What is Geiger Muller Counter?
It is a well-known device used for the detection and measurement of ionizing radiations such as α- particles, β- particles, and γ- rays. It is also known as the G. M counter.
- 1) Principle
- 2) History of Geiger Muller Counter
- 3) Construction of Geiger Muller Counter
- 4) Working of Geiger Muller Counter
- 5) Dead Time
- 6) Spurious (false) counts
- 7) Quenching of Discharge
- 8) Self-Quenching
- 9) Electronic Quenching
- 10) Uses
- 11) MCQs
- 12) Summary: Geiger Muller Counter
- 13) You may also like to learn:
Principle
The cylinder is filled with gas and has electrodes with voltage but no flowing current. The discharge in the tube is produced when ionization is produced by incident radiations. The intensity of the radiation field is measured as several pulses per second.
History of Geiger Muller Counter
In 1928, a German Physicist and his student made a particle detector. They developed sealed tubes and used basic ionizing principles. from that time, now onwards, G. M Counter is used to measure, and detect ionizing radiation in nuclear physics and industries.
Construction of Geiger Muller Counter
It consists of a fine central wire usually made of tungsten acting as an anode in a hollow metal cylinder acting as a cathode. A suitable mixture of gas is filled in the cylinder. Nowadays 90% argon and 10% ethyl alcohol are added.
These are added at 1/10th (0.1) of atmospheric pressure. At one end, there is a mica window, which allows the entry of alpha and beta particles into the tube. The other end is sealed with a non-conducting material. This carries the connecting pins for two electrodes.
Working of Geiger Muller Counter
When ionizing radiation enters the counter, primary ionization occurs and a couple of ions are produced. These ions are accelerated with higher energy by a strong electric field and they collide with other molecules causing ionization and these ions are multiplied by further collisions. Hence cascade of electrons is produced in a brief interval of time.
This cascade of electrons reaching the anode generates a current pulse to pass through an external resistor. This is amplified by electronic circuits and is utilized to run an electronic counter.
The counts in the counter are directly proportional to the intensity of the ionizing radiation. The ionization of the gas is independent of the kind of incident radiation. For this reason, the G.M. counter does not distinguish the kind of radiation that enters the chamber.
Dead Time
The positive ions take several hundred times as long to reach the outer cathode since positive ions are very enormous and massive than electrons. During this time further incoming particles cannot be counted. This time is called the dead time (10-4 s) of the counter.
Spurious (false) counts
When positive ions strike the cathode, secondary electrons are given off from the surface. These electrons could be accelerated to give additional spurious counts.
Quenching of Discharge
In order to record the chain of particles (i.e., ionizing particles), it is vital that the electric discharge brought on by the very first ionizing particle is completely quenched prior to the arrival of a new particle.
Though quenching relies on a variety of factors like voltage, shape, and size of the counter it is the mixture of the gas which is of vital importance. Alcohol has a low ionization capacity of 11.3 eV and Argon has an ionization potential of 15.7 eV. Argon ions get neutralized by acquiring an electron from the alcohol molecules.
This suggests that the ions reaching the cathode are especially that of alcohol, where they get neutralized. The energy which these ions bring is absorbed in dissociating the alcohol molecule. The alcohol vapor likewise takes in the ultraviolet photons released throughout the cascade and avoid them from ejecting photoelectrons from the cathode.
This shows that ethyl alcohol has two specific functions. Initially, it prevents the photoelectric result of the quanta at the cathode. Second, it avoids the production of secondary electrons, when the positive ions strike the cathode.
Self-Quenching
The spurious counts are prevented by mixing a percentage of quenching gas with the principal gas. The quenching gas needs to have an ionization capacity lower than that of inert principal gas.
Hence, the ions of quenching gas reach the cathode before the principal gas ions. When they reach near the cathode, they capture electrons and end up being neutral molecules. Following neutralization, the excess energy of the quenching particles is dissipated in the dissociation of particles instead of in the release of electrons from the cathode.
For example, bromine gas is added to neon gas. The bromine molecules absorb energy from the ions or secondary electrons and dissociate into bromine atoms. The atoms then easily recombine into particles again for the next pulse.
The gas quenching is called self-quenching. Although all commercial Geiger tubes are self-quenched, it prevails practice to use electronic quenching.
Electronic Quenching
For this function, a large negative voltage is applied to the anode instantly after recording the output pulse. This minimizes the electrical field below the critical value for ionization by collision.
The negative voltage stays up until all the positive ions are gathered at the cathode hence preventing secondary pulses.
Uses
It is utilized to determine the range or penetration power of ionizing particles.
The reduction in the count rate by placing metal plates between the source and the tube helps to estimate the penetration power of incident radiation.
MCQs
- What is the primary purpose of a Geiger Muller Counter?
- A) To measure magnetic fields
- B) To detect and measure ionizing radiations
- C) To detect radio waves
- D) To measure atmospheric pressure
- Answer: B) To detect and measure ionizing radiations
- Who developed the Geiger Muller Counter?
- A) Albert Einstein
- B) Wilhelm Conrad Roentgen
- C) Otto Hahn and Fritz Strassmann
- D) Hans Geiger and Walther Muller
- Answer: D) Hans Geiger and Walther Muller
- What is the primary principle behind the operation of a Geiger Muller Counter?
- A) Magnetic induction
- B) Ionization of gas by incident radiation
- C) Optical absorption
- D) Electrical conduction
- Answer: B) Ionization of gas by incident radiation
- What gas mixture is commonly used in a Geiger Muller Counter?
- What is the role of the mica window in a Geiger Muller Counter?
- A) To emit radiation
- B) To seal the counter
- C) To allow the entry of alpha and beta particles
- D) To release ions
- Answer: C) To allow the entry of alpha and beta particles
- What happens when ionizing radiation enters the Geiger Muller Counter?
- A) It creates a vacuum
- B) It produces a magnetic field
- C) It ionizes the gas, creating a cascade of electrons
- D) It generates light
- Answer: C) It ionizes the gas, creating a cascade of electrons
- What is the purpose of the external resistor in a Geiger Muller Counter?
- A) To measure temperature
- B) To amplify the current pulse
- C) To regulate voltage
- D) To emit radiation
- Answer: B) To amplify the current pulse
- What is the dead time of a Geiger Muller Counter?
- A) The time required for assembly
- B) The time during which further incoming particles cannot be counted
- C) The time for calibration
- D) The time for dismantling
- Answer: B) The time during which further incoming particles cannot be counted
- What is a spurious count in a Geiger Muller Counter?
- A) Accidental release of radiation
- B) False counts caused by positive ions striking the cathode
- C) Secondary radiation
- D) Low battery signal
- Answer: B) False counts caused by positive ions striking the cathode
- What is the function of the quenching gas in a Geiger Muller Counter?
- A) To increase sensitivity
- B) To decrease sensitivity
- C) To regulate temperature
- D) To generate radiation
- Answer: B) To decrease sensitivity
- What is self-quenching in a Geiger Muller Counter?
- A) The counter shuts down automatically
- B) The counter repairs itself
- C) A process where a quenching gas is mixed with the principal gas
- D) A process where the counter emits radiation
- Answer: C) A process where a quenching gas is mixed with the principal gas
- What is the purpose of electronic quenching in a Geiger Muller Counter?
- A) To generate electricity
- B) To increase sensitivity
- C) To prevent secondary pulses
- D) To detect radiation
- Answer: C) To prevent secondary pulses
- What is the primary application of a Geiger Muller Counter?
- A) To measure atmospheric pressure
- B) To detect radio waves
- C) To determine the penetration power of ionizing particles
- D) To measure temperature
- Answer: C) To determine the penetration power of ionizing particles
- How is the reduction in count rate achieved in a Geiger Muller Counter to estimate penetration power?
- A) By increasing the voltage
- B) By reducing the gas pressure
- C) By introducing metal plates between the source and the tube
- D) By decreasing the temperature
- Answer: C) By introducing metal plates between the source and the tube
- What is the primary function of the Geiger Muller Counter?
- A) To measure sound intensity
- B) To detect and measure ionizing radiation
- C) To measure temperature fluctuations
- D) To detect magnetic fields
- Answer: B) To detect and measure ionizing radiation
- Who were the scientists responsible for the discovery of the Geiger Muller Counter?
- A) Albert Einstein and Niels Bohr
- B) Hans Geiger and Walther Muller
- C) Marie Curie and Pierre Curie
- D) Otto Hahn and Fritz Strassmann
- Answer: B) Hans Geiger and Walther Muller
- What gas mixture is commonly used in the Geiger Muller Counter?
- A) Oxygen and carbon dioxide
- B) Nitrogen and helium
- C) Argon and ethyl alcohol
- D) Hydrogen and methane
- Answer: C) Argon and ethyl alcohol
- What happens when ionizing radiation enters the Geiger Muller Counter?
- A) It creates a magnetic field
- B) It ionizes the gas, leading to a cascade of electrons
- C) It emits light
- D) It generates heat
- Answer: B) It ionizes the gas, leading to a cascade of electrons
- What is the primary purpose of the external resistor in the Geiger Muller Counter?
- A) To regulate voltage
- B) To amplify the current pulse
- C) To emit radiation
- D) To measure temperature
- Answer: B) To amplify the current pulse
- What is the dead time of the Geiger Muller Counter?
- A) The time required for calibration
- B) The time during which no radiation is detected
- C) The time during which further incoming particles cannot be counted
- D) The time taken for assembly
- Answer: C) The time during which further incoming particles cannot be counted
- What is the primary application of the Geiger Muller Counter?
- A) To measure atmospheric pressure
- B) To detect radio waves
- C) To determine the penetration power of ionizing particles
- D) To measure sound intensity
- Answer: C) To determine the penetration power of ionizing particles
Summary: Geiger Muller Counter
The Geiger Muller Counter, also known as the G. M counter, is a vital device used for detecting and measuring ionizing radiations such as α- particles, β- particles, and γ- rays. It operates on the principle of ionization, where incident radiation ionizes gas within the counter, generating electron cascades that produce measurable current pulses.
Key Components:
- Principle: The counter’s principle involves the creation of a discharge within a gas-filled cylinder when ionizing radiation interacts with the gas.
- History: Invented in 1928 by German physicists, the Geiger Muller Counter has since been instrumental in nuclear physics and various industries.
- Construction: It comprises a central wire acting as an anode, a metal cylinder acting as a cathode, and a gas mixture (commonly 90% argon and 10% ethyl alcohol) filled at low pressure.
- Working: Ionization from incident radiation leads to electron cascades, generating current pulses that are amplified and counted electronically.
- Dead Time: The time during which further incoming particles cannot be counted due to the slow movement of positive ions is known as dead time.
- Quenching: Measures such as gas mixture selection and electronic quenching prevent spurious counts and ensure accurate detection.
- Uses: The counter is primarily employed to determine the penetration power of ionizing particles, crucial in various scientific and industrial applications.
The Geiger Muller Counter’s reliability and versatility make it indispensable in radiation detection and measurement, contributing significantly to nuclear research and safety protocols.