Nuclear Instruments and Methods in Physics Research A310 (1991) 665-671) North-Holland ~-- i[ I ~ &MlffNOOS M ~ • ,~,grl A, Sintered Halon as a diffuse reflecting liner for light integration boxes S. Nutter, C.R. Bower, M.W. Gebhard, R.M. Heinz and G. Spiczak Department of Physics, hidiana Unirersity Bloomington, IN 47405, USA Received 2 July 1991 Sintered Halon G-80, a polytetrafluoroethylene (PTFE) resin available commercially in powdered form, is a superb diffuse reflector. Sintered Halon has > 96% absolute reflectance at wavelengths between 300 nm and 380 nm, and about 98% absolute reflectance above 380 rim. It is chemically inert, is mechanically strong, does not fluoresce, and the high reflectance is e&~yto maintain over time. We have used sintered Halon as a lining in light integration boxes of water Cherenkov detectors. Preparation involves packing Halon powder into a mold to a specific density, then heating the material almost to the melting point (sintering). A method of preparing sintered Halon is presented. Physical properties of sintered Halon have been measured and we present those results here as well. 1. Introduction Motivation for developing a water insoluble mate- rial with a high diffuse reflectance in the UV wave- lengths arose in the design of the balloon-borne cosmic ray experiment PBAR [1-3] (which measured the flux of antiprotons high in the Earth's atmosphere). The resulting material, sintered Halon, was used subse- quently in the SMILI experiment (Superconducting Magnetic Instrument for Light Isotopes) [4]. These experiments called for Cherenkov detectors with a kinetic energ~ threshold between 400 and 700 MeV per nucleon [MeV/n]. This energy range re- quires a Cherenkov radiator with an index of refraction between 1.2 and 1.4. The most common substances with indices of refraction in this region are clear liq- uids. One such liquid, highly purified water, is an ideal Cherenkov radiator with an index of refraction of 1.34 (threshold of 470 MeV/n) at wavelengths around 400 nm. Distilled deionized water is successfully used as a Cherenkov radiator in the IMB detector. The IMB collaboration has found distilled deionized water to have attenuation lengths of more than five meters _from 350 nm to 700 nm, with maximal attenuation length of about 40 m at 400 nm [5]. The PBAR and SMILI Cherenkov detectors each required a light integration box to isotropize the Cherenkov light. Isotropization was obtained through the use of a diffuse reflector lining the interior of the box. Photomultiplier tubes (PMT), placed such that they viewed the light integration region, collected the light. Unlike previous Cherenkov detector designs which had the radiator adjacent to an air-filled integra- tion region (e.g. ref. [6]), the integration box was com- pletely filled with the liquid Cherenkov medium to avoid barriers (i.e. potential photon absorbers) be- tween the Cherenkov medium, where the light is pro- duced, and the integration region, where the light is isotropized and collected. Such barriers would other- wise be necessary, in the case of liquid Cherenkov mediums in a balloon flight, where agitation of the payload occurs frequently. An early prototype of our detector is described by Bower et al. [7]. Considering that a Cherenkov photon will bounce several times before reaching a PMT, a high value for the reflectance of the lining is essential to the light collection efficiency of the box. The high reflectance is especially important in the UV and visible portion of the spectrum between 300 nm and 500 nm (a require- ment imposed first by the fact that, in water, the majority of the Cherenkov radiation occurs in the [IV, and second by the quantum efficiency of the photomul- tiplier tubes viewing the integration region). Typical highly reflective coatings in the past have been paints based on BaSO 4 powder [8]. The re- flectance of BaSO4 is 99% at 500 nm, while the re- flectance of the paint at this wavelength is 97.3% [9]. (See fig. 1.) Unfortunately, the paint is water soluble, making it unsuitable for our application. The National Institute for Standards and Technol- ogy (NIST) has measured the reflectance of powdered Halon [10]. Halon G-80, a polytetrafluoroethylene (PTFE) resin, is one of many Teflon-like substances [11]. Halon is ideal for use in light integration boxes of Elsevier Science Publishers B.V.