16O-8Be break-up states and cluster structure of 24Mg

High-spin states of ²⁴Mg produced in the ¹⁶O + ¹²C interaction and decaying into the ¹⁶O_g.s. + ⁸Be_g.s. channel have been observed in the excitation region between 35 and 52 MeV. Spins have been assigned on the basis of the analysis of the measured angular correlations. Some of these states with positive parity correspond to the known resonances of the ¹²C( ¹²C, ⁸Be_g.s.) ¹⁶O reaction belonging to the ¹⁶O-2α rotational band of ²⁴Mg. Moreover other resonances show up at higher excitation energy with an energy-spin relationship again suggesting a ¹⁶O-2α cluster structure for the associated configuration. Received: 6 July 2001 / Accepted: 16 October 2001     

Dirac coupled-channels analysis of inelastic scattering of 800 MeV polarized protons from 16O, 24Mg and 26Mg

Coupled-channels calculations using the Dirac phenomenology have been performed to analyse cross sections and analyzing power data for inelastic scattering of 800 MeV protons from low-lying states in ¹⁶O, ²⁴Mg and ²⁶Mg. Large negative reals scalar potentials, large positive real vector potentials and large negative imaginary vector potentials were obtained. Considerable improvements were obtained using the Dirac equation compared to the classical Schrödinger calculations. These best fits have been found compatible with an imaginary scalar potential equal to zero (U_sⁱ=0) and therefore with less parameters (nine) than usually required in the classical calculations (twelve).     

Nonstatistical structures in the excitation functions of20Ne(16O,12C)24Mg and20Ne(16O,20Ne)16O reactions

The data on the excitation functions of²⁰Ne(¹⁶O,¹²C)²⁴Mg,²⁰Ne(¹⁶O,¹²C)²⁴Mg^*(1.37, 2⁺),²⁰Ne(¹⁶O,¹²C)²⁴Mg^*(4.12, 4⁺+4.24, 2⁺) +²⁰Ne(¹⁶O,¹²C^*(4.44, 2⁺))²⁴Mg,²⁰Ne(¹⁶O,¹²C)²⁴Mg^*(6.01, 4⁺+6.43, 0⁺),²⁰Ne(¹⁶O,²⁰Ne)¹⁶O,²⁰Ne(¹⁶O,²⁰Ne^*(1.63, 2⁺))¹⁶O, and²⁰Ne(¹⁶O,²⁰Ne^*(4.25, 4⁺))¹⁶O reactions atθ _lab=13° fromE _c.m.=22.8 to 38.6 MeV have been subjected to a statistical analysis comprising of the calculations of the distribution of cross sections, deviation functions, cross-correlation functions, summed excitation functions, cross-channel correlation coefficients and coherence widths. The analysis confirms the existence of nonstatistical structures atE _c.m.=24.6, 27.8, 31.7 and 35.5 MeV, and identifies a new structure of the same nature atE _c.m. =25.6 MeV.     

Cross section measurements for the forward elastic scattering of 13 MeV 6,7Li and 24 MeV 16O from 9Be, 12C, 16O and 28Si

Absolute elastic cross sections have been measured at 12.45°, 16.45°, 20.45° and 28.0° for targets of ⁹Be, ¹²C, ¹⁶O, ²⁸Si and ¹⁹⁷Au being bombarded by beams of 13 MeV ^6,7Li and 24 MeV ¹⁶O. The reliability of a method previously proposed for low mass multielemental analysis [1,2] depends to some extent on the precision and accuracy of the cross sections reported here. The ¹⁶O-beam reactions, except the ¹⁶O + ⁹Be reaction, are found to be consistent with the Rutherford predictions. This allows us to obtain target thickness independent cross sections with uncertainties of approximately 7% for the reactions initiated with ^6,7Li beams. Most of these cross sections are found to be non-Rutherford. Comparisons made with previous investigations give strong support to the cross section values obtained in this work.     

Three-body decays of light nuclei: 6Be, 8Li, 9Be, 12O, 16Ne, and 17Ne

The theoretical approach to the two-proton radioactivity and three-body decays developed in (L.V. Grigorenko, R.C. Johnson, I.G. Mukha, I.J. Thompson, M.V. Zhukov, Phys. Rev. Lett. 85, 22 (2000) and to be published in Phys. Rev. C) is applied to the range of light nuclear systems. We study nuclear structures, widths, and momentum correlations for the decay fragments. Strong contradictions with experiment, as well as effects of special interest, are found in ¹²O and ¹⁶Ne nuclei. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: l.grigorenko@surrey.ac.uk     

Statistical analysis of24Mg+24Mg elastic and inelastic scattering

The data on the excitation functions of²⁴Mg+²⁴Mg elastic and inelastic (²⁴Mg +²⁴Mg^*(2⁺),²⁴Mg^*(2⁺)+²⁴Mg^*(2⁺),²⁴Mg+²⁴Mg^*(4⁺),²⁴Mg^*(4⁺)+²⁴Mg^*(2⁺),²⁴Mg+²⁴Mg^*(6⁺)) scattering fromE _c.m=42 to 56 MeV have been subjected to a statistical analysis consisting of calculations of deviation function, cross-correlation function, cross-channel correlation coefficients, coherence widths, and the distribution of cross sections. On the basis of the analysis resonant structures atE _c.m=45.70, 46.65, 47.35 and 47.75 MeV have been confirmed. Two new resonant structures atE _c.m=44.55 and 50.50 MeV have been identified.     

7Be(p, γ)8 B cross section from indirect breakup experiments

The amplification of the E2 and M1 electromagnetic transitions in the radiative capture reaction ⁷Be(p, γ)⁸ B is investigated. This amplification occurs in Coulomb dissociation experiments. Interference between the excitation amplitudes of E1, E2 and M1 multipolarities leads to anisotropy effects in the angular distribution of the fragments. Dynamical coupling which could affect the direct correspondence between the Coulomb breakup and the radiative capture cross sections are studied by means of a coupled-channels approach.     

7 Be(p, γ) 8 B cross section from indirect breakup experiments

The amplification of the E2 and M1 electromagnetic transitions in the radiative capture reaction⁷ Be(p, γ)⁸ B is investigated. This amplification occurs in Coulomb dissociation experiments. Interference between the excitation amplitudes of E1, E2 and M1 multipolarities leads to anisotropy effects in the angular distribution of the fragments. Dynamical coupling which could affect the direct correspondence between the Coulomb breakup and the radiative capture cross sections are studied by means of a coupled-channels approach.     

The pair field of rotating 24Mg

The structure of the pair field of rotating ²⁴Mg is investigated for various choices of the parameters of the surface delta interaction, which was adopted as the generating two-body force. With realistic force parameters, yielding a pure T = 0 pairing at all angular momenta, the empirical yrast line up to I = 8 is well reproduced by the calculations.     

The 5v 3 bands of 18O enriched ozone: line positions of 16O16O18O, 16O18O16O, 16O18O18O and 18O16O18O

The four 5v ₃ bands of ¹⁸O enriched ozone have been observed and analysed for the first time. Two species (¹⁶O¹⁸O¹⁶O and ¹⁸O¹⁶O¹⁸O) belong to the C_2v symmetry group and two other (¹⁸O¹⁸O¹⁶O and ¹⁶O¹⁶O¹⁸O) to the C_s symmetry group. They have been recorded at a resolution of 0.008 cm^?1 with a pathlength of 32.16 m. Despite the very weak absorptions observed, almost 250 energy levels have been derived for each of the 4 species, with J ? 35 and K _a ? 13, and suitable sets of Hamiltonian parameters have been determined. For 3 species it has been necessary to account for the resonance between the (005) and (311) states to correctly reproduce the spectra observed. These resonances, anharmonic for C_2v, and hybrid (both anhar-monic and Coriolis) for C_s symmetry confirm the accidentally extremely strong coupling between the (005) and (311) states for ¹⁶O₃, due in that case to the very close distance between unperturbed energy levels. This work also confirms the excellent prediction of band centres of these four species derived from the recently determined isotopically invariant molecular potential function.     

The (12C, 8Be) reaction on 12C, 16O, 24,26Mg, 40,48Ca, 54Fe AND 58Ni between 50 and 65 MeV bombarding energy

Absolute cross sections have been measured for the (¹²C, ⁸Be_g.s.) reaction from the target nuclei ¹²C, ¹⁶O, ²⁴Mg, ²⁶Mg, ⁴⁰Ca, ⁴⁸Ca, ⁵⁴Fe and ⁵⁸Ni at various energies between 50 and 65 MeV bombarding energy (lab) using a highly efficient detection system for ⁸Be. The results are presented in form of particle spectra and angular distributions. Except for the lightest target nuclei ¹²C and ¹⁶O, the cross sections decrease rapidly with angle and a one-step direct reaction mechanism is indicated. Satisfactory agreement of the data is obtained with DWBA calculations, using the finite range computer code LOLA of DeVries which treats recoil effects exactly. The spectroscopic factors extracted for the (¹²C, ⁸Be) reaction are close to those obtained from (⁶Li, d), (⁷Li, t) and (¹⁶O, ¹²C) reactions. The selective excitation of the same final states in all of these reactions, as far as data are available, and the close agreement of the spectroscopic factors are interpreted as evidence for a rather simple α-transfer in these reactions in contrast to a more complicated transfer of four nucleons.     

Fragment angular correlation in the breakup of 16O ions at 95 MeV/A

Fragment angular correlations in projectile breakup reactions are very sensitive to interference between different multipolarities entering in the excitation-dissociation process of the projectile. In particular, it was proposed to disentangle L = 1 and L = 2 contributions in direct breakup reactions of ¹⁶O with low relative energy between the α and ¹²C fragment, which are of astrophysical interest. We studied the experimental aspects of extracting those angular correlations in extreme kinematical conditions usually encountered in breakup experiments of astrophysical interest. The breakup of 95 MeV/A ¹⁶O projectiles induced by a ²⁰⁸Pb target was measured using the high-resolution spectrometer SPEG at Ganil for the coincident detection of the fragments. Sequential breakup via the 12.53 MeV level of ¹⁶O is analyzed in this framework and it favors an one-step M2 excitation of this level.     

Ca，Be在镁合金中的阻燃作用

建立了镁合金的晶体,液态及其固/液界面模型.采用递归法计算了Ca,Be在α-Mg、固/液界面、镁液态中的环境敏感镶嵌能,定义并计算了Mg,Ca及Be与氧的原子亲和能.计算结果表明：Ca,Be在镁晶体中的环境敏感镶嵌能较高,不能稳定固溶于晶体中,因此在固体中的溶解度较小.合金凝固时Ca,Be扩散到环境能较低的液体中,向液面聚集.由于Ca,Be与氧的原子亲和能低于镁与氧的亲和能,聚集在液体表面的Ca,Be将优先与氧结合,生成致密的镁与合金元素的混合氧化物,阻止镁合金燃烧. 关键词： 电子结构 阻燃 Mg合金     

The optical properties of Be,Mg and Zn-diffused gallium phosphide

Interest in the Ga-site acceptors Be and Mg was stimulated by the possibility that they might produce efficient luminescence on association with O, analogous to the well-known red Zn-O luminescence in GaP but at higher transition energy. Attention was directed to diffusion doping by Be and Mg of GaP O-doped during growth because the reactivity of Be and Mg with O renders double doping during crystal growth very difficult. Structured green donor-acceptor pair spectra were observed at 1.6°K from many Be-diffused crystals, yielding an accurate measure of (E_A)_BE, 50 ± 1 meV. Moderately efficient orange-red luminescence was also observed below ∼ 100 °K from these crystals, but the intensity of this luminescence decreased rapidly to negligible levels by ∼ 200°K. This luminescence also contains sharp structure at 1.6°K, of a form characteristic of the decay of excitons bound to complex centres. Many sharp phonon replicas occur, involving local modes as well as characteristic GaP modes. One set of no-phonon lines, at least, near 2.19 eV, shows zero-field splitting, luminescence decay times and behaviour in magnetic and external strain fields characteristic of exciton decay at a centre with <100>; or <111>-type symmetry axes, containing no extra electronic particles. The exciton state is split by 2.4 meV by J-J coupling, and the axial field of the centre splits the hole states by ∼ 1.0 meV. These bound excitons are specifically characteristics of diffused GaP and appear analogous to bound excitons observed below 2.12 eV in Zn-diffused GaP. It is probable that the relevant centres contain diffusion components such as Be or Zn interstitials and improbable that O_P is involved. By contrast, weak orange bound exciton luminescence observed in Mg-diffused GaP does involve O, presumably as O_P. No analysis of the magneto-optical behaviour of this Mg-related bound exciton was possible in our crystals, so its symmetry axis was not established. It is possible that this is the Mg_Ga-O_P bound exciton. If so, the two-fold reductions in the exciton localisation energy from ∼ 0.32 eV to ∼ 0.15 eV and in the mass of the Ga-site substituent has produced dramatic changes in the form of the phonon cooperation between the Zn-O and “Mg-O” excitons. The “Mg-O” exciton luminescence is not dominant in our crystals, even at low temperature. The exciton state is again split by a local crystal field as well as by J-J coupling, but here the former splitting is predominant; 2_∈0 = 3.9 meV, Δ = 0.60 meV.     

Direct and sequential processes in the 16O(7Li, 7Be)16N reaction

Calculations are presented for the ¹⁶O(⁷Li, ⁷Be)¹⁶N charge exchange reaction. For the direct reaction, the tensor interaction leads to form factors with large spin and angular momentum transfer which are important for the unnatural parity states. The (⁷Li, ⁶Li)(⁶Li,⁷Be) sequential process exhibits a strong J-dependence and is the dominant contribution to the natural parity states.