434
18.
THE DETECTION OF PARTICLES
The importance of direct observation of the drop—track of a single
ionizing particle in the study of the nature of the particle and its
collisions with other particles is immediately obvious.
It is easier to secure good tracks of the heavily ionizing radia'
tions, such as alpha particles, than of the less ionizing radiations,
such as electrons.
The chamber need not be as clean nor the
illumination as intense for alpha tracks as for electron tracks.
In order to secure ne, sharp tracks: (1) the gas must be satu—
rated and at a constant, uniform temperature before the expansion,
(2) the particles must pass through the chamber at approximately
the time of the expansion and (3) the expansion must occur quickly
'
and be of a denite amount.
18—16.
The
Design and Operation of Expansion Cham—
bers.16“24— The Expansion Ratio. —The ratio of the nal to
the initial volume V/Vg of the chamber is called the isothermal
expansion ratio (R). When a simple piston of the type shown in
gure 18—13 is used, R is given as the ratio of the depth of the
chamber after the expansion to the depth before the piston moved
out.
In machines, such as those using a exible diaphragm piston,
where it is difcult to determine the volumes, R can be obtained
from the pressures of the gas in the chamber and the application
of Boyle’s (isothermal) law. Thus, the gas is compressed and
allowed to attain a constant temperature (in approximately one
minute).
Then, the total pressure of the gas and vapor
is
measured with a manometer.
Subtracting the vapor pressure
at
the existing temperature gives the initial gas pressure (Po). The
chamber is then expanded and allowed to reach thermal equilib—
rium, when the manometer reading is taken and corrected for the
vapor pressure to give the nal gas pressure (P).
Then
R = V/Vg = P0/P.
(18—7)
If good tracks are to be obtained, the expansion ratio must be
accurately adjusted to a particular value for each gas-vapor
com—
bination. This is especially true for electron tracks.
The table
on
page 435 shows optimum values of R as observed experi—
mentally by different workers.
It is seen that the volume increase
for the argon—alcohol combination is about ten per
cent.
The Gas and Vapor.——The gas—vapor combination should have
as small an expansion ratio as possible in order to shorten the expan—
18.
THE DETECTION OF PARTICLES
The importance of direct observation of the drop—track of a single
ionizing particle in the study of the nature of the particle and its
collisions with other particles is immediately obvious.
It is easier to secure good tracks of the heavily ionizing radia'
tions, such as alpha particles, than of the less ionizing radiations,
such as electrons.
The chamber need not be as clean nor the
illumination as intense for alpha tracks as for electron tracks.
In order to secure ne, sharp tracks: (1) the gas must be satu—
rated and at a constant, uniform temperature before the expansion,
(2) the particles must pass through the chamber at approximately
the time of the expansion and (3) the expansion must occur quickly
'
and be of a denite amount.
18—16.
The
Design and Operation of Expansion Cham—
bers.16“24— The Expansion Ratio. —The ratio of the nal to
the initial volume V/Vg of the chamber is called the isothermal
expansion ratio (R). When a simple piston of the type shown in
gure 18—13 is used, R is given as the ratio of the depth of the
chamber after the expansion to the depth before the piston moved
out.
In machines, such as those using a exible diaphragm piston,
where it is difcult to determine the volumes, R can be obtained
from the pressures of the gas in the chamber and the application
of Boyle’s (isothermal) law. Thus, the gas is compressed and
allowed to attain a constant temperature (in approximately one
minute).
Then, the total pressure of the gas and vapor
is
measured with a manometer.
Subtracting the vapor pressure
at
the existing temperature gives the initial gas pressure (Po). The
chamber is then expanded and allowed to reach thermal equilib—
rium, when the manometer reading is taken and corrected for the
vapor pressure to give the nal gas pressure (P).
Then
R = V/Vg = P0/P.
(18—7)
If good tracks are to be obtained, the expansion ratio must be
accurately adjusted to a particular value for each gas-vapor
com—
bination. This is especially true for electron tracks.
The table
on
page 435 shows optimum values of R as observed experi—
mentally by different workers.
It is seen that the volume increase
for the argon—alcohol combination is about ten per
cent.
The Gas and Vapor.——The gas—vapor combination should have
as small an expansion ratio as possible in order to shorten the expan—