66
5 Ionization Counters
5.8 Proportional Counter
Proportional counter operates in the region C (Fig. 5.2). Like the ionization counter,
in proportional counter also, a voltage is applied between the anode (also known as
collector electrode) and the chamber wall (i.e., the cathode). Radiation is allowed to
enter the chamber, where it creates ion-pairs due to ionization of enclosed gas. Since
the potential applied between the two electrodes is larger than what is applied in
the ionization counter, production of secondary ionization cannot be ruled out. The
number of these secondary ion-pairs becomes as high as 10
5 to 10
6 times the number
of primary ion-pairs formed. These ions when deposited at the electrodes causes
production of pulse height to the order of millivolts. The pulse height produced due
to cosmic radiations remains of the order of microvolt (this aspect will be discussed
later). Unlike the ionization counter, since the pulse height produced in this counter
is in range of millivolts, we do not need a sophisticated amplifier, and hence the cost
of this type of instrument is comparatively less.
5.8.1 Design of a Gas Flow Proportional Counter
Proportional counter can be designed in a variety of ways. Basically, there are two
types of proportional counters, one with enclosed gas and hence has a window,
and the another with continuous gas flow and without window. The construction of
the former is very similar to Geiger counter (will be discussed later). The gas flow
proportional counters have various geometries, usually not cylindrical. The anode is
a fine tungsten wire, usually 0.001
in diameter. This wire is formed either into a
loop or wielded to a terminal, which is supported by a single insulator or suspended
as a straight wire between two insulators. The gas flowing through the counter is a
mixture of 10 : 90 methane : argon or 4 : 96 isobutane : helium. However, in principle,
other types of combination of inert gases can also be used provided the energy for
removal of electron from their outermost orbital is lower than 30 eV. Description of
a counter with a straight wire supported by two insulators is shown in Fig. 5.7. It
consists of a half semi-spherical cylinder, with a hole in the center (Fig. 5.7). The
sample holder has a thread through which the platform (Y ) can be threaded into the
counter, so that it is very near the central anode wire (L), or it can be threaded out up
to a window point (Z ), through which the radioactive sample to be counted can be
loaded or unloaded from platform (Y ). The size of the hole of window (Z ) is such
that when sample is near the central wire, the flat portion of one half semi-spherical
cylinder is fully closed.
The ionizing gases are allowed to pass through (G) and get released into the
atmosphere (if radioactive substance is non-volatile) through (H ). In case of volatile
radioactive materials, the gas has to be filtered by bubbling through a suitable solutions to remove radioactive material. It may be advisable, therefore, to connect the
outlet (H ) to a water bubbler. The out let of the bubbler is connected with a long
5 Ionization Counters
5.8 Proportional Counter
Proportional counter operates in the region C (Fig. 5.2). Like the ionization counter,
in proportional counter also, a voltage is applied between the anode (also known as
collector electrode) and the chamber wall (i.e., the cathode). Radiation is allowed to
enter the chamber, where it creates ion-pairs due to ionization of enclosed gas. Since
the potential applied between the two electrodes is larger than what is applied in
the ionization counter, production of secondary ionization cannot be ruled out. The
number of these secondary ion-pairs becomes as high as 10
5 to 10
6 times the number
of primary ion-pairs formed. These ions when deposited at the electrodes causes
production of pulse height to the order of millivolts. The pulse height produced due
to cosmic radiations remains of the order of microvolt (this aspect will be discussed
later). Unlike the ionization counter, since the pulse height produced in this counter
is in range of millivolts, we do not need a sophisticated amplifier, and hence the cost
of this type of instrument is comparatively less.
5.8.1 Design of a Gas Flow Proportional Counter
Proportional counter can be designed in a variety of ways. Basically, there are two
types of proportional counters, one with enclosed gas and hence has a window,
and the another with continuous gas flow and without window. The construction of
the former is very similar to Geiger counter (will be discussed later). The gas flow
proportional counters have various geometries, usually not cylindrical. The anode is
a fine tungsten wire, usually 0.001
in diameter. This wire is formed either into a
loop or wielded to a terminal, which is supported by a single insulator or suspended
as a straight wire between two insulators. The gas flowing through the counter is a
mixture of 10 : 90 methane : argon or 4 : 96 isobutane : helium. However, in principle,
other types of combination of inert gases can also be used provided the energy for
removal of electron from their outermost orbital is lower than 30 eV. Description of
a counter with a straight wire supported by two insulators is shown in Fig. 5.7. It
consists of a half semi-spherical cylinder, with a hole in the center (Fig. 5.7). The
sample holder has a thread through which the platform (Y ) can be threaded into the
counter, so that it is very near the central anode wire (L), or it can be threaded out up
to a window point (Z ), through which the radioactive sample to be counted can be
loaded or unloaded from platform (Y ). The size of the hole of window (Z ) is such
that when sample is near the central wire, the flat portion of one half semi-spherical
cylinder is fully closed.
The ionizing gases are allowed to pass through (G) and get released into the
atmosphere (if radioactive substance is non-volatile) through (H ). In case of volatile
radioactive materials, the gas has to be filtered by bubbling through a suitable solutions to remove radioactive material. It may be advisable, therefore, to connect the
outlet (H ) to a water bubbler. The out let of the bubbler is connected with a long