428
18.
THE DETECTION OF PARTICLES
If the voltage on a counter tube is increased, the observed
background (B) will also increase.
In other words, the apparatus
becomes more sensitive and weaken radiations are detected.
This is shown in curve B of gure 18—11.
As an example, the residual or background count amounted
to 6 to 15 counts per minute, according to the applied voltage, for
a Geiger—Müller tube whose cathode was 6 centimeters long and '
1 centimeter in diameter (sectional area of 6 square centimeters).
This amounts to 1 to 2%— counts per minute per square centimeter.
In a second tube, 12 centimeters long and 2 centimeters in diameter (area 24 cm.2), B varied from 35 to 45, i.e., from 1% to 2 counts
per
minute per square centimeter.
In general, a tube which has
_
‘
been prepared so as to be essentially free (itself) from radioactive
contaminations and is placed in an
“
uncontaminated
”
room
will have a background of approximately one to three residual
counts per
minute per square centimeter.
If two tubes are placed a few centimeters apart, with their axes
parallel and in a horizontal plane and connected in a coincidence
circuit, chance or aceia’em‘al counts will occur which are not due to
the passage of a single particle through both tubes.12
18—11.
The Counting-rate Voltage Curve—Let a radioactive
.
source of moderate and constant intensity be placed near a counter
tube and the average number of counts per minute (N) be obtained
from readings taken over a comparatively long time interval.
Then,* the average activity (Ng) of thesource is”
No =
B,
(18—1)
where B is the average background counting rate.
Figure 18—11 shows a typical curve of counting rates When
_
_
different voltages are applied to the counter tube. The operating '
voltage (V) is usually set somewherè within the range PP, since,
,»
Over this plateau the applied voltage mayvary without causing
,
large changes in the counting rate. The width of the plateau
…
* It is here assumed that N is large compared with B.. The case of a weak
.
is discussed in sections 18—12 and 11—11 and the caSeof a very strong source
1s treated in section-18—13.
Unless the counter apparatus is sluggish, equation
18—1 is identical With equation 11—23 (ñ = Zi — Z,’WhCI‘C 71 is the activity of the
…
source).
.
’
’
'.
18.
THE DETECTION OF PARTICLES
If the voltage on a counter tube is increased, the observed
background (B) will also increase.
In other words, the apparatus
becomes more sensitive and weaken radiations are detected.
This is shown in curve B of gure 18—11.
As an example, the residual or background count amounted
to 6 to 15 counts per minute, according to the applied voltage, for
a Geiger—Müller tube whose cathode was 6 centimeters long and '
1 centimeter in diameter (sectional area of 6 square centimeters).
This amounts to 1 to 2%— counts per minute per square centimeter.
In a second tube, 12 centimeters long and 2 centimeters in diameter (area 24 cm.2), B varied from 35 to 45, i.e., from 1% to 2 counts
per
minute per square centimeter.
In general, a tube which has
_
‘
been prepared so as to be essentially free (itself) from radioactive
contaminations and is placed in an
“
uncontaminated
”
room
will have a background of approximately one to three residual
counts per
minute per square centimeter.
If two tubes are placed a few centimeters apart, with their axes
parallel and in a horizontal plane and connected in a coincidence
circuit, chance or aceia’em‘al counts will occur which are not due to
the passage of a single particle through both tubes.12
18—11.
The Counting-rate Voltage Curve—Let a radioactive
.
source of moderate and constant intensity be placed near a counter
tube and the average number of counts per minute (N) be obtained
from readings taken over a comparatively long time interval.
Then,* the average activity (Ng) of thesource is”
No =
B,
(18—1)
where B is the average background counting rate.
Figure 18—11 shows a typical curve of counting rates When
_
_
different voltages are applied to the counter tube. The operating '
voltage (V) is usually set somewherè within the range PP, since,
,»
Over this plateau the applied voltage mayvary without causing
,
large changes in the counting rate. The width of the plateau
…
* It is here assumed that N is large compared with B.. The case of a weak
.
is discussed in sections 18—12 and 11—11 and the caSeof a very strong source
1s treated in section-18—13.
Unless the counter apparatus is sluggish, equation
18—1 is identical With equation 11—23 (ñ = Zi — Z,’WhCI‘C 71 is the activity of the
…
source).
.
’
’
'.