Population of a low-spin positive-parity band from high-spin intruder states in 177Au : The two-state mixing effect

The extremely neutron-deficient isotopes 177,179Au were studied by means of in-beam γ-ray spectroscopy. Specific tagging techniques, α-decay tagging in 177Au and isomer tagging in 179Au, were used for these studies. Feeding of positive-parity, nearly spherical states, which are associated with 2d3/2 and 3s1/2 proton-hole configurations, from the 1i13/2 proton-intruder configuration was observed in 177Au. Such a decay path has no precedent in odd-Au isotopes and it is explained by the effect of mixing of wave functions of the initial state

On the imprecisions that may be induced when applying the Blaugrund approximation for the analysis of Doppler-shift attenuation lifetime measurements

It is shown that the Blaugrund approximation could have led to some imprecise lifetime determinations in the past which used the Doppler-shift attenuation method (DSAM). Comparison with Monte Carlo simulations of the slowing-down process show that there is not an easy way to judge using them on the reliability of old data

Collectivity of the 2(1)(+) state in Z <= 82 even-even nuclei probed by a ratio involving dynamic and static electromagnetic E2 moments: Evolution of the quadrupole degrees of freedom and a new signature for shape coexistence

We present a new approach to investigate the evolution of collectivity on the nuclear map based on the ratio R = B(E2; 2(1)(+) -> 0(1)(+))(exp)/B(E2;2(1)(+) -> 0(1)(-))(Q), where the quantity in the denominator is calculated using the quadrupole moment of the 2(1)(+) level when known. The systematics of such ratios in even-even nuclei reveals interesting features including signatures for particular collective excitations and sensitivity to shell- and subshell closures. A reproduction of these data by theoretical models is highly desirable with the potential to shed more light on the interplay between collective and single-particle motions. In particular, a new signature for shape coexistence was found: R < 1

Development of the multi-purpose Cologne Compact Differential Plunger (CoCoDiff) for the measurement of nuclear level lifetimes with the Recoil Distance Doppler-shift method

A new 3-foil plunger device, the Cologne Compact Differential (CoCoDiff) plunger has been built and commissioned. Due to its compact size, it can be used together with many different spectrometers and auxiliary detectors. As a commissioning experiment, level lifetimes of the 2+1 and the 4+1 excited states of 50Cr have been measured, using the Differential Decay Curve method (DDCM). A derivation is given on how this method can be applied to a differential plunger measurement, in order to measure distances for lifetimes from two different regions of sensitivity at the same time. The commissioning experiment took place at the Cologne FN tandem accelerator, using the reaction 24Mg(32S,4p2n)50Cr. Lifetimes, deduced from this measurement, agree well with literature values from earlier measurements

Operating the 120 degrees Dipol-Magnet at the CologneAMS in a gas-filled mode

Routine measurements of the last years at the 6 MV AMS system showed a good isobar suppression for all light isotopes, measured with the Degrader-Foil-Method, especially Cl-36 and Be-10. However, the efficiency is reduced due to the charge state distribution generated by the degrader-foil. For measurements injecting atomic Al-26, the ionization efficiency becomes the bottleneck, which leads to a relatively low ion source output. In the case of AlO-, the output is expected about one order of magnitude higher. However, to use AlO- the isobar interference of Mg-26 will become an issue. It has to be suppressed in several steps, e.g. by different dE/dx methods like TOF, degrader-foil or gas-filled magnet. In order to minimize the efficiency losses generated by the degrader-foil method, the 120 degrees-magnet at the 6 MV AMS system was modified to enable a gas-filled operation, and therefore focus the beam to a mean charge state. A constraint for all modification of the detection beam line was that the operation of the degrader-foil method is not affected. In this paper, we report on the performed construction work, as well as test measurements for Al-27 at 40 MeV ion energy. Isobar separation for Cl-36 and S-36 was measured as a function of the gas pressure. We compare measured data with Monte Carlo simulations performed with a computer code developed by our group. In addition, we will present a new dedicated gas ionization chamber designed for particle detection directly after the gas-filled magnet

On the time response of background obtained in gamma-ray spectroscopy experiments using LaBr3(Ce)detectors with different shielding

Employing the gamma-gamma fast-timing technique with LaBr3(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 gamma-gamma cascade. Our aim was to identify the components of the gamma-ray background by systematic gamma-gamma fast-timing measurements using different types of gamma shielding within a large gamma-ray spectrometer. The energy dependent physical zero-time response was measured using background-free full-energy peak events from the Eu-152 gamma-ray source. This is compared with the time response of the (Compton-) background distribution as obtained using the prompt Co-60 gamma-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 gamma rays generated in the materials of the spectrometer around the detector. Due to the additional time-of-flight of the Compton-scattered gamma rays, this low-energy background is largely delayed. Compared with a bare cylindrical 1.5 in. x 1.5 in. LaBr3(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. (C) 2015 Elsevier B.V. All rights reserved

Lifetime measurements with improved precision in S-30,S-32 and possible influence of large-scale clustering on the appearance of strongly deformed states

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 S-32, the consideration of some excitations in this nucleus as based on the existence of two doubly magic O-16 clusters offers a possible solution to the long-standing problem of missing quadrupole collectivity in shell-model calculations aiming to describe electromagnetic E-2 transition strengths and quadrupole moments. To experimentally check again this collectivity, lifetime measurements were performed on S-30 and S-32. In S-32, at least, the reproduction of a number of observables supports the cluster scenario. Additionally, the superdeformed yrast band in Zr-80 could find an explanation as based on the relative motion of two Ca-40 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

New lifetime measurements for the lowest quadrupole states in Ne-20,(22) and possible explanations of the high collectivity of the depopulating E2 transitions

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 Ne-20(,22). 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 Ne-20 while in Ne-22 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 a-cluster states of, e.g., the type alpha circle times 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 Ne-20

A dedicated AMS setup for medium mass isotopes at the Cologne FN tandem accelerator

AMS measurements of medium mass isotopes, e.g. of Mn-53 and Fe-60, are gaining interest in various fields of operation, especially geoscience. Therefore a dedicated AMS setup has been built at the Cologne 10 MV FN tandem accelerator. This setup is designed to obtain a sufficient suppression of the stable isobars at energies around 100 MeV. In this contribution we report on the actual status of the new setup and the first in-beam tests of its individual components. The isobar suppression is done with (dE/dx) techniques using combinations of energy degrader foils with an electrostatic analyzer (ESA) and a time of flight (ToF) system, as well as a (dE/dx),E gas ionization detector. Furthermore, the upgraded ion source and its negative ion yield measurement for MnO- are presented. (C) 2017 Elsevier B.V. All rights reserved

Shape coexistence and collective low-spin states in Sn-112,Sn-114 studied with the (p,p'y) Doppler-shift attenuation coincidence technique

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 gamma rays have been performed at the Institute for Nuclear Physics of the University of Cologne to excite low-spin states in Sn-112 and Sn-114 to determine their lifetimes and extract reduced transition strengths B(Pi L) Methods: The combined spectroscopy setup SONIC@HORUS has been used to detect the scattered protons and the emitted y rays of excited states in coincidence. The novel (p,p'gamma) Doppler-shift attenuation (DSA) coincidence technique was employed to measure sub-ps nuclear level lifetimes. Results: Seventy-four (74) level lifetimes tau 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 Sn-114 for the first time. The 1(-) candidate in Sn-114 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 Sn-112,Sn-114. 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 Sn-114. The general existence of QOC states in Sn-112,Sn-114 is supported by the observation of QOC candidates with J not equal 1