12/8/04 B. Azmoun 1

CsI Photocathode Production and Testing

Bob Azmoun

Brookhaven National Lab

12/8/04

12/8/04 B. Azmoun 2

CsI Deposition @ Stony Brook

Method• Use high purity CsI (Scintillator grade)

• High Vacuum (1E-7 Torr) [diffusion pump w/ N2 trap]

• Thoroughly clean vessel and all components• Bake the CsI

•Mask substrates•Evaporate very small amount of CsI

• This vaporizes any contaminants on molybdenum boat and/or outer surface of CsI crystals• Vessel walls coated with CsI will also act as a “getter”

• Warm up substrates before, during and after evaporation•Withholds water and contaminants from condensing onto the substrates before deposition, and onto the CsI after deposition

• Thickness monitor• Quartz crystal oscillator, Al foil control substrate

• Transportation/ Storage of Photocathodes (vacuum, gas flow)

Objective: To develop and test the technique of CsI evaporation at Stony Brook for production of CsI coated GEM foils for HBD prototypes and possibly even the final detector.

12/8/04 B. Azmoun 3

Dimensions of Evaporator Vessel

Diameter ~ 43cm

maskElectrodes/CsI crystal/Molybdenum Boat

Max. Height~80cm

Scaffolding: Photocathode substrate/Thickness monitors

Bell Jar

12/8/04 B. Azmoun 4

Thickness Uniformity

r

Underside of Scaffolding

Au-Ni-Cu clad G-10 PhotocathodeSubstrates

Al Foil Control Substratesfor measuring CsI thickness

Quartz crystal thickness monitor

Thickness Uniformity of Evaporated CsI

0

500

1000

1500

2000

2500

0 5 10 15 20

Radius from Center of Circular Vessel of Evaporator [cm]Cs

I Thi

ckne

ss [A

ngst

rom

s]

* Height of substrates above CsI crystal = 31.75'', * CsI molybdenum boat dimensions (i.e. size of evaporation source)~1.0''x0.5''

Quartz Crystal Thickness Monitor

CsI Thickness Target: 2000 Angstroms

12/8/04 B. Azmoun 5

Testing Apparatus @ BNL• Absolute QE of produced Photocathodes Vs Wavelength• Photoelectron signal in CF4 (CF4 transmittance)• GEM systematics (gain, stability, etc.)• Systematic Measurements of CsI Coated Triple GEM (pe collection efficiency Vs ED, etc)

Beam splitter/Collimator

MgF2 Windows

VUV beam

Photocathode orTriple GEM

Absorption Length~ 30cm

Independent Lamp Monitor

Photocathode Test

Vacuum Vessel (“cross”)

12/8/04 B. Azmoun 6

Preliminary MeasurementsVUV Wavelength Scan (D2 lamp): Absolute

Photoe lectron Current from CsI Sample Photocathode

0

50

100

150

200

250

300

1100 1300 1500 1700 1900Wavelength [Angstroms]

Ph

oto

elec

tro

n

Cu

rren

t [p

A] Vacuum Scan

CF4 Scan

Photoelectron Collection Efficiency Vs Collection Field

0

20

40

60

80

100

0 0.5 1 1.5

Collection Field [kV/cm]

Pho

toel

ectr

on C

olle

ctio

n E

ffici

ncy

w.r

.t. P

E c

oll.

in V

acuu

m [%

]

Gas: CF4 [Before Polish]

Vacuum [Before Polish]

Gas: CF4 [After Polish, P=30.98"Hg]

Gas: CF4 [After Polish, P=31.56"Hg]

Vacuum [After Polish]

CsI Photocathode Output after exposure to CF4(VUV Scans taken over a 10 day period during continuous CF4 flow )

-20

0

20

40

60

80

100

120

1100 1200 1300 1400 1500 1600 1700 1800 1900

Wavelength [Angstsom s]

~CsI

Pho

toca

thod

e C

urre

nt

[arb

. Uni

ts]

(11/29/04)

(12/03/04)

1st Scan started 1 hour af terstart of CF4 f low (11/24/04)

Ratio of thePhotoelectron Current in Gas and in Vacuum

0

20

40

60

80

100

120

1100 1300 1500 1700 1900

Wavelength [Angstroms]

Ph

otoe

lect

ron

Rat

io: G

as/V

acu

um

[%] CsI

PMT Monitor

12/8/04 B. Azmoun 7

Initial Use for CsI Photocathodes

1. Preliminary Tests in Idealistic Stainless steal vessel (Cross)• GEM + CsI + CF4 + single pad + gas tight vessel (minimal out-

gassing, high purity gas)• Systematic measurements of CsI coated triple GEM detector (Gain,

stability, QE measurement, transmittance, pe collection efficiency Vs ED, etc)

2. Install (10x10cm) CsI coated GEM into “simple” HBD prototype with “realistic vessel” (made of G-10) • GEM + CsI + CF4 + 16 pads (~1x1”) + ~50cm radiator

• In Lab tests: gas quality (H2O/O2 ppm’s, transmittance), performance of GEMs/CsI in more realistic conditions (gain,

stability, aging studies of CsI QE in gas, etc) • Test at Light Source: N0 measurement• Test inside PHENIX IR: Stability of GEMs, background, etc. ???