Yes the rubidium 87 isotope is technically radioactive as it does decay and have a half life. However it takes special techniques to measure as the activity level is so low. A piece of granite probably has more radioactivity than the tiny amount in a rubidium standard. This link gives some more details as well as a maximum daily requirement, mind the cautionary note! http://www.agilent.com/metrology/hazardous.shtml Also on Tom Van Baaks website are some PIX of Rb cells I took. I need to add the tiny one from an LPRO is recently acquired it's about 1/4" square! http://www.leapsecond.com/pages/rb-cells/ Cheers! Corby ____________________________________________________________ Save $10 on Flowers and Gifts! Shop now at www.ftd.com/16714 http://thirdpartyoffers.juno.com/TGL2141/fc/Dh4GRr5bIdw4RkUexf7ZwViCySzybvV1z4ExZZ5r9xmUJg9zxeH0Gyp8K1S/