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The Tagger Microscope is expected to have a time resolution of 200 ps or better. This specification imposes a strict demand on the light pulse rise time uncertainty because it introduces trigger time uncertainty when Constant Fraction Trigger (CFT) point is used.
The Tagger Microscope is expected to have a time resolution of 200 ps or better. This specification imposes a strict demand on the light pulse rise time uncertainty because it introduces trigger time uncertainty when Constant Fraction Trigger (CFT) point is used.
= Scintillation Pulse Time Resolution =
= Scintillation Pulse Time Uncertainty =
The scintillations in the fiber are distributed exponentially with a characteristic decay time <math>\tau</math> = 2.7 for [http://www.detectors.saint-gobain.com/Data/Element/Product/product.asp?ele_ch_id=P0000000000000001909 BCF-10/20]. Since the variance is <math>\tau^2</math> (and is additive over independent events) the time uncertainty for N generated photons becomes <math>\tau/N^{1/2}</math>. Thus, in order of priority, this relation implies
The scintillations in the fiber are distributed exponentially with a characteristic decay time <math>\tau</math> = 2.7 for [http://www.detectors.saint-gobain.com/Data/Element/Product/product.asp?ele_ch_id=P0000000000000001909 BCF-10/20]. Since the variance is <math>\tau^2</math> (and is additive over independent events) the time uncertainty for N generated photons becomes <math>\tau/N^{1/2}</math>. Thus, in order of priority, this relation implies
# scintillating fiber with smallest decay time must be procured
# scintillating fiber with smallest decay time must be procured
# photon capture and detection efficiency must be optimized
# photon capture and detection efficiency must be optimized
= Photon Capture =
Taking the accepted photon yield in plastic scintillator to be 8000γ/MeV, the following considerations control the total photon capture:
* taking the energy deposition in plastic (~1 <math>g/cm^3</math>) to saturate at ~2 MeV/cm with 2 cm length of scintillator yields 4 MeV
* The critical angle in multi-clad BCF scintillating fibers is 27.4<sup>o</sup> C resulting in forward capture of 5.6%
* BCF-98 optical fiber transmits 83% and 92% for spectra of BCF-10 and BCF-20 respectively (Efficiency curves shown in adjacent figure)