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Abstract A new device, the GALILEO plunger, has been developed for lifetime measurement studies using the GALILEO γ -ray detector array at the Laboratori Nazionali di Legnaro (LNL) in Legnaro, Italy. This plunger device holds two stretched metal foils in the beam path for the use of the recoil distance Doppler-shift technique in order to measure lifetimes of nuclear excited states in the range of 1 ps to 1 ns. The compact design allows one to combine different auxiliary detectors with the plunger device inside the target chamber and takes care of high γ -ray transparency. The design and control of the device are presented in this paper, together with lifetimes of excited states in 180Pt measured within the first in-beam commissioning run. [ABSTRACT FROM AUTHOR]

Shell model and multiparticle-multihole configuration-mixing calculations indicate a missing quadrupole collectivity needed to explain the transition strengths in the even-even Ne isotopes. Even with a significant scaling of the effective charges all details of the data cannot be reproduced. The effect is very pronounced in 20,22Ne. Therefore new lifetime measurements in these nuclei were undertaken by us with the Doppler-shift attenuation method to recheck experimentally earlier findings. They confirmed the previous result in the case of 20Ne while in 22Ne the 2+¹ level lifetime was found to be shorter by 43%. It turned out that in the latter nucleus the lifetime of the 4+¹ level also has to be corrected. The B(E2; 2+¹ → 0+¹) reduced transition strengths derived from the newly determined lifetimes as well as those for the heavier Ne isotopes are reasonably described by involving a mixing of α-cluster states of, e.g., the type α ⊗ 16O with normal, nearly spherical states. An extended version of the coupled-cluster effective interaction method published earlier describes well the spin dependence of the B(E2)'s in 20Ne. [ABSTRACT FROM AUTHOR]

Lifetime measurements of excited states in 178Hg have been performed using the 103Rh(78Kr,p2n) reaction at a beam energy of 354 MeV. The recoil-decay tagging (RDT) technique was applied to select the 178Hg nuclei and associate the prompt γ rays with the correlated characteristic ground-state α decay. Lifetimes of the four lowest yrast states of 178Hg have been determined using the recoil distance Doppler-shift (RDDS) method. The experimental data are compared to theoretical predictions with focus on shape coexistence. The results confirm the shift of the deformed prolate structures to higher lying states but also indicate their increasing deformation with decreasing neutron number. [ABSTRACT FROM AUTHOR]

Lifetime measurements have been performed using the 98Mo(86Kr,4n)180Pt reaction at a beam energy of 380 MeV, and the recoil distance Doppler-shift method. In a second experiment the 168Yb(16O,4n)180Pt reaction at a beam energy of 88 MeV using the Ge-gated γ-γ fast timing technique was used to determine lifetimes. Lifetimes of the four lowest yrast states of 180Pt have been determined. The experimental data are compared to calculations within the framework of the interacting boson model and the general collective model. Both models predict a deformed ground state and are consistent with all the remaining experimental results. [ABSTRACT FROM AUTHOR]

The formation of large size clusters and/or their relative motion as possible excitation mode are suggested to be closely related to the origin of deformation in specific cases. In 32S, the consideration of some excitations in this nucleus as based on the existence of two doubly magic 16O clusters offers a possible solution to the long-standing problem of missing quadrupole collectivity in shell-model calculations aiming to describe electromagnetic E2 transition strengths and quadrupole moments. To experimentally check again this collectivity, lifetime measurements were performed on 30S and 32S. In 32S, at least, the reproduction of a number of observables supports the cluster scenario. Additionally, the superdeformed yrast band in 80Zr could find an explanation as based on the relative motion of two 40Ca clusters. Simultaneously, a necessity arises to revise experimentally some lifetimes derived in the past within the Blaugrund approximation when using the Doppler-shift attenuation method. [ABSTRACT FROM AUTHOR]

Lifetimes of the 2 1 + states in 44 Ti, 48,50 Cr, and 52 Fe were determined with high accuracy exploiting the recoil distance Doppler-shift method. The reduced E2 transition strengths of 44 Ti and 52 Fe differ considerably from previously known values. A systematic increase in collectivity is found for the N = Z nuclei compared to neighboring isotopes. The B ( E2 ) values along the Ti, Cr, and Fe isotopic chains are compared to shell-model calculations employing established interactions for the 0 f 1 p shell, as well as a novel effective shell-model Hamiltonian starting from a realistic nucleon–nucleon potential. The theoretical approaches underestimate the B ( E2 ) values for the lower-mass Ti isotopes. Strong indication is found for particle-hole cross-shell configurations, recently corroborated by similar results for the neighboring isotone 42 Ca. [ABSTRACT FROM AUTHOR]

Employing the γ-γ fast-timing technique with LaBr 3 (Ce) scintillator detectors allows the direct determination of lifetimes of nuclear excited states with a lower limit of about 5 ps. This limit is increased as soon as background is present in the coincidence spectra underneath the full-energy peaks of the γ-γ cascade. Our aim was to identify the components of the γ-ray background by systematic γ-γ fast-timing measurements using different types of γ shielding within a large γ-ray spectrometer. The energy dependent physical zero-time response was measured using background-free full-energy peak events from the 152 Eu γ-ray source. This is compared with the time response of the (Compton-) background distribution as obtained using the prompt 60 Co γ-ray source. The time response of the typical Compton background is about 15 ps faster than the time response of background-free full-energy peak events. Below about 500 keV, a second type of background contributes by the detection of Compton-scattered γ rays generated in the materials of the spectrometer around the detector. Due to the additional time-of-flight of the Compton-scattered γ rays, this low-energy background is largely delayed. Compared with a bare cylindrical 1.5 in . × 1.5 in . LaBr 3 (Ce) detector, the BGO-shielded detector in the Compton-suppression mode improves the peak-to-total ratio by a factor of 1.66(5), while the Pb-shielded detector only slightly reduces the low-energy background. [ABSTRACT FROM AUTHOR]

We present the results from an experiment dedicated to search for quadrupole-collective isovector valence-shell excitations, states with so-called mixed proton-neutron symmetry (MSS), of 212Po. This nucleus was studied in an α-transfer reaction. The lifetimes of two short-lived excited states, candidates for the one-phonon MSS, were determined by utilizing the Doppler shift attenuation method. The experimental results are in qualitative agreement with a simple single-j shell model calculation, which, together with the observed lack of quadrupole collectivity, indicates that the isovector nature of low-lying states is a property of the leading single-particle valence shell configuration. [ABSTRACT FROM AUTHOR]

Lifetime measurements of excited states in doubly-magic 56Ni have been performed exploiting the Doppler-shift attenuation method in order to determine reduced transition probabilities. For the 4 1 + and 6 1 + states, the deduced B (E2) values are compared with results from shell-model calculations employing the GXPF1A and the modern PFSDG-U interactions. In addition, valence ab-initio calculations were performed using a novel realistic Hamiltonian derived from chiral perturbation theory including three-body potential contributions and are confronted with the experimental findings. The new results show maximum E2 strength in comparison with known values along the N = 28 chain of isotones. The results corroborate the high collectivity for the double shell closure at N = Z = 28 which was anticipated from the large B (E2 ; 2 1 + → 0 g. s. +) value despite the considerable increase of its excitation energy as compared to neighboring semi-magic nuclei. Based on similarities in the shell structures of the self-conjugate doubly-magic nuclei 56Ni and 100Sn, the new values could be an indication for an expected comparable collective behavior of the 6 1 + state in 100Sn. [ABSTRACT FROM AUTHOR]

A new method of extracting the γ -ray intensities necessary to perform lifetime measurements using the differential decay curve method (DDCM) is presented in this work, the unresolved Doppler-shifted components method (UDCM). The UDCM allows for a DDCM analysis to be performed using a γ -ray transition for which the fully Doppler-shifted and degraded components are unresolvable in energy and so are detected as a single peak. This technique was used to measure the known lifetime of the yrast 2 1 + state in 50 Mn with a depopulating transition that does not have resolvable fully Doppler-shifted and degraded components. The lifetime measured through applying the UDCM was consistent with the standard DDCM measurement of the 2 1 + state. Use of the UDCM allows for DDCM lifetime measurements to be made using transitions of smaller γ -ray energies, smaller recoil velocities and, in some cases, with a smaller uncertainty. In contrast to a standard DDCM analysis, a UDCM analysis is also independent of the widths of the fully Doppler-shifted and degraded components and as a result they do not need to be determined. [ABSTRACT FROM AUTHOR]

Lifetimes of excited states in 211At were measured using the electronic γ-γ fast timing technique. The nucleus of interest was populated in a 208Pb(6Li,3n)211At fusion-evaporation reaction at the FN Tandem accelerator of the Institute for Nuclear Physics, University of Cologne. The lifetimes of the 17/2-1 and 23/2-1 states were determined, together with an upper limit for the 13/2-1 state. The experimental results are compared to two shell-model calculations, one using a semiempirical interaction for three particles in a single j=9/2 shell and the other using the modified Kuo-Herling interaction in a multi-j model space. [ABSTRACT FROM AUTHOR]

Lifetimes of 2+1 and 4+1 states in 190,192,194,196Hg and of some negative parity band members were measured using the γ-γ fast-timing technique with a high-purity germanium and LaBr3(Ce) detector array. The excited states were populated via fusion-evaporation reactions using the Tandem Van de Graaff accelerator of the Institute of Nuclear Physics in Cologne, Germany. The derived reduced transition probabilities of the 2+1→0+1 and 4+1→2+1 transitions are discussed in the framework of the interacting boson approximation with two models using configuration mixing: a phenomenological one and a microscopical one. Both models describe the observed quantities of the nuclei within the experimental uncertainties. [ABSTRACT FROM AUTHOR]

Background: The semimagic Sn (Z=50) isotopes have been subject to many nuclear-structure studies. Signatures of shape coexistence have been observed and attributed to two-proton-two-hole (2p-2h) excitations across the Z=50 shell closure. In addition, many low-lying nuclear-structure features have been observed which have effectively constrained theoretical models in the past. One example are so-called quadrupole-octupole coupled states (QOC) caused by the coupling of the collective quadrupole and octupole phonons. Purpose: Proton-scattering experiments followed by the coincident spectroscopy of γ rays have been performed at the Institute for Nuclear Physics of the University of Cologne to excite low-spin states in 112Sn and 114Sn to determine their lifetimes and extract reduced transition strengths B(IIL). Methods: The combined spectroscopy setup SONIC@HORUS has been used to detect the scattered protons and the emitted γ rays of excited states in coincidence. The novel (p,p'γ) Doppler-shift attenuation (DSA) coincidence technique was employed to measure sub-ps nuclear level lifetimes. Results: Seventy-four (74) level lifetimes t of states with J=0-6 were determined. In addition, branching ratios were deduced which allowed the investigation of the intruder configuration in both nuclei. Here, sd IBM-2 mixing calculations were added which support the coexistence of the two configurations. Furthermore, members of the expected QOC quintuplet are proposed in 114Sn for the first time. The 1- candidate in 114Sn fits perfectly into the systematics observed for the other stable Sn isotopes. Conclusions: The E2 transition strengths observed for the low-spin members of the so-called intruder band support the existence of shape coexistence in 112,114Sn. The collectivity in this configuration is comparable to the one observed in the Pd nuclei, i.e., the 0p-4h nuclei. Strong mixing between the 0+ states of the normal and intruder configuration might be observed in 114Sn. The general existence of QOC states in 112,114Sn is supported by the observation of QOC candidates with J≠1. [ABSTRACT FROM AUTHOR]

An experiment using the electronic γ-γ fast-timing technique was performed to measure lifetimes of the yrast states in 170Yb. The lifetime of the yrast 2+ state was determined using the slope method. The value of τ=2.33(3) ns is in good agreement with the lifetimes measured using other techniques. The lifetimes of the first 4+ and 6+ states are determined using the generalized centroid difference method. The derived B(E2) values are compared to calculations done using the confined beta soft model and show good agreement with the experimental values. These calculations were extended to the isotonic chain N=100 around 170Yb and show a good quantitative description of the collectivity observed along it. [ABSTRACT FROM AUTHOR]

The ^{244}Pu(^{50}Ti,xn)^{294-x}Lv reaction was investigated at Lawrence Berkeley National Laboratory's 88-Inch Cyclotron. The experiment was aimed at the production of a superheavy element with Z≥114 by irradiating an actinide target with a beam heavier than ^{48}Ca. Produced Lv ions were separated from the unwanted beam and nuclear reaction products using the Berkeley Gas-filled Separator and implanted into a newly commissioned focal-plane detector system. Two decay chains were observed and assigned to the decay of ^{290}Lv. The production cross section was measured to be σ_{prod}=0.44(_{-0.28}^{+0.58})  pb at a center-of-target center-of-mass energy of 220(3) MeV. This represents the first published measurement of the production of a superheavy element near the "island of stability," with a beam of ^{50}Ti and is an essential precursor in the pursuit of searching for new elements beyond Z=118.

The ^{22}Mg(α,p)^{25}Al reaction rate has been identified as a major source of uncertainty for understanding the nucleosynthesis flow in Type-I x-ray bursts. We report a direct measurement of the energy- and angle-integrated cross sections of this reaction in a 3.3-6.9 MeV center-of-mass energy range using the MUlti-Sampling Ionization Chamber (MUSIC). The new ^{22}Mg(α,p)^{25}Al reaction rate is a factor of ∼4 higher than the previous direct measurement of this reaction within temperatures relevant for x-ray bursts, resulting in the ^{22}Mg waiting point of x-ray burst nucleosynthesis flow to be significantly bypassed via the (α,p) reaction.

The lifetimes of the first excited 2^{+}, 4^{+}, and 6^{+} states in ^{98}Zr were measured with the recoil-distance Doppler shift method in an experiment performed at GANIL. Excited states in ^{98}Zr were populated using the fission reaction between a 6.2  MeV/u ^{238}U beam and a ^{9}Be target. The γ rays were detected with the EXOGAM array in correlation with the fission fragments identified by mass and atomic number in the VAMOS++ spectrometer. Our result shows a very small B(E2;2_{1}^{+}→0_{1}^{+}) value in ^{98}Zr, thereby confirming the very sudden onset of collectivity at N=60. The experimental results are compared to large-scale Monte Carlo shell model and beyond-mean-field calculations. The present results indicate the coexistence of two additional deformed shapes in this nucleus along with the spherical ground state.