Answer to Question #13827 Submitted to "Ask the Experts"

Why does organic quenching have a lesser slope than halogen quenched in the Geiger-Mueller (G-M) counter?

I shall assume that when you refer to "a lesser slope" your reference is to the slope of the plateau that is generated when one plots count rate against applied voltage for a given radiation source.

As you are likely aware, halogen quenching agents such as bromine or chlorine are favored over organic molecules, such as ethanol, probably largely because the organic quenchers gradually get destroyed as they dissociate at the cathode as they become neutralized; the molecular fragments produced are not able to recombine to regenerate the original molecules. In the case of halogen quenchers, however, the ionized halogen molecule will dissociate as it becomes neutralized at the cathode, but the atoms are able to recombine to yield back the original molecule. This difference makes for typically much longer lifetimes for the halogen-quenched tubes than for the organic-quenched varieties.

An additional advantage of the halogen quench agents is that they are easier to ionize than the organic species, and this effect allows halogen-quenched tubes to be operated at lower voltages than the organic-quenched tubes (often less than 600 volts compared to about 1,000 volts for typical organic-quench tubes). This is true since a lesser electric field is required to allow the energy transfer from an ionized primary gas molecule, usually argon, to produce ionization of the quench gas. There is some slight disadvantage also associated with the lower voltage, however. This has to do with the shape of the electric field lines in many G-M tubes. The tubes are often cylindrical with a central wire or rod-type anode. The anode-to-cathode electric field lines near the ends of the anode are often distorted, representing longer pathlengths for ions to follow and reduced electric field strengths. Other G-M tube shapes may not use a typical central anode but often still exhibit field line distortions between parts of the anode and cathode.

Much of the slope in such G-M probes results from the loss of full ion collection at the lower voltages with noticeably increasing collection as voltages increase to reduce ion recombination. If the halogen-quenched tube is operating at a lower voltage throughout the plateau region, compared to an organic quenched tube, the relative change in ion collection (and associated count rate) may be appreciably greater for a given voltage increase than would be the case for a tube operating at a higher voltage. For example, a 50-volt change from 500 to 550 volts represents a 10% increase in voltage which would be reflected in increased ion collection near the ends of the tube. The same increase in a higher voltage tube from 900 to 950 volts represents a smaller 5.5% increase with an expectedly lower relative change in count rate. The effect may be enhanced by the added characteristic of a lower ionization potential for the halogen compared to the organic quench agents, making an increase in voltage relatively more effective in yielding ionized quench agent arriving at the cathode.

I hope this addresses your concerns.

George Chabot, PhD, CHP