Nick Connor

Cherenkov detectors may be classified as either imaging or threshold types, depending on whether they do or do not make use of Cherenkov angle (θ) information. In the simple case of a threshold detector the mass-dependent threshold energy allows the discrimination between a lighter particle (which does radiate) and a heavier particle (which does not radiate) of the same energy or momentum. Imaging counters may be used to track particles as well as identify them. Although devices using Cherenkov radiation are often thought of as particle identification (PID) detectors, in practice, they are widely used over a much broader range of applications; including:

• fast particle counters
• hadronic particle identification
• tracking detectors performing complete event reconstruction.

Types of Cherenkov Detectors – Examples

• RICH. Practical multi-track Ring-Imaging Cherenkov detectors (generically called RICH counters) are a more recent development. In a RICH detector, a cone of Cherenkov light is produced when a high speed charged particle traverses a suitable medium, often called radiator. This light cone is detected on a position sensitive planar photon detector, which allows reconstructing a ring or disc, the radius of which is a measure for the Cherenkov emission angle. For example, the LHCb experiment on the Large Hadron Collider uses two RICH detectors for differentiating between pions and kaons.
• Super-Kamiokande. Super-Kamiokande is an underground neutrino observatory, which uses large water Cherenkov counters to detect high-energy neutrinos to search for proton decay, study solar and atmospheric neutrinos, and keep watch for supernovae in the Milky Way Galaxy. It consists of a cylindrical stainless steel tank about 40 m (131 ft) in height and diameter holding 50,000 tons of ultrapure water. Mounted on an inside superstructure are about 13,000 photomultiplier tubes that detect light from Cherenkov radiation.

See also:

Cherenkov Detectors

A Cherenkov detector is a particle detector, which is based on detection of Cherenkov radiation (visible light or UV photons). In contrast to a scintillation counter the light production is instantaneous. Typical scintillator detectors have a decay time measured in microseconds while Cherenkov radiation is nearly instantaneous and with fast pulse processing equipment can be measured in picoseconds. Cherenkov counters are used mainly for the particle identification, i.e. for the determination of particle masses. Cherenkov counters contain two main elements:

• a radiator through which the charged particle passes,
• a photodetector (e.g. a photomultiplier tube – PMT).

The Cherenkov radiation, which is produced in the radiator, is electromagnetic radiation emitted when a charged particle (such as an electron) moves through a dielectric medium faster than the phase velocity of light in that medium. Particles exceeding the phase velocity of light result in polarization along the axis of motion creating a dipole field. When this field collapses, an electromagnetic pulse (Cherenkov radiation) is emitted in the forward direction. It is similar to the bow wave produced by a boat travelling faster than the speed of water waves. Cherenkov radiation occurs only if the particle’s speed is higher than the phase velocity of light in the material. Even at high energies the energy lost by Cherenkov radiation is much less than that by the other mechanisms (collisions, bremsstrahlung). It is named after Soviet physicist Pavel Alekseyevich Cherenkov, who shared the Nobel Prize in physics in 1958 with Ilya Frank and Igor Tamm for the discovery of Cherenkov radiation, made in 1934.

Cherenkov radiation is commonly produced in dielectric materials through Compton electrons or pair-production electrons and positrons. The intensity of light produced by this process is much less than that of luminescence (the basis for scintillation detector operation) requiring more sensitive optical photon detection equipment such as low light photo-multiplier tubes (PMT). Cherenkov counters may be classified as either imaging or threshold types, depending on whether they do or do not make use of Cherenkov angle (θ) information. In the simple case of a threshold detector the mass-dependent threshold energy allows the discrimination between a lighter particle (which does radiate) and a heavier particle (which does not radiate) of the same energy or momentum. Imaging counters may be used to track particles as well as identify them. Although devices using Cherenkov radiation are often thought of as particle identification (PID) detectors, in practice, they are widely used over a much broader range of applications; including:

• fast particle counters
• hadronic particle identification
• tracking detectors performing complete event reconstruction.

Types of Cherenkov Detectors – Examples

• RICH. Practical multi-track Ring-Imaging Cherenkov detectors (generically called RICH counters) are a more recent development. In a RICH detector, a cone of Cherenkov light is produced when a high speed charged particle traverses a suitable medium, often called radiator. This light cone is detected on a position sensitive planar photon detector, which allows reconstructing a ring or disc, the radius of which is a measure for the Cherenkov emission angle. For example, the LHCb experiment on the Large Hadron Collider uses two RICH detectors for differentiating between pions and kaons.
• Super-Kamiokande. Super-Kamiokande is an underground neutrino observatory, which uses large water Cherenkov counters to detect high-energy neutrinos to search for proton decay, study solar and atmospheric neutrinos, and keep watch for supernovae in the Milky Way Galaxy. It consists of a cylindrical stainless steel tank about 40 m (131 ft) in height and diameter holding 50,000 tons of ultrapure water. Mounted on an inside superstructure are about 13,000 photomultiplier tubes that detect light from Cherenkov radiation.

See also:

Radiation Detection

Detection of X-rays is very specific, because high-energy photons interact differently with matter. High-energy photons can travel thousands of feet in air and can easily pass through various materials. Moreover, high-energy photons can ionize atoms indirectly and directly (despite they are electrically neutral) through the photoelectric effect and the Compton effect. But secondary (indirect) ionization is much more significant.

In order to describe principles of detection of high-energy photons, we have to understand the interaction of radiation with matter. Each type particle interacts in a different way, therefore we must describe interactions of high-energy photons (radiation as a flow of these rays) separately.

See also: X-Rays

Interaction of X-rays with Matter

Although a large number of possible interactions are known, there are three key interaction mechanisms with matter. The strength of these interactions depends on the energy of the X-rays and the elemental composition of the material, but not much on chemical properties, since the X-ray photon energy is much higher than chemical binding energies. The photoelectric absorbtion dominates at low-energies of X-rays, while Compton scattering dominates at higher energies.

• Photoelectric effect
• Compton scattering
• Rayleigh scattering

The photon is completely absorbed in photoelectric effect, while only partial energy is deposited in any given Compton scattering. The probability of photoelectric absorption (dominates at lower X-rays energies) per unit mass is approximately proportional to:

τ(photoelectric) = constant x Z^N/E^3.5

where Z is the atomic number, the exponent n varies between 4 and 5. E is the energy of the incident photon. The probability of Compton scattering per one interaction with an atom increases linearly with atomic number Z, because it depends on the number of electrons, which are available for scattering in the target atom.

Detectors of X-Rays

Detectors may be also categorized according to sensitive materials and methods that can be utilized to make a measurement:

• Gaseous Ionization Detectors
• Scintillation Detectors
• Semiconductor Detectors

Detection of X-Rays using Ionization Chamber

Gamma rays have very little trouble in penetrating the metal walls of the chamber. Therefore, ionization chambers may be used to detect gamma radiation and X-rays collectively known as photons, and for this the windowless tube is used. Ionization chambers have a good uniform response to radiation over a wide range of energies and are the preferred means of measuring high levels of gamma radiation. Some problems are caused by the fact, that alpha particles are more ionising than beta particles and than gamma rays, so more current is produced in the ionization chamber region by alpha than beta and gamma. Gamma rays deposit significantly lower amount of energy to the detector than other particles.

Detection of X-Rays using Geiger Counter

Geiger counter can detect ionizing radiation such as alpha and beta particles, neutrons, X-rays 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.

The high amplification factor of the Geiger counter is the major advantage over the ionization chamber. Geiger counter is therefore a much more sensitive device than other chambers. It is often used in the detection of low-level gamma rays and beta particles for this reason.

Windowless type

Gamma rays have very little trouble in penetrating the metal walls of the chamber. Therefore, Geiger counters may be used to detect gamma radiation and X-rays (thin-walled tubes) collectively known as photons, and for this the windowless tube is used.

• A thick walled tube is used for gamma radiation detection above energies of about 25 KeV, this type generally has an overall wall thickness of about 1-2 mm of chrome steel.
• A thin walled tube is used for low energy photons (X-rays or gamma rays) and high energy beta particles. The transition from thin walled to thick walled design takes place at the 300–400 keV energy levels. Above these levels thick walled designs are used, and beneath these levels the direct gas ionisation effect is predominant.

Detection of X-rays using Scintillation Counter

Scintillation counters are used to measure radiation in a variety of applications including hand held radiation survey meters, personnel and environmental monitoring for radioactive contamination, medical imaging, radiometric assay, nuclear security and nuclear plant safety. They are widely used because they can be made inexpensively yet with good efficiency, and can measure both the intensity and the energy of incident radiation.

Scintillation counters can be used to detect alpha, beta, X-rays and gamma radiation. They can be used also for detection of neutrons. For these purposes, different scintillators are used.

• X-Rays. High-Z materials are best suited as scintillators for the detection of gamma rays. The most widely used scintillation material is NaI(Tl) (thallium-doped sodium iodide). The iodine provides most of the stopping power in sodium iodide (since it has a high Z = 53). These crystalline scintillators are characterized by high density, high atomic number, and pulse decay times of approximately 1 microsecond (~ 10^-6 sec). Scintillation in inorganic crystals is typically slower than in organic ones. They exhibit high efficiency for detection of gamma rays and are capable of handling high count rates. Inorganic crystals can be cut to small sizes and arranged in an array configuration so as to provide position sensitivity. This feature is widely used in medical imaging to detect X-rays or gamma rays. Inorganic scintillators are better at detecting gamma rays and X-rays. This is due to their high density and atomic number which gives a high electron density.

Detection of X-Rays using Semiconductors – HPGe Detectors

High-purity germanium detectors (HPGe detectors) are the best solution for precise gamma and x-ray spectroscopy.

As was written, the study and analysis of gamma ray spectra for scientific and technical use is called gamma spectroscopy, and gamma ray spectrometers are the instruments which observe and collect such data. A gamma ray spectrometer (GRS) is a sophisticated device for measuring the energy distribution of gamma radiation. For the measurement of gamma rays above several hundred keV, there are two detector categories of major importance, inorganic scintillators as NaI(Tl) and semiconductor detectors. If a perfect energy resolution is required, we have to use germanium-based detector, such as the HPGe detector. Germanium-based semiconductor detectors are most commonly used where a very good energy resolution is required, especially for gamma spectroscopy, as well as x-ray spectroscopy. In gamma spectroscopy, germanium is preferred due to its atomic number being much higher than silicon and which increases the probability of gamma ray interaction. Moreover, germanium has lower average energy necessary to create an electron-hole pair, which is 3.6 eV for silicon and 2.9 eV for germanium. This also provides the latter a better resolution in energy. The FWHM (full width at half maximum) for germanium detectors is a function of energy. For a 1.3 MeV photon, the FWHM is 2.1 keV, which is very low.

See also:

Detectors

Detection of gamma radiation is very specific, because gamma rays interact differently with matter. Gamma rays can travel thousands of feet in air and can easily pass through various materials. Moreover, gamma rays can ionize atoms indirectly and directly (despite they are electrically neutral) through the photoelectric effect and the Compton effect. But secondary (indirect) ionization is much more significant.

In order to describe principles of detection of gamma  radiation, we have to understand the interaction of radiation with matter. Each type particle interacts in a different way, therefore we must describe interactions of gamma rays (radiation as a flow of these rays) separately.

Interaction of Gamma Radiation with Matter

Gamma rays consist of high energy photons. These photons are particles/waves (Wave-Particle Duality) without rest mass or electrical charge. They can travel 10 meters or more in air. This is a long distance compared to alpha or beta particles. However, gamma rays deposit less energy along their paths. Lead, water, and concrete stop gamma radiation. Photons (gamma rays and X-rays) can ionize atoms directly through the Photoelectric effect and the Compton effect, where the relatively energetic electron is produced. The secondary electron will go on to produce multiple ionization events, therefore the secondary (indirect) ionization is much more significant.

Although a large number of possible interactions are known, there are three key interaction mechanisms  with matter.

• Photoelectric effect
• Compton scattering
• Pair production

The photon is completely absorbed in photoelectric effect and pair production, while only partial energy is deposited in any given Compton scattering. The probability of photoelectric absorption (dominates at lower gamma rays energies) per unit mass is approximately proportional to:

τ(photoelectric) = constant x Z^N/E^3.5

where Z is the atomic number, the exponent n varies between 4 and 5. E is the energy of the incident photon. The probability of Compton scattering per one interaction with an atom increases linearly with atomic number Z, because it depends on the number of electrons, which are available for scattering in the target atom. The probability of pair production (dominates at higher gamma rays energies), characterized by cross section, is a very complicated function based on quantum mechanics. In general the cross section increases approximately with the square of atomic number (σp ~ Z²) and increases with photon energy, but this dependence is much more complex.

As a result, effective sensitive material for gamma radiation detection is in most cases based on use of materials with two following material properties:

• high-density of material.
• high atomic number of material  (high Z materials)

Detectors of Gamma Radiation

Detectors may be also categorized according to sensitive materials and methods that can be utilized to make a measurement:

• Gaseous Ionization Detectors
• Scintillation Detectors
• Semiconductor Detectors

Detection of Gamma Radiation using Ionization Chamber

Gamma rays have very little trouble in penetrating the metal walls of the chamber. Therefore, ionization chambers may be used to detect gamma radiation and X-rays collectively known as photons, and for this the windowless tube is used. Ionization chambers have a good uniform response to radiation over a wide range of energies and are the preferred means of measuring high levels of gamma radiation. Some problems are caused by the fact, that alpha particles are more ionising than beta particles and than gamma rays, so more current is produced in the ionization chamber region by alpha than beta and gamma. Gamma rays deposit significantly lower amount of energy to the detector than other particles.

The efficiency of the chamber can be further increased by the use of a high pressure gas. Typically a pressure of 8-10 atmospheres can be used, and various noble gases are employed. For example, high-pressure xenon (HPXe) ionization chambers are ideal for use in uncontrolled environments, as a detector’s response has been shown to be uniform over large temperature ranges (20–170°C). The higher pressure results in a greater gas density and thereby a greater chance of collision with the fill gas and ion-pair creation by incident gamma radiation. Because of the increased wall thickness required to withstand this high pressure, only gamma radiation can be detected. These detectors are used in survey meters and for environmental monitoring.

Detection of Gamma Radiation using Geiger Counter

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.

The high amplification factor of the Geiger counter is the major advantage over the ionization chamber. Geiger counter is therefore a much more sensitive device than other chambers. It is often used in the detection of low-level gamma rays and beta particles for this reason.

 

Detection of Gamma Radiation using Scintillation Counter

 

Scintillation counters are used to measure radiation in a variety of applications including hand held radiation survey meters, personnel and environmental monitoring for radioactive contamination, medical imaging, radiometric assay, nuclear security and nuclear plant safety. They are widely used because they can be made inexpensively yet with good efficiency, and can measure both the intensity and the energy of incident radiation.

Scintillation counters can be used to detect alpha, beta, gamma radiation. They can be used also for detection of neutrons. For these purposes, different scintillators are used.

• Gamma Rays. High-Z materials are best suited as scintillators for the detection of gamma rays. The most widely used scintillation material is NaI(Tl) (thallium-doped sodium iodide). The iodine provides most of the stopping power in sodium iodide (since it has a high Z = 53). These crystalline scintillators are characterized by high density, high atomic number, and pulse decay times of approximately 1 microsecond (~ 10^-6 sec). Scintillation in inorganic crystals is typically slower than in organic ones. They exhibit high efficiency for detection of gamma rays and are capable of handling high count rates. Inorganic crystals can be cut to small sizes and arranged in an array configuration so as to provide position sensitivity. This feature is widely used in medical imaging to detect X-rays or gamma rays. Inorganic scintillators are better at detecting gamma rays and X-rays. This is due to their high density and atomic number which gives a high electron density.

Detection of Gamma Radiation using Semiconductors – HPGe Detectors

High-purity germanium detectors (HPGe detectors) are the best solution for precise gamma and x-ray spectroscopy.

As was written, the study and analysis of gamma ray spectra for scientific and technical use is called gamma spectroscopy, and gamma ray spectrometers are the instruments which observe and collect such data. A gamma ray spectrometer (GRS) is a sophisticated device for measuring the energy distribution of gamma radiation. For the measurement of gamma rays above several hundred keV, there are two detector categories of major importance, inorganic scintillators as NaI(Tl) and semiconductor detectors. If a perfect energy resolution is required, we have to use germanium-based detector, such as the HPGe detector. Germanium-based semiconductor detectors are most commonly used where a very good energy resolution is required, especially for gamma spectroscopy, as well as x-ray spectroscopy. In gamma spectroscopy, germanium is preferred due to its atomic number being much higher than silicon and which increases the probability of gamma ray interaction. Moreover, germanium has lower average energy necessary to create an electron-hole pair, which is 3.6 eV for silicon and 2.9 eV for germanium. This also provides the latter a better resolution in energy. The FWHM (full width at half maximum) for germanium detectors is a function of energy. For a 1.3 MeV photon, the FWHM is 2.1 keV, which is very low.

See also:

Detectors

Detection of beta radiation is very specific, because beta particles are more penetrating than alpha particles. On the other hand a thin aluminum plate can stop them.

In order to describe principles of detection of beta radiation, we have to understand the interaction of radiation with matter. Each type particle interacts in a different way, therefore we must describe interaction of beta particles (radiation as a flow of these particles) separately.

Interaction of Beta Particles with Matter

Beta radiation consist of free electrons or positrons at relativistic speeds. Beta particles (electrons) are much smaller than alpha particles. They carry a single negative charge. They are more penetrating than alpha particles, but a thin aluminium plate can stop them. They can travel several meters in air but deposit less energy at any one point along their paths than alpha particles. Similarly as for heavy charged particles, beta particles transfer energy by:

• Excitation. The charged particle can transfer energy to the atom, raising electrons to a higher energy levels.
• Ionization. Ionization can occur, when the charged particle have enough energy to remove an electron. This results in a creation of ion pairs in surrounding matter.

In addition to these interactions, beta particles also lose energy by radiative process known as the bremsstrahlung. From classical theory, when a charged particle is accelerated or decelerated, it must radiate energy and the deceleration radiation is known as the bremsstrahlung (“braking radiation”).

Summary of types of interactions:

• Inelastic collisions with atomic electrons (Excitation and Ionization)
• Elastic scattering off nuclei
• Bremsstrahlung
• Cherenkov radiation.
• Annihilation (only positrons)

Nature of an interaction of a beta radiation with matter is different from the alpha radiation, despite the fact that beta particles are also charged particles. In comparison with alpha particles, beta particles have much lower mass and they reach mostly relativistic energies. Their mass is equal to the mass of the orbital electrons with which they are interacting and unlike the alpha particle a much larger fraction of its kinetic energy can be lost in a single interaction. Since the beta particles mostly reach relativistic energies, the nonrelativistic Bethe formula cannot be used. For high energy electrons an similar expression has also been derived by Bethe to describe the specific energy loss due to excitation and ionization (the “collisional losses”).

Moreover, beta particles can interact via electron-nuclear interaction (elastic scattering off nuclei), which can significantly change the direction of beta particle. Therefore their path is not so straightforward. The beta particles follow a very zig-zag path through absorbing material, this resulting path of particle is longer than the linear penetration (range) into the material.

Detectors of Beta Radiation

Detectors may be also categorized according to sensitive materials and methods that can be utilized to make a measurement:

• Gaseous Ionization Detectors
• Scintillation Detectors
• Semiconductor Detectors

Detection of Beta Radiation using Ionization Chamber

For alpha and beta particles to be detected by ionization chambers, they must be provided with 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.

Ionization chamber may be, for example, used for the measurement of tritium in the air.  These devices are known as tritium-in-air monitors. Tritium is a radioactive isotope, but it emits a very weak form of radiation, a low-energy beta particle that is similar to an electron. It is a pure beta emitter (i.e. beta emitter without an accompanying gamma radiation). The electron’s kinetic energy varies, with an average of 5.7 keV, while the remaining energy is carried off by the nearly undetectable electron antineutrino. Such a very low energy of electron causes, that the electron cannot penetrate the skin or even does not travel very far in air. Beta particles from tritium can penetrate only about 6.0 mm of air. It is practically impossible to design a detector whose walls these beta particles can penetrate. Instead, tritium-in-air monitor pumps the tritium-contaminated air right through an ionization chamber, so that all of the energy of the beta particles can be usefully converted to producing ion pairs inside the chamber.

Detection of Beta Radiation using Scintillation Counter

Scintillation counters are used to measure radiation in a variety of applications including hand held radiation survey meters, personnel and environmental monitoring for radioactive contamination, medical imaging, radiometric assay, nuclear security and nuclear plant safety. They are widely used because they can be made inexpensively yet with good efficiency, and can measure both the intensity and the energy of incident radiation.

Scintillation counters can be used to detect alpha, beta, gamma radiation. They can be used also for detection of neutrons. For these purposes, different scintillators are used.

• Beta Particles. For detection of beta particles, organic scintillators can be used. Pure organic crystals include crystals of anthracene, stilbene and naphthalene. The decay time of this type of phosphor is approximately 10 nanoseconds. This type of crystal is frequently used in the detection of beta particles. Organic scintillators, having a lower Z than inorganic crystals, are best suited for the detection of low-energy (< 10 MeV) beta particles.

Detection of Beta Radiation using Semiconductors – Silicon Strip Detectors

Silicon-based detectors are very good for tracking charged particles. A silicon strip detector is an arrangement of strip like shaped implants acting as charge collecting electrodes.

Silicon strip detectors 5 x 5 cm² in area are quite common and are used in series (just like planes of MWPCs) to determine charged-particle trajectories to position-accuracies of the order of several μm in the transverse direction.  Placed on a low doped fully depleted silicon wafer these implants form a one-dimensional array of diodes. By connecting each of the metalized strips to a charge sensitive amplifier a position sensitive detector is built. Two dimensional position measurements can be achieved by applying an additional strip like doping on the wafer backside by use of a double sided technology.  Such devices can be used to measure small impact parameters and thereby determine whether some charged particle originated from a primary collision or was the decay product of a primary particle that traveled a small distance from the original interaction, and then decayed.

See also:

Detectors

Indirect ionizing radiation consist of electrically neutral particles and therefore does not ionize matter directly. The bulk of the ionization effects are due to secondary ionizations.

Detection of Gamma Radiation

Detection of gamma radiation is very specific, because gamma rays interact differently with matter. Gamma rays can travel thousands of feet in air and can easily pass through various materials. Moreover, gamma rays can ionize atoms indirectly and directly (despite they are electrically neutral) through the photoelectric effect and the Compton effect. But secondary (indirect) ionization is much more significant.

In order to describe principles of detection of gamma  radiation, we have to understand the interaction of radiation with matter. Each type particle interacts in a different way, therefore we must describe interactions of gamma rays (radiation as a flow of these rays) separately.

Detection of Gamma Radiation using Ionization Chamber

Gamma rays have very little trouble in penetrating the metal walls of the chamber. Therefore, ionization chambers may be used to detect gamma radiation and X-rays collectively known as photons, and for this the windowless tube is used. Ionization chambers have a good uniform response to radiation over a wide range of energies and are the preferred means of measuring high levels of gamma radiation. Some problems are caused by the fact, that alpha particles are more ionising than beta particles and than gamma rays, so more current is produced in the ionization chamber region by alpha than beta and gamma. Gamma rays deposit significantly lower amount of energy to the detector than other particles.

The efficiency of the chamber can be further increased by the use of a high pressure gas. Typically a pressure of 8-10 atmospheres can be used, and various noble gases are employed. For example, high-pressure xenon (HPXe) ionization chambers are ideal for use in uncontrolled environments, as a detector’s response has been shown to be uniform over large temperature ranges (20–170°C). The higher pressure results in a greater gas density and thereby a greater chance of collision with the fill gas and ion-pair creation by incident gamma radiation. Because of the increased wall thickness required to withstand this high pressure, only gamma radiation can be detected. These detectors are used in survey meters and for environmental monitoring.

Detection of neutrons is very specific, since the neutrons are electrically neutral particles, thus they are mainly subject to strong nuclear forces but not to electric forces. Therefore neutrons are not directly ionizing and they have usually to be converted into charged particles before they can be detected. Generally every type of neutron detector must be equipped with converter (to convert neutron radiation to common detectable radiation) and one of the conventional radiation detectors (scintillation detector, gaseous detector, semiconductor detector, etc.).

Detection of Neutrons using Ionization Chamber

Ionization chambers are often used as the charged particle detection device. For example, if the inner surface of the ionization chamber is coated with a thin coat of boron, the (n,alpha) reaction can take place. Most of (n,alpha) reactions of thermal neutrons are 10B(n,alpha)7Li reactions accompanied by 0.48 MeV 

Moreover, isotope boron-10 has 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 they react with the boron atoms in the detector gas. Either method may be used to detect neutrons in nuclear reactor. It must be noted, BF₃ counters are usually operated in the proportional region.

See also:

Detectors

Charged particles (atomic nuclei, electrons, positrons, protons, muons, etc.) can ionize atoms directly by fundamental interaction through the Coulomb force if it carries sufficient kinetic energy. These particles must be moving at relativistic speeds to reach the required kinetic energy. Even photons (gamma rays and X-rays) can ionize atoms directly (despite they are electrically neutral) through the Photoelectric effect and the Compton effect, but secondary (indirect) ionization is much more significant.

Detection of Alpha Particles

Detection of alpha radiation is very specific, because alpha particles travel only a few centimeters in air but deposit all their energies along their short paths, thus the amount of energy transferred is very high.

In order to describe principles of detection of alpha radiation, we have to understand the interaction of radiation with matter. Each type particle interacts in a different way, therefore we must describe interaction of alpha particles (radiation as a flow of these particles) separately.

Detection of Alpha using Semiconductors – Silicon Strip Detectors

Silicon-based detectors are very good for tracking charged particles. A silicon strip detector is an arrangement of strip like shaped implants acting as charge collecting electrodes.

Silicon strip detectors 5 x 5 cm² in area are quite common and are used in series (just like planes of MWPCs) to determine charged-particle trajectories to position-accuracies of the order of several μm in the transverse direction.  Placed on a low doped fully depleted silicon wafer these implants form a one-dimensional array of diodes. By connecting each of the metalized strips to a charge sensitive amplifier a position sensitive detector is built. Two dimensional position measurements can be achieved by applying an additional strip like doping on the wafer backside by use of a double sided technology.  Such devices can be used to measure small impact parameters and thereby determine whether some charged particle originated from a primary collision or was the decay product of a primary particle that traveled a small distance from the original interaction, and then decayed.

Detection of Beta Particles

Detection of beta radiation is very specific, because beta particles are more penetrating than alpha particles. On the other hand a thin aluminum plate can stop them.

In order to describe principles of detection of beta radiation, we have to understand the interaction of radiation with matter. Each type particle interacts in a different way, therefore we must describe interaction of beta particles (radiation as a flow of these particles) separately.

Detection of Beta Radiation using Scintillation Counter

Scintillation counters are used to measure radiation in a variety of applications including hand held radiation survey meters, personnel and environmental monitoring for radioactive contamination, medical imaging, radiometric assay, nuclear security and nuclear plant safety. They are widely used because they can be made inexpensively yet with good efficiency, and can measure both the intensity and the energy of incident radiation.

Scintillation counters can be used to detect alpha, beta, gamma radiation. They can be used also for detection of neutrons. For these purposes, different scintillators are used.

• Beta Particles. For detection of beta particles, organic scintillators can be used. Pure organic crystals include crystals of anthracene, stilbene and naphthalene. The decay time of this type of phosphor is approximately 10 nanoseconds. This type of crystal is frequently used in the detection of beta particles. Organic scintillators, having a lower Z than inorganic crystals, are best suited for the detection of low-energy (< 10 MeV) beta particles.

See also:

Detectors

Detection of alpha radiation is very specific, because alpha particles travel only a few centimeters in air but deposit all their energies along their short paths, thus the amount of energy transferred is very high.

In order to describe principles of detection of alpha radiation, we have to understand the interaction of radiation with matter. Each type particle interacts in a different way, therefore we must describe interaction of alpha particles (radiation as a flow of these particles) separately.

Interaction of Heavy Charged Particles with Matter

Alpha radiation consist of alpha particles at high energy/speed. The production of alpha particles is termed alpha decay. Alpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus. Alpha particles are relatively large and carry a double positive charge. They are not very penetrating and a piece of paper can stop them. In general, heavy charged particles transfer energy mostly by:

• Excitation. The charged particle can transfer energy to the atom, raising electrons to a higher energy levels.
• Ionization. Ionization can occur, when the charged particle have enough energy to remove an electron. This results in a creation of ion pairs in surrounding matter.

The distance required to bring the particle to rest is referred to as its range. The range of heavy charged particles in solids amounts to only a few microns, and thus most of the energy of these particles is converted to heat very close to the point of its creation. In case of gases the range increases to a few centimeters in dependence of gas parameters (density, type of gas etc.)  This distance is very important for detectors and significantly determines the design of all detectors. In the materials, the trajectory of heavy charged particles are not greatly affected, because they mostly interact with light atomic electrons. Other charged particles, such as the protons behave similarly with one exception – for lighter charged particles the ranges are somewhat longer.

A convenient variable that describes the ionization properties of surrounding medium is the stopping power. The classical expression that describes the specific energy loss is known as the Bethe formula. For alpha particles and heavier particles the stopping power of most materials is very high for heavy charged particles and these particles have very short ranges. For example, the range of a 5 MeV alpha particle is approximately only 0,002 cm in aluminium alloy. Most alpha particles can be stopped by an ordinary sheet of paper or living tissue.

Detectors of Alpha Radiation

Detectors may be also categorized according to sensitive materials and methods that can be utilized to make a measurement:

• Gaseous Ionization Detectors
• Scintillation Detectors
• Semiconductor Detectors

Detection of Alpha Radiation using Ionization Chamber

For alpha and beta particles to be detected by ionization chambers, they must be provided with 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. But it does not mean, alpha radiation cannot be detected by an ionization chamber.

For example, in some kind of smoke detectors, you can meet man-made radionuclides such as americium-241, which is a source of alpha particles. The smoke detector has two ionization chambers, one open to the air, and a reference chamber which does not allow the entry of particles. The radioactive source emits alpha particles into both chambers, which ionizes some air molecules. The free-air chamber allows the entry of smoke particles to the sensitive volume and to change attenuation of alpha particles. If any smoke particles enter the free-air chamber, some of the ions will attach to the particles and not be available to carry the current in that chamber. An electronic circuit detects that a current difference has developed between the open and sealed chambers, and sounds the alarm.

Detection of Alpha Radiation using Geiger-Mueller Counter

Geiger counters are mainly used for portable instrumentation due to its sensitivity, simple counting circuit, and ability to detect low-level radiation. Although the major use of Geiger counters is probably in individual particle detection, they are also found in gamma survey meters. They are able to detect almost all types of radiation, but there are slight differences in the Geiger-Mueller tube. However, the Geiger-Müller tube produces a pulse output which is the same magnitude for all detected radiation, so a Geiger counter with an end window tube cannot distinguish between alpha and beta particles.

End-Window type

For alpha and beta particles to be detected by Geiger counters, they must be provided with 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 to allow low-energy beta particles (e.g. from carbon-14) to enter the detector. The efficiency reduction for alpha is due to the attenuation effect of the end window, though distance from the surface being checked also has a significant effect, and ideally a source of alpha radiation should be less than 10mm from the detector due to attenuation in air.

Detection of Alpha using Scintillation Counter

Scintillation counters are used to measure radiation in a variety of applications including hand held radiation survey meters, personnel and environmental monitoring for radioactive contamination, medical imaging, radiometric assay, nuclear security and nuclear plant safety. They are widely used because they can be made inexpensively yet with good efficiency, and can measure both the intensity and the energy of incident radiation.

Scintillation counters can be used to detect alpha, beta, gamma radiation. They can be used also for detection of neutrons. For these purposes, different scintillators are used:

Alpha Particles and Heavy Ions. Due to the very high ionizing power of heavy ions, scintillation counters are usually not ideal for the detection of heavy ions. For equal energies, a proton will produce 1/4 to 1/2 the light of an electron, while alpha particles will produce only about 1/10 the light. Where needed, inorganic crystals, e.g. CsI(Tl), ZnS(Ag) (typically used in thin sheets as α-particle monitors), should be preferred to organic materials. Pure CsI is a fast and dense scintillating material with relatively low light yield that increases significantly with cooling. The drawbacks of CsI are a high temperature gradient and a slight hygroscopicity.

Detection of Alpha using Semiconductors – Silicon Strip Detectors

Silicon-based detectors are very good for tracking charged particles. A silicon strip detector is an arrangement of strip like shaped implants acting as charge collecting electrodes.

Silicon strip detectors 5 x 5 cm² in area are quite common and are used in series (just like planes of MWPCs) to determine charged-particle trajectories to position-accuracies of the order of several μm in the transverse direction.  Placed on a low doped fully depleted silicon wafer these implants form a one-dimensional array of diodes. By connecting each of the metalized strips to a charge sensitive amplifier a position sensitive detector is built. Two dimensional position measurements can be achieved by applying an additional strip like doping on the wafer backside by use of a double sided technology.  Such devices can be used to measure small impact parameters and thereby determine whether some charged particle originated from a primary collision or was the decay product of a primary particle that traveled a small distance from the original interaction, and then decayed.

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Detectors

One of three main types of detectors, which record different types of signals, is a counter. The activity or intensity of radiation is measured in counts per second (cps), which expresses a rate of counts per unit time as registered by a radiation monitoring instrument. In general, commonly used quantities are:

• counts per minute (cpm)
• counts per second (cps) 

The best known counter is the Geiger-Müller counter. In radiation counters the generated signal from the incident radiation is created by the counting of the number of interactions occurring at the sensitive volume of the detector.

Unit of counts per second, which is detected by a device is somehow proportional to the activity of a measured sample. But note that, this proportionality is determined also by the distance between the detector and the sample and also by the detection efficiency.

What is one count

Let assume gaseous ionization detectors. Basic gaseous ionization detector consists of a chamber that is filled with a suitable medium (air or a special fill gas) that can be easily ionized. As a general rule, the center wire is the positive electrode (anode) and the outer cylinder is the negative electrode (cathode), so that (negative) electrons are attracted to the center wire and positive ions are attracted to the outer cylinder. The anode is at a positive voltage with respect to the detector wall. As ionizing radiation enters the gas between the electrodes, a finite number of ion-pairs are formed. Under the influence of the electric field, the positive ions will move toward the negatively charged electrode (outer cylinder), and the negative ions (electrons) will migrate toward the positive electrode (central wire). The collection of these ions will produce a charge on the electrodes and an electrical pulse across the detection circuit. However it is a small signal, this signal can be amplified, and then recorded as one count.

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Detectors

Ionization Chambers

The ionization chamber, also known as the ion chamber, is electrical device that detects various types of ionizing radiation. The voltage of detector is adjusted so that the conditions correspond to the ionization region. The voltage is not high enough 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 which affects the accuracy of the Geiger-Mueller tube at high dose rates. This is due to the fact, there is no inherent amplification of signal in the operating medium and therefore these types of 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, 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 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 a distortion of the electric field.
• 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 Neutron 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 take place. 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 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 they react with the boron atoms in the detector gas. Either method may be used to detect neutrons in nuclear reactor.

Disadvantages of Ionization Chambers

• No Charge Amplification. Detectors in the ionization region operate at a low electric field strength, selected such that no gas multiplication takes place. The charge collected (output signal) is independent of the applied voltage and for single minimum-ionizing particles tends to be quite small and usually require special low-noise amplifiers for attaining efficient operating performance. In air, the average energy needed to produce an ion is about 34 eV, therefore a 1 MeV radiation completely absorbed in the detector produces about 3 x 10⁴ pair of ions. However it is a small signal, this signal 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 significantly lower amount of energy to the detector than other particles. The efficiency of the chamber can be further increased by the use of a high pressure gas.
• For alpha and beta particles to be detected by ionization chambers, they must be provided with 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.

Semiconductor Detectors

A semiconductor detector is a radiation detector which is based on a semiconductor, such as silicon or germanium to measure the effect of incident charged particles or photons. Semiconductor detectors are widely used in radiation protection, assay of radioactive materials and physics research because they have some unique features, can be made inexpensively yet with good efficiency, and can measure both the intensity and the energy of incident radiation. These detectors are employed to measure the energy of the radiation and for identification of particles. Of the available semiconductor materials, silicon is mainly used for charged particle detectors (especially for tracking charged particles) and soft X-ray detectors while germanium is widely used for gamma ray spectroscopy.

Advantages of HPGe Detectors

• Higher atomic number. Germanium is preferred due to its atomic number being much higher than silicon and which increases the probability of gamma ray interaction.
• Germanium has lower average energy necessary to create an electron-hole pair, which is 3.6 eV for silicon and 2.9 eV for germanium.
• Very good energy resolution. The FWHM for germanium detectors is a function of energy. For a 1.3 MeV photon, the FWHM is 2.1 keV, which is very low.
• Large Crystals. While silicon-based detectors cannot be thicker than a few millimeters, germanium can have a depleted, sensitive thickness of centimeters, and therefore can be used as a total absorption detector for gamma rays up to few MeV.

Disadvantages of HPGe Detectors

• Cooling. The major drawback of HPGe detectors is that they must be cooled to liquid nitrogen temperatures. Because germanium has relatively low band gap, these detectors must be cooled in order to reduce the thermal generation of charge carriers to an acceptable level. Otherwise, leakage current induced noise destroys the energy resolution of the detector. Recall, the band gap (a distance between valence and conduction band) is very low for germanium (Egap= 0.67 eV). Cooling to liquid nitrogen temperature (-195.8°C; -320°F) reduces thermal excitations of valence electrons so that only a gamma ray interaction can give an electron the energy necessary to cross the band gap and reach the conduction band.
• Price. The disadvantage is that germanium detectors are much more expensive than ionization chambers or scintillation counters.

Advantages of Silicon Detectors

• Compared with gaseous ionization detectors, the density of a semiconductor detector is very high, and charged particles of high energy can give off their energy in a semiconductor of relatively small dimensions.
• Silicon has a high density of  2.329 g/cm³ and therefore the average energy loss per unit of length allows building thin detectors (e.g. 300 µm) that still produce measurable signals. For example, in case of minimum ionizing particle (MIP) the energy loss is 390 eV/µm. The silicon detectors are mechanically rigid and therefore no special supporting structures are needed.
• Silicon-based detectors are very good for tracking charged particles, they constitute a substantial part of detection system at the LHC in CERN.
• Silicon detectors can be used in strong magnetic fields.

Disadvantages of Silicon Detectors

• Price. The disadvantage is that silicon detectors are much more expensive than cloud chambers or wire chambers.
• Degradation. They also suffer degradation over time from radiation, however this can be greatly reduced thanks to the Lazarus effect.
• High FWHM. In gamma spectroscopy, germanium is preferred due to its atomic number being much higher than silicon and which increases the probability of gamma ray interaction. Moreover, germanium has lower average energy necessary to create an electron-hole pair, which is 3.6 eV for silicon and 2.9 eV for germanium. This also provides the latter a better resolution in energy.

See also:

Semiconductor Detectors