Radiation is all around us. To help us to detect radiation, there are many apparatus designed
for this purpose. Here are some of the more common apparatus.
Photographic film
Photographic film can be used to detect radiation. The extent of blackening of the film shows
the level of exposure to [nuclear radiation].
Diffusion Cloud Chamber
The cloud chamber was invented by C.T.R. Wilson,
who got the idea from watching the froming of
clouds when he was on holiday on the Scottish mountains.
The bottom compartment is cooled by dry ice and
stays around at -78 degree Celsius; while the top
compartment is filled with air which is at room
temperature. A felt ring at the top of the chamber is soaked
in alcohol. A weak radioactive source emits radiation
producing ions along its path. The alcohol vapourise
in the warm region , diffuses downwards from the top
and condensed around the ions. These tiny alcohol
drops shows up as a track in the light.
Cross section of a cloud chamber
For better viewing, the perspex lid of the chamber should be rubbed frequently with a
cloth in order to create an electric field.
Spark Counter
This consists of a postively charged fine wire
stretched just below a piece of earthed wire
gauze. And a high voltage between the gauze
and the wire is adjusted until it is nearly
sparking. When a radioactive source is
brought near the wire gauze, sparks are
produced, in a random fashion. Due to
radiation from the source ionises the air between
the gauze and the wire, as a result, sparks are
produced under high voltage in the presence of
ionised particles.
Geiger-Muller (GM) Tube
cross section of a GM tube
This is a metal tube filled with argon gas at low pressure. At one end, it has a very thin mica-
window through which radiation can enter. Along the center of the tube, there is a wire. A
voltage of 400V is applied across the metal tube and the central wire. When radiation
enters the tube, it ionizes the argon gas inside, thus setting off a spark. A quenching gas
(typically a halogen) stops the flow of current a few microseconds later. Thus, a pulse
of current flows.
The GM tube is connected either to a SCALER or a RATEMETER. The scaler records the
total number of pulses or counts. The ratemeter gives the count rate in counts per second
(or minute). It is usually connected to a loudspeaker which gives a "click" for each pulse. The
GM tube connected to a scaler or ratemeter is usually referred to as a Geiger counter (or GM
counter).
Geiger Muller tube connected to a scaler
*back* (C) 1999 ThinkQuest Team 27954