Hello, This is my first post here, and I must say I'm in trouble caused by the last problem in my homework assignment. I live in Denmark so excuse me if some of the terms are not correct English. What I do know: Source: Ti-204, Decay: Beta-, Activity: 3,1MBq, Half-life period: 3,8 years The relationship between paperthickness (x) and intensity (measured decay/sec) is declining linearly (not sure what it's called). I've measured a, the coefficient, to: -40 counts/mm. The problem is this: At a measurement 25 months after the source started decaying, the detector measures 478 decays/sec. - What is the thickness of the paper? I am troubled because I'm not sure of the relationship between the measured decay (Intensity) and the other parametres. I know what the activity should be, but not the intensity. Any help would be appreciated. /Thomas M.