Radiation Detectors
Table of Contents
Comparison of Common Dosimeters
| Device -Accuracy | Common Uses | Advantages | Limitations |
|---|---|---|---|
| Ion Chambers ±1% | -Reference Dosimetry -Percent Depth Dose Distributions | -Best understood -Sub 1% accuracy possible -Low energy dependence | -Size limitations -ADCL calibration required |
| Diode Detectors ±2-3% | -Small field dosimetry -Array devices -Electron PDD | -Small volume -Rapid readout -No external bias | -Temperature dependence (0.5%/C) -Dose rate dependence -Energy dependence |
| Film ±2-5% | -Planar dose distributions -Electron PDD | -Best spatial resolution (μm) -Large area measurement -Persistent dose record -Tissue equivalent (radiochromic only) | -Delayed readout -Batch-to-batch variation -Chemical development (radiographic only) |
| Luminescent Dosimeters ±3% | -In Vivo Dosimetry -Personnel dosimeters -End-to-end testing (IROC) | -Small size -Low MV energy dependence | -Delayed readout -Signal loss over time -Supralinear response with accumulated dose |
| MOSFET Detectors | -In vivo dosimetry -Small Field Dosimetry -Surface dose | -Extremely small effective volume -Permanent dose record -Instant readout | -Finite life (~100Gy) -Energy Dependence -Temperature Dependence -Sensitivity changes with accumulated dose |
| Plastic Scintillators | -Small Field Dosimetry -Array Measurements -Electron measurements | -Small volume -Near water equivalent -Dose and rate independent | -Noise, especially Cherenkov Radiation -Sensitivity change with plastic yellowing -New technology, few vendors |
Ionization Chambers
Overview
An ionization chamber consists of a gas filled cavity surrounded by two electrodes of opposite polarity and an electrometer. The electric field established between the electrodes accelerates the radiation produced ions to be collected by the electrodes. This charge is read out by the electrometer and may be converted to absorbed dose.
Three common types of ionization chambers are used in medical physics for reference dosimetry: cylindrical, plane parallel and free air chambers.
Cylindrical Chambers
Cylindrical chambers are most commonly used in reference dosimetry applications of MV photons and electrons above about 6MeV. Cylindrical chambers, especially farmer chambers, are well characterized and are considered the gold standard of clinical reference dosimetry. Because the of their axial design, the effective point of measurement is upstream of the central axis of the chamber by 0.6rcav for photons and 0.5rcav for electrons.
Plane Parallel Chambers
Plane parallel, sometimes called parallel plate, ionization chambers are commonly used in low energy (<6MeV) electron dosimetry as well as in applications where precise measurement location is valued such as measurement of electron percent depth dose distributions. The key advantage of plane parallel ionization chambers is the effective point of measurement is the front (most upstream) plane of the chamber.
Free Air Chambers
Free air ionization chambers are the instrument of definition for the unit of the Roentgen and, as such, are tied fundamentally to absorbed dose. This makes free air ionization chambers the reference dosimeter of choice for Accredited Dosimetry Calibration Laboratories (ADCLs) but their large size makes them unsuitable for clinical applications.
Farmer Chambers
Key Point: Farmer chambers are thimble ionization chambers widely used in reference dosimetry. Questions regarding farmer chamber design and operation are common on board exams.
- Typical Sensitive Volume: 0.6cc (approximately cylindrical, 0.3cm radius, 2cm length)
- Typical Response: 20nC/Gy
- Effective Point of Measurement:
- Photons: 0.6rcav (~0.18cm) upstream of central axis
- Electrons: 0.5rcav (~0.15cm) upstream of central axis
Theory of Operation: Cavity Theory
Cavity theory is the basis of operation for ionization chambers used in reference dosimetry. Cavity theory relates measured dose in a cavity, such as an ion chamber, to dose at the same point in the medium in absence of the cavity.
Bragg-Gray Cavity Theory
Bragg-Gray (BG) theory relates dose to the medium, Dmed, to dose to the cavity fill gas, Dgas, via the ratio of mass collision stopping powers between the medium and gas, 
.
Bragg-Gray Assumptions
- Charged particle equilibrium (CPE) or transient charged particle equilibrium (TCPE) exist
- All electrons causing ionization in the cavity arise from phantom material
- Secondary electron spectrum is unchanged by presence of the cavity
- Energy of secondary electrons created inside the cavity are deposited locally
- Neglects secondary electrons (delta rays) generated within the cavity as a result of interactions with scattered electrons
Bragg-Gray Limitations
Because of contradictory and non-physical assumptions, Bragg-Gray theory is only an approximate solution for physical systems.
Assumptions 2 and 3 imply a need for a small cavity volume while requirement 4 requires a large volume to collect all electrons. These conditions cannot be met simultaneously.
Requirement 3, that the spectrum be unchanged, would mean that no energy could be collected to rigorously meet this theory. This is generally disregarded as the effect is minimal with a small cavity.
Spencer-Attix Cavity Theory
The Spencer-Attix formulation of cavity theory resolves the issues of the Bragg-Gray so that it applies for small cavities.
is the ratio of restricted mass collision stopping power from the medium to the cavity fill gas.- Restricted mass collision stopping power uses a cutoff energy, Δ, which removes the requirement that secondary electrons deposit their energy locally.
is the ratio of Spencer-Attix Requirements
- Requires charged particle equilibrium (CPE) or transient charged particle equilibrium (TCPE)
- All electrons causing ionization in the cavity arise from phantom material
- Secondary electron spectrum is unchanged by presence of the cavity
Burlin Cavity Theory
The Burlin formulation generalizes cavity theory for large and small cavities.
- is the ratio of restricted mass collision stopping power from the medium to the cavity fill gas.
- d is a parameter related to cavity size
- d = 1 for small cavities
- d approaches 0 for large cavities
- is the ratio of mass energy absorption coefficients from the medium to gas.
is the ratio of mass energy absorption coefficients from the medium to gas.Burlin Requirements
- Charged particle equilibrium (CPE) exists in medium and cavity
Restricted Mass Collision Stopping Power (L/ρ)
Restricted mass collision stopping power introduces a cutoff energy, Δ, typically taken to be 10-20keV.
- Electrons with energy <Δ are assumed to deposit their energy where created
- Electrons with energy >Δ dissipate their energy through the Continuous Slowing Down Approximation (CSDA)
Key Point: Bragg-Gray cavity theory provides an approximate theory of operation for ionization but requires contradictory assumptions. Spencer-Attix and Burlin theories improve upon this by assuming that low energy electrons deposit their energy locally.
Gas Amplification Curve
The response of an ionization chamber is heavily dependent on the voltage applied between the outer electrode and the central electrode. The gas amplification curve describes the behavior of an ionization chamber as a function of applied voltage.
Region I: Recombination
Response in this region is voltage dependent as well as energy dependent because a large number of ions recombine prior to collection.
Region II: Ionization region
Sufficient voltage to prevent recombination but insufficient voltage to produce secondary ionizations. This is the region used in clinical ionization chambers because the measured signal is directly proportional to the number of ionizations produced by incident radiation.
Region III: Proportional region
Response is proportional to energy collected and to the applied voltage. Proportional counters operate in this voltage range.
Region IV: Limited proportionality region
Response to collected energy diminishes while response to applied voltage increases. This voltage region is not used.
Region V: Geiger-Muller region
Townsend avalanche creates a large number of secondary avalanches. Like an explosion burns until the fuel source runs out, a Townsend avalanche produces secondary electrons until the electrons neutralize the local electric field preventing further ion production.
Region VI: Continuous discharge region
The chamber continually arcs due to excessive applied voltage.
Diode Detectors
Overview
Diode detectors are small volume solid state detectors used in small field dosimetry, array measurements, and in-vivo dosimetry. The small active volume of diode detectors makes them especially valuable in use cases where volume averaging may seriously obscure results (i.e. small fields, penumbra profile).
Diodes exhibit temperature, energy, dose rate, and orientation dependence.
Design
A diode detector consists of a diode (die), attached leads, a buildup cap. An electrometer is used to read out accumulated charge.
Diode (Die)
The die is the active component of the dosimeter. Diodes are comprised of a P-type or N-type semiconductor junction. The die is most often constructed of doped silicon or germanium.
Doping is the introduction of impurities to the semiconductor to produce either P-type (electron acceptor) or N-type (electron donor) semiconductors.
Buildup Cap
Surface diodes are constructed with a buildup cap surrounding the diode. This cap attenuates an incident beam allowing the device to measure surface dose.
Scanning diodes are often constructed without a buildup cap.
Leads
Metallic leads are attached to the die allowing for readout by the electrometer. Construction of these leads can add greatly to the orientation dependence of the detector.
Theory of Operation
PN Junctions and the Depletion Zone
There are two broad types of semiconductors: electron donors, referred to as N-type, and electron acceptors, referred to as P-type. When a P-type and an N-type semiconductor are placed in contact, electrons tend to flow from the N-type to the P-type semiconductor. This electron flow continues until a retarding electric field forms at the junction of the P- and N-type semiconductors, preventing further ion exchange. This region of retarding electric field is known as the depletion zone.
Key Point: The depletion zone is the active region of a diode detector. Because this region is only a small fraction of the die, volume averaging of a diode detector is very small.
Key Point: Semiconductors are materials with a small band gap that may be overcome by excited electrons. Doping materials can impact the size of the bandgap and hence the properties of the semiconductor.
Radiation Measurement
- Radiation incident upon the sensitive volume (i.e. the depletion zone) liberates ions.
- For electron measurement, the incident electrons may be directly captured.
- Ions are influenced by the strong electric field in the depletion zone. This propels the ions creating a current or accumulating a net charge.
- An electrometer is used to measure accumulated charge or induced current. This measurement is correlated with absorbed dose via a calibration constant.
Reverse Biasing
Some diode detectors may also be operated in a "reversed bias" mode in which a negative voltage is applied to the P side and a positive voltage is applied to the N side. This increases the diode's sensitivity and collection efficiency (less recombination) but increases the volume of the depletion zone (more volume averaging).
Film
Overview
Film may be used in radiation measurement, especially for measurement of relative dose distributions. The dosimetric accuracy of film is, however, limited to 2-5% because film measurement is subject to many compounding sources of error (measurement conditions, development, readout, etc).
There are two distinct types of film: radiographic film and radiochoromic film. Radiographic film is similar to that used in older radiography applications and requires chemical development. Chemical development adds cost and introduces significant variability in dose measurement.
Radiochromic film is self-developing via a polymerization reaction. This self-development feature has cause radiochromic film to largely supersede radiographic film for dosimetry purposes.
Dose Measurement
- Film is oriented with appropriate side facing beam and irradiated.
- Film may also be oriented parallel to the field for percent depth dose measurements.
- If radiographic, film is chemically processed. If radiochromic, time delay after exposure as image producing reactions occur.
- Film is read out on calibrated scanner.
- This will measure optical density.
- Optical density is converted to absorbed dose via calibration.
Optical Density
Optical Density (OD), the log base 10 of the fraction of light transmitted through an unexposed film to the light transmitted after exposure.
- I0 is the light collected without film.
- I is the light collected after passing through film.
- T is the transmittance; the ratio of the light transmitted through film to the light transmitted without film.
Hunter and Driffield (H&D) Curves
H&D curves are used to relate the exposure or dose to optical density. Most H&D curves used for dosimetry purposes report optical density as a function of log base 10 of dose.
Film Speed: Fast film will have a greater increase in optical density with dose but will also have a more limited range of optical densities. Slower film provides better range and dose resolution. Faster film is more appropriate for dose reduction in imaging purposes.
Gradient: The slope of the curve in the linear region. Fast films have a higher gradient.
Latitude: The range of exposures in the linear region. This is also referred to as the range.
Linear Region: This is the useful region in which optical density is proportional to log of exposure.
Toe, Shoulder: These regions are not clinically useful as the relationship between exposure and optical density is non-linear.
Fog: Darkening of the film due to background radiation or light exposure (for radiographic film).
Base: The natural attenuation of the film without exposure or fog.
Types of Film
| Dose Accuracy | Advantages | Disadvantages | |
|---|---|---|---|
| Radiographic | 3-5% |
|
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| Radiochromic | 2-3% |
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Radiographic Film
Radiation Measurement Process
- When radiation hits the film, loosely bound electrons are freed.
- These electrons aggregate around impurities and form a negative charge.
- This negative charge attracts the Ag+ ions leaving behind neutral (metallic) silver. This is the latent image.
- Latent image will be magnified a billion fold (~109)
- The film is developed in a 4-step process
- The developer is applied to the emulsion. This greatly amplifies the amount of metallic silver and the latent image.
- Acetic acid (referred to as the stop bath) is applied which stops further development.
- The fixer (Sodium Thiosulphate) dissolves all undeveloped grains thereby fixing the image.
- The image is washed to remove chemicals and dried.
- Finally, the film is read out on a calibrated optical digitizer.
Key Point: The latent image consists of metallic silver atoms and is not visible without development. Latent images can be formed by as few as 10 silver atoms. There are more commonly many thousands of metallic silver atoms in the latent image because a single X-ray quanta is able to produce over 1000 silver atoms.
Radiographic Film Construction
Plastic Base: provides the structure of the film.
Silver Bromide Emulsion: is the active layer of the film consisting of Ag+ and Br- ions.
- Grains are 0.1 – 3 μm in diameter
- 109 to 1010 grains per square centimeter
Gelatin Protective Layer: serves to keep the silver bromide grains well dispersed and to protect unexposed grains during development.
Radiochromic Film
Radiochromic, sometimes referred to as GafChromic, film used a radiation induced polymerization action to produce darkening of the film proportional to absorbed dose.
Radiation Measurement Process
- Radiation incident on the active layer induces a polymerization reaction.
- As the active layer polymerizes, it becomes partially opaque in proportion to the incident dose. This process continues for several hours and the time between measurement and readout must be controlled.
- After 1-24 hours, the film is read out on a calibrated optical digitizer. Near immediate readout is possible but results in decreased measurement precision.
Radiochromic Film Construction
Base Layer: Polyester (Mylar) base provides structure to the film.
Active Layer: Consists of radio-sensitive chemicals which polymerize into optically opaque polymers upon irradiation. This is a chemical reaction which takes approximately 24 hours to complete, although most of the development occurs in the first hour.
Symmetrical and Non-Symmetrical Designs: Although most current films are constructed symmetrically, older films have been constructed asymmetrically. Asymmetrical construction requires attention to orientation during measurement and readout to avoid introduction of systematic error.
EBT 2 is a common example of asymmetrical construction. Although it was later shown that this had little influence on measured dose distribution, EBT 3 was introduced with a symmetrical design.
Optical Digitizers and Film Scanners
Optical digitizers, sometimes referred to as film scanners, are used to convert the film optical density distribution to digital form for analysis. Digitizers typically consist of a light source and a CCD or SMOS image sensor. After digitization, the optical density map may be converted to a dose distribution by means of a calibration curve.
Luminescent Dosimeters
Overview
Luminescent dosimeters are crystal structures which are able to trap and store energy when irradiated. This energy is later released via luminescence in the form of visible light. Measurement of emitted light may be used to determine the dose delivered to the dosimeter.
TLDs vs OSLDs
| Accuracy | Chemical Composition | Advantages | Disadvantages | |
|---|---|---|---|---|
| Thermoluminescent Dosimeters (TLD) | ~3% | LiF:Mg, TI |
|
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| Optically Stimulated Luminescent Dosimeters (OSLD) | ~3% | Al2O3:C |
|
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Dose Measurement
- Incident ionizing radiation creates an electron-hole pair in the crystal structure.
- The liberated electron is promoted to the conduction band and migrates to the electron trap. At the same time the hole migrates along the valence band to a hole trap.
- Energy (in the form of heat for a TLD or light for an OSLD) is imparted to the electron and hole allowing them to escape their traps. This causes the electron-hole pair to recombine at a luminescent center and release light.
- Emitted light is measured by a photomultiplier tube or camera (CCD or CMOS). This light is used to determine absorbed dose.
Crystal Structure
Energy Bands: The crystalline structure of luminescent dosimeters gives rise to delocalized electronic states referred to as energy bands.
Valence Band is the band of electron orbitals that can donate an electron to the conduction band when excited. This is simply the outermost electron orbital that is occupied by electrons.
Conduction Band is the energy band that excited electrons can enter when leaving the valence band. Electrons in the conduction band are able to move freely producing electric current. Simply put, the conduction band is able to conduct current.
Band Gaps are the difference in energy between the valence and conduction bands. The size of the band gap determines the minimum energy required for an excited valence electron to enter the conduction band. Band gap size determines whether the material is a conductor (no band gap), a semiconductor, or an insulator (large band gap).
Crystal Lattice Imperfections: Imperfections in the crystal lattice create occupiable energy levels withinthe band gap. These energy levels are known as traps because electrons can remain trapped in these energy levels until freed. There are three important types of imperfections:
Trap Centers: are meta-stable energy states for charge carriers (electrons and holes) elevated out of the valence band. Trap centers allow the detector to hold some of the absorbed dose energy until readout.
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- Depth of the trap relates to the amount of energy required to escape the trap. (i.e. shallow traps may be escaped under ambient conditions but deep traps require a great deal of heat/light energy for the charge carrier to escape).
- F-center (the most common trap) is characterized by an anion vacancy in the crystal.
- Mangesium dopants add trap centers.
![Schematic of an F-center: (a) diagram of a simple electron trap. The square represents an ion vacancy and the e represents the electron filling the vacancy. (b) Diagram of the electron in a force field. The trapped electron is actually distributed among the surrounding positive cations. [DeWerd and Stoebe (1972)]](https://mk0oncologymedi6ds2v.kinstacdn.com/wp-content/uploads/2018/07/F-center-1.png "F-center")
Luminescent (Recombination) Centers: are imperfections that allow electron/hole pairs to recombine and, in the process, emit the light used to determine absorbed dose. Trapped electrons must be excited by light or heat to enter a luminescent center.
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- Titanium dopants add luminescence centers
Competitive Centers: also trap charge carriers but do not contribute to luminescence other than removing the charge from being able to recombine. Filling of competitive centers is responsible for the increase in sensitivity of TLDs at high doses (supralinear response).
Doping: Imperfections can be intentionally added to a crystal through a process called doping. Doping allows control over the properties of a luminescent dosimeter by modifying the content of trap, luminescent and competitive centers. Common dopants include Mg, Ti and C.
Key Point: Luminescent detectors exploit crystalline imperfections to trap radiation energy within the crystal. This energy can then be released as light and used to determine absorbed dose.
Thermoluminescent Dosimeters (TLD)
Optical Readout
Heating causes the TLD to emit photons which are measured in real time using a photomultiplier tube (PMT) or optical camera (CCD or CMOS). Heaters may either use an ohmically heated plate or heated nitrogen gas.
Glow Curve
A glow curve is a graph of luminescence as a function of TLD temperature.
The probability of escaping a trap center increases with temperature and decreases with trap depth. Cumulative probability of escaping a center is then proportional to both temperature and time under the temperature. This results in distinct glow peaks on the glow curve. Use of a consistent heating protocol (rate of temperature increase) is also important for this reason.
Reusing TLDs
Annealing (Preparation for Reuse)
Annealing is a heating process by which trap centers are emptied and redistributed through out the crystal lattice. As with readout, consistent heating protocol during annealing is important to assure consistent TLD response.
Below is one common annealing protocol:
- Heat to 400°C for 1 hour to reset lattice/impurity structure.
- Reduce heat to 80°C for 24 hours to rearrange the traps that result in peak 2 (glow curve image).
Supralinearity
The response of TLDs increases (i.e. TLDs become more sensitive) with repeated use. This increases sensitivity, known as supralinearity, is caused by a decrease in the availability and efficiency of competitive centers. There are two reasons for this decrease:
- As competitive centers become increasingly populated, the odds of a charge carrier encountering an available competitive center decreases.
- Energy distance between neighboring trap and luminescence centers decreases. This decreases probability of the charge carrier encountering a competitive center once freed from a trap.
Optically Stimulated Luminescent Dosimeters (OSLD)
Optical Readout
Light emission is stimulated by illuminating the crystal. Illumination may be supplied by lasers, LEDs or fluorescent lamps. Luminescent photons may be measured with a photomultiplier tube or camera (CCD or CMOS).
The entire illumination process takes only about 1 second and releases only about 0.05% of the stored luminescence. This allows multiple readouts of the same measurement.
Light Source Rejection
The wavelength of illumination light source is different than that of the luminescent photons. This allows the photometer to reject photons emitting from the light source by means of a filter. Light source photons may also be rejected from measurement using temporal filtering in which the light source is flashed rapidly and measurements are made when the light is off.
Reusing OSLDs
Bleaching
Bleaching is the optical treatment of an OSLD with light from a halogen lamp, fluorescent lamp, or green LED (fitted w/yellow filter). Bleaching empties most trap centers and prepares the device for reuse.
Importantly, deep trap centers will not be emptied during bleaching. This causes a change in OSLD sensitivity over time. To avoid this, an OSLD may be annealed at 900°C to empty deep traps.
MOSFET Detectors
Metal Oxide Semiconductor Field Effect Transistor (MOSFET) detectors are a semiconductor based radiation detector used for small field dosimetry, in-vivo dosimetry, and profile measurements. The key advantage of a MOSFET detector is the extremely small collecting volume, but the devices are hampered by accumulated damage limiting their effective lives to about 100Gy.
Construction
A MOSFET consists of a semiconductor of three leads called the drain, the source and the gate. MOSFETs may be either P-channel or N-channel but P-channel MOSFETs are the most common in dosimeters.
In a P-channel MOSFET, the source and drain are constructed of a P-type (electron acceptor) semiconductor while the gate is constructed of an N-type (electron donor) semiconductor. In an N-channel MOSFET, the construction material types are reversed. The source and drain are embedded in a silicon substrate and are insulated from the gate by a layer of silicon dioxide (SiO2).
The sensitive volume of a MOSFET detector is defined by the silicon dioxide insulator which traps electron-hole pairs during irradiation. Thickness of the silicon dioxide insulator (typically 0.1-1μm) impacts the sensitivity of the MOSFET with thicker layers being more sensitive.
Theory of Operation
A MOSFET can be thought of as a voltage-controlled solid-state switch. When the voltage applied to the gate is too low, the region between the drain and the source contains an excess of electric charge which prevents current flow. When a negative voltage in excess of the threshold voltage is applied to the gate, these electrons are repelled and an inversion layer (sometimes called the channel) forms. This inversion layer allows a current to pass from drain to source.
Note: The above refers to P-channel MOSFET designs which are most common in dosimetry applications. For N-channel MOSFETs, it is excess holes that block the current with VG = 0 and when a positive voltage is applied to the gate (VG > VThreshold) an N-channel forms allowing current to flow.
Dose Measurement
MOSFETs determine dose through measurement in change of threshold voltage as follows:
- As a MOSFET is irradiated, electron-hole pairs are created in the SiO2 layer.
- Holes then migrate to the interface between the SiO2 and the N-type substrate.
- The increase in holes in the interface region retard the electric field induced by applying the negative voltage to the gate. This means that a greater voltage must be applied to the gate to induce current flow between the source and the drain (i.e. it takes a higher voltage to turn the MOSFET on).
- Change in threshold voltage is proportional to the change in number of trapped holes and thus, to absorbed dose.
Key Point: Unlike TLDs or OSLDs which are able to empty their traps, MOSFET detectors accumulate trap filling, reducing their effective usable life to ~100Gy. This property also allows them to act as a persistent dose record.
MOSFET Modes of Operation
Single-bias, single MOSFET: One MOSFET on a chip operated in active mode
- Active mode is applying a negative bias to gate during irradiation
- High temperature dependence
- Sensitivity varies over life of device
Unbiased single MOSFET: One MOSFET on a chip operated in passive mode
- Does not require attached leads during irradiation
- Frequently used as disposable detector
- High temperature dependence
- Instability of response
- Shorter linearity range the biased MOSFET
Dual-bias, dual MOSFET: Two MOSFETs on the same chip operate at different gate biases
- Improves the following
- Improves sensitivity
- Improves reproducibility
- Improves stability
- Reduces temperature dependence
Plastic Scintillators
Plastic scintillation dosimeters emit light when irradiated. Scintillation may be read out by a photodetector and correlated with absorbed dose. Plastic scintillators have several desirable qualities including near tissue equivalency, small size limited by the ability to detect small amounts of light, and energy independence in the MV range. These qualities make scintillators valuable detectors with applications in small field dosimetry, high resolution detector arrays (IMRT QA), and, because the collision stopping power ratio of plastic is similar to water, electron measurements. Problems, such as low signal-to-noise ratio and yellowing of plastic materials under irradiation, are likely to be addressed as these detectors develop.
Dose Measurement
- Radiation induces scintillation in the sensitive volume.
- Scintillation light is transmitted by the light pipe to the photodetector.
- Background measurement may be transmitted by a second parallel light pipe.
- Photodetector converts light to electrical signal
- Readout device removes background signal, determines dose
- Optical Filtration
- Temporal avoidance of Cherenkov signal
Construction
A plastic scintillation detector consists of a scintillating fiber (sensitive volume), a light pipe (used to transmit scintillation photons), a photodetector, and an electrometer for readout.
Scintillating fiber: the sensitive volume of the detector which emits light when irradiated. There are two main types of scintillating fibers:
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- Inorganic scintillators: Light arises from impurities or defects in crystal lattice.
- Organic scintillators: Light arises from excitation of the molecules themselves.
Scintillation Dopants: Many scintillators directly emit UV light which has a short attenuation length. This requires a fluorine dopant which converts the UV light to visible light (typically blue, green, or orange) for measurement.
Light pipe: a plastic or air-filled fiber that conducts the light from scintillator to photodetector. One common design is the dual light pipe design which improves signal-to-noise ratio by allowing removal of Chernekov Radiation. In the dual light pipe design, one light pipe will connects the scintillator to a photodetector. A second light pipe is placed near the scintillator, but shielded from it. The second light pipe connects to a second photodetector, providing a background (Cherenkov) reading.
Photodetector: digitizes photon signal for readout. Photodetector may be a photomultiplier tube, a photodiode or a camera (CCD or CMOS).
Signal-to-Noise Ratio
The biggest hurdle in use of plastic scintillators for precision dosimetry is the inherently low SNR of such detectors. There are three main sources of noise for scintillators:
Cherenkov Radiation: Cherenkov radiation arising in the inactive regions of the detector may account for as much as 3% of the signal for X-ray beams and 12% of the signal for electron beams. Three techniques are employed to reduce Cherenkov signal:
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- Dual light pipe design
- Second background light pipe runs parallel to the primary light pipe but does not connect to the scintillator. Background pipe signal may be subtracted from the primary light pipe signal reducing noise.
- Optical filtration
- High wavelength scintillator may be used with a high pass optical filter to reduce Cherenkov signal by 50%-82%.2
- Typical Cherenkov emission spectrum overlaps with the spectrum of scintillation light making wavelength discrimination impractical.
- High wavelength scintillator may be used with a high pass optical filter to reduce Cherenkov signal by 50%-82%.2
- Temporal discrimination
- The scintillation signal lasts about 500 ns longer than the signal from Cherenkov radiation. This may be used to distinguish between Cherenkov and scintillation signal even when the optical spectrum of the signals overlap.
- Dual light pipe design
Dark Current: the small electric current that flows through photodetectors even when no photons are entering the device. Dark current may generally be subtracted during measurement as a background signal.
Direct Interactions between Radiation and Detector: Ideally, the photodetector would only generate a signal when photons originating from the scintillation fiber enter the detector. Photodetector may, however, also generate a signal when quanta of the radiation of interest enters the photodetector and hence contributes to noise. Direct interactions are often minimized by removing the photodetector from the treatment volume. This presents further challenges however in that the light pipe must then be longer, which increases its signal attenuation and decreases collection efficiency.
Other Scintillation Materials
While plastic scintillators function well for the detection of gamma and beta particles, other scintillation materials are also commonly used.
- NaI(Tl): Low energy (up to about 360keV) photon detection.
- ZnS: Detection of alpha particles.
Neutron Detectors
Bubble Detectors
Bubble detectors consist of a clear plastic tube filled with a gel polymer. When fast neutrons are incident upon the polymer, they superheat the polymer creating bubbles. These bubbles remain suspended in the gel and may be counted to determine neutron fluence or absorbed dose. Bubble chambers are able to measure both fast and, with the addition of a neutron absorber such as chlorine, thermal neutrons. These features make bubble detectors well suited for in-vault neutron measurements.
Dose Measurement
- Bubble detector cap is unscrewed reducing the chamber pressure and allowing measurement.
- Detector is exposed to neutrons. Neutron interactions superheat the polymer forming bubbles.
- Bubbles are counted either by eye or optically. The number of bubbles is correlated with fluence or absorbed dose.
- Bubble detector is reset by screwing down cap causing the internal pressure to increase.
Advantages
- Useful for in-vault measurements
- Good photon rejection
- Suitable for high dose rate
- No down time
- Small size
- Usable for personnel dosimetry
- Reusable
- Available in various energy ranges from thermal to fast
Disadvantages
- May have strong temperature dependence (5% per C).
- This can be reduced by adding volatile liquid to the chamber whose vapor pressure compensates for temperature sensitivity.
- Bubble overlap makes readout difficult in high dose measurements.
- May lose sensitivity over time due to medium degradation.
Activation Foils
Activation detectors are materials which become radioactive when exposed to neutrons. Determination of neutron absorbed dose is made by measuring decay products of the activation foil (typically β- or γ-rays).
Dose Measurement
- Activation foil is placed in area with neutron presence for set period of time.
- Once exposure is complete, the foil must be taken to a sensitive detector to read out. This is typically done with thin window Geiger counter (for β- emitting foils) or a NaI Crystal or Gi(Li) detector (γ emitting foils).
- Measured foil activity is used to determine neutron fluence or dose.
Types of Activation Foils
Thermal Neutron Detectors
Operate by capturing neutrons, resulting in active daughter nuclide.
Most common detectors are Indium (In-116m, T1/2 = 54 minutes) and gold (Au-198, T1/2 = 2.7 days)
Threshold Activation Detectors
Detectors which require some minimum (threshold) energy to produce the desired reaction. Most often used to measure fast neutron flux.
The most common detector for fast neutron flux around accelerators is Sulfer-32 which has a 2.7MeV threshold for the 32S(n,p)32P reaction.
Moderated Foil Detectors
Another technique for measuring fast neutrons is to surround a thermal neutron detector with a moderator. This reduces the neutron’s energy, allowing detection.
A polyethylene cylinder 15cm in diameter surrounding the foil is used to reduce the fast neutron energy. This cylinder is covered with 0.5-0.8mm of Cadmium which absorbs all incident thermal neutrons. In this way, only fast neutrons are detected.
Advantages
- A variety of materials are available allowing measurement of thermal and fast neutrons.
- Inexpensive allowing many to be used.
- Typically reusable after decay.
- Small allowing for collection of geometric information.
- Collect integrating information.
Disadvantages
- Unwanted activation products can interfere with readout.
- No instantaneous readout.
- Requires additional counting equipment to read out.
- Readout must be performed prior to significant decay.
Knowledge Test
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