Instrumentation and Methods A) Ion Chamber Measurements All environmental exposure rate measurements were obtained using a Reuter Stokes environmental radiation monitor model RSS-111. The instrument is designed to measure environmental radiation as low as 100 uRad/year. The RSS-111 consists of a spherical high pressure ion chamber filled to 25 atmospheres of argon. Incident radiation produces ion pairs within the active volume of the chamber which result in a current flow. The current flow is measured by an electrometer and is directly related to the free air exposure rate (1). The active volume of the stainless steel ionization chamber is known to +01%. The current produced inthe chamber is a function of incident radiation from an external field, cosmic ray-response and contamination found in the stainless steel. The equation relating instrument response to energy of the incident radiation is: Rj = Kj Lj] + Rw + Ke Ic where Rj = current produced in the chamber by the incident gamma field Kj = proportionality constant stating the variability of instrument response to the energy of the incident gamma field Ij = intensity of the gamma field in uR/hr Ra = current produced by activity in the stainless steel Ke = proportionality constant for cosmic rays Ic = intensity of cosmic rays