Time of Flight: the - PowerPoint Presentation

Slide1

Time of Flight: the scintillator perspective

Paul Lecoq

CERN, GenevaSlide2

Where is the limit?

Philips and Siemens TOF PET

achieve

550 to 650ps timing

resolution

About 9cm

localization

along

the LOR

Can

we

approach

the

limit

of 100ps (1.5cm)?

Can

scintillators

satisfy

this

goal?Slide3

For the scintillator the important parameters are

Time structure of the pulse

Light

yield

Light transport affecting pulse shape, photon statistics and LY

Timing

parameters

decay

time of

the

fast

component

Photodetector

excess

noise factor

number

of

photoelectrons generated by the fast component

General

assumption

,

based

on

Hyman

theorySlide4

Light output: LYSO example

Statistics

on about 1000 LYSO pixels 2x2x20mm

3

produced by CPI for the ClearPEM-Sonic project (CERIMED)

Mean

value = 18615 ph/MeV

For 511

KeV

and 25%QE: 2378

phe

Assuming

ENF= 1.1

Nphe

/ENF ≈ 2200

pheSlide5

t

= 40 ns

N

phe

t

= 40 ns

N

phe

N

phe

N

phe

Statistical limit on timing resolution

W(Q,t

) is the time interval distribution between photoelectrons

= the probability

density that the interval between event Q-1 and event Q is

t

=

time resolution when the signal is triggered on the

Q

th

photoelectron

LSO

N

phe

=

2200Slide6

Light generation

Rare Earth

4f

5dSlide7

Rise time is as important as decay time

Rise timeSlide8

Photon counting approach

LYSO, 2200pe

detected

,

t

d

=40ns

t

r=0ns

t

r=0.2ns

tr=0.5ns

t

r

=1nsSlide9

Cross-Luminescent crystals (very

fast

,

low

LY)BaF2 (1400ph/MeV) but 600ps decay time produces more photons in the first ns (1100) than

LSO (670)!Direct bandgap

semiconductors S. Derenzo, SCINT2001Sub-ns band-to-band

recombination in ZnO, CuI,PbI2, HgI

2NanocrystalsBright and sub-ns

emission due to quantum confinement

Faster

than Ce3+?Intrinsic limit

at 17ns

Pr

3+

Pr

3+

5d-4f transition

is

always

1.55eV

higher

than

for Ce

3+Slide10

Material

Density (g/cm

3

)

Radiation length X

0

(cm)

Refraction index n

Critical angle

Fondamental absorption (nm)

Cerenkov

threshold energy for

e

(

KeV

)

Recoil

e

range

a

bove

C threshold (

m

m)

#

C photons / 511KeV

g

ray

*

PbWO

4

8.28

0.89

2.2

63°

370

63

513

21

LSO:Ce

7.4

1.14

1.82

57°

190

101

527

15

LuAG:Ce

6.73

1.41

1.84

57°

177

97

582

22

LuAP:Ce

8.34

1.1

1.95

59°

146

84

487

28

Ultimately

fast

using

Cerenkov

emission

?

Even

low

enegy

g

ray

produce

Cerenkov

emission

in dense,

high

n

materials

This emission is instantaneous with a 1/l2 spectrum

*

Low

wavelength

cut-off

set

at

250nm for

calculations

on LSO,

LuAG

and

LuAP

Ce absorption bands

subtracted

from

Cerenkov

transparency

windowSlide11

22Na

PMT left (2150V)

PMT right (1500V)

LuAG

2013 (

undoped

-> shows no scintillation)

LSO 1121

8

cm

8

cm

Crystals wrapped on

5 sides with

teflon

.

Scope

Coincidence:

Th_left

=-4mV,

th_right

=-500mV

CFD

LuAG

Cerenkov

/LYSO Scintillation

coincidence

measurement

FWHM=374ps

LuAG

=259ps

FWHM=650ps

LuAG

=587psSlide12

Light Transport-49° <

θ

< 49°

Fast

forward detection 17.2%131° < θ < 229°

Delayed back detection

17.2%57° < θ < 123° Fast escape on the sides 54.5% 49° < θ

< 57° and 123° < θ < 131°

infinite bouncing 11.1%

For a 2x2x20 mm

3

LSO

crystal

Maximum time

spread

related to difference in travel path

is

424

ps

peak

to

peak

≈

162

ps

FWHMSlide13

Photonic crystals to improve light extraction

Periodic

medium

allowing

to couple light propagation modes

inside

and

outside

the

crystal

M.

Kronberger

, E.

Auffray

, P. Lecoq, Probing the concept of Photonics Crystals on Scintillating Materials

TNS on

Nucl

. Sc. Vol.55, Nb3, June 2008,

p

. 1102-1106

24%

34%Slide14

LuAP

Light gain

2.1

LYSO

Light gain

2.08

BGO

Light gain

2.11

LuAG:Ce

Light gain

1.92

Expected

Light Output Gain

for

different

crystals

Litrani

+ CAMFR simulationSlide15

How does the PhC work?

Section of the plane crystal- air interface: (EM –

fieldplot

)

Crystal- air interface with

PhC

grating:

θ

>

θ

c

Total Reflection at the interface

Extracted Mode

θ

>

θ

c

Diffracted modes interfere constructively in the

PhC

- grating and are therefore able to escape the CrystalSlide16

PhC fabricationNano

Lithography

PhC

is produced in cooperation with the INL (

Institut des Nanotechnologies de Lyon) Three step approach:Sputter deposition of an auxiliary layer

Electron beam lithography (EBL)

Reactive ion etching (RIE)

RAITH® lithography kit:Slide17

PhC fabrication

Reactive Ion Etching (RIE)

Chemically reactive plasma removes Si

3

N4 not covered by the resist

Change the composition of the reactive plasma to remove the resist (PMMA) without etching the Si

3N4

x

z

y

Scintillator

ITO

Si

3

N

4

a

Hole depth:

300nm

h

ole diameter:

200nm

x

z

y

Scintillator

ITO

Si

3

N

4

Ion Bombardment

PMMA ResistSlide18

PhC fabrication

Results

Scanning Electron Images:

a = 340nm

D = 200nmSlide19

Use larger LYSO crystal: 10x10mm2 to

avoid

edge

effects6 different patches (2.6mm x 1.2mm) and 1 (1.2mm x 0.3mm) of different PhC

patterns

PhC first results

0°

45°

PreliminarySlide20

PhC improves light extraction eficiency

But

also

collimation of the

extracted lightSlide21

ConclusionsTiming

resolution

improves

with lower thresholdUltimate resolution

implies single photon

countingHigh light yield is mandatory100’000ph/MeV achievable with scintillatorsShort

decay time15-20ns is the

limit for bright scintillators (LaBr

3)

1ns achievable but with poor LY

Crossluminescent materialsSeverely quenched

self-activated scintillators

SHORT RISE TIMEDifficult to break the barrier of 100psSlide22

New approaches?

Conclusions

Crystals

with

a highly

populated donor band (ZnO)Metamaterials loaded with quantum dotsMake use of

Cerenkov lightImprove light collection with

photonic crystalsSlide23

Our TeamCERNEtiennette Auffray

Stefan

Gundacker

Hartmut

HillemannsPierre JarronArno KnapitschPaul Lecoq

Tom MeyerKristof PauwelsFrançois

Powolny

Nanotechnology

Institute, Lyon

Jean-Louis Leclercq

Xavier

Letartre

Christian

Seassal