., the charge-exchange rescattering and the spin-flip rescattering. By taking into account the rescattering effect on the DWIA amplitude, we calculate the differential cross sections of reactions 12C(γ, π+)12B, 13C(γ, π-)13N and 15N(γ, π-)15O. Our results show that the rescattering processes give large effect on the negative pion photoproductions on 13C and 15N, while they give small effect on the positive pion photoproductions on 12C. This feature strongly depends upon the nuclear structure.

A test for the Cohen-Kurath wave functions in (p, d)-reactions

The experimental data for (p, d)-reactions on the 1p-shell nuclei atEp −33 MeV are used to test the Cohen and Kurath wave functions and the intermediate coupling. The experimental excitation energies and their order for all law transitions of the 1p-shell nuclei are compared with the theoretical predictions of Cohen and Kurath. The integrated cross-sections σ(0–90‡) for these transitions are also compared with their theoretical predictions for one-neutron pick-up. The good agreement between these data indicates a success for the Cohen-Kurath wave functions and for their model of calculations.

Spectroscopic amplitudes for two-cluster transfer between 1p-shell nuclei

Spectroscopic amplitudes for separation of two clusters from 1p-shell nuclei were calculated. The calculations were performed with the aid of intermediate-coupling shell model wavefunctions. The population of final states produced by simultaneous transfer of two clusters from 1p-shell nuclei was discussed. The localization of relative clusters in 1p-shell nuclei was considered.

P-shell nuclei in a (0+2)

An empirical effective interaction for the {ital A}=4--16 {ital p}-shell nuclei in the complete (0+2){h bar}{omega} model space is presented, which also reproduces the predominant 2{h bar}{omega} intruder states. The results are not spoiled by the mixing catastrophe. The inclusion of 2{h bar}{omega} states also leads to a new feature not encountered before in smaller model spaces, i.e., the appearance of radially excited states. The latter can be observed experimentally in certain cases.

P-shell nuclei in a (0+2)

Several observables for the A=4--16 p-shell nuclei, resulting from a calculation in the complete (0+2)\ensuremath{\Elzxh}\ensuremath{\omega} model space, are presented. It is shown that the 2\ensuremath{\Elzxh}\ensuremath{\omega} admixtures are substantial. The calculated values of these observables are compared with the experimental data. Not only can the properties of many intruder states be reproduced correctly, but also the description of predominantly 0\ensuremath{\Elzxh}\ensuremath{\omega} states is often improved considerably.