436
18.
THE DETECTION OF PARTlCLES
for example, by cams on the shaft of the driving mechanism. A
chamber may
be placed between two counter tubes connected in a
coincidence circuit (see gure 18—10).
If the output
of this circuit
is used to start an expansion, the particles recorded will be, for the
most part,
those which have passed through the chamber in line
with the counter tubes.
Continuously active cloud chambers have been devised but
have not yet proven
of practical value.
The electricalelâ used to remove the ions is not critical and
may range
from 20 to 300 volts.
It is removed or reduced to a
small value during the expansion.
If a strong eld is used during
the expansion, it is possible to separate the positive and negative
ions to such an extent that each track is doubled.
From the sepa—
ration of the tracks and the mobilities of the ions, it is possible to
compute the age of the track.
Older tracks are wider.“
The
“
sweep
”
eld is often applied between the piston and a ring of
metal or colloidal graphite (Aquadag) around the top
of the
chamber.
It is better to use wires or plates on insulating supports
so as
to avoid distortion of the tracks by surface charges on the
chamber walls.
The time during which an expansion takes place should be
short.
This is necessary, not only to secure a sufcient drop in
temperature to cause condensation of the vapor on nuclei but also
to prevent the continued motion of the gas
from distorting the
newly formed track. The expansion time is usually of the order
of magnitude of 0.01 second,23 although as long as 0.25 second has
been used.22
Flexible diaphragms and metal bellows can be
moved quickly whereas heavy pistons and water surfaces are
comparatively sluggish.
If a particle passes through the chamber appreciably before
the expansion takes place, the tracks will be broad and dense. If
the ions are formed in the chamber too late, the tracks are thin.
The sensitive lime, during Which reasonably sharp tracks can be
formed, is usually a fraction of a second, although in one design22
it amounted to 1.5—2 seconds.
A suitable tim°e interval must be used between successive expan—
si0ns.
A certain period of time must be allowed for the old di“Op$
to be removed by the electric eld or by precipitation, some of the
liquid must re—evaporate and thermal equilibrium must be restored.
18.
THE DETECTION OF PARTlCLES
for example, by cams on the shaft of the driving mechanism. A
chamber may
be placed between two counter tubes connected in a
coincidence circuit (see gure 18—10).
If the output
of this circuit
is used to start an expansion, the particles recorded will be, for the
most part,
those which have passed through the chamber in line
with the counter tubes.
Continuously active cloud chambers have been devised but
have not yet proven
of practical value.
The electricalelâ used to remove the ions is not critical and
may range
from 20 to 300 volts.
It is removed or reduced to a
small value during the expansion.
If a strong eld is used during
the expansion, it is possible to separate the positive and negative
ions to such an extent that each track is doubled.
From the sepa—
ration of the tracks and the mobilities of the ions, it is possible to
compute the age of the track.
Older tracks are wider.“
The
“
sweep
”
eld is often applied between the piston and a ring of
metal or colloidal graphite (Aquadag) around the top
of the
chamber.
It is better to use wires or plates on insulating supports
so as
to avoid distortion of the tracks by surface charges on the
chamber walls.
The time during which an expansion takes place should be
short.
This is necessary, not only to secure a sufcient drop in
temperature to cause condensation of the vapor on nuclei but also
to prevent the continued motion of the gas
from distorting the
newly formed track. The expansion time is usually of the order
of magnitude of 0.01 second,23 although as long as 0.25 second has
been used.22
Flexible diaphragms and metal bellows can be
moved quickly whereas heavy pistons and water surfaces are
comparatively sluggish.
If a particle passes through the chamber appreciably before
the expansion takes place, the tracks will be broad and dense. If
the ions are formed in the chamber too late, the tracks are thin.
The sensitive lime, during Which reasonably sharp tracks can be
formed, is usually a fraction of a second, although in one design22
it amounted to 1.5—2 seconds.
A suitable tim°e interval must be used between successive expan—
si0ns.
A certain period of time must be allowed for the old di“Op$
to be removed by the electric eld or by precipitation, some of the
liquid must re—evaporate and thermal equilibrium must be restored.