Determine Operating Voltage of G.M. Counter

APPARATUS REQUIRED

A Geiger muller counter tube with stand scalar unit having arrangement for extra high voltage D.C supply fitted with voltmeter, a stopwatch and a radioactive source.

THEORY

A Geiger muller counter tube consists of a glass cylinder C with a metal coating M on the inside and Co-axial wire anode A (usually tungsten) suspended along its axis. The cylinder is filled with a gas such as Argon, mixed with halogen vapor at a reduced pressure of 10 cm of mercury. The tube may have a very thin Mica and window w shielded by a protective gauge p. The metal tube is maintained at a negative potential of about 1000 volt with respect to the wire. The potential difference becomes M and A should be greater than certain threshold values but less than the voltage which would cause a continuous discharge of electricity through the gas. Under this condition a single ionizing particle like p- particle or a r-ray photons entering the counter through the end window will produce ionization of the gas Molecules. The ions are accelerated by the electric field and produce more ions by collision causing ionization current to built up rapidly which gives rise to an avalanche. The avalanche in turn produces an external resistance R. The resolving drop in potential across R reduces the potential across the tube below the threshold value and the current decay rapidly on the circuit has a small time constant. There is therefore a momentary source of current which is refered to as an electric pulse. Each pulse corresponds to one particle mentoring the counter.

The slope of plateau is given by

slope = N₂ -N₁/(V₂ -V₁)M X100…………..(i)

Where M= N₂ +N₁ / 2 N₂ and N₂ are counts

Per minute at voltage V₁ and V₂ respectively

The operating voltage of the G.M counter is given by

V= V₁+⅓(V₂-V₁)

from graph we get,

V₁ = 550

V₂ = 950

Now, V= V₁+⅓(V₂-V₁)

= 550+ 1/3(950-550)

= 550+ 1/3 X400

= 550+ 133.33

= 683.33V

OBSERVATION

Observation table:

S.N.	Voltage	Observed counting rate with radioactive source (i)	Observed counting rate with radioactive source (N)	Observed counting rate with radioactive source mean (M)	Only background (b)	Corrected counting rate (poise sec)
1	300	0	0	0	0	0
2	350	0	0	0	14	-14
3	400	322	223	222.5	14	308.5
4	450	539	364	351.5	13	338.5
5	500	350	355	362.5	64	288.5
6	550	311	350	330.5	9	321.5
7	600	344	352	348	160	108
8	650	331	335	333	11	322
9	700	332	317	324.5	15	39.5
10	750	332	339	335.5	12	323.5
11	800	358	332	345	17	32.8
12	850	318	348	333	15	318
13	900	344	349	346.5	8	338.5
14	950	368	423	395.5	13	382.5
15	1000	3574	5018	4296	46	4250

CALCULATION

V =V₁ +1/3 (V₂-V₁)

= 683.33V

RESULT

Hence the required operating voltage of G.M Counter is 683.33Volt.

PRECAUTIONS

i) The radioactive material Should be handled Carefully by a pair of tongs or forceps.

ii) The counting Switch should be always kept on so that the counting rate does not exceed. Excessive voltage will damage the G.M counter.

iii) The radioactive source should be placed in the source holder just below the Geiger tube below, increasing the high voltage slowly.

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