Wavelength Shifters as (new) light sensors

Transcription

1 Wavelength Shifters as (new) light sensors Markus Voge, Marek Kowalski, Sebastian Böser (Bonn University, Bonn, Germany) PINGU workshop, Amsterdam, March 2011

2 The Goal Detection of (extra-galactic) supernova neutrinos Few Mton effective volume for 10 MeV neutrinos need dense instrumentation in large volume

3 The Goal Detection of (extra-galactic) supernova neutrinos Few Mton effective volume for 10 MeV neutrinos need dense instrumentation in large volume Use PMTs?

4 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!???

5 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!??? $ $ $ $ $

6 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!??? $ $ $ $ $ $ $ $

7 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?

8 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

9 Maybe like this... PMT WLS

10 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

11 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

12 Wavelength Shifters (WLS) Fresnel equations describe transition between media Reflectivity: Transmissivity: This governs fraction of photons entering/leaving WLS nice = 1.31 nwls = 1.58 Ice WLS WLS Ice Total internal reflection for θ>56

13 Wavelength Shifters (WLS) Direct hit Wavelength-dependent absorption Reflected hit 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

14 Wavelength Shifters (WLS) Direct hit Wavelength-dependent absorption Reflected hit 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 ~56% ~23% ~17% 0 ~41%

15 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!

16 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

17 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?

18 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

19 Optics Lab Bonn

20 Optics Lab Bonn

21 Optics Lab Bonn

22 Optics Lab Bonn

23 Optics Lab Bonn

24 Optics Lab Bonn Spectrum: Input Output

25 Optics Lab Bonn Wavelength-dependent Response: Capture efficiency

26 Optics Lab Bonn Light attenuation in WLS bar

27 WLS Simulation Wrote MC code to trace photons in WLS

28 WLS Simulation Distribution of photon travel time in WLS Counts Unreflected photons End of WLS WLS bar: 2 m long, 10 cm wide ( w/o considering fluorescence molecule decay time (~7-10 ns) )

29 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...

30 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