I keep a couple of Geiger counters to hand and have others in long term storage.

The basic model uses a color coded bar graph display.
For those whose interest in radiation is surviving it, rather than academic study, the coarse resolution of a 7 stage display is fine.
GREEN=NORMAL
YELLOW=PREPARE TO PANIC
RED=PANIC

geiger counter

The instrument fits in a shirt pocket and is very easy to use, I understand that the emergency services use this model.
It uses standard 9 volt batteries.

I also have a couple of more sophisticated Geiger counters, but no point in recommending them as they are no longer made.