One gram of cadmium contains (.1222/113)(6.02E23) = 6.5E20 atoms of Cd-113. A half-life of 7.7E15 years corresponds to a decay constant of (ln 2) / (7.7E15) = 9E-17 per year. Conclusion: each gram of Cd will experience 58600 (+/- 250 or so ... a perfect example of a Poisson distributed random variable in nature!) beta decays per year, or an average "time between clicks" of 9 minutes. How much Cd is in a cellphone battery? That I don't know. Typical NiCd battery is 18% Cd by weight; I don't have a cell phone, so taking the battery out of your phone, weighing it, and doing the mulitiplication is left as an exercise to the reader. Curious coincidence: 77x9=693. Sheer speculation: it's the decay constant, not the half-life, that you can actually experimentally measure, for a long-lived nuclide. Do you suppose they actually measured the rate to be 9.0E-17, or was it something they could only determine to one significant figure? Gordon Bower --------------------------------------------------------------------- beta decay (e-) Assume 50 grams of Cd, there 6.11 grams of Cd-113 6.11 gm/2 = 3.055 gm in 7.7*10^15 3.055 gram is 3.055/113 -6.02*10^23 = 1.628*10^22 Cd Atoms 1.628*10^22 / 7.7*10^15 yr yr/365.25 day day/ 24 hr = .067 Hz 0.067 Hz * 3600 = 241 ... about 240 clicks per hour Matt Coury --------------------------------------