Development and Performance Development and Performance study of a study of a

Thick Gas Electron Multiplier Thick Gas Electron Multiplier (THGEM) based Radiation (THGEM) based Radiation

DetectorDetector

1Department of Instrumentation and Applied Physics, IISc, Bangalore

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What is a Gas Electron Multiplier (GEM)

Copper

Hole

Drift electrode

Strip Readout electrode

Top electrode

Bottom electrodeInsulator

Drift region

Induction region

Hole

Incoming radiation

Primary electrons

Electron avalanche

GAS VOLUME

Vdrift

VGEM

Vanode

Top view of GEM Electric field lines of GEM

Pulse amplitude variation with voltageWorking principle of GEM

E-field distribution in GEM

50µm

100µm300µm

What is Thick GEM (THGEM )? Its advantages over standard GEM

Simulated electron avalanche inside standard GEM (Gain ~103 )

Simulated electron avalanche inside THGEM (Gain ~105 )

E-field distribution in and around THGEM for different insulator thicknesses

Schematic representation of THGEM

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Z-a

xis

HOLE THGEM

X-axis

Drift electrode

Collection electrode

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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

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

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 THGEMEffect 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

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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

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Poor energy resolution

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Burnt areas in working THGEM due to discharges

Discharge happened due to rim offset

Improper etching leads to sparks

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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

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Steps of fabrication of THGEM

UV light

Photolithography (Side A)

Insulator (FR4)

Photoresistcopper

Drilling of holes in Cu clad PCB

Photomask

Photoresist coating on both sides

UV light

Photolithography of side B

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Developing

Copper etching

Photo resist removal

THGEM with etched rim

Steps of fabrication continued….

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Pictures of the fabricated THGEM and the readout electrode

THGEM with electrodes

Top electrode

Bottom electrodeTHGEM as seen under the microscope

Hole with no offset Hole with rim offset

THGEM active area

Readout Structure

Hexagonal arrangement

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THGEM mounted inside the Test chamber

THGEM

Detector chamber

Electrical feed throughs

Mounting support

O-ring

(for vacuum sealing)

THGEM as a THGEM as a

UV photon detectorUV photon detector

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UV photons

Photoelectrons

Electron multiplication

Signal generation

THGEM

Readout strips

CsI photocathode

Gas molecules

Backscattering of photoelectron

Working principle of UV photon detector

Edrift

Induction field

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Preparation of CsI photocathodeWhy 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

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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

hhPhotocathode

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

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Performance study of the photocathode

Effect of Electric field on photocurrent. Effect of annealing on photocathode performance

Setup used for semitransparent configuration Setup used for Ref configuration

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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.

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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

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Effect of vacuum treatment on the photoemission property of the thick CsI film

Photocurrent increases with the duration of evacuation

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Effect on microstructure of the film

30 minutes of

evacuation

45 minutes of

evacualtion

1.5 hour of evacuation

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Performance study of the UV photon detector

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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

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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

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Conflicting parameters affecting ETE and photoelectron yield

•Drift field

•Gas pressure

Simulation and experimental verification of these factors have been studied in detail

photocathodeDrift gap

Drift field

Readout electrode

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Hole

Coarse meshing

Fine meshingInsulator

Air block

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Steps of simulation for ETE estimationTHGEM 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 detectorSimulated Electron drift path

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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

Edrift

A

-Vc

-Vt

Photocurrent measurement setup THGEM bottom current measurement setup

Edrift

28ETE is the ratio of THGEM bottom current and the total photocurrent

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Test chamber for UV photon detection

Experimental setup for UV photon detection

UV lamp

Photocathode mounting arrangement

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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

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Drift field (kV/cm)

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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

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• 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

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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.

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Effect of gas pressure on ETE at multiplication regime

Effect of higher dipole field on drift field.

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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)

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Effect of Drift gap on ETE

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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

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•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 revealsStudy 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 .

Electron Spectra from a UV photon detector

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Charge output Voltage output

THGEM

detector

PreamplifierLinear amplifier and pulse shaper Oscilloscope

Multi channel analyzerComputer

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

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Output of the photon detector as seen in the oscilloscope

Large aperture Pin hole aperture

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Electron spectra obtained from PHA for different VTHGEM

The Electron Spectra

At higher multiplication voltage, the curve deviates from its exponential behavior, due to secondary effects.

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Secondary effects in UV Photon Detector

Avalanche induced Photon feedback (observed at high gain)

Photocathode

Readout electrode

Electron multiplication

Photon feedback

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

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Photon feedback effect observed in a single THGEM

Excess pulses due to photon feedback at different gain

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Effect of VTHGEM 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.

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THGEM as an X-ray detector

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

THGEM as an X-ray detector

46Preamplifier 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

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Effective gain plot for various VTHGEMGain stability curve for flood and collimated source

Performance study of the X-ray detector

Pulse height spectrum for X-rays from a Fe55 source

Effect of dipole field on energy resolution

Time (mins)

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Performance study…..

Effect of Drift Field on Energy Resolution

At low multiplication At high multiplication

Energy linearity study with THGEM

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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.

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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 ~103 in a single stage with energy resolution ~ 14% achieved so far .

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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.

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Publications1. 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 13th 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)

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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.

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Thank You