Conservation of radioactive objects

Valerie Tomlinson <vtomlinso<-at->aucklandmuseum<.>com>

Sophie Rowe <rswr2<-at->cam<.>ac<.>uk> writes

>The Polar Museum holds a radioactive compass with radium paint on
>the inside of the lid, and this radium paint is now beginning to
>flake and contaminate the inner surfaces of the object ...

A World War 1 compass with radioluminescent paint was treated in the
summer of 2015 in the Objects lab at the Canadian Conservation
Institute.  Dose rates were measured with a Victoreen 190 survey
meter, and measured a maximum dose rate of 380 microsievert/h when
the probe was placed directly above the exposed paint.  The dosage
rate dropped to 50-60 microsievert/hr when the compass was closed,
and went down to 2 microsievert/hr when the probe was positioned 30
cm away. The dosage rates were within the accepted limits of the
Annual Dose Equivalent Limit regulations for Canadian workers and it
was determined safe to treat the compass.  Personal protective
equipment, including a lead apron, was worn at all times as an extra
precaution.

Although the relatively low radiation values detected were specific
to this object, it is significant to note the degree of shielding
that was provided by the compass case itself, and that a distance of
30 cm from the compass provided enough shielding for the dosage rate
to drop to the equivalent of background radiation.  A small storage
box (with ventilation holes to prevent the buildup of radon gas) was
constructed for further shielding.  The box was labeled with a large
image of the compass (to prevent unnecessary opening and handling)
as well as radiation hazard warning labels.  When the compass was
returned to the client, it was advised that handling of the object
be kept to a minimum and that the compass be kept closed whenever
possible.

The paint was analysed by micro x-ray fluorescence spectroscopy at
CCI.  The analysis provided significant historical information about
the compass.  While radium itself was not detected, its decay
products (thorium, lead) were found to be present.  The earliest
radioluminescent paints used radium-226 which has a half-life of
1600 years.  In the late 1910s, radium-228 came into use with a
half-life of 5.8 years and thorium-228 as its first relatively
stable decay intermediate.  Our detection of thorium by XRF analysis
was therefore a useful indicator in dating the manufacture of the
compass.

The luminescence of these paints was a result of zinc sulphide
crystals being excited by the radiation of trace amounts of radium
and emitting electrons that would reach activator sites of either
copper, manganese, or silver.  The activator sites after being
excited would release photons, causing the paint to glow.  The metal
used as the activator determined the colour of the luminescence; in
the case of the WW1 compass, copper was determined to most likely be
the activator metal, which elucidated that the compass would have
likely glowed green.

Like the luminescent paint on the compass at the Polar Museum, the
paint on the WW1 compass was also cracked and flaking.  Luminescent
paints generally become brittle and friable as they age, as they
contained little binder to begin with so that it would not absorb
the radiation before it could reach the zinc sulphide crystals.
Most of the flaking paint was contained and sealed beneath the bezel
glass of the compass.  It was decided to leave these flakes where
they were, as they were safely contained and were not at risk of
becoming airborne.  The paint exposed on the underside of the lid
was consolidated with a coating of a 20-30% Paraloid B-72 solution
in acetone and ethanol.  The coating will delay the fragmentation of
the paint, as well as provide some added shielding.

We felt it was worth taking these measures to preserve the paint, as
the traces of information it contains give it considerable value.
Research of the manufacturing of these compasses underlined the
historic value of the paint, as it serves as a tangible reminder of
the women who lost or shortened their lives by applying it.  There
are lessons that can still be learned from their tragic story;
thorough investigation of the health effects of new technology-based
products remains a relevant and important issue today.

Sophia Zweifel
Postgraduate Intern
The Canadian Conservation Institute


                                  ***
                  Conservation DistList Instance 30:8
                   Distributed: Sunday, July 10, 2016
                        Message Id: cdl-30-8-002
                                  ***