Wavelength Shifters as (new) light sensors

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)




● Use PMTs?

Markus Voge (Bonn)




● Use PMTs?● Need many!

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Markus Voge (Bonn)





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Markus Voge (Bonn)





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Markus Voge (Bonn)




● Increase photosensitive area without PMTs?

Markus Voge (Bonn)




● 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


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


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● Spectrum:

Input

Output

Markus Voge (Bonn)

Optics Lab Bonn● Wavelength-dependent Response:

Cap

ture

effi

cie n

cy

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Optics Lab Bonn● Light attenuation in WLS bar

Markus Voge (Bonn)

WLS Simulation● Wrote MC code to trace photons in WLS

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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

Wavelength Shifters as (new) light sensors

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