Here's a puzzle; impute the gamma ray energy if you know the transmission factor thru (thick) concrete. T = B exp(-mu x t) is the simple equation, but B is a function of energy, so you cant just solve for mu and go the mu vs. E curve. Anybody confronted this one? (Distance-- r squared --effects can be ignored.) Joel I. Cehn, CHP joelc at alum.wpi.edu