I wonder if it might be a better idea to put the lead on the outside of the steel shield rather than on the inside. It would depend on which material has the most residual activity. 240 cpm still sounds to me like a fairly high background, and I wonder if residual radiation from the lead is causing this. If the residual radiation from the steel is lower, it would serve to filter out any beta radiation from the lead. I read some time ago about some medical facilities that used armor plating from pre-WWII battleships to shield their detector vaults, as the background radiation due to fallout was higher in post-WWII manufactured material. I guess it goes to show that if you're working at low count rates, anything can chime in and contribute to the background.
BTW, 30% efficiency sounds really attractive. Even without calibration,the plastic looks like a nice go-no go indicator.
Created on Tuesday, January 16, 2001 8:05 PM EDT by Richard L. Hester