Coulomb-excitation experiments are performed with postaccelerated beams of neutron-deficient Po196,198,200,202 isotopes at the REX-ISOLDE facility. A set of matrix elements, coupling the low-lying states in these isotopes, is extracted. In the two heaviest isotopes, Po200,202, the transitional and diagonal matrix elements of the 2+1 state are determined. In Po196,198 multistep Coulomb excitation is observed, populating the 4+1,0+2, and 2+2 states. The experimental results are compared to the results from the measurement of mean-square charge radii in polonium isotopes, confirming the onset of deformation from Po196 onwards. Three model descriptions are used to compare to the data. Calculations with the beyond-mean-field model, the interacting boson model, and the general Bohr Hamiltonian model show partial agreement with the experimental data. Finally, calculations with a phenomenological two-level mixing model hint at the mixing of a spherical structure with a weakly deformed rotational structure.We acknowledge the support of the ISOLDE Collaboration and technical teams and, especially, the support of RILIS and REX. This work was supported by FWO-Vlaanderen (Belgium), by GOA/2010/010 (BOF KU Leuven), by the Interuniversity Attraction Poles Programme initiated by the Belgian Science Policy Office (BriX network P7/12), by the European Commission within the Seventh Framework Programme through I3-ENSAR (Contract No. RII3-CT-2010-262010), by the German BMBF under Contract Nos. 05P12PKFNE, 06DA9036I 05P12RDCIA, and 05P12RDCIB, by the UK Science and Technology Facilities Council, by the Spanish MINECO under Project No. FIS2011-28738-C02-02, by Narodowe Centrum Nauki (Polish Center for Scientific Research) Grant No. UMO-2013/10/M/ST2/00427, by the Academy of Finland (Contract No. 131665), and by the European Commission through the Marie Curie Actions call PIEFGA-2008-219174 (J.P.)

Exclusive cross sections and momentum distributions have been measured for quasifree one-neutron knockout reactions from a 54 Ca beam striking on a liquid hydrogen target at ∼200 MeV=u. A significantly larger cross section to the p 3=2 state compared to the f 5=2 state observed in the excitation of 53 Ca provides direct evidence for the nature of the N ¼ 34 shell closure. This finding corroborates the arising of a new shell closure in neutron-rich calcium isotopes. The distorted-wave impulse approximation reaction formalism with shell model calculations using the effective GXPF1Bs interaction and ab initio calculations concur our experimental findings. Obtained transverse and parallel momentum distributions demonstrate the sensitivity of quasifree one-neutron knockout in inverse kinematics on a thick liquid hydrogen target with the reaction vertex reconstructed to final state spin-parity assignments.

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Detailed spectroscopy of the neutron-unbound nucleus 28 F has been performed for the first time following proton/neutron removal from 29 Ne/ 29 F beams at energies around 230 MeV/nucleon. The invariant-mass spectra were reconstructed for both the 27 F ( * ) + n and 26 F ( * ) + 2n coincidences and revealed a series of well-defined resonances. A near-threshold state was observed in both reactions and is identified as the 28 F ground state, with Sn( 28 F) = −199(6) keV, while analysis of the 2n decay channel allowed a considerably improved Sn( 27 F) = 1620(60) keV to be deduced. Comparison with shell-model predictions and eikonal-model reaction calculations have allowed spin-parity assignments to be proposed for some of the lower-lying levels of 28 F. Importantly, in the case of the ground state, the reconstructed 27 F+n momentum distribution following neutron removal from 29 F indicates that PACS numbers:

We report the first measurement of low-energy proton-capture cross sections of 124 Xe in a heavyion storage ring. 124 Xe 54+ ions of five different beam energies between 5.5 AMeV and 8 AMeV were stored to collide with a windowless hydrogen target. The 125 Cs reaction products were directly detected. The interaction energies are located on the high energy tail of the Gamow window for hot, explosive scenarios such as supernovae and X-ray binaries. The results serve as an important test of predicted astrophysical reaction rates in this mass range. Good agreement in the prediction of the astrophysically important proton width at low energy is found, with only a 30% difference between measurement and theory. Larger deviations are found above the neutron emission threshold, where also neutron-and γ-widths significantly impact the cross sections. The newly established experimental method is a very powerful tool to investigate nuclear reactions on rare ion beams at low center-of-mass energies.Charged-particle induced reactions like (p,γ) and (α,γ) and their reverse reactions play a central role in the quantitative description of explosive scenarios like supernovae [1] or X-ray binaries [2], where temperatures above 1 GK can be reached. The energy interval in which the reactions most likely occur under astrophysical conditions is called the Gamow window [3,4]. Experimentalists usually face two major challenges when approaching the Gamow window: firstly, the relatively low center-of-mass energies of only a few MeV or less, and secondly, the rapid decrease of cross sections with energy. The high stopping power connected to low-energy beams typically limits the amount of target material, and thus the achievable luminosity. A measurement of small cross sections, on the contrary, requires high luminosities.The description of charged-particle processes in explosive nucleosynthesis -e.g., the γ process occurring in core-collapse and thermonuclear supernovae [5-7] and the rp process on the surface of mass-accreting neutron stars [8] -requires large reaction networks including very short-lived nuclei. Experimental data are extremely scarce [9], especially in the mass region A > 70, and the modelling relies on calculated cross sections. It is therefore essential to test the theory and its central input parameters. In this Letter we report the first study of the 124 Xe(p,γ) 125 Cs reaction. The cross section is measured on the high energy tail of the Gamow peak, which is located between 2.74 and 5.42 MeV at 3.5 GK in the γ process [4]. While the 124 Xe(p,γ) reaction serves as a major milestone for improving the experimental technique