Luminescent materials for dosimetric applications

Transcription

1 Luminescent materials for dosimetric applications Adrie J.J. Bos Delft University of Technology, The Netherlands Delft University of Technology Challenge the future

2 Basic assumption: There is a relation between: light yield and absorbed dose 2

3 Steps in TL Dosimetry 1. Irradiation 2. Storage 3. Read-out γ, X-rays, e -, p, HCP λ emission Luminescent material ionisation storage heat 3

4 Steps in OSL Dosimetry laser γ, X-rays, e -, p, HCP λ stimulation > λ emission Luminescent material Ionisation Storage Read-out Q1: How is it energytically possible that E em > E stim? 4

5 Which luminescent material is a good dosimetric material? Application area Type of radiation Type of dosimetry 5

6 Application areas Personnel dosimetry Extremity Whole body Environmental dosimetry Terrestrial Space Medical dosimetry Radiotherapy Diagnostic radiology Nuclear medicine High dose dosimetry Radiation processing Nuclear reactors 6

7 Type of radiation Low LET radiation photons betas electrons High LET radiation protons Heavy Charged Particles (HCP) neutrons UV radiation 7

8 Type of Dosimetry Passive Thermoluminescence Dosimetry (TLD) Optically Stimulated Luminescence Dosimetry (OSLD) Electron Paramagnetic Resonance (EPR) Film Dosimetry Active Fiber optic dosimetry Source: Akselrod, et al. Radiat. Meas. 41(2007)S78 8

9 General requirements Sensitivity Linearity Appropriate energy dependence independence radiation energy tissue equivalent Long term stability Reproducible Easy to re-set, low residual 9

10 Specific requirements Match luminescence spectrum with maximum sensitivity of PM tube Mechanically strong Chemically inert No effect of day light (for TL) Radiation resistant Batch homogeneity Simple reuse Low production price 10

11 Application areas Dose range (Gy) Personnel dosimetry Extremity Whole body Environmental dosimetry Terrestrial Space Medical dosimetry Radiotherapy Diagnostic radiology Nuclear medicine High dose dosimetry Radiation processing Nuclear reactors

12 Application areas Dose range (Gy) Personnel dosimetry , +50 Extremity Whole body Environmental dosimetry ±30 Terrestrial Space Medical dosimetry Radiotherapy ? Diagnostic radiology Nuclear medicine High dose dosimetry 15 Radiation processing Nuclear reactors Uncertainty 1SD (%) Q2: Which uncertainty is required in radiotherapy? 12

13 Application areas Dose range (Gy) Personnel dosimetry , +50 Extremity Whole body Environmental dosimetry ±30 Terrestrial Space Medical dosimetry Radiotherapy Diagnostic radiology Nuclear medicine High dose dosimetry 15 Radiation processing Nuclear reactors Uncertainty 1SD (%) 13

14 Application areas Dose range (Gy) Uncertaint 1SD (%) Fading Personnel dosimetry , Extremity Whole body Environmental dosimetry ±30 ++ Terrestrial Space Medical dosimetry Radiotherapy Diagnostic radiology Nuclear medicine High dose dosimetry 15 - Radiation processing Nuclear reactors

15 Application areas Dose range (Gy) Uncertainty 1 SD (%) Fading TE Personnel dosimetry , Extremity Whole body Environmental dosimetry ±30 ++ ± Terrestrial Space Medical dosimetry Radiotherapy Diagnostic radiology Nuclear medicine High dose dosimetry Radiation processing Nuclear reactors

16 Tissue equivalence for Photons D TL = Φ E ( µ en/ ρ) TL (1) D = Φ E ( µ / ρ) (2) tissue en tissue From (1) and (2): 7.21 Material Z eff Soft tissue 7.35 LiF 8.31 Water 7.51 Air 7.77 BeO Li 2 B 4 O CaF D D TL tissue = ( µ en/ ρ) ( µ / ρ) en TL tissue 16

17 Tissue equivalence for neutrons 10 1 tissue O H 10 0 tissue C D K =Φ E ( µ tr/ ρ) K = E ( µ tr/ ρ ) Φ Kerma coefficient (pgy cm 2 ) C H N N mass fraction H (0,101) C (0,111) N (0,026) (NH 4 ) 2 BeF 4 :Tl + (NH 4 ) 2 SiF 6 :Tl O O (0,762) Neutron energy (ev) 17

18 Luminescence efficiency How efficiently transform TL/OSL materials absorbed energy into light? η = i energy emitted energy absorbed = Nphotons md h ν 18

19 Luminescence phenomena, applications and typical efficiencies Type of luminescence Induced by Application Efficiency (%) Black body radiation Heat Tungsten filament lamp 5% Photoluminescence Photons Fluorescent lamp 20% Cathodoluminescence Electrons Television screen 10% Electroluminescence Electric field LED, flat panel display % Thermoluminescence Ionising radiation Dosimetry, Dating Q3: η TLD-100? A: 0.01% B: 0.10% C: 1.0% D: 10% 19

20 Steps in the energy conversion process creation of electron-hole pairs (n eh E γ /W eh =hν/βe g ) thermalisation and trapping (η tr ) release of charge carriers from the trap (p) transfer to luminescent centre (S) de-excitation of the Luminescent Centre (Q) escape from the sample (η esc ) hν η = η psqη i tr esc β Eg E g η tr p S Cond. band Q η esc Valence band 20

21 Luminescence efficiency hν η = η psqη i tr esc β Eg Suppose: η tr = 1, p = 1, S = 1, Q = 1, η esc = 1 η i,max = hν β E g E g η tr p S Cond. band Q Valence band 21

22 η i,max and η exp for some TL materials E g hν η i,max η exp TL material ev β nm ev % % LiF:Mg,Ti LiF:Mg,Cu,P 0.91 CaF 2 :Dy(TLD-200) CaF 2 :Cu,Ho CaF 2 :Tm 0.29 CaF 2 :Mn 0.44 KMgF 3 :Ce BeO CaSO 4 :Dy CaSO 4 :Mn 1.2 Al 2 O 3 :C Li 2 B 4 O 7 :Mn C (diamond) Average

23 Efficiency various steps creation e-h pairs ~13% trapping (η tr ):? release of charge carriers from the trap (p) ++ transfer to luminescent centre (S) - + de-excitation of the Luminescent Centre (Q) - + escape from the sample (η esc ) + Conclusion: the trapping efficiency plays a dominant role in the overall effciency 23

24 Well known TL/OSL materials LiF family CaSO 4 :RE (RE= Dy, Tm, Sm) CaF 2 :Mn Li 2 B 4 O 7 :Mn MgB 4 O 7 :Dy,Na Al 2 O 3 :C BeO Al 2 O 3 :C BeO SrS:Ce,Sm Q4: Which class of materials do not show TL? 24

25 LiF family LiF: Mg,Ti Patent 1963 Thermo Electron (TLD-100,TLD-600, TLD-700) TLD Poland: LiF:MT LiF: Mg,Cu,P First mentioned by Nakajima et al SSDL, Beijng, Cina: GR200 Nemoto, Japan, NTL-500 Thermo Electron: TLD100H TLD Poland: LiF:MCP 25

26 Glow curves LiF LiF:Mg,Cu,P LiF:Mg,Ti Source: Bilski., Radiat. Prot. Dosim. 100(2002)199 26

27 Dose response TL( D) TL( D0 ) f ( D) = D D Linear: f(d) = 1 supralinear: f(d) >1 sublinear: f(d) < 1 0 Source: P. Olko, in: Y.S. Horowitz: Microdosimetric response (Elsevier, Amsterdam, 2006) 27

28 Role of dopants LiF:Mg,Ti LiF:Mg,Cu.P Mg: % Mg: 0.2% Ti: ppm P: 1 4% Cu: % Anneal procedure 1 h 400 C 10 C 24 h 80 C 28

29 TLD cycle 29

30 Glow curve TLD-100 after storage 30

31 Requirements OSL material Sensitivity Trapping centres: i) Thermally stable ii) Optically accessible Good separation between emission and stimulation bands Dose erasure by optically bleaching 31

32 OSL reader (schematic) 1 mw laser: photons cm -2 s -1 OSL-signal: < 10 6 photons cm -2 s -1 Q5: What is the role of the colour filter? 32

33 Al 2 O 3 :C before 1990: Al 2 O 3 known as TL material 1990: anion-deficient Al 2 O 3 :C developed as high sensitive TL material 1995: OSL material Application in Personnel dosimetry Environmental dosimetry Retrospectrive dosimetry Dating applications (equivalent to quartz) Q6: What is the role of C? A: Trapping centre B: Luminescent centre C: None of both 33

34 Emission and stimulation spectra of Al 2 O 3 :C Emission intensity (a.u.) Emission wavelength (nm) e - + F + F * F + hν 420 nm Source: Botter-Jensen et al. Radiat. Meas. 22 (1997)

35 OSL-signal of Al2O3:C OSL signal (a.u) 10 5 stimulatie mgy λ stimulation = = 470 nm nm mgy Time Tijd (s) (s) 35

36 Dose response Kodak XV-2 film 120 kvp 280 kvp 60 Co 6MV photons 20 MeV electron Source: Mota, et al. Phys. Med. Biol. 35 (1990) 565 Source: Akselrod and McKeever, Radiat. Protec. Dosim. 81 (1999)

37 Environmental dosimetry Source: Bøtter-Jensen et al. Radiat. Meas. 27 (1997)

38 BeO Known for a long time as TL material (used in routine personnel dosimetry by the ENEA-Italy dosimetry service) Relatively low TL sensitivity Strong TL self-absorption Strong supralinearity Highly tissue equivalent; Z eff (BeO) = 7.2 ; Z eff (tissue) = 7.4 Rediscovered as OSL material (Bulur, Sommer et al.) 38

39 BeO: : OSL dose response OSL signal (counts/channel) BeO Batch 5 1 mgy 2 mgy 4 mgy 8 mgy 16 mgy 32 mgy 64 mgy 128 mgy 256 mgy Integrated luminescence (counts) 10 8 BeO batch 5 OSL (9-509 s) Time (s) Dose (mgy) 39

40 BeO: : dose response Integrated luminescence (counts) 10 8 BeO batch TL ( C) OSL (9-509 s) Integrated Luminescence (counts) Batch 5 (OSL: s) Batch 5 (TL: C) Dose (mgy) Dose (Gy) 40

41 Cu + doped fused quartz λ stimulation = 790 nm X = 2 R Source: Justus et al Radiat. Protec. Dosim. 81(1999)5 41

42 MgO:Tb 3+ OSL signal (arb. units) OSL from λa stimulation "Single = Grain" 470 nm Irradiation: 90 Y/ 90 Sr 0 mgy 1 mgy 2 mgy 4 mgy 8 mgy 16 mgy 32 mgy 64 mgy 128 mgy 256 mgy 512 mgy mgy 2048 mgy 4096 mgy 8192 mgy Integrated OSL signal Time (s) Dose (mgy) 42

43 MgO:Tb 3+ : Fading Normalised signal OSL TL peak 1+2 TL peak Temperature ( C) Time (h) 3 43

44 SrS:Ce,Sm λstimulation= 1.04 µm Irradiation: UV (350 nm) Source: Lapraz te al. Phys. Stat. Sol. (a) 203 (2006)

45 SrS:Ce,Sm comparison with Al2O3:C Emission and stimulation bands are well separated No deep traps, SrS is fully reset after reading Strong fading a = TL signal; b = OSL signal Source: Lapraz et al. Phys. Stat. Sol. (a) 203 (2006) 3793 Benoit et al. University of Montpellier, France 45

46 Chemical Vapor Deposition (CVD) Diamond Attractive features: Tissue equivalent Chemical inertness Radiation hardness Physically robust Safe for in vivo use 46

47 Chemical Vapour Deposition Diamond B doped B doped Source: Goncalves et al Optical Materials 27(2005)

48 Problems with CVD Diamond Fading of the TL signal Sensitivity to daylight Poor linearity Poor reproducibility 48

49 Concluding remarks The development of new dosimetric materials goes slowly The market is dominated by LiF and Al 2 O 3 Even of well-known dosimetric materials the exact mechanism is unknown The key issue in developing an improved dosimetric material is a high concentration of stable trapping centres that can be synthesized in a reproducible way 49