Response of liquid scintillator assemblies as a function of angular orientation S.F. Naeem n , M. Scarpelli, E. Miller, S.D. Clarke, S.A. Pozzi Department of Nuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor, MI 48109, USA article info Article history: Received 4 December 2013 Received in revised form 12 February 2014 Accepted 12 February 2014 Available online 3 March 2014 Keywords: Liquid scintillator detectors Inert gas bubble Scintillation light Optical photons Geant4 simulations BK7 light guide abstract Liquid scintillator detector assemblies contain an inert nitrogen expansion volume to allow for expansion of the liquid with changing temperature. Measurements and Geant4 Monte Carlo simulations are performed to study the dependence of pulse height distribution shapes as a function of detector angle for two liquid scintillators assemblies filled with 97% organic-liquid cocktail and a 3% expansion volume. A 12.7-cm diameter by 12.7-cm long and a 7.6-cm diameter by 9.1-cm long EJ-309 liquid scintillator assemblies are investigated using a 137 Cs gamma-ray source. Aside from the differences in dimensions, the detector assemblies also differed in the design of the active detector volume: there is no light guide in the 12.7-cm-diameter detector assembly, whereas the 7.6-cm-diameter detector contains a BK7 light guide between the scintillation liquid and optical coupling to the photomultiplier tube. Results for the 12.7-cm-diameter detector show a decrease in the position of the Compton edge ranges from 4% to 40% at detector orientations where the expansion volume exists between scintillating medium and the photomultiplier tube. Results for the 7.6-cm-diameter detector show that the position of the Compton edge is relatively unaffected at all detector orientations due to the presence of light guide. & 2014 Elsevier B.V. All rights reserved. 1. Introduction Many techniques have been developed for medical, spectro- scopy, nuclear-nonproliferation, and particle physics applications that utilize organic-liquid scintillators as radiation detection devices [1–4]. Organic-liquid scintillators are primarily composed of hydrogen (H) and carbon (C) components in varying propor- tions. Furthermore, liquid scintillators can discriminate particles in a mixed radiation field, and pulse height distribution (PHD) spectra of particles of interest can be extracted by using various pulse shape discrimination algorithms. Scintillation light (optical photons) is produced as a result of secondary charged particles production from incoming radiation in the scintillating cocktail. The transportation of scintillation light from the scintillating cocktail to the photomultiplier tube (PMT) depends upon the optical properties of various media used in the detector assembly. The optical photons are eventually collected by the photocathode in the PMT and amplified into a measurable voltage pulse. Because oxygen can act as a quenching agent in liquid scintil- lators, it is necessary to seal the scintillation liquid in an air-tight container. Furthermore, liquid scintillators contain an infused inert nitrogen expansion volume to allow thermal expansion of the liquid volume under different environmental conditions. This expansion volume, however, affects scintillation light transport as demonstrated by Naeem et al. [5] and Xufei et al. [6]. The presence of the expansion volume decreases scintillation light collection in the PMT at specific detector orientations. This effect requires the detector to be calibrated with respect to detector orientation. Approaches exist to eliminate the effect of expansion volume in scintillation light collection. For example, utilization of external expansion pockets not only addresses thermal expansion of the scintillating cocktail, but also eliminates the effect of expansion volume in scintillation light collection. Another technique, as utilized in the organic-liquid scintillator assembly manufactured by Scionix [7] presented in this paper, is inserting a light guide between scintillating cocktail and optical coupling connecting to the PMT. The purpose of this paper is to systematically quantify the effect of detector orientation with and without the light guide in the resulting detector response through Monte Carlo methods and experimental validation. 2. Methods and materials 2.1. Detector assemblies PHD spectra and their respective Compton edges are compared for two liquid scintillator assemblies as a function of detector Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/nima Nuclear Instruments and Methods in Physics Research A http://dx.doi.org/10.1016/j.nima.2014.02.050 0168-9002 & 2014 Elsevier B.V. All rights reserved. n Corresponding author. Tel./fax: þ1 734 764 2409. E-mail address: sfnaeem@umich.edu (S.F. Naeem). Nuclear Instruments and Methods in Physics Research A 749 (2014) 35–41