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GEIGER-MLLER DETECTOR
G-M Detector Summary

Instrumentation and Control 2 of 2
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Radiation Detectors GEIGER-MÜLLER DETECTOR The number of electrons collected by a gas-filled detector varies as applied voltage is increased. Once the voltage is increased beyond the proportional region, another flat portion of the curve is  reached;  this  is  known  as  the  Geiger-Müller  region.    The  Geiger-Müller  region  has  two important characteristics: The number of electrons produced is independent of applied voltage. The number of electrons produced is independent of the number of electrons produced by the initial radiation. This means that the radiation producing one electron will have the same size pulse as radiation producing hundreds or thousands of electrons.  The reason for this characteristic is related to the way in which electrons are collected. When a gamma produces an electron, the electron moves rapidly toward the positively charged central wire.   As the electron nears the wire, its velocity increases.   At some point its velocity is great enough to cause additional ionizations.   As the electrons approach the central wire, the additional ionizations produce a larger number of electrons in the vicinity of the central wire. As discussed before, for each electron produced there is a positive ion produced.  As the applied voltage is increased, the number of positive ions near the central wire increases, and a positively charged  cloud  (called  a  positive  ion  sheath)  forms  around  the  central  wire.   The  positive  ion sheath reduces the field strength of the central wire and prevents further electrons from reaching the  wire.    It  might  appear  that  a  positive  ion  sheath  would  increase  the  effect  of  the  positive central wire, but this is not true; the positive potential is applied to the very thin central wire that makes the strength of the electric field very high.  The positive ion sheath makes the central wire appear much thicker and reduces the field strength.   This phenomenon is called the detector’s space charge.  The positive ions will migrate toward the negative chamber picking up electrons. As in a proportional counter, this transfer of electrons can release energy, causing ionization and the liberation of an electron.   In order to prevent this secondary pulse, a quenching gas is used, usually an organic compound. The G-M counter produces many more electrons than does a proportional counter; therefore, it is a much more sensitive device.   It is often used in the detection of low-level gamma rays and beta  particles  for  this  reason.    Electrons  produced  in  a  G-M  tube  are  collected  very  rapidly, usually within a fraction of a microsecond.  The output of the G-M detector is a pulse charge and is often large enough to drive a meter without additional amplification.   Because the same size pulse  is  produced  regardless  of  the  amount  of  initial  ionization,  the  G-M  counter  cannot distinguish  radiation  of  different  energies  or  types.   This  is  the  reason  G-M  counters  are  not adaptable   for   use   as   neutron   detectors. The   G-M   detector   is   mainly   used   for   portable instrumentation  due  to  its  sensitivity,  simple  counting  circuit,  and  ability  to  detect  low-level radiation. Rev. 0 Page 43 IC-06







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