APPLICATION OF CALCULATED PHYSICAL ADSORPTION-ISOTHERMS TO A RADON SENSOR

Recently a radon sensor using a glass scintillator in a fiber bundle s tructure was developed at Rutgers University. This instrument employs a flow of atmospheric air carrying the radon gas and its progeny in tr ace amounts through the bundle at room temperature. A central problem of interpretation is to distinguish whether the radioactive Rn-222 ato ms whose a decay is detected in the sensor are in the gas phase in the narrow spaces between the fibers or are adsorbed on the surfaces of t he fibers. At the 43rd National Symposium of the American Vacuum Socie ty (1996), calculated physical adsorption isotherms of radon were repo rted over a wide range of pressures, coverages, and temperatures. The latter explicitly included room temperature (300 K). The question aris es as to whether this calculated isotherm is applicable to the Rutgers sensor. From radioactive considerations alone, in one experiment, it is estimated that there is either an upper bound of 5.5 X 10(-10) Torr on the partial pressure of Rn or an upper bound of 3.9 X 10(-11) mono layers on the coverage. Application of the theoretical isotherms to th is experiment yields a unique value of 4.8 X 10(-10) Torr at a coverag e of 5 X 10(-12) monolayers (i.e., 13% of all atoms adsorbed). A caref ul record of counting rates during temperature variations around room temperature was kept. These variations were between 292 and 308 K, and counting rates varied by 26% being highest when the temperature was l ower. application of the theoretical isotherms at these two temperatur es gave good agreement with the counting rates. (C) 1998 American Vacu um Society.