In general, the Geiger counter and also the ionization chamber 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, pulse mode almost always uses proportional counters or Geiger counters. Detectors of ionizing radiation can be used both for activity measurements as well as for dose measurements. The dose can be obtained with knowledge about the energy needed to form a pair of ions.
Geiger Counter
The Geiger counter, also known as the Geiger-Mueller counter, is an electrical device that detects various types of ionizing radiation. This device is named after the two physicists who invented the counter in 1928, and 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 the 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 detector’s signal-to-noise ratio, subsequent electronic amplification is usually not required.
- Simplicity. G-M counters are mainly used for portable instrumentation due to their 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 on the nature of the ionization that caused the pulse. G-M detectors can not discriminate against different types of radiation (α, β, γ) or 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 cannot measure high radiation rates due to the “dead time” of the tube.
Ionization Chamber
The ionization chamber, also known as the ion chamber, is an electrical device that detects various types of ionizing radiation. The voltage of the detector is adjusted so that the conditions correspond to the ionization region, and the voltage is insufficient to produce gas amplification (secondary ionization).
Advantages of Ionization Chambers
- Current mode. Ionization chambers are preferred for high radiation dose rates because they have no “dead time,” a phenomenon that affects the accuracy of the Geiger-Mueller tube at high dose rates. This is because there is no inherent amplification of signal in the operating medium; therefore, these counters do not require much time to recover from large currents. In addition, because there is no amplification, they provide excellent energy resolution, which is limited primarily by electronic noise. Ionization chambers can be operated in current or pulse mode. In contrast, pulse mode almost always uses proportional counters or Geiger counters. Detectors of ionizing radiation can be used both for activity measurements as well as for dose measurements. The dose can be obtained with knowledge about the energy needed to form a pair of ions. The flat plate design is preferred because it has a well-defined active volume and ensures that ions will not collect on the insulators and cause electric field distortion.
- Simplicity. Output current is independent of detector operating voltage. Observe the flat region of the curve in the ion chamber region. As a result, less regulated and thereby less expensive and more portable power supplies can be used with ion chamber instruments and still offer a reasonably accurate response.
- Neutron Detection. In nuclear reactors, ionization chambers in current mode are often used to detect neutrons and belong to the Nuclear Instrumentation System (NIS). For example, if the inner surface of the ionization chamber is coated with a thin coat of boron, the (n, alpha) reaction can occur. Most of (n,alpha) reactions of thermal neutrons are 10B(n,alpha)7Li reactions accompanied by 0.48 MeV gamma emission. Moreover, isotope boron-10 has a high (n, alpha) reaction cross-section along the entire neutron energy spectrum. The alpha particle causes ionization within the chamber, and ejected electrons cause further secondary ionizations. Another method for detecting neutrons using an ionization chamber is to use the gas boron trifluoride (BF3) instead of air in the chamber. The incoming neutrons produce alpha particles when reacting with the boron atoms in the detector gas. Either method may be used to detect neutrons in a nuclear reactor.
Disadvantages of Ionization Chambers
- No Charge Amplification. Detectors in the ionization region operate at a low electric field strength, so no gas multiplication occurs. The charge collected (output signal) is independent of the applied voltage. Single minimum-ionizing particles tend to be quite small and usually require special low-noise amplifiers for efficient operating performance. The average energy needed to produce an ion in the air is about 34 eV. Therefore a 1 MeV radiation completely absorbed in the detector produces about 3 x 104 pairs of ions. However, it is a small signal that can be considerably amplified using standard electronics. A current of 1 micro-ampere consists of about 1012 electrons per second.
- Low Density. Gamma rays deposit a significantly lower amount of energy into the detector than other particles. The efficiency of the chamber can be further increased by the use of high-pressure gas.
- For alpha and beta particles to be detected by ionization chambers, they must be given a thin window. This “end-window” must be thin enough for the alpha and beta particles to penetrate. However, a window of almost any thickness will prevent an alpha particle from entering the chamber. The window is usually made of mica with a density of about 1.5 – 2.0 mg/cm2.
