Apparatus:
Geiger Muller Tube (as Mallard MX 180) tube holder and leads Universal castle
Co 60 radio-active () source along with its lifting tools time scaler and stop watch
Diagram:
Theory:
Geiger Muller tube consists of a discharge tube containing a cylindrical cathode made of a thin metallic sheet and an anode consisting of a wire (usually of tungsten) stretched or suspended along the axis of the tube (Fig.21.3). The tube may have a very thin mice window W shielded by a protective gauze. The tube is filled with some inert gas such as argon at a pressure of a few cms of mercury along with traces of alcohol to quench the discharge.
A suitable potential which is greater than a certain threshold value but less than the voltage which would cause a continuous discharge of electricity through the gas, is applied across the anode and the cathode of GM tube. Entry of a charged particle (,
) or photon in the tube through the window, causes ionization of gas molecules. The ions thus formed are accelerated by the electric field towards their respective electrodes. In this process more ions are produced by collision causing ionization current to build up rapidly which gives rise to an avalanche. The avalanche in turn produces a current through the external resistance R. The resulting potential change across R reduces the potential across the tube below the threshold value and the current decays rapidly. There is, therefore a momentary surge of current through R which is known as an electric pulse. Each also corresponds to one particle entering the tube. The pulses are counted by a scaler.
For a given radioactive source placed near the GM tube, the count rate i.e., the number of counts per minute depends upon the voltage applied across its electrodes. If we record the count rate as a function of voltage applied and plot it against voltage, we get a curve as shown in Fig. 21.2. The voltage corresponding to point B known as Geiger threshold. The potential range BD is called the Geiger plateau. The relative plateau slope (RPS) is the average percentage change in the count rate per unit increment of applied voltage.
If the applied voltage is increased beyond C, the counts increase rapidly (Fig. 21.2) The tube is usually operated in the region BC. Operation beyond C is not desirable because it damage the tube.
Geiger Muller Tube (as Mullard MX 180), tube holder and leads Universal castle
Co 60 radio-active () source along with its lifting tools time scaler and stop watch
Diagram
Theory
Geiger Muller tube consists of a discharge tube containing a cylindrical cathode made of a thin metallic sheet and an anode consisting of a wire (usually of tungsten) stretched or suspended along the axis of the tube (Fig.21.3). The tube may have a very thin mice window W shielded by a protective gauze. The tube is filled with some inert gas such as argon at a pressure of a few cms of mercury along with traces of alcohol to quench the discharge.
A suitable potential which is greater than a certain threshold value but less than the voltage which would cause a continuous discharge of electricity through the gas, is applied across the anode and the cathode of GM tube. Entry of a charged particle (,
) or photon in the tube through the window, causes ionization of gas molecules. The ions thus formed are accelerated by the electric field towards their respective electrodes. In this process more ions are produced by collision causing ionization current to build up rapidly which gives rise to an avalanche. The avalanche in turn produces a current through the external resistance R. The resulting potential change across R reduces the potential across the tube below the threshold value and the current decays rapidly. There is, therefore a momentary surge of current through R which is known as an electric pulse. Each also corresponds to one particle entering the tube. The pulses are counted by a scaler.
For a given radioactive source placed near the GM tube, the count rate i.e., the number of counts per minute depends upon the voltage applied across its electrodes. If we record the count rate as a function of voltage applied and plot it against voltage, we get a curve as shown in Fig. 21.2. The voltage corresponding to point B known as Geiger threshold. The potential range BD is called the Geiger plateau. The relative plateau slope (RPS) is the average percentage change in the count rate per unit increment of applied voltage.
If the applied voltage is increased beyond C, the counts increase rapidly (Fig. 21.2)
The tube is usually operated in the region BC. Operation beyond C is not desirable because it damage the tube.
Geiger Muller Tube (as Mullard MX 180), tube holder and leads Universal castle
Co 60 radio-active () source along with its lifting tools time scaler and stop watch
Diagram
Theory
Geiger Muller tube consists of a discharge tube containing a cylindrical cathode made of a thin metallic sheet and an anode consisting of a wire (usually of tungsten) stretched or suspended along the axis of the tube (Fig.21.3). The tube may have a very thin mice window W shielded by a protective gauze. The tube is filled with some inert gas such as argon at a pressure of a few cms of mercury along with traces of alcohol to quench the discharge.
A suitable potential which is greater than a certain threshold value but less than the voltage which would cause a continuous discharge of electricity through the gas, is applied across the anode and the cathode of GM tube. Entry of a charged particle (,
) or photon in the tube through the window, causes ionization of gas molecules. The ions thus formed are accelerated by the electric field towards their respective electrodes. In this process more ions are produced by collision causing ionization current to build up rapidly which gives rise to an avalanche. The avalanche in turn produces a current through the external resistance R. The resulting potential change across R reduces the potential across the tube below the threshold value and the current decays rapidly. There is, therefore a momentary surge of current through R which is known as an electric pulse. Each also corresponds to one particle entering the tube. The pulses are counted by a scaler.
For a given radioactive source placed near the GM tube, the count rate i.e., the number of counts per minute depends upon the voltage applied across its electrodes. If we record the count rate as a function of voltage applied and plot it against voltage, we get a curve as shown in Fig. 21.2. The voltage corresponding to point B known as Geiger threshold. The potential range BD is called the Geiger plateau. The relative plateau slope (RPS) is the average percentage change in the count rate per unit increment of applied voltage.
If the applied voltage is increased beyond C, the counts increase rapidly (Fig. 21.2)The tube is usually operated in the region BC. Operation beyond C is not desirable because it damage the tube.The GM tube is mounted in a casing called universal castle (Fig. 21.1) which consists of a lead box which shelves are provided. The radioactive source is placed on a slider which can be slide on a shelf a desired from the GM tube
Procedure:
1. Set up the apparatus as shown in Fig.21.1, and place the radioactive source on one of the shelves the castle underneath the GM tube. The HT power supply that provides the voltage across the electrodes the GM tube is contained in the scaler unit. The connecting cable transfers that voltage from the scaler to the GM tube and it is also communicates the pulse generate by the tube to the scaler unit for the purpose of counting.
2. Switch on the scaler and wait for 5 minutes for it to warm up. Then adjust the applied voltage till the scaler shows some response. Note this voltage and determine the number of counts registered in a certain interval of time say 2 minutes. Repeat this step and take the mean. Using this mean calculate the count rate i.e., the number of counts per minute. The applied voltage is read by a meter on the panel of the scaler unit.3.
3. Increase the applied potential in steps of 5 volts and for each value of applied voltage determine the count rate as in step No.2 till the Geiger threshold is reached. Thereafter increase the applied voltage I n step of 20 volts. While increasing the voltage in steps of 20 volts, discontinue the reading immediately as soon as the count rate shoes a rapid increase otherwise the GM tube will be damaged.4.
4. Plot the count rate against the applied voltage. Form the graph determine the Geiger threshold Geiger plateau and relative plateau slope (RPS). Tabulate the observation as given below
Observation & Calculation:
From the graph:
Geiger threshold voltage = OA = _ volts
Precaution:
- Handle the radioactive source cautiously by proper lifting tool.
- To avoid spurious count, all connection should be tight.
- In the plateau region, discontinue taking the readings immediately as soon as the count show a rapid increase.
Viva voce:
| Q.1: What is meant by Geiger Muller counter? Ans: it is a device used for detection and counting of charged particles and photons. Q.2: What is meant by characteristic of a GM counter? Ans: It is a graph between applied voltage and the number of counts per minute. Q.3: Define Geiger threshold and Geiger plateau? Ans: The point above which the number of counts per minute becomes almost independent of the applied voltage is known Geiger threshold. The more or less horizontal region of the characteristic of Geiger tube is called the Geiger plateau. | Q.4: What is meant by quenching? Ans: It is the elimination of secondary discharges after an incident particle has been detected. It is achieved in two ways. One by connecting a high resistance between the cathode and the anode in the circuit and the other by filling the counter with a mixture of argon and ethyl alcohol. Former method is known as external quenching and the latter as self-quenching. Q.5: What is a scaler? Ans: It is a device which directly records the counts of a counter tube pluses. |
