STUDY AND USE OF GEIGER MULLER COUNTER
All methods of detection are based on the ability of radiation to cause ionization (ie) to produce charged particles from the original neutral atoms and molecules.
A detection system consists of two parts
1) A device which responds to the nuclear radiation and
2) A measuring part to indicate this response of various medium in which the ionization takes place.
The following are the important media generally used for radiation detection.
1) Gases (Ion chamber, G.M. counters and proportional counter)
2) Scintillation media (Na Anthracene)
3) Photographic emulsion (Films)
4) Solid state dosimeters (semi conductors and Glass dosimeter)
Geiger Mueller Counter (GM)
The (GM) counter is most widely used. It is a cylindrical tube with a central anode made of tungsten wire (0.02 to 0.1 mm diameter) running all along. This helps to produce a high electrical field near the anode. The cylindrical cathode sheath is usually of metal. The tube is filled with argon gas and also quenching gas like ethyl alcohol or halogen vapour. The quenching gas is used to avoid pulses due to bombardment of anode on cathode. (i.e) secondary discharge. The optimum operative voltage (300 to 1400 volts), shelf factor, dead time in seconds and background counts are to be determined. The G.M. tube also consists of a diaphragm (made of mica/aluminum/glass). ß waves enter through the diapharam collide with inert gas (argon) and produces avalance. After avalance in the counter, the fast moving electrons are immediately collected by the central anode and the slow moving positive charged ions reaches later to the cathode (after some micro seconds). The efficiency of G.M. counter for ß particle is nearly 100% whereas for gamma radiation the efficiency is only 90% Detection of natural activity of K salts in G.M. counter
Potassium Salts tested K37 K38 K39 K40 K41 K42 K43 K45
Half life 1.2S 7.5M stable 1.4x108y stable 1.4H 22.4H 18M
Procedure
Different K salts are weighed and taken in planchets and counts /100 seconds are taken and for all salts the counts (counts/sec=sp. activity) are recorded.
Observation and Calculation
Operative voltage = 1245
Shelf No = 3
Dead time in seconds = 0.00208
Background counts/100s = 28
Sl. No Name of Percentage Wt. of salt Wt. of K Observed counts True counts True counts % of K K salt of K (g) in sample per 100 sec per 100 sec
KCl 52.43 2.7 1.42 93 65 1.23
KMnO4 24.75 3.9 0.965 70 42 1.69
KBr 32.86 3.8 1.25 88 60 1.83
K2SO4 44.87 5.2 2.33 87 59 1.31
K2Cr2O7 26.58 4.3 1.14 67 39 1.46
K2CrO4 40.27 4.3 1.73 91 63 1.56
K2CO3 56.5 2.9 1.64 98 70 1.23
Inference: The true counts divided by % of K is a constant for the various K salts.
Notes:
1.The voltage at which appreciable counts are noted is starting voltage (sv); the voltage at which maximum counts observed is threshold (T.V) voltage. The optimum operative voltage is TV+100.
2. Shelf factor: The position of windows is important to allow the radiation source to pass through the diapharam
Dead time: The time lag between two consecutive readings recorded in the counter.