88
5 Ionization Counters
preset time is taken as the dead time of the counter, for calculation purposes. The
latter method simplifies the counting procedure, otherwise, every time one uses the
G.M. counter, its dead time has to be determined, which is a very tedious procedure
as will be seen from the foregoing discussions.
1. Measuring activities of two radioactive samples of almost same activity: Two
sources of long-lived radioactive isotopes with almost same activities are prepared
for this purpose. They are made in such a way that the base of the source is a
thin polythene sheet so that thickness of sources is almost negligible (therefore,
absorption of radiation by the polythene sheet can be neglected).
32 P is taken as a
source of measurement, but other sources like
89 Sr can also be used. The idea is
that the source should be, as far as possible, a strong β-emitter. The activities of
the two sources are measured separately by the G.M. counter whose dead time is
to be determined. Let these activities be “m 1 ” and “m 2 ” for sources number one
and two. Then activities of both sources are measured together. Let this activity
be “m 12 ”. Let us assume that “n 1 ”, “n 2 ”, and “n 12 ” are true activities of the source
no. 1, 2 and when both were counted together respectively. Likewise, let “n b ”
and “m b ” be the true and measured background activities, respectively. Since the
background activities measured for all these measurements would be same, we
can say that m 1 = m 2 and likewise n 1 = n 2 . If τ be the dead time of the G.M.
counter (in the unit of second), the time lost in counting various activities can be
calculated as follows:
Time lost in counting activity (m 1 ) = m 1 × τ
The counter was thus operative for a period of (1 − m 1 × τ ) seconds during which
m 1 activity was recorded. Thus, actual counts/sec., i.e.,
n 1 =
m 1
(1 − m 1 × τ )
per second
(5.4)
Similarly,
n 2 =
m 2
(1 − m 2 × τ )
per second
(5.5)
and
n 12 =
m 12
(1 − m 12 × τ )
per second
(5.6)
and
n b =
m b
(1 − m b × τ )
per second
(5.7)
5 Ionization Counters
preset time is taken as the dead time of the counter, for calculation purposes. The
latter method simplifies the counting procedure, otherwise, every time one uses the
G.M. counter, its dead time has to be determined, which is a very tedious procedure
as will be seen from the foregoing discussions.
1. Measuring activities of two radioactive samples of almost same activity: Two
sources of long-lived radioactive isotopes with almost same activities are prepared
for this purpose. They are made in such a way that the base of the source is a
thin polythene sheet so that thickness of sources is almost negligible (therefore,
absorption of radiation by the polythene sheet can be neglected).
32 P is taken as a
source of measurement, but other sources like
89 Sr can also be used. The idea is
that the source should be, as far as possible, a strong β-emitter. The activities of
the two sources are measured separately by the G.M. counter whose dead time is
to be determined. Let these activities be “m 1 ” and “m 2 ” for sources number one
and two. Then activities of both sources are measured together. Let this activity
be “m 12 ”. Let us assume that “n 1 ”, “n 2 ”, and “n 12 ” are true activities of the source
no. 1, 2 and when both were counted together respectively. Likewise, let “n b ”
and “m b ” be the true and measured background activities, respectively. Since the
background activities measured for all these measurements would be same, we
can say that m 1 = m 2 and likewise n 1 = n 2 . If τ be the dead time of the G.M.
counter (in the unit of second), the time lost in counting various activities can be
calculated as follows:
Time lost in counting activity (m 1 ) = m 1 × τ
The counter was thus operative for a period of (1 − m 1 × τ ) seconds during which
m 1 activity was recorded. Thus, actual counts/sec., i.e.,
n 1 =
m 1
(1 − m 1 × τ )
per second
(5.4)
Similarly,
n 2 =
m 2
(1 − m 2 × τ )
per second
(5.5)
and
n 12 =
m 12
(1 − m 12 × τ )
per second
(5.6)
and
n b =
m b
(1 − m b × τ )
per second
(5.7)