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Optical Plastic Components
Light guides and wavelength shifters provide useful modes of light collection for scintillators. They convey scintillation photons to the readout device. Key performance parameters are good optical transmission across a broad range of wavelengths and highly polished surfaces to promote total internal reflection.
Light Guides for plastic scintillators are commonly made from colorless acrylic plastic. We off two types, BC-800 Ultraviolet Transmitting and BC-802 Ultraviolet Absorbing. Both are cast from decolorized methyl methacrylate monomer. The difference is no ultraviolet absorbing additives are present in BC-800 but are present in BC-802.
Plastic light pipes often are used with plastic and liquid organic scintillators to:
- Provide a PMT mounting surface
- Guide the scintillating light to the photocathode
- Back-off the PMT where the scintillator is in a strong magnetic field
- Minimize pulse height variation
Specifications
- BC-800 UV Transmitting acrylic plastic is the recommended light material for scintillators having emission spectra in the near UV. These include NaI(Tl) and BC-418, BC-420 and BC-422 plastic scintillators.
- BC-800 is useful as a radiator for Cerenkov radiation
- BC-802 UV absorbing acrylic plastic is general-purpose, non-UVT, PMMA plastic for most scintillators and is the recommended light guide material for scintillators having emission spectra in the “blue” i.e. 420 nm.
Design Options
Typical light guide geometries include:
- Right Cylinders used when the light pipe diameter is the same as the scintillator diameter
- Tapered Cones are transition pieces between square-to-round or round -to-round cross-sections
- "Fish-Tail" are transition pieces from thin, rectangular cross-sections to round cross-sections
- Adiabatic provide the most uniform light transmission from the scintillator exit end to the PMT; the cross-section areas of the input and PMT faces are equal. These are typically best on wide scintillators and are the most expensive.
Available in cast sheets and rods having size tolerances typical of commercial acrylic products.
All light guides are custom designed to suit the particular scintillator geometry and experimental constraints. We recommend that, for scintillators <6 mm thick, a fish tail light pipe have a groove machined into its edge which joins the scintillator. The scintillator edge fits into the groove to improve the mechanical strength of the joint. Also, a disk that matches the diameter of the PMT is coupled to the light pipe’s other end to act as the PMT mounting surface.
The length of a fish tail or adiabatic light pipe is generally equal to the width of the scintillator, for scintillators 15.2 cm wide or greater.
The length of a fish tail or adiabatic light pipe is generally equal to the width of the scintillator, for scintillators 15.2 cm wide or greater.
Related Resources
Wavelength shifter (WLS) plastic bars are made of solid organic materials (polymers) which absorb light of one wavelength and re-emit the light at a longer wavelength. A portion of the re-emitted light is transmitted by total internal reflection along the WLS bar to be read out at the ends.
WLS characteristic makes them useful as light collecting elements (similar in function to light pipes) in calorimeter detectors. Their use reduces the overall size and complexity of the detector.
Often used with scintillator shower stacks, single WLS bars are air-coupled to a stack or plane of scintillator strips. The scintillation light is essentially turned 90° in a very compact structure. However, there is a typical 75% loss of signal amplitude in such a system.
Specifications
- BC-480 Blue-emitting shifts from near UV (300-360 nm) to 425 nm
- BC480 provides a significant improvement over standard UVT acrylic as a Cerenkov radiating medium. The more intense Cerenkov light produced at shorter wavelengths is absorbed and re-emitted in the 425nm spectral region. Minimum Detectable Cerenkov Wavelength, 260 nm
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BC-482A Green-emitting shifts from 420 to 500 nm
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for use with BC-408 and BC-412 plastic scintillators
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BC-484 Green-emitting shifts from 380 to 435 nm
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for use with BC-414 plastic scintillator
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Design Options
Supplied as cast sheets, rough cut or diamond milled or polished rods
Various standard cast sizes:- 0.5 - 1 mm thick 30 x 60 cm (0.020 – 0.040” thick 11.81” x 23.62”)
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1.5 mm thick 30 x 101 cm (0.059” thick 11.81” x 39.76”)
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2 mm thick 45 x 101 cm (0.078” thick 17.72” x 39.76”)
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3 mm thick 63 x 101 cm (0.118” thick 24.8” x 39.76”)
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5, 6.4, 10, 12.7, 20, 25, 38 and 50 mm thick 63 x 203 cm
(0.197”, 0.25”, 3.937”, 0.5”, 0.787”, 0.984”, 1.496”, 1.968” thick 24.8” X 79.92”) -
75, 100, 125, 150 mm thick 60 x 101 cm (2.953”, 3.937”, 4.921”, 5.906” thick 23.62” x 39.76”)
| BC-480 | BC-482A | BC-484 | Shared General Tech Data | ||
|---|---|---|---|---|---|
| Scintillation Properties | Blue | Green | Blue | Base | Polyvinyltoluene |
| Decay Time (ns) | 5 | 12 | 3 | Density (g/cc) | 1.03 |
| Light Attenuation Length, cm | 400 | 350 | Softening Point | 70ºC | |
| Absorption Peak, nm | 420 | 375 | Refractive index | 1.58 | |
| Wavelength of Max. Emission, nm | 425 | 494 | 430 | Expansion Coefficient (perºC,<67ºC) | ~7.8X10-5 |
| Solubility | Soluble in aromatic solvents, chlorinated solvents, acetone, etc. Unaffected by water, dilute acids, lower alcohols, alkalis and pure silicone fluids or grease. | ||||