I have an old Goverment Geiger counter, uncalibrated since its about 40 years old, But the G Muller tube seems in good condition.
I have been taking some comparative readings of some common substances, which surprised me

A Colemans unused gas mantle produced no additional reading above my background radiation level (approx 20 CPM)
BUT an old Indian gas Mantle shot up to about 200 CPM. Presumably it had some Thorium in it. I assume modern gas mantles dont use the stuff?

An old primitive anti-static brush for cleaning Vinyl Records, increased my background count by a small amount, when placed against the GM tube. Polonium?

My kitchen smoke detector, surprisingly did not increase the background radiation.......until I broke open the plastic which held the detector. When I got a reading of 1200 CPM approx. I guess the plastic stops the low energy Alpha particles.
Got to buy a new detector now.

I have not found any old clocks or watch dials, as yet, that were painted with Radium, to glow in the dark.

The Salt substitute, Potassium Chloride, which your Doctor will advise you to take to protect your heart instead of Salt (Sodium Chloride)
Surprisingly, almost doubled my background radiation level. Could this be due to a natural contamination by Potassium-40? I'm not sure, but a ref book states that Potassium-40 has a half life of 1.2 billion years?

A old Victorian green fired fireplace tile, supposed to be colored with Uranium Oxide showed no Geiger counter response.

I have found an old Radio Valve that shows a slight increase above background. Presumably due
to the gettering agent, or its filament. Make of valve unknown.

And lastly the item I read about which started me off on my radio-active search.
Brazil Nuts ! Yes, I read that they absorbed Radio-activity quite naturally.
The half dozen I bought, showed no increase above
background. Not to be outdone I intend to by a sackful around Christmas time? Or get a more sensitive Geiger counter.

http://www.orau.org/ptp/collection/consumer%20products/brazilnuts.htm

Anybody aware of any other items I can test?
Evaporated human wee-wee glows in the dark, any one know why?

CRC Handbook of Chemistry and Physics. Isotope Table.

Modern gas mantles use inner transition metal oxides to increase thermal optical emissivity. They are not radioactive like thorium oxide-based mantles, but they are not as efficient as light emitters, either.

Polonium anti-stats have a rather short half-life. Po is an alpha-emitter and won't give much GM response. Check out the rest of the decay chain and half-lives.

K-40 in potassium chloride will give you lots of crickets. Look at its decay branching, too - lots of energy in its emissions no matter how it goes. Scan a banana.

Do a nice field survey including drywall (uranium segregating with calcium from phosphate works giving gypsum as byproduct), water sofeners (recycled cooling loop polishing resins from nuclear reactors), concrete (hot aggregate), and granite.

Rebar, too - Co-60 in the furnace liner to tell when its ceramic thermal insulation is failing. Recycled metal is always a fun look.

Just about the only large scale application that is intrinsically radiation-free is automobile catalytic converters. Give 'em a look. Trust nobody.

Urine contains fluorescent compounds that are the result of hemoglobin metabolism, among others. If you have mice, they lack a sphincter and so dribble constantly. You can track their progress using black light in a darkened room. Scare yourself good that way.

As for radioactivity, try some lead, fishing weights or bullets or whatever lead you might find lying around. If you want shielding try lexan (tm) (plexiglass). It stops even hard beta particles if thick enough.

When I was working with isotopes we were given a Coleman lantern mantle as a check to see that the GM counter was working. Gave a nice rise in clicks. Also, be aware that "no response at all", i.e., no clicks, means the detector is swamped and you best beat a hasty and cautious retreat from the high radiation. But I'm sure you knew that.

Hmmm, so the device that is supposed to warn you of high radiation fails to operate in an environment of high radioactivity.

Quite a design feature:-)

Errr, no, not exactly Kate. The natural radioactivity of the items I have mentioned are so low that you have to put the G Muller tube, up close and against the items before it registers.
Were there to be an atomic strike, or leak from an Atomic Energy establishment, it would light up and click like a stick on a picket fence, I hope.

You are right up to a point, Mike. Wwhen there is a lot of radioactivity, the receptor doesn't have time to recharge between discharges, and your receptor is "Swamped" with radiation, "maxed out" if you will, and is silent. If you don't even hear the occasional click from cosmic ray background radiation, you are in a high radiation field and the need to leave is paramount. If your GM counter is silent, do not be complacent, take action. Of course, checking the connections would be a good idea if the problem persists.