1 RICH 2013 Long term experience with CsI photocathodes in gas photon detectors S. Dalla Torre on behalf of the Trieste-COMPASS group

2 RICH 2013 RICH2013, Hayama 1-6/12/2013 Experience with CsI photocathodesSilvia DALLA TORRE OUTLINE  INTRODUCTION  DETERMINING THE EVOLUTION OF THE CsI EFFECTIVE QE  The principle  The set-up : COMPASS RICH-1  DATA, ANALYSIS, RESULTS  CONCLUSIONS

3 RICH 2013 RICHes & GASEOUS PDs RICH2013, Hayama 1-6/12/2013 Experience with CsI photocathodesSilvia DALLA TORRE Gaseous PDs offer:   operation in magnetic field   the most effective solution for what concerns the cost of large detector area application   minimum material budget when the photon detectors have to seat in the experiment acceptance an essential ingredient for RICH development OMEGA DELPHI CLEO III 1 st generation: converting vapours (TMAE, TEA) 2 nd generation: MWPCs with CsI PCs COMPASS RICH-1 ALICE-HMPID 3 nd generation: MPGDs with CsI PCs PHENIX HBD THGEM- based PDs (COMPASS,ALICE)

4 RICH 2013 CsI Sensitivity RICH2013, Hayama 1-6/12/2013 Experience with CsI photocathodesSilvia DALLA TORRE The best quartz cuts here A. Di Mauro, NIM A 525 (2004) 173. VUV domain only

5 RICH 2013 CsI & GAS 1/2 RICH2013, Hayama 1-6/12/2013 Experience with CsI photocathodesSilvia DALLA TORRE  Photoelectron extraction from a CsI film, the role of gas and E M. Alexeev et al., NIM A 623 (2010) 129 C. D. R. Azevedo et al., 2010 JINST 5 P01002 A. A.Breskin et al., B. B.NIM A 367 (1995) 342 VACUUM

6 RICH 2013 CsI & GAS 2/2 RICH2013, Hayama 1-6/12/2013 Experience with CsI photocathodesSilvia DALLA TORRE Why CsI ?  the most robust solid-state photoconverting material among the typical ones, thanks to a higher work function (PC stability under ion bombardment)   CsI exhibits a relative chemical robustness against oxygen and water, as compared to other photoconverters:  its quantum efficiency is preserved in case of short exposure to air  what about long term PC stability ? detector stability over years Accelerated ageing test, H. Hoedlmoser et al., NIM A 574 (2007) mC/cm mC/cm 2

7 RICH 2013 RICH2013, Hayama 1-6/12/2013 Experience with CsI photocathodesSilvia DALLA TORRE OUTLINE  INTRODUCTION  DETERMINING THE EVOLUTION OF THE CsI EFFECTIVE QE  The principle  The set-up : COMPASS RICH-1  DATA, ANALYSIS, RESULTS  CONCLUSIONS

8 RICH 2013 LONG TERM CsI-PC STABILITY RICH2013, Hayama 1-6/12/2013 Experience with CsI photocathodesSilvia DALLA TORRE Principle  Determine the rate of converted photons vs time  COMPASS RICH-1: MWPCs with CsI PCs   Extract the mean number of detected photons per particle from the data collected during six years of COMPASS data taking  Determine the rate evolution

9 RICH 2013 COMPASS RICH-1 in short 1/5 RICH2013, Hayama 1-6/12/2013 Experience with CsI photocathodesSilvia DALLA TORRE hadron PID from 3 to 60 GeV/c acceptance: H: 500 mrad V: 400 mrad trigger rates: up to ~100 KHz beam rates up to ~10 8 Hz material in the beam region: 2.4% X o material in the acceptance: 22% X o detector designed in 1996 in operation since 2002 upgraded in 2006 total investment: ~ 4 M € radiator: C 4 F 10 5 m 6 m 3 m MWPC’s + CsI UV mirror wall Al vessel radiator gas: C 4 F 10 with two kind of photon detectors providing: PMT’s beam pipe

10 RICH 2013 COMPASS RICH-1 in short 2/5 RICH2013, Hayama 1-6/12/2013 Experience with CsI photocathodesSilvia DALLA TORRE  -K separation, CL > 95% up to 45 GeV/c  -K separation, CL > 90% up to 60 GeV/c

11 RICH 2013 COMPASS RICH-1 in short 3/5 RICH2013, Hayama 1-6/12/2013 Experience with CsI photocathodesSilvia DALLA TORRE 12 outer CsI cathodes organised in 2-cathode MWPC

12 RICH 2013 COMPASS RICH-1 in short 4/5 RICH2013, Hayama 1-6/12/2013 Experience with CsI photocathodesSilvia DALLA TORRE About MWPCs with CsI photocathodes in COMPASS RICH-1   Operated in pure methane   Oxygen and water vapour contamination always below 5 ppm   Low amplification imposed by CsI (< 5 x 10 4)   FE chip: APV   FE ballistic deficit ~ 33%

13 RICH 2013 COMPASS RICH-1 in short 5/5 RICH2013, Hayama 1-6/12/2013 Experience with CsI photocathodesSilvia DALLA TORRE RATES IN THE EXTERNAL DETECTORS during SPS spills: Photoelectrons < 10 Hz / cm 2 MIPs < 10 Hz / cm 2  Integrated charge over 6 years < 10  C/cm 2 A typical event

14 RICH 2013 RICH2013, Hayama 1-6/12/2013 Experience with CsI photocathodesSilvia DALLA TORRE OUTLINE  INTRODUCTION  DETERMINING THE EVOLUTION OF THE CsI EFFECTIVE QE  The principle  The set-up : COMPASS RICH-1  DATA, ANALYSIS, RESULTS  CONCLUSIONS

15 RICH 2013 DATA SAMPLES RICH2013, Hayama 1-6/12/2013 Experience with CsI photocathodesSilvia DALLA TORRE 6 consecutive years of COMPASS data taking up to 15 days of data taking per sample identical cuts applied to the reconstructed particle trajectories (track quality) only identified  ’s

16 RICH 2013 WHICH CHERENKOV RINGS ? p (GeV/c) distributions (2006 data) RICH2013, Hayama 1-6/12/2013 Experience with CsI photocathodesSilvia DALLA TORRE  (mrad) distributions (2006 data)

17 RICH 2013 DATA ANALYSIS 1/4   only identified  ’s   only rings fully contained in a single cathode (independent data sets)   count N ph (detected photons) of each ring (background cannot be disentangled at this stage)   1 mrad wide  Ch bins  N =   Cherenkov photons : Poisson distribution  detected photons : Poisson distribution   At least 1 detected photon  measured N is biased, corrected measured:  ’  solving numerically the equation RICH2013, Hayama 1-6/12/2013 Experience with CsI photocathodesSilvia DALLA TORRE corrected

18 RICH 2013 DATA ANALYSIS 2/4 Fit the distribution use N(  Ch = 55.2 mrad) RICH2013, Hayama 1-6/12/2013 Experience with CsI photocathodesSilvia DALLA TORRE Frank and Tamm distribution Background (flat distribution assumed)

19 RICH 2013 DATA ANALYSIS 3/4 RICH2013, Hayama 1-6/12/2013 Experience with CsI photocathodesSilvia DALLA TORRE Distributions & fits (2006 data)  2 theoretical NDF = 18 Fit results stable versus:  fit range variations  Different ring selection: at least 2, 3 photons at least 2, 3 photons  estimated systematic error: 0.7 x  stat

20 RICH 2013 FIT CHECKS RICH2013, Hayama 1-6/12/2013 Experience with CsI photocathodesSilvia DALLA TORRE Rings with al least photons photons Fit ranges:  25 – 45 mrad (25 – 40 mard)  20 – 40 mrad (20 – 35 mard)  15 – 35 mrad (15 – 30 mard)

21 RICH 2013 DATA ANALYSIS 4/4 RICH2013, Hayama 1-6/12/2013 Experience with CsI photocathodesSilvia DALLA TORRE N(  Ch = 55.2 mrad) vsyear (statistical errors) Frank and Tamm background Too few photons: not used in the following MEAN OVER 11 CATHODES (statistical & systematic errors)

22 RICH 2013 COMPARING THE 6 DATA SETS RICH2013, Hayama 1-6/12/2013 Experience with CsI photocathodesSilvia DALLA TORRE POTENTIAL SOURCES OF SYSTEMATIC EFFECTS  different beams ( , h) and beam momentum   different background and secondary multiplicities  signal and background contributions are separated in the analysis  HV fixed, -20V (over 2kV) in 2010, 2011:  HV fixed, -20V (over 2kV) in 2010, 2011: small effect ~ 2.3% N decrease  differences in parameters:   T, P, electronics threshold  systematic effect estimated  procedure self-correcting for the variation of the refractive index (P, T, residual N 2 content)   n-1 = ( ) *   gas transparency:  negligible variations (constant monitoring) transmission through 1,87 m, corresponding to: H 2 O: ~1 ppm, O 2: ~3 ppm wavelength (nm) transmission

23 RICH 2013 RICH2013, Hayama 1-6/12/2013 Experience with CsI photocathodesSilvia DALLA TORRE OUTLINE  INTRODUCTION  DETERMINING THE EVOLUTION OF THE CsI EFFECTIVE QE  The principle  The set-up : COMPASS RICH-1  DATA, ANALYSIS, RESULTS  CONCLUSIONS

24 RICH 2013 CONCLUSIONS 1/2   Compatible with no CsI QE variation   Max CsI QE variation rate : 2.3 % / year [1 “COMPASS year” : ~6 months of data taking]   For the 3 most critical cathodes: Max CsI QE variation rate : 5.2 % / year RICH2013, Hayama 1-6/12/2013 Experience with CsI photocathodesSilvia DALLA TORRE Errors enlarged to get 50% probability for the linear fit corrected for corrected for lower HV (-20V)

25 RICH 2013 CONCLUSIONS 1/2 RICH2013, Hayama 1-6/12/2013 Experience with CsI photocathodesSilvia DALLA TORRE CsI photocathodes used in gas photon detectors are robustCsI photocathodes used in gas photon detectors are robust they can preserve their QE over periods of years with at most a modest decrease if exposed tothey can preserve their QE over periods of years with at most a modest decrease if exposed to atmospheres characterized by very low oxygen and water vapour contaminationatmospheres characterized by very low oxygen and water vapour contamination low integral ion bombardmentlow integral ion bombardment positive information supporting the effort for novel MPGD- based photon detectors with CsI photocathodespositive information supporting the effort for novel MPGD- based photon detectors with CsI photocathodes

26 RICH 2013 RICH2013, Hayama 1-6/12/2013 Experience with CsI photocathodesSilvia DALLA TORRE OUTLINE THANK YOU

27 RICH 2013 RICH2013, Hayama 1-6/12/2013 Experience with CsI photocathodesSilvia DALLA TORRE OUTLINE SPARE SLIDES

28 RICH 2013 MINIMUM n. of PHOTONS/ring RICH2013, Hayama 1-6/12/2013 Experience with CsI photocathodesSilvia DALLA TORRE Rings with al least 123photons

29 RICH 2013 Different fit ranges RICH2013, Hayama 1-6/12/2013 Experience with CsI photocathodesSilvia DALLA TORRE Fit ranges:  25 – 45 mrad (25 – 40 mard)  20 – 40 mrad (20 – 35 mard)  15 – 35 mrad (15 – 30 mard)

30 RICH 2013, over years RICH2013, Hayama 1-6/12/2013 Experience with CsI photocathodesSilvia DALLA TORRE(mbar) Top detector set - T (degrees) Bottom detector set - T (degrees) P, T averaged over the data samples used in this analysis