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In the meteorites trace elements have been determined by instrumental and radiochemical N.A.A. and the cosmogenic radioactivity of ²²Na, ²⁶Al, ⁴⁸V, and ⁵⁴Mn are measured by low-level gamma spectrometry in the two meteorites Hohenlangenbeck and Trebbin fallen November 1985 and March 1988 onto the territory of the GDR. The contents of Rb and Cs in the meteorite Hohenlangenbeck are relatively high indicating a low condensation temperature. Becuase of the small activity ratio of ²²Na/²⁶Al a lower particle flux in its orbit during the last five years is assumed.     

Total strength of the magnetic dipole resonance in <Superscript>27</Superscript>Al

The γ decay of the resonance-like structure observed in the ²⁶Mg(pγ)²⁷ Al reaction in the energy range E _p = 0.8–3.0 MeV of accelerated protons has been investigated. The M1 resonance on the ground and excited states of ²⁷Al with E* = 844 and 1014 keV is identified. The total strength of the M1 resonance on the ground state of this nucleus is determined. The position and total strength of this resonance are explained taking into account pairing forces.     

Search for low-energy resonances in 27Al(p, α)24Mg

The known low-energy resonances in ²⁷Al(p, γ)²⁸Si at e_R = 203, 223, 293 and 327 keV have been searched for in the ²⁷Al(p, α)²⁴Mg reaction. The observed upper limits for the resonance strength together with the results from previous work show that the MgAl cycle is not closed at all stellar temperatures of interest and that in the important temperature range near T₉ = 0.1 the (p, γ) reaction proceeds at least 100 times faster than the (p, α) reaction. The results might be of importance for the explanation of extinct ²⁶Al in meteoritic samples as well as of the 4 solar masses of ²⁶Al found in the interstellar medium via γ-astronomy.     

Two-proton radioactivity search

Two-proton radioactivity, a spontaneous breakup of elements with emission of two protons, was predicted to exist near the proton drip line by V.I. Goldansky long time ago. The recent theoretical and experimental progress in a search for such an exotic nuclear decay is reviewed. In theory, the new three-body model which treats two-proton radioactivity as a genuine three-particle nuclear decay is considered. In experiment, the first evidence for two-proton decay of ⁴⁵Fe is described. Four atoms of ⁴⁵Fe, produced at the fragment separator of GSI, decayed via particle emission with a total energy of 1.1(1) MeV and a half-life of 3.2 _?1.0 ^+2.6 ms. A possible experiment for a direct observation of two-proton emission from the ground state of ¹⁹Mg is considered for its decay in-flight. The half-life of ¹⁹Mg, as well as proton-proton correlations, might be derived from the distribution of the ¹⁹Mg decay vertices extrapolated from the measured trajectories of all fragments.     

The β-decay of 22Al

In an experiment performed at the LISE3 facility of GANIL, we studied the decay of ²²Al produced by the fragmentation of a ³⁶Ar primary beam. A β-decay half-life of T_1/2 = 91.1±0.5ms was measured. The β-delayed one- and two-proton emission as well as β-α and β-delayed γ-decays were measured and allowed us to establish a partial decay scheme for this nucleus. New levels were determined in the daughter nucleus ²²Mg. The comparison with model calculations strongly favours a spin-parity of I^π = 4⁺ for the ground state of ²²Al.     

Evidence for critical angular momenta in the formation of 26Al via the 14N + 12C channel

States in ²⁰Ne have been studied through the ¹²C(¹⁴N, ⁶Li)²⁰Ne reaction. Excitation functions have been measured from 20 MeV to 60 MeV in steps of 5 MeV at different angles for ²⁰Ne states up to 10 MeV excitation energy. States of ²⁴Mg have been also populated using the ¹²C(¹⁴N, d)²⁴Mg reaction; excitation functions of ²⁴Mg states up to 9 MeV excitation energies as well as angular distributions at 35 MeV bombarding energy have been obtained. Comparisons of data with Hauser-Feshbach calculations show clearly that the compound nucleus mechanism is the main process for both ¹²C(¹⁴N, ⁶Li)²⁰Ne and ¹²C(¹⁴N, d)²⁴Mg reactions. Strong evidence has been provided for inhibition of the ²⁶Al compound nucleus formation for angular momenta higher than critical values. The location of the yrast line in the ²⁶Al nucleus is discussed.     

The photoproton decay of 27Al

Photoproton spectra from ²⁷Al were measured from 14.8 to 27.6 MeV excitation energy in 400 keV steps. From these, high resolution photoproton cross sections to low-lying states of the residual ²⁶Mg nucleus were deduced. The large integrated cross sections to these states justify the interpretation of the de-excitation γ-ray measurements which consistently indicate strong population of low-lying residual states following photodisintegration. Further, the results are discussed with reference to the particle-hole model. By examination of the microscopic configurations of the possible GDR states an explanation is proposed for the differences in the cross sections to various residual states. Finally, the results are compared with spectroscopic factors determined from pickup reactions on ²⁷Al.     

Fusion Reaction of ^16O＋^14N and Its Implication for the Production of ^26Al in Explosive Oxygen Burning

We suggest that the fusion reaction ^16O＋^14N may be a new way to produce ^26Al in interstellar medium. Adopting different mixing modes, we investigate the impact on the production of ^26Al in explosive oxygen burning and find that the result is extremely sensitive to mixing mechanisms. In some cases, we obtain an encouraging result, for example, the greatest final abundance of ^26Al reaches 7.779×10^-6, which means that the explosive oxygen burning may be a new origin of ^26 Al.     

Short lifetimes in 26Al

Mean lifetimes of levels in ²⁶Al have been measured using the Doppler-shift attenuation (DSA) method and the reaction ²⁵Mg(p, γ)²⁶Al. The lifetime values or limits were determined for 34 bound levels below the excitation energy of 6 MeV; the lifetimes of 13 levels and upper limits of 3 levels are reported for the first time. For the effective stopping of recoils, the targets were prepared by implanting ²⁵Mg into Ta backings. The Monte Carlo method and the experimental stopping power were used in the DSA analysis.     

The resonance-like structure observed in the <Superscript>25</Superscript>Mg(p, γ)<Superscript>26</Superscript>Al reaction

The gamma decay of resonance-like structure (RLS) observed in the ²⁵Mg(p, γ)²⁶Al reaction in the region of excitation energies of 7–9 MeV is studied, and the excitation function of this reaction is measured. The resonance transition strengths of states in the 1.4–2.0 MeV range of accelerated proton energies are determined. The resulting strength distributions of M1 transitions between the resonance states and the low-lying bound states in ²⁶Al are of a resonance nature. The position of the center of gravity of magnetic dipole resonance for the ground state in ²⁶Al is 7.92 MeV, and the total MDR transition strength is 5.7 MeV × μ _N ² .     

The decay of the giant dipole resonance in 26Mg to T = 32 states of 25Mg

The population of $\text{T =}\text{3}\text{2}$ states in ²⁵Mg from the giant dipole resonance in ²⁶Mg was observed by the ²⁶Mg (γ, nγ′) reaction. The previously reported result, that neutron decay between 18.9 MeV and 23.0 MeV proceeds mainly to these states, could not be confirmed. Therefore the isospin splitting of the giant resonance of ²⁶Mg remains questionable.     

Constraints on the universe as a numerical simulation

The level scheme above the proton threshold in ²⁶Si is crucial for evaluating the ²⁵Al(p, γ)²⁶Si stellar reaction, which is important for understanding the astrophysical origin of the long-lived cosmic radioactivity ²⁶Al(T _1/2 = 7.17 × 10⁵ y) in the Galaxy. The excited states in ²⁶Si have been studied using an in-beam γ-ray spectroscopy technique with the ²⁴Mg(³He, nγ)²⁶Si reaction. γ-rays with energies up to 4.6 MeV emitted from excited states in ²⁶Si have been measured using large volume HPGe detectors. The spin-parity of one of the most important states reported recently at 5890.0keV has been assigned as 0⁺ by γ-γ angular correlation measurements in this work.     

Hydrogen burning of 24, 25, 26Mg in explosive carbon burning

The absolute resonance strengths of the ^24–26Mg(p, γ)^25–27Al reactions have been studied. The absolute values S_c.m.=1.03 ± 0.11, 2.3 ± 0.2 and 2.2 ± 0.2 eV for the frequently used standard resonances at E_p=823, 434 and 454 keV in the reactions ^24–26Mg(p, γ)^25–27Al, respectively, have been determined. In addition, absolute values for use as standards were determined for the 684 keV ²⁵Mg(p, γ)²⁶Al and 840 and 1966 keV ²⁶Mg(p, γ)²⁷Al resonances. New relative values were determined for the strengths of the if²⁵Mg(p, γ)²⁶Al resonances at ifE_{p = 0.3–1.7 MeV} and the ²⁶Mg(p, γ)²⁷Al resonances at E_p = 0.3–2.1 MeV. The resonance energies and total widths obtained in the present work are also given. The branching ratios and angular distributions of the γ-rays used in the absolute strength determinations were obtained in the same set-up as the absolute strength measurement. Using the revised resonance strengths, the astrophysical reaction rates of hydrogen burning of ^24–26Mg in explosive carbon burning in the stellar temperature region of T = (0.1–5.0) × 10⁹ K are compared with Hauser-Feshbach calculations. The present reaction rates are also discussed in the frame of the MgAl cycle.     

Lifetime measurements in 29 Al

The lifetimes of the first six excited states of ²⁹Al have been measured by the DSA method with two different reactions,²⁶Mg(α, p)²⁹Al and²⁷Al(t, p)²⁹Al. The recoil distance method was also used to remeasure the mean lifetime of the first excited state, leading to a value τ_m = 6.5 ± 0.5 ps. Experimental results are compared with shell model calculations.     

A study of the 26Mg(12C, 12B)26Al charge-exchange reaction

The reaction ²⁶Mg(¹²C, ¹²B)²⁶A1(5⁺, 3⁺) has been studied using a beam of 102 MeV of ¹²C. Shell-model, microscopic direct model and finite-range coupled reaction channel (CRC) calculations including recoil effects, have been performed, for comparison with the experimental data. DWBA calculations were performed for the intermediate states of interest in the ¹¹B + ²⁷Al and in the ¹³C + ²⁵Mg channels and these results were also compared with the experimental ones. The dominant reaction mechanism for ²⁶Mg(¹²C, ¹²B)²⁶Al(5⁺, 3⁺) appears to be the sequential mode.     

Coupled-channel description of the photodisintegration of 24Mg, 28Si, and 32S nuclei within the intermediate-coupling scheme

The coupled-channel method is used to calculate photonuclear reactions on ²⁴Mg, ²⁸Si, and ³²S nuclei within the intermediate-coupling scheme. The origin of relatively narrow peaks corresponding to photoabsorption on these nuclei is studied. Partial channels of giant-dipole-resonance decay are considered. It is shown that the splitting of the giant dipole resonance in the ²⁴Mg nucleus into two broad maxima is due to the deformation of its surface.     

Some proposals of new experiments on cluster radioactivity

Measurement of elastic scattering and fusion-fission cross sections of the cluster decay products is proposed for the study of the mechanism of cluster radioactivity. The obtained data on ¹²C+²⁰⁸Pb allow to select between different theoretical models. The experiment on search of the exotic nucleus ¹¹²Ba and its cluster decay is discussed.     

Evidence for excited states in 95Ag

The first evidence for excited states in ⁹⁵Ag is presented. ⁹⁵Ag is the heaviest T _z = 1/2 nucleus for which gamma-rays have been identified. The reaction ⁴⁰Ca(⁵⁸Ni, 1p2n)⁹⁵Ag was used in the experiment, which resulted in the assignment of three gamma-rays to ⁹⁵Ag. A detector system consisting of the detector arrays Euroball, Neutron Wall and Euclides was used to detect gamma-rays, neutrons and charged particles, respectively. Received: 31 May 2002 / Accepted: 8 July 2002     

Proton threshold states in 26Al

The energy levels of ²⁶Al between E_x = 6.3 and 6.5 MeV, corresponding to proton threshold energies in the ²⁵Mg + p reaction from $\text{E}{}_{\mathrm{<mtext>p</mtext>}}\text{= 0}\text{to}\text{200}\text{keV}\text{, have been investigated using the reactions}{}^{27}\text{Al(su3}$He, α)²⁶Al and ²⁴Mg(³He, p)²⁶Al. Despite early work reporting a doublet at E_x = 6346 keV and E_x = 6362 keV, most subsequent work reported a single state with conflicting spin and parity assignments. Our work clearly establishes the presence of the doublet and resolves the conflicts. We find that there are six states in this excitation energy region, including a new state at E_x = 6410 ± 5 keV. The l-values leading to possible spin and parity assignments for all these states have been made using DWBA. We conclude however that only three of these six states may contribute to the production of ²⁶Al by the ²⁵Mg(p, γ) reaction in the MgAl cycle and the new structure of ²⁶Al reported here can substantially increase the production rate of ²⁶Al in stellar reactions.     

A DWBA formalism for knockout-exchange reactions of composite nuclei

A DWBA formulation of the knockout-exchange scattering of composite projectiles having zero internal orbital angular momentum is presented. The model problem assumes a target nucleus heavy enough to allow the neglect of its recoil. The formalism accommodates any form of nucleon-nucleon potential containing central, tensor and spin-orbit terms which are treated on the same footing, with no multipole expansions. The nonlocal radial integrals assume transition nucleon exchange density matrices characteristic of finite potential well shell-model target and cluster model projectile wave functions. The usefulness of the formalism is demonstrated with a PWBA numerical application to the ²⁶Mg(⁶Li, ⁶He)²⁶Al reaction.