States In 27Al Research Articles

Electromagnetic multipoles of positive parity states in 27Al by elastic and inelastic electron scattering

Nuclear structure of 27Al nucleus has been investigated using shell model with self-consistent Hartree–Fock calculations. Shell model calculations are performed with full sd model space for positive parity states. For all selected excited states, Skyrme interactions are adopted to generate from them a one-body potential in Hartrre-Fock theory to calculate the single-particle matrix elements and compared with those of the harmonic oscillator. The deduced results are discussed for the longitudinal and transverse form factors and compared with the available experimental data. It has been found that the core-polarization effects are essential in obtaining a reasonable description of the data.

Role of hyperfine interaction in Landé [formula omitted]-factors of [formula omitted] clock states

In the weak-magnetic-field approximation, we calculated the hyperfine-induced Landé g-factors of the 3P0o clock states in 27Al+ and 87Sr by using the multi-configuration Dirac–Hartree–Fock method. The present results, δghfs(1)(3P0o)=−1.183(6)×10−3 for 27Al+ and δghfs(1)(3P0o)=7.78(30)×10−5 for 87Sr, agree with experimental values very well. The influence of relativistic effects and electron correlations on the involving hyperfine interaction and Zeeman matrix elements, and energy separations were investigated in detail. We also estimated the contributions from perturbing states to the g-factors concerned so as to truncate the summation over the perturbing states without loss of accuracy. Based on these, the computational uncertainties were systematically analyzed for the hyperfine-induced Landé g-factors of the 3P0o clock states in these two atomic systems.

The 26Al(p, γ) 27Si reaction at low stellar temperature

Shell-model calculations have been used to predict the locations of states in 27Si which are analogous to well-studied states in 27Al. From this, we have determined the resonance properties of the known states in 27Si near the 26Al + p threshold. The resulting thermonuclear reaction rate is uncertain by about a factor of ten at low temperatures, but it appears that the 26Al(p, γ) 27Si reaction is too slow to destroy a significant amount of 26Al at these temperatures.

Measurements of differential cross-section ratios for single-nucleon transfer reaction pairs near A = 25

Differential cross sections for the (d, p), ( 3He, d), (α, t), and (α, 3He) reactions involving seventy-one residual states in 23Na, 25Mg, 25Al, and 27Al have been measured at a forward angle with incident energies of 17.5, 20.2, 34.8, and 34.8 MeV, respectively. The ratio of cross-section pairs involving formation of the same residual state is determined for forty-five cases where both the angular momentum transfer and single-particle spectroscopic strength have been previously established. These are compared to values calculated with conventional distorted-wave Born approximation analyses, and the utility of this technique for identifying some levels which are possible s- or p-wave resonances is demonstrated and discussed for states in the vicinity of proton thresholds. An application is made involving proton threshold states in 27Al.

Proton-threshold states in27Al and the production of 27Al at low stellar temperatures

The 26Mg(3He, d)27Al reaction has been employed to measure excitation energies and proton spectroscopic factors for states corresponding to 26Mg + p resonances in the vicinity of the proton-capture threshold. The width ratio Γγ/Γ was measured for three previously established resonances via a study of the 26Mg(3He, dγ)27Al reaction, and corresponding values of the proton widths were obtained. Combining this information establishes strengths for four of the states lying within 150 keV of the proton threshold. A 26Mg + p reaction rate is deduced, and its astrophysical implications are discussed.

Proton threshold states in 27Si and the destruction of 26Al at low stellar temperatures

Using the 27Al( 3He, t) 27Si reaction, we have located and measured accurate excitation energies for seven states in 27Si between the 26Al + p threshold and the lowest observed 26 Al( p, γ) 27 Si resonance. By examining the mirror states in 27Al via the 27Mg( 3He, d) 27Al and 26 Mg(α, t) 27 Al reactions, we find that in the 26AI + p channel there can be at most only one s-wave resonance that lies below the lowest directly observed resonance; this possible l = 0 resonance most probably corresponds to either the 7589, 7557 or 7533 keV state in 27Si. The remaining six states are most probably formed via l ⩾ 2 proton capture. Upper limits for the reduced proton widths of these states have been used to estimate an upper limit for the 26 Al( p, γ) 27 Si thermonuclear reaction rate.

Hyperfine structure studies in2D and4P states of27Al,69,71Ga and115In

Hyperfine structure studies in2D and4P states of27Al,69,71Ga and115In

Properties of unbound high-spin states in27Al

Selectively excited high-spin states in27Al have been located by the reaction12C(16O,p)27Al. The excitation functions of these states have been measured at incident energiesEcm=18.7 to 30.1 MeV at intervals of 150 keV. They exhibit maxima of presumably non-statistical origin. At four different energies the subsequent decay of the27Al states has been studied by detecting the final heavy recoils with the Munich recoil spectrometer in coincidence with the proton emitted in the first stage of the reaction. Using this new method branching ratios of theγ-, α-, andn-decay have been measured. Based on angular correlation arguments spins up to 27/2ħ have been determined within the experimental accuracy of 1–2ħ. The observations suggest a superdeformed shape of27Al at least in some of the states.

Dsa measurement of short lifetimes in 27Al

Lifetimes or upper limits of 17 bound states in 27Al have been measured using the Doppler shift attenuation method applied to the 26Mg(p, γ) 27 Al reaction in the proton energy range 1.4–2.2 MeV. For the effective stopping of recoils, the targets were prepared by implanting 26Mg into tantalum backings. The Monte Carlo method and the experimental stopping values were used in the DSA analysis. In the Monte Carlo simulations the scattering angles of recoiling ions were calculated directly from the Thomas-Fermi interaction potential, rather than from the LSS theory.

Mean lives of excited states in 28Al

Abstract The mean lives of excited states in 28Al formed in the 27Al(d,p) reaction have been determined from the measured Doppler shift of γ-rays emitted in coincidence with the protons. The multipole order and maximum admixture of higher order γ-radiation are determined. In several cases the result leads to a definite assignment or strongly favoured assumptions for the spin value of the excited state. A total of 28 levels below 5.45 MeV excitation energy have been studied. Previously mean lives have been reported for twelve of these levels. The experimental results indicate that several closely spaced doublets may be explained as a s 1 2 neutron coupled to states in 27Al.

Inelastic scattering of electrons from 27Al

Electron scattering form factors have been measured for the excitation of 2.21 MeV( 7 2 + ) 2.734 MeV ( 5 2 +) and 3.00 MeV ( 9 2 +) states in 27Al using values of momentum transfer between 0.31 and 1.05 fm −1. Reduced ground-state transition probabilities and transition radii are obtained for the 2.21 and 3.00 MeV states. Intermediate-coupling calculations are shown to reproduce the energy spectrum of 27A1 but not the transition matrix elements and the form factors. Much better agreement is obtained using a large basis shell-model calculation.

Absolute excitation energy measurements using an electrostatic analyzer

The excitation energies of several states in 6Li, 11B, 12C, 27Al and 56Fe have been measured in terms of the standard meter and the standard volt. The energy of the electron beam from a FN Tandem Accelerator was measured absolutely with a 61 cm radius electrostatic analyzer to an uncertainty of 0.037%. Thin target bremsstrahlung produced by this beam was used to excite the states in question. The excitation energies are as follows: (2126.5 ± 1.5) keV for the 2.12 MeV state in 11B; (2984.2 ± 1.5) keV for the 2.98 MeV state in 27Al; (3451.3 ± 1.7) keV for the 3.45 MeV state in 56Fe; (3569.0 ± 1.7) keV for the 3.56 MeV state in 6Li; (4439.4 ± 1.6) keV for the 4.44 MeV state in 12C; (4446.4 ± 1.8) keV for the 4.44 MeV state in 11B; and (5021.1 ± 1.8) keV for the 5.02 MeV state in 11B. These results are compared with particle-reaction and gamma ray spectroscopy measurements.

High-spin states in27Al

The24Mg(α, p γ) reaction has been studied atα-particle energies of 13, 15, and 15.2 MeV. Using method II of Litherland and Ferguson spin assignments or restrictions thereof could be made to the27Al levels atE x =5500 keV (I=11/2, 7/2), 6950 keV (I=11/2, 7/2), 7226 keV (I=9/2), 7286 keV (I=9/2–13/2), 7399 keV (I=11/2, 7/2) and 7443 keV (I=13/2, 9/2). Using the Doppler-shift attenuation method a lifetimeτ=20–50 fs was obtained for theE x =7399 keV level and a limitτ<20 fs for the levels atE x =7443, 7286, 7226, 6950, 6718, and 6287 keV, respectively. Further spin assignments in27Al were obtained by a study of the26Mg(p, γ) reaction at the Ep=2174 keV resonance. Fromγ-ray angular distributions the resonance spin was determined as I=9/2 (previous assignmentI=7/2) and the spin of theE x =5959 keV level was determined asI=7/2. Tentative spin assignments were made to the levels atE x =6537 keV (I=7/2) and 6718 keV (I=9/2). From the combined evidence of the24Mg(α, p) and26Mg(p, γ) reactions the spin of the 6287 keV level was found to beI=7/2. The results are compared with shell model calculations and the Nilsson model.

Consistency study of DSA lifetime measurements

The mean life of the 1.01 MeV state in 27Al has been measured by the recoil distance method to be 2.37 ± 0.12 ps. Doppler-shift line-shape analysis in five backings gives values 2.27 ± 0.04 (Ca), 2.13 ± 0.07 (Ti), 1.73 ± 0.08 (Cu), 2.15 ± 0.04 (Ba), 1.68 ± 0.06 (Ta), 1.77 ± 0.07 ps (Au), where the quoted errors are statistical only. This dependence of the DSA values on atomic number agrees with previous measurements of this effect. The data support the suggestion that the longer USA values are correct. A complete method for analysis of recoil distance data is described and checked against measurements of the 27Al 0.84 MeV and 24Mg 1.37 MeV states.

Reactions 19F(γ,xγ′) and 31P(γ,xγ′)

The spectra of deexcitation gamma rays following photodisintegration of 19F and 31P have been measured to study further the reaction channels involved in photodisintegration of nonclosed shell nuclei. Besides (γ,n) and (γ,p) decays to excited states of the residual nuclei, strong (γ,α) channels from 19F to the first excited state of 15N and from 31P to the first two excited states of 27Al were observed.

Shell-model calculations for masses 27, 28 and 29: electromagnetic transition rates and multipole moments

Electromagnetic transition probabilities, multipole moments and log ƒt values have been calculated from many-particle shell-model wave functions in a truncated 1 d 5 2 2 s 1 2 1 d 3 2 config space, with a maximum of four holes in the 1 d 5 2 subshell. The electric quadrupole transition strengths and moments are reproduced very well in a least-squares fit to 74 experimental data with one parameter, for the isoscalar effective charge, yielding the values e p = 1.6 e and e n = 0.6 e. The results for magnetic dipole transition strengths and moments follow from adjusting two effective reduced single-particle matrix elements in separate least-squares fits to 17 experimental data in A = 27 nuclei and 21 data in A = 29 nuclei. The average absolute deviations between theory and experiment for E2 and M1 transition strengths are 3.0 and 0.05 W.u., while the average measured strengths are 7.7 and 0.08 W.u., respectively. Transitions from excited states above E x = 4.8 MeV in 27A1 and from some low-lying states in 27Al and 28Si are poorly reproduced by the present model. Calculated strengths of transitions from analogue states are given. Previous conclusions about the single-particle character of M1 transitions and the collective behaviour of E2 transitions are confirmed. The experimental data of seven A = 27–29 nuclei are well reproduced in one general treatment with an appreciably lower number of free parameters than are required to obtain comparable results in collective model calculations.

Investigation of the 6477 keV state in 27Al with a Ge(Li) γ-ray polarimeter

Abstract The E x = 6477 keV level in 27 Al was populated at the E p = 2293 keV, J π = 7 2 − , T = 3 2 resonance in the 26 Mg(p, γ) reaction. The angular distribution and polarization of the R → 6477 keV γ-ray transition yields a unique spin-parity assignment J π (6477) = case 72 − . The lifetime of the E x = 6477 keV level was measured, using the Doppler-shift attenuation method, as τ = 21 ± 6 fs.

A test of the collective correlations calculation of the dipole states of28Si

The relative populations of final States in27Al following the28Si (γ,p)27Al reaction are calculated in the framework of the collective correlations model of the giant resonance. Satisfactory agreement with experiment is found for all final states up to 2.7 MeV; however the population of the 2.98 MeV (3/2+) final state, which is the only one based on the two-phonon states of28Si, disagrees with experiment by a factor of 20.

The 23Na(α,γ)27Al reaction. A survey of the reaction and some properties of the 27Al nucleus

The 23Na(α,γ) reaction has been studied in the region Eα = 2–3 MeV. The γ-ray spectra from a total of 16 resonances have been surveyed using a 40 cm3 Ge(Li) detector. Excitation energies, γ-ray decay modes, branching ratios, and γ-ray yields of these resonances are given. Below Eα = 2.7 MeV, all resonances have [Formula: see text] and the α-particle capture proceeds with [Formula: see text]. The resonances at Eα = 2.696 and 2.797 MeV have strong branches to the Jπ = 11/2+ state in 27Al at 4.509 MeV excitation energy. A measurement of γ-ray angular distributions at these resonances yields Jπ(2.969 res.) = 9/2+, 11/2 and Jπ(2.797 res.) = 9/2+ for the resonances, and Jπ(5.671) = 9/2+, 7/2 and Jπ(4.580) = 9/2+, 7/2 for bound states in 27Al. Investigation of this latter level at the Ep = 2.114 MeV resonance in the 26Mg(p,γ)27Al reaction restricts the spin to 7/2. New excited states in 27Al were found at Ex = 5.504, 5.671, and 7.815 MeV. A state at Ex = 5.499 MeV was confirmed.

A study of the 26Mg( 3He, d) 27Al reaction

Angular distributions of deuterons from the 26Mg( 3He, d) 27Al reaction have been analysed with DWBA calculations to extract spectroscopic factors for most states in 27Al up to 4.58 MeV excitation. Ground state occupation numbers for d 5 2 , s 1 2 and d 3 2 orbits in the 26Mg ground state, and the single-particle centroids in 27Al are obtained. The results are in reasonable agreement with the excited core model of the low-lying states of 27Al. The negative parity state at 4.05 MeV in 27Al is shown to be a 1p hole state rather than a 2p particle state. Finally, a previous estimate of the excitation of d 3 2 particles in the ground state of 28Si is shown to be questionable.