A recent posting to the Sextants@yahoogroups.com asked about renewing the illumination of the US Navy Mk V bubble chamber. I think I am correct in saying that this is identical to the AN 5851 chamber illumination. A little oddly, there was no attempt to have electrical illumination. Instead, self-luminous radium paint was used. Now radium has a half life of nearly 1600 years, so plainly it is not the radium that has decayed, but the luminescent compound mixed with it. Replacing the paint with a modern photoluminescent paint is not difficult, but some minimal commonsense precautions should be taken to avoid swallowing or breathing in the old paint.
The bubble chamber, incidentally, is one of the easier ones to service, as the manufacturer sensibly provided a plug in the form of a taper pin to allow refilling. The top and bottom of the inside of chamber need to be scrupulously clean before refilling and this is not so simple.
Let us assume, however, that you have a bubble in a chamber that is free from debris. The first step is to remove the chamber together with the eyepiece prism by removing the three screws shown in the following photograph:
Removal of the eyepiece prism is completed by inverting the bubble chamber and removing two more screws, as shown in the next photograph:
The radium paint is contained beneath the black washer-like disc that is secured by four tiny countersunk screws. Removing these allows it to be carefully prised from its seat. The paint can then be removed from the underside of the washer and from the underside of the bubble chamber bottom glass by softening and dissolving the paint with acetone. Certainly you should take every precaution not to inhale or swallow any. How you dispose of the paint is up to you. The level of radioactivity is low and there is only a small quantity, so disposing in the sea is unlikely to do any harm to anyone. A local university physics department or the radio therapy unit of a hospital might be prepared to advise.
Replace the paint on the underside of the washer with, first, a layer of white paint and then a layer of modern photoluminescent paint. A layer of the latter (but not the white paint) can also be applied to the bottom glass beneath the washer – the thicker the layer of the luminescent paint, the better the illumination. Re-assembly is the reverse of dis-assembly.
To charge the photoluminescent paint, shine a bright light, say, a 40 watt bulb, onto the index prism for 5 minutes or so. How long the light lasts depends on how long you charge the paint for and on the grade of the paint.
You might wonder whether overpainting the radium paint with a paint containing copper-doped zinc sulphide would renew the radioluminescence. The answer seems to be “No” because the alpha particles emitted by the radium penetrate poorly and the ZnS.Cu needs to be mixed intimately with the radium.
Post me a comment if you would like me to deal with cleaning and refilling the bubble chamber. It is not as simple as replacing the paint, but needs no very special tools, notwithstanding the instructions given in the original overhaul manual.