410
17.
SMALL CURRENT TECHNIQUE
IONIZATION CHAMBERS
Ionization Chambers for Use With X-rays.—The mea—
surement of the intensity of an X—ray beam by means of an ionization chamber has been discussed in the chapter on >;—rays, particularly in section 9—4.
lonization
chambers
are
shown in
gures 9—6, 9—7 and 9—10, while some details of their construction
have been given in experiment 9—3.
In addition, we may add the
following notes.
In designing a chamber, be sure that an electric field exists
over the region where the beam is absorbed.
lf a soft x=ray beam
enters
the chamber and is almost all absorbed in the first few
centimeters of gas, then the collecting rod must also be located in
the front end of the chamber.
For the usual wave-length range, a chamber of 20 centimeters
length Will serve to absorb practically all radiation if the proper
gas is used, and 15 centimeters is satisfactory for most uses.
Now, there are sudden kicks observed in the electrometer, even
when no X-rays are present.
These are mostly due to alpha particle emission from the walls of the chamber.
The number of
these spuriou_s impulses is greater with long chambers. There is
a further factor, indicating the use of short chambers.
Consider
a chamber of a given length, having capacity C between the collector and the walls, Which absorbs 50 per cent of the incident
radiation.
The
current
produced will
be
{ = Q [ : CV,/f.
A similar chamber twice as long will absorb 75 per
cent of the
radiation, an increase of50 per cent, as we know from the exponen—
tial absorption law. However, it will have twice the capacity,
an increase of 100 per
cent.
It follows that the output
current of
the longer chamber Will be only 75 per cent that of the shorter
one.
Actually, the capacity to be considered is not only that of
the chamber but also includes the xed value of the measuring cir—
cuit, so that the current from the longer chamber is not so small as
the value just computed. Also, if the radiation which is absorbed
by the short chamber is but a small percentage, say one per cent,
of that
in the original beam, then greater length does help
materially.
The diameter of the chamber depends on the area of the X—ray
beam.
The beam should not strike the inner walls nor the col—
17.
SMALL CURRENT TECHNIQUE
IONIZATION CHAMBERS
Ionization Chambers for Use With X-rays.—The mea—
surement of the intensity of an X—ray beam by means of an ionization chamber has been discussed in the chapter on >;—rays, particularly in section 9—4.
lonization
chambers
are
shown in
gures 9—6, 9—7 and 9—10, while some details of their construction
have been given in experiment 9—3.
In addition, we may add the
following notes.
In designing a chamber, be sure that an electric field exists
over the region where the beam is absorbed.
lf a soft x=ray beam
enters
the chamber and is almost all absorbed in the first few
centimeters of gas, then the collecting rod must also be located in
the front end of the chamber.
For the usual wave-length range, a chamber of 20 centimeters
length Will serve to absorb practically all radiation if the proper
gas is used, and 15 centimeters is satisfactory for most uses.
Now, there are sudden kicks observed in the electrometer, even
when no X-rays are present.
These are mostly due to alpha particle emission from the walls of the chamber.
The number of
these spuriou_s impulses is greater with long chambers. There is
a further factor, indicating the use of short chambers.
Consider
a chamber of a given length, having capacity C between the collector and the walls, Which absorbs 50 per cent of the incident
radiation.
The
current
produced will
be
{ = Q [ : CV,/f.
A similar chamber twice as long will absorb 75 per
cent of the
radiation, an increase of50 per cent, as we know from the exponen—
tial absorption law. However, it will have twice the capacity,
an increase of 100 per
cent.
It follows that the output
current of
the longer chamber Will be only 75 per cent that of the shorter
one.
Actually, the capacity to be considered is not only that of
the chamber but also includes the xed value of the measuring cir—
cuit, so that the current from the longer chamber is not so small as
the value just computed. Also, if the radiation which is absorbed
by the short chamber is but a small percentage, say one per cent,
of that
in the original beam, then greater length does help
materially.
The diameter of the chamber depends on the area of the X—ray
beam.
The beam should not strike the inner walls nor the col—