Geiger-Müller Theory of Operation
SN-7298A Theory of Operation
The SN-7928A is a Geiger-Müller tube consisting of a lead-lined aluminum tube with a 446 stainless steel anode coating and a wire cathode passing up the center of the tube. The tube is filled with a low-pressure (<0.1 atm) mixture of helium and chlorine gases. The electrodes are at a potential difference of ~515 V.
When no ionizing radiation impacts the tube no current flows due to the very low conduction of the inert gas; however, when alpha, beta radiation with energy above 2.5 MeV, and/or x-rays penetrate the mica window of the tube, the helium gas molecules are ionized, creating both positive helium ions and electrons. The strong electric field on the tube's electrodes accelerates the ions towards the cathode and the electrons towards the anode. Along the way, the ion pairs gain sufficient energy to ionize further gas molecules through collisions with other argon atoms, creating an avalanche cascade of 106 to 108 charged particles and ultraviolet radiation.
The intense electric field near the anode collects the electrons to the anode and repels the positive ions. Electron mobility in the tube (104 m/s) is 104 times higher than that for positive ions, meaning that the electrons are collected within microseconds, while the positive ions surrounding the center wire are accelerated over milliseconds toward the cathode.
The temporary presence of a positive space charge surrounding the central anode terminates the production of additional avalanches by reducing the field gradient near the center wire below the avalanche threshold.
If it were not for the quenching halogen (chlorine) gas, ions would reach the cathode with sufficient energy to liberate new electrons, starting the avalanche process all over again, producing a continuous discharge; however, the halogen vapor has a lower ionization potential ( < 10 eV) than that of the fill gas (26.4 eV). Upon collision with a vapor molecule the fill gas ion gives up ~ 10 eV to the quench vapor molecule which then quickly dissociates rather than losing its energy by radiative emission. The remainder of the partially neutralized vapor-atom energy (~ 4 eV) produces a UV photon that is strongly absorbed by the molecules and prevented from reaching the cathode. Any quench vapor that might be accelerated and impact the cathode dissociates on contact.
Creation Date: 09/12/2008