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

14th CBM collaboration meeting, Split, October 2009 C. Höhne/ M. Dürr 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

14th CBM collaboration meeting, Split, October 2009 C. Höhne/ M. Dürr 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 < < 660 nm (calibrated diode as reference) relative q.e. measurement for 150 nm < < 500 nm (compare measurement to reference PMT) stored at GSI under CO 2 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)

14th CBM collaboration meeting, Split, October 2009 C. Höhne/ M. Dürr 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]

14th CBM collaboration meeting, Split, October 2009 C. Höhne/ M. Dürr 5 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 absorption fluorescence Example: p-Terphenyl WLS films

14th CBM collaboration meeting, Split, October 2009 C. Höhne/ M. Dürr 6 Simulation P. Solevi (CERN): 0.8  m 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 ~ 350 nm) compares well with observation (~0.2)

14th CBM collaboration meeting, Split, October 2009 C. Höhne/ M. Dürr 7 Cherenkov light spectrum number of photons produced for a particle with charge ze, a radiator of length L and refraction index n( ) … and in dependence on energy E=h =hc/ … integrated (assume n(E) ~ const):

14th CBM collaboration meeting, Split, October 2009 C. Höhne/ M. Dürr 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

14th CBM collaboration meeting, Split, October 2009 C. Höhne/ M. Dürr 9 Gain factor in photoelectrons gain factor of appr. 1.7 for 200 nm < < 660 nm no aging effect from Feb 08 to Jun 08 use PM1 as example, same analysis/ studies done for PM2-PM5 PM1 – 235  g/cm 2 p- terphenyl coverage

14th CBM collaboration meeting, Split, October 2009 C. Höhne/ M. Dürr 10 Combined q.e. (abs. + rel. meas.) – log scale relative q.e. scaled to absolute q.e. by a factor determined for the -range ( /500) nm continuous drop of q.e. for ( ) nm (wls covered and uncovered) wls covered PMT: effect of p-terphenyl film or reference PMT (window material, wls coverage)?

14th CBM collaboration meeting, Split, October 2009 C. Höhne/ M. Dürr 11 Gain factor in photoelectrons although agreement in single -intervals is matter of discussions integral fits well and allows extension of measured range appr. factor 2 in gain compared to uncovered PMT! absorption edge for CO 2 ~ 175 nm mirror reflectivity drops at ~ 180 nm (prototype from FLABEG) be careful: … factor 2 compared to PMT with borosilicate glass! … maybe (30-50)% only for UV glass 275 nm 200 nm CBM RICH 180 nm → 6.9 eV 180 nm

14th CBM collaboration meeting, Split, October 2009 C. Höhne/ M. Dürr 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 C 6 H 5 C 6 H 4 C 6 H 5 TPB (1,1,4,4 tetraphenyl- 1,3-butadiene)

14th CBM collaboration meeting, Split, October 2009 C. Höhne/ M. Dürr 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  g/cm 2 TPB 92  g/cm 2

14th CBM collaboration meeting, Split, October 2009 C. Höhne/ M. Dürr 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!

14th CBM collaboration meeting, Split, October 2009 C. Höhne/ M. Dürr 15 Uncertainties (II) PM1 PM1 raw = untreated TPB coverage cleaning measurement of “cleaned A” and “… B” directly behind each other! ±10% uncertainty in q.e. measurement → ±6% difference in integral 10% error reasonable (partially additional uncertainties in normalization…)

14th CBM collaboration meeting, Split, October 2009 C. Höhne/ M. Dürr 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  g/cm 2 absorption spectrum (literature): sample preparation M. v. Stenis (CERN)

14th CBM collaboration meeting, Split, October 2009 C. Höhne/ M. Dürr 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

14th CBM collaboration meeting, Split, October 2009 C. Höhne/ M. Dürr 18 Thickness dependence no systematic dependence beyond uncertainties observed (see shaded box) ±10%

14th CBM collaboration meeting, Split, October 2009 C. Höhne/ M. Dürr 19 Thickness dependence (II) no systematic dependence beyond uncertainties observed (see shaded box) ±10%

14th CBM collaboration meeting, Split, October 2009 C. Höhne/ M. Dürr 20 Summary – WLS film studies absolute and relative q.e. measurements combined drop for > 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 < 150 (180) nm no significant thickness dependence observed for wls films > 63  g/cm 2 (~0.5  m 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)

14th CBM collaboration meeting, Split, October 2009 C. Höhne/ M. Dürr 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

14th CBM collaboration meeting, Split, October 2009 C. Höhne/ M. Dürr 22 extra slides

14th CBM collaboration meeting, Split, October 2009 C. Höhne/ M. Dürr 23 Relative quantum efficiency for < 200 nm q.e. measurement done relative to a reference PMT known from absolute q.e. measurement: q.e. for ( ) nm appr. the same with and without p-terphenyl expectation only holds approximately for ( ) nm details of reference PMT? … asked for …

14th CBM collaboration meeting, Split, October 2009 C. Höhne/ M. Dürr 24 Relative quantum efficiency (II) simple scaling possible between absolute and relative q.e. measurement? test for -range where abs. q.e. measurement available (raw PMTs): … holds approximately, however changes easily by 30% …nevertheless give it a try

14th CBM collaboration meeting, Split, October 2009 C. Höhne/ M. Dürr 25 Combined q.e. – lin scale (30-40)% differences easily between absolute and relative measurement however: large changes for relative q.e. itself

14th CBM collaboration meeting, Split, October 2009 C. Höhne/ M. Dürr 26 Thickness dependence? experience of A. Braem: p-terphenyl coverage with 100  g/cm  shows best results P. Baillon et al (NIM 126 (1975) 13): same effect if coverage > 25  g/cm 2 preliminary results of own study (see previous reports of P. Koczon) indicate largest gain for appr. 100  g/cm 2 coverage however: so far only q.e. in separate -bins compared directly large dependence on normalization → compare integrated gains

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