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
PD June 13-14 , 2007 Perugia - Italy
Roberto Pani
[email protected]
Scintillation crystal readout technique
Individual
Coupling
Pixellated
crystal
PD June 13-14 , 2007 Perugia - Italy
Light
Sharing
Continuous
crystal
Roberto Pani
[email protected]
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)
PD June 13-14 , 2007 Perugia - Italy
Roberto Pani
[email protected]
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
PD June 13-14 , 2007 Perugia - Italy
Roberto Pani
[email protected]
Light sharing technique
Scintillation light flash on
photocathode
X & Y Position Centroid Algorithm
Si i ni
Position: X =
Si ni
E  Si ni
Energy:
Charge distribution sampling
by anode array
Anode array (Hamamatsu H8500)
k
1
2
3
4
5
6
7
8
nik
1 2 3 4 5 6 7 8…i
ni   nik
k
PD June 13-14 , 2007 Perugia - Italy
Roberto Pani
[email protected]
Position determination in light sharing
technique
Image PSF
1mm FWHM
one γ-ray
interaction
 im 
 lightPSF
n phe
Many γ-ray
interactions
Scintillation light PSF
15 mm FWHM
counts (a.u.)
5000
height analisys
ER 
3750
2500
1250
1
n phe
0
0
100
200
Pulse height (a.u.)
PD June 13-14 , 2007 Perugia - Italy
300
SR 
measured position
(mm)
Co57 pulse
50
Position linearity
40
30
l
20
10
dX
dx
0
 image
0
l
10
20
30
40
mechanical position (mm)
Roberto Pani
[email protected]
50
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
PD June 13-14 , 2007 Perugia - Italy
Roberto Pani
[email protected]
Point Spread Function and critical angle c
Planar crystal / PMT
glass window
n2
c  sen ( )  52o
n0
1
PD June 13-14 , 2007 Perugia - Italy
Pixellated crystal / PMT glass window
Light output angle < 45°
Roberto Pani
[email protected]
Pixellated scintillation crystal
NaI:Tl
1m m x 1mm x 4 mm
+
H8500 MAPMT
Pixel Spatial resolution < 1.3 mm
Image Spatial resolution > 1.3 mm
50
Poor energy resolution ~ 14%
800
400
30
Counts
counts
40
20
10
0
0
0
100
200
300
image pixel
PD June 13-14 , 2007 Perugia - Italy
400
500
600
0
500
channel
1000
1500
Roberto Pani
[email protected]
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
500
450
700
Very good linearity !!!
400
Image Position (chn)
600
500
400
300
200
350
300
250
200
150
100
100
50
0
50
100
150
200
250
300
350
400
450
500
0
0
5
10
15
20
25
30
35
40
45
Mechanical Position (mm)
PD June 13-14 , 2007 Perugia - Italy
Roberto Pani
[email protected]
50
Modulation Transfer Function

MTF  f    LSF x  e2ifx dx

Detector assembly:
(1.5 mm hole, 22 cm lenght)
Multi-anode read-out
1
MTF (norm)
MAPMT Hamamatsu H8500
LEGP collimator
Crystal samples:
MC simulation LEGP
LaBr3(Ce) continuous crystal
NaI(Tl) 1,1 mm pixel array
0.75
0.5
0.25
LaBr3:Ce continuous, 5mm thick
NaI:Tl array, 1.1mm pixel 1.3 pitch
0
0
0.2
0.4
0.6
Spatial Frequencies (mm-1)
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
PD June 13-14 , 2007 Perugia - Italy
Roberto Pani
[email protected]
0.8
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.
There are few predictions if energy resolution or light output dominates the
intrinsic spatial resolution in light sharing
 Low afterglow
for high counting rate
PD June 13-14 , 2007 Perugia - Italy
Roberto Pani
[email protected]
Scintillator crystals for SPECT
Density
(g/cm3)
Atten.len.
@
140keV*
(mm)
Z eff.
Photofraction
(%)
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
40,000
27
(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
-
-
472
535
PD June 13-14 , 2007 Perugia - Italy
Roberto Pani
[email protected]
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
PD June 13-14 , 2007 Perugia - Italy
Roberto Pani
[email protected]
Scintillator crystals for PET
Light yield
(ph/MeV)
Decay
time
(ns)
Relative
coinc.
efficiency
Coinc.
timing res.
(ps)
ΔE/E
@511
keV
(PMT)
43
9,000
300
100%
3000
10%
65
34
26,000
40
90%
300
10%
420
54
-
30,000
40
-
-
11%
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
5.60
472
535
-
29
90,000
30
73%
200
<15%
Z eff.
Photofraction
(%)
480
83
7.4
420
7.1
LaCl3:Ce
Density
(g/cm3)
Emiss.
max
(nm)
BGO
7.1
Lu2SiO5:Ce
(LSO)
Lu2(1-x)Y2xSiO5:Ce
(LYSO)
PD June 13-14 , 2007 Perugia - Italy
Roberto Pani
[email protected]
Energy Resolution
Re ( E ) 
E
E
    ENC

 2.355  ( Re.sci )  
  N    N  M
ph  
ph

2
Intrinsic Scintillation Contribute




2
Electronic noise
 Non homogeneities
Photodetector and preamplifier system
 Non proportionality of scintillation response
Statistical generation of the signal
[Equivalent noise charge
– E. Gatti, NIM Phys Res 1990 ]
Nph: number of photons in a scintillation flash
 : worsening of the Poisson behaviour
 : Quantum Efficiency
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
PD June 13-14 , 2007 Perugia - Italy
Roberto Pani
[email protected]
Intrinsic Scintillator Energy Resolution
Crystal
Nph/
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
0
PMT
XP2254B Philips
Allier (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
YAP
21000
10300
4.3
2.5
2.3
2.6
PMT XP20Y0
Photonis
APD –
6307073500
Adv.Phot.Inc.
Moszynski (2006)
Moszynski (2000)
1.3
1.25
Relative Light Yield
1.2
1.15
1.1
1.05
1
0.95
0.9
NaI(Tl)A
LaBr3(Ce)B
0.85
0.8
Luminosity (phe @ 662keV - PMT 25% QE)
W.Moses, NIM A, 487 (2002)
PD June 13-14 , 2007 Perugia - Italy
1
10
A
Energy (keV)
100
1000
Roberto
Pani
– Prescott and Narayan, NIM
A, 75 (1969)
– G.Bizarri, [email protected]
TNS, Vol 53,02 (2006)
B
Is the QE really useful?
1° PMT HIGH QE:
Hamamatsu R7600-200
10%
1%
 QE max. = 41.6 % 10
@ 380 nm
 Number of dinode = 10
 Gain= 2.0 E+06 @ HV= -700 V
PD June 13-14 , 2007 Perugia - Italy
100%
Energy Resolution
Energy Resolution
100%
Crystal Test: LaBr3:Ce Cylinder
(½”Ø  ½” thickness)
R7600-200 (QE
R7600-200 (QE = 41%)
R6231
standard
R6231 standard PMT (QE
= 30%)
H8500 MA-PMT (QE =H8500
22%) MA-PMT
10%
1%
10
100
100
Energy (keV)
Energy (keV)
Roberto Pani
[email protected]
1000
Is the QE really useful?
Pulse heigh Resolution & Coincidence
Resolving Time:
2° PMT HIGH QE:
Hamamatsu R8900-00-C12
Crystal TEST: LSO 4 x 4 x 20 mm3
Source
: Na22 (E @ 511keV)
PHR (%)
 QE max. = 42 % @ 350 nm
 Number of dinode = 12
 Gain= 1.0 E+06 @ HV= -800 V
PMT position
CRT (psec)
Standard
HIGH QE
Position
Type
Type
(QE=22%)
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
15.4
590
*Courtesy
of 17.1
Hamamatsu Photonics
K.K.
(Iwata City 530
- Japan)
PD June 13-14 , 2007 Perugia - Italy
Roberto Pani
[email protected]
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
300
No glass window
Q.E = 0.60 – Phe n° = 5102
100
90
250
200
80
150
70
900
60
100
800
50
40
700
30
600
20
500
10
400
0
50
0
S.R.= 0.75 mm
E.R. = 2.3%
300
S.R.=0.82 mm
E.R. = 5.1 %
( 6.9 % including intrinsic energy
resolution of LaBr3:Ce)
200
100
0
S.R.= 0.60 mm
E.R. = 1.4 %
( 4.8 % including intrinsic energy
resolution of LaBr3:Ce)
PD June 13-14 , 2007 Perugia - Italy
( 5.1 % including intrinsic energy
resolution of LaBr3:Ce)
Roberto Pani
[email protected]
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
PD June 13-14 , 2007 Perugia - Italy
Roberto Pani
[email protected]