Geiger Muller - Gas Filled Detector

Geiger Muller - Gas Filled Detector

Geiger Muller or counters are one of the kind of gas filled detector;that operate by using the ionising nature of alpha, beta and gamma radiation;neutron sensitive devices can also be produced;typically by introducing boron;which interacts with neutrons to produce secondary ionization particles that trigger the counter response. The GM tube is a shield metal tube cylinder with low pressure inert gas such as Argon or Neon;a thin metal wire runs down the center of the tube; which is electrically insulated from outer cylinder till the end of the tube; the front of the tube is shield with radiation window,that is specific to typical radiation that is to be detected by the counter;a thin micro window is to be used if the tube is to be sensitive to alpha particles and low energy beta particles;both of which have low penetrating power;a thicker window of different material such as glass or thin sheet of metal is to be used to measure high energy beta particles while for gamma rays.

The tube is often shield without a window;in such tubes, the detection occurs when high energy photons liberate electrons from the tube outerwall,the inner wire and outer wall are maintained at a pd of about 1kV and in the absence of radiation, no current can flow through inert gas b/w the central anode and the outer cathode; the connections are made by wires into a connection housing;fixed over the rare of the tube;and the outer tube guard is actually screwed onto the tube to protect the actual GM tube;this tube guard can be open at the end or can be covered with an energy filter;to change the energy and particle sensitivity of the device or a careful calibration is used, it allows for ambient dose measurement;rather than ambient count measurement to be made;the wires are connected to the tube to the control electronics;by power to perform counting operations and allow functions such as conversion from counts to dose, data logging, data averaging and driving the display;the tube works on the basis of gas amplification,incoming radiation ionizes some of the inert detector gas resulting in a free electron and a positively charged ion; the electric field inside the tube attract the charged ions towards outer cathode while the electrons are attracted towards central anode, as the electrons encounter the anode, the electrons experience the growing strength so that their accelerating forces increases, near the anode the acceleration is such that enough energy that it either excites the electrons in other atoms of the detector gas,or ionize them completely, excited electrons quickly decay releasing photons that can trigger ionization farther along the tube while the electrons free due to ionization can go on to cause further ionization leading to an exponential growth; this is often referred to as avalanche effect.

The charge migration in the tube leads to reduction of potential in an anode and increase in potential of the cathode either of which may be detected as a signal by the counter electronics,as the negative charge around the anode increases, the effective electric field is reduced, eventually this reduction is such that further avalanche are not possible, and the tube can no longer detect radiation,this step persist until the sufficient electrons have recombined with the anode, and the positive gas ions recombine with the cathode so that the field is recoverable enough in strength to trigger another avalanche,this is so k/a dead time of the detector,the time after detection the counter is insensitive to further events and this existence means that the detector count rates must be corrected to give the actual count rate,after the dead time further detection are possible with reduced signal strength, the total time that elapses before the full strength signal is produced by subsequent events is k/a recovery time.

Part 2:
The recombination of positive gas ions at cathode may be problematic as the ions may be neutralized in excited state or dislodge electrons at the cathode,when in excited state the atoms will eventually decay to ground state by emitting a photon and these photons and the dislodged electrons may be the cause of reionisation of the gas triggering another avalanche so the single detection event will lead to a continuous discharge, to prevent this a quenching mechanism is used, the quenching may be electronic so the electric field is removed fro a short period of time following an event to prevent further discharge or may be inherent in the design by mixing quenching gases with the detection gases such quenching gases are easier to ionize than detection gas so that during migration to the cathode, the detection gas is neutralized by the quenching gas which then becomes the positive ion, migrating to the cathode, when the charged quenching gas ions recombines with the cathode, it does so in the ground state so that further avalanche discharge is avoided.

The most effective quenching gas are the organic compounds,but these are dissociated irreversibly during quenching which gives the tube operating limiting longevity, an alternative is to use halogen gas which is recovered full at cathode, so avoid the removal of quenching gas, the raw output from GM tube shows a fraction of radiation counts per sec which is modified by taking in account of the dead time to give actual counts per sec; if the radiation type and energy are known, then the counts per sec can be calibrated so that the unique gives an equivalent dose rate,this is not the best method for dose determination from an unknown source as in the GM tube signal pool side is relatively insensitive to the instant radiation energy type, so the energy deposited is difficult to determine,the use of counts per sec or dose rate depends on large extent on circumstances of use, on both modes the radiation are being detected, in the former the activity is displayed while in the later, conversion is made to indicate the energy deposition rate,

For wide spread contamination by a radioactive material the energy rich in the GM tube will be small as the inverse square law and transient absorption remove all of the fraction of the instant particles however radiation from contamination can be measurable as the increase in the background radiation level which can easily be expressed in the increase in the no of counts per sec, as a practical radiation detection device,GM tube is not considered a natural choice for measuring the radiation than pulse device such as linac, the reason is the dead time is much longer than the pulse width duration, the detector will only pick up a single event rather than a bunch of electrons or photons and the tube will count the pulses not the radiation leading to an underestimation of dose,if that time is significantly longer than bunch frequency, the detector will count only a fraction of bunches,leading to further underreading,if the operating characteristic of GM tube and linac are known, these effects can be compensated for so the GM tube could be used, this will mean however,different methods have been used to measure the different calibration methods to measure the data,

Reference:
https://www.youtube.com/watch?v=bcjMOr-qiwA
https://www.youtube.com/watch?v=1qRjSLqM4zg