C. Joram CERN / PH International Scoping Study CERN Meeting September 2005 1

Development of spherical HPDs Development of spherical HPDs

for a Water Cherenkov Detectorfor a Water Cherenkov Detector

(initiated by C2GT study)(initiated by C2GT study)

A. Braem, E. Chesi, Christian Joram, J. Séguinot, P. WeilhammerCERN / PH

representing the C2GT team

www.cern.ch/ssd/HPD

C. Joram CERN / PH International Scoping Study CERN Meeting September 2005 2

Hybrid Photon Detectors (HPD)Hybrid Photon Detectors (HPD)

V

photocathode

focusing electrodes

siliconsensor+ FEelectronics

e-

light quantum

segmentedsiliconsensor

Combination of sensitivity of PMT with excellent spatial and energy resolution of silicon sensor

Gain:

Gain is achieved in a single dissipative step !

eV6.3CUeG

UC = 20 kV G ~ 5000

GFG small compared to electronics

C. Joram CERN / PH International Scoping Study CERN Meeting September 2005 3

Classical HPD designsClassical HPD designs

• Proximity focused• 1:1 imaging• Operates in axial

magnetic fields.

• ‘Fountain’ focused• Demagnification D• No real focusing

ballistic point spread• Intolerant to magnetic fields

• ‘Cross’ focused• Demagnification D• Focusing leads to

small point spread• Intolerant to magnetic fields

C. Joram CERN / PH International Scoping Study CERN Meeting September 2005 4

Main advantages of HPD technologyMain advantages of HPD technology• Excellent signal definition• Allows for photon counting• Free choice of segmentation (50 m - 10 mm)• Uniform sensitivity and gain• no dead zones between pixels

2 p.e.

Drawbacks

• Rel. low gain (3000 - 8000) low noise electronics required

• Expressed sensitivity to magnetic fields

CMS HCAL, 19-pixel HPD (DEP, NL)

Pad HPD

Single padS/N up to 20

1 p.e.

2 p.e.

3 p.e.

Viking VA3 chip (peak = 1.3 s, 300 e- ENC)UC = -26 kV

elec

tron

ic n

oise

cut

• e- backscattering from Si surface continuous ‘background’

det (p.e.) ~ 85-95 %

pulse height (ADC counts)

C. Joram CERN / PH International Scoping Study CERN Meeting September 2005 5

Some prototype developments for physics experimentsSome prototype developments for physics experimentsdeveloped and built at CERN

NIM A 442 (2000) 128-135 NIM A 478 (2002) 400-403

www.cern.ch/ssd/HPD

5-inch 10-inch

127 mm Ø, D ~ 2.5,2048 channels 1mm2

bialkali photocathode

NIM A 504 (2003) 19 NIM A 518 (2004) 574-578

254 mm Ø, D ~ 4,2048 channels 1mm2

bialkali photocathode

C. Joram CERN / PH International Scoping Study CERN Meeting September 2005 6

e, ,

CC reactionsin H2O

segmented photosensitive ‘wall’about 250 × 250 m2

Fiducial detectorvolume ~ 1.5 Mt

(E below threshold for production)

~ 50 m

e±, ± 42°Che

renk

ov lig

ht

Cherenkov light

Principle of neutrino detection by Cherenkov effectin C2GT (CERN To Gulf of Taranto)

The wall is made of ~600 mechanical modules (10 x 10 m2), each carrying 49 optical modules.

10

m

A. Ball et al., C2GT, Memorandum, CERN-SPSC-2004-025, SPSC-M-723

A. Ball et al., Proc. of the RICH2004 conference, subm. to NIM A

C. Joram CERN / PH International Scoping Study CERN Meeting September 2005 7

The ideal photodetector for C2GTThe ideal photodetector for C2GT

• large size (>10”)

• spherical shape, must fit in a pressure sphere

• ± 120° angle of acceptance

• optimized QE for 300 < < 600 nm

• single photon sensitive

• timing resolution 1-2 ns

• no spatial resolution required

• electronics included

• cost-effective (need ~32.000 !)

• adapted to industrial fabrication

120°

42°

C. Joram CERN / PH International Scoping Study CERN Meeting September 2005 8

Design considerations Silicon or dynodes ?Silicon or dynodes ?

silicon dynodesAngular coverage > ± 90° Angular coverage < ± 60°

TTS < 1 ns (FWHM) ? TTS ~ 3 ns

det (1 p.e.) ~93% (back scattering) det (1 p.e.) ~90%

‘delicate’ components (Si, ceramics) only metal inside tube

no magnetic shielding needed (to be verified !)

mu-metal shielding needed

Uniform response over ± 90° Uniform response over ± 25°

If silicon, p-i-n diode or APD ?p-i-n diode or APD ?

p-i-n APD

Signal at 20 kV: 5·103 e, G = 1 2-3 ·105 e, G ~ 50

Cd = 35 pF/cm2 , ENF ~ 1 Cd = 300 - 1500 pF/cm2, ENF = 2 - 5

detector size ‘arbitrary’ few sizes available (1,3,5 mm Ø, 5x5 mm2)

C. Joram CERN / PH International Scoping Study CERN Meeting September 2005 9

432 mm

(17”)380 mm

PAHV

optical gel(refr. index matching + insulation)

benthos sphere(432 / 404)

electrical feed-throughsvalve

standard base plate of HPD 10” (prel. version).

C2GT Optical Module

15

joint Si sensor

ceramic support

C. Joram CERN / PH International Scoping Study CERN Meeting September 2005 10

ElectrostaticsElectrostatics

Simulations Simulations with SIMION 3Dwith SIMION 3D

110º

-150 -100 -50distance from centre (mm)

-15000

-10000

-5000

0

5000

10000

15000

(V

) or

E (V

/cm

)

~ 1/r

E ~ 1/r2

• Potential and field distribution similar to a point charge.

• Low field at photocathode (~100 V/cm),

• Very high field close to Si sensor (~10,000 V/cm). Try to reduce by a grounded field cage around Si sensor.

- 20 kV

120º

- 20.3 kV

C. Joram CERN / PH International Scoping Study CERN Meeting September 2005 11

9.0 9.5 10.0 10.5 11.0 11.5 12.0Transit Time (ns)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

rel.

freq

uenc

y (a

.u.)

= 0.15 nsRMS = 0.18 ns

angles > 110°

0 20 40 60 80 100 1209

10

11

12

Tra

nsit

time

(ns)

polar angle (deg.)

0 < < 120°

Transit TimeTransit Time

C. Joram CERN / PH International Scoping Study CERN Meeting September 2005 12

Effect of angular spread and magnetic fieldEffect of angular spread and magnetic field

• Ekin(t0) = 2 eV (conservative)

• -40º em 40º (rel. to surface)

0.45 Gauss

B

E~1/r2 produces a focusing effect

effect of earth magentic field seems

to be marginal

C. Joram CERN / PH International Scoping Study CERN Meeting September 2005 13

0 V

~23

mm

500 V/mm

1000 V/mm

1500 V/mm

2000 V/mm

Is E-field around Si sensor too high ?Is E-field around Si sensor too high ?

A grounded grid could help

0 V

Gradients at Si sensor reduced to ~500 V/mm. High field around grid, f(diam.).

-11.000 V

Gradients up to 2000 V/mm at edges of Si sensor

500 V/mm

Grid at 0 V

Grid at ‘natural’ potential: -11 kV Grid at 0 V

C. Joram CERN / PH International Scoping Study CERN Meeting September 2005 14

A ‘half-scale’ prototypeA ‘half-scale’ prototype208 mm (~8-inch)

Al coating2 rings

Development in collaboration with Photonis-DEP, C. Fontaine et al.

C. Joram CERN / PH International Scoping Study CERN Meeting September 2005 15

Enevlope fabricated by SVT, France.

Sphere blown from a glass cylinder !

C. Joram CERN / PH International Scoping Study CERN Meeting September 2005 16

Current status: All components ready ‘dry assembly’. Processing soon.

First tests will be done with a metal cube rather than Si sensors

C. Joram CERN / PH International Scoping Study CERN Meeting September 2005 17

Fabrication

Large number of tubes needed industrial fabrication internal process (as for large PMTs).

However, this would lead to a ‘pollution’ of Si-sensor and high field region with Cs/K/Sb. protect by mask (difficult!) use ‘hybrid’ process ?

Q.E. monitoring

C. Joram CERN / PH International Scoping Study CERN Meeting September 2005 18

Processing in the existing set-up at CERN, used for 5” and 10” HPDs

10ӯ

Only minor mechanical adaptations required.

5ӯ

heating element

glass envelope Sb source

K/Cs sources

base plate with pre-mounted sensor

C. Joram CERN / PH International Scoping Study CERN Meeting September 2005 19

SummarySummary

A spherical HPD has been designed which promises to meet the C2GT requirements

Strong featuresStrong features• Large acceptance• Low TTS• Good signal definition

IssuesIssues• Low noise and short peaking time for large detector capacitance • Industrialization of production

A half scale propotype tube is under construction in collaboration with Photonis to prove the basic characteristics of this design.