I'd just punt this one. Cover the lens so that the image is otherwise black, get a good calibrated Geiger counter and wander around taking pictures of your lens cover and noting the counter's readings.
Go back and count the light dots in your black photos and see if they correlate with the counter readings. Use your fancy exposure-time/fstop/ccd-size math after you're sure there's something going on.
For that matter, bring along a crummy cmos sensor camera as well. More sensors, more fun.
As a Brit living in the US it took me over a year (and several misunderstanding) to realize that there is a difference in the use of the word 'punt' between US and UK. The main confusion was between:
1. (Informal) To cease doing something; give up [1]
2. (Chiefly British Slang) To gamble [1]
Often the context allows for either of those to make sense. Off topic, I know, but maybe it will help others avoid the confusion.
Yes, thanks for that. I should know better by now. Once while living in Germany I asked someone in my German-peppered-with-english-words-for-german-words-I-didn't-know if they were concerned about the amount of preservatives in some sort of shelf stable cheese food.
They heard Präservativ when I probably meant Konservierungsmittel. Hilarity ensued.
However punting (as in American Football manoeuvre) is not that well known a term in the UK. Perhaps punting as gambling refers to standing upright in a flat-bottomed boat an hoping to remain dry?
It is a standard term in rugby, anyone in the UK who knows about that sport is familiar with that definition. It's more commonly just called a 'drop kick' but people would know what punt meant in the context of sport.
Yes, you could go on to create a chart that you could extrapolate future readings from, but I was just trying to answer the question of correlation between photo artifacts and actual radiation.
You'd want to go back with a plan and find a wide range of different radiation levels to get a calibrated scale worth using. Your iphone almost certainly has a wacky non-linear response to gamma rays.
Also make sure to get a variety of angles to the suspected source. The sensor is pretty planar, so will probably have a much smaller response when it's edge-on versus facing directly towards or away.
Go back and count the light dots in your black photos and see if they correlate with the counter readings. Use your fancy exposure-time/fstop/ccd-size math after you're sure there's something going on.
For that matter, bring along a crummy cmos sensor camera as well. More sensors, more fun.