The γ-γ fast-timing technique and the EXILL&FATIMA campaign

¹ Institut für Kernphysik, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
² LPSC, 53 avenue des Martyrs, 38026 Grenoble Cedex, France
³ National Centre for Nuclear Research, ul. Hoza 69, Warsaw, Poland
⁴ Institut Laue-Langevin, 71 avenue des Martyrs, 38042 Grenoble Cedex, France
⁵ Grand Accélérateur National d’Ions Lourds, CEA/CRF-CNRS/IN2P3, 14076 Caen Cedex 05, France
⁶ INFN, via Marzolo 8, 35131 Padova, Italy
⁷ Faculty of Physics, University of Warsaw, ul. Hoza 69, PL-00-681 Warsaw, Poland
⁸ Grupo de Física Nuclear, FAMN, Universidad Complutense, 28040 Madrid, Spain
⁹ Department of Physics, University of Surrey, Guildford GU2 7XH, United Kingdom
¹⁰ Faculty of Physics, University of Sofia “St. Kliment Ohridski”, 1164 Sofia, Bulgaria
¹¹ SCEM, University of Brighton, Lewes Road, Brighton, BN2 4GJ, United Kingdom
¹² Institut für Kernphysik, TU Darmstadt, Schlossgartenstr. 9, 64289 Darmstadt, Germany
¹³ CEA, IRFU, Université Paris-Saclay, 91191 Gif-sur-Yvette, France
¹⁴ Horia Hulubei NIPNE, 77125 Bucharest, Romania

^* e-mail: regis@ikp.uni-koeln.de
^** Deceased

Published online: 14 November 2018

Abstract

At the Institut Laue-Langevin in Grenoble, germanium-gated γ-γ fast-timing lifetime measurements of nuclear excited states in neutron-rich nuclei have been performed within a prompt γ-ray spectroscopy experimental campaign. We report on results obtained from the cold-neutron induced fission of ²³⁵U. The excited secondary fission products were stopped almost instantaneously within the thick target and the γ rays emitted were collected triggerlessly using the EXILL&FATIMA mixed array of HPGe and LaBr₃(Ce) detectors. Precise lifetimes could be determined by analysing the γ-γ time difference spectra using the generalized centroid difference method. This picosecondsensitive method provides many advantages and is briefly explained. Still, the major source of systematic errors is related to the contribution of time-correlated Compton background. The EXILL&FATIMA results are discussed with respect to the typical energy-dependent timing behaviour of the background. According to the time response of the background, appropriate methods and a time correction for the sub-nanosecond regime are proposed.