Geiger-Muller Counters Marisol Amaya Michael Blackmon A Geiger by xuyuzhu


									Geiger-Muller Counters
     Marisol Amaya

    Michael Blackmon
        A Geiger-Muller counter, also known as Geiger counter, is a scientific instrument use to
detect and measure alpha, beta, and gamma radiation. Geiger-Muller counters were invented by
H. Geiger and E.W. Muller in 1928. The tube was an improved version of an earlier counter
which was developed by Geiger in 1908. The new tube was invented by Geiger with the help of
his student Muller. A Geiger-Muller tube is a gas-filled detector, that is relatively inexpensive,
and is used as an survey meters and contamination monitors. The Geiger-Muller counter consists
of a pair electrodes surrounded by ionizing gas, which is usually Argon, Neon, and the halogen
elements. The pair of electrodes usually have a high voltage across them. The apparatus only
consists of two parts, the tube and the counter with a power supply, which normally haves
voltage controls and timer options.

        The Geiger-Mueller tube are most of the time cylindrical, with a wire down the center.
One end consists of a “window” which is make of a thin material, mica, that allows the entrance
of particles, usually alpha, and can be shielded easily. A wire, that runs lengthwise down the
center of the tube, is positively charged with a relatively high voltage and acts as an anode. The
tube itself acts as the cathode. The anode and cathode are connected to an electric circuit that
maintains the high voltage between them. For this to properly happen an appropriate voltage
must be maintain across the electrodes. If the voltage is too low, the electric field in the tube is
too weak to cause a current pulse. If the voltage is too high, the tube will undergo continuous
discharge, and the tube can be damaged. The larger tubes require larger voltages to produce the
necessary electric fields inside the tube. Most of the time the manufacture recommends the
correct voltage to use for the tube which is usually shown in the manual.

        When radiation from a particle(or ray) enters the tube and causes the gas to be ionized,
will results in an ion pair. Then an electron is knocked out of an ionized atom and a positive
atom is left. This is follow by the negative electrons traveling to the anode and the positive ions
traveling to the cathode. Even so it will take longer for the positive ions to reach the cathode
since electrons move 10⁴ times faster than them. Once the electrons have reached the central
wire and are collected they create a sharp pulse which is counted. After the pulse is counted, the
charged ions become neutralized, the Geiger-Muller counter is ready to record another pulse. In
order for the Geiger counter tube to restore itself quickly to its original state once the radiation
has entered, a gas is added to the tube.

        As the electrons travel toward the anode they ionize other atoms, which produces a
cascade of electrons called gas multiplication or a townsend avalanche. The multiplication factor
is typically 10⁶ to 10⁸ . The resulting discharge current causes the voltage between the anode
and cathode to drop. The counter (electric circuit) detects this voltage drop and recognizes it as a
signal of a particle’s presence.The positive ions may still have enough energy to start a new
cascade. One (early) method was external quenching which was done electronically by quickly
ramping down the voltage in the GM tube after a particle was detected. This means any more 9
electrons or positive ions created will not be accelerated towards the anode or cathode,
respectively. The electrons and ions would recombine and no more signals would be produced.
The modern method is called internal quenching. A small concentration of a polyatomic gas
(organic or halogen) is added to the gas in the GM tube. The quenching gas is selected to have a
lower ionization potential (~10 eV) than the fill gas (26.4 eV). When the positive ions collide
with the quenching gas’s molecules, they are slowed or absorbed by giving its energy to the
quenching molecule. They break down the gas molecules in the process (dissociation) instead of
ionizing the molecule. Any quenching molecule that may be accelerated to the cathode
dissociates upon impact producing no signal. If organic molecules are used, GM tubes must be
replaced as they loss they permanently break down over time (about one billion counts)

        Geiger-Muller counter are very popular due to their relatively low price compare to other
detectors. This is also back by how durable and easily portable they are. But even still they have
their disadvantages. One of their being that even though they can detect all types of radiations,
they cannot differentiate which type of radiation they are detecting. They are also unable to
determine the exact energy of the radiation that they have detected. And the major disadvantage
isof the Geiger-Muller counter is it very low efficiency. Even still the Geiger-Muller is still
useful for the great convenient it provides.

To top