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

  1. Radiation induces scintillation in the sensitive volume.
  2. Scintillation light is transmitted by the light pipe to the photodetector.
    • Background measurement may be transmitted by a second parallel light pipe.
  3. Photodetector converts light to electrical signal
  4. 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:

    • 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).

Dual light pipe design allows improved signal-to-noise ratio.

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:

    • 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.
    • 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.
Blue line: Combined signal of Scintillation and Cherenkov Red: Cherenkov radiation only Yellow: Scintillation only signal

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.

Comparison of Common Dosimeters

Device
-Accuracy
Common UsesAdvantagesLimitations
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

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