Nuclear Instruments and Methods in Physics Research A 580 (2007) 322–325 Measuring the angular profile of the reflection of xenon scintillation light C.P. Silva à , J. Pinto da Cunha, V. Chepel, A. Pereira, V. Solovov, P. Mendes, F. Neves, M.I. Lopes Department of Physics, LIP-Coimbra, University of Coimbra, 3004-516 Coimbra, Portugal Available online 21 May 2007 Abstract A chamber designed for measuring the angular distribution of reflected ultraviolet vacuum light is described. We report first measurements of the reflection profile of xenon scintillation light by a polished copper surface and by polytetrafluoroethylene (PTFE). r 2007 Elsevier B.V. All rights reserved. PACS: 78.20.Ci; 29.40.M Keywords: Reflectance; VUV; PTFE; Xenon scintillation; Liquid xenon 1. Introduction Liquid xenon detectors based on scintillation are being used (or developed) for several applications, in particular dark matter search, neutrino experiments or medical imaging [1]. The response of such detectors depends on both the properties of the liquid (absorption and Rayleigh scattering) and on the reflectance of the container surfaces. These data are of extreme importance for the optimisation of the scintillation detectors and particularly as an input for the Monte Carlo simulations. However, since the scintillation light is in vacuum ultraviolet (VUV) at l ’ 175 nm, the absorption in air is high, making the measurements difficult. The aim of this work is to measure the angular distributions of the xenon scintillation light reflected by materials used in liquid xenon chambers, especially Polytetrafluoroethylene (PTFE) which is used in the inner walls of liquid containers [2]. PTFE is known as a high reflectance diffuser having a reflectance of ’ 99% for wavelengths in the range from 350 to 1800 nm [3]. Much less, however, is known for wavelengths l ’ 175 nm concerning both the reflectance and the angular distribu- tion of the reflected light. Consequently, the Monte Carlo simulations usually assume a priori that the surface is purely Lambertian. 2. Set-up For these measurements we designed and built a vacuum chamber, 40  120  30 cm 3 , made of stainless steel (lateral walls) and aluminium (top and bottom). To reduce the random light background, the inner walls of the chamber and most of the pieces of equipment were covered with black paper of low reflectivity (Thorlabs BFP1). The layout of the chamber is shown in Fig. 1. The source of VUV light is a small cylindrical propor- tional counter with internal diameter of 40 mm and anode 0.5 mm filled with Xenon gas at 1 bar. An 241 Am source is mounted on one base at a distance of 1.5 cm from the central electrode emitting 5.5 MeV a-particles along the counter axis. Under an applied voltage of 1.3 kV the electrons extracted from the a-tracks produce pulses of VUV light in the vicinity of the anode due to secondary scintillation which escapes the counter through a quartz window. The spectrum of this light is similar to scintillation in liquid Xenon, both are peaked at 175 nm [4]. The charge signal taken from the central anode is used for triggering. ARTICLE IN PRESS www.elsevier.com/locate/nima 0168-9002/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2007.05.166 à Corresponding author. Tel.: +351 967144140. E-mail address: claudio@lipc.fis.uc.pt (C.P. Silva).