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Organic scintillators – light yield

Organic scintillators – light yield. Attenuation lengths are sensitive to optical clarity, surface quality (internal reflection) and impurities often deal with >meter lengths of scintillators, so these optical effects are important Aging and crazing (microcracks in surface) diminish light yield

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Organic scintillators – light yield

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  1. Phys 521A

  2. Organic scintillators – light yield • Attenuation lengths are sensitive to optical clarity, surface quality (internal reflection) and impurities • often deal with >meter lengths of scintillators, so these optical effects are important • Aging and crazing (microcracks in surface) diminish light yield • Radiation damage is complex; it affects both light yield and attenuation Phys 521A

  3. Examples of organic scintillators • Tiles are usually wrapped in reflective material and light-tight tape • Light guide to couple to PMT • Fiber readout also used Phys 521A

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  7. Inorganic scintillators • Higher densities, lower light yields relative to organics • Crystals – slow and expensive to grow • Light collection efficiency – optical transparency, surface quality, scattering centres • Signal develop-ment can be slow; 100s of ns organic ~1 ~40 ~2 ~80 ~2 tune ~25 no Phys 521A

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  9. Examples of inorganic scintillators BaF2 CsI(Tl) NaI(Tl) Lead Tungstate Phys 521A

  10. Ionization chambers Phys 521A

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  14. Charge transport in gases • Recall primary/total ionization (nT ~ 2-3 x nP) • Distribution of ejected electrons ~ 1/E2 (E=electron energy) • Ionization clusters form around energetic secondary electrons; can degrade time and position resolution Probability of producing an electron of energy≥E (left scale), and range of electrons inArgon at NTP (right scale) Phys 521A

  15. Drift velocity • Liberated electrons accelerated by applied field, but have small mean-time-between-collisions τ, sovd = e E τ / m << vrms (think Ohm’s law) • More sophisticated model (ω = Larmor freq eB/m): • Drift velocity depends on τ, i.e. on collisional cross-sections; very sensitive to gas mixture, contaminants • ExB force causes “Lorentz angle” (see above); Phys 521A

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