geiger müller counter - wordpress.com · 2018. 9. 8. · history •the german physicist hans...

Geiger Müller Counter

Dr. Ruchika Yadu

Introduction• The Geiger–Müller counter is an instrument used for measuring

ionizing radiation used widely in such applications as radiationdosimetry, radiological protection, experimental physics and thenuclear industry.

• It detects ionizing radiation such as alpha particles, beta particles andgamma rays using the ionization effect produced in a Geiger–Müllertube; which gives its name to the instrument .

• It is perhaps one of the world's best-known radiation detectioninstruments in wide and prominent use as a hand-held radiationsurvey instrument

Department of Biotechnology SAC Jabalpur

History• The German physicist Hans Wilhelm Geiger is best known as the inventor

of the Geiger counter to measure radiation. In 1908, Geiger introduced thefirst successful detector of individual alpha particles. Later versions of thiscounter were able to count beta particles and other ionizing radiation.

• The introduction in July 1928 of the Geiger-Müller counter marked theintroduction of modern electrical devices into radiation research.

• The original detection principle was discovered in 1908, but it was not untilthe development of the Geiger-Müller tube in 1928 that the Geiger-Müllercounter became a practical instrument.

• Since then it has been very popular due to its robust sensing element andrelatively low cost.

• However, there are limitations in measuring high radiation rates and theenergy of incident radiation.


Advantages

• They are relatively inexpensive

• They are durable and easily portable

• They can detect all types of radiation


Disadvantages

• They cannot differentiate which type of radiation is being detected.

• They cannot be used to determine the exact energy of the detected radiation

• They have a very low efficiency


PRINCIPLE

• Charged particles ionize the gas through which they pass ,theelectrons so produced during ionization get accelerated under highpotential and further produce ionization.

• Basically it consists of a pair of electrodes surrounded by a gas. Theelectrodes have a high voltage across them.

• The gas used is usually Helium or Argon. When radiation enters thetube it can ionize the gas. The ions (and electrons) are attracted tothe electrodes and an electric current is produced.

• A scaler counts the current pulses, and one obtains a "count"whenever radiation ionizes the gas.


Construction of GM counter

• The apparatus consists of two parts, the tube and the (counter +power supply).

• The Geiger-Mueller tube is usually cylindrical, with a wire down thecenter.

• The (counter + power supply) have voltage controls and timeroptions. A high voltage is established across the cylinder and the wire


Construction of GM counter• Cylinder(B): It is a sealed glass tube of radius 2-3 cm enclosing a co-axial metal

cylinder serving as a cathode.

• Thin metal wire: It is stretched along the axis of cylinder and serves as anode.The free end of anode is covered with glass bead to avoid corona dischargebuilding up at sharp points.

• Thin window made of mica or other materials provided for counting lesspenetrating radiation like α or β rays.

• Inert gas like, argon is filled in glass tube with vapors of some volatile compoundlike ethyl alcohol (quenching vapor) in ratio 10:1 is added to it. In some countersneon gas mixed with traces of halogen (e.g. Bromine as a quenching vapor ) isused.

• The gases must be free from electro negative impurities such as(CO2 , O2 )whichwill tend to form negative ions and initiate secondary avalanche.


General plan


• When ionizing radiation such as an alpha, beta or gamma

particle enters the tube, it can ionize some of the gas molecules

in the tube. From these ionized atoms, an electron is knocked

out of the atom, and the remaining atom is positively charged.

• The high voltage in the tube produces an electric field inside the

tube. The electrons that were knocked out of the atom are

attracted to the positive electrode, and the positively charged

ions are attracted to the negative electrode.

• This produces a pulse of current in the wires connecting the

electrodes, and this pulse is counted. After the pulse is counted,

the charged ions become neutralized, and the Geiger counter is

ready to record another pulse.

• In order for the Geiger counter tube to restore itself quickly to its

original state after radiation has entered, a gas is added to the

tube. Department of Biotechnology SAC Jabalpur

Working of GM Counter•Radiation (dark blue) is moving about randomly outside thedetector tube.

• Some of the radiation enters the window (gray) at the end of thetube.

• When radiation (dark blue) crash with gas molecules in the tube(orange), it causes ionization: some of the gas molecules are turnedinto positive ions (red) and electrons (yellow).

• The positive ions are attracted to the outside of the tube (lightblue).

• The electrons are attracted to a metal wire (red) running down theinside of the tube maintained at a high positive voltage.

• Many electrons travel down the wire making a burst of current ina circuit connected to it.

• The electrons make a meter needle deflect and, if a loudspeaker isconnected, you can hear a loud click every time particles aredetected.


Townsend discharge or Townsend avalanche

• The Townsend discharge or Townsend avalanche is a gas ionisationprocess where free electrons are accelerated by an electric field,collide with gas molecules, and consequently free additionalelectrons.

• Those electrons are in turn accelerated and free additional electrons.The result is an avalanche multiplication that permits electricalconduction through the gas.


QUENCHING:

• Upon reaching the electrodes the ions are neutralized. Some of theses ionshowever might escape and produce their ionization avalanche.

• GM tube might give a continuous discharge. In order to overcome this thetube is quenched by addition of suitable Quenching agent (butane,propane, ethanol, ethyl formate, chlorine, bromine).

• Quenching agent prevents continuous ionization.

• If continuous ionization is not prevented the detector will not respond toany other particle except the very first which enters the tube.

• Quenching even results in destruction of the quencher. Therefore GM tubebecomes useless after some time.


Quenching: Types

• Internal quenching: a small amount of gas(butane or ethanol) isadded into the GM counter is called internal quenching.

• External quenching: a digital circuit is connected with the GMcounter to reverse the polarity of the counter's electrodes, after thepulse is detected, to stop the secondary electrons.


Geiger Counter Units

• There are two types of UNITS used;

• Counts :- The counts display is the simplest and is the number ofionizing events displayed either as a count rate, commonly "countsper second“ , or as a total over a set time period (an integrated total).The counts readout is normally used when alpha or beta particles arebeing detected.

• Radiation Dose:- It is displayed in a unit such as the “Sievert” whichis normally used for measuring gamma or X-ray dose rates.


Readout

• The readout can be analog or digital, and increasingly, moderninstruments are offering serial communications with a host computeror network.

• There is usually an option to produce audible clicks representing thenumber of ionization events detected.

• This is the distinctive sound normally associated with hand held orportable Geiger counters.


Efficiency of the Geiger-counter:

• The efficiency of a detector is given by the ratio of the (number of particles of radiation detected)/(number of particles of radiation emitted):

• ε = number of particles of radiation detected

number of particles of radiation emitted


TYPES OF GM COUNTERBroadly, there are two main types of Geiger–Müller counter:

End Window Type: - • For alpha particles, low energy betaparticles, and low energy X-rays, the usual form is a cylindrical end-window tube.

• This type has a window at one end covered in a thin materialthrough which low-penetrating radiation can easily pass. The otherend houses the electrical connection to the anode.


• Pancake tube type: - It is a variant of End window type.The pancake GMtube is a truncated cylinder having the shape of a pill box. As is true of theend window GM tube, one end is covered with a thin (usually mica)window. The window thickness is 1.5 to 2.0 mg/cm2.

• The shape of the anode is unusual since it has a circular configuration in aplane parallel to the entrance window.

• The primary function of the pancake GM is the detection andmeasurement of beta emitting surface contamination. It also responds toalpha particles and gamma rays.


Windowless Type:- As the name suggests, this type of tube would nothave any windows and the thickness would be in the range of one totwo mm. This type of tube is used for detecting high penetratingradiations.

• 1. Thick walled • Used for high energy gamma detection, this typegenerally has an overall wall thickness of about 1-2 mm of chromesteel.

• Because most high energy gamma photons will pass through the lowdensity fill gas without interacting, the tube uses the interaction ofphotons on the molecules of the wall material to produce high energysecondary electrons within the wall.


• 2. Thin walled:- Thin walled tubes are used for:

• High energy beta detection, where the beta enters via the side of thetube and interacts directly with the gas,

• Low energy gamma and X-ray detection.

• The lower energy photons interact better with the filled gas so thisdesign concentrates on increasing the volume of the fill gas by using along thin walled tube and does not use the interaction of photons inthe tube wall.


Resolving time (Dead time)• After a count has been recorded, it takes the G-M tube a certain

amount of time to reset itself to be ready to record the next count.

• The resolving time or "dead time", T, of a detector is the time it takesfor the detector to "reset" itself. Since the detector is "not operating"while it is being reset, the measured activity is not the true activity ofthe sample.

• If the counting rate is high, then the effect of dead time is veryimportant.


ApplicationsParticle Detection:-

• GM Counter provide an easily used means of both detecting andmeasuring the following types of radiation:

Alpha particles

Beta particles

X rays

Gamma rays


Applications

Typical applications of GM includes the following which occur in and around nuclear power stations

Checking satisfactory performance of shielding.

Ensuring safety of working staff

Warning of any release of contamination

Recording exposure levels

Monitoring environment near nuclear installations

Detecting hazards for emergency service staff


Other Applications

• Measuring output from nuclear sources in various types of educational experimentation

• Tracking the radioactive isotope ‘labels’ frequently used as tracers in chemistry, agriculture, civil engineering, petroleum engineering and agriculture.


geiger müller counter - wordpress.com · 2018. 9. 8. · history •the german physicist hans...

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