The Generalized Centroid Difference method for lifetime measurements via gamma-gamma coincidences using large fast-timing arrays.

A novel method for direct electronic "fast-timing" lifetime measurements of nuclear excited states via gamma-gamma coincidences using an array equipped with N very fast high-resolution LaBr3(Ce) scintillator detectors is presented. The generalized centroid difference method provides two independent "start" and "stop" time spectra obtained without any correction by a superposition of the N(N - 1)/2 calibrated gamma-gamma time difference spectra of the N detector fast-timing system. The two fast-timing array time spectra correspond to a forward and reverse gating of a specific gamma-gamma cascade and the centroid difference as the time shift between the centroids of the two time spectra provides a picosecond-sensitive mirror-symmetric observable of the set-up. The energy-dependent mean prompt response difference between the start and stop events is calibrated and used as a single correction for lifetime determination. These combined fast-timing array mean gamma-gamma zero-time responses can be determined for 40 keV < E-gamma < 1.4 MeV with a precision better than 10 ps using a Eu-152 gamma-ray source. The new method is described with examples of (n,gamma) and (n,f,gamma) experiments performed at the intense cold-neutron beam facility PF1B of the Institut Laue-Langevin in Grenoble, France, using 16 LaBr3(Ce) detectors within the EXILL&FATIMA campaign in 2013. The results are discussed with respect to possible systematic errors induced by background contributions

Low-spin states in Ge-80 populated in the beta decay of the Ga-80 3(-) isomer

The structure of Ge-80 has been investigated at the ISOLDE facility at CERN. A previous study reported for the first time a low-lying 0(2)(+) intruder state at 639 keV, based on the coincidence with a previously unobserved 1764-keV gamma ray, and suggested it as evidence for shape coexistence in Ge-80. We used the beta decay from the 3(-) 22.4-keV state in Ga-80 to enhance the population of low-spin states in Ge-80, including any excited 0(+) level, and gamma gamma coincidences to investigate it. We observed a 1764-keV gamma ray in coincidence with strong transitions in Ge-80, thus not feeding the proposed 639-keV 0(2)(+). No connecting transitions from previously known levels to the 639-keV and 2403-keV 2(3)(+) states could be established either. Shell-model calculations for Ge isotopes and N = 48 isotones were performed. They succeed to explain most of the experimental levels, but fail to reproduce the presence of a 0(2)(+) state below approximate to 1200 keV in Ge-80. Our experimental findings and shell-model calculations are difficult to reconcile with a very low-lying 0(2)(+) state in Ge-80

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

Properties of low-lying states in Co-65 from lifetime measurements

The low-energy structure of Co-65 was studied by means of gamma(-) and fast-timing spectroscopy at the ISOLDE/CERN facility. The known level scheme of Co-65 populated following the beta(-) decay of Fe-65 was expanded. The experimental results were compared with large-scale shell-model calculations. The measured long lifetime of the (1/2(1)(-)) level confirms its nature as a highly collective state with proton excitations across the Z = 28 gap and neutrons across the N = 40 subshell

Low-lying isomeric states in 80Ga from the β- decay of 80Zn

A new level scheme of 80Ga has been determined. This nucleus was populated following the β− decay of 80Zn at ISOLDE, CERN. The proposed level scheme is significantly different compared to the previously reported one and contains 26 levels up to 3.4 MeV in excitation energy. The present study establishes that the previously identified 1.9-s β− -decaying 6− isomer is the ground state of 80Ga and the 1.3-s β− -decaying 3− isomer lies at an excitation energy of 22.4 keV. A new isomeric level was identified at 707.8 keV and its half-life was measured to be 18.3(5) ns, allowing the 685.4-keV transition de-exciting this state to be assigned anM2 multipolarity. The newly measured spectroscopic observables are compared with shell-model calculations using the jj44bpn and JUN45 interactions

The (n,gamma) campaigns at EXILL

© Owned by the authors, published by EDP Sciences, 2015. ESSN: 2100-014X Artículo firmado por 67 autores. This work was supported by NUPNET by contract 05P12PKNUF, 05P12RDNUP and DNC7RP01/4, by the Science and Technology Facilities Councils (UK), by the Spanish MINECO via the FPA2010-17142 and PRIPIMNUP-2011-1338 projects, and by the DFG by cluster of Excellence Origin and Structure of the Universe, grant KR 1796/2-1 and SFB634. This material is based in part upon work supported by the U.S.D.O.E., SC, Office of nuclear physics, under Award No. DE-FG02-91ER40609. The EXILL campaign would not have been possible without the support of several services at the ILL and the LPSC. We are grateful to the EXOGAM collaboration for the loan of the detectors, to GANIL for assistance during installation and dismantling, to the INFN Legnaro laboratory for the loan of the GASP detectors and to the FATIMA collaboration for the use of the LaBr3(Ce) scintillators. International Symposium on Capture Gamma-Ray Spectroscopy and Related Topics (CGS15)(15. 2014. Dresden, Alemania)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.NUPNETScience and Technology Facilities Councils, UKMINECO, SpainDFGU.S.D.O.E., SC, Office of nuclear physicsILLLPSCDepto. de Estructura de la Materia, Física Térmica y ElectrónicaFac. de Ciencias FísicasTRUEpu