Optical Absorption Spectra of Some Potentially Interesting Gases for Ceren - Kov Counters

For gas Cerenkov counters employing wavelength shifters coupled to normal-gas phototubes, the vacuum ultraviolet transmission of the filler gas is of great interest. We have measured transmission to 1100 A for an optical path of one atmospheremeter of methane, ethane, Freon 14, propane, butane, isobutane, nitrogen, sulfur, hexafluoride, Freon 22, and methyl chloride. Aging results on wavelength shifters are reported. (Submitted for publicat ion) * Work supported by the U. S. Atomic Energy Commission t Partially supported by the National Science Foundation under the Presidential Internship Program. + Present address: IBM-Watson Laboratories, Yorktown Heights, New York 10598, USA INTRODUCTION The use of Cerenkov counters is widespread and essential in many areas of high-energy physics. Use of quartz-windowed photomultiplier tubes enables the experimenter to use Cerenkov light from the visible to almost 2000 A. Recently the feasibility of using organic wavelength shifters in conjunction with normal glass phototubes was successfully demonstrated. 1 The effect of exposure of MgF2-coated samples of p-terphenyl and diphenyl stilbene to different gaseous environments was investigated. The gases were nitrogen, oxygen, and propane, at a pressure of about 2 atmospheres absolute. Within the experimental error (f 100/o) no aging effects could be detected over a period of three months. These wavelength-shifter materials convert photons in the wavelength range of 1100 to 3600 A into an emission centered around 3850 A. In view of this possible extension of the useful Cerenkov light range to 1100 A, it is of special interest to find potential gas fillers which might be transparent to light below 2000 A. In this paper, we report absorption spectra of some of these gases in the vacuum UV region. EXPERIMENTAL The usual method of measuring gas absorption in the vacuum UV range involves filling an appropriate LiF-windowed absorption cell with the gas of interest. These cells are available with only small optical paths, and they cannot be highly pressurized because of the fragility of the LiF windows. Since the objective of our experiments was to find the absorption properties of the gases in Cerenkov counter conditions, i. e. , long optical paths (1 m) and relatively high pressures (at least 1 atm), use of the commercially available cells would have involved large extrapolation with concomitant inaccuracies.