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First observations on CsI-coated ThGEM

in Trieste

presented by Gabriele Giacomini

phone meeting 06/08/2008

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In these preliminary measurements we used a deuterium lamp (AVALIGHT-DHS produced by AVANTES) to extract electrons from a CsI layer, coating the “top” surface of the ThGEM (type “C4”). This irradiation produces a DC current, which is measured by picoammeters ( Keithley and home-made) .

The amount of irradiation is not accurately known (since the lamp is not perfectly calibrated), then information about the gain cannot be extracted from these measurements.

Irradiation level is chosen to avoid a fast ageing of the CsI layer, known to

lose its quantum efficiency after a collected charge of ~ mC/cm2.

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ThGEM characteristics:ThGEM type C4:• Size = 3 X 3 cm2

• Rim = 100 m• Pitch = 0.8 mm• thickness = 0.4 mm

• Top coated by CsI acting as photocathode (over 5 m of Ni and 0.02 m of Au) ( good quantum efficiency for < 200 nm)

CsI is damaged by moisture exposure, then the right procedure is to always keep the CsI layer under vacuum or clean atmosphere (~ < 20 ppm H20 and O2).In our case, only after CsI deposition (at CERN) the ThGEM has been put under a vacuum container, then suffering a short exposure. During the mounting operations, the unavoidable air exposure has been minimized by flushing N2 over the CsI, besides keeping the whole duration as short as possible (< 10’). The CsI layer survived showing, as test # zero, sensitivity to environment solar UV and blindness to halogen lamp.

CsI quantum efficiency

nm(From L. Molnar – RICH 2007 Trieste)

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Ar/CO2 (70/30)

In (N2 while idle)Ar/CO2 out

(N2 while idle)

PicoAmmeter KEITHLEYHV in

Cathode wires (almost invisible)

THGEM top,coated with CsI

High quality quartz window(HERAUS Suprasil II)

The chamber

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The set-upAVALIGHT-DHSDeuterium lamp

The anode pads are covered by an Aluminum box, a BNC carries out the signal/current

Optic fiber and its holder. The beam out of the fiber diverges and irradiates the whole quartz window.

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Top: CsI coated

Bottom

Anode pads

Quartz window

Cathode wires10 M

10 M

10 M

1 M

- HV

- HV

- HV

Beam out from optic fiber

signal/current

gas ingas out

7 mm

0.4 mm

5 mm

Just a sketch …

h

e-

7

0

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0 500 1000 1500 2000 2500 3000 3500

V top (V)

I cath

od

e (

pA

)

• Photoelectrons collected by cathode (grounded by Keithley picoammeter) and • V = -100 V, so electrons drift only towards cathode wires.• anode GND

• Scan in Vtop (then in E drift),

keeping fixed V•1000 V ~ 1kV/cm Ar/CO2 1 atm

The collection efficiency is a monotonic function of the “Drift” Electric FieldThere are several effects:• e- extraction efficiency from CsI depends on drift/diffusion inside CsI itself •Potential barrier between CsI and “vacuum” depends on the Fields• higher fields decrease the detrimental effect of electron backscattering on gas atoms/ molecules, accounting for a charge loosing•Some gain at the cathode wires?

Measurement of the photocurrent (no gain on THGEM)collection efficiency

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Very rough estimation of the # of photons

A current of 10 pA ~108 e-/s, supposing 1/10th is extracted 109/s e- producedThe q.e. of CsI is very small at these wavelength (> 200 nm) - and it is wavelength dependent. So, let’s say we have 1012/s photons impinging the CsI layer…

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300

0 200 400 600 800 1000 1200

halogen = black body @ 3000 K

Deuterium

With a spectrophotometer (AVAspec - AVANTES) we acquire the irradiance spectra of the Halogen and deuterium lamps. The light of the halogen photogenerates a DC current in a photodiode: knowing the light absorption in silicon, we are able to state – in first approximation - the number of photons emitted by the halogen lamp.Comparing the spectra, we should know the photons emitted by the deuterium lamp (1.5e13 for <300nm).We don’t like the shoulder on the left of the Deuterium spectrum: maybe it’s an artifact of the spectrophotometer making this measurement useless and wrong…

An independent way to estimate the number of photons:

?

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V scan (gain from ThGEM)

0.1

1

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100

1000

10000

0 500 1000 1500 2000

V

I an

od

e (p

A)

In this set-up:•Electrons collected by anode

•Edrift (fixed) = 0 V/cm

(but we measured Ianode to be insensitive

to E drift )

•Einduction (fixed) = 4000 V/cm

•Scan in V, until multiplication occurs

The graph shows three regions:• V < 250 : partial collection of electrons inside the holes• 250 V < V < 1000 V : enhanced collection inside holes but gain still ~ 1• V > 1000 V : multiplication (we gain a factor 2 every 80 V)

In the last points the currents are huge (and time dependent) so we don’t stay for long times. - In this range, environment light too provides high current

The currents reported are read immediately after the opening of the shutter, the currents then decrease with time, due to the well known polarization effects.

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Wavelength responseTo study the response of CsI to a specific wavelength, we filtered the D2 light source with several narrow bandpass filters (450, 390, 340, 289, 239 nm).We measure again the anode current and we operate the THGEM while it multiplies:

V > 1000V

The beam spot irradiates the filter, while the environment light is blocked by the black paper sheet, covering the quartz window. The chamber is blind at 450, 390 and 340 nm (as expected from CsI) but shows significant sensitivity at 289 and, more, at 239 nm.

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1

10

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1000

1000 1200 1400 1600 1800 2000

V

I an

od

e (p

A)

239 nm

289 nm

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V induction = 2000V

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1000

1000 1200 1400 1600 1800V (top - bottom)

I (p

A)

top

bottom

anode

V induction = 3000 V

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1000

1000 1200 1400 1600 1800V

I (p

A)

top

bottom

anode

V scan - IIWith the 240 nm filter, we do again the V scan – for two different Vinduction –

reading the currents from anode, bottom, top and drift.

Vdrift = 3000 V Vdrift = 4000 V

Idrift is ~ 0, altough Edrift has a sign for which the cathode competes with the holes in

collecting the electrons (Vdrift is constant).

(Einduction=4kV/cm) (Einduction=6kV/cm)

Increasing the gain, more electrons to BOTTOM

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Induction scan

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E induction (kV/cm)

ra

tio

: I a

no

de

/I to

tal

Induction scan

V = 1550 VKeeping fix the V (the gain), we are interested to the ratio :

I anode / I total

e.g., to decide the voltages in order to get:• a good signal from the anode•a trigger from the bottom.

This ratio is a monotonic function of the induction field and saturates at quite high induction fields.

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-900

-700

-500

-300

-100

100

4850 4900 4950 5000

time

I an

od

e (p

A)

V = 1650 V, Vinduction = 3000 V

-160

4200 4300 4400 4500 4600 4700 4800

time

I an

od

e (p

A)

V = 1500V V = 1550V V = 1600VV = 1450V

Time development of the currents

The currents decrease after their initial value (nothing new)but for V > ~ 1600 V, where a positive feedback (due to ion/photon bombardement of the CsI layer) is triggered, leading to increasing currents (which seem to saturate).

Shutter open Shutter closed