Sd Shell Research Articles

MONOPOLE SHIFT IN ODD NEUTRON-RICH F ISOTOPES: A SHELL MODEL DESCRIPTION

Monopole shift in the proton states in odd neutron-rich F isotopes has been studied with neutron number changing from N = 10 to 20 for three different interactions. The variation of monopole shift point out towards appearance of shell closure at N = 14 and N = 16 and its disappearance at N = 20. Low lying states of odd F isotopes have also been calculated in the limited configuration space of sd shells for these interactions.

Charge density distributions for odd-A of 2s-1d shell nuclei

An analytical expression for the charge density distributions is derived based on the use of occupation numbers of the states and the single particle wave functions of the harmonic oscillator potential with size parameters chosen to reproduce the observed root mean square charge radii for all considered nuclei. The derived expression, which is applicable throughout the whole region of shell nuclei, has been employed in the calculations concerning the charge density distributions for odd- of shell nuclei, such as and nuclei. It is found that introducing an additional parameters, namely and which reflect the difference of the occupation numbers of the states from the prediction of the simple shell model leads to obtain a remarkable agreement between the calculated and experimental results of the charge density distributions throughout the whole range of

Charge density distributions for odd-A of 2s-1d shell nuclei

An analytical expression for the charge density distributions is derived based on the use of occupation numbers of the states and the single particle wave functions of the harmonic oscillator potential with size parameters chosen to reproduce the observed root mean square charge radii for all considered nuclei. The derived expression, which is applicable throughout the whole region of shell nuclei, has been employed in the calculations concerning the charge density distributions for odd- of shell nuclei, such as and nuclei. It is found that introducing an additional parameters, namely and which reflect the difference of the occupation numbers of the states from the prediction of the simple shell model leads to obtain a remarkable agreement between the calculated and experimental results of the charge density distributions throughout the whole range of

Approaching the “island of inversion”:P34

Yrast states in {sup 34}P were investigated using the {sup 18}O({sup 18}O,pn) reaction at energies of 20, 24, 25, 30, and 44 MeV at Florida State University and at Lawrence Berkeley National Laboratory. The level scheme was expanded, {gamma}-ray angular distributions were measured, and lifetimes were inferred with the Doppler-shift attenuation method by detecting decay protons in coincidence with one or more {gamma} rays. The results provide a clearer picture of the evolution of structure approaching the ''island of inversion,'' particularly how the one- and two-particle-hole (ph) states fall in energy with increasing neutron number approaching inversion. However, the agreement of the lowest few states with pure sd shell model predictions shows that the level scheme of {sup 34}P is not itself inverted. Rather, the accumulated evidence indicates that the 1-ph states start at 2.3 MeV. A good candidate for the lowest 2-ph state lies at 6236 keV, just below the neutron separation energy of 6291 keV. Shell model calculations made using a small modification of the WBP interaction reproduce the negative-parity, 1-ph states rather well.

Observation of narrow states in nuclei beyond the proton drip line:F15andNe16

Two high-lying states in F15 and Ne16, unbound with respect to one-proton (1p) and two-proton (2p) emissions, have been observed in the fragmentation of Ne17 at intermediate energies. They undergo mainly sequential emissions of protons via intermediate states in O14 and F15 and have decay energies of 7.8(2) and 7.6(2) MeV, respectively. The widths of the newly observed states in F15 and Ne16 are much smaller than the Wigner limits for single-particle configurations, of 0.4(4) and 0.8(+8−4) MeV, respectively. In addition, narrow widths of 0.2(2) MeV are derived for two other high-lying states in F15 with Qp of 4.9 and 6.4 MeV, which match features of the recently predicted narrow odd-parity F15 states with two valence protons in the sd shell. All energies and widths have been obtained by analyzing angular correlations of the decay products, p−p−O14 and p−p−N13, whose trajectories have been measured by a tracking technique with silicon microstrip detectors.Received 15 April 2009DOI:https://doi.org/10.1103/PhysRevC.79.061301©2009 American Physical Society

Angular momentum projected configuration interaction with realistic Hamiltonians

The projected configuration interaction method starts from a collection of mean-field wave functions and builds up correlated wave functions of good symmetry. It relies on the generator coordinator method techniques, but it improves the past approaches by a very efficient method of selecting the basis states. We use the same realistic Hamiltonians and model spaces as the configuration interaction (CI) method and compare the results with the full CI calculations in the sd and pf shell. Examples of {sup 24}Mg, {sup 28}Si, {sup 48}Cr, {sup 52}Fe, and {sup 56}Ni are discussed.

Orbital twist M2 mode in the 1ħω transitions from 1p-, sd-, and f2p-shell nuclei

The contributions of the orbital twist mode to the matrix elements and form factors of all 1ħω single-particle M2 transitions are analyzed for 1p, sd and f2p shell nuclei.

PROTON RESONANCE PROPERTIES IN LIGHT NUCLEI WITH MEAN-FIELD TYPE POTENTIALS

The properties of proton resonances in the p and sd shells are studied with a single-particle potential generated by self-consistent-mean field calculations. Besides, we also propose to calculate the proton-nucleus interaction by a double-folding procedure with the density of the daughter nucleus represented by the result of the same mean-field model. Without raising any adjustable parameter, these calculations provide reasonable estimates for the widths of observed proton resonances in light nuclei. The mean-field-based calculation is particularly useful for the predications of the properties of resonances in light exotic nuclei and the direct proton capture reactions of astrophysics.

Level density and shape changes in excited sd shell nuclei

In the present calculation we have used the Monte Carlo method of generating collective spin and total energy of the nucleus for various configurations of the system with N0 single particle states available for n number of particles. The different configurations (arrangements of occupied single particle states) leading to a particular energy E and spin J are then collected to get the density of states for the given energy E and spin J. We find that if we use the cranked Nilsson model single particle states for the rotational frequency Ω = 0.0ħω, 0.05ħω and 0.1ħω there is a shift in the maximum density of states Wmax with a tendency for the system to become more oblate or prolate depending on the shift in the maximum density of states as the angular momentum decreases or increases. The change in nuclear level density with collectivity, i.e. with the use of cranked Nilsson model single particle levels has been noticed.

Shell effects for isovector l-forbidden M1 transitions in odd nuclei

It is inferred from the analysis of the experimental data that the reduced probabilities of isovector l-forbidden M1 transitions achieve maximum values in nuclei with a magic core. The minimum values are related to γ transitions in nuclei belonging to the region of stable deformation with A ≈ 25. Isovector l-forbidden M1 transitions in mirror nuclei are explored. The ratio of the reduced probabilities for proton and neutron transitions shows the maximum deviation from the theoretical evaluations for the pair of nuclei 41Ca-41Sc consisting of the even-even magic core 40Ca and one nucleon outside the closed 1d2s shell.

Determination of theN=16Shell Closure at the Oxygen Drip Line

The neutron unbound ground state of (25)O (Z=8, N=17) was observed for the first time in a proton knockout reaction from a (26)F beam. A single resonance was found in the invariant mass spectrum corresponding to a neutron decay energy of 770_+20(-10) keV with a total width of 172(30) keV. The N=16 shell gap was established to be 4.86(13) MeV by the energy difference between the nu1s(1/2) and nu0d(3/2) orbitals. The neutron separation energies for (25)O agree with the calculations of the universal sd shell model interaction. This interaction incorrectly predicts an (26)O ground state that is bound to two-neutron decay by 1 MeV, leading to a discrepancy between the theoretical calculations and experiment as to the particle stability of (26)O. The observed decay width was found to be on the order of a factor of 2 larger than the calculated single-particle width using a Woods-Saxon potential.

Study of N=20 shell gap with 1H(28Ne,27,28Ne) reactions

This paper reports on the 1H(28Ne,28Ne) and 1H(28Ne,27Ne) reactions studied at intermediate energy using a liquid hydrogen target. From the cross section populating the first 2+ excited state of 28Ne, and using the previously determined B(E2) value, the neutron quadrupole transition matrix element has been calculated to be Mn=13.8 ±3.7 fm2. In the neutron knock-out reaction, two low-lying excited states were populated in 27Ne. Only one of them can be interpreted by the sd shell model while the additional state may intrude from the fp shell. These experimental observations are consistent with the presence of fp shell configurations at low excitation energy in 27,28Ne nuclei caused by a vanishing N=20 shell gap at Z=10.

Calculations of elastic and inelastic electron scattering on 19F using large-basis no core-shell model wave functions

Large basis no core shell model calculations are performed to study the elastic and inelastic electron scattering on 19F. All major shells s, p, sd and pf are considered with ( 0 + 2 ) ℏ ω truncations. Excitations out of major shell space are taken into account through a microscopic theory which allows particle–hole excitations from the sd and pf shell orbits to all higher orbits with 2 ℏ ω excitations. Calculations are presented for the transitions from J π T = 1 / 2 + 1 / 2 to J π T = 1 / 2 + 1 / 2 , 3 / 2 + 1 / 2 , 5 / 2 + 1 / 2 , 7 / 2 + 1 / 2 and 9 / 2 + 1 / 2 in 19F. Excitations out the no core shell model space are essential in obtaining a reasonable description of the longitudinal and transverse electron scattering form factors.

The competition between [formula omitted] and pair coupling in the many-fermion sd shell and interacting boson models

We consider the competition between SU ( 3 ) and pair coupling in the 2 s 1 d nuclear shell model. By working within the framework of the LST-coupling shell model with good supermultiplet symmetry, the problem appears as a competition between two dynamical subgroup chains U ( 6 ) ⊃ SU ( 3 ) ⊃ SO ( 3 ) and U ( 6 ) ⊃ O ( 6 ) ⊃ S O ( 3 ) which has also been studied in the context of the interacting boson model.

Description of superdeformed bands in lightN=Znuclei using the cranked Hartree-Fock-Bogoliubov method

Superdeformed states in light N=Z nuclei are studied by means of the self-consistent cranking calculation (i.e., the P+QQ model based on the cranked Hartree-Fock-Bogoliubov method). Analyses are given for two typical cases of superdeformed bands in the A similar or equal to 40 mass region, that is, bands where backbending is absent (Ca-40) or present (Ar-36). Investigations are carried out particularly for the following points: cross-shell excitations in the sd and pf shells; the role of the g(9/2) and d(5/2) orbitals; the effect of nuclear pairing; and the interplay between triaxiality and band termination.

Electromagnetic transitions in neutron-richF22

Electromagnetic transitions in {sup 22}F were investigated using the {sup 9}Be({sup 14}C, p) reaction at E{sub lab}=22 MeV. Proton-{gamma} and proton-{gamma}-{gamma} coincidences were measured using a segmented E-{delta}E Si telescope and the Florida State University {gamma} detector array. {gamma}-ray energies, intensities, and branching ratios were measured, and mean lifetimes were inferred using the Doppler-shift attenuation method. Several new states and many new {gamma}-decay branches were observed. The results are compared with sd shell model calculations using the USD, USDA, and USDB interactions. The excitation energies of the states below 2 MeV agree slightly better with the calculations using the USD interaction, while the levels between 2 and 4 MeV agree somewhat better with those using the newer USDA and USDB interactions.

Systematic studies on the exotic properties of isotopes from oxygen to calcium

Systematic calculations of the ground state properties of nuclei with proton number Z = 8 – 20 have been made by the macroscopic–microscopic (MM) model with isospin-dependent spin–orbit potential. Special emphases are placed on the exotic properties of neutron-rich nuclei, the evolution of deformations and the shell structures. The calculated binding energies agree well with the experimental data. The calculations well reproduce the deformations for both nuclei near stability-line and drip-line. The calculations extend the boundaries of the island of inversion and show that 28–32Ne, 29–33Na and 30–34Mg belong to the island. The large deformations and exotic properties of nuclei around N = 20 are discussed and their origin are explained. We stress that both protons and neutrons play important roles in the formation of the large deformations and the appearance of the island of inversion. The properties of nuclei around N = 28 are also discussed. The calculations show that the N = 28 shell seems to disappear in Mg, Al and Si isotopes. We show that most exotic properties of sd and pf shell nuclei can be understood in MM model.

Microscopic shell-model description of the exotic nucleus 16C

The structure of the neutron-rich carbon nucleus 16C is described by introducing a new microscopic shell model of no-core type. The model space is composed of the 0s, 0p, 1s0d, and 1p0f shells. The effective interaction is microscopically derived from the CD-Bonn potential and the Coulomb force through a unitary transformation theory. Calculated low-lying energy levels of 16C agree well with the experimental values. The B(E2;21+→01+) value is calculated with the bare charges. The anomalously hindered B(E2) value for 16C, measured recently, is well reproduced.

Spectroscopic Study of Neutron Shell Closures via Nucleon Transfer in the Near-Dripline NucleusO23

Neutron single particle energies have been measured in 23O using the 22O(d,p)23O*-->22O+n process. The energies of the resonant states have been deduced to be 4.00(2) MeV and 5.30(4) MeV. The first excited state can be assigned to the nu d3/2 single particle state from a comparison with shell model calculations. The measured 4.0 MeV energy difference between the nu s1/2 and nu d3/2 states gives the size of the N=16 shell gap which is in agreement with the recent USD05 ("universal" sd from 2005) shell model calculation, and is large enough to explain the unbound nature of the oxygen isotopes heavier than A=24. The resonance detected at 5.3 MeV can be assigned to a state out of the sd shell model space. Its energy corresponds to a approximately 1.3 MeV sized N=20 shell gap, therefore, the N=20 shell closure disappears at Z=8 in agreement with Monte Carlo shell model calculations using SDPF-M interaction.

Measurement of the electric quadrupole moment of 32Al

Abstract The electric quadrupole moment Q for the ground state of 32Al has been measured using the β-NMR technique. Spin-polarized 32Al nuclei were obtained from the fragmentation of 40Ar projectiles at E / A = 95 MeV / nucleon , and were implanted in a single crystal α-Al2O3 stopper. The quadrupole moment was deduced from the measured quadrupole coupling constant. The obtained value, | Q ( Al g.s. 32 ) | = 24 ( 2 ) mb , is remarkably small as compared with other Al isotopes, but is well explained by shell model calculations within the sd shell. The present result indicates that 32Al has a normal sd-shell structure, in contrast to the neighboring N = 19 isotone 31Mg for which a strongly deformed intruder ground state has been reported to occur.