The thing about using a radiation detector is that, what is it really allowing you to sense? At best all it would do is 'highlight' others normal appearing objects in a manner which isn't really that important to humans in everyday circumstances. It would be kind of like a human being able to sense two different shades of blue as well as red and green. Sure you could tell the difference between those two shades significantly better. but it provides no really beneficial sensory information, unless you work at a nuclear power plant or do radon gas inspections, and they already have plenty of devices which do that.
Ultrasonics would provide the most practical implementation I think. Maybe subsonics.. Using FFT algorithms you can stretch or compress ordinarily unnoticeable frequency bands into something the human ear can pick up, constructing one of these devices would be I think the most practical and useful thing you could do. They need a decent amount of processing power, but nothing a DSP or a PC couldn't do.
Cross correlating sensory information is another possible avenue you might want to investigate. Probably the most fascinating 'disorder' I've ever heard about, a very very small portion of the population has something called synesthesia. The primary symptom of this disorder is a person seeing letters or numbers in colour, and the same numbers/letters are always the same colour. This cross sensory overflow overrides the visual cortex's normal input from the eyes.
I've seen also a recent bionics project which takes a digital camera, feeds it through a DSP and outputs to a grid of sensors attached to a mouth piece (against the tongue) providing a slight electrical impulse processed from the camera, allowing blind people to taste their surroundings. Well enough at least to avoid large physical obstacles.
There are commercially available multi gas sensors, but they're very expensive. I think they have an Ismell on the market still that can create via software on the PC side several thousand different odors, from a stock of only a few base chemicals, sort of like a printer cartridge. Obviously applications for that kind of thing are very limited =)