Anomalous Effects in Thermoluminescence

Arkadiusz MandowskiJacek Orzechowski

Ewa Mandowska

Institute of PhysicsJan Długosz UniversityCzęstochowa, Poland

RAD 2012, NiRAD 2012, Nišš

Principles of luminescence dosimetry

Purpose:

Determination of dose of ionizing radiation using

optical (luminescence) techniques

Methods: Thermoluminescence (TL)

[thermal stimulation – heating] Optically Stimulated Luminescence (OSL)

[optical stimulation]

Principles of luminescence dosimetry

preparing a detector

irradiation

storage

luminescence readout (TL / OSL)

(signal reset)

Relaxation processes during thermoluminescence

Excitation perturbation of a solid from equilibrium; energy storage;

Metastable state very slow relaxation processes with respect to to given time scale (from minutes to centuries), practically undetectable

Heating fast relaxation, easy to detect, TL luminescence other properties for other TSR processes

TL kinetic theories

TL theoretical models

excited states

traps

deep traps

R C

S1

S2A

Examples of anomalous TL behaviour

• TL dose-rate effect

• first order shape of most TL glow peaks

• the occurrence of very high frequency factors

• dose-dependent peak parameters (peak positions, activation energies and frequency factors)

• anomalous heating-rate effect (total number of emitted photons increases with heating rate)

Examples of anomalous TL behaviour

• TL dose-rate effect

• first order shape of most TL glow peaks

• the occurrence of very high frequency factors

• dose-dependent peak parameters (peak positions, activation energies and frequency factors)

• anomalous heating-rate effect (total number of emitted photons increases with heating rate)

Classification of TL/OSL models

e-STM

LT

With respect to charge carriers type• one-carrier kinetics (e.g. active electrons)• two-carrier kinetics (active electrons and holes)

With respect to energy distribution• OTOR (one trap one recombination centre)• discrete distribution or traps and RCs• continuous energy distribution of traps and RCs

With respect to spatial distribution• geminate pairs T-RC• trap clusters• random distribution of traps and RCs

With respect to type of interaction• localized transitions• delocalized transitions (band-like)

The simple trap model (STM)(extended)

exp ( ),ii i i c i i i

En n n A N n

kTn

æ ö- ÷ç- = - -÷ç ÷çè ø&

,s s s ch B h n- =&

1 1

,q p

s i cs i

h n n M= =

= + +å å

s=1..q,

i=1..p,

conductionband

valenceband

shallowtraps

recom binationcentres

activetraps

deeptraps

n c

h v

M

E iD iA i

B j

h j

N i, n i

TL

TSC c

dhJ h

dtJ n

µ - º -

µ

&

The model of localized transitions (LT)

exp ,eE

n n AnkT

næ ö- ÷ç- = -÷ç ÷çè ø

&

,eh Bn- =&

,eh n n= +

conductionband

valenceband

recom binationcentres

activetraps

n c

h v

E iD iA i

B j

h j

n i

n e

Various topologies of delocalization

Clustering

LT STM

Displacement of charge carriers

The model of semi-localized transitions (SLT)

Clustering

Displacement of charge carriers

LT STMSLT

The model of semi-localized transitions (SLT)

va lence band

conduction band

trap

recom binationcentre

trap excitedlevel

The model of semi-localized transitions (SLT)

AD

B

h v

n c

n

h

n e

V

C

K

The model of semilocalized transitions (SLT)

Mandowski A 2005 J. Phys. D: Appl. Phys. 38, 17

AD

B

h v

n c

n

h

n e

V

C AD

B

n

h

n e

V

C AD

B

n

h

n e

V

C

K K K

T-RC units

01H

10H

00H

01E

10E

00E

T-RCT-RCT-RC

0 11 0,H H

0 11 0,E E

00H

00E

Horowitz et al. 2003 J. Phys. D: Appl. Phys. 36 446Picture by courtesy of prof. Horowitz and prof. Oster

TLD-100 (LiF:Mg,Ti)

SLT system – kinetics... ?

SLT = STM + LT ?

SLT = Semi-localized Transitions STM = Simple Trap Model LT = Localized Transitions

NO !

The model of semilocalized transitions (SLT)

TL kinetics for K=0

0 1 01 0 0

0 1 01 0 0

D VA

D VA

BC C C

H H H

E E E

¾¾® ¾¾®¬¾¾

¾¾® ¾¾®¬¾¾

¯ ¯ ¯]

( )0 0 11 1 0cH D Cn H AH=- + +&

( )1 0 10 1 0cH DH A B V Cn H= - + + +&

0 1 00 0 0cH VH Cn H= -&0 0 0 11 1 1 0cE Cn H DE AE= - +&

( )1 1 0 10 0 1 0cE Cn H DE A V E= + - +&

0 1 0 10 0 0 0cE BH Cn H VE= + +&

0 1 0 1 11 0 0 0 0( ) ( )c cn Cn H H H V H E=- + + + +&

10B BH=L

0 1 01 0 0( )C cCn H H H= + +L

Mandowski A 2005 J. Phys. D: Appl. Phys. 38, 17

The riddle of very high frequency factors

Bilski P, (2002) Radiat.Prot.Dosim. 100, 199-206

LiF:Mg,Ti

LiF:Mg,Cu,P

Unphysical values !(allowed 108 1014 s-1)

=1020 s-1

E=2.05 eV

=1021 s-1

E=2.29 eV

Anomalous peaks are very narrow !

The riddle of very high frequency factors explained by SLT energy configurations

- activation energies for various configurations may be different!

nmH

nmE

- states with charged recombination centres

- states with empty recombination centres

1 1( ), ( ) ( ), ( )D t V t D t V t®

2 2( ), ( ) ( ), ( )D t V t D t V t®

2 1E E ED º -

activation energy gain between charged and

non-charged T-RC unit

Casca

de d

etra

pp

ing

E=0.9 eV; EV=0.5 eV; = V=1010 s-1

Efit=1.65 eV; fit=2.01020 s-1

Efit=1.87 eV; fit=3.01024 s-1

Efit=1.90 eV; fit=1.91026 s-1

a

b

c

c

b

a

0.0

0.2

0.4

0.6

0.8

TL

[a.u

.]

c

a

320 340 360 380 400 420 440 460

T em pera ture [K ]

0.0

0.2

0.4

0.6

0.8

TL

[a.u

.]

0.0

0.2

0.4

0.6

0.8

TL

[a.u

.]

a

b

c

0.0

0.2

0.4

0.6

0.8

1.0

TL

[a.u

.]

E = 0 eVE V = 0.5 eVr = 0

E = -0 .1 eVE V = 0.5 eVr = 0

E = -0 .2 eVE V = 0.5 eVr = 0

E = -0 .3 eVE V = 0.5 eVr = 0

Cascade detrapping – how does it work?

Initally (at low temperatures) most of charge carrier transitions goes within localized pairs

A carrier (electron) thermally released to the conduction band recombines to an adjacent hole-electron pair

The remaining „lonely” electron having decreased activation energy is rapidly excited to the conduction band

The free carrier moves to an an adjacent hole-electron pair and the process repeats one again

E E

The heating-rate effect (normal)We measure TL intensity for various heating rates:

The number of emitted photons:

300 400 500 600Tem perature [K ]

0

2

4

6

8

10

TL

/ [

a.u

.]

w ithoutquenching

300 400 500 600Tem perature [K ]

0

2

4

6

8

10

TL/

[a

.u.]

w ithquenching

0 0 0

TL , ,k k kt T T

t T T

J T J Tn J t dt dT C W T dT

where: 1

, , 1 expW

C W T CkT

is the quenching function

The heating-rate effectin YPO4:Ce3+,Sm3+ (anomalous)

A.J.J. Bos et al., Radiat. Meas. (2010)

Explanation of the anomalous heating-rate effect by SLT model

Dorenbos, P., 2003b. J. Phys.: Condens. Matter 15, 8417–8434.

Explanation of the anomalous heating-rate effect by SLT model

Mandowski A, Bos A J J (2011), Radiation Measurements (doi:10.1016/j.radmeas.2011.05.018)

Experimental data in YPO4:Ce3+, Sm3+SLT modelling

Dose-rate effect by SLT model

-15 -10 -5 0 5 10 15log(G /G 0)

1.165

1.170

1.175

1.180

1.185

1.190R

ela

tive

pea

k m

axi

mu

m o

f

C [

a.u

.]

1.220

1.225

1.230

1.235

1.240

Rel

ativ

e ar

ea

und

er

C p

ea

k [a

.u.]

C_m ax

S C

Illustration of the dose rate effect. The TL output - two localized (1BL and 2BL) and one

delocalized peak (CL), calculated after three stages: excitation, fast relaxation and heating.

Conclusions

The model of semi-localized transitions model (SLT) offers simple explanation of some anomalous effects in thermoluminescence, including - anomalous heating rate effect - very high effective frequency factors (cascade detrapping mechanism)as well as- dose rate efect- first order shape of TL peaks, etc.

Other experimental data indicate the necessity of taking into account larger clusters of traps and RCs.

Anomalous Effects in ThermoluminescenceArkadiusz Mandowski, Jacek Orzechowski, Ewa Mandowska

Thank you!