Vintage radioactive wrist watches - Is Radium really nasty?

[Manolo Sobrino]

Jebem, I think no one really needs (nor really wants) to own any amount of radium just for the fun of it. Maybe you can keep it somehow safely now, but can you expect that it will be handled properly in the future?

Alpha emitters when inhaled or ingested become a "fully-blown" ionization source sitting within your living tissue, that's why they are powerful mutagens and even small amounts are dangerous (don't judge these things for their size, but for their activity). This radium paint can produce some nasty powder. The idea of radiological protection is to prevent any unnecessary exposure, more so beyond the general public/student/worker safety limits.

I wouldn't trust or use a G-M tube for dose measurements. Unfortunately DIY people talk about mSv all the time, they have no idea what they're talking about. The sievert is the unit of dose equivalent, which is the product of the absorbed dose by a dimensionless quality factor Q. Q = 1 for gammas and electrons, but it can be as high as 20 for high LET radiation, for instance alphas. What they mean is absorbed dose, which is measured in grays, 1 Gy = 1J/1kg = 100 rad. But in order to measure this you need to measure the absorbed energy, for instance with a chamber where you can apply the Bragg-Gray principle.

G-Ms actually detect radiation as radiation monitors, they count ionizing particle events that get into their chamber (that means that you need a thin window and being really close to the source for alpha and beta particles, because of air attenuation: the range of alpha particles in air is just a few cm), but can't give you any information about their energy or type because they operate in a kind of saturation plateau, so there is no way to measure the absorbed energy. I think there are some variants that claim to use "energy compensation" (this one probably isn't), I haven't seen one myself. People use ionization chambers (often inside phantoms) for these things.

So what are you measuring here?

Decay scheme Ra 226

Decay scheme Rn 222

Well, let's consider the Rad 226. Besides the 186 keV gamma emission of the Rn 222 that happens 3.5% of the time, which you detect mostly through Compton scattering electrons from the walls of the chamber (the G-M efficiency for gammas is a few percents), most of the counts are delta rays (secondary electrons) coming from the 4.6-4.8 MeV alphas in air.

You have have the decay chain of the daughter nuclide as well, Rn 222, which is a dense inert gas that will diffuse around. Its progeny, while emitting more alphas (more delta rays for the counter) and now betas too, will deposit nearby resulting in a certain amount of Pb 210 contamination (that will turn slowly into awful Po 210).

Notice that you have this in a plastic bag, then air, the pic shows the detector next to, but not over the source (it could be closer as well) and we have no idea about the efficiency of the detector (its geometric efficiency is hard to figure), or its calibration (what about dead time?). Those 7000cpm are surely proportional to the activity of the source, but we can't really tell their relation to it.

About your G-M tube, I've found this info:

http://gstube.com/data/2398/

http://mightyohm.com/blog/2014/11/a-spot...nter-tube/

According to this, at 500V you're probably operating it beyond its VDC rating (350V-475V), recommended 400V (there is a 260V min, 320V max starting voltage there). Above the G-M plateau the gas-filled cylindrical ionization chamber is not a G-M any more and you might break it, specially if the quench compound is worn out.

I don't understand the "Gamma Sensitivity Ra226 (cps/mR/hr)" figure. It makes sense it being higher than the one for Co 60, as Co 60 is a >1 MeV gamma emitter but I really don't know how to use that number. If someone does, please give us a hint. On the other hand, I wouldn't trust the specs of a radiation detector hopefully calibrated some 40 years ago.

Please, check this:

https://www.orau.org/ptp/collection/radi...ntinfo.htm

Quote:"NCRP Report 56 indicated that the estimated average dose equivalent to the gonads of an individual wearing a radium-containing watch was approximately 3 mrem per year. The estimated dose from a watch containing 4.5 uCi of radium, a very large amount, would be 310 mrem per year to the gonads. Note that the associated risk here is much lower than if the dose had been the same over the entire body.

In the previously mentioned study of pocket watches by the National Center for Radiological Health, measurements using TLDs and a human phantom indicated that the annual dose to the gonads was 60 mrem (assuming that the watch was worn 16 hours per day)."

As 1 Sv = 100 rem, that high activity watch would yield a dose equivalent of roughly 3 mSv/year to gonads. For the entire body it would be less because the tissue weighting factors are higher for gonads (w_T=0.20) than for instance, lungs (w_T=0.12) or skin (w_T=0.01). The ICRP recommended exposure limit for members of public is 1 mSv/year for whole body exposure. I guess watches are relatively safe to wear (not to tinker with) but, as you might take planes frequently and maybe have an X-rays or CT scan every once in a while, I think there's just no need for that.

My advice would be: use disposable gloves, put that bag in a sealed thick-plastic food container, throw in the gloves before closing it and (assuming you are in Lisbon) contact these people (my educated guess, I really don't know how things go in Portugal, for instance in Spain we have a state-owned company -Enresa- for this) they can measure/dispose of it:

http://www.itn.pt/sec/psr/pt_psr_rrr.htm

Let me recommend you a book on this (I used its full of typos 2nd edition some years ago):

James E. Turner - Atoms, Radiation, and Radiation Protection