Geiger counters (was: Re: Mystery metal)

According to Don Foreman <dforeman@xxxxxxxxxxxxxxxxxxxx>:

[ ... Geiger counters ... ]

Anybody know how these work?

Essentially, it is a cylindrical metal electrode surrounding a
central electrode, in a low pressure (near vacuum) gas (argon, possibly?)

A high voltage is applied across it, somewhat below the level at
which it will conduct on its own across the gap between the outer
electrode and the central one.

When an energetic particle (radiation) enters the zone between
the electrodes, it ionizes the gas to shorten the path, and a momentary
arc jumps across. The current which it represents is amplified to the
headphones (if any) plus integrated for the meter.

Note that the better tubes have a very thin window of quartz to
let the electrons through (beta particles, IIRC). The Alpha particles
and others can penetrate the metal side walls. My fancy Geiger counter
has a window which flips aside from the probe for that use. The probe
is used on the two most sensitive ranges. The other two ranges use a
much smaller Geiger tube with a known thickness of metal housing between
it and the outside (there is a special dent marking that spot for high
level measurements).

Is there a solid-state replacement for
a GM tube that works on low voltage?

Well ... a scintillation counter uses a crystal which emits tiny
flashes of light whenever struck by an energetic particle, and those
flashes are detected and amplified by a photomultipler tube (still needs
high voltage, though not as high). It *might* (these days) be possible
to use a solid state photosensor and amplifier -- I'm not sure whether
the light levels are sufficient for those, however. And I *think* that
the cost of the crystal is higher than the cost of the photomultiplier
tube and the associated electronics.

I hope that this helps.

Enjoy,
DoN.
--
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