Nick Connor

The relationship between the applied voltage and pulse height in a detector is very complex. Pulse height and the number of ion pairs collected are directly related. As was written, the voltages can vary widely depending upon the detector geometry and the gas type and pressure. The figure schematically indicates the different voltage regions for alpha, beta and gamma rays. There are six main practical operating regions, where three (ionization region, proportional and Geiger-Mueller region) are useful to detect ionizing radiation. These reqions are shown below. The alpha curve is higher than the beta and gamma curve from recombination region to part of limited proportionality region due to the larger number of ion pairs produced by the initial reaction of the incident radiation.

Ionization Region

In the ionization region, an increase in voltage does not cause a substantial increase in the number of ion-pairs collected. The number of ion-pairs collected by the electrodes is equal to the number of ion-pairs produced by the incident radiation, and is dependent on the type and energy of the particles or rays in the incident radiation. Therefore, in this region the curve is flat. The voltage must be higher than the point where dissociated ion-pairs can recombine. On the other hand, the voltage is not high enough to produce gas amplification (secondary ionization). Detectors in the ionization region operate at a low electric field strength, selected such that no gas multiplication takes place. Their current is independent of the applied voltage, and they 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.

 

See also:

Gaseous Detectors

The relationship between the applied voltage and pulse height in a detector is very complex. Pulse height and the number of ion pairs collected are directly related. As was written, the voltages can vary widely depending upon the detector geometry and the gas type and pressure. The figure schematically indicates the different voltage regions for alpha, beta and gamma rays. There are six main practical operating regions, where three (ionization, proportional and Geiger-Mueller region) are useful to detect ionizing radiation. These reqions are shown below. The alpha curve is higher than the beta and gamma curve from recombination region to part of limited proportionality region due to the larger number of ion pairs produced by the initial reaction of the incident radiation.

Recombination Region

At low voltage, the electric field is not large enough to accelerate electrons and ions. The electrons and ions can recombine soon after they are produced, and only a small fraction of the produced electrons and ions reach their respective electrodes. As the detector voltage is increased, however, an increasingly large fraction of the ions produced will reach the electrodes. This increase continues until the “saturation” voltage is attained. The range of operating voltage where this occurs is referred to as the recombination region. Detectors are not operated in this region, because neither the number of recombinations nor the number of ion-pairs initially produced can be determined accurately.

 

See also:

Gaseous Detectors

The relationship between the applied voltage and pulse height in a detector is very complex. Pulse height and the number of ion pairs collected are directly related. The Townsend avalanche or Townsend discharge significantly infuelnce the number of ion pairs collected in gaseous ionization detectors. The Townsend avalanche is a gas ionisation process where free electrons are accelerated by an electric field, collide with gas molecules, and consequently free secondary electrons. The result is an avalanche multiplication that permits electrical conduction through the gas.

Townsend Avalanche – Proportional Region

In the proportional region, the charge collected increases with a further increase in the detector voltage, while the number of primary ion-pairs remains unchanged. Increasing the voltage, provides the primary electrons with sufficient acceleration and energy so that they can ionize additional atoms of the medium. These secondary ions formed are also accelerated causing an effect known as Townsend avalanches, which creates a single large electrical pulse. Even though there is a large number of secondary ions (about 10³ – 10⁵) for each primary event, the chamber is always operated such that the number of secondary ions is proportional to the number of primary events. It is very important, because the primary ionization is dependent on the type and energy of the particles or rays in the intercepted radiation field. The number of ion pairs collected divided by the number of ion pairs produced by the primary ionization provides the gas amplification factor (denoted by A). The gas amplification that occurs in this region can increase the total amount of ionization to a measurable value. The process of charge amplification greatly improves the signal-to-noise ratio of the detector and reduces the subsequent electronic amplification required. 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. Proportional counter detection instruments are very sensitive to low levels of radiation. Moreover, proportional counters are capable of particle identification and energy measurement (spectroscopy). Different energies of radiation and different types of radiation can be distinguished by analyzing the pulse height, since they significantly differ in the primary ionization.

Townsend Avalanche – Geiger-Mueller 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. These avalanches can be triggered and propagated by photons emitted by atoms excited in the original avalanche. Since these photons are not affected by the electric field, they may interact far (e.g. laterally to the axis) from the primary avalanche, the entire Geiger tube is participating in the process. 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.

 

See also:

Gaseous Detectors

The relationship between the applied voltage and pulse height in a detector is very complex. Pulse height and the number of ion pairs collected are directly related. As was written, the voltages can vary widely depending upon the detector geometry and the gas type and pressure. The figure schematically indicates the different voltage regions for alpha, beta and gamma rays. There are six main practical operating regions, where three (ionization, proportional and Geiger-Mueller region) are useful to detect ionizing radiation. These reqions are shown below. The alpha curve is higher than the beta and gamma curve from recombination region to part of limited proportionality region due to the larger number of ion pairs produced by the initial reaction of the incident radiation.

 

• Recombination Region. At low voltage, the electric field is not large enough to accelerate electrons and ions. The electrons and ions can recombine soon after they are produced, and only a small fraction of the produced electrons and ions reach their respective electrodes. As the detector voltage is increased, however, an increasingly large fraction of the ions produced will reach the electrodes. This increase continues until the “saturation” voltage is attained. The range of operating voltage where this occurs is referred to as the recombination region. Detectors are not operated in this region, because neither the number of recombinations nor the number of ion-pairs initially produced can be determined accurately.
• Ionization Region. In the ionization region, an increase in voltage does not cause a substantial increase in the number of ion-pairs collected. The number of ion-pairs collected by the electrodes is equal to the number of ion-pairs produced by the incident radiation, and is dependent on the type and energy of the particles or rays in the incident radiation. Therefore, in this region the curve is flat. The voltage must be higher than the point where dissociated ion-pairs can recombine. On the other hand, the voltage is not high enough to produce gas amplification (secondary ionization). Detectors in the ionization region operate at a low electric field strength, selected such that no gas multiplication takes place. Their current is independent of the applied voltage, and they 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.

• 

  Proportional Region. In the proportional region, the charge collected increases with a further increase in the detector voltage, while the number of primary ion-pairs remains unchanged. Increasing the voltage, provides the primary electrons with sufficient acceleration and energy so that they can ionize additional atoms of the medium. These secondary ions formed are also accelerated causing an effect known as Townsend avalanches, which creates a single large electrical pulse. Even though there is a large number of secondary ions (about 10³ – 10⁵) for each primary event, the chamber is always operated such that the number of secondary ions is proportional to the number of primary events. It is very important, because the primary ionization is dependent on the type and energy of the particles or rays in the intercepted radiation field. The number of ion pairs collected divided by the number of ion pairs produced by the primary ionization provides the gas amplification factor (denoted by A). The gas amplification that occurs in this region can increase the total amount of ionization to a measurable value. The process of charge amplification greatly improves the signal-to-noise ratio of the detector and reduces the subsequent electronic amplification required. 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. Proportional counter detection instruments are very sensitive to low levels of radiation. Moreover, proportional counters are capable of particle identification and energy measurement (spectroscopy). Different energies of radiation and different types of radiation can be distinguished by analyzing the pulse height, since they significantly differ in the primary ionization.

• Limited Proportional Region. In the limited proportional region, the gas amplification factor does not continue to increase proportionally to the voltage. Additional ionizations and nonlinear effects causes, that there is no proportionality of the output signal to the deposited energy at a given applied voltage. The electric field in the chamber is distorted due to the high positive ion concentration. 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. This region is usually avoided as a detection region.

• 

  Geiger-Mueller Region. In the Geiger-Mueller region, the voltage and thus  the electric field is so strong that secondary avalanches can occur. These avalanches can be triggered and propagated by photons emitted by atoms excited in the original avalanche. Since these photons are not affected by the electric field, they may interact far (e.g. laterally to the axis) from the primary avalanche, the entire Geiger tube is participating in the process. 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.  Detectors, which are operated in the Geiger-Mueller region, are capable of detection of gamma rays, and also of all types of charged particles, that can enter the detector. These detectors are known as Geiger counters. Main advantage of these instruments is that they usually do not require any signal amplifiers. Since the positive ions do not move far from the avalanche region, a positively charged ion cloud disturbs the electric field and terminates the avalanche process. In practice the termination of the avalanche is improved by the use of “quenching” techniques. In contrast to proportional counters the energy or even incident radiation particle cannot not be distinguished by Geiger counters, since the output signal is independent of the amount and type of original ionization.

• Discharge Region. Finally, at still higher voltages (above Geiger-Mueller region), the electric field generates a continuous discharge of the medium, with the chamber no longer being sensitive to any incident ionization. This region is not used for detection or measurement of ionizing radiation. If the Geiger tube voltage is increased above the end of the plateau the count rate begins to increase rapidly again, until the onset of continuous discharge where the tube cannot detect radiation, and may be damaged.

Types of Detectors of Ionizing Radiation

As a result, there are three basic types of gaseous ionization detectors, which are categorized according to the voltage applied to the detector:

• ionization chambers,
• proportional counters,
• Geiger-Müller tubes.

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.

See also:

Gaseous Detectors

Principles of Detectors

In their basic principles of operation, most detectors of ionizing radiation follow similar characteristics. Detectors of ionizing radiation consist of two parts that are usually connected. The first part consists of a sensitive material, consisting of a compound that experiences changes when exposed to radiation. The other component is a device that converts these changes into measurable signals. All detectors require that radiation must deposit some of its energy in sensitive material that forms part of the instrument. The radiation enters the detector, interacts with atoms of the detector material and deposits some energy to sensitive material. Each event may generate a signal, which can be a pulse, hole, light signal, ion pairs in a gas, and many others. The main task is to generate sufficient signal, amplify it and to record it.

Description of Gaseous Ionization Detectors

The chamber has a cathode and an anode that are held at some large relative voltage, and the device is characterized by a capacitance that is determined by the geometry of the electrodes. As ionizing radiation enters the gas between the electrodes, a finite number of ion-pairs are formed. The behavior of the resultant ion-pairs is affected by the potential gradient of the electric field within the gas and the type and pressure of the fill gas. 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 electric field in this region keeps the ions from recombining with the electrons. The collection of these ions will produce a charge on the electrodes and an electrical pulse across the detection circuit. 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.

Types of Detectors of Ionizing Radiation

As a result, there are three basic types of gaseous ionization detectors, which are categorized according to the voltage applied to the detector:

• ionization chambers,
• proportional counters,
• Geiger-Müller tubes.

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.

See also:

Gaseous Detectors

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 using standard electronics.

Signal-to-Noise Ratio

Signal-to-noise ratio, SNR, is a measure used in science and engineering that compares the electrical output signal to the electrical noise generated in the cable run or in the instrumentation.

Signal-to-Noise Ratio – Germanium Detectors

Total absorption of a 1 MeV photon produces around 3 x 10⁵ electron-hole pairs. This value is minor in comparison the total number of free carriers in a 1 cm³ intrinsic semiconductor.

Particle passing through the detector ionizes the atoms of semiconductor, producing the electron-hole pairs. But in germanium-based detectors at room temperature, thermal excitation is dominant. It is caused by impurities, irregularity in structure lattice or by dopant. It strongly depends on the E_gap (a distance between valence and conduction band), which is very low for germanium (Egap= 0.67 eV). Since thermal excitation results in the detector noise, active cooling is required for some types of semiconductors (e.g. germanium).

Note that, a 1 cm³ sample of pure germanium at 20 °C contains about 4.2×10²² atoms, but also contains about 2.5 x 10¹³ free electrons and 2.5 x 10¹³ holes. . As can be seen, the signal to noise ratio (S/N) would be minimal. The addition of 0.001% of arsenic (an impurity) donates an extra 10¹⁷ free electrons in the same volume and the electrical conductivity is increased by a factor of 10,000. In doped material the signal to noise ratio (S/N) would be even smaller. Because germanium has relatively low band gap, these detectors must be cooled in order to reduce the thermal generation of charge carriers (thus reverse leakage current) to an acceptable level. Otherwise, leakage current induced noise destroys the energy resolution of the detector.

See also:

Detection

Discrimination in radiation detection circuits refers to the process of distinguishing between different types of radiation on the basis of pulse height. A discriminator circuit selects the minimum or maximum pulse height that is to be counted. For example a discriminator circuit is used in the neutron detector to distinguish the large pulses from alpha ionisation (neutron absorption in the detector produces alpha particles) from the much smaller ones that would be caused by gamma rays.

Special Reference: U.S. Department of Energy, Instrumantation and Control. DOE Fundamentals Handbook, Volume 2 of 2. June 1992.

See also:

Detection

Purpose of Radiation Detectors

There are three main types of detectors, which record different types of signals.

• Counter. The activity or intensity of radiation is measured in 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.
• Radiation Spectrometer. Spectrometers are devices designed to measure the spectral power distribution of a source. The incident radiation generates a signal that allows to determine the energy of the incident particle.
• Dosimeter. A radiation dosimeter is a device that measures exposure to ionizing radiation. Dosimeters usually record a dose, which is the absorbed radiation energy measured in grays (Gy) of the equivalent dose measured in sieverts (Sv) A personal dosimeter is dosimeter, that is worn at the surface of the body by the person being monitored, and it  records of the radiation dose received.

All these types of equipment require that the radiations result in observable changes in a compound (whether gas, liquid or solid). In their basic principles of operation, most detectors of ionizing radiation follow similar characteristics. Detectors of ionizing radiation consist of two parts that are usually connected. The first part consists of a sensitive material, consisting of a compound that experiences changes when exposed to radiation. The other component is a device that converts these changes into measurable signals. All detectors require that radiation must deposit some of its energy in sensitive material that forms part of the instrument. The radiation enters the detector, interacts with atoms of the detector material and deposits some energy to sensitive material. Each event may generate a signal, which can be a pulse, hole, light signal, ion pairs in a gas, and many others. The main task is to generate sufficient signal, amplify it and to record it.

Classification by Sensitive Material

We focus on the first part, which consists of a sensitive material. Detectors may be categorized according to sensitive materials and methods that can be utilized to make a measurement:

• Gaseous Ionization Detectors
• Scintillation Detectors
• Semiconductor Detectors
• Film Badges
• TLD

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 using standard electronics.

Classification of Radiation Detectors – Radiation Type

Principles and methods for detection of ionizing radiation are dependent on many factors. Type of radiation measured and detected is a key factor, and different types of detectors in different physical states (solid, liquid or gas) are used to measure selective types of ionizing radiation. It is important to distinguish between alpha or beta particles, X-rays or γ-rays and neutrons. Since each radiation interacts differently with matter, we cannot use, for example, neutron detector to detect beta radiation. Sometimes, detectors can detect various types of radiation, for example, neutron detectors are also able to detect gamma radiation. Usually, this feature is not desirable, and detectors must compensate the undesirable component of radiation.

In order to describe the classification of radiation detectors by the type of radiation, we have to understand the interactions of radiation with matter. Each type particle interacts in a different way, therefore we must describe interaction of particles (radiation as a flow of these particles) separately. For example charged particles with high energies can directly ionize atoms. On the other hand electrically neutral particles interacts only indirectly, but can also transfer some or all of their energies to the matter. This is the key feature of the categorization of radiation detectors. Simply, we cannot use a gamma rays detector to detect alpha radiation. Detectors can be categorized into two general types as follows:

• Detection of Directly Ionizing Radiation
  • Alpha radiation
  • Beta radiation
• Detection of Indirectly Ionizing Radiation
  • Gamma radiation
  • Neutron radiation
  • Neutrinos

See also:

Radiation Detection

Basic Principles of Detectors

There are three main types of detectors, which record different types of signals.

• Counter. The activity or intensity of radiation is measured in 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.
• Radiation Spectrometer. Spectrometers are devices designed to measure the spectral power distribution of a source. The incident radiation generates a signal that allows to determine the energy of the incident particle.
• Dosimeter. A radiation dosimeter is a device that measures exposure to ionizing radiation. Dosimeters usually record a dose, which is the absorbed radiation energy measured in grays (Gy) of the equivalent dose measured in sieverts (Sv) A personal dosimeter is dosimeter, that is worn at the surface of the body by the person being monitored, and it  records of the radiation dose received.

All these types of equipment require that the radiations result in observable changes in a compound (whether gas, liquid or solid). In their basic principles of operation, most detectors of ionizing radiation follow similar characteristics. Detectors of ionizing radiation consist of two parts that are usually connected. The first part consists of a sensitive material, consisting of a compound that experiences changes when exposed to radiation. The other component is a device that converts these changes into measurable signals. All detectors require that radiation must deposit some of its energy in sensitive material that forms part of the instrument. The radiation enters the detector, interacts with atoms of the detector material and deposits some energy to sensitive material. Each event may generate a signal, which can be a pulse, hole, light signal, ion pairs in a gas, and many others. The main task is to generate sufficient signal, amplify it and to record it.

In this section we describe a number of different types of radiation measuring instruments. We focus on the first part, which consists of a sensitive material. Detectors may be categorized according to sensitive materials and methods that can be utilized to make a measurement:

• Gaseous Ionization Detectors
• Scintillation Detectors
• Semiconductor Detectors
• Film Badges
• TLD

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 using standard electronics.

See also:

Radiation Detection

What is Ionization

Ionization is the process in which an atom or a molecule gains or loses electrons to form charged ion. Ionization can result from the loss of an electron after collisions with energetic subatomic particles, collisions with other atoms, molecules and ions, or through the interaction with electromagnetic radiation. In general, ionizing radiation is any radiation (particles or electromagnetic waves) that carries enough energy to knock electrons from atoms or molecules, thereby ionizing them. For ionizing radiation, the kinetic energy of particles (photons, electrons, etc.) is sufficient and the particle can ionize (to form ion by losing electrons) target atoms to form ions.

The boundary between ionizing and non-ionizing radiation is not sharply defined, since different molecules and atoms ionize at different energies. Gamma rays, X-rays, and the higher ultraviolet part of the spectrum are ionizing, whereas the lower ultraviolet, visible light (including laser light), infrared, microwaves, and radio waves are considered non-ionizing radiation.

Ionization Energy

Ionization energy, also called ionization potential, is the energy necessary to remove an electron from the neutral atom.

X + energy → X⁺ + e⁻

where X is any atom or molecule capable of being ionized, X⁺ is that atom or molecule with an electron removed (positive ion), and e⁻ is the removed electron.

A nitrogen atom, for example, requires the following ionization energy to remove the outermost electron.

N + IE → N⁺ + e⁻        IE = 14.5 eV

The ionization energy associated with removal of the first electron is most commonly used. The nth ionization energy refers to the amount of energy required to remove an electron from the species with a charge of (n-1).

1st ionization energy

X → X⁺ + e⁻

2nd ionization energy

X⁺ → X²⁺ + e⁻

3rd ionization energy

X²⁺ → X³⁺ + e⁻

Ionization Energy for different Elements

There is an ionization energy for each successive electron removed. The electrons that circle the nucleus move in fairly well-defined orbits. Some of these electrons are more tightly bound in the atom than others. For example, only 7.38 eV is required to remove the outermost electron from a lead atom, while 88,000 eV is required to remove the innermost electron. Helps to understand reactivity of elements (especially metals, which lose electrons).

In general, the ionization energy increases moving up a group and moving left to right across a period. Moreover:

• Ionization energy is lowest for the alkali metals which have a single electron outside a closed shell.
• Ionization energy increases across a row on the periodic maximum for the noble gases which have closed shells

For example, sodium requires only 496 kJ/mol or 5.14 eV/atom to ionize it. On the other hand neon, the noble gas, immediately preceding it in the periodic table, requires 2081 kJ/mol or 21.56 eV/atom.

See also:

Radiation Detection