Quantum Efficiency measurement system for large area CsI photodetector

1. Quantum Efficiency measurement system for large area CsI photodetector Francesco Cusanno INFN Roma I Gruppo Sanita’ on behalf of Hall A RICH collaboration • TJNAF - Hall A RICH • Evaporation system • The Q.E. measurement system: • Measure principles and procedure • UV source: deuterium lamp and filters • Reference PMT • Collection charge chamber • Results • Thickness dependence • Contamination and reconditioning • Conclusion and future work

2. , 1.5 cm , 0.5 cm TJNAF - Hall A RICH We built a proximity focusing RICH for Hall A at Thomas Jefferson National Accelerator Facility (TJNAF or Jefferson Lab) Rich in Hall A RICH specifications 190 cm RICH Phase space in Hall A 190 cm CsI PhotoCathode PC1 PC2 PC3 64.0 cm Cu – Ni – Au pad layers 40.3 cm

4. Evaporation system 10-6 mbar vacuum, 2 nm/s CsI deposition at T = 60 ºC (CERN experts indications). Vacuum - heating conditions start 15 – 24 h before evaporation. A post-evaporation heat treatment is done for 12 hours. Rotating mirror (CaF2) 120 cm UV source box 110 cm Photocathode PMT Collection chamber Movement system Crucible bars

5. Evaporation layout Crucibles positions 64,40 • PhotoCathode – crucibles plane distance: 42 cm • 4 mm Ni – 1 mm Au support • crucible quantity: 0.8 g weight each one, corresponding to ~ 320 nm thickness (expected and measured) Expected thickness Thickness (nm)

6. PD 3 Measurement principle The ratio A2/A1 = Q.E.(CsI)/Q.E. (PMT), indeed the wire chamber is in the vacuum (no charge amplification) and the anode and grid voltage allow to work in full collection regime for the chamber (and the PMT dynodes and anode are connected together to ground, so no charge amplification for PMT too; PMT supply is 78 V). A3 current (PhotoDiode in the optic box) monitors the UV source stability.

7. Measurement procedure • flow N2 to purge optical box (stable, > 5 Torr overpressure, starting at least 12 h, before measurement) • use batteries to reduce instruments electric ground floating • we use a supply-meter box, connected to the PD, PMT and the collection chamber and to the picoamp-meter KEITHLEY 485 • Current measurements: for any position • Select the PMT position • Read the PMT current • Change the mirror position toward the CsI plane • Read the chamber current • Change the position on the CsI plane

8. UV source Hamamatsu L2D2 lamp (C7860 power supply) 161 nm spectral peak; 3 filters (Acton Research Corporation), ~ 20 nm FWHM wide, centered at 160 – 185 – 200 nm) Optical box Filter rotating switch system, adjustable iris, (6 position, one is Al disk) Evaporator wall N2 flow tube

9. 160.8 nm Hamamatsu L2D2 Deuterium lamp

10. UV filters (Acton Research Co.) Peak 158.80 nm; spread: 25.20 nm FWHM Peak 198.40 nm; spread: 23.40 nm FWHM

11. Peak 185.80 nm; spread: 21.60 nm FWHM CsI Q.E. Q. E. (%) PMT- source convolution CsI - source convolution UV source spectrum (lamp + filter) PMT Q.E.

12. Q. E. (%) Q. E. (%)

13. PMT We use Electron Tubes Limited (ETL) 9402B PMT, Q.E. is known by ETL (single PMT) datasheets. ETL 9402B characteristics 9402 S/No. 38 PMT Q.E.

14. Collection charge chamber CsI Photocathode (0 V) vacuum 2 mm e- +133 V Anode wires 2 mm Grid (0 V) Light spot Anode wires collect electrons from the CsI plane The grid wires ‘stop’ electrons on the anode Anode wires are 20 mm diameter, grid wires are 50 mm diameter, anode and grid wires are crossed. Wire distance is 4 mm. Chamber collecting area is 50 mm x 50 mm (light spot is smaller, ~ 10 mm).

15. 160 nm Q.E. map, 300 nm uniform thickness Bad spot position (cm) Results • Usally 24 row (1.5 cm distance), some ‘cross’ checks 64.0 cm Measurement layout 40.3 cm We get a complete Q. E. map; in this case 1 bad spot (~ 1 cm2). If bad results we can repeat the evaporation (soon). Good uniformity on all the PC (total spread: 21.7 % -24.4 %, apart the bad spot; average Q. E.: 23.7 % ).

16. Correspondent pattern for all the wavelenghts Average Q. E. at 200 nm: 5.5 % Same pattern at 185 nm too, Average Q. E. at 185 nm: 11.6 %

17. Comparison of the results • On 3 different PCs we have similar results, Q. E. total spread < 10 % • Results may be specific to the substrate - support 15 % error bars are plotted • Fit using Jlab experimental RICH data will supply a check on the direct Q. E. measurement

18. 160 nm 185 nm 200 nm Thickness dependence Crucibles weight: 0; 0; 1.2 g; 1.2 g Expected thickness • Higher range was expected, probably we lost partially CsI due to a mismatching between crucible volume and CsI weight

19. 160 nm 185 nm 200 nm 160 nm 185 nm 200 nm After heating After heating After pumping After pumping Air exposure and reconditioning 22 h. air exposure (19.5 ºC, 41% relative humidity), 27 h. pumping has a small effect; 12 h. heating restore about 1/3 of the loss. Outgassing and reconditioning 25 not-pumping d., 0.25 mbar, reconditioning: 12 h. pumping + 14 h. heating 60ºC; second (longer) heating has no effect. 6 not-pumping d., 0.013 mbar, has similar effect than after pumping reconditioning in the previous case (50% loss) Possible interpretation is the ‘outgassing’ of organic particles from the substrate (it is cleaned by organic solvent before evaporation).

20. Conclusion and future work • The system performs reliable Q.E. evaluation and indicative absolute measurements on large photosensitive area, it can perform the measure since immediately after evaporation until immediately before the assembling of the PhotoCathode in the RICH chamber. • Therefore it can monitor the eventual decrease of the Q.E., in case of delay between evaporation and detector assembling. Also the system can perform thickness, air exposure, post heat treatment dependence study. • Preliminary thickness and air exposure tests seem to show that a thicker CsI has better Q.E. and it is less sensitive to air exposure. The Q.E. loss due to air exposure can be (at least partially) recovered by heating or pumping (dry gas flow).

21. Extrapolating the thickness dependence results (adding the ‘standard’ 300 nm result) it possible to expect higher Q. E. at higher thickness Expected thickness Crucibles weight: 0; 0; 1.6 g; 1.6 g

22. X-Rays scattering measurements 2 mm Au 10 mm Ni (7 mm requested) Support and temperature dependence • Support thickness: 7 mm Ni – 2 mm Au • Also we are interesting on T dependence study of CsI performances

23. Different T (and P) produces different growth