Measurement of picosecond lifetimes in neutron-rich Xe isotopes

Background: Lifetimes of nuclear excited states in fission fragments have been studied in the past following isotope separation, thus giving access mainly to the fragments' daughters and only to long-lived isomeric states in the primary fragments. For the first time now, short-lived excited states in the primary fragments, produced in neutron-induced prompt fission of U-235 and Pu-241, were studied within the EXILL&FATIMA campaign at the intense neutron-beam facility of the Institute Laue-Langevin in Grenoble. Purpose: We aim to investigate the quadrupole collective properties of neutron-rich even-even Xe-138,Xe-140,Xe-142 isotopes lying between the double shell closure N = 82 and Z = 50 and a deformed region with octupole collectivity. Method: The gamma rays emitted from the excited fragments were detected with a mixed array consisting of 8 HPGe EXOGAM Clover detectors (EXILL) and 16 LaBr3(Ce) fast scintillators (FATIMA). The detector system has the unique ability to select the interesting fragment making use of the high resolution of the HPGe detectors and determine subnanosecond lifetimes using the fast scintillators. For the analysis the generalized centroid difference method was used. Results: We show that quadrupole collectivity increases smoothly with increasing neutron number above the closed N = 82 neutron shell. Our measurements are complemented by state-of-the-art theory calculations based on shell-model descriptions. Conclusions: The observed smooth increase in quadrupole collectivity is similar to the evolution seen in the measured masses of the xenon isotopic chain and is well reproduced by theory. This behavior is in contrast to higher Z even-even nuclei where abrupt change in deformation occurs around N = 90

B(E2; 2_(1)^(+) → 0_(1)^(+)) value in Kr^(90)

A smooth onset of collectivity in Kr^(88,92,94,96) has been determined from reported B(E2; 2_(1)^(+) → 0_(1)^(+)) and E(2+ 1 ) values. This is in contrast to the sudden onset in even-even Zr, Mo, and Sr isotopes. Our objective was to complete the systematics by determining the B(E2; 2+ 1 → 0+ 1 ) value in Kr^(90), which was produced by cold-neutron-induced fission of 235U. The lifetime of the 2+ 1 state in Kr^(90) was measured via the electronic γ -γ timing technique using the EXILL and FATIMA spectrometers. Based on the measured mean lifetime of τ = 15(10) ps, the B(E2; 2_(1)^(+) → 0_(1)^(+)) value of 13^(+26)_(−5) W.u. in Kr^(90) is determined for the first time and the smooth onset of deformation in the even-even Kr isotopes beyond neutron number N = 50 is confirmed

Search for the ⁷³Gaground-state doublet splitting in the β decay of ⁷³Zn

The existence of two close-lying nuclear states in ⁷³Ga has recently been experimentally determined: a 1/2⁻ spin-parity for the ground state was measured in a laser spectroscopy experiment, while a J_π = 3/2⁻ level was observed in transfer reactions. This scenario is supported by Coulomb excitation studies, which set a limit for the energy splitting of 0.8 keV. In this work, we report on the study of the excited structure of ⁷³Ga populated in the β decay of ⁷³Zn produced at ISOLDE, CERN. Using β-gated, γ-ray singles, and γ –γ coincidences, we have searched for energy differences to try to delimit the ground-state energy splitting, providing a more stringent energy difference limit. Three new half-lives of excited states in ⁷³Ga have been measured using the fast-timing ;method with LaBr₃(Ce) detectors. From our study, we help clarify the excited structure of ⁷³G and we extend the existing ⁷³Zn decay to ⁷³Ga with 8 new energy levels and 35 γ transitions. We observe a 195-keV transition consistent with a γ ray de-exciting a short-lived state in the β-decay parent ⁷³Zn

The mutable nature of particle-core excitations with spin in the one-valence-proton nucleus Sb-133

The gamma-ray decay of excited states of the one-valence-proton nucleus Sb-133 has been studied using cold-neutron induced fission of U-235 and Pu-241 targets, during the EXILL campaign at the ILL reactor in Grenoble. By using a highly efficient HPGe array, coincidences between gamma-rays prompt with the fission event and those delayed up to several tens of microseconds were investigated, allowing to observe, for the first time, high-spin excited states above the 16.6 mu s isomer. Lifetimes analysis, performed by fast-timing techniques with LaBr3(Ce) scintillators, revealed a difference of almost two orders of magnitude in B(M1) strength for transitions between positive-parity medium-spin yrast states. The data are interpreted by a newly developed microscopic model which takes into account couplings between core excitations (both collective and non-collective) of the doubly magic nucleus Sn-132 and the valence proton, using Skyrme effective interaction in a consistent way. The results point to a fast change in the nature of particle-core excitations with increasing spin. (C) 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license

Abrupt shape transition at neutron number N=60: B(E2) values in Sr-94,Sr-96,Sr-98 from fast gamma-gamma timing

Lifetimes of low-lying yrast states in neutron-rich Sr-94,Sr-96,Sr-98 have been measured by Germanium-gated gamma-gamma fast timing with LaBr3(Ce) detectors using the EXILL& FATIMA spectrometer at the Institut Laue-Langevin. Sr fission products were generated using cold-neutron-induced fission of 235U and stopped almost instantaneously within the thick target. The experimental B(E2) values are compared with results of Monte Carlo shell-model calculations made without truncation on the occupation numbers of the orbits spanned by eight proton and eight neutron orbits and show good agreement. Similarly to the Zr isotopes, the abrupt shape transition in the Sr isotopes near neutron number N = 60 is identified as being caused by many-proton excitations to its g(9/2) orbit

Assessment of New Photosensors for Fast Timing Applications with Large Scintillator Detectors

Proceedings of: 2011 IEEE Nuclear Science Symposium and Medical Imaging Conference (NSS/MIC). Valencia, Spain, 23-29 October 2011The measurement of absolute nuclear transition probabilities is a very sensitive tool to study the structure of the atomic nucleus. Direct access to transition rates can be achieved via the lifetime of the nuclear levels de-populated in radioactive decay. The Advanced Time-Delayed (ATD) method, or Fast Timing, is a well-established technique to measure lifetimes down to a few ps. The development of the technique was based on the use of BaF_2 detectors, but a recent major breakthrough occurred with the introduction of LaBr_3(Ce) crystals, uniting excellent time response with much superior energy resolution than BaF_2 crystals. Relatively large LaBr_3(Ce) cylindrical detectors of typically 1.5"×1.5" are employed for fast timing, in combination with fast 2-inch photomultiplier tubes from Photonis such as the linear focused 8-stage XP20D0. Another option for a 2-inch fast phototube is the 8-stage Hamamatsu R9779, whose timing properties have already been tested with small LSO crystals. New possibilities are also offered by novel photosensors such as silicon photomultipliers, which are intrinsically fast. In this work we have investigated the performance of the Hamamatsu R9779 photomultiplier tube, and the viability of CeBr_3 crystals for fast-timing applications.This work was supported in part by Comunidad de Madrid (ARTEMIS S2009/DPI-1802) and the Spanish Ministry of Science and Innovation (grants FPA201O-l7l42 and CSPD-2007-00042, Ingenio2010).Publicad

Half-life of the 15/2(+) state of I-135: A test of E2 seniority relations

The half-life of the 15/ 2(1)(+) state of the 3-valence-proton nucleus I-135 has been measured to be 1.74(8) ns using the EXILL-FATIMA mixed array of Ge and LaBr3 detectors. The nuclei were produced following the cold neutron-induced fission of a U-235 target at the PF1B beam line of the Institut Laue-Langevin. The extracted B(E2; 15/2(+) -> 11/2(+)) value enabled a test of seniority relations for the first time between E2 transition rates. Large-scale shell-model calculations were performed for Te-134 and I-135, and reinterpreted in a single-orbit approach. The results show that the two-body component of the E2 operator can be large whereas energy shifts due to the three-body component of the effective interaction are small

Fast-timing study of the l -forbidden 12+→32+ M1 transition in Sn 129 FAST-TIMING STUDY of the l -FORBIDDEN ⋯ R. LICǎ et al.

© 2016 authors. Published by the American Physical Society.The levels in Sn129 populated from the β- decay of In129 isomers were investigated at the ISOLDE facility of CERN using the newly commissioned ISOLDE Decay Station (IDS). The lowest 12+ state and the 32+ ground state in Sn129 are expected to have configurations dominated by the neutron s12 (l=0) and d32 (l=2) single-particle states, respectively. Consequently, these states should be connected by a somewhat slow l-forbidden M1 transition. Using fast-timing spectroscopy we have measured the half-life of the 12+ 315.3-keV state, T12= 19(10) ps, which corresponds to a moderately fast M1 transition. Shell-model calculations using the CD-Bonn effective interaction, with standard effective charges and g factors, predict a 4-ns half-life for this level. We can reconcile the shell-model calculations to the measured T12 value by the renormalization of the M1 effective operator for neutron holes

The (n, gamma) campaigns at EXILL

At the PF1B cold neutron beam line at the Institut Laue Langevin, the EXILL array consisting of EXOGAM, GASP and ILL-Clover detectors was used to perform (n, gamma) measurements at very high coincidence rates. About ten different reactions were measured in autumn 2012 using a highly collimated cold neutron beam. In spring 2013, the EXOGAM array was combined with 16 LaBr3(Ce) scintillators in the EXILL&FATIMA campaign for the measurement of lifetimes using the generalised centroid difference method. We report on the properties of the set-ups and present first results from both campaigns