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

¹ IKP der Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
² Grupo de Fisica Nuclear, FAMN, Universidad Complutense, CEI Moncloa, 28040 Madrid, Spain
³ National Centre for Nuclear Research, BP1, ul. Hoza 69, PL-00-681, Warsaw, Poland
⁴ LPSC Grenoble, 53, rue des Martyrs, 38026 Grenoble Cedex, France
⁵ School of Engineering, University of the West of Scotland, Paisley, PA1 2BE, United Kingdom
⁶ Institut Laue-Langevin, CS 20156, 38042 Grenoble Cedex 9, France
⁷ GANIL, Bd Henri Becquerel, BP 55027, 14076 Grenoble Cedex 05, France
⁸ INFN, Sezione di Padova, Via Marzolo 8, 35131 Padova, Italy
⁹ Faculty of Physics, University of Warsaw, ul., Hoza 69, PL-00-681, Warsaw, Poland
¹⁰ SCEM, University of Brighton, Lewes Road, Brighton, BN2 4GJ, United Kingdom
¹¹ IKP der Technische Universität zu Darmstadt, Schlossgartenstr. 9, 64289 Darmstadt, Germany
¹² CEA de Saclay, Route Nationale, 91400 Gif-sur-Yvette, France
¹³ Faculty of Physics, University of Sofia, “St. Kliment Ohridski”, 1164 Sofia, Bulgaria
¹⁴ Department of Physics, University of Surrey, Guildford, GU2 7XH, United Kingdom
¹⁵ Horia Hulubei NIPNE, 77125 Bucharest, Romania
¹⁶ NPL, Teddington, Middlesex, TW11 0LW, United Kingdom
¹⁷ IPN de Lyon, 4, Rue Enrico Fermi, 69622 Villeurbanne Cedex, France

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

Published online: 28 May 2015

Abstract

A novel method for direct electronic “fast-timing” lifetime measurements of nuclear excited states via γ-γ 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 γ-γ 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 γ-γ 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 energydependent 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 γ-γ zero-time responses can be determined for 40 keV < E_γ < 1.4 MeV with a precision better than 10 ps using a ¹⁵²Eu γ-ray source. The new method is described with examples of (n,γ) and (n,f,γ) 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.