Status report on WLS studies and mirror development

1. Status report onWLS studies and mirror development P. Koczon, C. Höhne – GSI Darmstadt M. Dürr – HS Esslingen

2. Outline • Wavelength shifter studies • reminder of status, open questions • quantification of gain • extension of absolute q.e. measurements into the UV region • uncertainties • thickness dependence • fluorescence, SEM measurements • Mirror development for the CBM RICH detector • reminder and status

3. Status: WLS studies • February 2008 • wls coverages on 5 Photonis XP3102 photomultipliers tested at CERN in cooperation with A. Braem, M. v. Stenis, C. Joram (P. Koczon) • TPB, p-terphenyl (+ “Yellow-X”) applied with different thicknesses • absolute q.e. measurement for 200 nm < l < 660 nm (calibrated diode as reference) • relative q.e. measurement for 150 nm < l < 500 nm (compare measurement to reference PMT) • stored at GSI under CO2 atmosphere • June 2008 • q.e. of stored PMTs remeasured (ageing effect?) • thickness dependence study for p-terphenyl coverage • q.e. measurement of Teflon based wls films (proposal of IHEP Protvino)

4. Open questions • quantification of gain (reference?) • thickness dependence • combination of absolute and relative q.e. measurements? • uncertainties? • understanding of plateau? absolute q.e. wavelength [nm]

5. WLS films Wavelength shifting films – principle and application • Organic molecules absorbing in the short (UV) wavelength region • Strong fluorescence in visible region • Application via evaporation, spin coating/ dip coating Example: p-Terphenyl absorption fluorescence http://omlc.ogi.edu/spectra/PhotochemCAD/html/p-terphenyl.html

6. Simulation • P. Solevi (CERN): 0.8 mm WLS coverage on top of Borosilicate glass • appr. 60% of fluorescence photons reach PMT window with a difference of 0.3 mm (RMS) to absorption point of Cherenkov photon • → expect q.e. of 0.6∙0.3=0.18 for these photons (assume shift to l ~ 350 nm) • compares well with observation (~0.2)

7. Cherenkov light spectrum • number of photons produced for a particle with charge ze, a radiator of length L and refraction index n(l) • … and in dependence on energy E=hn=hc/l • … integrated (assume n(E) ~ const):

8. Detector efficiency • for the number of measured photoelectrons detector efficiencies (i.e. quantum efficiency) have to be considered: • the gain of using wls films can be quantified by comparing the integrals with and without their usage • normalize integral without wls-film to 1 • normalize integral with wls film to integral without

9. Gain factor in photoelectrons • gain factor of appr. 1.7 for 200 nm < l < 660 nm • no aging effect from Feb 08 to Jun 08 use PM1 as example, same analysis/ studies done for PM2-PM5 PM1 – 235 mg/cm2 p-terphenyl coverage

10. Combined q.e. (abs. + rel. meas.) – log scale • relative q.e. scaled to absolute q.e. by a factor determined for the l-range (350-450/500) nm • continuous drop of q.e. for (150-200) nm (wls covered and uncovered) • wls covered PMT: effect of p-terphenyl film or reference PMT (window material, wls coverage)?

11. Gain factor in photoelectrons • although agreement in single l-intervals is matter of discussions integral fits well and allows extension of measured range • appr. factor 2 in gain compared to uncovered PMT! be careful: … factor 2 compared to PMT with borosilicate glass! … maybe (30-50)% only for UV glass • absorption edge for CO2 ~ 175 nm • mirror reflectivity drops at ~ 180 nm (prototype from FLABEG) CBM RICH 180 nm → 6.9 eV 180 nm 200 nm 275 nm

12. Comparison of TPB and p-terphenyl • in literature both substances were reported so have a similar effect • clear difference seen in wavelength dependence p-Terphenyl C6H5C6H4C6H5 TPB (1,1,4,4 tetraphenyl-1,3-butadiene)

13. Comparison of TPB and p-terphenyl • difference can be quantified in gain factor: • 1.25 (TPB) compared to 1.65 (p-Terphenyl) pTer 100 mg/cm2 TPB 92 mg/cm2

14. Uncertainties • ±10% uncertainty in q.e. measurement PM1 PM1 raw = untreated TPB coverage cleaning measurement of “cleaned A” and “… B” directly behind each other!

15. Uncertainties (II) • ±10% uncertainty in q.e. measurement → ±6% difference in integral • 10% error reasonable (partially additional uncertainties in normalization…) PM1 PM1 raw = untreated TPB coverage cleaning measurement of “cleaned A” and “… B” directly behind each other!

16. Thickness dependence – Fluorescence measurements • fluorescence measurements with excitation at 230 nm and 280 nm • enhanced intensity for thicker films (results at 280 nm similar), almost all UV photons are absorbed for films > 100 mg/cm2 sample preparation M. v. Stenis (CERN) absorption spectrum (literature): http://omlc.ogi.edu/spectra/PhotochemCAD/html/p-terphenyl.html

17. Thickness dependence – SEM measurements • measurement of layer thickness • increased surface roughness for thicker layers • increased light scattering → less transparency for visible photons SEM measurements: J. Kraut, HS Esslingen

18. Thickness dependence • no systematic dependence beyond uncertainties observed (see shaded box) ±10%

19. Thickness dependence (II) • no systematic dependence beyond uncertainties observed (see shaded box) ±10%

20. Summary – WLS film studies • absolute and relative q.e. measurements combined • drop for l > 200 nm to be understood: effect of p-terphenyl film or reference PMT (window material, wls coverage) • gain of factor 2 in photoelectrons measured with p–Terphenyl coverage of PMT window (borosilicate) for l < 150 (180) nm • no significant thickness dependence observed for wls films > 63 mg/cm2 (~0.5 mm layer thickness) although expected from fluorescence measurements • next steps • fluorescence decay time? • application techniques, mechanical stability • long term stability (re-measure stored PMTs) • crosstalk on MAPMT H8500 promising results from MAPMT test (J. Eschke)

21. Mirror development • Status CBM collaboration meeting, March 2009: • FLABEG GmbH, Germany • good reflectivity of mirror samples • surface inhomognities on cm scale • Compas, Czech Republic • mirror prototype produced (3 mm thickness) • first tests performed on surface homogenity, reflectivity: promising! Early summer 2009: Mirror prototypes ordered – still waiting for delivery

22. extra slides

23. Relative quantum efficiency • for l < 200 nm q.e. measurement done relative to a reference PMT • known from absolute q.e. measurement: q.e. for (300-600) nm appr. the same with and without p-terphenyl • expectation only holds approximately for (300-450) nm details of reference PMT? … asked for …

24. Relative quantum efficiency (II) • simple scaling possible between absolute and relative q.e. measurement? • test for l-range where abs. q.e. measurement available (raw PMTs): … holds approximately, however changes easily by 30% …nevertheless give it a try

25. Combined q.e. – lin scale • (30-40)% differences easily between absolute and relative measurement • however: large changes for relative q.e. itself

26. Thickness dependence? • experience of A. Braem: p-terphenyl coverage with 100 mg/cm2 shows best results • P. Baillon et al (NIM 126 (1975) 13): same effect if coverage > 25 mg/cm2 • preliminary results of own study (see previous reports of P. Koczon) indicate largest gain for appr. 100 mg/cm2 coverage • however: so far only q.e. in separate l-bins compared directly • large dependence on normalization • → compare integrated gains

27. Piotr Koczoń,GSI CBMOctober 2008 • Monochromator • Vacuum • deep UV • relative QE • QE Test Bench (HPD) • Air • 200 nm cut off • abs. QE measurement