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

448 CHAPTER 9
Substituting the given values in the above equation, we obtain
˙D(0.1 MeV) = 8.1 × 10 −10 Gy/s
˙D(2.0 Mev) = 8.2 × 10 −10 Gy/s.
The dose rates for the two radiation fields are about the same. A particle-fluxmeasuring
instrument, however, such as a GM counter, would register about 20
times more for the 0.1-MeV radiation than for the higher energy radiation. Since
ionization chambers measure absorbed energy rather than individual particles and
are reasonably energy independent (Fig. 9-17), we use ionization-chamber-type instruments
when measuring radiation dose rates (unless a specific counter-type instrument
is calibrated for a specific gamma-ray energy, in which case it may be used
for dosimetry for that quantum energy only).
As shown in Example 9.3, particle-counting survey instruments, such as GM
counters and scintillation counters, are very energy dependent when used to measure
radiation dose (Fig. 9-18). The GM counter’s response is much flatter (although
still not flat) with the beta window closed than with the beta window open.
The reason for this is that the thicker wall, represented by the closed beta window,
absorbs more low-energy photons than does the open window. This is the
basis for energy-compensated GM detectors. An energy-compensated GM detector
has a wall thick enough to flatten the energy response to within about ±15% of
the response to 137 Cs, yet thin enough to detect photons down to about 50 keV
(Fig. 9-19).
Figure 9-17. Energy response curve for a general-purpose ionization-chamber-type survey meter with a 300mg/cm
2 -thick retractable beta shield. (Courtesy of Ludlum Measurements, Inc, Sweetwater, TX.)