P. Napiorkowski

Studies of pear-shaped nuclei using accelerated radioactive beams

There is strong circumstantial evidence that certain heavy, unstable atomic nuclei are ‘octupole deformed’, that is, distorted into a pear shape. This contrasts with the more prevalent rugby-ball shape of nuclei with reflection-symmetric, quadrupole deformations. The elusive octupole deformed nuclei are of importance for nuclear structure theory, and also in searches for physics beyond the standard model; any measurable electric-dipole moment (a signature of the latter) is expected to be amplified in such nuclei. Here we determine electric octupole transition strengths (a direct measure of octupole correlations) for short-lived isotopes of radon and radium. Coulomb excitation experiments were performed using accelerated beams of heavy, radioactive ions. Our data on 220Rn and 224Ra show clear evidence for stronger octupole deformation in the latter. The results enable discrimination between differing theoretical approaches to octupole correlations, and help to constrain suitable candidates for experimental studies of atomic electric-dipole moments that might reveal extensions to the standard model.

Experimental test of the polarization direction correlation method (PDCO)

Abstract The study of the polarization direction correlation method (PDCO) for γ quanta emitted from the nuclear states oriented in fusion-evaporation reactions is discussed with emphasis on making unique multipolarity assignments. The method is applied to the data coming from a typical experiment performed with the EUROGAM II array, where polarization-sensitive CLOVER detectors were used. The accuracy obtained in the experiment for the studied transitions was high enough to exclude, using the PDCO method, most of the ambiguities which occur if the assignments are made on the basis of angular correlation measurements alone.

Information on antiprotonic atoms and the nuclear periphery from the PS209 experiment

A. Trzcińska, J. Jastrzȩbski, T. Czosnyka, T. von Egidy, K. Gulda, F. J. Hartmann, J. Iwanicki, B. Ketzer, M. Kisieliński, B. K los, W. Kurcewicz P. Lubiński, P. J. Napiorkowski, L. Pieńkowski, R. Schmidt, E. Widmann Heavy Ion Laboratory, Warsaw University, PL-02-093 Warsaw, Poland Physik-Department, Technische Universität München, D-85747 Garching, Germany Institute of Experimental Physics, Warsaw University, PL-00-681, Warsaw, Poland Institute of Physics, University of Silesia, PL-40-007 Katowice, Poland CERN, CH-1211 Geneva 23, Switzerland

Electromagnetic structure of 98Mo

Abstract The nucleus 98 Mo was multiply Coulomb excited using 20 Ne, 84 Kr and 136 Xe beams. Eighteen E2 and M1 reduced matrix elements connecting 7 low-lying levels have been determined using the least-squares code GOSIA. The results are compared with the predictions of an extended version of the IBM1 model. The quadrupole sum rules approach was used to determine the shape parameters in two 0 + (ground and first excited) states. The rotational invariants 〈Q 2 〉 and 〈 cos 3δ〉 obtained show the shape coexistence in 98 Mo nucleus: the triaxial ground state and the prolate first excited state.

Neutron density distributions from antiprotonic Pb 208 and Bi 209 atoms

The x-ray cascade from antiprotonic atoms was studied for 208 Pb and 209 Bi. Widths and shifts of the levels due to the strong interaction were determined. Using modem antiproton-nucleus optical potentials, the neutron densities in the nuclear periphery were deduced. Assuming two-parameter Fermi distributions (2pF) describing the proton and neutron densities, the neutron rms radii were deduced for both nuclei. The difference of neutron and proton rms radii Δr nP equal to 0.16 ± (0.02) stat ± (0.04) syst fm for 208 Pb and 0.14 ± (0.04) stat ± (0.04) syst fm for 209 Bi were determined, and the assigned systematic errors are discussed. The Δr nP values and the deduced shapes of the neutron distributions are compared with mean field model calculations.

Nucleon density in the nuclear periphery determined with antiprotonic x rays: Cadmium and tin isotopes

The x-ray cascade from antiprotonic atoms was studied for 106Cd, 116Cd, 112Sn, 116Sn, 120Sn, and 124Sn. Widths and shifts of the levels due to strong interaction were deduced. Isotopic effects in the Cd and Sn isotopes are clearly seen. The results are used to investigate the nucleon density in the nuclear periphery. The deduced neutron distributions are compared with the results of the previously introduced radiochemical method and with HFB calculations.

Spectroscopic Quadrupole Moments in {96,98}Sr: Evidence for Shape Coexistence in Neutron-Rich Strontium Isotopes at N=60.

Neutron-rich {96,98}Sr isotopes have been investigated by safe Coulomb excitation of radioactive beams at the REX-ISOLDE facility. Reduced transition probabilities and spectroscopic quadrupole moments have been extracted from the differential Coulomb excitation cross sections. These results allow, for the first time, the drawing of definite conclusions about the shape coexistence of highly deformed prolate and spherical configurations. In particular, a very small mixing between the coexisting states is observed, contrary to other mass regions where strong mixing is present. Experimental results have been compared to beyond-mean-field calculations using the Gogny D1S interaction in a five-dimensional collective Hamiltonian formalism, which reproduce the shape change at N=60.

Analysis methods of safe Coulomb-excitation experiments with radioactive ion beams using the GOSIA code

Abstract.With the recent advances in radioactive ion beam technology, Coulomb excitation at safe energies becomes an important experimental tool in nuclear-structure physics. The usefulness of the technique to extract key information on the electromagnetic properties of nuclei has been demonstrated since the 1960s with stable beam and target combinations. New challenges present themselves when studying exotic nuclei with this technique, including dealing with low statistics or number of data points, absolute and relative normalisation of the measured cross-sections and a lack of complementary experimental data, such as excited-state lifetimes and branching ratios. This paper addresses some of these common issues and presents analysis techniques to extract transition strengths and quadrupole moments utilising the least-squares fit code, GOSIA.

El, E2, E3 and M1 information from heavy ion coulomb excitation

Abstract The richness of information pertaining to El, E2, E3 and M1 interaction deduced from Coulomb excitation experiments is illustrated. E2 and particularly M1 transition probabilities in 128 Xe are presented. Large set of E3 and E1 additionally to E2 transition probabilities in 226 Ra are shown.

Antiprotonic investigation of the nuclear periphery

Some results inferred from a program devoted to the study of the nuclear periphery using antiprotonic atoms are presented.