Geiger Counter

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Geiger Counter:

A Geiger counter is an instrument which is used for detecting and measuring ionizing radiation. It is also known as a Geiger–Muller counter; this is widely used in many applications like experimental physics, radiological protection, radiation dosimetry, nuclear industry and nuclear-industry.

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It detects ionizing-radiation such as alpha particles, beta particles and gamma particles using the ionization effect produced in the Geiger Muller tube by which the name of the instrument is known.


History:

Hans Geiger in 1908, under the supervision of Ernest Rutherford, developed an experimental technique for detecting alpha particles which became the basis for developing the Geiger–Müller tube in 1928. The basic ionization mechanism used was discovered by John Sealy Townsend between 1897 and 1901 and is known as the Townsend discharge.

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Geiger and Walther in 1928 developed the sealed Geiger–Müller tube which they used. It was small, rugged that not only could it detect alpha-beta radiation both, but also gamma radiation. So radiation-instruments could be produced relatively cheaply, and so the Geiger counter came into existence.


The modern type of Geiger counter uses the halogen tube, and these lightweight instruments offer an improvement. Better range of detection of multiple types of ionizing radiation, i.e. alpha, beta, X-rays, and gamma, all are available even in the same unit. Their features are easy to use, common-interface and compatible with a catalogue of optional accessories for wireless reach back, rechargeable batteries, also extension poles to maintain good ALARA (as low as reasonably achievable) principles.


Types and Applications:

An intended-detection application of a Geiger counter explains the tube design being used. Subsequently, there are many designs which may be generally categorized as end-window, or windowless also as thin-walled or thick-walled, and sometimes hybrids of these types.


Historical uses of the Geiger principle was for the alpha and beta particles detection. However, this instrument is still being used for this purpose today. Geiger counter shares wide applications as they are used as handheld radiation survey instruments and is probably one of the world's best instruments known for radiation detection.


Particle Detection

The "end-window" type of a GM tube has to be used for alpha particles, and low energy beta particles as the particles show a limited range and are easily stopped by the solid-material. Therefore, this tube requires a window which is thin enough to allow as many as possible of these particles. The window is usually made of mica with1.5 - 2.0 mg/cm2 density.


Alpha particles contain the shortest range, and to detect these the window should ideally be less than 10 mm of the radiation source. Geiger–Müller tube generates a pulse output that is the same magnitude for all radiations detected, hence the Geiger counter with an end window tube is not able to distinguish the alpha or beta particles.


The "pancake" GM tube is a type of end-window probe and designed with a larger detection area for quick checking. However, Atmospheric pressure against fill gas's low pressure minimizes the window size because of the limited strength of the membrane.

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Some beta-particles can also be detected by thin-walled "windowless" GM tubes that have no end-window, however, allow high energy beta-particles to pass through the tube. The tube walls have more stopping power in comparison to a thin end-window; they even allow these energetic particles to reach the fill gas.


‘End-window’ GM counters are still being used as a portable,  general-purpose, radioactive-contamination measurement also detection-instruments, due to their less cost, robustness and high detection-efficiency especially with energy-rich beta particles.


For differentiation between alpha and beta particles and to know the particle's energy information, the scintillation counters are used.

 

The Principle of Working of GM Counter:

The Geiger counter has a Geiger–Müller tube or the sensing element to detect the radiation and the processor that displays the outcome.


At lower pressure and high voltage, the Geiger–Müller tube is filled with a non-reactive gas like He, Ne, or Ar. The GM tube briefly conducts electrical charge when a particle or photon of incident radiation makes the gas ionized by an ionization process, and produced electrons move towards the Anode. They later generate secondary electrons after repeated collisions with the gas particles as their velocity is relatively high. These secondary electrons further generate more electrons in Geometric progression. As a result, the large ionization current is produced due to this massive multiplication action.

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Here the ionization is amplified more within the tube by the Townsend discharge-effect to produce an easily measured detection pulse, that is fed to the processor display. This large-pulse from the GM tube makes the Geiger counter comparatively cheap to manufacture because electronics are greatly simplified subsequently.


Readout:

Detected radiation readout method in Geiger counter are of two types, i.e. Counts and Radiation dose. There is a simple type of display unit which shows the number of ionizing events detected displayed as a count rate, like "counts per minute or seconds" or as the total number of counts over the set period.


This count readout is usually taken when alpha particles or beta particles are being detected normally. To achieve is a display of radiation-dose rate is more complicated. It displays in Sievert, normally used to measure gamma or X-ray dose rates. Presence of radiation can be detected by a GM tube, not its energy, that influences the radiation's ionizing effect. The electronic processor will apply only known factors to make this conversion, i.e., specific to every instrument and is determined by its design & calibrations.


The readout may be analogue or digital, and the modern instruments provide serial communications with a host network or computer. The option is there to generate audible clicks that represent ionization events number. It is the distinctive sound normally with handheld or portable Geiger counters.


Advantages of GM Counter:

  1. GM counter can count alpha particles, beta particles, gamma particles, and cosmic rays as well.

  2. They have high sensitivity.

  3. In this case power supply required not to be with precise regulation because the pulse height is constant almost throughout the range.

  4. As the output pulse is very high, hence amplification required is more subtle.

 

Disadvantages of GM Counter:

  1. GM counter can not measure energy due to a lack of differentiating abilities.

  2. Uncharged particles like neutrons cannot be detected.

  3. GM counters are less efficient due to its large paralysis time limits and also large dead-time.

  4. Quenching agents used in GM counters often decompose, which leads to the reduction in a lifetime. 

Thus, GM Counter is primarily used due to its advantages. However, GM counters are not free from disadvantages, its uses make it preferable over other radiation counters.