17—17.
IONIZATION CHAMBERS FOR X-RAYS 411
lector.
For uorescent X_rays, where the rays diverge from their
source, a conical chamber may be used.
The col/alor should be such as to keep the capacity as small as
possible.
lt is possible to obtain saturation (withdrawal of all
ions produced) with moderate voltages even when ne wires are
used.
For example, with a tube 20 centimeters long, 5 centi—
meters in inside diameter and with a number 40 copper wire collector, the potential needed for saturation is about 50 volts. With
a
similar tube and a sixteenth inch nickel wire, the saturation
potential is about 25 volts. Higher potentials, from 100 to
200 volts, are frequently used.
Concerning the choice of 77zez‘a/s to be used in the construction
of the chamber and the collector; copper is more likely to be free
from radioactive contaminations than brass; lead is very likely
to be contaminated; tin, rather than lead solder should be used.
The contaminations are largely on the surface of the walls and may
be reduced by sandpapering or by rolling sand around inside, on a
lathe.
ln the following comparison14 of the number of alpha
particles emitted from a given area of material per unit time, the
smaller the number, the more suitable the material for use in
chambers: cold rolled steel 12, commercial brass 15, pyrex glass
16, commercial platinum foil 29, commercially electroplated
nickel 31, commercial aluminum foil 34, electroplated copper
(CuCN) 37, and commercial tin foil 47.
For practical purposes,
there is no observable effect
the
polariz‘y of the applied potential; the collector may be either
positive or negative.
_
_
Amber is used as the insu/ation for the lead Wire from the collector.
The use of a guard ring will reduce the leakage a great
deal.
There is the central lead wire, some amber, the surrounding
metal guard ring, then a good insulator such as bakel1te or hard
rubber.
The guard is grounded.
The window, where the X-ray
beam enters the chamber, should
be as thin as possible, yet strong
not to burst under the
pressure dilïerence on its two srdes. Cellophao
0.008 centrmeter thick or aluminum 0.025 centrmeter th1ck Wlll serve for a
-
_ea
lu
â//_
W1nÎÎËZ ÎdÏisabîe t >o< âge a onesixteenth inch lead sheaz‘lz around
the chamber to avoid X—1‘aYS
from the rectier tubes, etc.
'
IONIZATION CHAMBERS FOR X-RAYS 411
lector.
For uorescent X_rays, where the rays diverge from their
source, a conical chamber may be used.
The col/alor should be such as to keep the capacity as small as
possible.
lt is possible to obtain saturation (withdrawal of all
ions produced) with moderate voltages even when ne wires are
used.
For example, with a tube 20 centimeters long, 5 centi—
meters in inside diameter and with a number 40 copper wire collector, the potential needed for saturation is about 50 volts. With
a
similar tube and a sixteenth inch nickel wire, the saturation
potential is about 25 volts. Higher potentials, from 100 to
200 volts, are frequently used.
Concerning the choice of 77zez‘a/s to be used in the construction
of the chamber and the collector; copper is more likely to be free
from radioactive contaminations than brass; lead is very likely
to be contaminated; tin, rather than lead solder should be used.
The contaminations are largely on the surface of the walls and may
be reduced by sandpapering or by rolling sand around inside, on a
lathe.
ln the following comparison14 of the number of alpha
particles emitted from a given area of material per unit time, the
smaller the number, the more suitable the material for use in
chambers: cold rolled steel 12, commercial brass 15, pyrex glass
16, commercial platinum foil 29, commercially electroplated
nickel 31, commercial aluminum foil 34, electroplated copper
(CuCN) 37, and commercial tin foil 47.
For practical purposes,
there is no observable effect
the
polariz‘y of the applied potential; the collector may be either
positive or negative.
_
_
Amber is used as the insu/ation for the lead Wire from the collector.
The use of a guard ring will reduce the leakage a great
deal.
There is the central lead wire, some amber, the surrounding
metal guard ring, then a good insulator such as bakel1te or hard
rubber.
The guard is grounded.
The window, where the X-ray
beam enters the chamber, should
be as thin as possible, yet strong
not to burst under the
pressure dilïerence on its two srdes. Cellophao
0.008 centrmeter thick or aluminum 0.025 centrmeter th1ck Wlll serve for a
-
_ea
lu
â//_
W1nÎÎËZ ÎdÏisabîe t >o< âge a onesixteenth inch lead sheaz‘lz around
the chamber to avoid X—1‘aYS
from the rectier tubes, etc.
'