86
5 Ionization Counters
to reach its original potential “P”. Regaining of its original potential follows exponentially with time (Fig. 5.19a, b). Time taken by the anode to reach to its original
potential (i.e., after electrons have been fully discharged at the anode) is related to
the time taken by Ar
+ to get discharged completely at the cathode. After complete
discharge of electrons and Ar
+ ions formed from a single radiation, the anode potential comes back to its original condition. At this stage if a second radiation enters
the counter, the entire process, as discussed earlier, is repeated, and the output of
negative pulses at the anode appears, as shown in Fig. 5.19b.
But, if a fresh radiation enters the counter, for example, at anode potential x
1
(i.e., much before the anode has regained its original potential), the radiation will
experience a lower operating potential than what it would have experienced, had the
anode reached its original potential “P”. Consequently, number ion-pairs or cloud of
charged avalanche formed too would be low. The number of electrons thus formed
would also be less, thereby lowering the anode potential by a lower magnitude (y).
In other words, anode potential will be lowered by a lower magnitude “y”, than what
a fresh radiation would have produced (x), had the anode potential been at “P”. As a
result, the time taken by the anode to regain its original potential will become larger
T y than T x (Fig. 5.19c). Moreover, the negative pulse height “y” recorded by the
second radiation would be smaller than the previous one, “x”. This would mean a
loss of count recorded in the scaler due to the anode potential being at “y” rather than
at the operating potential “P”. The situation becomes very complex when ionizing
particles keep on entering the counting chamber one after another at such an interval
that the anode gets no chance to recover to its original potential completely (Fig.
5.19d). That is to say that while the anode potential is regaining its original value, a
third radiation causes it to go down to “z” from “y”. In this case, the time required to
recover the potential also becomes large (i.e., T z ) and the counter operates at much
less anodic potential. The impact of anode potential has been seen in Fig. 5.17, where
we saw that if this anode potential is low, the count rate is also low. This means that
when the counter operates at lower potentials, we either loose activity while counting
the radioactive sample or the counter is not properly operating during this period or
the counter is dead for this period. Therefore, one can say that the counter is dead
for a period of T x or T y or T z , as the case may be.
A note of caution is required here. It may appear from the discussion that dead
time of the counter is a variable quantity depending upon the specific activity of
the radioactive sample. This is not true. In Fig. 5.19d, T x or T y or T z is shown to
explain the phenomena of dead time. T z represents a condition that of a typewriter
with many keys pressed together. Under T z condition no counting can be done. T x
is the actual dead time, and T y shows a condition when one starts losing the count
because radiations are entering within the counter’s dead time period. Dead time of
the counter, therefore, can be divided into two parts (Fig. 5.19e).
Dead time = Actual dead time (T D ) + Recovery time (T R )
5 Ionization Counters
to reach its original potential “P”. Regaining of its original potential follows exponentially with time (Fig. 5.19a, b). Time taken by the anode to reach to its original
potential (i.e., after electrons have been fully discharged at the anode) is related to
the time taken by Ar
+ to get discharged completely at the cathode. After complete
discharge of electrons and Ar
+ ions formed from a single radiation, the anode potential comes back to its original condition. At this stage if a second radiation enters
the counter, the entire process, as discussed earlier, is repeated, and the output of
negative pulses at the anode appears, as shown in Fig. 5.19b.
But, if a fresh radiation enters the counter, for example, at anode potential x
1
(i.e., much before the anode has regained its original potential), the radiation will
experience a lower operating potential than what it would have experienced, had the
anode reached its original potential “P”. Consequently, number ion-pairs or cloud of
charged avalanche formed too would be low. The number of electrons thus formed
would also be less, thereby lowering the anode potential by a lower magnitude (y).
In other words, anode potential will be lowered by a lower magnitude “y”, than what
a fresh radiation would have produced (x), had the anode potential been at “P”. As a
result, the time taken by the anode to regain its original potential will become larger
T y than T x (Fig. 5.19c). Moreover, the negative pulse height “y” recorded by the
second radiation would be smaller than the previous one, “x”. This would mean a
loss of count recorded in the scaler due to the anode potential being at “y” rather than
at the operating potential “P”. The situation becomes very complex when ionizing
particles keep on entering the counting chamber one after another at such an interval
that the anode gets no chance to recover to its original potential completely (Fig.
5.19d). That is to say that while the anode potential is regaining its original value, a
third radiation causes it to go down to “z” from “y”. In this case, the time required to
recover the potential also becomes large (i.e., T z ) and the counter operates at much
less anodic potential. The impact of anode potential has been seen in Fig. 5.17, where
we saw that if this anode potential is low, the count rate is also low. This means that
when the counter operates at lower potentials, we either loose activity while counting
the radioactive sample or the counter is not properly operating during this period or
the counter is dead for this period. Therefore, one can say that the counter is dead
for a period of T x or T y or T z , as the case may be.
A note of caution is required here. It may appear from the discussion that dead
time of the counter is a variable quantity depending upon the specific activity of
the radioactive sample. This is not true. In Fig. 5.19d, T x or T y or T z is shown to
explain the phenomena of dead time. T z represents a condition that of a typewriter
with many keys pressed together. Under T z condition no counting can be done. T x
is the actual dead time, and T y shows a condition when one starts losing the count
because radiations are entering within the counter’s dead time period. Dead time of
the counter, therefore, can be divided into two parts (Fig. 5.19e).
Dead time = Actual dead time (T D ) + Recovery time (T R )