5.9 Geiger–Müller Counter (G.M. Counter)
75
5.9 Geiger–Müller Counter (G.M. Counter)
The G.M. counter is widely used because of its good characteristics, high sensitivity,
and versatility for use with different types of radiation and energy ranges, large size
of the output signal, and a reasonable cost. This counter has been built and operated
successfully with tube having diameter of 1 ∼15 cm, containing ionizing gas (2 mm
to about 25 cm pressure of Hg) and with length from 1 to 50 cm. Nevertheless,
here we shall be discussing the description of an End-Window G.M. counter, which
is commonly used in radioactivity measurements. Since output pulses of the G.M.
counter are typically in the order of a volt or more, at the most, one stage of amplification is needed before they can be counted. This makes the counter inexpensive
and simple.
5.9.1 Design of the End-Window G.M. Counter
The G.M. counter tube, like proportional counter, consists of an envelope in which,
two electrodes and the appropriate filling gas are incorporated. The internal collector
electrode is a fine wire, which is a few hundredth of a mm in diameter. It is often made
of tungsten because of its strength and uniformity in diameter. The anode (sometime
called a collector) is usually a straight wire fastened from the insulators at both ends.
However, in the End-Window G.M. counter, the collector is supported at one end
only, while the free end is covered by a glass bead (which prevents point discharge
of electricity to the window). Configuration of the G.M. tube is usually cylindrical,
with the collector mounted coaxially. The other electrode, often referred to as the
cathode, is generally a part of the envelope of the tube. If the envelope is metallic, it
may serve directly as the cathode. If the envelope is of glass, its inside surface may
be covered with a conductive coating to form a cathode. Stainless steel, nickel, or
other materials are highly conductive and do not get oxidized quickly, hence makes
a suitable cathode surface.
The most common filling gas is the noble gases, particularly helium, argon, and
neon. Usually, a small percentage of additional gases are added for quenching purposes (this process is discussed in Sect. 5.13). One of the requirements for the satisfactory operation of a G.M. counter is that the electron attached to the enclosed
gas should be easily detachable (i.e., the gas should be easily ionizable) so that only
free electrons transfer negative charge to the anode. This is achieved by enclosing
a gas, which needs very low energy to remove electrons from its outermost shell.
The minimum energy required to ionize argon gas is 15.7 eV. This means that the
radiations having energies greater than this can be counted by this counter. Unlike the
gas flow proportional counter, with Geiger–Müller counter, the gas is permanently
sealed in the anode chamber. Therefore, sealing of gas is done such that one end of
the counter contains a very thin sheet of metal (usually aluminum) or mica sheet.
This end is referred as the window of the counter. The efficiency of counting with
75
5.9 Geiger–Müller Counter (G.M. Counter)
The G.M. counter is widely used because of its good characteristics, high sensitivity,
and versatility for use with different types of radiation and energy ranges, large size
of the output signal, and a reasonable cost. This counter has been built and operated
successfully with tube having diameter of 1 ∼15 cm, containing ionizing gas (2 mm
to about 25 cm pressure of Hg) and with length from 1 to 50 cm. Nevertheless,
here we shall be discussing the description of an End-Window G.M. counter, which
is commonly used in radioactivity measurements. Since output pulses of the G.M.
counter are typically in the order of a volt or more, at the most, one stage of amplification is needed before they can be counted. This makes the counter inexpensive
and simple.
5.9.1 Design of the End-Window G.M. Counter
The G.M. counter tube, like proportional counter, consists of an envelope in which,
two electrodes and the appropriate filling gas are incorporated. The internal collector
electrode is a fine wire, which is a few hundredth of a mm in diameter. It is often made
of tungsten because of its strength and uniformity in diameter. The anode (sometime
called a collector) is usually a straight wire fastened from the insulators at both ends.
However, in the End-Window G.M. counter, the collector is supported at one end
only, while the free end is covered by a glass bead (which prevents point discharge
of electricity to the window). Configuration of the G.M. tube is usually cylindrical,
with the collector mounted coaxially. The other electrode, often referred to as the
cathode, is generally a part of the envelope of the tube. If the envelope is metallic, it
may serve directly as the cathode. If the envelope is of glass, its inside surface may
be covered with a conductive coating to form a cathode. Stainless steel, nickel, or
other materials are highly conductive and do not get oxidized quickly, hence makes
a suitable cathode surface.
The most common filling gas is the noble gases, particularly helium, argon, and
neon. Usually, a small percentage of additional gases are added for quenching purposes (this process is discussed in Sect. 5.13). One of the requirements for the satisfactory operation of a G.M. counter is that the electron attached to the enclosed
gas should be easily detachable (i.e., the gas should be easily ionizable) so that only
free electrons transfer negative charge to the anode. This is achieved by enclosing
a gas, which needs very low energy to remove electrons from its outermost shell.
The minimum energy required to ionize argon gas is 15.7 eV. This means that the
radiations having energies greater than this can be counted by this counter. Unlike the
gas flow proportional counter, with Geiger–Müller counter, the gas is permanently
sealed in the anode chamber. Therefore, sealing of gas is done such that one end of
the counter contains a very thin sheet of metal (usually aluminum) or mica sheet.
This end is referred as the window of the counter. The efficiency of counting with