Photodisintegration of 28Si to excited residual states

Integrated cross sections up to 28 MeV have been measured to excited residual states in ²¹Si, ²⁷Al and ²⁴Mg, following the photodisintegration of ²⁸Si. From comparisons with spectroscopic factors of nucleon pickup reactions on ²⁸Si, leading to the same residual states, it is concluded that the photonucleon emission process is predominantly single particle in nature. Estimates of isospin mixing in the ²⁸Si giant dipole resonance are also given.     

A study of the states of 30Si with the 28Si(t,p)30Si and 29Si(d,p)30Si reactions

The excitation energies of the levels in ³⁰Si have been measured up to an excitation of 9.46 MeV with the ²⁸Si(t, p)³⁰Si reaction at a triton energy of 6.0 MeV. Angular distributions have been measured of proton groups from the ²⁸Si(t, p)³⁰Si and ²⁹Si(d, p)³⁰Si reactions in a multi-angle magnetic spectrograph. Triton bombarding energies of 10.5 and 12.1 MeV were used and the deuteron incident energy was 10.0 MeV. States in ³⁰Si up to an excitation of 8 MeV were observed. Spins and parities of several states have been assigned using an empirical method for the (t, p) results and using a DWBA analysis for the (d, p) distributions. Spectroscopic factors for twelve states were obtained from the latter analysis. Two of these disagree with theoretical predictions. The state previously reported at an excitation of 6.63 MeV in ³⁰Si was observed to be formed by a strong L = 0 transition in the (t, p) reaction and also by a strong l = 1 transition in the (d, p) reaction. We deduce that there are two closely spaced states at about this excitation, one having a spin and parity of 0⁺ and the other 0^?, 1^? or 2^?.     

Structure of 0+ states of 28Si and particle-hole conjugation

Full sd-shell calculations for the 0⁺ states of ²⁸Si have been performed in the SU(3) basis so that the intrinsic deformation of the shell model states can be deduced by inspection. The shell model Hamiltonian is decomposed in a symmetric part H_S and an antisymmetric part H_A with respect to particle-hole conjugation. It is shown that the splitting of prolate and oblate states is due to the spin-orbit part of H_A. The different prediction for ²⁸Si obtained with Kuo and with Preedom-Wildenthal matrix elements can be attributed to the difference in a single parameter: the strength of the spin-orbit part of H_A.     

Microscopic study of elastic 28Si28Si scattering and intermediate structure resonances in 56Ni

Elastic ²⁸Si²⁸Si scattering has been studied microscopically with special emphasis on quasimolecular and barrier resonances in high partial waves. Schematic application of the double resonance mechanism predicts intermediate structures in the excitation functions in qualitative agreement with the experimental data.     

Transient field measurements of g-factors for 28Si and 30Si

Transient field precession measurements have been performed on the first excited J^π = 2⁺ states of ²⁸Si and ³⁰Si with the IMPAC technique on recoil in magnetized iron. The results were analyzed with empirically adjusted Lindhard-Winther predictions. This yields g-factors of $\text{g = +0.56 ± 0.09}\text{and}\text{g = +0.56 ± 0.16}\text{for}{}^{28}\text{Si and}{}^{30}\text{Si}$, respectively. In the present cases the influence of static hyperfine fields is negligible due to the very short mean lives for ²⁸Si and ³⁰Si of 0.68 and 0.35 ps, respectively. The results are compared with theoretical calculations. Previous results for ²⁶Mg(2₁⁺) were reanalyzed with the more recent lifetime of τ_m = 0.72 ± 0.03 ps. The value of the g-factor becomes g = +0.82 ± 0.16.     

Strong evidence of alpha-particle doorway states in 28Si observed in alpha-decays to low-lying states of 24Mg

Four ratios of alpha-particle decay probabilities to specific members of the K-bands of ²⁴Mg were measured at bombarding energies E_cm = 19.7 to 28.2 MeV. The energies for enhancement of these ratios correspond quite well to the 0⁺ resonances reported.     

Alpha-heavy-ion angular correlations from 28Si + 12C

Alpha particles have been measured in coincidence with heavy recoil nuclei from the ²⁸Si + ¹²C reaction. At E_lab = 87 MeV angular correlations for alphas between 15° and 55° and heavy ions at angles ?9°, ?12° and ?15° have been taken. An excitation function of coincidence events with θ_α = 30° and θ_HI = ?12° has been measured for 84 MeV < E_lab < 91.5 MeV. The results are well described by a statistical-model calculation for compound nucleus decay. No evidence is found for additional processes.     

Yrast and high-spin states in 22Ne

High-spin states in ²²Ne have been investigated by the reactions ¹¹B(¹³C, d)²²Ne and ¹³(¹¹B, d)²²Ne up to $\text{E}{}^1\text{～- 19}\text{MeV}$. Yrast states were observed at 11.02 MeV (8⁺) and 15.46 MeV (10⁺) excitation energy. A backbending in ²²Ne is observed around spin 8⁺. The location of high-spin states I ≦ 10 is discussed in terms of the rotational band structure, Strutinsky-type calculations, and pure shell-model predictions.     

Isomers and high-spin states in 205At

The properties of high-spin states in ²⁰⁵At have been investigated to $\text{J ?}\text{37}\text{2}$ and an excitation energy of > 4.0 MeV. The properties of the $\text{29}\text{2}$⁺ isomer at 2340 keV have been established and those of the $\text{25}\text{2}$⁺ isomer at 2063 keV have been further investigated. In the present study the mean lives of these isomers have been determined as 11.2±0.2μs and 98±2 ns respectively. The M2 branch of the $\text{13}\text{2}$⁺ state at 970 keV to the $\text{9}\text{2}$⁺ ground state has been measured. This decay is attributed to a proton single-particle $\text{i}{}_{\mathrm{<mtext>13</mtext><mtext>2</mtext>}}\text{→}\text{h}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}$ transition. The trend of excitation energies of a number of proton excitations in the odd-mass nuclei from ²¹¹At to ²⁰¹At has been compared to the corepolarization model and the average $\text{ν}\text{f}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}{}^{\mathrm{?1}}\text{π}\text{f}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}$ interaction energy has been deduced. Comments are made upon probable configurations for many of the levels.     

Elastic scattering of neutrons from 28Si and 32S

The excitation functions in the elastic scattering of neutrons have been measured at several angles in the energy range 600–3600 keV. Data have been analyzed in terms of the Ericson fluctuation and energy-averaged angular distributions have been analyzed using the optical model including Hauser-Feshbach theory.     

Levels of 28Si from the 24Mg(α, α)24Mg and 24Mg(α, γ)28Si reactions

Excitation functions have been measured at six angles for ²⁴Mg(α, α)²⁴Mg up to E_α 4.94 MeV. Multi-level R-matrix analysis was performed for nineteen resonances. More than ha of the determined spin-parities and other resonance parameters are new values. The ²⁴Mg(α, γ)²⁸Si studies of Maas et al. were extended up to E_α = 5.13 MeV. Resonance strengths and branchin ratios were determined. The γ-ray angular distribution measurement at E_α = 3.79 MeV gives J^π = 2⁺ value for the resonance. A comparison of the results obtained in different reaction channel is given and the possibility of clusterisation in some excited states of ²⁸Si is discussed.     

Proton resonant and direct capture on 28Si

Excitation functions for the ²⁸Si(p, γ)²⁹P capture reaction have been measured in the proton energy range E_p = 1.3–2.3 mMeV. The analysis of the data reveals the presence of the direct capture process to the ground state and first excited state of ²⁹P. The strengths of the resonances at E_p = 1.65, 2.09 and 2.88 MeV and the spectroscopic factors of the ground state and first excited state in ²⁹P are deduced.     

Microscopic study of elastic 16O28Si scattering

Elastic ¹⁶O²⁸Si scattering is studied microscopically within the Generator Coordinate Method. A resonance analysis supplies a series of three rotational bands; the two excited bands agree very well with rotator-like structures found in experiment. The phase shifts do not show any parity dependence.     

Elastic transfer in the scattering of 16O by 28Si

It is shown that the interference of potential scattering and elastic transfer amplitudes can account for the recently observed gross structure in the excitation function for elastic scattering of ¹⁶O by ²⁸Si at 180°.     

Structure of high-spin states in 143Pm

High-spin states of ¹⁴³Pm have been studied in the reactions ¹⁴¹Pr(α, 2n)¹⁴³Pm and ¹⁴³Nd(d, 2n)¹⁴³Pm by means of in-beam spectroscopy. The level scheme, spin and parity assignments are based on results obtained from singles γ-ray spectra, conversion electron spectra, prompt and delayed γ-γ coincidences, γ-ray angular distribution and linear polarization measurements. Positive- and negative-parity states with energies up to 4580 keV and spins up to $\text{25}\text{2}$ have been established including 22 new levels. For two nanosecond isomeric states the nuclear spin precession in an external magnetic field was observed providing the following g-factors:

$\text{g(}\text{11?}\text{2}\text{, 959.7 keV)=1.14(9), g(}\text{15+}\text{2}\text{, 1898.3 keV)=1.00(7).}$

The experimental results are well understood by calculations which have been performed in the framework of the shell model (for positive-parity states of 11 valence protons above a Z = 50, N = 82 core) and of the cluster-vibration model (for 3 holes in a Z = 64, N = 82 core). In the case of positive-parity states no evidence for particle-core coupling could be found, while the negative-parity states could qualitatively be understood within the particle-core coupling picture.     

Yrast traps and high-spin states in 210Rn

Yrast and near-yrast states have been investigated in ²¹⁰Rn to high spin (J > 30) and high energy (E_x > 10 MeV). Three different (HI, xn) reactions were used to populate the states of interest and several different γ-ray spectroscopic techniques were utilized. Three high-spin yrast traps were discovered. Two de-excite by strong E3 transitions while the third decays mainly via an extremely inhibited E2 transition. The E3 decays are interpreted as allowed single-particle transitions between proton or neutron states above the ²⁰⁸Pb shell closure while the inhibited E2 transition is interpreted as indicating a substantial change in structure as the decay proceeds down the yrast line. The interpretation has been given in terms of shell-model calculations.     

Shape coexistence of high-spin isomeric states in proton-rich A~190 nuclei

High-spin isomeric states in proton-rich A~190 nuclei have been investigated using configuration-constrained calculations of potential-energy surfaces. The calculations reproduce reasonably the experimental data, and predict shape coexistence of high-spin isomeric states in light Po isotopes.     

Semiclassical analysis of the scattering of 16O by 28Si

Semiclassical analysis of the optical potentials which fit the recent Stony Brook-BNL ¹⁶O + ²⁸Si scattering data, shows that the scattering mechanism of these potentials at backward angles is essentially the same. Neither the reflection from the sharp surface of the surface transparent potential used by Dehnhard and Shkolnik, Golin and Kahana, nor the modified form factor in the potential used by Lee and Chan is essential to fit the angular distribution. Based on the uniform approximation, we derive analytic formula to understand the angular distribution at backward angles and to understand the oscillatory structure in the 180° excitation functions.     

Intermediate states with K = 2 symmetry for the electrofission of 28Si

An intermediate state for the recently observed electrofission of ²⁸Si is proposed on the basis of symmetry arguments and a simple variational calculation.