7.9 Low-Level Counting
135
7.9.1 Anti-coincidence Counting System
By an anti-coincidence counting system, background activity can be reduced to
0.1 cpm. This is usually done with a thin mica end-window G.M. counter, because
other counters need a large space and hence would be bulkier. A typical design of this
counter is shown in Fig. 7.7. Two G.M. counters A and B are used, with B having
a large aluminum window and a small-sized counter A (mica-type window). The
source to be counted is kept very close to counter A such that its radiation does not
pass through both counters simultaneously.
However, cosmic radiation or γ -radiation (emitted by isotope present in the lead
shield) can pass through both A and B counters simultaneously producing negative
pulses in both counters at the same time. The electronic arrangement is made, such
that if negative pulses are produced in both counters simultaneously, they are not
allowed to pass through the counting system (i.e., scaler) and are rejected, but if
produced in any one of the counters, they are allowed to enter the counting system
(i.e., to the scaler). Naturally, it will always be the inner counter, which will receive
radiations by the sample, unless it could penetrate the wall of the inner counter to
reach the second counter, which is difficult to encounter under the present geometrical
arrangement. Rejection of negative pulses or its acceptance is achieved with the help
of an anti-coincidence electronic circuit.
With a shielding of 3.5 cm of mercury and 20 cm of steel, it has been possible to
reduce the background to 0.77 cpm, or shielding of 3 cm of lead and 10 cm of steel
could give a background to about 1.3–1.5 cpm. These types of counting systems
Fig. 7.7 A schematic
diagram of an
anti-coincidence counting
system with two G.M.
counters under the
anti-coincidence condition.
A is a mica window G.M.
counter and B is a large
aluminum end-window G.M.
counter
Scaler
Anti
coincidence
Probe Dual
E.H.T.(V) Dual
a
*
Al-Window
Radioactive sample
Mica Window
Lead
shield
b
135
7.9.1 Anti-coincidence Counting System
By an anti-coincidence counting system, background activity can be reduced to
0.1 cpm. This is usually done with a thin mica end-window G.M. counter, because
other counters need a large space and hence would be bulkier. A typical design of this
counter is shown in Fig. 7.7. Two G.M. counters A and B are used, with B having
a large aluminum window and a small-sized counter A (mica-type window). The
source to be counted is kept very close to counter A such that its radiation does not
pass through both counters simultaneously.
However, cosmic radiation or γ -radiation (emitted by isotope present in the lead
shield) can pass through both A and B counters simultaneously producing negative
pulses in both counters at the same time. The electronic arrangement is made, such
that if negative pulses are produced in both counters simultaneously, they are not
allowed to pass through the counting system (i.e., scaler) and are rejected, but if
produced in any one of the counters, they are allowed to enter the counting system
(i.e., to the scaler). Naturally, it will always be the inner counter, which will receive
radiations by the sample, unless it could penetrate the wall of the inner counter to
reach the second counter, which is difficult to encounter under the present geometrical
arrangement. Rejection of negative pulses or its acceptance is achieved with the help
of an anti-coincidence electronic circuit.
With a shielding of 3.5 cm of mercury and 20 cm of steel, it has been possible to
reduce the background to 0.77 cpm, or shielding of 3 cm of lead and 10 cm of steel
could give a background to about 1.3–1.5 cpm. These types of counting systems
Fig. 7.7 A schematic
diagram of an
anti-coincidence counting
system with two G.M.
counters under the
anti-coincidence condition.
A is a mica window G.M.
counter and B is a large
aluminum end-window G.M.
counter
Scaler
Anti
coincidence
Probe Dual
E.H.T.(V) Dual
a
*
Al-Window
Radioactive sample
Mica Window
Lead
shield
b