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Pocket Chambers and Pocket Dosimeters |
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Paul Frame, Oak Ridge Associated Universities |
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Pocket chambers and pocket
dosimeters are small ionization chambers that, as the name implies, are
usually worn in the pocket. While they were designed to measure x-rays and
gamma ray exposures, they would also respond to betas above 1 MeV.
Neutron-sensitive versions were also available. The terms pocket
chamber and pocket dosimeter are often used interchangeably. The original
distinction between the two terms, used here, is rarely made anymore. In
part, this is due to the fact that the devices that I call pocket chambers
are rarely used any more. 1. Pocket Chambers Pocket chambers go by a
variety of names: indirect-reading dosimeters, non-self-reading dosimeters
and condenser-type pocket dosimeters. Prior to WW II, they were only used
to a limited extent, primarily in medical facilities and around
accelerators. The Manhattan Project however created a huge demand and they
were worn by almost everyone who might be exposed to radiation. A pocket chamber acts
as an air-filled condenser (capacitor) much like the thimble chambers used
in radiology. Prior to being
worn, it is given a charge with a charger-reader, e.g., the Victoreen
Minometer. Any
subsequent exposure to radiation ionizes the air inside the chamber and
this reduces the stored charge. In
order to quantify the exposure, the charge is measured and the decrease is
related to the exposure. Pocket chambers were approximately 4 - 5” long and 0.5” in diameter. An aluminum rod (ca. 0.0625” in diameter) running along the chamber axis served as one electrode, while the outer wall of the chamber served as the other electrode. The central electrode was suspended at each end with a polystyrene insulator and at one end it penetrated the insulator to serve as the charging contact. One problem with the early models involved the threaded caps that were used to protect the charging contact - they would wear and the metal fragments would get on the insulator. The graphite coating on the inside of the chamber wall caused a similar type of problem with some of the early models because it would sometimes flake off and short out the chamber. The early models were also susceptible to discharge as a result of mechanical shock because the central electrode would flex and contact the chamber wall. To solve this problem, later versions used a thicker central electrode and/or positioned a small insulating disk in the center of the electrode. Because of these problems, it was usual for a worker to wear two dosimeters and the lower of the two readings was considered the most accurate.
2. Pocket Dosimeters Like pocket chambers,
pocket dosimeters are known by a number of other names, e.g.,
direct-reading dosimeters, self-reading pocket dosimeters and pocket
electroscopes. They are actually quartz fiber electroscopes the sensing
element of which is a movable bow-shaped quartz fiber that is attached at
each end to a fixed post. The latter is also shaped like a bow (or
horseshoe). The dose is determined by looking through the eyepiece on one
end of the dosimeter, pointing the other end towards a light source, and
noting the position of the fiber on a scale. Until 1950 or so, the vast
majority of pocket dosimeters had a range up to 200 mR, although a few
high range versions were available for emergency situations. Higher range
versions became more readily available in the 1950s for military and civil
defense purposes. Pocket dosimeters tended to be slightly larger than pocket chambers. Their walls might be made of aluminum, bakelite, or some other type of plastic. If the material was not conductive, the inner surface of the chamber was coated with Aquadag (graphite). The central electrode was usually a phosphor bronze rod. This made pocket dosimeters more energy dependent than pocket chambers whose central electrodes were usually aluminum. Some dosimeters (e.g., Keleket Model K-145) employed boron-lined chambers which made them sensitive to thermal neutrons. Pocket dosimeters must be charged (ca. 150 – 200 volts) with some sort of charger, but they do not require another device to read them. This allows the worker to determine his or her exposure at any time, an important advantage when working in high radiation fields.
The first direct reading pocket dosimeters were built by Charlie Lauritsen at the California Institute of Technology. 3. Pocket Chambers (indirect-reading) vs Pocket Dosimeters (direct reading)
References Morgan, K.Z., Health Control and Nuclear Research, unpublished manuscript, ca. 1951. |
