[R-390] Panel meters, some numbers

[Tisha Hayes]

I was doing a clock restoration of a Ingraham "Atomic", eight day wind-up
clock. It was a $25 ebay purchase that was claimed to be working and
sometimes it did work so I knew the mainspring wasn't broken but that it
probably was dirty or gummed up.

These have radium paint on the hands and on the main numbers. They were
made from the late 1920's until the 1950's. Looking closely through the
glass I could see the radium paint but also that there were lots of "fleas"
floating around under the glass. Often these are caused by the highly
energetic alpha particle decay causing the zinc and radium to flake off. I
used to see something similar in the hot-cells at the lab I worked at
during my university internship.

I masked up, gloved up, went outside and worked on a HDPE (plastic) sheet
and disassembled the clock; First removing the winder and the back screws.

As I went I cleaned each part with 99% denatured ethanol on a cotton swab
with the plastic stick instead of the paper core. Dipping a new cotton swab
in alcohol while I wiped down each part;
1) The inside of the rear case cover
2) Clockwork mechanism (that someone had oiled, hence the clock being
gummed up and not working), but not getting cotton stuck in gears, or
springs, just to de-grease those parts, so this was not a thorough cleaning
3) Removing the front ring I swabbed the inside edge of the front ring.
4) The inside of the glass (quite dirty and lots of black residue came off,
it took two swabs to get this clean)
5) A slightly damp swab very gently on the face of the clock dial to remove
loose contaminants

I did a quick survey with an alpha particle detecting probe, then a beta
probe, a low energy scintillation detector probe and finally a G-M probe
for beta and gamma. other than the face the remaining counts were minimal
on the clock parts, except for the gearing where I would really need to
pull that and give it an ultrasonic bath.
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I got the clock working (what was my real intent of this procedure) but I
went ahead and used hemostats to pull off all of the cotton bits that were
still damp with alcohol (since I used ethanol instead of methanol the
evaporation rate is a bit less). I put all six of the swabs in a tiny
ziplock bag (the kind you get when you buy coax connectors).
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Since I deal with radiation on a quite regular basis I have some very
sophisticated equipment; CsI(Tl), NaI(Tl), LYSO, BF3, 3He, gas proportional
alpha and not just for counting but for isotope identification using gamma
ray spectroscopy. I can measure and quantify most anything I run across.
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What I suspected is that there would be a large number of radium
"daughters" that had contaminated the case of the clock. Some of this comes
from the physical disintegration of the radium/zinc paint and would be in
particulate form. The remainder would be from the immediate radium decay
product that is radon "gas". In an enclosed space the radon gas will sit
until it goes through its 3.8 day half lives that are the next step in the
decay chain.

After the radon decays it becomes Polonium 218 for a few minutes, then Lead
214 for a few more minutes, then Bismuth 214... and on and on. Some with
beta particle decays, some with alpha particles. After years it will become
stable lead or mercury. The inside of this clock is going to have an entire
array of these different elements that are all radioactive to varying
degrees.

In the wipings from the clock here is what I found for radiation
measurement;

With just a beta/gamma probe for a raw count I detected 790 uR/hr. That is
7.9 uSv/hr. I deducted the background level in my lab that is at 20 uR/hr
right now (some shielded samples at a distance in pigs)
Beta particles; 1.63K CPM  (about 27 Bq)
Alpha particles; 2.11K CPM (about 35 Bq)
I also detected some very low levels of X-ray or gamma radiation that was
above background but not in a way that I could quantify without doing
long-counts.

Since the gamma and X-ray was so low I would need to do a very long run of
scintillation counting to find out what isotopes were there. I am currently
out of liquid nitrogen to do anything with the HPGe detector.

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The take-away from this is that if you go cracking open the panel meters on
any military gear you should fully expect that those cases are contaminated
everywhere with the radium daughter products. Even if you have an old
yellow Civil Defense meter you are not going to be able to do anything more
than maybe detect a small fraction of the beta particles and none of the
isotopes that are throwing off alpha particles.

None of this is immediately dangerous; There are no levels that are going
to make your eyeballs explode or your hair to fall out. But you can readily
internally contaminate yourself by breathing in any loose particles and you
will definitely be coating your fingers with these daughter products and
then turning that in to internal contamination if you go and eat a cookie
or smoke a cigarette.


*Ms. Tisha Hayes, AA4HA*