Radium watch hands boley radiumzeiger

Radium 226 Paint on Watch hands

Recently I was allowed to take a look at set of radium watch hands. The watch hands were made from the early 1900s to the late 1970s. They are interesting because when they were new, they would glow bright green because of the radiation of the Ra-226 hitting the zinc sulfite in the paint.

Now, many years later, the paint only emits a tiny fraction of the light it was emitting when it was brand new. One might think that is because the Ra-226 isotope has decayed so much that it can not make the zinc sulfite glow anymore. We know that Ra-226 has a alf life of about 1600 years. So considering the watch hands were made in the 50s, the Ra-226 has only decayed by 2.2%! What has really decayed is the zinc sulfite in the paint being hit by electrons and Helium nuclei all the time.

The whole pack is a set of 48 pairs (96 total) of watch hands for wristband watches. The hands are 10mm to 14mm long and the longest paint strips are 6mm long. It must be noted that not all of them are painted, on some pairs only one is illuminated. The size of the pack looks just so much bigger on the pictures than it is in reality. It fits in the palm of ones hand quite well.

I took several measurements without taking them out. My Geiger Counter with a non compensated SBM-20 Tube placed in the middle of the pack averages at about 22 CPS (1320 CPM).

measuring radium watch hands with scintillation counter

My big 52×52 Nai(Tl) gamma scintillation probe attached to a Ludlum Model 3 reads 110.000 CPM with a average backround reading of 6.000 CPM.

We can now very roughly assume, that my scintillation counter detects all of the radiation going to one side. To detect the true CPM which would be much higher because of the shielded betas, we need a 4-pi proportional counter. Bout to roughly estimate the radium contents of all the 98 watch hands, we go on with our value of 10400CPM to one side (2-pi). That would be 3466 CPS to all sides. The specific acticity of 1g Ra-226 is 3.655*10^10. That means one gram of Ra-226 decays 36.55 billion times per second.

Now that we can set both numbers in contrast we can say that there are at least 94.83ng of Ra-226 in the whole pack of watch hands.