Geiger muller counter

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                                   

1.                 KCl                52.43                   2.7                   1.42                            93                               65                                         1.23

2.              KMnO4                      24.75                   3.9                  0.965                           70                               42                                         1.69

3.                 KBr                32.86                   3.8                  1.25                             88                              60                                         1.83

4.                 K2SO4                    44.87                   5.2                   2.33                             87                             59                                         1.31

5.                K2Cr2O7                 26.58                   4.3                   1.14                             67                             39                                         1.46

6.                 K2CrO4                 40.27                   4.3                    1.73                           91                              63                                         1.56

7.                 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.