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

Evaluation of Radiation Safety Measures 683
Figure 13-4. Energy dependence of a Geiger counter survey meter. The meter reading is multiplied
by the correction factor appropriate to the quantum energy in order to obtain the true exposure rate.
(Courtesy of Electronica Lombarda SPA.)
the appropriate one being selected by the surveyor, who turns the time constant
selector switch to the desired value.
4. Energy dependence. Most radiation-measuring instruments have a limited span
of energy over which the radiation dose is accurately measured. One of the
figures of merit of a radiation dosimeter is the energy range over which the
instrument is useful. This information must be known by the health physicist in
order to choose a proper instrument for a particular application or to interpret
the measurements properly. The energy dependence is usually specified by the
manufacturer as “accurate to ±10% of the true value from 80 keV to 2 MeV”
or by means of an energy-dependence curve (Fig. 13-4). The magnitude of the
errors that can arise when the energy dependence factor is overlooked is shown
in Table 13-2.
TABLE 13-2. Energy Dependence of Dose–rate Response of GM and Scintillation
Counters—Meter Reading for a True Exposure Rate of 1 mR/h
GAMMA-RAY GM COUNTER SCINTILLATION
ISOTOPE ENERGY (MeV) (mR/h) COUNTER (mR/h)
60 Co 1.25 1.15 0.6
226 Ra 0.84 1.0 0.96
137 Cs 0.661 0.92 1.39
198 Au 0.411 0.82 2.65
203 Hg 0.279 1.29 7.5
141 Ce 0.145 2.4 14.1
241 Am 0.06 6.0 9.8
Reproduced with permission from Peirson DH. Interpretation of instrument readings: Pulse-counting instruments. Phys
Med Biol. 1963; 7(4):445–453. Copyright c○ 1963 Institute of Physics Publishing.