Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!ames!lll-winken!xanth!mcnc!godot!bourland
From: bourland@godot.radonc.unc.edu (J. Daniel Bourland)
Newsgroups: sci.electronics
Subject: Re: Radiation Detectors/Counters
Keywords: geiger counters
Message-ID: <1025@godot.radonc.unc.edu>
Date: 26 Apr 89 12:48:20 GMT
References: <5499@lynx.UUCP>
Reply-To: bourland@godot.UUCP (J. Daniel Bourland)
Distribution: usa
Organization: Radiation Oncology NCMH/UNC, Chapel Hill, NC
Lines: 48

A geiger-muller counter (GM) will count any ionizing radiation that is
able to penetrate the wall of the detector.  This includes gamma and x
rays down to 10's of keV, higher energy beta (not from tritium) and
few alphas.  Some GMs have a thin window in the end of the tube (a thin-
end window GM) which allow much more beta and more alphas to enter
the tube and be counted.

In terms of hazard, gamma, x, beta, alpha, and neutrons can all be
of sufficient danger if the exposure route is the "right" one as well
as the dose rate.  For instance, with gammas the risk is for external
exposure although internal exposures can occur.  With alphas the risk
is for internal exposure of the lung through inhalation.  So it can
vary - you should be careful about saying this one is safe and this one
is bad.

The roentgen is a measurement of ionization per mass of air, specifically
2.58 x 10^-4 coul/kg of air (a coulomb is a lot of charge) and is valid
only for photons.  Your GM counter's output is one click per ionizing
event, regardless of the amount of energy deposited, thus the GM
really just gives you counts per minute or hour or something like that.
The GM does not discriminate between photon, beta or alpha radiation
in terms of output - if the particle makes and ionizing event in the
GM, it is counted.  For this reason, the R/hr scale is true only for
the isotope for which the GM is calibrated (usually Cs-137, Co-60 or
Ra-226), but it is still accurate for isotopes having energies near
to that of the calibration isotope.

With regard to the rem, the rem is a unit for radiation protection purposes,
and is the dose in rads times the quality factor (QF) for the radiation
in question.  QF is an indicator of biological effect, relative to the
effect for photons or electrons (which have a QF of 1).  For neutrons
QF = 10, for alhpas QF = 10 to 20.  As it turns out, 1 rad is about
1 roetgen.  So a rem about equals a roetgen if the QF is 1, which works
for photons and electrons.  For alphas and neutrons, it will depend on
the QF, which does vary with energy.

Your GM will detect neutrons if the interact in the wall of the detector.
Neutron detectors are usually surrounded by a sphere of plastic to
stop the neutrons for counting - it is very tricky dosimetry.

All for now.  For those participating in this mini-series on radiation
instrumentation and dosimetry, please send $10.00 to the address below.
The honor code is in effect.  Thank you.

Dan Bourland, Physicist		bourland@godot.radonc.unc.edu
Dept of Radiation Oncology
CB 7512
Chapel Hill, NC 27599