84
5 Ionization Counters
such that statistical fluctuation gets minimized. If scaling of Y -axis is small, flat
plateau may appear as shown in Fig. 5.18A though calculated slope may be 0.03
per volt. However, in order to observe visibly a plateau, list all count rates which
appear almost same, take their average value, and then take the square root of the
value. For plotting the data, make the scale of Y -axis such that 1 cm (or 1 inch
as the case may be) represents a value almost equal to the square root of average
count rate. If count rates are plotted under this condition, graph A can be made
to look like D provided that the counter is free from other defects as mentioned
earlier.
5.12 Dead Time of Geiger–Müller Counter
5.12.1 What is a Dead Time?
In previous discussions, we mentioned the term “Dead Time” and related it to the
ability to measure the magnitude of activity of the radioactive sample. In this section,
we shall take up this issue again and discuss its implications in a much greater depth.
The concept of dead time is slightly abstract and may be difficult to understand,
specially, if reader does not have much knowledge about the operation of electronic
instruments. Therefore, an attempt to explain the concept of dead time is best by
taking a simple example of a typewriter machine. When one key of a typewriter
is pressed, a lever of alphabet, etc., is ejected giving an impression of the same
on the paper. While learning typing, we often tend to press too many keys quickly
and we know what happens then; the keys get jammed. Therefore, the typewriter
stops functioning unless the keys are brought back to their original positions. We
soon realize that a minimum time must lapse between pressings of two keys. This
minimum duration required by the machine to operate is called the “dead time” of
the typewriter. During this period, the machine cannot accept new commands. In
other words, the total time required by the key to come back to its original position
is the “dead time” of the machine. This dead time includes two factors: “operation
time” (i.e., time taken by the lever to reach the paper soon after the key was pressed)
and “recovery time” (i.e., time required by the lever to come back to its original
position). Therefore, any machine working on a periodic mode will require a definite
period to complete its one full cycle of operation before the next set of operation can
start, and this total period is called the dead time of the machine. Likewise, after a
Geiger–Müller or any ionization counter has received a radiation, it requires a definite
period of rest before it can accept a new radiation to be recorded. The impact of this
behaviour on operation of the counter is explained in forthcoming sections.
5 Ionization Counters
such that statistical fluctuation gets minimized. If scaling of Y -axis is small, flat
plateau may appear as shown in Fig. 5.18A though calculated slope may be 0.03
per volt. However, in order to observe visibly a plateau, list all count rates which
appear almost same, take their average value, and then take the square root of the
value. For plotting the data, make the scale of Y -axis such that 1 cm (or 1 inch
as the case may be) represents a value almost equal to the square root of average
count rate. If count rates are plotted under this condition, graph A can be made
to look like D provided that the counter is free from other defects as mentioned
earlier.
5.12 Dead Time of Geiger–Müller Counter
5.12.1 What is a Dead Time?
In previous discussions, we mentioned the term “Dead Time” and related it to the
ability to measure the magnitude of activity of the radioactive sample. In this section,
we shall take up this issue again and discuss its implications in a much greater depth.
The concept of dead time is slightly abstract and may be difficult to understand,
specially, if reader does not have much knowledge about the operation of electronic
instruments. Therefore, an attempt to explain the concept of dead time is best by
taking a simple example of a typewriter machine. When one key of a typewriter
is pressed, a lever of alphabet, etc., is ejected giving an impression of the same
on the paper. While learning typing, we often tend to press too many keys quickly
and we know what happens then; the keys get jammed. Therefore, the typewriter
stops functioning unless the keys are brought back to their original positions. We
soon realize that a minimum time must lapse between pressings of two keys. This
minimum duration required by the machine to operate is called the “dead time” of
the typewriter. During this period, the machine cannot accept new commands. In
other words, the total time required by the key to come back to its original position
is the “dead time” of the machine. This dead time includes two factors: “operation
time” (i.e., time taken by the lever to reach the paper soon after the key was pressed)
and “recovery time” (i.e., time required by the lever to come back to its original
position). Therefore, any machine working on a periodic mode will require a definite
period to complete its one full cycle of operation before the next set of operation can
start, and this total period is called the dead time of the machine. Likewise, after a
Geiger–Müller or any ionization counter has received a radiation, it requires a definite
period of rest before it can accept a new radiation to be recorded. The impact of this
behaviour on operation of the counter is explained in forthcoming sections.