1. Anomalous Effects in Thermoluminescence Arkadiusz Mandowski Jacek Orzechowski Ewa Mandowska Institute of Physics Jan Długosz University Częstochowa, Poland

2. 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]

3. Principles of luminescence dosimetry preparing a detector irradiation storage luminescence readout (TL / OSL) (signal reset)

4. 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

5. TL theoretical models

6. 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)

7. 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)

8. 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)

9. The simple trap model (STM) (extended) s=1..q, i=1..p,

10. The model of localized transitions (LT)

11. Various topologies of delocalization Clustering  LT STM Displacement of charge carriers 

12. The model of semi-localized transitions (SLT) Clustering  Displacement of charge carriers  LT STM SLT

13. The model of semi-localized transitions (SLT)

15. The model of semilocalized transitions (SLT) Mandowski A 2005 J. Phys. D: Appl. Phys. 38, 17 T-RC units T-RC

16. Horowitz et al. 2003 J. Phys. D: Appl. Phys. 36 446 Picture by courtesy of prof. Horowitz and prof. Oster TLD-100 (LiF:Mg,Ti)

17. SLT system – kinetics... ? SLT = STM + LT ? SLT = Semi-localized Transitions STM = Simple Trap Model LT = Localized Transitions NO !

18. The model of semilocalized transitions (SLT) TL kinetics for K=0 Mandowski A 2005 J. Phys. D: Appl. Phys. 38, 17

19. 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 10 8   10 14 s -1 ) =10 20 s -1 E=2.05 eV =10 21 s -1 E=2.29 eV Anomalous peaks are very narrow !

20. The riddle of very high frequency factors explained by SLT energy configurations - activation energies for various configurations may be different! - states with charged recombination centres - states with empty recombination centres activation energy gain between charged and non-charged T-RC unit

21. Cascade detrapping E=0.9 eV; E V =0.5 eV; = V =10 10 s -1 E fit =1.65 eV; fit =2.0  10 20 s -1 E fit =1.87 eV; fit =3.0  10 24 s -1 E fit =1.90 eV; fit =1.9  10 26 s -1

22. 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

23. The heating-rate effect (normal) We measure TL intensity for various heating rates: The number of emitted photons: where:is the quenching function

24. The heating-rate effect in YPO 4 :Ce 3+,Sm 3+ (anomalous) A.J.J. Bos et al., Radiat. Meas. (2010)

25. Explanation of the anomalous heating-rate effect by SLT model Dorenbos, P., 2003b. J. Phys.: Condens. Matter 15, 8417–8434.

26. 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 YPO 4 :Ce 3+, Sm 3+ SLT modelling

27. Dose-rate effect by SLT model

28. 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.

29. Anomalous Effects in Thermoluminescence Arkadiusz Mandowski, Jacek Orzechowski, Ewa Mandowska Thank you!