Answer to Question #334 Submitted to "Ask the Experts"

Q

I recently purchased a luminous disk from a collector of military relics (WWII era) which is about an inch and one half in diameter. The disk has a brass backing and a glass front which is riveted to the brass plate. Under the glass is a disk which is about an inch in diameter and the disk still glows actively, despite its age. I'm told that parachutists of the 82nd Airborne Division used the discs to mark helmets, gear, and uniforms during WWII. The fellow who sold me the disk still has a lead-lined box which held up to four of the disks, and the box is stamped with the warning that a disk should not be worn on one's person for more than 24 hours consecutively. When I tested the disk with my geiger counter, the needle pinned to the extreme right on the bottom three scales and the audible clicks disappeared into a roar. The reading was far greater than any I have found from a luminous clock or watch face or from a WWII aircraft gauge. I am curious about the composition, manufacture, and radioactive safety of this collectible. Is the luminous medium a zinc sulfide/radium salt mixture or some other composite? How much radioactive material is in the disk (one source I consulted earlier said that he thought each disk contained 7 micrograms of ²²⁶Ra, but another individual told me that the disk probably does not contain that much radium). Is it safe to own these items? I would appreciate any information you can provide.

A

These disks are frequently referred to as "deck markers" and they were widely used by the various branches of the armed forces. Being radioluminescent, they provided a passive continuous source of low-level light for nighttime maneuvers. Until the mid 1960s, the radioluminescent material consisted of a mix of ²²⁶Ra and zinc sulfide (ZnS). Later versions used ⁹⁰Sr as the radioactive material. The type of disks you are describing varied in size and activity. From what I have seen, the activities ranged from 5 to 15 microcuries (one microcurie of ²²⁶Ra is equivalent to one microgram). A good guess therefore would be 10 uCi. My personal opinion is that devices with this amount of radium should be turned over to the state radiation control program (often these programs are part of the state's Health Department). Sooner or later, someone is likely to open them up and the resulting mess could require a relatively expensive cleanup. I get offered these things on a regular basis and I always turn them down. Quite frankly, they aren't that interesting.

Paul Frame, CHP, PhD