Calculate the beta absorbed dose-rate of common pure beta emitters and a few beta-gamma emitters, given the activity and the isotope. If the isotope and dose-rate are known, the activity may be calculated. The method used is extrapolation tables from the 'Handbook of Health Physics and Radiological Health.' Plane source for spills may be selected and backscatter factors are included in the calculation. Tritium (H-3) is included by request even though the energy and distance traveled are extremely low. Sr-90 and Y-90 may be calculated separately or in equilibrium together. Disclaimer: This calculator should not be used to calculate skin dose. The formulas were designed to give you dose-rates that might be measured by a scientist or a technician. The closest distance a typical ion chamber can get is around 2.5 cm (distance to center of chamber, the calibration point). If you want to calculate contact skin doses or dose-rates at close distances, you should not use this calculator and should consider a software package such as VarSkin