Geometry
Variations in sample volumes or in geometric configurations of the con-
tainer can affect the accuracy of measurements in a dose calibrator, partic-
ularly for low-energy radiations. Thus, 1 mCi (37 MBq) in 1-ml or 30-ml
volume, or 1 mCi (37 MBq) in 1-cc syringe, 10-cc syringe, or 10-cc vial, or in
containers of different materials (plastic or glass) may give different read-
ings in the dose calibrator. Correction factors must be determined for these
geometric variations and applied to the measured activities, if the error
exceeds ±10%.
Pocket Dosimeter
The pocket dosimeter operates on the principle of a charged electroscope
equipped with a scale inside. It consists of a quartz fiber electroscope inside
the chamber. Initially, the dosimeter is fully charged by means of an exter-
nal power supply (a dosimeter charger), and the scale then reads zero. After
exposure to radiation, charge is lost, and the loss of charge is proportional
to the amount of radiation exposure, which is read on the inside scale in
mR. This reading can be seen through a viewing window at the end of the
dosimeter. After complete discharge of the dosimeter, it can be charged and
used again. It is primarily used to determine personnel exposure while
working with radiation and has the advantage of giving immediate read-
ings. These dosimeters are available in full-scale readings of 200 mR,
500 mR, and 1 R. Discharge due to leakage is the major disadvantage of
these dosimeters.
Geiger–Müller Counters
The Geiger–Müller (GM) counter operates in the Geiger region of the
voltage, as shown in Figure 7.2. As already mentioned, in this region, an
avalanche of ionizations occurs as a result of high voltage. Once an ioniza-
tion is initiated, the avalanche of ionizations can lead to repetitive dis-
charges unless the process is interrupted by the quenching technique. An
electronic technique of quenching can be applied in which the voltage
applied to the GM tube is temporarily reduced below the Geiger region
until all ion pairs return to their de-excited states. This happens in a few
tenths of a millisecond. The original voltage is then restored for the detec-
tion of the next event. This technique is no longer in use.
The common technique of quenching is to add a small quantity of a
quenching gas to the counting gas. Either organic solvent vapors (e.g., ethyl
alcohol, xylene, or isobutane) or halogen gases (chlorine or bromine) are
commonly used as the quenching gas. These molecules transfer electrons to
the “positive” ion cloud and become themselves ionized. Ionized molecules
of the quenching gas migrate to and dislodge electrons from the cathode.
Geiger–Müller Counters 77