Schlein (1992) offers the following rule of thumb: Dose Rate a 1 cm from a beta point source (neglecting air & self absorption): DR(1 cm) = 300 rad/hr per mCi He states that dose rate at 1 cm from a beta point source varies only slightly with beta energy. I could not find a rule of thumb relating distributed beta surface activity to absorbed dose. Tony Sorensen Varian Oncology Systems