arXiv subjects

N. Pietralla

Publications and source records attributed to N. Pietralla.

At least 73 records · Page 4Linked to original sources

arXiv2010

First Application of Pulse-Shape Analysis to Silicon Micro-Strip Detectors

The method of pulse-shape analysis (PSA) for particle identification (PID) was applied to a double-sided silicon strip detector (DSSD) with a strip pitch of 300 \{mu}m. We present the results of test measurements with particles from the reactions of a 70 MeV 12C beam impinging on a mylar target. Good separation between protons and alpha particles down to 3 MeV has been obtained when excluding the interstrip events of the DSSD from the analysis.

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arXiv2010

Transition probabilities in the X(5) candidate $^{122}$Ba

To investigate the possible X(5) character of 122Ba, suggested by the ground state band energy pattern, the lifetimes of the lowest yrast states of 122Ba have been measured, via the Recoil Distance Doppler-Shift method. The relevant levels have been populated by using the 108Cd(16O,2n)122Ba and the 112Sn(13C,3n)122Ba reactions. The B(E2) values deduced in the present work are compared to the predictions of the X(5) model and to calculations performed in the framework of the IBA-1 and IBA-2 models.

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arXiv2010

Fermionic-bosonic couplings in a weakly-deformed odd-mass nucleus, $^{93}_{41}$Nb

A comprehensive decay scheme of $^{93}$Nb below 2 MeV has been constructed from information obtained with the $^{93}$Nb(n,n$^\prime$$γ$) and $^{94}$Zr(p,2n$γ$$γ$)$^{93}$Nb reactions. Branching ratios, lifetimes, transition multipolarities and spin assignments have been determined. From $M1$ and $E2$ strengths, fermionic-bosonic excitations of isoscalar and isovector character have been identified from the weak coupling $\pi1g_{9/2}$$\otimes$$^{92}_{40}$Zr and $\pi2p_{1/2}^{-1}$$\otimes$$^{94}_{42}$Mo configurations. A microscopic interpretation of such excitations is attained from shell-model calculations using low-momentum effective interactions.

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arXiv2006

Microscopic Restoration of Proton-Neutron Mixed Symmetry in Weakly Collective Nuclei

Starting from the microscopic low-momentum nucleon-nucleon interaction V{low k}, we present the first systematic shell model study of magnetic moments and magnetic dipole transition strengths of the basic low-energy one-quadrupole phonon excitations in nearly-spherical nuclei. Studying in particular the even-even N=52 isotones from 92Zr to 100Cd, we find the predicted evolution of the predominantly proton-neutron non-symmetric state reveals a restoration of collective proton-neutron mixed-symmetry structure near mid-shell. This provides the first explanation for the existence of pronounced collective mixed-symmetry structures in weakly-collective nuclei.

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arXiv2006

Polarization and relaxation of radon

Investigations of the polarization and relaxation of $^{209}$Rn by spin exchange with laser optically pumped rubidium are reported. On the order of one million atoms per shot were collected in coated and uncoated glass cells. Gamma-ray anisotropies were measured as a signal of the alignment (second order moment of the polarization) resulting from the combination of polarization and quadrupole relaxation at the cell walls. The temperature dependence over the range 130$^\circ$C to 220$^\circ$C shows the anisotropies increasing with increasing temperature as the ratio of the spin exchange polarization rate to the wall relaxation rate increases faster than the rubidium polarization decreases. Polarization relaxation rates for coated and uncoated cells are presented. In addition, improved limits on the multipole mixing ratios of some of the main gamma-ray transitions have been extracted. These results are promising for electric dipole moment measurements of octupole-deformed $^{223}$Rn and other isotopes, provided sufficient quantities of the rare isotopes can be produced.

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arXiv2006

Gamma-soft Analog of the Confined Beta-soft Rotor Model

A gamma-soft analog of the confined beta-soft (CBS) rotor model is developed, by using a gamma-independent displaced infinite well beta-potential in the Bohr Hamiltonian, for which exact separation of variables is possible. Level schemes interpolating between the E(5) critical point symmetry (with R(4/2)=E(4)/E(2)= 2.20) and the O(5) gamma-soft rotor (with R(4/2)=2.50) are obtained, exhibiting a crossover of excited 0+ bandheads which leads to agreement with the general trends of first excited 0+ states in this region and is observed experimentally in 128-Xe and 130-Xe.

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arXiv2006

Identification of mixed-symmetry states in an odd-mass nearly-spherical nucleus

The low-spin structure of 93Nb has been studied using the (n,n' gamma) reaction at neutron energies ranging from 1.5 to 3.0 MeV and the 94Zr(p,2n gamma)93Nb reaction at bombarding energies from 11.5 to 19 MeV. States at 1779.7 and 1840.6 keV, respectively, are proposed as mixed-symmetry states associated with the coupling of a proton hole in the p_1/2 orbit to the 2+_1,ms state in 94Mo. These assignments are derived from the observed M1 and E2 transition strengths to the symmetric one-phonon states, energy systematics, spins and parities, and comparison with shell model calculations.

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arXiv2006

The i11/2 f5/2 and i11/2 p3/2 neutron particle-hole multiplets in 208Pb

Inelastic proton scattering via isobaric analog resonances allows to derive rather complete information about neutron particle-hole states. We applied this method to the doubly-magic nucleus 208Pb by measuring angular distributions of 208Pb(p, p') on top of the isobaric analog resonances in 209Bi with the Q3D magnetic spectrograph at München. We identify the six states of the i11/2 f5/2 multiplet and the four states of the i11/2 p3/2 multiplet in the energy range 4.6 MeV < Ex < 5.3 MeV. Firm spin assignments for the ten states are given, some of them new. Additional measurements of the reaction 207Pb(d, p) confirm the fragmented i11/2 p1/2 multiplet.

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arXiv2005

Parity assignments in 172,174Yb using polarized photons and the K quantum number in rare earth nuclei

The 100 % polarized photon beam at the High Intensity gamma-ray Source (HIgS) at Duke University has been used to determine the parity of six dipole excitations between 2.9 and 3.6 MeV in the deformed nuclei 172,174 Yb in photon scattering (g,g') experiments. The measured parities are compared with previous assignments based on the K quantum number that had been assigned in Nuclear Resonance Fluorescence (NRF) experiments by using the Alaga rules. A systematic survey of the relation between gamma-decay branching ratios and parity quantum numbers is given for the rare earth nuclei.

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arXiv2003

Experimental evidence for 56Ni-core breaking from the low-spin structure of the N=Z nucleus 58Cu

Low-spin states in the odd-odd N=Z nucleus 58Cu were investigated with the 58Ni(p,n gamma)58Cu fusion evaporation reaction at the FN-tandem accelerator in Cologne. Seventeen low spin states below 3.6 MeV and 17 new transitions were observed. Ten multipole mixing ratios and 17 gamma-branching ratios were determined for the first time. New detailed spectroscopic information on the 2+,2 state, the Isobaric Analogue State (IAS) of the 2+,1,T=1 state of 58Ni, makes 58Cu the heaviest odd-odd N=Z nucleus with known B(E2;2+,T=1 --> 0+,T=1) value. The 4^+ state at 2.751 MeV, observed here for the first time, is identified as the IAS of the 4+,1,T=1 state in 58Ni. The new data are compared to full pf-shell model calculations with the novel GXPF1 residual interaction and to calculations within a pf5/2 configurational space with a residual surface delta interaction. The role of the 56Ni core excitations for the low-spin structure in 58Cu is discussed.

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arXiv2003

Investigation of low spin states in 48-Cr with the MINIBALL spectrometer

Low spin states in the self-conjugate even-even nucleus 48-Cr were investigated using the MINIBALL gamma-ray spectrometer. At the FN tandem accelerator in Cologne the 46-Ti(3-He,n) reaction was used for the measurement of gamma-gamma coincidences for an excitation function from 7 to 12 MeV beam energy. 17 excited states were observed, nine for the first time by means of gamma-ray spectroscopy, and new spin assignments were made. No excited states apart from the ground band were observed below 3.4 MeV.

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arXiv2002

Extended M1 sum rule for excited symmetric and mixed-symmetry states in nuclei

A generalized M1 sum rule for orbital magnetic dipole strength from excited symmetric states to mixed-symmetry states is considered within the proton-neutron interacting boson model of even-even nuclei. Analytic expressions for the dominant terms in the B(M1) transition rates from the first and second $2^+$ states are derived in the U(5) and SO(6) dynamic symmetry limits of the model, and the applicability of a sum rule approach is examined at and in-between these limits. Lastly, the sum rule is applied to the new data on mixed-symmetry states of 94Mo and a quadrupole d-boson ratio $nd(0^+_1)/nd(2^+_2) \approx 0.6$ is obtained in a largely parameter-independent way

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arXiv2001

Quasideuteron states with deformed core

The M1 transitions between low-lying T=1 and T=0 states in deformed odd-odd N=Z nuclei are analyzed in the frames of the rotor-plus-particle model. Using the representation of an explicit coupling of angular momenta we show that strong coupling of the quasideuteron configurations to the axially deformed core results in a distribution of the total 0+ --> 1+ strength among a few low-lying 1+ states. Simple analytical formulae for B(M1) values are derived. The realization of the M1 sum rule for the low-lying 1+,T=0 states is indicated. The calculated B(M1) values are found to be in good agreement with experimental data and reveal specific features of collectivity in odd-odd N=Z nuclei.

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arXiv2000

Quasideuteron configurations in 46V and 58Cu

The data on low spin states in the odd-odd nuclei 46V and 58Cu investigated with the 46Ti(p,ngamma)46V, 32S(16O,pn)46V and 58Ni(p,ngamma)58Cu reactions at the FN-TANDEM accelerator in Cologne are reported. The states containing large quasideuteron components are identified from the strong isovector M1 transitions, from shell model calculations and from experimental data for low-lying states.

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arXiv2000

Low-Spin Spectroscopy of 50Mn

The data on low spin states in the odd-odd nucleus 50Mn investigated with the 50Cr(p,ngamma)50Mn fusion evaporation reaction at the FN-TANDEM accelerator in Cologne are reported. Shell model and collective rotational model interpretations of the data are given.

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arXiv2000

Shell model description of "mixed-symmetry" states in 94Mo

Shell model calculations have been performed for the nucleus 94Mo. The calculated excitation energies and electromagnetic properties of low-lying states are in good agreement with the data, which include states with mixed-symmetry (MS) assignments in previous interacting boson model studies. In the shell model large isoscalar E2 matrix elements are found between states with MS assignments indicating that they form a class of states with similar proton-neutron symmetry.

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arXiv2000

Quadrupole shape invariants in the interacting boson model

In terms of the Interacting Boson Model, shape invariants for the ground state, formed by quadrupole moments up to sixth order, are studied in the dynamical symmetry limits and, for the first time, over the whole structural range of the IBM-1. The results are related to the effective deformation parameters and their fluctuations in the geometrical model. New signatures that can distinguish vibrator and gamma-soft rotor structures, and one that is related to shape coexistence, are identified.

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arXiv2000

Search for the electric dipole excitations to the $3s_{1/2} \otimes [2^{+}_{1} \otimes 3^{-}_{1}]$ multiplet in $^{117}$Sn

The odd-mass $^{117}$Sn nucleus was investigated in nuclear resonance fluorescence experiments up to an endpoint energy of the incident photon spectrum of 4.1 MeV at the bremsstrahlung facility of the Stuttgart University. More than 50 mainly hitherto unknown levels were found. From the measurement of the scattering cross sections model independent absolute electric dipole excitation strengths were extracted. The measured angular distributions suggested the spins of 11 excited levels. Quasi-particle phonon model calculations including a complete configuration space were performed for the first time for a heavy odd-mass spherical nucleus. These calculations give a clear insight in the fragmentation and distribution of the $E1$, $M1$, and $E2$ excitation strength in the low energy region. It is proven that the $1^{-}$ component of the two-phonon $[2^{+}_{1} \otimes 3^{-}_{1}]$ quintuplet built on top of the $1/2^{+}$ ground state is strongly fragmented. The theoretical calculations are consistent with the experimental data.

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