Deformation effects in the 28Si+12C and 28Si+28Si reactions

The possible occurrence of highly deformed configurations is investigated in the ⁴⁰Ca and ⁵⁶Ni di-nuclear systems as formed in the ²⁸Si + ¹²C, ²⁸Si reactions by using the properties of emitted light charged particles. Inclusive as well as exclusive data of the heavy fragments and their associated light charged particles have been collected by using the ICARE charged particle multidetector array. The data are analysed by Monte Carlo CASCADE statistical-model calculations using a consistent set of parameters with spin-dependent level densities. Significant deformation effects at high spin are observed as well as an unexpected large ⁸Be cluster emission of a binary nature.     

Spin assignment of the 8.555 MeV state in28Si

Angular correlations between the gamma rays following the decay of the 8.555 MeV state in²⁸Si, known to have natural parity, were measured. The spin assignment of this state, found in this experiment, isJ ^π=6⁺. Energy sequence of the lowestJ ^π=2⁺, 4⁺ and 6⁺ states in²⁸Si compares well with the energy sequence of the ground state rotational bands in²⁰Ne and²⁴Mg.     

High spin states of some nuclei around the N=Z=28 double closed shell

A spectroscopic study is performed for high spin states of⁵⁵Fe,⁵⁵Co and⁵⁷Ni. To populate the investigated residues with a relevant cross section, the fusion evaporation reactions of³⁰Si(²⁸Si, 2pn)⁵⁵Fe,³⁰Si(²⁸Si, 2np)⁵⁵Co and⁴He(⁵⁴Fe,n)⁵⁷Ni were chosen. To identify the newγ transitions and to build the energy level schemes,γ — γ coincidence techniques together with excitation functions were employed. Angular distributions andγ — γ angular correlations allowed us to assign the spin values of the nuclear states. The previous level scheme of⁵⁵Fe is extended into the region between 6.5–11 MeV of excitation energy, up to spin 27/2, while the yrast decay paths of⁵⁷Ni and⁵⁵Co are reported here for the first time. Experimental data are fairly well reproduced by Glaudemans' shell model calculations.     

The (3He,t) reaction on 24Mg and 28Si

The (³He, t) charge exchange reaction on ²⁴Mg and ²⁸Si has been studied at 38.5 MeV. The angular distributions provide a number of new spin assignments in ²⁴Al and ²⁸P. A two-step excitation through intermediate α-particle channels was used to predict the ²⁴Al cross sections. The fits are fair in shape, but about a factor of three too large when a perfect overlap of initial and final shapes is assumed. The data to three 1⁺ states of ²⁸P are compared to the known electromagnetic reduced transition rate B(M1) to the analog states in ²⁸Si.     

Total cross sections for 4,6He + 28Si reactions measured at 10–28 MeV/A

New results on the energy dependence of the total cross section (σ _R) for ⁶He scattering on ²⁸Si in the incident energy range 10–28 MeV/A are obtained. The α-particle-production cross sections for the ²⁸Si(⁶He, ⁴He)X channel are measured as well. The secondary beam of ⁶He with an intensity of up to 5×10⁴ particle/s was generated by bombarding a thick beryllium target with ～32-MeV/A ⁷Li ions. In the energy region below 17 MeV/A, σ _R increases sharply. The experimental dependences of the total cross sections are compared with the results of σ _R calculations using the double-folding potential within the optical model. The energy dependence of σ _R for ⁶He differs from that for the neighboring nuclei, which can be associated with the structural features of the former nucleus. The energy spectra of α particles produced in the ⁶He interactions with silicon indicate two mechanisms of their production: transfer reaction and ⁶He breakup in the field of the ²⁸Si nucleus.     

A continuum calculation for the giant dipole resonance in28Si including the effect of collective correlations

Continuum shell model calculations for the giant dipole resonance in²⁸Si based on both the 1p 1h model and the collective correlation model are described and the results compared with experiment. The eigenchannel theory of nuclear reactions has been used to include the effect of the particle continuum.     

Towards 0.99999 28Si

A new approach for producing high-purity silicon with isotopic enrichment of ²⁸Si isotope is reported. The methods of centrifugal enrichment were modified to obtain the initial gaseous silicon tetrafluoride with a record-breaking enrichment of 0.99999664(11) with respect to ²⁸Si. The effective conversion of silicon tetrafluoride into elementary silicon with minimal isotopic dilution was achieved in an electron cyclotron resonance discharge plasma, sustained by gyrotron microwave radiation with a frequency of 24 GHz. We have experimentally demonstrated the deposition of the layers of microcrystalline ²⁸Si with enrichment of 0.999986 ± 0.000003, which is the best result at the present time.     

Features of <Emphasis Type="Italic">E</Emphasis>1 resonances in <Superscript>28</Superscript>Si and <Superscript>30</Superscript>Si nuclei

The structure of the E1 resonance in ²⁸Si and ³⁰Si isotopes is calculated within the particle–core coupling (PCC) version of the multiparticle shell model using experimental data on direct pickup reactions.     

Size and shape of the nuclear density distributions of20Ne and28Si from 104 MeV alpha particle scattering

Measured differential cross sections for the scattering of 104 MeV α particles from²⁰Ne and²⁸Si are analyzed on the basis of a deformed folding model. Rms-radii and values of the deformation parameters for the nuclear density distributions are extracted, and the effect of the size of the α particle is demonstrated. Previous discrepancies between (p,p′) and (α, α′) results are partly removed.     

The 28Si(3He,pp)29Si reaction induced by 33 MeV polarized 3He

Differential cross sections and analysing powers were measured for the ²⁸Si(³He, pp)²⁹Si reaction using a 33 MeV polarized ³He beam. The sequential breakup model successfully described the data as a function of both the emission angle and relative proton energy of the ²He cluster for several states in ²⁹Si. An unambiguous j-dependence of the analysing powers was observed and used to asign J^π values for some final states. A statistical analysis of the breakup continuum revealed that a large fraction of such events originate from a formation of the ³¹S compound system.     

The energy dependence of fusion in the 9Be+28Si system

The angular distributions of the energy spectra of the light charged particles (p, d and α) from the ⁹Be + ²⁸Si reaction have been measured in the energy range 12 ≦ E_lab ≦ 30 MeV. The particle evaporation spectra and the angular distributions were analyzed with a spin dependent statistical model. Angular distributions of ⁹Be ions elastically scattered on ²⁸Si have been measured at the energies 12 MeV, 17 MeV, 23 MeV and 30 MeV and were analysed, together with previously measured cross sections, with the optical model. The fusion cut-off angular momentum l_fus(E), the fusion cross section σ_fus(E) and the ratio σ_fus/σ_R^OM(E) were deduced. The excitation function for fusion was analyzed with the Glas and Mosel model. The parameters obtained from the fusion excitation function were compared with the corresponding ones from the ⁹Be + ²⁸Si optical-model interaction potential.     

Zu Kernreaktionen des Si28 mit 14 MeV-Neutronen

Charged particle spectra from the reactions Si²⁸(n, α), Si²⁸(n,p), Si²⁸(n, d) were measured by irradiating solid state detectors with 14 MeV neutrons. The background due to escape effects was reduced by pulse shape discrimination. From coincidence measurements between two detectors facing each other information on angular distributions and the spectrum of protons only was obtained.     

Comments on “the (3He,d) reaction on28Si”

An assertion made in a recent paper dealing with the²⁸Si(³He,d) reaction to the 1/2⁺ ground state of²⁹P, namely that the spectroscopic factor extracted in the conventional way using DWBA analyses has a strong energy dependence, is shown to be due to a complication of inconsistent DWBA analyses by various authors, rather than being due to any intrinsic feature of the²⁸Si(³He,d) reaction mechanism.     

Isotopically engineered Si and Ge for spintronics and quantum computation

Si, Ge as well as SiGe structures are the promising materials for spintronics and quantum computation due to the fact that in both crystals only one isotope (²⁹Si and ⁷³Ge) has nuclear spin. As a result, isotope engineering of Si and Ge permits to control the density of nuclear spins and vary the spin coherence time, a crucial parameter in spintronics. In the first part we discuss the NMR study of nuclear spin decoherence in Ge single crystals with different abundance of the ⁷³Ge isotope. It was observed that the slow component of the dephasing process is elongated with depletion of Ge crystal with isotope ⁷³Ge. The second part is devoted to the development of the Kane's model of nuclear spin-based quantum computer, which uses the nuclear spin of ³¹P impurity atoms in a ²⁸Si matrix as quantum bits (qubits). We discuss a new method of placing ³¹P atoms in a ²⁸Si based on neutron-transmutation-doping of isotopically engineered Si and Ge. In the proposed structure, interqubit coupling is due to indirect hyperfine interaction of ³¹P nuclear spins with electrons localized in a ²⁸Si quasi-one-dimensional nanowire, which allows one to control the coupling between distant qubits.     

The structure of28Si above 10 MeV excitation energy III: Level scheme and shell model interpretation

²⁸Si level scheme up to 14.5 MeV excitation energy is reevaluated using information from two preceding papers. It consists of approximately 250 levels which are almost completely characterized according to the quantum numbersI, π, T of the levels. The properties of positive-parity states are compared to the predictions of shell model calculations within the completes-d basis space using the unifieds-d shell Hamiltonian. A spectrum of 48 experimentalT=1 states between 9.3 and 16 MeV is reproduced with a rms deviation of only 150 keV. A calculation of radiative widths and γ-decay modes which uses free-nucleong-factors yields excellent agreement with experiment and confirms that quenching of M1 transitions is only marginal in²⁸Si. The detailed shell model analysis of theT=0 spectrum is extended to the limiting energy whereT=1 wave function admixtures, not contained in the theory, become important experimentally. This happens at 6–8 MeV above the yrast state, depending on the spin value. Altogether it appears that a spectrum of 171 levels below 14.5 MeV, which have positive or unassigned parity, is almost completely accounted for by the model. Apparent intruder states from outside thes-d shell space are observed atE _x =10 945 keV (I ^π=4⁺) and 12 860 keV (I ^π=6⁺) and are interpreted as members of aK ^π=0⁺ rotational band.     

Isobaric analogue states in28Si from (p, γ) reaction in27Al

Analogue states in²⁸Si are identified through (p, γ) reaction in²⁷Al in the energy rangeE _p =0.85–3.0 MeV. There appears hardly any indication of a mixing. Coulomb displacement energy ΔE _c for the isobaric pair²⁸Al-²⁸Si is obtained. A least squares analysis is also carried out on all the available experimental results of ΔE _c as a function of ¯Z/A ^1/3.     

Differences in the structure of isoscalar and isovector “stretched” excitations in 24Mg and 28Si

A comparison of theoretical distorted wave impulse approximation results with the experimental data for the excitation of “stretched” 6^?T = 0 and 6^?T = 1 levels in ²⁴Mg and ²⁸Si by 135 MeV protons suggests significant differences in the structure of the isoscalar and isovector levels. This is consistent with the results of a recent study of the (π, π′) reaction on ²⁸Si.     

90° excitation functions for elastic scattering of 28Si+16O and 28Si+12C

The gross structures in the excitation functions for elastic scattering of ²⁸Si+¹⁶O and ²⁸Si+ ¹²C, measured at 90°, where only even partial waves contribute, are found to be anti-correlated with those at 180°. Excitation functions at both angles, including the enhanced gross structure at 180°, can be reproduced by calculations using a potential which attracts odd partial waves more than even partial waves.     

Sub- and above-barrier fusion of loosely bound 6Li with 28Si

Fusion excitation functions are measured for the system ⁶Li + ²⁸Si using the characteristic $ \gamma$ -ray method, encompassing both the sub-barrier and above-barrier regions, viz, E _lab = 7-24 MeV. Two separate experiments were performed, one for the above-barrier region ( E _lab = 11-24 MeV) and another for the below-barrier region ( E _lab = 7-10 MeV). The results were compared with our previously measured fusion cross-section for the ⁷Li + ²⁸Si system. We observed the enhancement of the fusion cross-section at sub-barrier regions for both ⁶Li and ⁷Li , but the yield was substantially larger for ⁶Li . However, for well-above-barrier regions, a similar type of suppression was identified for both the systems.     

The 28Si(7Li, 7Be)28Al reaction at 36 MeV

The ²⁸Si(⁷Li, ⁷Be)²⁸Al reaction has been investigated at E_7Li = 36 MeV. States and groups of states were observed up to 5.3 MeV excitation in the ²⁸Al+⁷Be system. Experimental angular distribution for unresolved doublets of states at ≈ 0.0 and 0.44 MeV excitation, corresponding to ⁷Be in its ground state (⁷Be₀) and first excited states (⁷Be₁) with ²⁸Al in its ground state (3⁺) and first excited state (0.031 MeV, 2⁺) are compared with microscopic distorted wave approximation calculations.