Photodetector requirements for gamma ray imaging with scintillation crystals Roberto Pani

PD June 13-14 , 2007 Perugia - ItalyRoberto Pani

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Photodetector requirements for gamma ray imaging with

scintillation crystalsRoberto Pani

INFN and Sapienza-University of Rome Italy

1th Workshop on “Photo Detection”

June 13 - 14 , 2007 Perugia, Italy


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Individual Coupling

LightSharing

Pixellatedcrystal

Continuouscrystal

Scintillation crystal readout technique


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Individual coupling technique

Munich APD PET*

4 x 8 APD Array (Hamamatsu Photonics)2 x 2 x 6 mm3 LSO individual coupledIntrinsic FWHM ~ 1.2 mm

* Courtesy of Roger Lecomte – Université de Sherbrooke (Québec, Canada)


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Individual coupling technique High packing fraction > 80%

Spatial resolution limited by crystal pixel size ( 1mm tomography, > 1mm planar image) Electronic readout up to 20000 chains (SPET)

Single photoelectron readout not needed

Low noise to allow 140 keV photon energy detection

High gain (104 or more) not needed

Energy resolution depending on scintillation crystal / photodetector


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Light sharing technique

1 2 3 4 5 6 7 8 … i

k

k

ii nn

kin1

2345678

Anode array (Hamamatsu H8500)Charge distribution sampling by anode array

Position:

Energy:

i i ni

i ni X =

i ni

X & Y Position Centroid AlgorithmScintillation light flash on photocathode

k


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phe

lightPSFim n

phenER 1

0

1250

2500

3750

5000

0 100 200 300

Pulse height (a.u.)

coun

ts (a

.u.)

Co57 pulse height analisys

0

10

20

30

40

50

0 10 20 30 40 50mechanical position (mm)

mea

sure

d po

sitio

n (m

m)

Position linearity

dxdX

l

Image PSF 1mm FWHM

Many γ-ray interactions

one γ-ray interaction

Scintillation light PSF 15 mm FWHM

lSR image

Position determination in light sharing technique


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Light sharing technique

Spatial resolution limited by crystal pixel size ( scintillation array)

Spatial resolution not limited for continuous crystal

Low number of electronic chains

Single photoelectron readout needed

Energy resolution depending on scintillation crystal /photodetector

High gain ( >104 ) is needed

Timing/rise time < 500 ps for ToF


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Point Spread Function and critical angle c

oc n

nsen 52)(0

21

Pixellated crystal / PMT glass window

Light output angle < 45°

Planar crystal / PMT glass window


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Pixellated scintillation crystal

NaI:Tl1m m x 1mm x 4 mm

+H8500 MAPMT

0

400

800

0 500 1000 1500channel

Cou

nts

0

10

20

30

40

50

0 100 200 300 400 500 600

image pixel

coun

ts

Pixel Spatial resolution < 1.3 mm Poor energy resolution ~ 14%Image Spatial resolution > 1.3 mm


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Continuous scintillator crystal1.5 mm step scannig – 0.4 mm Ø Tc99m point source

LaBr3:Ce49 mm x 49 mm x 4 mm+ 3 mm glass window

H8500 MAPMT

0

50

100

150

200

250

300

350

400

450

500

0 5 10 15 20 25 30 35 40 45 50

Mechanical Position (mm)

Imag

e Po

sitio

n (c

hn)

0

100

200

300

400

500

600

700

50 100 150 200 250 300 350 400 450 500

Best Values:

Energy resolution = 9.6 % (@ 1000V) Overall Spatial Resolution= 1.1 mm Intrinsic Spatial Resolution= 1.0 mm

Very good linearity !!!


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0

0.25

0.5

0.75

1

0 0.2 0.4 0.6 0.8Spatial Frequencies (mm-1)

MTF

(nor

m)

MC simulation LEGPLaBr3(Ce) continuous crystalNaI(Tl) 1,1 mm pixel array

MTF for Continuous CrystalSpatial Resolution limited to LEGPEnhancement in Contrast - increased AUC (Area Under Curve) NO restrictions in image digitization (Nyquist frequency not limited from image pixel)Continuous position responseIncreased detection efficiency

Detector assembly:MAPMT Hamamatsu H8500LEGP collimator (1.5 mm hole, 22 cm lenght)Multi-anode read-out

dxexLSFfMTF fxi2

Crystal samples:LaBr3:Ce continuous, 5mm thickNaI:Tl array, 1.1mm pixel 1.3 pitch

Modulation Transfer Function


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Scintillation crystal: requirements for SPECT (@140 keV)

Z 40 → Photofraction greater than 70%

High density (> 3 gr/cc) → Reduction of crystal thickness to obtain 80-90% efficiency ( important for light collection) Refraction index close to 1.5 → To avoid light loosing due to critical angle (continuous crystal) Decay time 1 s→ To obtain 200 kHz max.

High luminous efficiency (> 20000 at suitable wavelength) → To improve: Decoding crystal pixel in scintillation array

Spatial resolution, in continuous crystal Energy resolution.

Low afterglow for high counting rate

There are few predictions if energy resolution or light output dominates the intrinsic spatial resolution in light sharing


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Density

(g/cm3)

Atten.len. @

140keV*(mm)

Z eff.

Photo-fraction

(%)

Light yield(ph/MeV)

Decay time (ns)

Refr.index

ΔE/E(@140keV

)(PMT)

Emiss max(nm)

NaI:Tl 3.67 3.76 51.0 84 41,000 230 1.85 9% 410

CsI(Tl) 4.51 2.55 52.0 86 66,000 630 1.80 14% 565

YAP 5.50 10.00 36.0 50 21,000 27 1.95 20% 350

LaCl3:Ce 3.86 4.22 49.5 80 40,00027

(65%)1.90 8% 350

LaBr3:Ce 5.07 3.32 47.4 79 63,000 16 (97%) 1.90 6% 380

LuI3:Ce 5.60 1.70 - 90 90,000 30 - -472535

Scintillator crystals for SPECT


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Scintillation crystal: requirements for PET (@ 511 keV)

Z 50 → Photofraction greater than 30%

High density ( >7 gr/cc) → To obtain, in 30 mm crystal length, 50% coincidence efficiency and reduction parallax error for small animal imaging. Scintillation decay time 300 ns → To allow good coincidence time resolution. Time resolution better than 0.5 ns can reduce random coincidences (50 % in a 3D PET) and time of flight can be realized.High luminous efficiency > 8000 ph/MeV →

To enable block detectors with a greater number of pixel (from 8 8 BGO to 16 16 LaBr3(Ce) crystal pixel/module).

Improvement in energy resolution reduces scatter background (25% Compton scattering / 25% “true” events in a 3D PET).

Low afterglow for high counting rate


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Density

(g/cm3)

Emiss. max(nm)

Z eff.Photo-

fraction (%)

Light yield(ph/MeV)

Decay time (ns)

Relative coinc.

efficiency

Coinc. timing res.

(ps)

ΔE/E @511

keV(PMT)

BGO 7.1 480 83 43 9,000 300 100% 3000 10%

Lu2SiO5:Ce

(LSO)7.4 420 65 34 26,000 40 90% 300 10%

Lu2(1-x)Y2xSiO5:Ce

(LYSO)7.1 420 54 - 30,000 40 - - 11%

LaCl3:Ce 3.86 350 49.5 15 46,000 20 (65%) 36% 265 4%

LaBr3:Ce 5.07 380 47.4 14 63,000 16 (97%) 42% 260 3%

LuI3:Ce 5.60472535

- 29 90,000 30 73% 200 <15%

Scintillator crystals for PET


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2

2. )(355.2)(

MNENC

NR

EE

ERphph

sciee

Intrinsic Scintillation Contribute Non homogeneities Non proportionality of scintillation response

Statistical generation of the signalNph: number of photons in a scintillation flash worsening of the Poisson behaviour Quantum Efficiency

Electronic noisePhotodetector and preamplifier system

[Equivalent noise charge – E. Gatti, NIM Phys Res 1990 ]

PMT P-I-N APD SSD SiPM 1.25 1 2 1 1

at pk 30% 80% 80% > 80% 60%

M 5 105 1 < 103 1 104-105

ENC/M 0 370 20 25 0

Energy Resolution


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B – G.Bizarri, IEEE TNS, Vol 53,02 (2006)

LaBr3(Ce)BNaI(Tl)A

W.Moses, NIM A, 487 (2002)

Luminosity (phe @ 662keV - PMT 25% QE)

CrystalNph/MeV

Nel

@ 662 keV

ER(%)@ 662 keV

ERscint.

(%)ERst

(%)ERnoise

(%)Light detector Ref.

NaI(Tl) 40000 6000 6.7 5.9 3.2 0 PMT typical

CsI(Tl) 65000 6000 6.6 5.8 3.2 0PMT

XP2254B PhilipsAllier (1998)

CsI(Tl) 65000 26000 4.3 3.8 1.5 1.2 SDD Fiorini (1997)

LaBr3(Ce) 63000 12000 3.6 2.2 2.5 0 PMT XP20Y0 Photonis Moszynski (2006)

LaBr3(Ce) 63000 19000 2.7 2.0 1.7 0.5 SDD Fiorini (2006)

LSO 2000 5300 8.8 7.8 3.6 0 PMT XP20Y0 Photonis Moszynski (2006)

BGO 9000 880 11.7 5.8 8.0 0 PMT XP20Y0 Photonis Moszynski (2006)

YAP 21000 10300 4.3 2.5 2.3 2.6APD –

6307073500 Adv.Phot.Inc.

Moszynski (2000)

Intrinsic Scintillator Energy Resolution


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Is the QE really useful?

Crystal Test: LaBr3:Ce Cylinder (½”Ø ½” thickness)

1 inch

1 inc

h

1° PMT HIGH QE:Hamamatsu R7600-200

QE max. = 41.6 % @ 380 nm Number of dinode = 10 Gain= 2.0 E+06 @ HV= -700 V

1%

10%

100%

10 100 1000

Energy (keV)

R7600-200 (QE = 41%)R6231 standard PMT (QE = 30%)H8500 MA-PMT (QE = 22%)

1%

10%

100%

10 100 1000

Ene

rgy

Res

olut

ion


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Is the QE really useful?Pulse heigh Resolution & Coincidence

Resolving Time:

Crystal TEST: LSO 4 x 4 x 20 mm3 Source : Na22 (E @ 511keV)

PHR (%) CRT (psec)

PositionStandard

Type (QE=22%)

HIGH QE Type

Standard Type

(QE=22%)

HIGH QE Type

A 15.1 14.0 460 400

B 16.0 14.5 500 440

C 16.4 14.8 520 460

D 15.8 14.8 550 480

E 16.0 15.4 600 510

F 17.1 15.4 590 530

2° PMT HIGH QE:Hamamatsu R8900-00-C12

QE max. = 42 % @ 350 nm Number of dinode = 12 Gain= 1.0 E+06 @ HV= -800 V

PMT position

*Courtesy of Hamamatsu Photonics K.K. (Iwata City - Japan)


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Critical Angle & Q.E. :MC Simulation GEANT 4 Scintillation crystal : LaBr3:Ce continuous crystal

50 x 50 x 4 mm3 ( white entrance face – black edges) 8 8 Photodetector array ( 6.0 mm pitch) 140 keV photon energy

3 mm glass window Q.E = 0.22 – Phe n°=1153

No glass window Q.E = 0.60 – Phe n° = 5102

No glass window Q.E = 0.22 – Phe n°=1860

S.R.= 0.60 mmE.R. = 1.4 %

( 4.8 % including intrinsic energy resolution of LaBr3:Ce)

S.R.=0.82 mmE.R. = 5.1 %


S.R.= 0.75 mmE.R. = 2.3%



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Conclusion

LaBr3:Ce seems a very promising crystal for SPET ( PET ToF)

application

Light sharing on continuous crystal requires position sensitive

photodetectors with superior performances

Intrinsic energy resolution of scintillators can seriously limit the energy

resolution response of a high Q.E. photodetectors

Removing glass window( critical angle) in scintillator coupling, could

strongly enhance imaging performances

Photodetector requirements for gamma ray imaging with scintillation crystals Roberto Pani

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