Radiation Detectors: Geiger Counters, Cloud Chambers, and More
Radiation Detectors: A Detailed Look
Geiger Counter
A Geiger counter consists of a central wire inside a hollow metal cylinder filled with gas at low pressure. An electrical voltage is applied between the cylinder and the wire, making the wire more positive than the cylinder. When radiation enters the tube and ionizes an atom in the gas, the released electron is attracted towards the positively charged central wire.
As the electrons accelerate toward the wire, they collide with other atoms, releasing more electrons. This process creates a cascade of electrons moving toward the wire, causing a short pulse of electrical current. This pulse activates a timer device connected to the tube. When amplified, this current pulse produces the characteristic “crack” associated with radiation counters.
Cloud Chamber
A cloud chamber reveals the visible path of ionizing radiation as traces of fog. It comprises a glass cylindrical chamber, sealed at the top by a glass window and at the bottom by a mobile piston. The chamber can be saturated with water vapor or alcohol by adjusting the piston.
A radioactive sample is placed inside the chamber. When radiation passes through the chamber, ions are produced along its trajectory. If the saturated air in the chamber is cooled rapidly by moving the piston, tiny droplets of moisture condense on these ions, forming visible steam trails that indicate the radiation’s path. These are analogous to the contrails formed by ice crystals in the sky behind jets.
Bubble Chamber
The particle tracks in a bubble chamber are formed by tiny gas bubbles in liquid hydrogen. The liquid hydrogen is heated under pressure inside a glass and stainless steel chamber to a temperature just below its boiling point.
If the chamber pressure is suddenly lowered in the brief moment after a particle passes through, it produces a thin trail of bubbles along the particle’s trajectory. Before the liquid erupts and boils, pictures are taken of the particle’s short track. Placing the bubble chamber in a magnetic field allows for the determination of the charge and mass of the particles.
Spark Chamber
A spark chamber is a counter consisting of a set of closely spaced parallel plates. Every third plate is grounded, while the intermediate plates are maintained at a high voltage (approximately 10 kV). As charged particles pass through the chamber, ions are produced in the gas between the plates.
The discharge along the path of the ions creates a visible spark between pairs of plates. A series of sparks indicates the particle’s trajectory. A variation of this design is the streamer chamber, which consists of only two plates separated by a larger distance. The electric shock in the streamer chamber tracks the trajectory of charged particles.
Scintillation Counter
A scintillation counter relies on the principle that certain substances are easily excited and emit light when charged particles or gamma rays pass through them. These tiny flashes of light, or scintillations, are converted into electrical signals by special photomultiplier tubes.
A scintillation counter is much more sensitive to gamma rays than a Geiger counter and can measure the energy of the charged particles or gamma rays absorbed in the detector.