Answer to Question #133 Submitted to "Ask the Experts"

Q

I am not sure if you can help, but it has come to my attention that gauges from aircraft are radioactive and the older they get the greater the radioactivity. Since I work on aircraft I am rather concerned. My friend has some old gauges that he brought in for testing with a Geiger Meter and to our surprise it pinned the Meter. I am concerned as to what type of radiation these gauges produce and if we have any concern for alarm.

A

For many years radium was used to make luminescent dials for various purposes. The radium was mixed into a paint with a phosphorescent agent that emitted light after absorbing the radiation from radium's decay process. Radium and its progeny are a part of the natural uranium decay chain but have been concentrated from its natural state for this use. Radium (and its progeny) emit alpha, beta, and gamma radiation. All of the alpha radiation and most of the beta radiation is stopped by the glass cover of the instrument. The gamma radiation passes through the glass and is easily detected by most radiation meters. At 1 foot from the instrument, the radiation is usually 30 times less than at the instrument's surface.

Typical radium dial gauge found in older aircraft.	Baby Ben" clock and pressure gauge with radium dials

For comparison, the radiation at the face of the clock pictured above is 0.5 mR/h and at the gauge is twice that amount. At 1 foot, the radiation is less than 0.03 mR/h for both. At a distance of 2 feet, the radiation is difficult to distinguish from normal background radiation. In most instances there is no reason to be concerned. These devices should not be disassembled or the glass face removed. If the face becomes broken, using disposable gloves, seal the device in a plastic bag and dispose of it in accordance with your local requirements. The devices should not be allowed to accumulate in large quantities (20 or more). The amount of radiation emitted is more closely related to the amount of radium paint than the age or the size of the instrument. The radium paint is usually "off-white" to brown as it ages. Look for the radium paint on the hands, numerals, and tick marks. While modern (since mid-1960s) instruments do not have radium, many use promethium or tritium to achieve the same glow. They are also radioactive but the radiation emitted is insignificant. — Michael Murray, ORNL, email: MurrayME@ornl.gov