430
18.
THE DETECTION OF PARTICLES
and was made of 100 mesh copper gauze.
The relative sensitivity
was a maximum when the tube was lled with dry air at 6.5 centimeters of mercury, the over-voltage was 170 and the circuit was
of the type shown in gure 18—7.
_
18—13.
The Efciency of Counting.—The eiciency of counting (E) is dened as the ratio of the number of observed counts per
unit time (No) to the number of ionizing particles (7?) which passed
through the counter tube during that time. Thus,
E = No/ñ.
(18—2)
If 1000 particles pass through and only 900 are recorded, the
efciency is 0.9 or 90 per
cent.
Let three counter tubes be placed one above the other, with
axes parallel, and connected in a triple coincidence circuit. The
over-all efciency would then be
E; = E1E253 = Nt/ñ,
(18—3)
where E1, E2 and E3 are the respective eiciencies of the individual
counters and Nt is the observed triple counting rate.
If, now,
the center tube be disconnected (but left in place), the efciency
of the double coincidence circuit will be
Ed : E1E3 = Nd/ñ,
(18-4)
where Nd is the observed double counting rate.
From the last
two equations, we see that
E2 = Aït/Nd.
(18—5)
The e’iciency of a counter tube may, therefore, be determined
12
by placing it as the center tube in a triple counter and observing
the counting rates, corrected for accidentals, when the tube is
connected to its amplier (N,) and when it is not (Nd). In
similar manner, by interchanging the tubes, El and E3 may
be
determined.
The inactive time (0) of a counter is dened as the time interval
during Which it is insensitive to ionizing particles. With a high
speed circuit and a rapid countenmeter, a is (probably) largely
the time required for the restoration of the voltage across the
counter tube after its discharge.
If No is the number of counts
18.
THE DETECTION OF PARTICLES
and was made of 100 mesh copper gauze.
The relative sensitivity
was a maximum when the tube was lled with dry air at 6.5 centimeters of mercury, the over-voltage was 170 and the circuit was
of the type shown in gure 18—7.
_
18—13.
The Efciency of Counting.—The eiciency of counting (E) is dened as the ratio of the number of observed counts per
unit time (No) to the number of ionizing particles (7?) which passed
through the counter tube during that time. Thus,
E = No/ñ.
(18—2)
If 1000 particles pass through and only 900 are recorded, the
efciency is 0.9 or 90 per
cent.
Let three counter tubes be placed one above the other, with
axes parallel, and connected in a triple coincidence circuit. The
over-all efciency would then be
E; = E1E253 = Nt/ñ,
(18—3)
where E1, E2 and E3 are the respective eiciencies of the individual
counters and Nt is the observed triple counting rate.
If, now,
the center tube be disconnected (but left in place), the efciency
of the double coincidence circuit will be
Ed : E1E3 = Nd/ñ,
(18-4)
where Nd is the observed double counting rate.
From the last
two equations, we see that
E2 = Aït/Nd.
(18—5)
The e’iciency of a counter tube may, therefore, be determined
12
by placing it as the center tube in a triple counter and observing
the counting rates, corrected for accidentals, when the tube is
connected to its amplier (N,) and when it is not (Nd). In
similar manner, by interchanging the tubes, El and E3 may
be
determined.
The inactive time (0) of a counter is dened as the time interval
during Which it is insensitive to ionizing particles. With a high
speed circuit and a rapid countenmeter, a is (probably) largely
the time required for the restoration of the voltage across the
counter tube after its discharge.
If No is the number of counts