5.8 Proportional Counter
73
radiation decreases with the reciprocal of the square of distance. Hence, closer it
is to the counter the greater is its chances of interacting with the ionizing gas.
• Due to the flow of gas in the counter, ions produced near the anode wire are
constantly swept away, thus the anode remains practically free from the cloud of
electrons to receive fresh ion pair produced by the radiation. This makes the dead
time of the counter (this is related to the time for which the anode is covered with
electrons of the ion-pairs produced by radiation) very small (of the order of 0.1
microsec.). The small dead time of counter permits the counting of high specific
activity without much loss of activity. One could record with a proportional counter
an activity of about 10
6 cpm, which is the upper limit of activity normally used in
radiochemical works.
• Radioactive samples mixed with α-emitter and β-emitter can be counted without
chemically separating the two isotopes if a pulse height analyzer is used.
• For counting of α-particles, no shielding of the counter from cosmic radiation is
needed, whereas for β-particles, a small shielding by lead bricks is required.
• A gaseous sample of radioactive substance can be counted by the gas flow proportional counter. This is achieved by allowing the radioactive gas to flow with argon
methane mixture slowly through the counter. For example,
14 C present in CO 2 as
14 CO 2 in a sample, or
3 H present in CH 4 as C 3 H 4 , etc., can easily be detected by
this counter. Because of this feature, the gas flow proportional counter can be connected with the gas–liquid chromatography instrument to detect any radioactive
species present in the sample. In other words, instead of using a thermal detector
or a flame ionization detector in the gas–liquid chromatography, one can use gas
flow proportional counter as a detector for detecting the radioactive samples. It is
important to realize that the conventional detector of gas–liquid chromatography
can detect samples up to 10
−6 M concentration, whereas gas flow proportional
counter can detect samples up to 10
−12 M concentration.
• Geometrical efficiency of counting (i.e., total solid angle that can be covered by
the counter to count the activity of radiation) can be improved from 2π to 4π , by
constructing a counter as shown in Fig. 5.12.
Typical spherical type of gas flow proportional counters for 2π and 4π geometrical
efficiency, as shown in Fig. 5.12, are self-explanatory. In this type of counter, the
radioactive sample is kept in one plane. For such counter, either hanging type anode
or horizontal type anode are used. The source tray is made of a polythene sheet on
which the source is placed. When this counter (which is equivalent to 2π geometry)
is duplicated, as shown in Fig. 5.12, 4π geometry can be achieved. The source is kept
on one side of the polythene sheet, such that upper side of the counter behaves like
windowless counter while the bottom counter behaves like a counter with a window
of thickness equivalent to thickness of the polythene sheet.
• A proportional counter can be used to calculate the absolute activity (activity with
100% counting efficiency). However, some corrections are needed to account for
the following factors:
73
radiation decreases with the reciprocal of the square of distance. Hence, closer it
is to the counter the greater is its chances of interacting with the ionizing gas.
• Due to the flow of gas in the counter, ions produced near the anode wire are
constantly swept away, thus the anode remains practically free from the cloud of
electrons to receive fresh ion pair produced by the radiation. This makes the dead
time of the counter (this is related to the time for which the anode is covered with
electrons of the ion-pairs produced by radiation) very small (of the order of 0.1
microsec.). The small dead time of counter permits the counting of high specific
activity without much loss of activity. One could record with a proportional counter
an activity of about 10
6 cpm, which is the upper limit of activity normally used in
radiochemical works.
• Radioactive samples mixed with α-emitter and β-emitter can be counted without
chemically separating the two isotopes if a pulse height analyzer is used.
• For counting of α-particles, no shielding of the counter from cosmic radiation is
needed, whereas for β-particles, a small shielding by lead bricks is required.
• A gaseous sample of radioactive substance can be counted by the gas flow proportional counter. This is achieved by allowing the radioactive gas to flow with argon
methane mixture slowly through the counter. For example,
14 C present in CO 2 as
14 CO 2 in a sample, or
3 H present in CH 4 as C 3 H 4 , etc., can easily be detected by
this counter. Because of this feature, the gas flow proportional counter can be connected with the gas–liquid chromatography instrument to detect any radioactive
species present in the sample. In other words, instead of using a thermal detector
or a flame ionization detector in the gas–liquid chromatography, one can use gas
flow proportional counter as a detector for detecting the radioactive samples. It is
important to realize that the conventional detector of gas–liquid chromatography
can detect samples up to 10
−6 M concentration, whereas gas flow proportional
counter can detect samples up to 10
−12 M concentration.
• Geometrical efficiency of counting (i.e., total solid angle that can be covered by
the counter to count the activity of radiation) can be improved from 2π to 4π , by
constructing a counter as shown in Fig. 5.12.
Typical spherical type of gas flow proportional counters for 2π and 4π geometrical
efficiency, as shown in Fig. 5.12, are self-explanatory. In this type of counter, the
radioactive sample is kept in one plane. For such counter, either hanging type anode
or horizontal type anode are used. The source tray is made of a polythene sheet on
which the source is placed. When this counter (which is equivalent to 2π geometry)
is duplicated, as shown in Fig. 5.12, 4π geometry can be achieved. The source is kept
on one side of the polythene sheet, such that upper side of the counter behaves like
windowless counter while the bottom counter behaves like a counter with a window
of thickness equivalent to thickness of the polythene sheet.
• A proportional counter can be used to calculate the absolute activity (activity with
100% counting efficiency). However, some corrections are needed to account for
the following factors: