A popular feature of most wrist watches is the ability of the hands and hour markers to glow in the dark. This feature makes it possible for the wearer to read the time when sufficient light may not be readily available. We take this for granted now, but in the early twentieth century glow in the dark paint was an exciting new technology.
Today watch dials and hands are treated with a paint known in the industry as Super LumiNova. The paint contains a photoluminescent compound that is excited when it interacts with light particles of a certain wavelength, this causes the compound itself to emit light for a period of time. Super LumiNova is non-radioactive, although like all paint I wouldn’t suggest ingesting it.
Watch dials and hands from the 1960’s and earlier are often treated with paint that contains Radium-226 as the energizing agent for the photoluminescent compound. The decay of the Radium isotope results in the emission of alpha particles which bombard the adjacent photoluminescent compound, the compound reacts by emitting a pleasant green glow. Since Radium-226 has a half life of 1600 years, these watches will continue to glow until the photoluminescent compound breaks down (usually after several years).
Radium paint has a distinct advantage over Super LumiNova in that it will continue to glow brightly through the night without being subjected to a “recharging” period under natural light. The drawback is that Radium-226 is radioactive and generally you don’t want to have anything radioactive too close to your body. Ingesting Radium based paint can be fatal as the Radium Girls discovered.
I’ve been aware of the danger Radium paint posed since before I began repairing watches but lately considered that perhaps there was more I could do to protect my myself (and my family) from those dangers. As a result of this anxiety, I invested in a small geiger counter/dosimeter to get a general idea of the scope of any contamination and also to try and quantify the danger each Radium treated part might present.
I purchased from Amazon a Quarta Radex RD1503+ geiger counter/radiation detector. The device detects beta, gamma, and x-ray emissions (this obviously leaves out the detection of alpha particles but equipment capable of measuring alpha emission is quite expensive and outside of my budget). The decay of Radium-226 produces gamma rays and further down the decay chain beta particles are emitted.
I must admit quite a bit of apprehension while waiting for the tool to arrive in the post. Had I been a complete knucklehead and put my family at risk? Did I take enough precautions in dealing dealing with contaminated watch parts? Were my precautions effective? When my purchase finally arrived and I was able to put these concerns to rest. What I discovered was sometimes heartening and other times unsettling.
From how I understand it, normal background radiation should register in the 0.08 to 0.10 microsievert range. Immediately after unpacking the RD1503+ I checked my workspace which registered at 0.13 microsieverts. While this is a little bit high, the rest of the house (and immediate vicinity outdoors) registered approximately the same level.
My recently serviced Pierce watches registered an elevated level of radiation but not beyond my expectations. Most have had the radium paint removed from the hands but the dials all have been left as they were.
Placing the most recently serviced Pierce next to the geiger counter/dosimeter resulted in the alarm sounding. This one doesn’t yet have the crystal installed and it’s quite apparent by the reading.
The sensing tube for the dosimeter is located on the left side of the unit. Gamma radiation will pass through most everything quite easily but can be stopped by dense materials such as metal. This made me wonder what the reading would be if I pointed the dial directly at the detecting tube.
With the crystal removed from the watch, the meter registered the highest detectible level of radiation- 9.99 microsieverts. The alarm sounded constantly. I quickly packed the watch away and considered investing in a lead lined box. Testing Pierce No. 4 in the same way resulted in a much lower reading though- apparently the crystal makes a big difference.
This may suggest that much of the radiation emitted is in the beta range which is a little less energetic than those gamma rays. Regardless, less is more when it comes to radiation exposure.
I ended by testing my brass tweezers which do most of the work and was overall pleased by the reading.
Just for fun I tested my daily-wearer which has Tritium based paint (used in watchmaking from the 1960’s to the 1980’s). Tritium is also radioactive but the decay half-life is only twelve years and Tritium undergoes beta decay which is not quite as dangerous as alpha decay. I wasn’t expecting a strong reading since the watch dates from 1967 but as you can see the Tritium has not lost all of it’s radioactivity yet.
So what is the takeaway from all of this? Even though the numbers on the dials may not glow in the dark anymore, the Radium paint is still about as radioactive as the day it was applied and this makes it quite dangerous to handle. Having the geiger counter handy is a good way of quantifying that danger and serves as a powerful reminder to take the necessary precautions to keep everyone safe.
When properly maintained and with the acrylic crystal installed these watches are probably safe enough to handle but the toxicity of what lies beneath that crystal should not be taken lightly. As the old saying goes, “An ounce of prevention is worth a pound of cure”.
Following are some interesting links and a video that helps visualize and quantify the danger posed by Radium-226 and radioactive materials in general. The first, How Much Radiation is too Much? A Handy Guide, is a link from the US Public Broadcasting System (PBS) that includes a handy guide to determining what a dangerous level of radiation exposure would be. Second, the US Nuclear Regulatory Commission FAQ Regarding Radium-226, publishes the guidelines with respect to ownership of products with radium based paint. Lastly is a video from a favored YouTube channel of mine, Veritasium, regarding radiation exposure. Enjoy.
2 thoughts on “Radioactive Dials”
I just read your article, I believe alpha particles are less energetic than beta particles, meaning alpha molecules produce less damage. The series particles/energy from radioactive decay is from lowest energy to highest energy: alpha – beta – gamma.
I’m responding late but yes, you are correct. Beta particles are more energetic than Alpha particles however alpha particles will put quite the hurt on you if ingested and stored in your body. The human body sees Radium as Calcium and stores it in your bones, which is not a good place for it to be. Radium goes through several iterations of decay resulting in the emission of both alpha and beta particles.