Created in the 1990s, the Atom-Tag detector enjoyed a certain success due to its compactness and simplicity. However, it has not been commercially available since the late 2000s. 

Production of this detector has recently resumed, this time in Australia. It still bears the same name and is as compact and practical as ever: just charge it up and it's ready to go! It's compatible with the OpenRadiation application. And it has two advantages: apart from DIY kit detectors, it's the cheapest on the market, and it's very reliable! 

If you'd like to order one, here's the link: https://www.gammaspectacular.com/blue/gamma-spectroscopy/radiation-detectors-and-dosimeters/atom-tag . Remember, it's the energy-compensated model that you need to order. 

Photo credit : @Jean-Marc Bertho