A Compton-background-correction method for fast-timing measurements using LaBr₃(Ce) detectors

: The case of 114Pd

Abstract

Information about the structural evolution that occurs in the deformed areas of the nuclear chart can be obtained from measuring the energies and lifetimes of the excited states. Level lifetimes in nuclei belonging to the deformed regions around mass numbers A ~ 110 and A ~ 150 were measured in a fast-timing experiment performed at the Argonne National Laboratory between December 2015 and January 2016. The nuclei of interest were populated in the spontaneous fission of a ²⁵²Cf source which was placed at the focus of the Gammasphere array. Half of the detectors in the array (51 HPGe) were coupled with a fast timing array, made of 25 LaBr₃(Ce) scintillator detectors to obtain a 76-detector hybrid array able to cover the full solid angle. This was the first time that the Gammasphere array was successfully coupled with an array containing such a large number of LaBr3(Ce) detectors and also the first time that a fully digital acquisition system was used for this scintillator array.

The data collected during the month-long experiment showed low statistics and very low peak-to-background ratios.  Since the fast-timing technique with LaBr₃(Ce) detectors aims to measure lifetimes of the order of tens of picoseconds this bad data quality led to the need  for an accurate background characterization. The background correction methods used in the literature have been developed for much cleaner data.  This work introduces a new background  correction approach, supported  by  a  very  simple  but  solid  mathematical  basis. This background correction procedure is then combined with the Generalized Centroid Difference method,  to measure the lifetimes of the first three excited levels in the yrast band of 100Zr and 114Pd.  The τ  values of 855(44) ps, 34(11) ps and ≤ 26 ps obtained for the 2₁+, 4₁+ and 6₁+ states in 100Zr,  respectively, agree well with the literature values.  The lifetims of τ  = 25(12) ps has been measured for the first time for the 4+ state in  114Pd and an upper limit of 15 ps is suggested for the lifetime of the 6+  state.  The two new  lifetimes are discussed in the context of IBM and Shell Model calculations and a reasonable agreement is found.

Date of Award	Jan 2019
Original language	English
Awarding Institution	University of Brighton
Supervisor	Alison Bruce (Supervisor), Zsolt Podolyák (Supervisor) & Matthias Rudigier (Supervisor)