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1th Workshop on “Photo Detection” June 13 - 14 , 2007 Perugia, Italy. Photodetector requirements for gamma ray imaging with scintillation crystals Roberto Pani INFN and Sapienza-University of Rome Italy. Scintillation crystal readout technique. Light Sharing. Individual Coupling.

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slide1

1th Workshop on

“Photo Detection”

June 13 - 14 , 2007 Perugia, Italy

Photodetector requirements for gamma ray imaging with scintillation crystals

Roberto Pani

INFN and Sapienza-University of Rome Italy

slide2

Scintillation crystal readout technique

Light

Sharing

Individual

Coupling

Continuous

crystal

Pixellated

crystal

slide3

Individual coupling technique

Munich APD PET*

4 x 8 APD Array (Hamamatsu Photonics)

2 x 2 x 6 mm3 LSO individual coupled

Intrinsic FWHM ~ 1.2 mm

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

slide4

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
slide5

X & Y Position Centroid Algorithm

Si i ni

Position:

X =

Si ni

E = Si ni

Energy:

Light sharing technique

Scintillation light flash on

photocathode

Anode array (Hamamatsu H8500)

Charge distribution sampling

by anode array

1

2

3

4

5

6

7

8

k

1 2 3 4 5 6 7 8 … i

slide6

Image PSF1mm FWHM

oneγ-ray interaction

Manyγ-rayinteractions

Scintillation light PSF 15 mm FWHM

Position linearity

Co57 pulse height analisys

Position determination in light sharing technique

slide7

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
slide8

Point Spread Function and critical angle c

Planar crystal / PMT glass window

Pixellated crystal / PMT glass window

Light output angle < 45°

slide9

Pixellated scintillation crystal

NaI:Tl

1m m x 1mm x 4 mm

+

H8500 MAPMT

Poor energy resolution ~ 14%

Pixel Spatial resolution < 1.3 mm

Image Spatial resolution > 1.3 mm

slide10

Continuous scintillator crystal

1.5 mm step scannig – 0.4 mm Ø Tc99m point source

LaBr3:Ce

49 mm x 49 mm x 4 mm

+ 3 mm glass window

H8500 MAPMT

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

Very good linearity !!!

slide11

Detector assembly:

  • MAPMT Hamamatsu H8500
  • LEGP collimator
  • (1.5 mm hole, 22 cm lenght)
  • Multi-anode read-out
  • Crystal samples:
  • LaBr3:Ce continuous, 5mm thick
  • NaI:Tl array, 1.1mm pixel 1.3 pitch
  • MTF for Continuous Crystal
  • Spatial Resolution limited to LEGP
  • Enhancement in Contrast - increased AUC (Area Under Curve)
  • NO restrictions in image digitization (Nyquist frequency not limited from image pixel)
  • Continuous position response
  • Increased detection efficiency

Modulation Transfer Function

slide12

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 ms→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.
  • Lowafterglow for high counting rate

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

slide14

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).
  •  Improvementin energy resolution reduces scatter background (25%
  • Compton scattering / 25% “true” events in a 3D PET).
  • Lowafterglow for high counting rate
slide16

Energy Resolution

  • Intrinsic Scintillation Contribute
  • Non homogeneities
  • Non proportionality of scintillation response

Electronic noise

Photodetector and preamplifier system

[Equivalent noise charge

– E. Gatti, NIM Phys Res 1990]

Statistical generation of the signal

Nph: number of photons in a scintillation flash

a : worsening of the Poisson behaviour

h : Quantum Efficiency

slide17

Intrinsic Scintillator Energy Resolution

NaI(Tl)A

LaBr3(Ce)B

A – Prescott and Narayan, NIM A, 75 (1969)

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

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

W.Moses, NIM A, 487 (2002)

is the qe really useful

1 inch

1 inch

Is the QE really useful?

1° PMT HIGH QE:

Hamamatsu R7600-200

Crystal Test: LaBr3:Ce Cylinder

(½”Ø  ½” thickness)

  • QE max. = 41.6 % @ 380 nm
  • Number of dinode = 10
  • Gain= 2.0 E+06 @ HV= -700 V
is the qe really useful1

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
Is the QE really useful?

Pulse heigh Resolution & Coincidence Resolving Time:

Crystal TEST: LSO 4 x 4 x 20 mm3

Source : Na22 (Eg @ 511keV)

PMT position

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

slide20

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

No glass window

Q.E = 0.22 –Phe n°=1860

3 mm glass window

Q.E = 0.22 – Phe n°=1153

No glass window

Q.E = 0.60 – Phe n° = 5102

S.R.= 0.75mm

E.R. = 2.3%

( 5.1 % including intrinsic energy resolution of LaBr3:Ce)

S.R.=0.82 mm

E.R. = 5.1 %

( 6.9 % including intrinsic energy resolution of LaBr3:Ce)

S.R.= 0.60 mm

E.R. = 1.4 %

( 4.8 % including intrinsic energy

resolution of LaBr3:Ce)

slide21

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