Optical Properties of the DIRC Fused Silica Cherenkov Radiator

TL;DR

This paper discusses the development and testing of fused silica bars for the DIRC Cherenkov detector, focusing on optical properties, material selection, and manufacturing methods to ensure high performance in particle identification.

Contribution

It presents new methods for selecting radiation-hard, optically uniform fused silica and evaluates their suitability for use in the DIRC detector system.

Findings

Fused silica bars can meet stringent optical and radiation hardness requirements.

Measurement of wavelength-dependent internal reflection coefficients.

Development of effective radiator support and optical coupling techniques.

Abstract

The DIRC is a new type of Cherenkov detector that is successfully operating as the hadronic particle identification system for the BABAR experiment at SLAC. The fused silica bars that serve as the DIRC's Cherenkov radiators must transmit the light over long optical pathlengths with a large number of internal reflections. This imposes a number of stringent and novel requirements on the bar properties. This note summarizes a large amount of R&D that was performed both to develop specifications and production methods and to determine whether commercially produced bars could meet the requirements. One of the major outcomes of this R&D work is an understanding of methods to select radiation hard and optically uniform fused silica material. Others include measurement of the wavelength dependency of the internal reflection coefficient, and its sensitivity to surface contaminants, development of radiator support methods, and selection of good optical glue.