The exposure rate (mR/hr or counts/min) given by the GM tube for x-
rays or g-rays depends on the energy of the photons, because they primar-
ily interact with the walls of the tube rather than with the gas volume. The
GM tubes are made of aluminum or steel with atomic number Zhigher
than that of air. Since at low energies the photons primarily interact via the
photoelectric process, which is proportional to Z^3 , the exposure rate mea-
sured by the GM counter will be overestimated. On the other hand, at
medium energies, the Compton interaction predominates, which is inde-
pendent of Zand will therefore give correct reading of exposure rate. Since
most GM counters are commonly calibrated for 662-keV photons of^137 Cs,
they give only crude estimations of exposure rates for energies less than
150 keV and must be calibrated at these energies. However, the so-called
energy-compensated detectors have been devised in which a thin layer of
high Zmaterial such as tin is placed around the inside of the detector. The
increased photoelectric absorption of low energy photons in tin significantly
flattens the energy response of the detector. A disadvantage of this detec-
tor is that the low energy photon sensitivity is greatly reduced.
The GM survey meters are more sensitive than ionization chambers by
a factor of about 10. Because voltage pulses generated in GM tubes are
independent of the energy deposited, they cannot discriminate between
energies and types of radiations. These counters are almost 100% efficient
for counting b-particles but have only 1% to 2% efficiency for counting g-
and x-rays. The dead time, or resolving time (Chapter 8), of the GM coun-
ters is about 100 to 500 microseconds. This limits the count rates to about
15,000–20,000 counts per minute (cpm) for these counters, and at higher
activities they tend to saturate, thus losing counts. The GM counters are
normally used for area survey for contamination with low-level activity.
According to the NRC regulations, these survey meters must be calibrated
annually with standard calibrated sources such as^137 Cs.
Questions
- Describe the principles of gas-filled detectors.
- What are the differences between an ionization chamber and a
Geiger–Müller counter? - What is the function of a push-button isotope selector on a dose
calibrator? - Can you discriminate between 140-keV g-rays, 364-keV g-rays, and
5-MeV a-particles using a GM counter? - What type of instruments would you use for:
(a) Survey of the laboratory?
(b) X-ray beam exposure?
(c) Area survey around x-ray room?
(d) Spill of 50mCi (1.85 MBq) of^201 Tl?
Questions 79