58
5 Ionization Counters
primary or secondary, gets accelerated enough to cause a chain of ionization process.
Because of this, an avalanche of ions is created in the tube. In other words, radiation
of any type (i.e., irrespective of α-particles, β-particles, or γ -rays) producing even
a single ion-pair in the chamber would be sufficient to cause collection of almost
same amount of charge at the electrode as a particle giving rise to several ionpairs. Its amount does not depend on the type of radiation or energy of the particle.
In this region, charge collected at the electrode increases slowly with increase in
voltage. This region is known as the Geiger region (E). This region, therefore,
cannot differentiate ionizations produced by either α-particles or β-particles or γ -
radiations.
5.3.1.6 Continuous Spark Region
Any further increase in potential beyond the Geiger region causes a continuous spark
between the anode and the cathode. This region is called Spark region where generally, applied voltage is in excess of a few kV. Usually, high voltage pulses produced
by the ionizing radiation with a very short rise time can be counted in this region.
Because as soon as such pulses are applied to the counter, instantaneously current
is produced, which is measured. Counters operating in this region are called Spark
chambers. They can be made of various designs. This is truly a “do-it yourself”
detection system to meet the specific requirements of the user.
It would seem that the process of secondary ion production which is caused by
acceleration of ions toward the collecting electrode would continue to increase the
charge indefinitely as voltage is increased higher and higher (i.e., beyond region F).
But it is not so. On increasing the voltage ionization and discharge of ion-pairs tend
to spread over a successively larger portion of the tube. This limits the magnitude of
electrostatic field, which can be generated in the tube. Furthermore, since electrons
are lighter than positive ions (i.e., Ar
+ ion), the former are swept out of the collecting
gas very rapidly, leaving behind an atmosphere of positive ions (positive ion sheath)
around the anode. These ions give rise to what is known as “space charge effect”.
This positive ion sheath surrounds the central electrode (i.e., the anode) reducing
effective magnitude of electrostatic field intensity between the anode and the cathode
of the chamber. These two factors limit the number of ions, which can be collected at
the anode. Because of these two factors, the multiplication of secondary ionization
cannot continue to increase indefinitely with increasing voltage.
5.4 Nature of Gas to be Used in Ionization Chamber
It is also important to realize that in principle, one could use any gas in the ionization
counter, but it must meet the following requirements:
5 Ionization Counters
primary or secondary, gets accelerated enough to cause a chain of ionization process.
Because of this, an avalanche of ions is created in the tube. In other words, radiation
of any type (i.e., irrespective of α-particles, β-particles, or γ -rays) producing even
a single ion-pair in the chamber would be sufficient to cause collection of almost
same amount of charge at the electrode as a particle giving rise to several ionpairs. Its amount does not depend on the type of radiation or energy of the particle.
In this region, charge collected at the electrode increases slowly with increase in
voltage. This region is known as the Geiger region (E). This region, therefore,
cannot differentiate ionizations produced by either α-particles or β-particles or γ -
radiations.
5.3.1.6 Continuous Spark Region
Any further increase in potential beyond the Geiger region causes a continuous spark
between the anode and the cathode. This region is called Spark region where generally, applied voltage is in excess of a few kV. Usually, high voltage pulses produced
by the ionizing radiation with a very short rise time can be counted in this region.
Because as soon as such pulses are applied to the counter, instantaneously current
is produced, which is measured. Counters operating in this region are called Spark
chambers. They can be made of various designs. This is truly a “do-it yourself”
detection system to meet the specific requirements of the user.
It would seem that the process of secondary ion production which is caused by
acceleration of ions toward the collecting electrode would continue to increase the
charge indefinitely as voltage is increased higher and higher (i.e., beyond region F).
But it is not so. On increasing the voltage ionization and discharge of ion-pairs tend
to spread over a successively larger portion of the tube. This limits the magnitude of
electrostatic field, which can be generated in the tube. Furthermore, since electrons
are lighter than positive ions (i.e., Ar
+ ion), the former are swept out of the collecting
gas very rapidly, leaving behind an atmosphere of positive ions (positive ion sheath)
around the anode. These ions give rise to what is known as “space charge effect”.
This positive ion sheath surrounds the central electrode (i.e., the anode) reducing
effective magnitude of electrostatic field intensity between the anode and the cathode
of the chamber. These two factors limit the number of ions, which can be collected at
the anode. Because of these two factors, the multiplication of secondary ionization
cannot continue to increase indefinitely with increasing voltage.
5.4 Nature of Gas to be Used in Ionization Chamber
It is also important to realize that in principle, one could use any gas in the ionization
counter, but it must meet the following requirements: