part, we're not detecting radioactivity. you want to know how much exposure have you had. >> a machine doesn't tell you how much exposure you've had three days ago? >> no. think of it like a camera. you can hear it clicking off as it's giving readings here, which are all low. they're significantly low as compared to anything that would trigger an alarm. that measures alpha, beta and gamma rays. this is just measuring the air constantly around us, 13 microrads per hour, and that's, again, a very low number. >> professor, how does radiation spread over distance? if you are -- the u.s. government, the obama administration has said that americans should be no closer than 50 miles to this nuclear plant. that's a greater distance than the japanese government has been using over the last several days.