In general, the Geiger counter and also the proportional counter are types of gaseous ionization detectors. These can be categorized according to the voltage applied to the detector:
As with other detectors, ionization chambers can be operated in current or pulse mode. In contrast, proportional counters or Geiger counters are almost always used in pulse mode. Detectors of ionizing radiation can be used both for activity measurements as well as for dose measurement. With knowledge about the energy needed to form an pair of ions – the dose can be obtained.
The Geiger counter, also known as the Geiger-Mueller counter, is electrical device that detects various types of ionizing radiation. This device is named after the two physicists who invented the counter in 1928. Mueller was a student of Hans Geiger. Geiger counter is widely used in applications such as radiation dosimetry, radiological protection, experimental physics, and the nuclear industry. A Geiger counter consists of a Geiger-Müller tube (the sensing element which detects the radiation) and the processing electronics, which displays the result.
Geiger counter can detect ionizing radiation such as alpha and beta particles, neutrons, and gamma rays using the ionization effect produced in a Geiger–Müller tube, which gives its name to the instrument. The voltage of detector is adjusted so that the conditions correspond to the Geiger-Mueller region.
Advantages of Geiger-Mueller Counter
- High Amplification. A strong signal (the amplification factor can reach about 1010) is produced by these avalanches with shape and height independently of the primary ionization and the energy of the detected photon. The voltage pulse in this case would be a large and easily detectable ≈ 1.6 V. The technical advantage of a Geiger counter is its simplicity of construction and its insensitivity to small voltage fluctuations. Since the process of charge amplification greatly improves the signal-to-noise ratio of the detector, the subsequent electronic amplification is usually not required.
- Simplicity. G-M counters are mainly used for portable instrumentation due to its sensitivity, simple counting circuit, and ability to detect low-level radiation. G-M detectors are generally more sensitive to low energy and low intensity radiations than are proportional or ion chamber detectors.
- Simpler Electronics. G-M detectors can be used with simpler electronics packages. The input
sensitivity of a typical G-M survey instrument is 300-800 millivolt, while the input
sensitivity of a typical proportional survey instrument is 2 millivolt.
Disadvantages of Geiger-Mueller Counter
- No particle identification, no energy resolution. Since the pulse height is independent of the type and energy of radiation, discrimination is not possible. There is no information whatsoever on the nature of the ionization that caused the pulse. G-M detectors can not discriminate against different types of radiation (α, β, γ), nor against various radiation energies. This is because the size of the avalanche is independent of the primary ionization which created it.
- Dead Time. Because of the large avalanche induced by any ionization, a Geiger counter takes a long time (about 1 ms) to recover between successive pulses. Therefore, Geiger counters are not able to measure high radiation rates due to the “dead time” of the tube.
A proportional counter, also known as the proportional detector, is an electrical device that detects various types of ionizing radiation. The voltage of detector is adjusted so that the conditions correspond to the proportional region. In this region, the voltage is high enough to provide the primary electrons with sufficient acceleration and energy so that they can ionize additional atoms of the medium. These secondary ions (gas amplification) formed are also accelerated causing an effect known as Townsend avalanches, which creates a single large electrical pulse.
Advantages of Proportional Counters
- Amplification. Gaseous proportional counters usually operate in high electric fields of the order of 10 kV/cm and achieve typical amplification factors of about 105. Since the amplification factor is strongly dependent on the applied voltage, the charge collected (output signal) is also dependent on the applied voltage and proportional counters require constant voltage. The high amplification factor of the proportional counter is the major advantage over the ionization chamber.
- Sensitivity. The process of charge amplification greatly improves the signal-to-noise ratio of the detector and reduces the subsequent electronic amplification required. Since the process of charge amplification greatly improves the signal-to-noise ratio of the detector, the subsequent electronic amplification is usually not required. Proportional counter detection instruments are very sensitive to low levels of radiation. Moreover, when measuring current output, a proportional detector is useful for dose rates
since the output signal is proportional to the energy deposited by ionization and therefore proportional to the dose rate.
- Spectroscopy. By proper functional arrangements, modifications, and biasing, the proportional counter can be used to detect alpha, beta, gamma, or neutron radiation in mixed radiation fields. Moreover, proportional counters are capable of particle identification and energy measurement (spectroscopy). The pulse height reflects the energy deposited by the incident radiation in the detector gas. As such, it is possible to distinguish the larger pulses produced by alpha particles from the smaller pulses produced by beta particles or gamma rays.
Disadvantages of Proportional Counters
- Constant Voltage. When instruments are operated in the proportional region, the voltage must be kept constant. If a voltage remains constant the gas amplification factor also does not change. The main drawback to using proportional counters in portable instruments is that they require a very stable power supply and amplifier to ensure constant operating conditions (in the middle of the proportional region). This is difficult to provide in a portable instrument, and that is why proportional counters tend to be used more in fixed or lab instruments.
- Quenching. For each electron collected in the chamber, there is a positively charged gas ion left over. These gas ions are heavy compared to an electron and move much more slowly. Free electrons are much lighter than the positive ions, thus, they are drawn toward the positive central electrode much faster than the positive ions are drawn to the chamber wall. The resulting cloud of positive ions near the electrode leads to distortions in gas multiplication. In practice the termination of the avalanche is improved by the use of “quenching” techniques.