1. Thin films technology for RICH detectors Functionality Production technologies Performance Presented at the CBM-RICH workshop 06-07 March 2006 André Braem, CERN, PH-DT2 department

2. A. Braem, CERN PH-DT2 CBM-RICH workshop March 2006 Thin films in RICH detectors The light yield is directly proportional to the performance of the coatings Reflective coating (R~90%) Anti-reflective coating (T 92%  ~98%) Photocathode (QE ~25%) Wavelength shifter

3. A. Braem, CERN PH-DT2 CBM-RICH workshop March 2006 Reflective coatings Substrate (glass, Be, plastics…) Adherence and barrier layer Metallic reflector Protection and reflectance enhancement dielectric layers

4. A. Braem, CERN PH-DT2 CBM-RICH workshop March 2006 Adherence and barrier under-layer  A thin (~20nm) layer of Chromium or Nickel is generally used to promote adherence on most substrates.  A barrier layer (SiOx, CrOx…) is mandatory when the substrate material risks to react with the metallic reflective layer. Inter-diffusion of aluminum (reflective layer) and gold (replicated substrate) : Cr + SiO diffusion barrier (CF mirror of CERES inner RICH) A B C 2 months at 100˚C : Au Cr + SiO Al + MgF 2 glass Al + MgF 2 Au glass Al + MgF 2 glass

5. A. Braem, CERN PH-DT2 CBM-RICH workshop March 2006 Metallic reflective coatings  Aluminum is the best metallic reflector for a broad band reflectivity in the far UV. 220 <  < 600 nm  R~90% in VUV the reflectivity of aluminum is strongly dependent on: -The production parameters such as vacuum quality, deposition rate etc.. -The substrate roughness (<1.5nm rms) -The surface oxidation  a protective layer is required. Magnesium fluoride is commonly used as single protective layer for VUV mirrors “Standard” VUV coatings for Cerenkov detectors: DELPHI, CERES, HADES, COMPASS…

6. A. Braem, CERN PH-DT2 CBM-RICH workshop March 2006 Metal multi-dielectrics reflective coatings  Reflectivity enhancement at given wavelength by exploiting interferences  Aluminum over coated with n pairs of transparent films of high (H) and low (L) refractive index. Al reflector Cr adherence layer n pairs LH photons substrate low index high index low index high index  inc.  Dielectric films like SiO 2, MgF 2 (L-materials) or HfO 2, Nb 2 O 5, TiO 2 (H- materials) are used.  Hard mirror surface can be achieved  good mechanical protection  Technology limited for  > 220nm due to the lack of H-materials which are transparent in VUV.

7. A. Braem, CERN PH-DT2 CBM-RICH workshop March 2006 Metal multi-dielectrics reflective coatings  Simulated reflectivity of aluminium + pairs of SiO2 – HfO2 layers optimized for = 300nm

8. A. Braem, CERN PH-DT2 CBM-RICH workshop March 2006 Reflective coating optimized for LHCb RICH2 = 0.176 = 0.149 620 413 310 248 206 Detection efficiency of an HPD detector (quartz window), with and without double reflection from the coated mirror,  inc. = 30º. Reflectivity of Al + 1 pair SiO 2 /HfO 2 on glass,   nc. = 30º Absorbance in HfO 2 film ! [nm] Measurement Simulation

9. A. Braem, CERN PH-DT2 CBM-RICH workshop March 2006 Anti reflective single layer coatings  MgF2 is generally selected for single layer broad band AR coatings On quartz: Optimum n 1 = 1.22 ! n MgF 2 (250nm) =1.412 !  The surface reflectivity is reduced by a factor 2.  Low refractive index (1.2 < n < 1.4) can be obtained with porous sol gel silica coatings. Best performance if

10. A. Braem, CERN PH-DT2 CBM-RICH workshop March 2006 Anti reflective multi-layers coatings  Pairs of low and high refractive index materials  Many solutions are available in coating industry for UV-VIS light.  Low residual reflectivity but in a reduced band width !

11. A. Braem, CERN PH-DT2 CBM-RICH workshop March 2006 Coating technology Thickness monitor Substrate (rotation)  Metals and dielectrics are evaporated in a high vacuum deposition plant. Aluminium is evaporated from a Tungsten filament Dielectrics are evaporated from an electron gun source

12. A. Braem, CERN PH-DT2 CBM-RICH workshop March 2006 Deposition parameters LayerRate [nm/s]Thickness [nm]Pressure [mb] Cr0.2101x10 -7 Al>20852x10 -7 MgF 2 1.531 1 2x10 -7 SiO 2 0.2 38 2 2x10 -5 (O 2 ) HfO 2 0.228 2 2x 10 -5 (O 2 ) 1 Optimized for =160nm 2 Optimized for =275nm - Substrate roughness - Residual pressure - Aluminium deposition rate - Delay between Aluminium and MgF 2 depositions  Well known technology available from most industrial partners But for optimal reflectivity in VUV some critical parameters must be well under control:

13. A. Braem, CERN PH-DT2 CBM-RICH workshop March 2006 Series production of mirrors for LHCb RICH2 @ CERN Average reflectivity (250-350 nm)

14. A. Braem, CERN PH-DT2 CBM-RICH workshop March 2006 VUV reflectivity measurements  Essential for the production of VUV mirrors ! 250225207190183175155 nm

15. A. Braem, CERN PH-DT2 CBM-RICH workshop March 2006 Photocathodes for RICH detectors 1). Alkali halide in gas photodetectors - Large area CsI reflective photocathodes - Sensitive in the 7.75 - 6.2 eV range - Robust and transferable (in moisture free environment) e-e- h 2). Bi-alkali Antimonide in vacuum tubes - Semitransparent encapsulated photocathodes - Sensitive in the 2 – 4 eV range (K 2 CsSb + UV extended glass window) h e-e-

16. A. Braem, CERN PH-DT2 CBM-RICH workshop March 2006 HPDs : PC and detector assembly inside vacuum chamber CsI PC: coating under vacuum and detector assembly under gas PCB production CsI depositionPC transfer & storage detector assembly & operation Need state of the art technologies (UHV, chemistry, thin film coating, vacuum sealing, encapsulated electronics) Operational photon-detector Focusing electrodes Silicon sensor FE electronics Vacuum seal Photocathode processing Photocathodes for RICH detectors

17. A. Braem, CERN PH-DT2 CBM-RICH workshop March 2006 CsI Photocathodes production Remote controlled enclosure box protective box pcb substrate  Uniform deposition of 300 nm CsI Deposition rate: ~1nm/s Substrate temperature: 60˚C Pressure ~6 x 10 -7 mb Heat post-treatment: 60˚C, 8-12hrs  CsI PC transferred in a protection box under Argon atmosphere after quality control 4 CsI sources + shutters Thickness monitor PCB substrate  Vacuum evaporation of CsI powder from 4 sources.

18. A. Braem, CERN PH-DT2 CBM-RICH workshop March 2006 CsI photo-current measurements QE obtained from test beam measurement on 6 CsI PCs Photo-current scan on PC46: Mean value = 3.71 min-max variation 6% All CsI data from H.Hoedlmoser CERN ALICE/HMPID

19. A. Braem, CERN PH-DT2 CBM-RICH workshop March 2006 Series production of CsI PCs Initial current level before heat enhancement phase Current level after enhancement phase

20. A. Braem, CERN PH-DT2 CBM-RICH workshop March 2006 Development of HPD vacuum tubes at CERN 5” HPD Indium joint h e-e- Si sensor Front end electronicsIn silicon: 3.6eV  1 e/h pair 20keV  5000 e/h dV = 20kV E e = 20 keV 10” HPD PET HPDSpherical HPD Bi-alkali photocathode

21. A. Braem, CERN PH-DT2 CBM-RICH workshop March 2006 The HPD development plant  “External” photocathode process  Ultra-high vacuum technology

22. A. Braem, CERN PH-DT2 CBM-RICH workshop March 2006 Radial dependence of HPD (PC68) QE for =230, 290 and 350 nm. QE of a K 2 CsSb photocathode (HPD PC87) Performance of bi-alkali photocathodes

23. A. Braem, CERN PH-DT2 CBM-RICH workshop March 2006 Wavelength shifters coatings  A dedicated plant has been set-up for coatings on PMTs Vacuum evaporation from a molybdenum crucible: Pressure ~5x10 -5 mb Thickness >1  m Rate > 10 nm/s Vaporization temp. < 200˚C  Weak mechanical resistance A protective layer of 30nm MgF 2 is required !  P-Terphenyl: Absorbtion range 110-360 nm ; emission peak at 385 nm  Inhomogeneous coatings on glass surfaces  Alternative coating method:  Solvent spray with transparent binder

24. A. Braem, CERN PH-DT2 CBM-RICH workshop March 2006 Performance of P-Terphenyl Publication of G.J.Davis NIM B 117(1996) 421-427 Ext. QE: N ph emitted (4  ) / N ph incident External QE of p-terphenyl at ~optimal thickness P-terphenyl evaporated at pressure of 1-1.5 x 10 -1 Torr Deterioration in efficiency of p-terphenyl after a six month period (175 nm incident light)

25. A. Braem, CERN PH-DT2 CBM-RICH workshop March 2006 Summary and conclusions  The light yield is directly proportional to the performance of the coatings Coatings exist for high reflectivity, AR, photosensitivity, WLS For detectors in the visible/near UV range standard industrial solutions exist. For applications in the far UV / VUV technical challenge and cost increase drastically. Reliability and long term stability become issues.  Functional coatings play an important role at various places in a Cherenkov detector. The light yield is directly proportional to the performance of the coatings.  Coatings exist for high reflectivity, Anti-reflection, photosensitivity and Wavelength shifting.  For detectors in the visible/near UV range standard industrial solutions are available.  For applications in the far UV / VUV technical challenge and cost increase drastically. Reliability and long term stability become issues.

26. A. Braem, CERN PH-DT2 CBM-RICH workshop March 2006 Spares

27. A. Braem, CERN PH-DT2 CBM-RICH workshop March 2006 CsI quality control: VUV-scanner Reference CsI PM PC current reading D 2 light source + 100V PM Translation stage  2d scan of photo-current across the whole photocathode Relative measurement to a reference CsI photomultiplier

28. A. Braem, CERN PH-DT2 CBM-RICH workshop March 2006 CsI deposition performed at 60°C All PCs have similar initial response Heat post treatment at 60°C for 24hrs The PC response increases 20-50% during the first hours. Heat post-treatment

29. A. Braem, CERN PH-DT2 CBM-RICH workshop March 2006 Heat post-treatment Decreasing response observed on some PCs !  The presence of residual water in the vacuum chamber is believed to strongly influence the photo-emission properties of the highly hygroscopic CsI film !