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

Status report onWLS studies and mirror development

P. Koczon, C. Höhne – GSI Darmstadt

M. Dürr – HS Esslingen

2 14th CBM collaboration meeting, Split, October 2009 C. Höhne/ M. Dürr

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


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 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)


Open questions

• quantification of gain (reference?)

• thickness dependence

• combination of absolute and relative q.e. measurements?

• uncertainties?

• understanding of plateau?

abso

lute

q.e

.

wavelength [nm]


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

absorptionfluorescence

Example: p-Terphenyl

http://omlc.ogi.edu/spectra/PhotochemCAD/html/p-terphenyl.html

WLS films


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)


Cherenkov light spectrum

)(

11

2222

2

n

zL

d

dN

• 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/

112222

2

)(

11370

)(

11

cmeV

EnL

Enc

zL

dE

dN

• … integrated (assume n(E) ~ const):

)(1

1 1222

22

1

EEnc

zLdE

dE

dNN

E

E


Detector efficiency

• for the number of measured photoelectrons detector efficiencies (i.e. quantum efficiency) have to be considered:

2

1

2

1

).(.det..

E

E E

ep dEEeqconstdEdE

dNN

E

• 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


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/cm2 p-terphenyl coverage


Combined q.e. (abs. + rel. meas.) – log scale

• relative q.e. scaled to absolute q.e. by a factor determined for the -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)?


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 CO2 ~ 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


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)


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/cm2

TPB 92 g/cm2


Uncertainties

• ±10% uncertainty in q.e. measurement

PM1

PM1 raw = untreated

TPB coverage

cleaning

measurement of “cleaned A” and “… B” directly behind each other!


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…)


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/cm2

http://omlc.ogi.edu/spectra/PhotochemCAD/html/p-terphenyl.html

absorption spectrum (literature):

sample preparation M. v. Stenis (CERN)


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


Thickness dependence

• no systematic dependence beyond uncertainties observed (see shaded box)

±10%


Thickness dependence (II)

• no systematic dependence beyond uncertainties observed (see shaded box)

±10%


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/cm2 (~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)


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


extra slides


Relative quantum efficiency

• for < 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 …


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):

?absrefPM

PMrel QEconst

I

IQE

?constI

IQE

PM

refPMabs

… holds approximately, however changes easily by 30%

…nevertheless give it a try


Combined q.e. – lin scale

• (30-40)% differences easily between absolute and relative measurement

• however: large changes for relative q.e. itself


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/cm2

• preliminary results of own study (see previous reports of P. Koczon) indicate largest gain for appr. 100 g/cm2 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- 200 nm cut off- abs. QE measurement

Monochromator-Vacuum- deep UV- relative QE

Documents

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