AXIAL PET - HPD --------------------

Tests WLS - Readout axial PET - Bari - Janvier 2007

AXIAL PET - HPD

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AXIAL COORDINATE RECONSTRUCTION WITH WLS STRIPS

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1) Introduction

2) Experimental test set-up

3) Readout of the LYSO crystal bars

Performances : luminosity – Energy resolution

4) Readout of the WLS strips

Performances : pe statistic – σz resolution

5) Expected performances of a brain PET scanner

6) WLS readout with SiPM

J. Seguinot


HPD2

HPD1

x

y

z

HPD2

HPD1

Original concept ;

N1

N2

Z = 0.5·(λeff.ln(N1/N2) + Lc)

σz =(λeff/√2No)sqrt{ ez/λeff + e(L-z)/λeff }

For Lc = 10 cm and λeff ~ 7 to 8 cm : σz ~4 mm.

N1 = (No/2)·e-z/λeff

N2 = (No/2)·e-(L-z)/λeff

Npe(z) = N1 + N2

No (nber of pe’s detected for λeff = ∞) characterizes the luminosity of the LYSO cristal matrix.

Energy resolution : The statistical term σE/E = ENF/sqrt( Npe) is limited by the value of λeff required to minimize σz.

New concept :

would allow a digital z reconstruction by mean of a transversal readout of each layer of the LYSO crystal bar matrix by mean of WLS strips (33 x 13 strips – L = 6 cm, w=3mm).

The principle is well known and largely used in calorimetry

For a WLS strip width w = 3 mm, σz ≤ w/√12 =.9 mm

As λeff ~ 400 mm for polished crystal bars, the energy resolution will also be significantly improved with respect to the original concept.

(was proposed by D.Schinzel)


Transmisson coefficient (Fresnel corrected) ELJEN WLS (10x) sheets

00.10.20.30.40.50.60.70.80.9

1

200 300 400 500 600lambda (nm)

T

TR 0.7 FC

TR 1.0 FC

TR 1.5 FC

EJ-280 wavelength shifting plastics

Emission peak : 490 nm Absorption peak : 425 nm Quantum efficiency : 0.86 Decay time : 8.5 ns Density : 1.02 – ref. index : 1.58

LYSO emission peak

Absorption coefficient ~ 2.5 mm-1

Physical properties :

Using a reflector on the opposite side of the strip more than 80% of the light emitted by the LYSO crystals is absorbed in a WLS of 0.7 mm.


Light attenuation length in the ELJEN EJ-280 WLS strips

The mean light intensity transmitted over 3 cm (6 cm is the max. length of the strips) is 78 to 88% for a thickness of 0.7 and 1 mm respectively.


vacuum pump (turbo)P < 10-5 mbar

DAQReadout

card

Pulsed LED (blue)

MgF2

sapphire

mirror

Si sensor (300 m)208 pads (4×4 mm2)

VME

-UPC = 0 -20 kV

collimator

H2 self triggered U.V flash lampΔt ~ 10 nsf ~ 40 Hz

collimator

U.V light beam – Ф ~ 2 mm

Advantages :

1) The el. beam energy can be varied by adjusting the nber of pe’s emitted from the PC and the acceleration voltage up to 30 kV.

2) Possibility to scan a surface by moving the mirror.

PC

EXPERIMENTAL SET UP FOR HPD - TESTSThe principle is similar for the tests of the WLS strips : a narrow electron beam (Ф ~ 2mm – Δt ~ 10 ns) is generated from the photoemission of a CsI-PC. The light source is a self triggered UV flash lamp.

Inconvenient :

The set-up must be vacuum pumped to < 10-5 mbar.


WLS strips – ELJEN – 1 mm thick, 3 x 60 mm2

Polished LYSO bars – 3x3x100 mm3 - delivered by St Gobain (Fr)

10 mm PMT Hamamatsu R1650 (inside the enceinte)

PMT Photonis XP 3102 (outside the enceinte)Support of the PC (10 nm CsI

deposited on a CaF2 crystal)

Position adjustable U.V light spot beam (~2mm in diameter - Δt~ 10 ns) scanning normal to the WLS strips

Set-up is turbo-pumped at ~ 10 exp-6 mbar

- 25 kV Sapphire window (1mm)


PMT – Hamamatsu R1650

LYSO – crystal bar 3.2x3.2x10 cm3

PMT – Photonis XP3102

Support CaF2 crystal coated with a transmittive CsI photocathode (10 nm thickness)

U.V light spot beam

2 mm diam.- Δt ~10 ns

- 25 kV

Anode at gnd potential with a mesh (T=.90) above the crystals

Set-up turbo-pumped at ~10 exp.(-6) mbar

The extremities opposite to the PMTs of the LYSO bars and of the WLS strips

are aluminized

All the components of the set-up have been produced by the workshop in Bari.

Sapphire window


Gain PMT = 6. 106

Readout of the LYSO bars


Gain PMT = 1.3.106

Readout of the WLS strips


Crystal LYSO

Single photoelectron response

~45 photoelectrons from scintillation are detected without back scattering – Relative Light Output (RLO) = 0.8

X-HPD DEVELOPMENT

Aluminized cylindrical crystal : Ф=12 mm, h= 18 mm


Typical pulses from the readout of the LYSO bars


The dispersion reflects the statistic on the nber of pe’s emitted from the CsI-PC

850 pe’s

WLS test set-up

Response of the LYSO bars for Uacc=25 kV and UPMT= - 1 kV

( ADC counts )


ESTIMATIONS OF THE PERFORMANCES OF THE LYSO BARS :

Photomultiplier Photonis XP 3102 : Gain G = 6·106 at – 1 kV

Attenuation signal = 18 db ( ~ factor 8)

ADC : sensitivity = 50 fC/ADC count

Electron beam : The number of primary pe’s emitted from the PC is determined by the relation,

Npe = (<μ>/σ)2 where <μ> is the mean value of the LYSO charge distribution of ecart type σ.

Uacc = acceleration voltage (kV)

RLO = Relative Light Output of the scintillation yield with the acceleration voltage (0.8 at 25 kV)

Npe(LYSO) = {8 x 50· <μ>· 10-15} / G x 1.6·10-19 = 2.5·106·<μ>/ G

= (No/2) · {e-z/λeff + R · e-(L – z)/λeff}

R = reflectivity at the end bar

Experimental conditions : z = 6 cm, L = 10 cm, λeff = 40 cm, R = 0.8 ?

The formula above, if correct, shows that the number of detected pe’s is~ independent of z. This is very good for the discrimination in energy needed for the trigger and, moreover, the resolution in energy is minimized.

At 511 keV equivalent energy,

No = 1794·<μ> / (Npe·G · Uacc·RLO)

with Uacc (kV) and G PMT = 6.10-6


scanning across to the LYSO bars

0

200

400

600

800

1000

1200

1400

60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77

z (mm)

Ch

arg

e (A

DC

co

un

ts)

Series1

-U (pc) = 25 kV

U(PMT)=1. kV

On the plateau, 12 ± .4 primary pe’s from the PC are detected and the mean charge (1189 ± 24 ADC counts) corresponds to about 500 pe’s from the scintillation light in the LYSO crystal bars, i.e ~40 pe’s/ inc.pe, as expected from the previous tests for the X-HPD project.

Assuming an effective light attenuation length λeff of 40 cm for polished crystal bars and a reflectivity of 80% of the Al coating at the end of the bars opposite to the PMT, one can estimate No to ~1400.

LYSO

Data :12/9/2006

The shape is exactly what one expectsfor a total crystal width of 6.4 mm and

a beam spot of ~ 2mm in diameter


Npe(LYSO) vs Uacc

0

100

200

300

400

500

600

700

800

900

1000

5 10 15 20 25 30 35

Uacc (kV)

Np

e (L

YS

O)

Npe inc. = 17.3 ± 0.7 for 10≤ Uacc ≤ 27.5

LSF : -113.1 + 36.51 x U(kV)

Cut-off : 3.1 kV

120 keV350 keV

Data : 1/3/2007


No x RLO vs Uacc

800

900

1000

1100

1200

1300

1400

1500

1600

5 10 15 20 25 30

Uacc (kV)

No

x R

LO

LSF

Data

RLO (Relative Light Output)

RLO = 0.8

RLO = 0.7

LUMINOSITY LYSO : No ~ 1600

Data : 1/3/2007

In our publication (Il Nuovo Cimento Vol.29 C, N. 4) we have quoted 1525 for LSO assuming a QE of 30%


Typical readout pulses from the WLS strips

Sum of channels 1 and 2


Charge distribution : sum of the two WLS strips response.

40 pe’s

Uacc = 25 kV

(ADC counts)


PERFORMANCES OF THE WLS STRIPS :

Photomultipliers : Hamamatsu R1650 at 1.2 kV

Gain PM = 1.3 106

ADC sensitivity 50 fC/ADC count

<QWLS> : mean charge in ADC count

(Npe)WLS = 0.24 · <QWLS>

At 511 keV equivalent energy :

(Npe)WLS = 122.6 · <QWLS> / (Npe·Uacc·RLO)

The ratio :

R = (Npe)WLS/ (Npe)LYSO = 0.58 · <QWLS>/<μLYSO>

(PMT (LYSO) at -1 kV and the PMT(LYSO) at -1.2 kV).

is a rough estimation of the ratio of the detected photon yields

assuming :

QE PMT(LYSO) at 425 nm = .30 QE PMT(WLS) at 500 nm = .15


Scanning across the WLS strips

0

10

20

30

40

50

60

70

80

90

50 55 60 65 70 75 80 85

z (mm)

Ch

arg

e A

DC

co

un

ts

Series1

Series2

Strip # 1 Strip # 2

At z = 64 mm (centre of strip #1) ) about 60 % of the total charge is detected, i.e ~ 40 pe’s at 511 keV and 40% on strip#2 i.e ~20 pe’s.

3 mm

Data 12/19/2006

<Npe inc.> = 7.3 ± 1.3

WLS

As both strips are hit when scanning over the total width covered by the strips, a precise digital z reconstruction cannot be obtained unless to apply a high discrimination level proportional to the level of light detected.

Indeed, the aperture of the light cone at the input of the WLS is large (~400) and the spot size is still increased because the scintillation light in these tests is generated on the opposite side of the strips since the range of electrons of 25 keV in the crystals is very small.


Scanning normal to the WLS strips

0

2

4

6

8

10

12

14

16

18

20

50 55 60 65 70 75 80

z (m m )

Q(A

DC

co

un

ts)/

ph

oto

elec

tro

n

Series1

6 mm WLS strips

At z = 66 mm,

Q(WLS)/inc. pe = 16 – 4 (bkg subst.) ADC counts

For a PM gain = 1.3.106 :

Npe(WLS)/20 keV inc. pe = 2.9

i.e, ~ 70 pe’s at 511 keV for a 1 mm WLS strip and a photon detector quantum efficiency of ~15%.

Data : 12/19/2006

Uacc = 25 kV

The tail is a bkg due to photons which escape from the lateral surface of the LYSO bars.

Q = Q2 + Q31 mm WLS strips


Q wls(ADC counts) vs Uacc

40

60

80

100

120

140

160

180

200

5 10 15 20 25 30

Uacc (kV)

Qw

ls (A

DC

cou

nts)

Data : 1/3/2007

Z mir = 67 mm


Npe (WLS) vs Uacc

0

5

10

15

20

25

30

35

40

45

50

5 10 15 20 25 30

Uacc (kV)

Np

e (

WL

S)

LSF : -4.5 + 1.676 x Uacc(kV)

<Npe> inc.=17.3 ± 0.7

E=120 keV E=350 keV

Data : 1/3/2007

Cut-off : 2.7 kV

Z mir = 67 mm

At 511 keV : Npe (WLS) ~ 60

1 mm WLS strips


Z RECONSTRUCTION

52

54

56

58

60

62

64

66

59 61 63 65 67 69 71 73 75

z mirror (mm)

z re

con

stru

cted

(m

m)

Q2 Q3

WLS

ZREC = 60 + (Q2 – Q3) / (Q2 + Q3)

Q2 = 0

Q3 = 0

LSF : m = 0.9

The slope m = 0.9 instead of 1 expected is due to the different detection efficiency of the WLS strips.

Data - 12/22/2006

ANALOGUE Z RECONSTRUCTION


Z (mm)

Uacc = 25 kV

As the z reconstruction is, “de facto”, analogue, the resolution σZ varies as 1/√Npe(WLS) or 1/√E, and is estimated to ~ 1mm for 511 keV gamma from the extrapolation of the next measurement.

( E = 350 keV )


Resolution z

0

0.5

1

1.5

2

2.5

3

0.1 0.15 0.2 0.25 0.3 0.35 0.4

1 / sqrt Npe(WLS)

Sig

ma

z (m

m)

LSF : σz = -0.21 + 9.785/√Npe

10 kV

27.5 kV

Data : 1/3/2007

350 keV

120 keV


From our publication in Il Nuovo Cimento Vol. 29 C,N. 4

HPD – PET LYSO 1.85 2.35 2.1 ~ 9 mm3 (Digital z reconstruction) FWHM values

Expected

This is ~ physical limit


PET-HPD with LYSO and digital z reconstruction

ΔE/E (511 keV) = 9 %

ΔV (mm3) = 9

ΔE/E . ΔV (%.mm3) = 81

FoM ~ 3


x

y

z

The concept would allow a better azimuthal coverage with a single HPD.


READOUT OF THE WLS STRIPS BY MEAN OF SiPM’s

We need a specific SiPM design :

• Active area : 3 x 1 mm2 (cross section of the WLS strip 3 x 0.7 mm2)

• Nber of pixels : 600 – i.e 200 / mm2 (dynamic range is sufficient)

• Pixel size : 70 x 70 μm2

• Sensitivity to single photons (R quenching ~ 250 k)

Advantages:

By increasing the pixel size :

1) One increases the active area. Consequently the quantum efficiency could, hopefully, be better than 30% at 500 nm ( ? ) instead of ~15% for a bi-alkali PC, a large gain, especially for the detection of Compton events.

2) The capacitance per pixel being higher, the gain would be increased.


SLIDES FOR COMPLEMENT OF INFORMATION


0

20

40

60

80

100

120

140

55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75

Q2+Q3

Q2-zero_scaled

Q3-zero

Z (mm)

Ch

arg

e (

AD

C c

ou

nts

)


Charge ( V.s.)

Gain PMT = 3.10 exp.6

AXIAL PET - HPD --------------------

Documents

wls readout

wls strips performances

transversal readout

axial pet hpd

wls strip width w

lyso crystal bars performances

luminosity energy resolution

pe statistic z resolution

Transcript of AXIAL PET - HPD --------------------

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