The very neutron-rich oxygen isotopes 25

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The degree of collectivity in the neutron-rich nuclei 32 Mg and 34 Mg has been determined via intermediateenergy Coulomb excitation in inverse kinematics. Measured energies of the first excited 2 + states and reduced electric quadrupole transition probabilities B(E2; 0 + g.s. → 2 + 1 ) are presented for 32 Mg and 34 Mg. The results agree with previous measurements and confirm the placement of both nuclei within the "island of inversion."

The 112,120 Sn(γ, γ ) reactions below the neutron separation energies have been studied at the superconducting Darmstadt electron linear accelerator S-DALINAC for different endpoint energies of the incident bremsstrahlung spectrum. Dipole strength distributions are extracted for 112 Sn up to 9.5 MeV and for 120 Sn up to 9.1 MeV. A concentration of dipole excitations is observed between 5 and 8 MeV in both nuclei. Missing strength due to unobserved decays to excited states is estimated in a statistical model. A fluctuation analysis is applied to the photon scattering spectra to extract the amount of the unresolved strength hidden in background due to fragmentation. The strength distributions are discussed within different model approaches such as the quasiparticle-phonon model and the relativistic time blocking approximation allowing for an inclusion of complex configurations beyond the initial particle-hole states. While a satisfactory description of the fragmentation can be achieved for sufficently large model spaces, the predicted centroids and total electric dipole strengths for stable tin isotopes strongly depend on the assumptions about the underlying mean field.

Quasifree one-proton knockout reactions have been employed in inverse kinematics for a systematic study of the structure of stable and exotic oxygen isotopes at the R^{3}B/LAND setup with incident beam energies in the range of 300-450  MeV/u. The oxygen isotopic chain offers a large variation of separation energies that allows for a quantitative understanding of single-particle strength with changing isospin asymmetry. Quasifree knockout reactions provide a complementary approach to intermediate-energy one-nucleon removal reactions. Inclusive cross sections for quasifree knockout reactions of the type ^{A}O(p,2p)^{A-1}N have been determined and compared to calculations based on the eikonal reaction theory. The reduction factors for the single-particle strength with respect to the independent-particle model were obtained and compared to state-of-the-art ab initio predictions. The results do not show any significant dependence on proton-neutron asymmetry.

The reaction 9 Be 28 Mg; 26 Ne X has been studied at 82 MeV=nucleon together with two similar cases, 30 Mg and 34 Si. Strong evidence that the reactions are direct is offered by the parallel-momentum distributions of the reaction residues and by the inclusive cross sections. The pattern of the partial cross sections for 28 Mg suggests the presence of correlations. A preliminary theoretical discussion based on eikonal reaction theory and the many-body shell model is presented. The reaction holds great promise for the study of neutron-rich nuclei.

The nucleosynthesis of heavy proton-rich nuclei in a stellar photon bath at temperatures of the astrophysical γ-process was investigated where the photon bath was simulated by the superposition of bremsstrahlung spectra with different endpoint energies. The method was applied to derive (γ,n) cross sections and reaction rates for several platinum isotopes.PACS numbers: 26.30.+k, 98.80.Ft, 26.45.+h The trans-iron nuclei have been synthesized by neutron capture in the s-and r-processes, except the p-nuclei (p for proton-rich), with relative abundances of the order of 0.01 to 1% [1]. The main production mechanism of the p-nuclei is assumed to be photodisintegration in the γ-process, i.e. by (γ,n), (γ,p), and (γ,α) reactions induced on heavier seed nuclei synthesized in the s-and r-processes. Typical parameters for the γ-process are temperatures of 2 ≤ T 9 ≤ 3 (T 9 is the temperature in units of 10 9 K), densities ρ ≈ 10 6 g/cm 3 , and time scales τ in the order of seconds. Several astrophysical sites for the γ-process have been proposed, whereby the oxygenand neon-rich layers of type II supernovae seem to be good candidates. However, no definite conclusions have been reached yet [1], predominantly due to the lack of experimental data for the cross sections and reaction rates of the γ-induced reactions at astrophysically relevant energies. All reaction rates have been derived theoretically using statistical model calculations [1-6].The energy distribution of a thermal photon bath at a temperature T is given by the Planck distributionwhere n γ (E, T ) is the number of γ-rays at energy E per unit of volume and energy interval. In a photon-induced reaction B(γ,x)A the distribution leads to a temperature dependent decay rate λ(T ) of the initial nucleus Bwith the speed of light c and the cross section of the γ-induced reaction σ (γ,x) (E). Obviously, λ is also the production rate of the residual nucleus A. In the following we will focus on photodisintegration by the (γ,n) reactions. A large number of (γ,n) cross sections has been measured over the years [7,8]. However, most of the data have been obtained around the giant dipole resonance (GDR), i.e. at energies much higher than those in stars, and practically no data exist for the p-nuclei. The integrand in Eq. (2) is given by the product of the γ flux c n γ (E, T ), which decreases steeply with increasing energy E, and the cross section σ (γ,x) (E), which increases with E approaching the GDR region. The product leads then to a window at an effective energy E eff with a width ∆ similar to the Gamow window for charged-particle-induced reactions. If one assumes a typical threshold behavior of the (γ,n) cross section close to the threshold energy E thr , the effective energy is approximately given by E eff = E thr + 1 2 kT , and the typical width ∆ is in the order of 1 MeV (Fig. 1). 191 Pt reaction (E thr = 8676 keV) in a thermal photon bath with temperature T9 = 2.5. The Planck distribution nγ (E, T ) (dotted line) and the (γ,n) cross section σ(E) (dashed line) are giv...

Intermediate-energy Coulomb excitation is a key method to investigate collectivity in exotic nuclei far from ␤ stability. We report on the measurement of the absolute B(E2;0 1 ϩ →2 1 ϩ ) excitation strength in 46 Ar for five different minimum impact parameters. Our findings underline the validity, feasibility, and perspective of this technique for the study of exotic nuclei also in the regime of higher beam energy.