11

The X-HPD: Development of a large spherical hybrid photodetector

A. Braem +, C. Joram +, J. Séguinot +, L. Pierre *, P. Lavoute *

+ CERN, Geneva (CH) * Photonis SAS, Brive (F)

Introduction: C2GT, requirements, Hybrid tubes

The X-HPD concept

Proto 0: A first sealed prototype with metal anode

Proto 1: Towards a full X-HPD

Summary and Outlook

22

e, ,

CC reactionsin H2O

segmented photosensitive ‘wall’about 250 × 250 m2

Fiducial detectormass ~ 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., Eur. Phys. J. C (2007)

33

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°

44

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

55

Concept of a spherical tube with central spacial anode

• Radial electric field negligible transit time spread ~100% collection efficiency no magnetic shielding required

• Large viewing angle (d ~ 3)

• Possibility of anode segmentationimaging capability (limited!)

• Sensitivity gain through ‘Double-cathode effect’

T ~ 0.4 QE

QE

66

Silicon is a good anode material for a HPD

• Gain = e·UC-A / 3.6 eV ~ 5000 for 20 kV

• Back scattering rel. small BS = 0.18 (20 kV)

• However: large tube requires large Si sensor

large capacitance (35 pF/cm2)

ns timing only with segmented sensor (few pF)

Scintillator as alternative anode ? Readout by conv. PMT.• Idea is not new ! Philips SMART tube, Lake Baikal QUASAR tube.

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

both tubes used thindisks of P47 phosphor (YSO:Ce powder)Philips made

~30 tubes(1980s-1992)

200 tubesin operation since 1998

77

PMTXP2982

pressure sphere

The Philips SMART Tube The QUASAR 370 Tube

G. van Aller et al. A "smart" 35cm Diameter Photomultiplier. Helvetia Physica Acta, 59, 1119 ff., 1986.

gprimary ~ 35, t ~ 2.5 ns / Npe

R. Bagduev et al., Nucl. Instr. Meth. A 420 (1999) 138

88

SMART and Quasar tubes were the first X-HPDs

• however the use of a disk shaped scintillator didn’t allow to exploit full potential.

• need to use fast scintillator, e.g. cube or cylinder.

• Require• high light yield high gain• short decay time• Low Z preferable low back scattering coefficient • Emission around = 400 nm

LY (LY ( / keV / keV nsns ZZeffeff BSBS emissionemission (nm) (nm)

YAP:CeYAP:Ce 1818 2727 3232 ~0.35~0.35 370370

LYSO:CeLYSO:Ce 2525 ~40~40 6464 ~0.45~0.45 420420

LaBrLaBr33:Ce:Ce 6363 3030 4747 ~0.4~0.4 360360

LaBr3 is quite hygroscopic

• side and top require conductive and reflective coating define el. potential, avoid light leaking out.

• Problem: crystals have high refr. index (~1.8) large fraction of light is trapped inside crystal due to total internal refection.

99

X-HPD ½ scale prototype (208 mm Ø)

LYSO 12 Ø × 18 mm

1010

Electrostatics (SIMION 3D)

Expected performance• Viewing angle 120°• X-tal hit probability ~ 100%• flight time spread ~0.4 ns (RMS)

Simulation (SIMION 3D). 1/2 scale prototype. electron flight time

0

1

2

3

4

5

6

7

8

0 30 60 90 120

polar angle (degr.)

flig

ht t

ime

(n

s)

mean = 6.1 nsTTS = 0.37 ns (RMS)

Can be furtherimproved by optimizingbulb geometry.

1111

First build a HPD with metal-cube anode (1 cm3)‘Proto 0’

0

10

20

30

40

50

60

300 350 400 450 500 550 600

(nm)

QE

(%

)

0

0.5

1

1.5

2

2.5

3

ratio

QE top QE side

Built in CERN plant.

0

0.2

0.4

0.6

0.8

1

1.2

-180 -135 -90 -45 0 45 90 135 180

f,q (degrees)

rel.

sen

sitiv

ity

f

q

measured at Photonis

A. Braem et al., NIM A 570 (2007) 467-474

1212

Prepare and characterize components for real X-HPD ‘Proto 1’

PMTPhotonis XP3102 (25 mm Ø)

electrical feedthrough Al coated

1313Calibration: 1 p.e. = 0.0514·109 Vs

ENF ~ 1.15

PMT XP3102 has verygood signal properties.

1414

Test set-up

Lab e- accelerator

based on pulsedphotoelectronsfrom a CsI cathode.

DAQ on digital scope.LeCroy Waverunner LT344.

vacuum pump (turbo)P < 10-5 mbar

Pulsed H2 flash lamp(VUV)

MgF2

sapphirewindow

mirror

-UPC = 0 -30 kV

collimator

CsI photocathode

X-HPD anode

1515

PM T pulse (LYSO, Ee = 20 keV)

-100

-80

-60

-40

-20

0

20

-50 0 50 100 150 200 250

time (ns)

pu

lse

he

igh

t (m

V) data

fit (45 ns)

Pulse shape on scope.

Contributions from individual photoelectrons clearly visible

1616

Charge amplitude

for single photoelectrons… and multi-photoelectrons

modest photo counting ability

1717

0

5

10

15

20

25

30

0.0 5.0 10.0 15.0 20.0 25.0 30.0

U (kV)

N p

.e.

0.0

0.2

0.4

0.6

0.8

1.0

Rel

. Lig

ht O

utpu

t

LYSOdata

rel. light output of LSO (literature)

fit (relative light output)

Calibration of charge amplitude (for single incident photoelectrons) using single photon response of PMT

= primary gain of X-HPD

1818

LYSO, Ee = 27.5 keV

Fit = Gauss + Exp.

Wrote a very simple M.C.:

Ingredients: BS + E’e spectrum + statistics

Input: <Np.e.>, Poisson

e-

BS BS

Ee

E’e lost ?

PMT

e- back-scattering from LYSO

Single photoelectron spectra are well described by Gaussian + Exponential.

Where does the non-Gaussian part come from ?

Ee-E’e

E.H. Darlington, J. Phys. D, Vol. 8 (1975) 85

Expect to loose <10% of single photoelectrons by a 3 pedestal cut.

single p.e. detection efficiency of X-HPD ~90%

1919

What happens to back scattered electrons in radial E-field of X-HPD ?• Low energy electrons (E < 5 keV) are reabsorbed within << 1 ns.• For E > 5 keV re-absorption (within ~1 ns) or loss. Depends on hit position and

emission angle.

X-tal top

X-tal side

2020

First photoelectron timing

y = 8.03x-0.47

y = 36.40x-0.42

1.5

1.61.7

1.8

1.92

2.1

2.2

2.32.4

2.5

12.5 15 17.5 20 22.5 25 27.5 30

U (kV)

RMS

time

reso

l. (n

s)

8

8.5

9

9.5

10

10.5

11

11.5

12

time

(ns)

LYSO crystal

Timing studies with first electron method

clock start clock stop

2121

Include time calculation in M.C. Needed 1 ns overall jitter to describe data.

M.C. shows stronger tails than actually measured, because all BS electrons are assumed to be lost.

2222

Summary and Outlook

X-HPD • is an attractive concept for

large area photodetection• promises higher

performance than classicalPMT

• proof of principle OK• components characterized

Next steps• Make X-HPD with LYSO

X-anode in ~2-3 weeks• Test it at CERN and at

Photonis + …• Improve light extraction

from LYSO crystal. Ideas exist.

2323

Back-up Transparencies

2424

• Ee = 25 keV - 0.5 keV (E-loss in Al layer) × 25 ph/kev (scint. yield)

612 ph

• d/4 = 0.16(qcrit = 33.5°) 98 ph

• Fresnel reflection losses ~ -10% 90 ph

• <QE> ~ 0.25 22.5 p.e.

X-tal with Al coating

MgF2 anti-reflection coating at exit side

Window (+ opt. grease)

Light guide (Al coated)

PMT window (+ opt. grease)

Photocathode

Estimate of the photoelectric yield

2525

LSO crystal with PMT readout. The plot shows the reconstructed number of photoelectrons as function of the applied voltage at the CsI cathode. Full and open symbols correspond to UPMT = 1000 V and 900 V, respectively. The fits to the data are explained in the text. The relative light output of LSO [15][16] is shown as dashed line (secondary axis).

0

5

10

15

20

25

30

35

40

45

0.0 10.0 20.0 30.0U (kV)

N p

.e.

0.0

0.2

0.4

0.6

0.8

1.0

Re

l. L

igh

t Ou

tpu

t LSO

YAP

rel. light output (LSO)

A. Braem et al., NIM A 570 (2007) 467-474

Crystals coupled via sapphire window to PMT. Use of optical grease.

2626LSO crystal read out with PMT. As previous figure, however the light intensity of the flash tube was increased. On average there is about 1 photoelectron. The pedestal peak is cut at 250 counts.

A. Braem et al., NIM A 570 (2007) 467-474

LSO crystal coupled via sapphire window to PMT. Use of optical grease.

2727

PhotonisXP 1807

PhotonisXP 1806

PhotonisXP 1805

PhotonisXP 1804

q ~ 30° q ~ 35°q ~ 35°

q ~ 35° q ~ 31° q ~ 31°

2828

CERN CERN photocathode photocathode ‘transfer’ plant ‘transfer’ plant