Analysis of Saint-Gobain Scintillating Optical Fibers for use as Wave Guides for Cerenkov- Ring-Imaging-MPPC Array By Chris Ketter 26 Sept. 2014

Introduction The Hamamatsu MPPCs make a good candidate for detecting single photons produced by Cerenkov radiation, while within a magnetic field. The drawback is a small detector area (1x1 mm) and large footprint (6x5 mm). R. Dolenec et al. investigated the use of cone-shaped light guides underneath their Aerogel (n=1.03) radiator. They improved their LGP from 1.6 to 3.7 γ/ring, a 230% improvement. This presentation investigates the possibility of using plastic fibers as light guides to “fan-out” Cerenkov- photons onto the MPPCs.

Overview What fibers are available from Saint- Gobain and what is their optical acceptance? What kind of radiator will meet the needs? –θ_cerenkov < θ_max of fibers? –works with acrylic adhesive? other adhesives? Luminosity predictions?

Plastic Scintillating Fibers Two general types available with an assortment of sizes and shapes. Cladding: –Single-Clad: NA = 0.58, so θ_max = 35° –Multi-Clad: NA = 0.74, so θ_max =48° Shapes: –Round: (3.44%)/(5.6%) trapping efficiency for single-/multi-clad respectively. –Square: (4.4%)/(7.3%) respectively. Others: –BCF-91A shifts blue to green with 12 ns decay time. –BCF-92 shifts blue to green with a 2.7 ns decay time. –BCF-98 is designed for waveguide applications.

Fiber Construction Core: –Polystyrene –n = 1.60 Cladding: –Acrylic –n = 1.49 –3% or 4% thickness (round or square, respectively) Secondary Cladding: –Fluor-Acrylic –n = 1.42 –1% or 2% thickness (round or square, respectively)

Wavelength-Shifting Fibers Emission Spectra MPPC peak sensitivity around 450~500 nm. Cernerkov phontons peak in UV range, and fall like 1/λ^2 Either of these would help shift Cerenkov photons toward the MPPC’s sensitive range. BCF-92 has it’s absorption peak nearer the UV region and is faster than its counterpart, BCF-91a. Drawback: wavelength shifting fibers are not scintillators, so the center would have to be reconstructed from tracker data. Reference: Saint-Gobain datasheet

Radiator Given that the maximum acceptance of the fibers is 35° and 48° for single- and multi-clad fibers, we want our index to be sufficiently small. The acceptance angle for our trackers will be ~A°, so we want to be able to accept muons skewed by up to ~A°. Theta should be less than 48° – A°. If theta is too small, then we’ll need a thicker radiator and hence will loose resolution on the ring diameter. nθ