General Instructions - 3B SCIENTIFIC PHYSICS U8483220 Instrucciones De Uso

4. Operating principle
Experiments by R. von Helmholtz in 1887 showed that
ions in an atmosphere supersaturated with water
vapour act as condensation centres around which
cloud droplets form. The charged particles emitted
from radioactive elements generate large numbers of
ion pairs along their paths in the surrounding atmos-
phere. If the air is supersaturated with water vapour,
the ions act as condensation centres, and with suit-
able illumination the tracks of the particles become
visible as fine vapour trails ("condensation trails").
In the cloud chamber the supersaturation of the sur-
rounding air is produced by sudden expansion and
resultant cooling of the gas filling.
5. Operation

5.1 General instructions

1. When the cloud chamber is being closed, the
knurled screws must be tightened firmly to ensure an
airtight seal. By immersing the chamber under water
and squeezing the rubber bellows, any leakage will
become apparent.
2. It is essential for the cloud chamber to be kept free
of dust particles. When withdrawing the radiation
cartridge from the cloud chamber, the filling nozzle
must be closed with a rubber bung. The risk of con-
tamination is especially great when the chamber is
taken apart. Therefore, do not open the chamber
more often than is necessary, and before reassem-
bling it, clean it thoroughly with a damp chamois-
leather.
3. The cloud chamber remains usable for a very long
time if the radiation cartridge remains attached to the
filling nozzle or the nozzle is closed by an air-tight
bung.
4. The radiation cartridge is tightly sealed to prevent
any emanation. Even when it remains in the cloud
chamber for a long time, there is no risk of radioac-
tive contamination.
5. The accurately parallel cover plate allows particle
tracks to be photographed with no optical distortion.
For this the illumination should be arranged, using
apertures, so that the light beam does not fall on the
black base-plate.
6. If a deposit of moisture forms on the Plexiglas plate
during storage or due to uneven heating by the illu-
minating lights, it can be eliminated by placing a
warm woollen cloth over the plate.
5.2 Experiment procedure
•
Using a pipette, introduce the cloud chamber
fluid (about 10 to 20 drops) into the chamber
through the filling nozzle, and distribute it evenly
by shaking.
•
Screw the radiation cartridge into the filling noz-
zle, after first using a screwdriver or flat object to
rotate the cartridge shaft so that its flattened end
faces towards the middle of the chamber.
•
Align the cloud chamber horizontally by clamping
it on a stand.
•
Set up the illumination so that the light beam
enters the chamber from the side at about 90
the direction of the radiation from the radioactive
source.
•
Rub the cover plate with a woollen cloth, without
applying pressure.
•
Squeeze the rubber bellows tightly, hold for 1 to 2
seconds, then release.
On releasing the rubber bellows, the tracks of the
α−particles become visible as vapour trails. They
slowly disappear after 1 to 2 seconds. The process can
be repeated after waiting only a few seconds.
•
By tilting the cloud chamber, bring the absorp-
tion foil into the path of the radiation and ob-
serve the absorption of the α−particles on paper.
5.3 Comments
1. When the cover plate is rubbed, an electric field is
generated between it and the base-plate, which
purges the chamber of residual ions, which would
interfere with the experiment by causing a haze. If the
photographs obtained after repeated operation of the
rubber bellows are blurred, the cover plate needs to
be rubbed again.
2. In the photographs obtained from the cloud cham-
ber, it can clearly be seen that the trails are of differ-
ent lengths. A large fraction of them are only about
half as long as the longest ones. From the different
lengths of the trails, it can be concluded that the
particles are emitted at differing velocities.
Each α-emitting substance (nuclide) is characterised
by a unique emission energy, and a corresponding
range of penetration through air. The α-particles from
radium 226 have a range of 3.6 cm (at atmospheric
pressure). The α−particles with the long trails are
emitted by a decay product (Ra A, range 6.3 cm). The
radioactive material in the radiation cartridge is sur-
rounded by an extremely thin metal foil. Conse-
quently, the observed ranges are slightly smaller than
the values given in the tables.
If an α−particle collides with an atomic nucleus in its
flight, its direction is changed and the affected nu-
cleus is set in motion, thus producing a trail of its
own. Such collisions are very rare, and therefore you
will be very lucky if you are able to observe such an
event.
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