Levels In 28Si Research Articles

The structure of28Si above 10 MeV excitation energy II: Assignments of quantum numbers

New assignments of quantum numbers have been obtained for more than 50 levels in28Si between 8953 and 15915 keV excitation energy. They are based on the measurement of γ-ray angular distributions or anisotropies on 29 resonances of the27Al(p, γ) reaction and on the24Mg(a, γ) resonance atEα=3355 keV. A total of 15 high-spin states withI=5–7 has been obtained and the spectrum ofT=1 states has been identified up to 15915 keV excitation energy and a maximum spinI=6.

Radiative decay of the 15.235 MeV level in28Si

In27Al(p, γ)28Si reaction, a narrow resonance was located for the incident proton energyEp=3.785 MeV. It is tentatively identified as the analogue of the 5.91 MeV level in28Al. It is found to populate only levels of high spin (J≥3) and the angular distributions are consistent with spin-parity assignment ofJπ=3+ for the resonance. Gamma-decay strengths and branching ratios are deduced. The transition strengths from the otherJπ=3+,T=1 are compiled and compared against the strength expected from the positron decay of the ground state of28P (Jπ=3+). It is apparent that there is considerable configuration mixing among theJπ=3+,T=1 levels in28Si.

Search for missing predicted high-spin states in28Si

New shell model calculations have predicted several high-spin (I π=5+ and 6+) levels in28Si near 10 MeV excitation energy which are missing from or ambiguous in existing experimental studies. Angular distributions, linear polarizations and Doppler-shifts ofγ-rays have been measured for theγ-decay of theE p=1,911 and 2,073 KeV resonances of the27Al(p, γ) reaction in an attempt to discover these missing states or confirm the discrepancies between experiment and theory. The excitation energies and spin-parities of the resonances were determined as 13,424.4±0.2 keV,I π=5+ and 13,582.3±0.5 keV,I π=6+. States populated in theγ-decay of these resonances were assigned spins and parities as follows: 11,777 keV,I π=5+; 11,331 keV,I π=6+; 10,417 keV,I π=5+; 9,417 keV,I π=4+ and 8,945 keV,I π=5+. On the basis ofγ-ray transition rates T=1 is assigned to the 13,424 keV level and T=0 to the 10,417 and 11,777 keV levels. With the new data excellent agreement is achieved between the experimental spectrum of28Si and the new shell model predictions. These data provide evidence for aK π=3+ rotational band comprised by the 6,276, 6,889, 8,945 and 11,331 keV levels. This band emerges also from the shell model wave functions as do theK π=0+ bands based on the ground state and the 6,691 keV state.

Coexistence of three different shapes in 28Si

Extensive agreement is found between the experimental levels in 28Si and the combined SU3-Hartree-Fock and the Nilsson-Strutinsky theoretical treatments. Of particular interest is a K = 0 + band representing a third shape with ϵ ∼ 0.45 and γ ∼ 15° which coexists with the previously known oblate and prolate bands. This new band crosses the prolate and oblate bands in the vicinity of J π = 8 + producing a strong backbend.

Delayed particle emission following the β + decay of 28P and 32Cl

Thin sources of 28P and 32Cl were prepared by depositing recoil atoms from the reactions 28Si(p, n) 28P and 32S(p, n) 32Cl with a helium jet onto a fast transport tape. Delayed particle spectra were measured with high resolution using Si(Au) surface barrier detectors. Six proton groups and ten α-particle groups in the energy ranges 0.4 MeV < E p < 1.3 MeV and 1.3 MeV < E α < 2.7 MeV were assigned to the decay of 28P. Six proton groups and eleven α-particle groups in the energy ranges 0.7 MeV < E p < 1.9 MeV and 1.5 MeV < E α < 3.6 MeV were identified in the decay of 32Cl. Absolute intensities of the individual proton and α-particle transitions were determined and corresponding upper limits for the log ft values of the precursor β + transitions were calculated. The total proton and α-particle branching ratios are 1.3 × 10 −5 and 8.6 × 10 −6 for 28P and 2.6 × 10 −4 and 5.4 × 10 −4 for 32Cl, respectively. The observed particle groups were assigned to excited levels known from resonance reaction studies in 28Si and 32S. Ratios of the partial widths for α-particle and proton emission, Γ α/Γ p were determined for four levels in 28Si and for one level in 32S; upper limits are given for several other levels. New evidence for spin-parity assignments is discussed. Partial decay schemes are presented for 28P and 32Cl.

The α-width of the lowest T = 2 state in 28Si

We have studied the E x = 15.22 MeV, J π = 0 + T = 2 level in 28Si as an isospin forbidden resonance in 24Mg(α, α 0) 24Mg and obtained Γ α, c.m. = 222 ± 22 eV. This corresponds to a reduced width ≈ 1.4 × 10 −4 times the Wigner limit, which is the largest known reduced α 0 width of a 0 + T = 2 state in the A = 4 n T z = 0 nuclei. We have also established the existence of a J π = 0 + state at E x = 15.12 MeV with Γ c.m. = 11 ± 2 keV and Γ x/ Γ = 0.7 ± 0.1. Narrow resonances at E x = 15.08, 15.29 and 15.31 MeV are also observed.

Measurement of compound nuclear lifetimes by the crystal blocking technique

The crystal blocking technique has been used to measure the lifetime of the 12.44 MeV 2 + level in 28Si excited in the 27Al(p, α) 24Mg resonance reaction at E p = 885 keV. Both 〈110〉 axial and {111} planar blocking effects in Al single crystals have been investigated. Measurements have been made for two reaction depths, 1000 and 4000 A. Various methods of extracting the lifetime, involving both analytical calculations and computer simulation, are compared. The theoretical change in volume of an axial blocking dip due to a lifetime effect has been calculated in the continuum approximation. The results obtained from the different methods of analysis agree, the average value of the lifetime being 28 as. Although the results show no systematic dependence on reaction depth, pronounced depth effects are evident in computer simulations of the planar blocking dips.

The transfer reactions induced by 38 MeV 19F ions on 27Al

The reactions induced by 38 MeV 19F on 27Al have been studied using the Tandem Van de Graaff accelerator at Harwell together with a mass identification system based on a combination of a Buechner magnetic spectrograph and position sensitive silicon detectors in the focal plane of the magnet. Comparison of the elastic scattering measurements with Coulomb cross sections provided an absolute normalisation to the measured reaction yields. Transfer reactions were measured as a function of emitted particle energy and angle for single nucleon pick-up and stripping and for 2, 3 and 4 particle stripping. Essential features of the experimental technique are outlined and the experimental results are presented. The significance of the energy spectra and angular distributions are discussed. The results of DWBA analyses of one-nucleon transfer reactions yield spectroscopic factors for levels in 28Si, 28Al and 26Al which are in fair agreement with those obtained in other studies.

The 27Al(d, nγ) Reaction and Bound Isobaric Analogue States in 28Si

The 27Al(d,nγ)28Si reaction has been used to study characteristics of bound isobaric analogue levels in 28Si. Neutron energies were determined by time-of-flight techniques using a pulsed incident beam and a large liquid scintillation detector. The neutron detector was placed at 0° thus accentuating lp = 0 transfer. Gamma rays in coincidence with neutrons were detected by a coaxial Ge(Li) spectrometer. The results show good agreement with previously measured branching ratios for the 6 277, 8 589, 9 316 and 9 381 keV levels in 28Si. New information is obtained for the analogue of the 2 139 keV level in 28Al. The excitation energy is found to be 11 434.1 ± 0.7 keV and gamma-ray transitions are observed to T = 0 as well as T = 1 levels. An upper limit for the lifetime of 100 fs is estimated from the experimental results.

The lifetime of the 8543 and 8945 keV levels in 28Si

Abstract The lifetimes of the 8543 and 8945 keV levels in 28 Si have been measured by the Doppler shift technique using the 27 Al(p, γ) 28 Si reaction resonances at E p = 3096 and 2677 keV, respectively. Both centroid shift and lineshape analysis were used, and the results lead to a value of τ = 15 ± 3 fs for the 8543 keV level and τ = 104 ± 8 fs for the 8945 keV level. The results are compared with previous measurements and available theoretical calculations.

Experimental evidence for the validity of discontinuous stopping in DSAM analysis

Different methods used for the analysis of Doppler shifted gamma-ray lines are compared via the determination of the lifetime of the 4.62 MeV (4 + level in 28Si. This level is excited using the 27Al(p, γ) reaction with E p = 767 keV and 1589 keV. The shape and shift of the gamma-ray due to the transition 4.62 MeV (4 + → 1.78 MeV (2 +) in 28 Si was measured. The best fit is obtained when discontinuous stopping for the recoiled ion is assumed.

Ground State Radiation Width of the 10.18 MeV, Jπ = 3−, State in 28Si Deduced From 100 MeV Proton Inelastic Scattering

Differential cross section data for 100 MeV protons inelastically scattered from 6Li, 9Be, 12C, 24Mg, and 28Si are compared with the corresponding E2 and E3 transition rates and a strong correlation is shown to exist between (dσ/dΩ)max and Γγ0 if the shape of the proton angular distribution is characteristic of single excitation. The persistence of the correlation to transitions with Γγ0/Γγ &lt; 0.1 allows an estimate of Γγ0 = 8.6 ± 5 × 10−4 eV for the 10.18 MeV, Jπ = 3−, level in 28Si.

Decay of the 15.22 MeV, T=2 state in 28Si

The γ-ray decay of the lowest T=2 state (0 +) in 28Si at has been investigated with a Ge(Li) detector using the reaction 24 Mg(α,γ) 28 Si . Transitions to 1 +, T=1 levels in 28Si at 11445 ± 3 (40%), 10901 ± 3 (43%), 10724 ± 3 (9%, T=(1)) and 10598 ± 3 (8%, T=(1)) keV excitation were observed and a comparison is made with a simple shell-model description of the T=2 and T=1 levels. Limits have been placed on Γ γ 1 and Γ p o and values are given for Γ, Γ α 1 , Γ p 1 and Γ p 2 .

Resonance levels in 28Si from 27Al+p reactions

By measuring the proton elastic scattering from 27A1 with surface barrier detectors, 18 resonance levels in 28Si were observed in the energy range E p = 1.1–1.8 MeV. For all resonances, the proton orbital momenta were determined, and for 12 resonances unique spin assignments were made on the basis of these and previous (p, γ) and (p, α) measurements. Partial proton widths are given for all resonances. The yield of inelastically scattered protons from 27Al and of the emitted α-particles to the first excited state in 24Mg was also observed. Resonance strengths were determined relative to the known strength of the 1184 keV resonance in the 27Al(p,γ) 28Si reaction. Seven resonances can be classified as probable analogues of the 3.46, 3.59, 3.67, 3.88, 3.93 and 4.03 MeV states in 28A1. For all T = 1 resonances spectroscopic factors were deduced.

28Si Levels from 29Si(τ, α) 28Si reaction

The levels in 28Si up to 12 MeV were studied in the 29Si(τ, α) 28Si reaction. A distorted-wave analysis was performed to determine the angular momentum transfers leading to several levels in 28Si whose spins and parities had not been determined uniquely. Tentative spin and parity assignments were made for those states. The results are compared with the published information about the levels of 28Si.

The 29Si(p, d) 28Si and 30Si(p,t) 28Si reactions and the mixing of 1p hole and 2p particle configurations in 28Si

Levels in 28Si with excitation energies up to 12 MeV have been investigated with the 29Si(p,d) 28Si and 30Si(p, t) 28Si reactions at an incident energy of 35 MeV. Angular momenta and spectroscopic factors have been obtained from a DWBA analysis of the angular distributions. Spin and parity assignments of 2 − for the 9.76 MeV level and (1, 2) − for the 9.93 MeV level of 28Si have been established. The usefulness of the 30Si(p, t) 28Si reaction as a spectroscopic tool appears to be limited since states with unnatural parity were excited with similar cross sections as most natural parity states. The emphasis lies on a discussion of the negative parity states. Considerable l = 1 spectroscopic strength was found at energies close to the expected position of 1p hole states. Arguments are brought forward in favour of a 1p assignment to the l = 1 transfer. A strong mixing of 1p hole and 2p particle configurations is concluded for the negative parity states of 28Si.

Study of the Ep = 1.724 MeV Resonance in the 27Al(p, γ) 28Si Reaction

The 13.245 MeV level in 28Si has been populated by resonant proton capture at 1.724 MeV in the 27Al(p,γ)28Si reaction. Gamma-ray angular distributions and linear polarizations have been measured respectively with a coaxial and a planar Ge(Li) counter. Branching ratios have been extracted from the angular distributions. Spins have been assigned to the levels of 28Si with the help of the angular distribution and linear polarization results. The spin and parity of the 13.245 and 9.699 MeV levels are most probably 5− with T = 1 and 0 respectively. The assignment Jπ = 4− for the 8.411 MeV level is in agreement with the present data. A spin and parity of 3− for both the 13.245 and 9.699 MeV levels also fits the data but with more adjustable parameters. Arguments are presented to show that these assignments are unlikely. Relative measurements of the gamma-ray yields from a thick target give a value of (7.5 ± 1.5) eV for (2J + 1)ΓγΓp/Γ of the resonance. The M1 transition strength between the 13.245 and the 9.699 MeV levels is then deduced to be (0.7 ± 0.15) W.u. The known properties of the 9.699 MeV level suggest that it may be associated with an E5 oscillation of the ground state of 28Si.

Mesures des vies moyennes des premiers états excités Du Noyau 28Si

The lifetimes of eleven levels in 28Si have been measured with the Doppler shift attenuation method. The states were excited in the 27Al(p, γ) 28Si reaction. A decay scheme is given for the resonance at E p = 774 keV. The levels at 8.947 and 10.430 MeV were populated at the E p = 2054 keV resonance. The results are compared with rotational-model and Hartree-Fock calculations.

Inelastic scattering of 100 MeV protons from Mg and Si using a Ge(Li) total absorption proton counter

A cooled Ge(Li) detector was used as the total absorption proton counter for the energy analysis of 100 MeV protons scattered from targets of enriched magnesium (99.7 % 24Mg) and silicon (natural isotopic composition). A total energy resolution of 400 keV (FWHM) was achieved; it is comparable to the best resolution presently attainable with magnetic spectrometers in the 100–200 MeV energy region. The measurements cover the 10°–60° lab angular region; and angular distributions are presented for 23 peaks in the energy spectra. A qualitative discussion of the excited states which correspond to the observed peaks is based mainly on the shapes of the distributions. Comparisons are made with results from similar experiments at 55 MeV and 185 MeV. Shapes of angular distributions for strongly excited 2+, 3− and 0+ transitions are independent of energy, but significant differences exist for 4+ transitions. The excitation of a previously unknown level in 28Si at 11.45±0.12 MeV energy has been observed. The level most probably has Jπ = 5−.

Properties of the 9.699 MeV level in 28Si

The decay of the 9.699 MeV level in 28Si has been studied using the reaction 27Al(p,γ)28Si at Ep = 1724 keV. Gamma rays from the decay of the 9.699 MeV level were observed with large-volume Ge(Li) detectors. Branching ratios for the decay of the 9.699 MeV level have been obtained and a lower limit for the lifetime of this level has been measured to be [Formula: see text] using the Doppler-shift attenuation method.