Most people think a Geiger counter is expensive to make or difficult to get. But a Spanish company has designed one that you can build yourself and has sent dozens of them to Japan.
Using an Arduino microcomputer, Libelium, a Spanish company that makes communications hardware, designed a Geiger counter that can be built and tested by home hobbyists (or bought preassembled). The item appears on the Cooking Hacks web site.
Libelium's development team posted the schematics and software, along with suggestions for testing the device. A discussion board linked to the page has several people offering suggestions for improving the device. Arduino computers are open-source, so there is no proprietary technology involved.
The Libelium team says they wanted to help the people in Japan who fell victim to the Tohoku earthquake and attendant nuclear disaster. So the first set of detectors made was sent to Japan. As technical guys, we feel the responsibility of providing our support in those areas where we can contribute. As a result, first batch has been shipped to Japan at no charge to the Tokyo Hackerspace and other working groups.
A Geiger counter consists of a tube of gas at low pressure (typically a noble gas such as helium or argon) connected to an electrode. When ionizing radiation - either alpha particles, beta particles, neutrons or gamma rays enter the tube, they hit the atoms of the gas and release electrons, which hit other atoms of gas and release more of them. The result is a signal carried by current that can be processed to power a display that shows much radiation there is.
The counters from Cooking Hacks will show beta and gamma radiation, and can be fitted with tubes that have a thin mica window at one end to show alpha particles.
The radiation sensor boards are sold for €65 ($93) and a sensor board plus the Geiger tube can be had for €95 ($135). They can only be pre-ordered as the first series has already been shipped to Japan.