Radiation Detection, Survey Instruments and Dosimetry
Instrument types
Ionization chambers (including pressurized chambers) give accurate, relatively energy-independent exposure/dose-rate readings and are the usual choice for radiography surveys. Geiger-Mueller (GM) detectors are very sensitive but energy dependent, have dead time, and can saturate and read falsely low or zero in intense fields - never rely on a GM alone near an unshielded source. Scintillation detectors (NaI(Tl)) are efficient for low-level and spectral work; proportional counters discriminate radiation types and are used for contamination. GM pancake probes are standard for wipe/leak-test screening.
Calibration and checks
Survey instruments must be calibrated at intervals not exceeding 6 months and after repair, using a source traceable to a national standard, and must be accurate within plus or minus 20 percent of the true value. Operability checks (battery, response to a check source, zero) are performed before each day of use.
Dosimetry: film, TLD and OSL badges are processed by an accredited laboratory, worn as the license specifies, and are the legal dose record. A direct-reading pocket dosimeter (typically 0-200 mR range) and an audible alarm ratemeter are used during radiography but are not substitutes for the badge.
Traps
Using a dosimeter as a survey meter, or a survey meter as a personnel record. Assuming a meter reading of zero means no radiation (saturation). Confusing the 6-month instrument calibration with the 6-month leak test. Forgetting the 20 percent tolerance when judging a calibration.
Worked example
A meter calibrated within 20 percent reads 10.0 mR/h; the true value lies between 8.0 and 12.0 mR/h. If the same meter shows 0.5 mR/h at 2 m, you expect 0.125 mR/h at 4 m - a very different number signals an inverse-square error or instrument problem.