Fluke Biomedical 90-12 - Manual (Page 11)

Theory of Operation
General
3
3-1
Section 3
Theory of Operation
3.1 General
X-rays, gamma rays, beta rays and alpha particles produce charged particles in a gas. These charged
particles are negatively charged electrons and positively charged ions. An electron and a positively
charged ion are called an ion pair. Collecting these ion pairs formed in a gas filled chamber permits
quantification of the incident radiation.
Charged particles, such as alphas and betas, produce ion pairs through direct action, either by colliding
with electrons in matter or the interaction of electrical fields. This process is called ionization.
Uncharged particles, such as photons, produce ion pairs through indirect action. They interact with matter
to form charged particles, which then produce ion pairs as described above.
The number of ion pairs formed in a gas depends upon the type and energy of the incident radiation and
the nature of the gas. Each different gas has its own ionization potential, that is, the amount of energy
necessary to cause one ion pair to form.
If left to themselves, negative electrons and positive ions will recombine to form neutral atoms again.
However, if an electric field is applied, via the application of a high voltage to the gas, then the negative
electrons will move toward the positive electrode and the positive ions will move toward the negative elec-
trode. This causes a current to flow.
This current is usually very small, usually too small to measure under ordinary conditions, therefore, some
method for amplifying the current is needed. It is desirable to operate the detector in such a way that each
individual interaction process gives rise to a detectable pulse. This is accomplished via the external circuit
design and through gas multiplication.
A few free electrons, under the influence a strong enough electric field, may have enough kinetic energy
that when they collide with neutral atoms they also ionize these atoms. If this process begins and repeats
itself many times it produces a cascade of electrons. This cascade works like an avalanche and produces
a pulse of many charged electrons. This is called gas multiplication.
In a Geiger Mueller tube the formation of only one free electron in the gas volume is usually enough to
initiate an avalanche.
The Geiger Mueller pulse or avalanche ends when a high enough positive ion concentration is formed in
the gas to reduce the intensity of the electric field. Any ionization event that occurs during this time will not
produce an avalanche or pulse. During this time the tube is said to be "dead." After a short time period
these ions dissipate enabling the tube to respond normally once more.
Geiger Mueller tubes are filled with some small amount of "quench" gas to help end the pulse period by
absorbing some of the free energy in the system.
If the voltage on the Geiger Mueller tube is too high, or the radiation rate is too high, the Geiger Mueller
tube will continuously discharge because a continuous arc of current will have been formed.