Like old wrist watches with “glow in the dark” hands and numerals, some meters of that era had radium paint on their needles and scales. As you know, a Radium-containing compound was mixed with a phosphorescent material that emitted visible light without the need to “charge” the paint with an external light source. In this case, the Alpha particles (helium nuclei) emitted by the decaying Radium 226 bombarded the phosphorescent material which converted the Alpha particles’ energy into visible light. For a long time. The phosphor material was typically Zinc Sulphide crystals.
The meter shown above has Radium paint on the needle and the 3 scale lines. This paint is usually a pale yellow to light brown color from the ones I have seen. This particular meter emits 0.5 mR/hr, about 50 times the normally low background in this area but still below that of a pre – 1990’s Coleman camping lantern mantle.
(Lantern mantles had naturally-occurring radioactive Thorium 232 in them. Thorium has a very high melting temperature and emits a very bright white light when heated by the burning lantern fuel. When Coleman stopped selling the Thorium mantles the replacement product apparently used Yttrium instead. I have both types; the newer mantles put out about 60% of the light the Thorium mantles do. These old – style Thorium mantles were routinely used by health-safety personnel as “check sources” to verify the proper operation of geiger counters.)
Above: The meter in my SCR-284 is relatively “hot” radioactivity-wise. More so than any other radio or meter that I have. Probably because there is a relatively large amount of paint on the scale. It puts out about 6 mR/hour as measured with an Eberline E-120 Geiger counter (with detector probe HP-190A) contacting the meter glass. Radium 226 is primarily an Alpha emitter but it does emit some Beta and Gamma radiation and it stays “hot” for a long time (half life of 1620 years as it decays into Radon gas). It finally damaged the phosphor paint it was intended to excite in this meter. Very, very dim glow these days; the glow is gone but the Radium is still there. Like a radioactive home smoke detector, these things are not very dangerous if left intact. Don’t disassemble it…
These measurements are primarily from Gamma radiation – Alpha’s won’t penetrate the meter glass, nor will most Beta’s; sorry, no Neutrons. My R-390A Receiver meters also had detectable levels of radiation: The Carrier Level meter read 1.2 mR/hr, the Line meter read 0.4 mR/hr. My TBY meter read 1 mR/hr, my GRC-9′s labeling each read just background. Background radiation during these tests was 0.01 mR/hr. Probably no radium paint used on my GRC-9’s – your mileage may vary. They utilize the newer type of phosphorescent paint – you must “charge” it with a strong light source. There is no radioactive material in that type of paint.
Above: The unusual panel meter on the TBY Receiver-Transmitter. A little radium paint on the “center” square and needle. Aim here.
I remember that when I was a kid I looked at my fathers wristwatch hands under a low power microscope. In the dark, you could see the individual flashes of light emitted from the paint as an Alpha particle slammed into a phosphor crystal. Scintillating!
Radiation follows the inverse square law. If you double the distance from the source, the radiation flux is reduced by a factor of 4. Just like with explosives, distance is your friend when dealing with sources. If you are VERY close, say by eating or inhaling the paint, you may have a serious health issue. Keep them sealed. Just be aware.