1 Prepared By Najat AL-Twarqi
Gas Ionization Geiger Muller Detector
History:
Hans Geiger developed a device (that would later be called the
"Geiger counter") in 1908 together with Ernest Rutherford. This counter was only capable of detecting alpha particles. In 1928, Geiger and Walther Muller improved the counter so that it could detect all kinds of ionizing radiation.
Description:
A Geiger counter (Geiger-Muller tube) is a device used for the detection and measurement of all types of radiation alpha, beta and gamma radiation.
Basically, the Geiger counter consists of two electrodes with a gas at reduce pressure between the electrodes. The gas used is usually Helium or Argon. The outer (negative) electrode ( or cathode ) is usually a cylinder, while the inner ( positive ) electrode ( or anode) is a thin wire positioned in the center of the cylinder. The voltage between these two electrodes is maintained at such a value that virtually any ionizing particle entering the Geiger tube will cause an electrical avalanche within the tube. The Geiger tube has a thin window at one end through which the ionizing radiation enters shown in Fig.1.
2 Prepared By Najat AL-Twarqi Fig.1
Operation:
When an ionizing radiation such as Gamma, Beta or Alpha passes through the tube, some of the gas molecules are ionized, creating positively charged ions and electrons, the strong electric field created by the tube's electrodes will accelerate the positive ions toward the cathode and the electrons toward the anode. As the ion pairs accelerate toward the cathode they gain enough energy to ionize further gas molecules through collision, creating an avalanche of charged particles, the electrons will be collected by the anode and will form an intense pulse of current that will be measured and counted as a single particle or a single photon.
The positive charged ions will be accelerated toward the cathode gaining energy and ionizing more gas molecule producing more electrons in the process, any sort of radiation that will arrive during this period, no pulse would be formed because the counter is insensitive; the insensitivity lasts for a certain time, called the dead time of the counter.
HV Cylindrical (cathode)
Wire (anode ) Window
α ,β or γ radiations
+ ــ
3 Prepared By Najat AL-Twarqi
To prevent the current from flowing continuously there are several methods to quench or stop the discharge:
1. In external quenching, the operating voltage of the counter is decreased.
2. The internal or self quenching is accomplished by adding small amount of an organic or halogen gas.
The quenching gas ( Halogen ) is selected to have a lower ionization potential then the fill gas, when the positive ions collide with the quenching gas molecules, they are slowed or absorbed by giving its energy to the quenching molecules, they break down the gas molecules in the process ( dissociation ) instead of ionizing them. Any quenching molecule that may be accelerated toward the cathode dissociates upon impact producing no signal, and then they will naturally recombine. If an organic gas molecules are used in the quenching process GM tube must be replaced as they permanently break down over time.
Most of the Geiger counters will not detect neutrons, since they don't ionize the gas, but this can be achieved by coating the metal tube with boron the neutrons will interact with the boron producing alpha particles that will trigger the avalanche process.
The type used in experiments is ST-150 nuclear lab station.
Advantages:
1- It is operation is simple.
2- It provided a very strong signal so that it does not need to a preamplifier.
3- It can be used with any kind of radiation.
4- It is inexpensive.
Disadvantages:
1- It does not differentiate between kinds of radiation or energies; it tells only that a certain number of particles entered the detector during its operation.
2- It has a relatively long dead time ( 200 to 300 µs ).
