166 Chapter 3. Gas Filled Detectors
3.3.D RegionofLimitedProportionality
As we increase the bias voltage, more and more charges are produced inside the active
volume of the detector. Now since heavy positive charges move much slower than
the electrons, they tend to form a cloud of positive charges between the electrodes.
This cloud acts as a shield to the electric field and reduces the effective field seen
by the charges. As a consequence the proportionality of the total number of charges
produces to the initial number of charges is not guaranteed. This region is therefore
termed as the region oflimited proportionality(see Fig.3.3.1). Since the loss of
proportionality means loss of linearity, radiation detectors are not operated in this
region.
3.3.E Geiger-MuellerRegion......................
Increasing the voltage further may increase the local electric field to such high values
that an extremely severe avalanche occurs in the gas, producing very large number
of charge pairs. Consequently a very large pulse of several volts is seen in the
readout electronics. This is the onset of the so calledGeiger-Mueller region.Inthis
region, it is possible to count individual incident particles since each particle causes
a breakdown and a large pulse. Since the output pulse is neither proportional to the
deposited energy nor dependent on the type of radiation, the detectors operated in
this region are not appropriate for spectroscopy.
There is also a significantdead timeassociated with such detectors. Dead time
is the time during which the detector is essentially dead. This can happen due to
the detector itself or the associated electronics. If there is a large accumulation of
positive charges, it can reduce the internal electric field to a value that it can no
longer favor avalanche multiplication. If radiation produces charge pairs during this
time, the charges do not get multiplied and no pulse is generated. The detector
starts working again as soon as most of the positive charges have been collected by
the respective electrode. Dead time will be discussed in some detail later in the
chapter.
The multiplication of charges in a GM detector is so intense that sometimes it is
termed asbreakdownof the gas. We will discuss this phenomenon in the following
section.
E.1 Breakdown
The large number of ions created during the avalanche drift much slower than the
electrons and therefore take longer to reach the cathode. When these heavy positive
charges strike the cathode wall, they can release more ions from the cathode material
into the gas. The efficiencyγof this process is generally less than 10%.γis known
as thesecond Townsend coefficient. At moderate voltages,γis not high enough
to make significant increase in charge population. However at higher voltages the
secondary ion emission probability increases, deteriorating the linearity of the output
pulse with applied voltage. Further voltage increase may start discharge in the gas.
At this point the current goes to very high values and is limited only by the external
circuitry, that is, the height of the pulse becomes independent of the initial number of
electron ion pairs. Geiger tubes, that we will visit later in the chapter, are operated
in this region.