Introduction to Health Physics: Fourth Edition - Ruang Baca FMIPA UB

704 CHAPTER 13
by gamma-ray spectroscopy. The adsorbed radon activity is determined by placing
the canister directly on a scintillation counter and counting the gammas from the
adsorbed radon and its progeny. The charcoal adsorption system is calibrated by
measuring the activity after exposing the canister to known concentrations of radon.
Corrections for temperature and relative humidity must be made since these factors
influence radon adsorption by the activated charcoal. The short half-life of radon
requires that the exposed canister be measured as soon as possible after sampling is
completed. The sensitivity of this method is about 0.5 pCi Rn/L (18.5 Bq/m 3 ), or
about 2.5 × 10 −3 working level (WL).
Alpha-Track Detectors
Long-term average Rn concentrations can be determined with alpha-track detectors.
An alpha-track detector is a piece of plastic that suffers radiation damage when it
is exposed to alpha radiation. In use, the detector is enclosed in a filter-covered
container, and air diffuses through the filter into the container. Alpha particles from
the airborne radon and its progeny strike the detector, and produce submicroscopic
tracks in the plastic. At the end of the exposure period, the container is sealed and
brought to the laboratory. There, the detector is removed from the container and
the alpha tracks are “developed” by placing the detector into a caustic solution.
This enlarges the submicroscopic tracks so that they are visible with a microscope.
The track density is directly proportional to the total radon exposure. Because the
alpha-track detector records the effects of radiation rather than to the radiation,
and because the sensitivity and precision of the measurement depend on the total
number of alpha tracks, long-term average radon concentrations can be measured
simply by leaving the detector in place for a long period of time. The practical
sensitivity of the system is about 0.2 pCi/L-month.
Electret Ion Chambers
An electret ion chamber (EIC) is an integrating monitor whose operating principle is
similar to that of a capacitor-type pocket dosimeter. An electret is an electrostatically
charged plastic (usually Teflon r○) disc. In an EIC for measuring atmospheric radon,
the electret is placed into a chamber that connects with the atmosphere through
a filtered portal that is kept covered until the start of a measurement. During the
measurement, usually a period of 3–7 days, the filtered portal is open and radon
gas diffuses through the filter (whose function is to prevent airborne particles from
entering into the ion chamber) into the chamber. The alphas from the radon and
its progeny ionize the air in the chamber, and the ions neutralize charges on the
electret’s surface. The electret charge is measured before and after the exposure,
and the rate of change of the charge (change divided by the time of exposure)
is proportional to the average concentration of radon during the measurement
period. Electrets are available for short-term exposures of 2–7 days, and for longterm
exposures of 1–12 months. The sensitivity of this method is about 5 Bq/m 3
(0.14 pCi/L) for a 7-day measurement.
Grab Sample (Kusnetz Method)
If we wish to know the radon levels in an occupational setting, where higher radon
levels may be expected, such as in a mine, in a gypsum factory, or in a phosphate
ore processing plant, we can take a grab sample and analyze it for radon progeny. A