Plastic scintillation dosimetry is a technique allowing to measure and verify dose delivered into patient body during cancer treatment. Plastic scintillation dosimeter is composed with a few millimeters piece of plastic scintillator glued to optical fiber. Scintillator is covered by reflecting foil or painted with white reflecting paint to increase light signal entering to optical fiber. Scintillator converts absorbed radiation into visible light. Light emitted by scintillator is transported by optical fiber to light detector which converts it into electrical pulses collected by acquisition electronics. Part of dosimeter from scintillator to light detector is covered by light-tight housing to decrease influence of ambient light.
Advantages of plastic scintillation dosimeters are response independent of total dose, dose rate and angle of incidence, radiation hardness – resistance to signal loss up to 10,000 Gray of irradiation. In general the amount of light in plastic scintillators is not proportional to the absorbed dose. Therefore the application of plastic scintillators as dosimeters will require careful calibration.
Scintillators used in scintillation dosimetry should have density similar to density of human body about 1 g/cm3. Plastic scintillators manufactured from polystyrene (PS) or polyvinyltoluene (PVT) have density in range from 1.02 to 1.06 g/cm3. Other important property is chemical composition and low atomic number of scintillators components. Plastic scintillators have mainly carbon and hydrogen in their structure and its atomic composition is similar to water-based human tissue. Difference is that oxygen is main component in tissue and carbon is main component in plastic scintillator.
Optical properties of plastic scintillators are suitable for construction of dosimeters. The conversion efficiency of gamma radiation to visible light called light output is around 10,000 light photons per 1 MeV of absorbed gamma radiation. Maxima of emission spectra of most commercial scintillator types are centered in the range from 390 to 435 nm (blue light) where maximum of quantum efficiency of light detectors is the highest.
Rise and decay time of plastic scintillator for dosimeter construction is not a major factor contrary to the time-of-flight positron emission tomography and total-body PET scanners. In dose measurements charge of signal is collected. Many wavelength shifters (WLS) with various fluorescent decay times can be used in scintillators composition.
Plastic scintillators usually consist of three components. Polymer base absorbing radiation and two fluorescent substances dissolved in polymer. First substance absorbs deposited energy from the polymer and emits ultraviolet light. Second substance, WLS, absorbs ultraviolet light and emits blue light. UV-emitting fluorescent dye is added because polymer has low quantum efficiency of fluorescence. Adding few percent of this dye transfers excitation energy from polymer to UV-emitting dye, which has high fluorescence quantum yield. WLS is added up to 0.1% to shift emission spectra to visible light were polymers are more transparent and blue light in scintillator can travel up to several meters.
Most of the used wavelength shifters are well known as laser dyes and are incorporated in commercial plastic scintillators. For example BDB was used in SCSN-38 polystyrene scintillator and BBOT was added to SCSN-81 polystyrene scintillator, both made by Kuraray in Japan. POPOP is a WLS in UPS 923A polystyrene scintillator and in BC-400 polyvinyltoluene scintillator.
MDAC and DPA were used in polystyrene-based plastic scintillators with pulse shape discrimination capability. Lumogen F Violet 570 (LFV 570) was used in fluorescent polymer optical fibers and radiation hard polysiloxane scintillator. PBBO and DMPOPOP were incorporated in liquid scintillators.
Purpose of this work is to measure light output of polystyrene-based plastic scintillators with additions of fixed concentration of many chemical types of WLS. Manufactured scintillators have fixed concentration of one of the best UV-emitting dye and due to the addition of WLS emit light in blue part of visible spectra.