I'd been thinking about this myself, that is, low cost geiger counters. Terrible tragedy, your assistance may be very important.. it's not cool finding out about the Strontium after you drank the water.
I notice that you've used a GM tube, I'd been looking around and seen these as a likely good sensor:
but I have a question.. though obviously not as accurate, wouldn't it be possible to fabricate an ionizing radiation detector from the ground up, using a photoresistor/transistor, a light-sealed container, and standard phospors? Basically a poor-man's photomultiplier?
As I understand it, Beta on up should cause, say, a fluorescent tube to DIMLY illuminate when and where an energetic ionizing radiation particle strikes, knocking up that energy level and then emitting a photon. Is there a reason why we can't make a LOT less costly sensor, even if it isn't as accurate, by adding a darlington phototransistor to a plastic can holding a CFL bulb? Beta is nothing more than high-energy electrons, and commonly available phosphors react very nicely to that energy range, it's what makes them useful for light bulbs.
In my mind, either this (or a Neon, like an NE-2) bulb ought to be usable as a crude sensor for at least Gamma and high energy Beta.. Alpha would be stopped by the plastic and glass, as well as low-energy Beta... but honestly, neither of them are of great concern. It's that high-energy stuff that knocks bits around in you chromosomes that you need to avoid.
In looking at this stuff, it's the Geiger-Meuller tube that costs so much and is likely to break.. and are designed for lab measurements. What I am envisioning is a lot simpler.. a "safe/not safe" sensor you could place in proximity to food, water, whatever for a short time and not be working blind, so to speak. A simple threshhold level sensor, rather than a quantative measurement as can be had from GM tubes.
I would think there would be a number of semiconductor devices which ought to react to ionizing radiation, not the least of which being Cadmium Sulfide cells and Silicon photocells and that type of device. Any of these things should "notice" ionization, that's how they operate.
If a $50 fragile GM tube could be replaced with a $.20 CDS cell, and that would tell you if your glass of water was safe to drink.. there's a workable "emergency, triage" type device that could really make a difference in this type of disaster. Are GM tubes really still the best we have?
What about the reverse of the old smoke detectors? As I remember, they depended somehow on ionozation of smoke particles from a (weakly) radioactive source. If smoke+ionizing radiation is easily detectable, maybe smoke makes a cheap but inaccurate ionizing radiation detector?