increase in photon collection from a yap:ce matrix coupled to wave lenght shifting fibres n. belcari...

Increase in Photon Collection from a YAP:Ce MatrixIncrease in Photon Collection from a YAP:Ce Matrix

Coupled to Wave Lenght Shifting FibresCoupled to Wave Lenght Shifting Fibres

N. Belcaria, A. Del Guerraa, A. Vaianoa, C. Damianib , G. Di Domenicob, G. Zavattinib

a Department of Physics, University of Pisa and INFN, Pisab Department of Physics, University of Ferrara and INFN, Ferrara

e-mail: [email protected]

mailto:[email protected]

Two coincidence photons (511 keV)

Interaction within the detector

Point of positron annihilation

PET PrincipleDetermination of the Point of Interactionby means of a matrix of scintillating finger crystalsIncident photon

Pixel X&Y coordinates

Determination of the lines of flight

Image reconstruction

Position sensitive readout

WLS Fibres in PET ApplicationsWLS Fibres in PET Applications

CsI(Na)

CsI(Na)

CsI(Na)

Light Mixer

PMT 1 PMT 2

X-Fibres

Z-Fibres

Y-Fibres

White Reflector

W.Worstell et al. IEEE Trans. Nucl. Sci. VOL.45, NO.6, DECEMBER 1998

LSO

LSO

LSO

LSOY-Fibres

X-Fibres

Light Pipes

M.B.Williams et al. IEEE Trans. Nucl . Sci. VOL.45, No.2, APRIL 1998

61 Pixel HPD

61 Pixel HPD

LSO crystals

CsI

H.Herbert et al.IEEE Medical Imaging Conference Record,Seattle 1999

YAP-(S)PET scanner

PSPMTs

YAP:CeScintillator(2x2x30 mm)

• Dual Modality (PET/SPECT)• New Scintillator (YAP:Ce)• Position Sensitive PMT• Small Animal Imaging

4x4x3cm4x4x3cm33 YAP:Ce Matrix YAP:Ce Matrix

Name

Chemical formula

Density Effective atomic number

Refractive index

Scintillation decay time

Peak emission wavelenght

Light Yield

Yttrium Aluminum Yttrium Aluminum Oxide Perovskite Oxide Perovskite

YAlOYAlO33:Ce:Ce++

5.35 5.35 g/cmg/cm33

2626

1.951.95

27ns27ns

17000 ph17000 ph /MeV/MeV

Material

Refractive index

Peak absorbance wavelenght

Core

Cladding

CoreCladding

CoreCladding

1.05 1.05 g/cmg/cm33

1.591.591.491.49

350nm350nm

440nm440nm

Density1.19 1.19 g/cmg/cm33

WLS Fibre Kuraray SCSF-78WLS Fibre Kuraray SCSF-78

370nm370nm

PolymethilmethacrylatePolymethilmethacrylate

PolystyrenePolystyrene

Peak emission wavelenght

Section Size

SCSF-38

SCSF-78

1mm x 1mm1mm x 1mm

2mm x 2mm2mm x 2mm

Position MeasurementSet Up

Position MeasurementSet Up

PSPMT R2486 #2

YAP:Ce 5x5 crystal matrix

PMT Philips XP2020 #3

YAP:Ce 19 mm

10 mm thick22 Nasource

SCSF 78square section

fibres

PSPMT R2486 #1

crystal image (PSPMT #1)

Fibre image (PSPMT #2)

Event: (#1) AND (#2) AND (#3)

Kuraray WLS Fibres Attenuation LengthKuraray WLS Fibres Attenuation Length

1 m long, 2 mm thick SCSF 78 1 m long, 1 mm thick SCSF 38

FIT: I = P1exp(-P2x) + P3exp(-P4x)

Evaluation of the Light Yield of the SystemEvaluation of the Light Yield of the System(YAP crystal + WLS Fibre + HPD)(YAP crystal + WLS Fibre + HPD)

14

18%

70%

17.3%

80%

84%

96%

90%

5.7%

17000 /MeV

0.511 MeV

14 p.e. ( both sides )

74 ph ( both sides )

44 ph ( One side )

255 ph ( one side )

319 ph ( one side )

379 ph ( one side )

421 ph ( one side )

468 ph ( one side )

8687 ph. ( one side )

Expected Photoelectrons

Detector Quantum Efficiency

Reflection At The Fibre Aluminized End

Fibre Quantum Efficiency

YAP Emission -Fibre Absorption Overlapping

Light Transmitted at the Air-Fibre Interface

Light Transmitted at the YAP-Air Interface

Light Escaping From One Crystal End (geometry + attenuation)

YAP Emission

Energy Deposit

Light escaping from One Fibre End(geometry + attenuation)

Experimental Set-up

System Light Yield MeasurementSystem Light Yield Measurement

Single Pixel HPD

22Na Black Tape

Photopeakevent

selection

HPD spectrum

PS-PMT Spectrum

YAP:Ce 5x5crystal matrix

(10x10x30 mm3 )

Hamamatsu R2486 PMT

2x2x1 mm2 square section fiber

Using WLS Fibres and an HPD to read-out a YAP:Ce matrix we found a signal of about

10 p.e. for 511 keV events.

The discrepancy with the expected signal could be due to imperfection in the set-up

The readout of a YAP:Ce matrix by WLS Fibres is then feasible but:

Low light yield Þ low detection efficiency (expecially for lower energy events)

We need to increase the light yield!We need to increase the light yield!

Where are weWhere are we

P(0 p.e.) 8.2% reduction in single detection efficiency of about 8.2% for 122keV events.

Cuttingangle

New shape of the finger crystal:New shape of the finger crystal:"V-cut""V-cut"

Uniform scintillation

Total light

Total reflection

Reflective layer

The best cutting angle () is 45°

“Flat”

“V-cut”

Comparison between “Flat” and “V-cut 45°” finger crystalComparison between “Flat” and “V-cut 45°” finger crystal

"Uniform*" 5.4 9.5 (+76%){Light (%) Flat "V-cut"

RT: 8.2Al: 1.3

* Uniform scintillation within the crystal

(conservative choice)

Extracted light:Monte Carlo simulation

Practical use of“V-cut 45°” crystals

Problems

Very difficult to be produced High cost ( +70%)

A new solution: "Half Pipe-cut" A new solution: "Half Pipe-cut"

"Uniform" 5.4 8.2 (+52%){RT: 7.7Al: 1.6

Light (%) Flat "Half Pipe -cut"

"Real"* 5.7 9.3 (+63%)

RT: 6.9Al: 1.3

{

Advantages:

Easier production Not so expensive Easier coupling with more versatile

*Considering an exponential attenuation of 511 keV photons within the YAP:Ce (=2.7 cm)

Round WLS fibres

Crystal -4 -3 -2 -1 0 1 2 3 4

25.9

2.8 2.8

30.9

9.7 9.7 3.9 3.9 1.6 1.6

0.5

0.5

Bac

k-re

flec

ted

ligh

t (%

)

Coupling YAP-WLS Fibres: Air or Optical Grease?Coupling YAP-WLS Fibres: Air or Optical Grease?

AIR

OPTICAL GREASE

Core: Clad :

AIR

GREASE

3.1

3.1

15.4

0

81.5

96.9

18.5

Trasmitted Lost

Core:

Clad :

3.1

% %

Light trasmission in the fiber

AIR

GREASE

% of light emerging from the crystal

Light trasmission in the crystal Back-reflection into the crystals

Combined factor

AIR

GREASE

9.3

20.5

9.3% 18.5% = 1.7%

20.5% 3.1% (5.3%*)= 0.6% (1.1%*)

(*Using a double cladding fibre)

Evaluation of the Light Yieldof the System Evaluation of the Light Yieldof the System half-pipe shaped YAP crystal (with air interface) + WLS Fibre + HPDhalf-pipe shaped YAP crystal (with air interface) + WLS Fibre + HPD

23 (14)

18%

70%

17.3%

80%

84%

96%

90%

9.3 %

17000 /MeV

0.511 MeV

23 p.e. ( both sides )

126 ph. ( both sides )

81 ph. ( One side )







Expected Photoelectrons

Detector Quantum Efficiency

Reflection At The Fibre Aluminized End

Fibre Quantum Efficiency

YAP Emission -Fibre Absorption Overlapping

Light Transmitted at the Air-Fibre Interface

Light Transmitted at the YAP- Air Interface

Light Escaping From One Crystal End (geometry + attenuation)

YAP Emission

Energy Deposit

Light escaping from One Fibre End*(geometry + attenuation)

* For sake of semplicity we used the same parameters as the square section fibres

10 mm

2mm diameter WLS Fibre

30 m

m To PSPMT

to 61 pixel HPD

Light yield measurement Test of various WLS fibres Imaging capabilities

To be done:

Future studies: Future studies: 555 YAP:Ce "Half-Pipe" Crystal Matrix5 YAP:Ce "Half-Pipe" Crystal Matrix

Conclusions Conclusions We simulated various geometry and coupling metods

• Using “Half-Pipe” shaped crystals and “air” coupling we expect:

23 23 p.e. (instead of 1414 p.e.) for 511keV events.

This fact could helps in reducing the efficiency loss due to the low light yield:

e.g., for 122 keV we expect to measure 4.1 4.1 p.e. (instead of 2.52.5 p.e.)

corresponding to a reduction in single detection efficiency of 1.7% 1.7% (instead of 8.2% 8.2%)

• Experimental measurements are needed.

increase in photon collection from a yap:ce matrix coupled to wave lenght shifting fibres n. belcari...

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