Challenge the future

DelftUniversity ofTechnology

Inorganic Scintillators for the detection of ionizing radiation

P. Dorenbos

Inorganic Scintillators for the detection ofIonizing radiationP. Dorenbos

Scintillator applications• security• medical diagnostics• industry + science• space explorations• high energy physics

Topics

• How it works?• The light output How many photons are emitted?• The luminescence What is color and speed of emission?• The scintillation mechanism What happens on an atomic scale?

• How it performs?• Energy resolution What is the status and are the limits?• Time resolution What is the status and are the limits?

• How it is used?• Current developments What are the hot topics today?

The scintillation process

ionizingradiation

γ-photonneutronparticle

inorganiccompound(crystal) =host lattice

ionizationtrack

photon flash

luminescenceCentre

1MeV 50000 photonsin between a lot is happening!

electric signal

X-rayGamma-ray

ΑlphaProtonElectron

Radiation detector

Schematic of a radiation detector

scintillator photon detector

Scintillators Light sensors

inorganic crystals/powder organic plastics/crystals glass liquid gas

Human eyePhotomultiplier tubessilicon photo diodesSi-PMTsCCDsgas-filled detectors

The Bolognian stone

Human eye1900 1920 1940 1960 1980 2000 2020

CaWO4ZnS:Ag

NaI:TlCdWO4CsI:TlCsFCsILiI:Eu

silicate glass:CeCaF2:EuZnO:GaCdS:InCsI:NaBaF2 (slow)Bi4Ge3O12

YAlO3:CeBaF2( fast)

(Y,Gd)2O3:CeCeF3PbWO4Lu2SiO5:CeLuAlO3:Ce

RbGd2Br7:CeLaCl3:CeLaBr3:Ce

year

History of scintillator discovery

Invention ofPhotomultipliertube

1900 1920 1940 1960 1980 2000 2020

CaWO4ZnS:Ag

NaI:TlCdWO4CsI:TlCsFCsILiI:Eu

silicate glass:CeCaF2:EuZnO:GaCdS:InCsI:NaBaF2 (slow)Bi4Ge3O12

YAlO3:CeBaF2( fast)

(Y,Gd)2O3:CeCeF3PbWO4Lu2SiO5:CeLuAlO3:Ce

RbGd2Br7:CeLaCl3:CeLaBr3:Ce

year

History of scintillator discoveryTU-Delft discoveries

Main drivers of scintillation research

• Homeland security• High resolution scintillators for radioisotope identification• Shortage of 3Hethermal neutron scintillators

• Space explorations• High resolution, low intrinsic count rate scintillators

• Medical diagnostics• TOF-PET very fast risetime scintillators• SPECT

• High energy physics• High density and high radiation hardness• Large volumes at low cost

Important gamma ray scintillator parameters

• High light output Y (photons/MeV)• fast scintillation speed τs (ns)• low energy resolution RFWHM (%)• high density for γ detection ρ (g/cm3)• large size of crystal 10-100-1000 cm3

• low cost per cm3

• low afterglow (low phosphorescence)• low background count rate (low intrinsic activity)

• absence of radioactive isotopes

Importance depends on application

NaI (at 80 K) 3.67 1.75 60 303 76000NaI(Tl) 3.67 1.75 230 415 38000CsI(Tl) 4.51 1.75 3340 540 65000BaF2 4.89 1.5 630 310 9500(valence e− to core) 0.6 220 1400Bi4Ge3O12 (BGO) 7.13 2.15 300 480 8200PbWO4Lu2SiO5:Ce (LSO)YAlO3:Ce (YAP)LaCl3:CeLaBr3:Ce

8.287.45.373.75.1

2.201.81.951.82.1

1047273517

470420370350380

10025000180005000070000

scintillator ρ n τ λmax Nphot/MeV

Properties of inorganic scintillators

dr. A. Gektin and prof. C.W.E. van Eijk with a NaI:Tl ingot

The growth of NaI:Tl scintillation crystals

Examples or bare scintillation crystals

CsI(Tl)

NaI(Tl)

CdWO4BGO

Examples of packed crystals

NaI(Tl) crystalsGamma camera crystal

NaI:Tl+ crystal 20-60 cm diameter and 2-25 mm thick

7 to 61 photomultipliers

The scintillation process

ionizingradiation

γ-photonneutronparticle

inorganiccompound(crystal) =host lattice

ionizationtrack

photon flash

luminescenceCentre

1MeV 50000 photons

A lot!What is happening in between?

The scintillation process

• Three phases1. The interaction phase + thermalization phase (ps)2. The charge carrier and energy migration phase (ps-ns-ms)3. The luminescence phase (ns-μs)

photon

Valence band

Cond. band

Luminescence center (LC)

Eg=3-10 eV

VIS 400 - 800 nm 3.1 - 1.6 eVUV 180 - 400 nm 6.9 - 3.1 eVVUV < 180 nm > 6.9 eV

Phase I: The interaction phase• Gamma ray interaction energetic electron

• The photo-electric effect Zeff3-4 (dominant <100 keV)

• The Compton scattering ρ (dominant around 1MeV)• Pair production Eγ >1.02 MeV

140 keV

511 keV

Z(I) =53Z(Cs) =55Z(La) =57Z(Gd) =64Z(Lu) =71Z(Pb) =82Z(Bi) =83

The ionization track in scintillators• 10 keV – 10 MeV X-ray or gamma-rays produce fast primary

electrons

24 2 2 20

2 2 20

24 ln ln 1

particle chargeparticle velocity

N,Z number density and atomic number atomsI average ionization potential atoms

m vdE e z v vNZdx m v I c c

ezv

π − = − − −

==

==

The Bethe formula

Smaller energy smaller v higher dE/dxmore ionizations/volume

• Multiple interactions• One scintillation event can produce separate ionization tracks in the

scintillator• Everything takes place in the 1-10 ps time scale• Separate flashes add to one scintillation flash

• High energy physics GeV-TeV shower of events

Some physical properties of other semiconductorsEnergy needed for electron-hole pair generation

Linear relationship between Eion and Eg (In figure β=2.8)

The ionization track and electron-hole creation

ph

2.5 50.000 /MeV

8 eV

N

ehion g

ehg

eh

E EN

E E

NE

SQN

γ γ

β

β

= =

≈ ⇒ ==

=

• S is the transfer efficiency from track to luminescence center• Q is quantum efficiency of luminescence center

eE

The ideal scintillator has S=Q=1

2 3 4 5 6 7 8 9 10 11 12 13 140

20

40

60

80

100

120

140

Lu2S3:Ce

K2LaCl5:Ce

LuI3:Ce

Gd2O2S:Tb

LiI:Eu

NaI:(80K)

NaI:TlLaCl3

LaCl3

CsILaBr3

ZnS:Ag

bromides oxides fluoride sulfides chlorides iodides

band gap (eV)

Yiel

d (p

hoto

ns/k

eV)

Lu2SiO5:Ce

RbGd2Br7:Ce

CaF2:Eu

6102.5 gap

YE

=

Ultimate scintillator region

Latest developments in scintillation research

Latest materials developments- Elpasolites (Radiation Monitoring Devices, dr. K. Shah)- Eu2+ doped halides (University of Berkeley, prof. Derenzo, prof. Moses)- Lu3Al5O12:Pr3+ (Tohuku Univ, prof. Yoshikawa, Inst. of Phys. Prague, dr. M. Nikl)

2 3 4 5 6 7 8 9 10 11 12 13 140

20

40

60

80

100

120

140

CsBa2Br

5:Eu

Cs2LiYCl

6:Ce

Cs2NaLaI

6:Ce

band gap (eV)

Yiel

d (p

hoto

ns/k

eV)

SrI2:Eu2+

Cs2LiLaBr

6:Ce

Lu3Al5O12:Pr3+

CsBa2I5:Eu

BaBrI:Eu

Current developments SrI2:Eu

Cherepy et al. Appl. Phys. Lett. 92 (2008) 083508

Current developments Ba2CsI5:Eu2+

Bourret-Courchesne et al. Nucl. Instr. Meth. A612 (2009) 138

Current developments BaBrI3:Eu2+

Bourret-Courchesne et al. Nucl. Instr. Meth. A613 (2010) 95

Recently more materials were discovered. All have their pros and cons.

Fast and good energy resolution

But high intrinsic background (Lu)

Radiometrics : scintillators 2

The scintillation process; phase III

• Three phases1. The interaction phase + thermalization (ps)2. The charge carrier and energy migration phase (ns-ms)3. The luminescence phase (ns-μs)

photon

Valence band

Cond. band

Eg=3-10 eV

S

Q Ideal scintillatorS=Q=1

The most popular activator ions

• The 6s2-ions Tl+, Pb2+, Bi3+

• NaI:Tl, CsI:Tl, Bi4Ge3O12, PbWO4

• The lanthanide ions Ce3+, Pr3+, Eu2+

• YAlO3:Ce, Y3Al5O12:Ce, Lu2SiO5:Ce, LuAlO3:Ce, LaCl3:Ce, LaBr3:Ce, LiI:Eu, Lu3Al5O12:Pr3+

The luminescence phase

Tl, Pb, Bi

ground state configuration 6s2 1S0excited state configuration 6s6p 1P1 singlet

3P23P1 triplet3P0

2S+1LJ

Selection rules Allowed transitions∆J = 0, ±1 (not J = 0 → J’ = 0), ∆S = 0

Singlet → singlet allowed, fastTriplet → singlet forbidden, slow

1P1

3P2

3P1

3P0

1S0

relaxation

NaI:Tl τ=230 nsCsI:Tl τ=3.3 µs

6s2-ions Tl+, Pb2+, Bi3+

34

Lanthanide luminescence

• electron configuration [Xe]4fn5d0

• 4f electrons are shielded from the crystalline field. Level energies independent on host lattice

• 5d electron strong interaction with crystal field. Level energies depend on host crystal

0 1 2 3 40

1

2

3

4Radial distribution functions for free Ce3+

5d0

4fn5s2

5p6

r [Angstrom]

rad.

dist

r. fu

nctio

n (r2 R2 (r)

)

0

1

2

3

4

5

6

7

8

9

10

11

12eV

LuErHoDyTbPm TmNdPrCe YbGdSm Eu

The Dieke diagram of free ion trivalent lanthanide 4fn and 5d levels

2 3 4 5 6 7

excitation

Inte

nsity

(arb

itrar

y un

its)

Energy in eV

NaLaF4:Ce

LaCl3:Ce

LaBr3:Ce

emission

CrystalFieldsplitting

Stokes shift

The emission wavelength

• Electrons either in the ground state or excited state of a luminescence center interact with the surrounding atoms in a compound

• 4f electrons have negligible interaction• 5d electrons (also 6s and 6p electrons of 6s2-ions) have strong

interaction• Interaction depends on

• Crystal structure, • coordination number, bond lengths etc.

• The chemical properties of the anions• F-, Cl-, Br-, I- and O2-, S2-

• The emission of Ce3+ may vary from 300 nm in fluoride compounds up to 600 nm in sulfide compounds

Characteristic emission spectra Ce, Pr, Nd

10 20 30 40 50 60

0.4

0.8 LaF3Nd

wavenumber [103 cm-1]

0.4

0.84f2→4f2

4f3→4f3

LiLuF4Y3Al5O12

Pr

yiel

d [a

rb. u

nits

]

0.4

0.8

LiYF4Lu2S3

Ce

800 600 400 200

wavelength [nm]

0

1

2

3

4

5

6

7

8

9

10

11

12eV

LuErHoDyTbPm TmNdPrCe YbGdSm Eu

Emission and quantum efficiency

200 400 600 800 10000

20

40

60

80

Photomultipliertube

SiliconPhotodiode

wavelength [nm]qu

antu

m e

fficie

ncy

[%]

10 20 30 40 50 60

0.4

0.8 LaF3Nd

wavenumber [103 cm-1]

0.4

0.84f2→4f2

4f3→4f3

LiLuF4Y3Al5O12

Pr

yiel

d [a

rb. u

nits

]

0.4

0.8

LiYF4Lu2S3

Ce

800 600 400 200

wavelength [nm]

Ce3+ doped lanthanide compounds are ideal for gamma ray scintillators

• Fast dipole and spin allowed 5d-4f emission (15-60 ns)• Emission wavelength matches sensitivity of

photomultiplier tubes• High thermal stability of emission• Absence of slow 4f-4f emission• Ce3+ is a good hole trap which is regarded as a first

important step in the scintillation process• Lanthanide (La, Gd, Lu) based host lattices are dense

providing efficient gamma ray detection materials• LaBr3:Ce, Gd2SiO5:Ce, Lu2SiO5:Ce

Stokes shift and self-absorption

• A scintillator crystal needs to be transparent to its own scintillation light

• A photon emitted by a luminescence center can be re-absorbed by another luminescence center of the same type

• Probability increases with• Larger activator concentration• Higher temperature (broadening of emission and absorption

bands)• Larger crystal size• Smaller value for the Stokes shift

• Stokes shift = energy difference between emission and first absorption band of luminescence center

2 3 4 5 6 7

excitation

Inte

nsity

(arb

itrar

y un

its)

Energy in eV

NaLaF4

LaCl3

LaBr3

emission

CrystalFieldsplitting

Stokes shift

Configuration coordinate

Ener

gy E

0.1 fs

1 ps

> ns

Stokes shift and self-absorption

Stokes shift is the energy lostdue to lattice relaxation

300 350 400 450 5000

5

10

15

20

25

30

300K 350K 400K 450K 500K 550K 600K

Wavelength (nm)

Inte

nsity

(Arb

. uni

ts)

Aspects of self-absorption in LaBr3:5% Ce

• High energy part of emission re-absorbed by Ce3+

• Re-emission by Ce3+

• emission profile changes• scintillation decay time lengthening• scintillation light losses (because Q<1)

100 200 300 400 500 600 700121416182022242628303234

0 50 100 150

0.01

0.1

1

Deca

y tim

e (n

s)

Temperature (K)

500 K300 K

80 K

Time (ns)

700 K

Thermal quenchingImportant for:

• luminescence efficiency Q• stable scintillator operation against temperature fluctuations• high temperature applications e.g. oil-well logging

Thermal quenching of luminescence

Configuration coordinate

Ener

gy E

0.1 fs

1 ps

> ns

( ) 1

10

( )exp qq

B

TQ T E

sk T

ν ν

ν νν

τ ττ τ

−

−

Γ= = =

−∆Γ +Γ +

Γν

Γq

Configuration coordinate

Ener

gy E

0.1 fs> ns

Conduction band states

Luy et al. J. Lumin. 48/49 (1991) 251

0.3 eV1.7 eV1.2 eV0.8 eV0.8 eV

YLiF4

LaF3

Y3Al5O12

Thermal quenching of Ce3+ emission

If quenching via CB-states than 5d-level location important

So far we have treated:

• Phase I: Interaction of a gamma photon with scintillators• Creation of the ionization track• Relation between number of ionizations and band gap

• Phase III: The luminescence centers• s2-elements (Tl, Pb, Bi) and lanthanides (Ce3+, Pr3+)• Wavelength of emission, self-absorption, Stokes shift• Thermal stability and thermal quenching

• Next, Phase II: scintillation mechanisms• Intrinsic scintillators

• Self trapped exciton and core valence luminescence• Activated scintillators

• energy migration from ionization track to luminescence centers• LaCl3:Ce and LaBr3:Ce as an example

Core valence luminescence

cation cond. band

anion valence band

cation valence band

core-valence-luminescenceEvc

Eg

condition for CVL: Evc < Eg

STE

BaF2 - fast UV luminescenceN.N. Ershov, N.G. Zakharov, P.A. RodnyiOpt. Spektrosk. 53(1982)89-93

STE

fast

0.6 µs

780 ps

CVL: 1400 ph/MeV 800 ps

STE: 104 ph/MeV 600 ns

fast

STE

200 300 400λ (nm)

Self trapped exciton and Core valence luminescence in BaF2

What is a self trapped exciton?

• hole in valence band is shared between two anions forming an X2

- molecule like defect (Vk center)

• electron in an orbit around the Vk center

Scintillation mechanism in LaCl3 and LaBr3

Ce3+ + h Ce4+

Ce4+ + e (Ce3+)* photon17 ns fast scintillation component

e + h exciton STESTE STE emission (or loss)μs slow scintillation componentSTECe transfer

X-ray excited luminescence LaCl3:0.6% Ce3+

300 400 500 600

T=400KT=375KT=350KT=325KT=300KT=275KT=250KT=225KT=200KT=175KT=135K

Wavelength (nm)

150 200 250 300 350 4000

20

40

60

80

100

TotalSTECe3+

Rela

tive

Inte

nsity

(%)

Temperature (K)

Competition STE emission and Ce emission as function of temperature

STE

250 300 350 400 450 500 550

200 K

175 K135 K

400 K

b)a)LaCl30.6 % Ce

wavelength (nm)

250 300 350 400 450 500 550100 K

400 K

LaCl34 % Ce

wavelength (nm)250 300 350 400 450 500 550

100 K

400 K

LaCl310 % Ce

c)

wavelength (nm)

Competition STE emission and Ce emission as function of temperature and concentration

Summary scintillation processes in La-halides

• Direct capture of electron and holes• Fast Ce3+ emission decay component of 16 ns

• Creation of self trapped excitons• Slow STE emission• Thermally activated STE migration

• Energy transfer from STE to Ce3+

• Slow Ce3+ scintillation components

Different competing scintillation processes depending on• Ce concentration • on temperatures

Further reading: G. Bizarri, P. Dorenbos, Phys. Rev. B 75, 184302 2000

0 100 200 300 400 500 600 700 800

Coun

ts

Energy (keV)

NaI:Tl+

0 100 200 300 400 500 600 700 800

Coun

ts

Energy (keV)

LaBr3:Ce3+ 5.1 g/cm3

17 ns

R = 2.9 %

Y = 70000 ph/MeV

3.6 g/cm3

230 ns

R = 6.5 %

Y= 40000 ph/MeV

LaBr3:Ce3+; record low energy resolution at 662 keV

0 500 1000 1500 2000 2500 30000

2

4

6

energy (keV)

coun

ts (a

rb.u

nits

)

spectrum due to 1.37 MeV + 2.75 MeV gamma source

Photoelectric interaction Total absorption peaks

Compton scatteringCompton edgesCompton continua

222.52 and 1.16 MeV

511 2C

EE

Eγ

γ

= =+

Anatomy of a pulse height spectrum from 24Na-source measured with LaBr3:5%Ce

0 500 1000 1500 2000 2500 30000

2

4

6

energy (keV)

coun

ts (a

rb.u

nits

)

Pair creation• e- + e+ +Ekin (=Eγ -1.02 MeV)• e+ + e- 2 x 511 keV γ photons

• 511 keV escape peak at 2.24 MeV• double escape at 1.73 MeV

Compton scattering ofannihilation quanta with escape of scattered gammaphotons

Anatomy of a pulse height spectrum from 24Na-source measured with LaBr3:5%Ce

511 keVescape

Two remaining features at511 keV and around 250 keV

Back scatter peaks due togamma rays scattered from materials outside the scintillator• Compton scattered gamma’s• annihilation gamma 511 keV

Anatomy of a pulse height spectrum from 24Na-source measured with LaBr3:5%Ce

0 500 1000 1500 2000 2500 30000

2

4

6

energy (keV)

coun

ts (a

rb.u

nits

)

BrilLanCeTM trademark of La-halide scintillators

Discovered by TU-Delft

LaCl3:Ce crystal growth progressFrom labsize to 4”x6” scintillators crystals

LaBr3:Ce for nuclide identification

Comparison NaI:Tl, LaBr3:Ce, HP-Ge

0.1

1

10

100

0 1000 2000 3000 4000 5000 6000 7000 8000 9000

NaILaBr3HPGe

Cou

nt ra

te (s

-1)

Energy (keV)

LaBr3:Ce in 2011 on mission to Phobos (Mars)and in 2014 on mission to Mercury

LaBr3 detectorfor thePhobos-Gruntmission

LaBr3 detector for theBepiColombo mission

Aspects that determine resolution

( )

2 2 2 2stat inhom nonprop

stat1% 235 0 2FWHM PMT

PMTdph

R R R R

vR v .N

= + +

+= ≈

• the fundamental limit is governed by statistics• you need to detect as many photons as possible

bright scintillator efficient low noise photon detector

The ideal scintillator has S=Q=1

2 3 4 5 6 7 8 9 10 11 12 13 140

20

40

60

80

100

120

140

Lu2S3:Ce

K2LaCl5:Ce

LuI3:Ce

Gd2O2S:Tb

LiI:Eu

NaI:(80K)

NaI:TlLaCl3

LaCl3

CsILaBr3

ZnS:Ag

bromides oxides fluoride sulfides chlorides iodides

band gap (eV)

Yiel

d (p

hoto

ns/k

eV)

Lu2SiO5:Ce

RbGd2Br7:Ce

CaF2:Eu

6102.5 gap

YE

=

Ultimate scintillator region

Energy resolution @ 662 keV

stat1 0.22.35

dph

RN+

=

• YAlO3:Ce, Lu3Al5O12:Pr, LaCl3:Ce, LaBr3:Ce, SrI2:Eu are reasonably close to fundamental limit.

• Lu2SiO5, NaI:Tl, CsI:Tl, strong deviation

0 5000 10000 15000 200000

2

4

6

8

10

reso

lutio

n (%

) at 6

62 k

eV

number of detected photons at 662 keV

LaCl

3

LaBr3

SrI2:Eu

BaF 2

LSO

:CeBG

O

CsI:T

l

NaI:T

l

YAlO

3:Ce

Lu3A

l 4O12

:Pr

RstatWhat causes the strong deviation?

Aspects that determine resolution

( )

2 2 2 2stat inhom nonprop

stat1% 235 0 2FWHM PMT

PMTdph

R R R R

vR v .N

= + +

+= ≈

• when the number of created photons is not proportional with the energy of the primary electron

additional contribution to resolution (Rnonprop)• topic of current interest

for study, we need a tunable β-source inside scintillator!

The Compton Coincidence Technique• Developed by Valentine and Rooney

• Nucl. Instr. and Meth A353 (1994) 37; IEEE Trans.Nucl.Sci.43(1996)1271

• Provides electron response curves

Advantage• intrinsic tunable β-source• energy range 6-400 keV

( )( )20

'1 1 cos

'C

hhh m c

E h h

ννν ϑ

ν ν

=+ −

= −

Cherepy et al. IEEE Trans. Nucl. Sci. 56(3) (2009) 873

Scintillator response to electrons NaI:Tl. LSO:Ce StronglyNon-proportional

LaBr3:CeLaCl3:CeYAlO3:Ceproportional

?

K-dip spectroscopy method

EX=EK+∆E

Ephe=EX-EK=∆E

Energy photo-electron = EX-E keVYield (photo-electron)= total yield – yield@EK keV

We have created a tunable internal β-sourceElectron response from ~0.1 to (100-EK) keV

Tunable synchrotron X-ray source• Synchrotron X-ray beamline HASYLAB, DESY, Hamburg• 9-100 keV monochromatic and tunable

• 5-15 eV energy resolution • Pencil beam

• we always excite the same volume element of the scintillator• minimum contribution to resolution from scintillator inhomogeneity

0.1 1 100

20

40

60

80

100Lu

3Al

5O

12:Pr

Gd2SiO

5:Ce

Lu2SiO

5:Ce

Lu2Si

2O

7:Ce

Rela

tive

light

yie

ld, %

K-photoelectron energy, keV

Joining K-dip with SLYNCI data100eV – 400keVElectron response curves20 different scintillators

K-electron spectroscopy

Compton-electron spectroscopy

• Can we go below 2% energy resolution at 662 keV?• We need bright scintillators combined with low noise photon detectors

• We need proportional scintillators

0 5000 10000 15000 200000

2

4

6

8

10

reso

lutio

n (%

) at 6

62 k

eV

number of detected photons at 662 keVLa

Cl3

LaBr3

SrI2:Eu

BaF 2

LSO

:CeBG

O

CsI:T

l

NaI:T

l

YAlO

3:Ce

Lu3A

l 4O12

:Pr

Rstat

Radiometrics : scintillators 2

Time resolution

Time resolution = accuracy with which the moment of interactioncan be determined

It depends:• scintillation speed (decay time and rise time)

• more ph/MeV• photon detector time resolution• electronics

Timing resolution

STE

fast

0.6 µs

780 ps

BaF2

The steepness of the rising pulse is important

0 50 100 150 200 250 3001

10

100

1000

10000

LaBr3

4%Ce17 ns

LaBr3:0.5% Ce

NaI:Tl230 ns

inte

nsity

(arb

. uni

ts)

time (ns)

Scintillator risetime studies

-100 0 100 200 300 400 500 600 700 800 900 10000

100

200

300

400

Phot

on c

ount

s

Time [picosecond]

CaGa2S4: Ce3+ [host excited] LSO:Ce3+ [5d excited]

fs laser excitation Pulsed X-ray excitationLaBr3:Ce

Typical numbers for time resolution with PMTs and 1 MeV gamma’s :

NaI(Tl) : 1 ns

BGO : 2-4 ns

BaF2 : 200 ps

Plastics : 150 ps

Radiometrics : scintillators 2

Fast timing is important for• positron emission tomography• positron life time studies• time of flight applications

Time resolution LaBr3:Ceat 511 keV with PMTs

LaBr3 TOF-PET scanner

First prototype LaBr3TOF-PET-scanner (2008-2009)(dr. J. Karp Pennsylvanian State University)(and Philips Medical Systems)

Contains 38880LaBr3 crystals

Time-of flight positronEmission tomography scanner• 511 keV annihilation photons• coincident detection• time resolution 200 ps

First TOF PET-image with LaBr3:Ce(October 2009)

Advantage TOF-PET strong false signal reduction especially importantFor fat patients

-300 -200 -100 0 100 200 3000

100

200

300

400

Time Difference (ps) N

umbe

r of E

ntrie

s

DataGaussian Fit

-300 -200 -100 0 100 200 3000

100

200

300

400

500

600

700

Time Difference (ps)

Num

ber o

f Ent

ries

DataGaussian Fit

Timing Spectra with SiPM – CRT at Optimum Threshold Settings

FWHM = 100 ps FWHM = 172 ps

LYSOLaBr3:5%Ce

Intrinsic activity of Lu2SiO5:Ce

With abundance of 2.6% 176Lu with half life of 3.8x1010 years an intrinsic activity of 290 cnts/cm3 is obtained176Lu isotopes emit 89, 202, plus 307 keV γ-photons plus emission ofa β- with maximum energy of 596 keV (plus a neutrino).

Intrinsic activity of LaBr3:Ce

With abundance of 0.089% 138La with half life of 1.0x1011 years an intrinsic activity of 1.4 cnts/cm3 is obtained

138La emits either a 1.436 MeV gamma plus an 32 keV X-ray from Baor it emits a β- with Emax=255keV plus a789 keV gamma.

Final remarks

• Ideal scintillator does not exist• compromise based on the application

• High resolution for spectroscopy (isotope identification)• proportional scintillators

• extremely fast rise time for timing and TOF applications• low intrinsic count rate for space explorations• hight density at low cost

• for calorimeters• current topics

• understanding non-proportionality• Eu2+ and Pr3+ activated scintillators• scintillator pulse rise time studies

Acknowledgements

• Some of the materials for these slides were provided by• Dr. P. Schotanus, company Scionix, The Netherlands• Dr. E. Mattmann, company Saint Gobain crystals, France• Dr. A. Owens and F. Quarati, European Space Agency, Netherlands• Dr. J Karp, Pennsylvanian State University, USA

References

• Glodo et al. IEEE TNS, 52 (2005) 1805.

• Radiation Detection and Measurements, G. Knoll, John Wiley&Sons, Inc

• Saint Gobain brochures on scintillators, http://www.detectors.saint-gobain.com/

• N. Cherepy, S.A. Payne, et al. IEEE Trans. Nucl. Sci. 56(3) (2009) 873

• I.V. Khodyuk, J.T.M. de Haas, P. Dorenbos, IEEE Trans. Nucl. Science 57(2010) 1175-1181

• G. Bizarri, P. Dorenbos, Phys. Rev. B. 75 (2007) 184302.

• P. Dorenbos, IEEE Trans. Nucl. Sci. 57 (2010) 1162-1167.

• P. Dorenbos, J.T.M. de Haas, C.W.E. van Eijk, IEEE Trans. Nucl. Sci. 51(3) (2004) 1289-1296.

• M.J. Weber, J. Lumin. 100 (2002) 35.

Thermal neutron scintillators• Efficient thermal neutron detection

• high capture cross section• suitable isotopes and isotope enrichment

• absence of competing capture processes• gamma ray back ground rejection

• thin layers of low density• pulse height discrimination• pulse shape discrimination

Thermal neutron capture reactions• n + 6Li 3H + α + 4.79 MeV• n + 10B 11B 7Li + α + 2.78 MeV (7%)

7Li* + α + 2.30 MeV (93%)

Scintill. Dopant/ λem Light yield τDensity ρ ρZeff4

host conc-mol% nm photons per nsg/cm3 (x 10-6)neutron MeV gamma

Abs. Lengthat 1.8 Å

mm

6Li-glass 2.5 0.52 Ce 395 6.000 4.000 756LiI 4.1 0.54 31 Eu 470 50,000 12,000 1.4 x 103

6LiF/ZnS 2.6 0.8 1.2 Ag 450 160,000 75,000 10 x 103

6Li6Gd(BO3)3 3.5 0.35 25 Ce 385/415 40,000 20,000 200/800

inorganic thermal-neutron scintillators

n + 6Li (7.5%*) → 3H (2.75 MeV) + 4He (2.05 MeV) σ = 940 b at 1.8 Å

Pulse shape discrimination due to core-valence luminescence in LiBaF3:Ce3+

0 200 400 600 800 1000 1200

energy [keV]

100

101

102

103

104

coun

ts

1

2

Spectra of radiation emitted by a Pu-Be source shielded with paraffine and 6 cm of lead

# 1 is without discrimination, # 2 with discrimination. The neutron peak at 750 keV gamma-equivalent energy is almost free from background

LiBaF3:Ce scintillator

neutrons

CVL does not respond to 3H (2.75 MeV) + 4He (2.05 MeV)

C.M. Combes et al, NIM A416(1998)364

0 100 200 300 400 500 6000

1

2

3

slow component

τ= 85 ns

CVLτ=4 ns

Inte

nsity

(arb

. un.

)

time (ns)

Thermal neutron detection with Cs2LiYCl6:0.1% Ce3+

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.50

1

2

3

4

5

6Cs2LiYCl6:0.1% Ce3+

thermal neutronpeak662 keV

γ-peak

coun

ts (1

03 )

γ-equivalent energy (MeV)

22000 ph/MeV 70000 ph/neutronpulse shape pluspulse heightdiscrimination

ρ = 3.3.g/cm3

R=9%

α/β=0.66

Thermal neutron scintillators

Selection of the scintillator based on thermal neutron detection efficiency

σ (barn)

Cs2LiYBr6

Rb2LiYBr6

Li2NaYBr6

K2LiYBr6

Rb2LiLaBr6

Cs2LiLuI6

Rb2LiYI6

6Li - nat/%Σσ

71/41

71/62

142/77

71/60

71/58

71/29

71/65

6Li - 95enr/%Σσ

895/90

895/95

1790/98

895/95

895/95

895/84

895/96

Absorption length0.18 nm - 95enr

(mm)

3.7

3.5

1.7

3.5

3.5

3.9

3.4

Rb2LiYBr6: 0.1% Ce3+

Rb2LiYBr6: 0.5% Ce3+

Rb2LiYBr6: 1% Ce3+

Rb2LiYBr6: 5% Ce3+

Compounds Photons/neutron α/β ratio Peak resolution%

59,000 ± 5,90083,000 ± 8,30049,000 ± 4,90051,000 ± 5,10039,000 ± 3,90042,000 ± 4,200

0.740.750.790.820.740.80

3.65.48.54.25.87.8

Thermal neutron scintillation light yield and peak resolution Rb2LiYBr6:Ce3+

6LiI: Eu 4.1 0.54 470 50,000 12,000 0.86 1.4 x 103

6LiF/ZnS:Ag 2.6 0.8 450 160,000 75,000 0.45 10 x 103

Scintill. λem α/βratio

Light yield τDensity ρhost nm photons per ns

g/cm3neutron MeV gamma

Abs. Lengthat 1.8 Å

mm

Li -Glass:Ce 395 ~6,000 ~4,000 752.56 0.52

PeakResolution

(%)

13-22 %

3.9 %

-

0.31

Cs2LiYBr6:Ce3+ 4.1 3.7 389 73,000 20,000 0.76 89,

2.5 x 103

Rb2LiYBr6:Ce3+ 4.8 3.5 385 83,000 23,000 0.74 42,140

1.6 x 103

Li2NaYBr6:Ce3+ 2.9 1.7 380 39,000 12,400 0.66 30,560

2.6 x 103

4.6 %

3.6 %

-