12-Basic Radiation Detection: Gas-filled detectors: GM Counters


Now let’s make another pass, looking at
the particular regions of operation, and I’ll give you a bit more information. In the ion chamber region, the output signal
is proportional to the amount of charge deposited in the sensitive volume that is in turn proportional
to the incident particle energy. In principle, this could be used to identify
the type of particle and its energy, but the signal is not large, and only strongly ionizing
particles—for example, alpha proton fission fragments and heavy ions—are detected. For typical ion chambers, only particles with
energies of 10 keV or greater are detected. Ion chambers come in two basic designs: pulse
counting ion chambers and integrated ionization chambers. In the first, the pulses and their height
are tallied electronically and in the latter case, the current itself is monitored. Most ion chambers are cylindrical, but some
use parallel plates, and some use a combination to help detect alphas and betas while still
doing a good job with photons. Most ion chambers use air as the counting
gas. It’s cheap and readily available. There’s often a window for alpha and beta,
so that these particles can more easily get into the sensitive volume of the detector. A piece of paper stops an alpha particle,
and a few millimeters of aluminum stops most beta particles. If I add boron to the sensitive volume, the
neutrons interact with this boron and create charged particles. These detectors can be used to monitor neutron
fluence rates at a reactor, for example. There are several design paths to minimize
the entrance of gamma rays with these detectors, including decreasing the amount of chamber
gas or increasing the amount of boron, among other ways. We’ll talk about it more in a minute. With proportional counters, we can identify
the type of particle, whether it is alpha, beta, or gamma, and its energy. For the counting gases, helium or argon for
alpha, beta, or gamma or boron trifluoride for neutrons are the most common. Detection of low-energy particles, below 10
keV, is possible due to the gas amplification. To stop a Geiger-Mueller discharge, a quenching
gas, often methane, is added to the counting gas. This helps resolution but limits the lifetime
of proportional counters for all except gas flow counters. Geiger-Mueller counters work in the GM region,
where the number of electrons produced is independent of the applied voltage above the
GM threshold, and the number of electrons produced in the gas is independent of the
number of electrons produced in the initial radiation. Identification of the type of radiation, the
number of original ion pairs, is impossible; therefore, the only information is about the
number of particles. The GM detector would discharge continuously
after the first pulse, except by positive ions around the central anode wire to form
a sheath around the wire, reducing the apparent field strength. Also, a quench gas is normally added. Halogens can be used, and they have the advantage
of recombining later in the tube. Other quench gases can be used, but if they
don’t re-form after use, then they limit the tube lifetime due to degradation of the
fill gas. GM counters are limited to low count rates
due to the large dead time for each count. Again, we’ll talk about this in a minute. Despite these limitations, GMs are widely
used to look for contamination or to guard against the release of radioactive material
because they are sensitive, portable, they have simple counting circuits and the ability
to detect very low levels of radiation.

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