Measuring Light Reflectance of BGO Crystal Surfaces

Abstract

Abstract-A scintillating crystal's surface reflectance has to be well understood in order to accurately predict and optimize the crystal's light collection through Monte Carlo simulations. In this paper, we measure the inner surface reflectance properties for BGO. The measurements include BGO crystals with a mechanically polished surface, rough-cut surface, and chemically etched surface, and with various reflectors attached, both aircoupled and with coupling compound. The measurements are performed with a laser aimed at the center of a hemispherical shaped BGO crystal. The hemispherical shape eliminates any non-perpendicular angles for light entering and exiting the crystal. The reflected light is collected with an array of photodiodes. The laser can be set at an arbitrary angle, and the photodiode array is rotated to fully cover 2π of solid angle. The current produced in the photodiodes is readout with a digital multimeter connected through a multiplexer. The two rows of photodiodes achieve 5-degree by 4-degree resolution, and the current measurement has a dynamic range of 10 5 :1. The acquired data was not described by the commonly assumed linear combination of specular and diffuse (Lambertian) distributions, except for a very few surfaces. Surface roughness proved to be the most important parameter when choosing crystal setup. The reflector choice was of less importance and of almost no consequence for rough-cut surfaces. Pure specular reflection distribution for all incidence angles was measured for polished surfaces with VM2000 film, while the most Lambertian distribution for any surface finish was measured for titanium dioxide paint. The distributions acquired in this paper will be used to create more accurate Monte Carlo models for light reflection distribution within BGO crystals.

Simulating Scintillator Light Collection Using Measured Optical Reflectance

“…This does not fully agree with our measured data, as for instance a Lambertian reflector (e.g., Teflon ® tape) attached to a BGO surface exhibits Lambertian and specular lobe reflection distributions for low incidence angles and a specular lobe (turning into a specular spike) reflection distribution for high (very high) incidence angles [11]. The Lambertian portion of the reflection distribution at low incidence angles is most likely due to the Lambertian reflector underneath the surface, and the surface produces the specular lobe reflection distribution.…”

“…The instrument has been used to measure the reflectance of BGO crystals with polished, etched, and rough-cut surfaces, and various reflectors ( [11]. Each of these surface-reflector measurements consist of a set of angular reflection distributions produced by a set of laser incidence angles.…”

“…The measured data was acquired from laser incidence angles in 4º steps, from 2º to 82º [11]. This data needed to be resampled to every whole angle, including the endpoints of perpendicular incidence (θ = 0º) and parallel incidence (θ = 90º), i.e., θ = [0º:1º:90º], for accurate simulations.…”

“…We have previously demonstrated that not all light distributions can be described in this way [10], [11]. In particular, some commonly used reflector materials, e.g., Tyvek ® , Lumirror ® , and Melinex ® , have reflection distributions that are not linear combinations of these components.…”

“…The aim of this work is to improve the accuracy of existing optical Monte Carlo simulations by using measured data from BGO reflectance measurements [11]. In addition, the usability of the current codes is improved, as the user does not need the detailed knowledge of surface roughness, standard deviation of reflected light spread, etc.…”