This idea came late in the design phase of the sensor helmet. As it turns out, commercial sensors for detecting radiation aren’t as tiny as sensors for detecting light, sound, motion, etc. Also, they are 10 times the cost of other conventional sensors. The cost and size would prohibit the sensor being integrated with the sensor helmet, but it could still be used for specific mapping purposes.
I was able to initially source detectors online from resellers like Sparkfun and Adafruit. There are also a large list of results when searching online with the title of this post. One of the results that caught my interest was the RDTN.org effort in response to the Fukushima Daiichi nuclear disaster that occurred in Japan. They had small portable detectors that they were building and deploying in areas throughout Japan. These small devices would link up the readings to the internet and provide a collective map of readings using Xively. They succeeded in getting funding and have since changed there name to Safecast.
I had in the past made a detector using a transistor and simple circuitry, but this turns out to be nothing more than a amplified EMF detector. I would still like to connect a bar graph LED to that project, but it is not a reliable means to detecting background radiation.
I had found another project that looked interesting. This device has the means to provide location detection by using layers of tubes. The size and power required for this would limit it to fixed readings, but the design method was something I thought was worth mentioning here. The other detector that looked promising for small applications was the Scintillation counter. I have yet to find one designed for use with an Arduino.