11-Basic Radiation Detection: Gas-filled detectors: Ionization and Gas Amplification

Now let’s go look at the construction details
of gas-filled detectors. Usually these detectors are cylinders with
an anode wire along the center line of the detector. The electrons are collected by the anode,
and the large gas ions migrate to the walls of the detector, the cathode, and are neutralized
there by the electrons returning back through the electronics. If the voltage between the anode and the cathode
is sufficiently high, then recombination is stopped, and the electrons drift towards the
anode wire, having multiple collisions with gas molecules in the detector. These electrons reach the anode wire, not
all at the same time, but over the course of a few milliseconds because of the random
nature of how often they collided with gas molecules, how much energy was taken away
during the collisions, where in the sensitive volume each electron was created, and other
factors. With the voltage set only to stop recombinations,
the electrons collected are those created directly by the interaction of the ionizing
radiation with the detector, or primary ionization. If we raise the voltage difference between
the anode and the cathode, something interesting begins to happen. An electron, after being stopped by a collision
with a gas molecule, can gain enough energy from the electric field between hits to cause
an ionization the next time it hits a gas molecule, creating secondary ionization. This multiplication of electrons by secondary
ionization is called gas amplification. The primary and secondary ionization are generally
indistinguishable, and what comes from the detector is just a larger pulse. This graph shows what happens to the detector
signal as the voltage difference between the anode and cathode is increased. With no, or very low, voltage, the ion electron
pair will simply recombine, and because scientists are such dull and boring folks, this is called
the recombination region. When enough voltage is applied to stop recombination,
but is not high enough to cause gas amplification, the detector is in the ion chamber region,
region two here on this graph. The two lines here represent the size of the
pulse coming from the detector. The lower line is set at some arbitrary amount
of charge, and the higher line is for double that amount of charge in the detector. As we can see, the pulses in the ion chamber
region are proportional to the amount of charge deposited in the detector’s sensitive volume. As the voltage is increased, gas amplification
starts, first, rarely, because only a few of the electrons will escape gas collisions
long enough to gain the energy they need to cause secondary ionization. But, finally at the end of the region three,
the proportional region on this graph, a significant fraction of the electrons do. Next is out region four, the region of limited
proportionality, where the height of the pulse does not necessarily correspond to the number
of primary ionizations, because the secondary ionizations are beginning to swamp the original
primary ionization signal. Region five is the Geiger-Mueller region,
and in this region, all of the information from the primary ionizations is lost. Whether one electron or billions of electrons
were created by a primary ionization, the gas amplification is so large that the primary
signal is insignificant. Above this is the region called, “Burning
your anode wire if you do.” This should be avoided if you want to keep
your detector working.

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