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#11a.     The Geiger Counter

  (Files in red–history)


9. Magnetic trapping

    9H. Poincaré, 1896

10. Trapped Motion

    10H. Einstein, 1910

10a. Particle Drift

11. Explorers 1/3

  11a. Geiger Counter

12. Rad. Belts

    12H. Argus 1958

12a. Inner Belt

12b. Outer Belt

13. Fast Particles

14. Synch. Orbit

15. Energy

        As noted in the section on plasmas, gases conduct electricity only when some of their atoms are ionized, i.e. are split up into free electrons and ions. Fast electrons and ions emitted by radioactive materials do ionize atoms with which they collide, and Hans Geiger, an associate of Rutherford (ions, history) used this property to invent a sensitive detector for such particles.

    A Geiger Counter
        A "Geiger counter" usually contains a metal tube with a thin metal wire along its middle, the space in between them sealed off and filled with a suitable gas, and with the wire at about +1000 volts relative to the tube.

        An ion or electron penetrating the tube (or an electron knocked out of the wall by X-rays or gamma rays) tears electrons off atoms in the gas, and because of the high positive voltage of the central wire, those electrons are then attracted to it. In doing so they gain energy, collide with atoms and release more electrons, until the process snowballs into an "avalanche" which produces an easily detectable pulse of current. With a suitable filling gas, the flow of electricity stops by itself, or else the electrical circuitry can help stop it.

        The instrument was called a "counter" because every particle passing it produced an identical pulse, allowing particles to be counted (usually electronically) but not telling anything about their identity or energy (except that they must have sufficient energy to penetrate the walls of the counter). Van Allen's counters were made of thin metal, with insulating plugs at the ends.

Questions from Users:
            ***     The Geiger counter (3 queries)
                  ***     Geiger counters for locating lost objects
                        ***     Geiger Counters (1)     (Followed by Geiger Counters (2)
                              ***     How to test a Geiger counter    

Last updated 25 November 2001
Re-formatted 3-11-2006