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Markus Voge (Bonn)
Wavelength Shifters as (new)
light sensors
Markus Voge, Marek Kowalski, Sebastian Böser(Bonn University, Bonn, Germany)
PINGU workshop, Amsterdam, March 2011
Markus Voge (Bonn)
The Goal● Detection of (extra-galactic) supernova
neutrinos● Few Mton effective volume for 10 MeV
neutrinos → need dense instrumentation in large volume
Markus Voge (Bonn)
The Goal● Detection of (extra-galactic) supernova
neutrinos● Few Mton effective volume for 10 MeV
neutrinos → need dense instrumentation in large volume
● Use PMTs?
Markus Voge (Bonn)
The Goal● Detection of (extra-galactic) supernova
neutrinos● Few Mton effective volume for 10 MeV
neutrinos → need dense instrumentation in large volume
● Use PMTs?● Need many!
???
Markus Voge (Bonn)
The Goal● Detection of (extra-galactic) supernova
neutrinos● Few Mton effective volume for 10 MeV
neutrinos → need dense instrumentation in large volume
● Use PMTs?● Need many!
???
$ $$
$$
Markus Voge (Bonn)
The Goal● Detection of (extra-galactic) supernova
neutrinos● Few Mton effective volume for 10 MeV
neutrinos → need dense instrumentation in large volume
● Use PMTs?● Need many!
???
$ $$
$$$$$
Markus Voge (Bonn)
The Goal● Detection of (extra-galactic) supernova
neutrinos● Few Mton effective volume for 10 MeV
neutrinos → need dense instrumentation in large volume
● Increase photosensitive area without PMTs?
Markus Voge (Bonn)
The Goal● Detection of (extra-galactic) supernova
neutrinos● Few Mton effective volume for 10 MeV
neutrinos → need dense instrumentation in large volume
● Increase photosensitive area without PMTs?● Idea: Use wavelength shifting plastic sheets to
collect photons● Guide photons onto fewer and smaller PMTs
Markus Voge (Bonn)
WLS
PMT
Maybe like this...
Markus Voge (Bonn)
Wavelength Shifters (WLS)● Std. technology in particle detectors to detect scintillation light
● Plastic sheet (PVT) doped with “ aromatic” molecules
● Molecules are excited by charged particle (= scintillation) or photon (= fluorescence) and re-emit photon with larger wavelength isotropically
www.eljentechnology.com
Markus Voge (Bonn)
Wavelength Shifters (WLS)● Concept:
● Blue (Cherenkov) photon enters WLS
● High attenuation for blue photon (~1-2 mm att. length)→ absorption→ fluorescence
● Green photon is emitted● Low attenuation for green photon
(few m att. length)● Green photon transmitted via total
internal reflection
● Result: large sensitive area, small readout area
www.eljentechnology.com
Markus Voge (Bonn)
Wavelength Shifters (WLS)● Fresnel equations describe
transition between media● Reflectivity: Transmissivity:
● This governs fraction of photons entering/leaving WLS
Total internal reflectionfor θ>56°
Ice → WLS WLS → Ice
nice
= 1.31
nWLS
= 1.58
Markus Voge (Bonn)
Wavelength Shifters (WLS)● Wavelength-dependent
absorption● Isotropic re-emission of
fluorescence photon with ~86% efficiency (otherwise, energy goes to phonons)
● Fraction ΔΩ/4π of photons reaching readout surface depends on geometry and material
● Tubular geometry (like infinitely wide box): significant fraction (~20-40%) reaches readout surface
www.eljentechnology.com
Reflected hit
Direct hit
Markus Voge (Bonn)
Wavelength Shifters (WLS)● Wavelength-dependent
absorption● Isotropic re-emission of
fluorescence photon with ~86% efficiency (otherwise, energy goes to phonons)
● Fraction ΔΩ/4π of photons reaching readout surface depends on geometry and material
● Tubular geometry (like infinitely wide box): significant fraction (~20-40%) reaches readout surface
0~23%
~56%
Direct hit
Reflected hit
~17%
~41%
Markus Voge (Bonn)
Possible geometries
Box (directly in ice)
Box in glass shell
Tube (directly in ice)
Best option: high difference in refractive indices, no absorption in glass shell, no radioactive background from glass
Tube in glass shell
Probably, glass shell needed for mechanical stability under 1-2 km of ice!
Markus Voge (Bonn)
Current activities● Selection of most efficient WLS material● Measurements of WLS sheets in optics lab to test feasibility
● Write small simulation code for better understanding of WLS
Markus Voge (Bonn)
Available materials● St. Gobain (formerly Bicron):
● BC-480: 330 nm → 425 nm
● BC-482A: 420 nm → 500 nm
● BC-484: 380 nm → 435 nm
● Eljen Technology:● EJ-280: 420 nm → 500 nm
● Evonik Industries?
http://prod.detectors.saint-gobain.com/
http://www.eljentechnology.com/
http://www.plexiglas-shop.com/
Markus Voge (Bonn)
Optics Lab Bonn● Small lab setup with Xe lamp, monochromator
● Old samples of WLS bars from Zeus experiment (>20 years old)
● Photo diode, spectrograph for readout● Measure spectra, efficiency, attenuation...● Waiting for new WLS samples
Markus Voge (Bonn)
Optics Lab Bonn
Markus Voge (Bonn)
Optics Lab Bonn
Markus Voge (Bonn)
Optics Lab Bonn
Markus Voge (Bonn)
Optics Lab Bonn
Markus Voge (Bonn)
Optics Lab Bonn
Markus Voge (Bonn)
Optics Lab Bonn● Spectrum:
Input
Output
Markus Voge (Bonn)
Optics Lab Bonn● Wavelength-dependent Response:
Cap
ture
effi
cie n
cy
Markus Voge (Bonn)
Optics Lab Bonn● Light attenuation in WLS bar
Markus Voge (Bonn)
WLS Simulation● Wrote MC code to trace photons in WLS
Markus Voge (Bonn)
WLS Simulation● Distribution of photon travel time in WLS
( w/o considering fluorescence molecule decay time (~7-10 ns) )
WLS bar: 2 m long, 10 cm wide
End of WLS
Unreflected photons
Cou
nts
Markus Voge (Bonn)
To do...● Get recent WLS samples for measurement● Test different materials● Work out geometry of modules● Test in ice/under pressure etc.?● Write more general simulation code to simulate more complex scenarios
● ...
Markus Voge (Bonn)
Summary● We want cost-effective large photosensitive area
● WLS are an interesting concept● Using the right geometry and material, promising efficiencies of several 10% are feasible