Polonium is an alpha emitter. Just that the alpha particle flux is quite high (1 mg of polonium releases a flux equal to 5 grams of radium). Only 1 in 100,000 alpha events is accompanied by a gamma (~800 keV), so a Geiger counter will not work. While a sheet of paper will stop alphas (as well as dead skin), once inside, exposed to live cells, the energetic alphas tear through cell membranes and all sorts of other damage.
Thank you for posting accurate information.