1. Development and Performance study of a Thick Gas Electron Multiplier (THGEM) based Radiation Detector 1 Department of Instrumentation and Applied Physics, IISc, Bangalore

2. 2 What is a Gas Electron Multiplier (GEM) Top view of GEM Electric field lines of GEM Pulse amplitude variation with voltageWorking principle of GEM E-field distribution in GEM

3. What is Thick GEM (THGEM )? Its advantages over standard GEM Simulated electron avalanche inside standard GEM (Gain ~10 3 ) Simulated electron avalanche inside THGEM (Gain ~10 5 ) E-field distribution in and around THGEM for different insulator thicknesses Schematic representation of THGEM 3 Z-axis HOLE THGEM X-axis Drift electrode Collection electrode

4. 4 Applications of THGEM 1.Gaseous photon detector 2.Soft X-ray detector 3.Neutron detector 4.Ring imaging Cherenkov detector 5.Tracking detectors operating in intense particle fluxes 6.X-ray imaging

5. Design optimization of THGEM using simulations Geometrical factors affecting electric field inside THGEM are Hole diameter Insulator (FR4) thickness Rim size Effect of hole diameter on E-max Effect of hole diameter and thickness on E-max It has been observed that t/d ~1 gives maximum gain. This is similar to as observed for standard GEM. 5

6. What is rim and its significance. A rim is provided around each hole for reducing the probability of discharges at higher operating voltages. This rim was created by chemical etching of copper. Schematic representation of rim in THGEM Effect of rim size on E-max as estimated by GARFIELD Due to technical limitation during fabrication often the rim centre is not aligned properly with the hole centre. This is known as rim offset 6

7. 7 Rim offset and it effect on the detector performance The precision of the rim w.r.t. the hole centre is very crucial in terms of the detector output-WHY? Hole Rim Rim with offset Rim without offset E-field distribution across the THGEM hole for rim with offset E-field distribution across the THGEM hole for rim without offset Better energy resolution due to gain uniformity 7 Poor energy resolution

8. 8 Burnt areas in working THGEM due to discharges Discharge happened due to rim offset Improper etching leads to sparks

9. 9 Fabrication of THGEM THGEM was fabricated at the PCB lab, ISAC. The design parameters of the fabricated THGEM are Hole diameter= 200  m (lesser diameter holes were not possible) Insulator thickness= 250  m Rim size= 100  m. Pitch= 450  m,550  m. Active area= 30mm x 30 mm 9

10. 10 Steps of fabrication of THGEM UV light Photolithography (Side A) Insulator (FR4) Photoresist copper Drilling of holes in Cu clad PCB Photomask Photoresist coating on both sides UV light Photolithography of side B

11. 11 Developing Copper etching Photo resist removal THGEM with etched rim Steps of fabrication continued….

12. 12 Pictures of the fabricated THGEM and the readout electrode THGEM with electrodes Top electrode Bottom electrode THGEM as seen under the microscope Hole with no offsetHole with rim offset THGEM active area Readout Structure Hexagonal arrangement

13. 13 THGEM mounted inside the Test chamber THGEM Detector chamber Electrical feed throughs Mounting support O-ring (for vacuum sealing)

14. THGEM as a UV photon detector 14

15. 15 UV photons Photoelectrons Electron multiplication Signal generation THGEM Readout strips CsI photocathode Gas molecules Backscattering of photoelectron Working principle of UV photon detector E drift Induction field

16. 16 Preparation of CsI photocathode Why CsI? Quantum efficiency of CsI ranges between 40% to 1% in the wavelength range of 150-220 nm The photocathode is prepared by the process of thermal evaporation. Thin film of CsI is deposited over the quartz substrate makes the photocathode for UV photon detection. Quantum efficiency of CsI Transmission curve for quartz

17. 17 Different photocathode configurations 1. Semitransparent configuration (CsI film thickness~ 30-50 nm) 2.Reflective configuration (CsI film thickness~300 nm) Readout electrode Electron avalanche Photoelectron h h Photocathod e Semitransparent Photocathode Reflective Photocathode Thus the performance study has been carried out for both the configuration The detection efficiency of the detector strongly depends on the photoemission property of the photocathode

18. 18 Performance study of the photocathode Effect of Electric field on photocurrent. Effect of annealing on photocathode performance Setup used for semitransparent configurationSetup used for Ref configuration

19. 19 Photocurrent variation with pressure Photocurrent variation with pressure for ST PC. Photocurrent variation with pressure for Ref PC. The reduction of photocurrent with pressure can be attributed to the backscattering phenomenon.

20. 20 Effect of moisture on CsI photocathode performance CsI is hygroscopic and thus moisture degrades the photoelectron yield from the photocathode surface Reason for the degradation For thin film (20-50nm), moisture present over the surface of the film absorbs the UV photons, degrading the photoemission property of the film For thick film (300 nm), in addition to adsorbed moisture at the surface there is penetration of water molecules into the bulk of the film Effect of vacuum treatment on thin CsI film exposed to moist air

21. 21 Effect of vacuum treatment on the photoemission property of the thick CsI film Photocurrent increases with the duration of evacuation

22. 22 Effect on microstructure of the film 30 minutes of evacuation 45 minutes of evacualtion 1.5 hour of evacuation

23. 23 Performance study of the UV photon detector

24. 24 Detection Efficiency of a UV photon detector (i) Maximum photoelectron yield from the photocathode surface. Efficient photon detection requires (ii) Efficient charge transfer (photoelectrons) from the photocathode surface to the THGEM hole for multiplication, which is termed as Electron Transfer Efficiency (ETE). Photoelectron yield depends on Electric field at the photocathode surface. Gas mixture (backscattering effect) Wavelength of the incident light Photocurrent measured with electric field at the photocathode surface

25. 25 What is ETE and its significance. It is the ratio of the number of electrons focused inside the THGEM hole and the total number of photoelectrons extracted from the photocathode surface. Better ETE means Better focusing of electrons inside the THGEM hole More number of electrons are subjected to avalanche multiplication Higher output signal obtained. Thus higher ETE means higher detection efficiency and hence higher sensitivity of the detector ETE depends on Drift field Gas mixture Gas pressure Drift gap Snapshot of Garfield plot showing efficient and inefficient focusing of electrons inside the THGEM hole

26. 26 Conflicting parameters affecting ETE and photoelectron yield Drift field Gas pressure Simulation and experimental verification of these factors have been studied in detail photocathode Drift gap Drift field Readout electrode 26

27. 27 Steps of simulation for ETE estimation THGEM structure modeled in ANSYS Field map files are exported to GARFIELD Gas mixture defined using MAGBOLTZ Uniform matrix of large number of electrons produced at the photocathode surface Each of the electrons was drifted from their starting point. Monte Carlo technique was used to simulate the drift path of the electron End coordinate of drifting electrons returned by GARFIELD was used to find ETE, backscattered and electrons stopping at the top metal electrode. Meshed structureSimulated detector Simulated Electron drift path

28. 28 Experimental procedure for ETE estimation Mounting the photocathode few mm above the THGEM inside the test chamber Evacuation and gas filling inside the chamber Measurement of photocurrent A V E drift A -Vc-Vc -Vt-Vt Photocurrent measurement setup THGEM bottom current measurement setup E drift 28 ETE is the ratio of THGEM bottom current and the total photocurrent

29. 29 Test chamber for UV photon detection Experimental setup for UV photon detection UV lamp Photocathode mounting arrangement

30. 30 Effect of drift field on ETE ETE decreases with drift field. Decrease in ETE with drift field is related to the increase in transverse diffusion coefficient Photocurrent increases with drift field. Increase in photocurrent is related to reduced backscattering. Thus optimization of drift field is very important for achieving maximum detection efficiency 30 Drift field (kV/cm)

31. 31 Optimization of Drift field for achieving maximum Detection Efficiency Optimum drift field below at null multiplication is in the range of 0.2-0.4 kV/cm Optimum drift field above multiplication is in the range of 1-1.6 kV/cm The optimum drift field shifts for higher voltages due to change in electron focusing property 31

32. 32 Photoelectron losses can arise due to Photoelectron backscattering Electrons terminate at the top metal electrode due to transverse diffusion Simulation results showing percentage of electrons lost due to various factors Photoelectron losses in the drift region

33. 33 Effect of gas pressure on ETE Increase in pressure increases ETE Number of backscattered electrons increase with pressure Increase of ETE with pressure is due to decrease of electron diffusion with pressure The percentage increase in ETE is more (75%) than percentage increase in backscattering (58%). Hence higher pressure is beneficial for higher detection efficiency.

34. 34 Effect of gas pressure on ETE at multiplication regime Effect of higher dipole field on drift field.

35. 35 Effect of gas mixture on ETE Increased quencher concentration increases ETE due to decrease in transverse diffusion coefficient (closed symbols show transverse diffusion coefficient) Gas mixture having smallest transverse diffusion coefficient has highest ETE (Open symbols show transverse diffusion coefficient)

36. 36 Effect of Drift gap on ETE 36 σ α (d) 1/2 d=drift distance from the point of origin of the electron. σ = diffusion width of the primary electron Simulated electron track for drift length smaller than 1 mm. Simulated electron track for longer drift length Effect of drift gap on ETE

37. 37 Detection efficiency of the UV photon detector strongly depends on drift parameters like drift field, gas mixture, gas pressure and drift gap. Study on detection efficiency reveals Drift field should be optimized considering the opposite dependency of the photoemission from the photocathode and the ETE. The optimum drift field value depends on the multiplication field present inside the THGEM hole. Higher ETE can be obtained for higher gas pressure, gas mixture with lower transverse diffusion coefficient and smaller drift gap. Simulation studies revealed that transverse diffusion has a major impact on deciding ETE and the ultimate detection efficiency.

38. E lectron Spectra from a UV photon detector 38 Charge output Voltage output THGEM detector Preamplifier Linear amplifier and pulse shaper Oscilloscope Multi channel analyzer Computer High voltage power supply Spectra were recorded in pulse counting mode. Each count in the spectrum corresponds to one photoelectron emitted from the photocathode. Schematic of the electronic chain used for acquiring electron spectra from the photon detector

39. 39 Output of the photon detector as seen in the oscilloscope Large aperturePin hole aperture

40. 40 Electron spectra obtained from PHA for different  V THGEM The Electron Spectra At higher multiplication voltage, the curve deviates from its exponential behavior, due to secondary effects.

41. 41 Secondary effects in UV Photon Detector Avalanche induced Photon feedback (observed at high gain) Photocathode Readout electrode Electron multiplication Photon feedback In Single THGEM configuration, photon feedback is damaging to the photocathode Photon feedback can be minimized in multi THGEM configuration Why photon feedback is harmful? Enhances photocathode ageing Produces unwanted secondary pulses

42. 42 Photon feedback effect observed in a single THGEM Excess pulses due to photon feedback at different gain

43. 43 Effect of  V THGEM on photon feedback at different drift fields Excess counts at the tail of the single electron spectrum are due to photon feedback effect. Photon feedback effect at low gain Photon feedback effect at medium gain Photon feedback effect at high gain Thus study of electron spectra reveals that secondary effect like Photon feedback is prominent at high gain. This effect can be minimized using THGEM in multistage configuration with low multiplication in the first stage.

44. 44 THGEM as an X-ray detector

45. 45 Test setup for detection of X-rays with THGEM.

46. THGEM as an X-ray detector 46 Preamplifier and amplifier output from a THGEM based X-ray detector Output of PC MCA Parameters studied are Effective gain Gain stability with time Energy resolution

47. 47 Effective gain plot for various  V THGEM Gain stability curve for flood and collimated source Performance study of the X-ray detector Pulse height spectrum for X-rays from a Fe 55 source Effect of dipole field on energy resolution Time (mins)

48. 48 Performance study….. Effect of Drift Field on Energy Resolution At low multiplicationAt high multiplication Energy linearity study with THGEM

49. 49 Conclusions The THGEM is fabricated with the design parameters optimized using simulations. For the application of the THGEM as a UV photon detector, CsI photocathode is prepared using thin film technology. The hygroscopic nature of CsI deteriorates the efficiency of the photocathode in the presence of moist air. Vacuum treatment of the thin CsI film restores a significant portion of its lost sensitivity. The efficiency of the detector is the product of the quantum efficiency of the photocathode and the ETE of the THGEM. The ETE and hence the detection efficiency of the UV photon detector strongly depends on the drift parameters like drift field, drift gap, gas mixture and gas pressure. The opposite dependency of the photocathode quantum efficiency and the ETE on drift field needs to be considered while choosing an optimum drift field value.

50. 50 Conclusions…… The sensitivity of the detector depends on the THGEM multiplication. THGEM operating in multistage configuration with low multiplication voltage at the first stage and optimized drift parameters ensures stable operation The single electron spectra obtained from the UV photon detector shows an exponentially decreasing distribution where each count corresponds to one single photoelectron emitted from the photocathode. THGEM as an X-ray detector also showed promising results. It achieved a gain of ~10 3 in a single stage with energy resolution ~ 14% achieved so far.

51. 51 Scope of future work Operating THGEM in multistage configuration for single photon detection under stable operating conditions The readout electrode presently in use is strip readout. This can be changed to pixelated readout for imaging application. This will make THGEM as a potential element for UV imaging application for example in astronomy. An extensive material characterization of CsI for enhanced quantum efficiency application has been planned. Also the development of a sealed GPM with semitransparent photocathode will be undertaken.

52. 52 Publications 1. Baishali G, Radhakrishna V., Koushal V., Rakhee K., and K. Rajanna “Study of electron focusing in Thick GEM based photon detectors using semi- transparent photocathodes” Nuclear Instruments and Methods in Physics Research A. (Article in Press) 2. Baishali G, Radhakrishna V and K. Rajanna “Effect of vacuum treatment on CsI photocathode performance in UV photon detector” Optical Material Express vol. 3 No.7 p 948. 3. Baishali G, Radhakrishna V, Koushal V, Rakhee K and Rajanna K. “Study on the Detection Efficiency of Gaseous Photomultipliers”. Proc. of SPIE Vol. 8727 p 523(Advanced Photon counting Technique VII, held at Baltimore, USA on 2013) 4. Baishali G., Radhakrishna V., K.Rajanna “3D simulation for maximizing Electron Transfer Efficiency in THICK GEMs” Proceedings of the 13 th ICATPP Conference (held at Como, Italy on 2011) 5. Baishali G., Rakhee K., Koushal V.,Radhakrishna V. and K. Rajanna “GEM design requirements for X-ray Polarimeter” presented at the National Space Science Symposium (held at Tirupati on Feb 2012)

53. 53 Acknowledgements (1)I am thankful to my guide and Chairman Prof. K.Rajanna for all the support and freedom he has given me all these years. (2)My sincere thanks to my mentor at ISAC Dr. Radhakrishna for his guidance and help without which my thesis would had been impossible. (3)I am thankful to Dr. Sreekumar and Dr. Seetha for giving me permission to come to ISAC and perform a major portion of my experiments. (4) I thank all my labmates and seniors at IISC for the lovely time I spent with them. (5)Thanks to our GEM Team members (Rakhee and Koushal) at ISAC who made my stay at ISAC a memorable one. (6)Thanks to all the staff of the IAP department office. (7)I extend my warm gratitude to all the scientists, engineers, JRFs at ISAC who made my stay at ISAC very comfortable and enjoyable one. (8)I would like to thank the scientists of the PCB lab, ISAC, who helped us in fabricating our THGEM. (9)Thanks to my family members for their support during this long journey.

54. 54 Thank You