TLD simplistic Model TLD is an Inorganic Crystal. The added trace impurities (Mg, Ti ) create discrete levels in the band-gap region, and thus play very important role. e Radiation Partially Full Heat Trap Lum. Ctr. ~ 400nm (3.1eV) for TLD-100 LiF (Mg,Ti) h Valance band Efficiency 0.04% for TLD-100, 1.2% for CaSo 4 (Mn) Imperfections (Defects) Plane Point Intrinsic (Thermal defects) Vacant lattice sites F center (-ve ion vacancy) Trapping center for e Radiation induced Interstitial atoms V center (+ve ion vacancy) Trapping center for hole Traps Extrinsic Lattice atoms are replaced by impurities activators Creates discrete energy levels in band gap Luminescence centers Lattice Vacancies Solid State by Kittle, Chapter-18
Lattice Vacancies Solid State by Kittle, Chapter-18 Schottky Defects: Most common in Alkali halides. Ion ends up on the surface. Frenkel Defects: Most common in silver halides. Ion takes an interstitial position. Defects are generated when KCl crystal is grown with controlled amounts of CaCl 2. TLD: Materials, Forms TLD-100: LiF(Mg,Ti) 6 Li/Li = 7%.Meas. e, x * (n,a) cross-section is TLD-600: LiF(Mg,Ti) 6 Li/Li = 96%.. Meas. n*, higher for 6 Li than 7 Li. TLD-700: LiF(Mg,Ti) 6 Li/Li = 0%..Meas. 6 Li+Thermal n= 3 H+ TLD-700H: LiF(Mg,Cu,P). -sensitivity 30 times better & better S/N. CaF 2 (Mn) ). 23 times sensitive than TLD-100, but light sensitive! CaSO 4 (Mn) ).. 70 times sensitive than TLD-100, but light sensitive, and fades more! TLD-800: Li 2 B 4 O 7 (Mn) 1/3 rd sensitive than TLD-100! Al 2 O 3 (?). NOTE: Li is close to tissue-eq. (Atomic # slightly > Tissue s) Forms Single xtal (normally for research) Powder 75-150 m Chips/Rods (pellets) compressed powder Teflon matrix (discs) TLD Reader proof encloser Current Integr & recorder N2 Flow NOTE: Nitrogen-Flow is maintained because O 2 produces own luminescence.
TLD-100 glow curve: Specific to anneal procedure (400 0 C for 1 hr, 100 0 C for 2 hr, let cool to T room : produces only peak # 2, 5 & 6) (400 0 C for 1 hr, abrupt cool to T room : produces 6 peaks) Signal 8.5y 80y 1d 3m 350-600 nm max @ 400 nm (3.1eV) 10 min Some 210 100 y 0 60 120 170 190 285 300 0 C Glow-curve shape depends on anneal procedure. Ti produces lumin. ctr. Mg ++ produces e-traps. Peaks 1, 6 are due to intrinsic defects. Peaks 2-5 are due to Mg ++ (Dosimetry normally uses peak #5). N 2 flow during heat-cycle eliminates non-radiation induced signal in high-temp. region. TLD glow-curve: Simplistic model Randall & Wilkins (1945) Assume: All electrons, released from traps, undergo thermoluminescence, and no re-trapping. Implies: Glow intensity is proportional to the rate of release of trapped electrons. E Radiation e Heat Trap e population n Lum. Ctr. h Valance band If at the trap at instant t, e population is n, then: De-population rate at the trap, dn n and also, Exp [-E/(KT)] With S as constant of proportionality = S n Exp [-E/(KT)].. (1) Rearrange left side, dn dt dt = ------- dt dn Assume ramp rate, = R (uniform): R dt = -------. (2) Separate variables, dn = S Exp [-E/(KT)] dt n R To obtain n, integrate, n dn S T - = Exp [-E/(KT)] dt n o n R T o Integration of both sides yields, -[ln (n) ln (no)] = S T Exp [-E/(KT)] dt R T o n -ln n o = ------- n S T = Exp Exp - E/ KT dt n 0 R T o Substitute n from this equation into equation (1): dn S no Increases with T - S T Exp Exp - E/ KT dt Exp [-E/(KT)]. (3) R To
Glow-curve Simplistic model (Cont d) Glow intensity, I dn dn = - C Use equation (3) = C S no Exp Exp - E/ KT dt Exp - E/ KT R T o.. (4) Glow Intensity constant with T with T The peak is caused by the competing factors I 0 T 0 T Glow-curve Simplistic model (Cont d) Competing Factors: Graph shows function Exp(-E/KT) increases with T. Exp(-E/KT) 1.0 0.0 T Therefore integral {Exp(-E/KT)}, call it y, increases with T. Hence the factor Exp(-y) decreases with increase in T. Plastic-Scintillator in Dosimetry-Use Ref: Beddar et al., Phys in Med & Bio 37 (1992) p1883-1900 & 1900-1913. Material: Kirov et al. (2-D dosimetry of brachy), Med.Phys 26 (1999) P 1515-1523. Properties: Plastic scintillator BC-400 from Bicron (Like NE-102 from NEL) peak ~ 423 nm, 1.032, T soften 750C Dimensions: (1mm dia, 4mm long) light-sealed with thin black slieve. Optical cables: Thinnest available 0.5mm Signal cable ( Optical fiber couples scintillator to a PMT). B.gr. cable to another PMT (used for subtraction of Cerenkov signal induced by the radiation in the signal cable) Import. charact: Stop-power ( en ) very close to H 2 O & no need of H 2 O-proofing. E-depend least (compared to air-ion chamber, TLD & Si diode. Radiation damage lower than diodes. Only 3% for 100Gy of 15 MeV e (9% for x-diode, 25% for e-diode) Rel-Measurements: See next slide
DD data with Scintillation Detector (compared to ion-chamber % diode) Diamond Detector Band gap: The insulator has band gap of ~ 5.6 ev, hence well suited for operation at high temperature up to 300 0 C. Conductivity increases with radiation dose (N 2 as an impurity plays role). Artificial diamond can better reproduce N 2 concentration. Dimensions: Thickness 0.1-0.3 mm, area 3-15 mm 2. Signal Typical specs: Bias +100 V (NOT 300v) 100 M Ohm E range 0.08-20 MV (photons) 4-20 MeV (electrons) Diamond Sensitivity 0.5-5 nc/r @ 60 Co. Cost $ ~ 7,000 Impt. Char: en very close to that of H 2 O. en change small over 0.1-20 MV ( 7% vs. diode 42%). Negligible directional dependence. Superior long-term stability & fast response time (~ns). Does show a small dep on D-rate, but corr. Can be applied (Laub et al. 1997)