To Study Characteristics of GM Counter

1 Abstract:

In this experiment we studied characteristics of Geiger Muller tube. Geiger counter is a detector which can detect a radiation and measure it activity rate. Characteristics of Geiger Muller counter are studied by plotting a graph between voltage on x axis and counts on y axis.

2 Objective:

The objective of this experiment is

· To study the characteristics of GM counter.

· To Determine the Plateau Curve of GM Counter

· To Calculate Standard Deviation

3 Introduction:

3.1.Radioactivity:

“Radioactivity defined as the spontaneous emission of particles or rays or both at the same time, from the decay of certain nuclides that these particles are, due to an adjustment of their internal structure.”

In nature there are some elements which are unstable. Unstable mean they possess some extra amount of energy in the nucleus. This energy makes them unstable. These elements emit energy by some process. This phenomenon is called radioactivity. Radioactivity was discovered by Becquerel in 1896 when he wrapped a photographic plate around uranium sample. The radiation emitted from uranium was later recognized as alpha particles. Later on, other type of radiations was also discovered known as beta particles and gamma particles. Unstable elements get rid of extra energy by some processes and they decay into some other element. This process is called radioactive decay and elements are called radioactive elements.

3.2.Types of Radioactive Decay:

There are three basic processes by which unstable elements decay to some other elements which may be stable.

3.2.1. Alpha decay:

Emission of alpha particles from a heavy unstable nucleus is called alpha decay. Alpha particles have two protons and two neutrons, so in this process atomic number of parent nucleus is changed by 2 and mass number is changed by 2. Some energy is also emitted in this process. Uranium, plutonium etc. are alpha particle emitters.

3.2.2. Beta decay:

Elements which emit beta particles from their nucleus are called beta emitters and process is called beta decay. In this process either a positive electron (positron) is emitted or a negative electron is emitted. In positron emission a proton is converted into neutron and positron is emitted. This process will convert atom to next below atomic number element. In electron emission process a neutron is converted into proton and an electron is produced. An electron has energy very less so it cannot stay inside nucleus so they are emitted and so atom is transformed into next above atomic number element. There is another process which takes place in beta decay that is called electron capture. In this process an electron from electronic shell is swallowed by nucleus and a proton is converted into neutron so atom is transmuted into next below atomic number element.

3.2.3. Gamma decay:

Atoms emitting gamma radiation is called gamma emitters and phenomena is called gamma decay. During some nuclear reactions or in some decay processes discussed above, atom after reaction may have some excess amount which it can emit through nuclear transitions, called gamma radiations. In gamma decay no nuclear transmutation takes place but atoms are only transformed from excited to ground states.

3.3.Geiger Muller Counter:

Geiger-Muller (GM) counters were invented by H. Geiger and E.W. Müller in 1928, and
are used to detect radioactive particles. Alpha, Gamma and Beta radiations are invisible to humans and exposure to these radiations can be hazardous to the health of living organisms. It is therefore extremely important that suitably designed detectors are available in order to gain information on the type and amount of radiation present.

3.3.1. Construction & Working:

It consists of a metallic chamber with a thin central tungsten wire insulated from the outer chamber. The central wire is at positive with respect to the outer chamber and hence the central wire acts as anode while the outer serves as cathode. If the outer chamber is made out of glass, then its inner surface is wanted with some conducting material to serve as cathode. Geiger-Muller Counter is usually filled with noble gases such as argon, neon etc.

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. For proper use of the Geiger counter, one must have the appropriate voltage across the electrodes. If the voltage is too low, the electric field in the tube is too weak to cause a current pulse. If the voltage is too high, the tube will undergo continuous discharge, and the tube can be damaged. Usually the manufacture recommends the correct voltage to use for the tube. Larger tubes require larger voltages to produce the necessary electric fields inside the tube.

3.3.2. Characteristics of GM tubes:-

The important parameters which decide the quality of functioning of Gm tubes are

Dead time

Recovery time

Plateau length &Plateau slope

3.3.2.1.Quenching of GM tube:

As ions and electrons are generated in gas present inside the GM tube. These ions and electrons move to opposite sides and reach the cathode and anode completing circuit and producing current pulse. These ions and electrons, under the action of high voltage, can get a large kinetic energy and when they strike the cathode or anode (for electron) they producing a sputtering effect. In effect it can eject electrons from cathode and these electrons and produces its own current. This process produces an unwanted current and in GM tube which make busy the counter and so next pulse is not recorded. To remove this process, heavy molecular mass molecules multi atom gas is added to GM tube. This gas is called quenching gas. This gas decrease the kinetic energy of ions striking the cathode and therefore sputtering process is avoided and this process is called quenching.

3.3.2.2.Dead time:

As, the positive ions take considerable time to reach cathode tube compared to electrons. The reason is that the mobility of electrons is about 1000 times greater than that of electrons.

Due to the low drift velocity of positive ions, there is formation of cloud of positive ions which tend to electric field opposite to that of actual field. This reduces the electric field intensity due to anode potential and thus affects gas multiplication factor. This in turn affects the pulse heights.

In high count rates, it is more worse that there is formation of dense positive cloud which makes the electric field intensity in the vicinity of anode wire reduce by great margin thus multiplication goes down by big margin. During this phase of detector, any new ionizing event caused by incoming particle cannot be recorded. Thus the time interval during which any event caused by newly incoming particle would not get counted and called as dead time of the country.

3.3.2.3.Recovery time:

After certain time, all the positive ions tend to reach cathode wall and thus the electric field begins to restore to actual value. When the electric field goes beyond a critical value there is again formation for pulses. But the process requires some time to give maximum pulse heights. Hence the total time required for GM tube to give maximum pulse height pulses is Recovery time.

3.3.2.4.Plateau length & Slope:

In order to decide the operating voltage of the GM tube, a graph between anode voltage (X axis) and count rate (Y axis) is plotted. After applying minimum voltage to initiate Geiger discharge, the no. of pulses shall remain same in fixed radiation field exposure. But due to formation of short pulses during recovery time there is variation in count rate. Thus one of the quality parameters deciding the operation of GM tube is that plateau slope shall be less. Usually 2-3% plateau slope is a good choice. As we go on applying voltage to the anode, the tube starts entering continuous discharge region. Thus the slope gets worsened. The region or length of voltage region during which the plateau slope remains in desired value is called as plateau length and usually the operating voltage is chosen at the midpoint of plateau length.

3.3.3. Limitation:

There are two main limitations of the Geiger counter. Because the output pulse from a Geiger-Muller tube is always the same magnitude regardless of the energy of the incident radiation, the tube cannot differentiate between radiation types. A further limitation is the inability to measure high radiation rates due to the "dead time" of the tube. This is an insensitive period after each ionization of the gas during which any further incident radiation will not result in a count.

Experimental Work

4. Apparatus:

1. Radioactive source

2. Geiger-Muller Counter

3. GM Tube

4. Computer with STX software

5. Power supply

6. Experimental Setup:

GM Tube and Counter:

Taking Reading From STX software:

7. Experimentation:

1. GM Tube is connected to the radiation counter which is already connected to a computer containing STX software used for detecting radiations.

2. Voltage range is set from 700-1000 in STX, and the interval to 10s.

3. Firstly the background radiation are detected and measured by STX without placing any radiation source in GM tube.

4. Noted Down the reading of counts

for different value of voltages.

5. Now a gamma source is placed within the Geiger Muller tube.

6. Radiation produced by the source are detected by the tube and measured by the computer as X.

7. Difference between the counts for without and with source is calculated as Real Counts Y.

8. Mean value is calculated as

9. Standard Deviation is calculated by using the following formula:

10. Finally a graph is plotted between voltage on x-axis and real counts on y-axis, known as plateau curve.

8. Observations and calculations

Sample Stage=3^rd s

Runs=1

N=No of observation

Sr.No.	Voltage	Counts Without Sample	Counts With Sample	Real Counts
Sr.No.	V		X
1	760	1	5	4	54	2916
2	770	4	19	15	43	1849
3	780	6	26	20	38	1444
4	790	4	29	25	33	1089
5	800	8	38	30	28	784
6	810	4	40	36	22	484
7	820	2	44	42	16	256
8	830	11	54	43	15	225
9	840	7	51	44	14	196
10	850	8	53	45	13	169
11	860	10	56	46	12	144
12	870	2	51	49	9	81
13	880	8	58	50	8	64
14	890	4	94	90	32	1024
15	900	9	101	92	34	1156
16	910	8	94	94	36	1296
17	920	7	104	97	39	1521
18	930	9	107	98	40	1600
19	940	7	105	98	40	1600
20	949	4	102	98	40	1600
21	960	9	109	100	42	1764

9. Graphical Representation

10. Results & Discussion:

Voltage is slowly varied and counting rate is measured. Counting rate against the increasing operating voltage give optimized operating voltage. For low voltages, no counts are recorded. This is because the electric field is too weak for even one pulse to be recorded. As the voltage is increased, eventually one obtains a counting rate. The voltage at which the GM tube just begins to count is called the starting potential. The counting rate quickly rises as the voltage is increased. The Plateau curve obtained has one extra region except the saturation, operating and breakdown region showing the steady flow of electrons in GM Tube.