Analysis of Saint-Gobain
Scintillating Optical Fibers for use
as Wave Guides for CerenkovRing-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” Cerenkovphotons onto the MPPCs.
Overview
• What fibers are available from SaintGobain 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
Reference: Saint-Gobain datasheet
• 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.
Radiator
θ
n
1.10
23.9
1.20
33.0
1.30
39.3
1.40
44.1
1.50
47.9
1.60
51.0
1.70
53.7
1.80
56.0
1.90
58.0
2.00
59.8
• Given that the maximum acceptance
of the fibers is 35° and 48° for singleand 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.