Everything about Geiger-m Ller Tube totally explained
A
Geiger-Müller tube (or
GM tube) is the sensing element of a
Geiger counter instrument that can detect a single particle of
ionizing radiation, and typically produce an audible click for each. It was named for
Hans Geiger who invented the device in
1908, and
Walther Müller who collaborated with Geiger in developing it further in
1928. It is a type of
gaseous ionization detector with an operating voltage in the
Geiger plateau.
The Geiger counter is sometimes used as a
hardware random number generator.
Description and operation
A Geiger-Müller tube consists of a
tube filled with an inert gas such as
helium,
neon or
argon, in some cases in a
Penning mixture, and an organic vapor or a halogen and contains electrodes, between which there's a voltage of several hundred
volts, but no
current flowing. The walls of the tube are either metal or the inside coated with metal or
graphite to form the
cathode while the
anode is a
wire passing up the center of the tube.
When
ionizing radiation passes through the tube, some of the gas molecules are ionized, creating positively charged
ions, and
electrons. The strong electric field created by the tube's electrodes accelerates the ions towards the cathode and the electrons towards the anode. The ion pairs gain sufficient energy to ionize further gas molecules through collisions on the way, creating an
avalanche of charged particles.
This results in a short, intense pulse of current which passes (or
cascades) from the negative electrode to the positive electrode and is measured or counted.
Most detectors include an
audio amplifier that produce an audible click on discharge. The number of pulses per
second measures the intensity of the radiation field. Some Geiger counters display an exposure rate (for example
mR·h), but this doesn't relate easily to a
dose rate as the instrument doesn't discriminate between radiation at different energy
GM tubes
The usual form of tube is an
end-window tube. This type is so-named because the tube has a window at one end through which ionizing radiation can easily penetrate. The other end normally has the electrical connectors. There are two types of end-window tubes: the
glass-mantle type and the
mica window type. The glass window type won't detect
alpha radiation since it's unable to penetrate the glass, but is usually cheaper and will usually detect
beta radiation and
X-rays. The mica window type will detect alpha radiation but is more fragile.
Most tubes will detect
gamma radiation, and usually
beta radiation above about 2.5
MeV. Geiger-Müller tubes won't normally detect
neutrons since these don't ionise the gas. However, neutron-sensitive tubes can be produced which either have the inside of the tube coated with
boron or contain
boron trifluoride or
helium-3 gas. The neutrons interact with the boron nuclei, producing alpha particles or with the helium-3 nuclei producing hydrogen and
tritium ions and electrons. These charged particles then trigger the normal avalanche process.
Quenching
To prevent the current from flowing continuously there are several techniques to stop, or
quench the discharge. Quenching is important because a single particle entering the tube is counted by a single discharge, and so it'll be unable to detect another particle until the discharge has been stopped, and because the tube is damaged by prolonged discharges.
External quenching uses external electronics to remove the high voltage between the electrodes.
Self-quenching or
internal-quenching tubes stop the discharge without external assistance, and contain a small amount of a polyatomic organic vapor such as
butane or
ethanol; or alternatively a
halogen such as
bromine or
chlorine. Ions will collide with quench gas molecules, and give up energy to them by causing them to
dissociate.
Invention of halogen tubes
The halogen tubes were invented by
Sidney H. Liebson in
1947, and are now the most common form, since the discharge mechanism takes advantage of the
metastable state of the inert gas atom to ionize the halogen molecule and produces a more efficient discharge which permits it to operate at much lower voltages, typically 400–600 volts instead of 900–1200 volts. It also has a longer life because the halogen ions can recombine whilst the organic vapor can't and is gradually destroyed by the discharge process (giving the latter a life of around 10
8 events).
Further Information
Get more info on 'Geiger-m Ller Tube'.
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