My Cellphone has a Sensor in It
By now, everyone realizes that cellular telephones (a.k.a., cellphones) have locator capability. This capability, known as Enhanced 911, is a consequence of the Wireless Communications and Public Safety Act of 1999. Cellphone location can be done using radio direction-finding from two or more cellphone towers, but some cellphones now have a GPS receiver built into them. Some of these GPS units are based on assisted GPS in which some location information is derived from proximity to a particular cellphone tower. Cellular companies weren't too reluctant to provide the Enhanced 911 feature, since location information can be used for profit; e.g., pumping ads for local restaurants to a cellphone screen. Since technology can be used for both good and evil, I keep my cellphone powered-down most of the time, and I'm a member of the Electronic Frontier Foundation.
Ephraim Fischbach, a Fellow of the American Physical Society, Professor of Physics at Purdue University, and author of an interesting paper on testing random numbers , has proposed a novel enhancement for cellphones . If radiation detectors can be built into cellphones, their data, combined with GPS data, can show the location and movement of radioactive materials. Radiation shielding is not 100% effective, so even when dangerous levels of radiation are prevented, there's still detectable radiation above the usual background level.
Andrew Longman, a Purdue alumnus who is working on instrumentation for the project, explains the rationale for the project quite simply. "The likely targets of a potential terrorist attack would be big cities with concentrated populations, and a system like this would make it very difficult for someone to go undetected with a radiological dirty bomb in such an area." AT&T is providing free data air time for the project which is funded by the Indiana Department of Transportation.
Testing was accomplished by installing a (safe) radiation source, detectable at a fifteen foot range, on the Purdue campus, and having people walk around the campus randomly. This source was much smaller than an expected "dirty bomb" source, and its position was determined easily. The system could be used to detect radioactive spills and other accidental radiation releases. There will, of course, be some false alarms, such as radioactive bananas. Bananas contain nearly half a gram of potassium. One isotope of potassium, 40K, has a natural abundance of 0.012% and a half-life of a little more than a billion years. 40K doesn't sound that active, but half a gram is still more than 1022 atoms, of which about 1018 are radioactive. This results in about a billion decays in a year, or about thirty decays per second from your luncheon banana.
1. S.J. Tu and E. Fischbach, "Geometric Random Inner Products: A Family of Tests for Random Number Generators," Phys. Rev. vol. E67 (2003), 016113.
2. Emil Venere and Elizabeth K. Gardner, "Cell phone sensors detect radiation to thwart nuclear terrorism" (Purdue University Press Release, January 22, 2008).