TG-51 PROTOCOL
C. SOLID STATE METHODS
There are several solid state systems available for the dosimetry of ionizing radiation. However, none of the systems is absolute—each needs calibration in a known radiation field before it can be used for the determination of absorbed dose.
There are two types of solid state dosimeters: (a) integrating-type dosimeters (thermolumi- nescent crystals, radiophotoluminescent glasses, optical density-type dosimeters such as glass
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ChaPter 8 Measurement of absorbed Dose 119
Radiation G Value (No./100 eV)
137Cs 15.3 ± 0.3
2 MV 15.4 ± 0.3
60Co 15.5 ± 0.2
4 MV 15.5 ± 0.3
5–10 MV 15.6 ± 0.4
11–30 MV 15.7 ± 0.6
(Data from ICrU. Radiation Dosimetry: X rays and Gamma Rays with Maximum Photon Energies between 0.6 and 50 MeV.
report 14. Bethesda, MD: International Commission on radiation Units and Measurements; 1969, with permission.) TABLE 8.5 Recommended G Values for the Ferrous Sulfate Dosimeter
(0.4 mol/L H2SO4) for Photon Beams
and film), and (b) electrical conductivity dosimeters (semiconductor junction detectors, induced conductivity in insulating materials). Of these, the most widely used systems for the measure- ment of absorbed dose are the thermoluminescent dosimeter (TLD), diodes, and film, which are described.
C.1. Thermoluminescence Dosimetry
Many crystalline materials exhibit the phenomenon of thermoluminescence (TL). When such a crystal is irradiated, a very minute fraction of the absorbed energy is stored in the crystal lattice.
Some of this energy can be recovered later as visible light if the material is heated. This phenom- enon of the release of visible photons by thermal means is known as TL.
The arrangement for measuring the TL output is shown schematically in Figure 8.10. The irradiated material is placed in a heater cup or planchet, where it is heated for a reproducible heating cycle. The emitted light is measured by a photomultiplier tube (PMT), which converts light into an electrical current. The current is then amplified and measured by a recorder or a counter.
There are several TL phosphors available, but the most noteworthy are lithium fluoride (LiF), lithium borate (Li2B4O7), and calcium fluoride (CaF2). Their dosimetric properties are listed in Table 8.6. Of these phosphors, LiF is most extensively studied and most frequently used for clinical dosimetry. LiF in its purest form exhibits relatively little TL. But the presence of a trace amount of impurities (e.g., magnesium) provides the radiation-induced TL. These impurities give rise to imperfections in the lattice structure of LiF and appear to be necessary for the appearance of the TL phenomenon.
C.2. Simplified Theory of Thermoluminescent Dosimetry
The chemical and physical theory of TLD is not exactly known, but simple models have been proposed to explain the phenomenon qualitatively. Figure 8.11 shows an energy-level diagram of an inorganic crystal exhibiting TL by ionizing radiation.
TL
Grounded TLD sample
High voltage
PMT
Recorder
Heater power supply
Amplifier
Figure 8.10. Schematic diagram showing apparatus for measuring thermoluminescence (tL). PMt, photomultiplier tube; tLD, thermoluminescent dosimeter.
120 Part I Basic Physics
TABLE 8.6 Characteristics of Various Phosphors
Characteristic LiF Li2B407:Mn CaF2:Mn CaF2:nat CaSo4:Mn
Density (g/cc) 2.64 2.3 3.18 3.18 2.61
effective atomic no. 8.2 7.4 16.3 16.3 15.3
tL emission spectra (a)
range 3,500–6,000 5,300–6,300 4,400–6,000 3,500–5,000 4,500–6,000
Maximum 4,000 6,050 5,000 3,800 5,000
temperature of main tL glow peak
195°C 200°C 260°C 260°C 110°C
efficiency at cobalt-60 (rela- tive to LiF)
1.0 0.3 3 23 70
energy response without added filter (30 keV/
cobalt-60)
1.25 0.9 13 13 10
Useful range Small, <5%/12 wk mr–106 r mr–3 × 105 r mr–104 r r–104 r
Fading mr–105 r 10% in first mo 10% in first mo No detectable
fading
50–60% in the first 24 h
Light sensitivity essentially none essentially none essentially none Yes Yes
Physical form Powder, extruded,
teflon embedded, silicon embedded, glass capillaries
Powder, teflon embedded
Powder, teflon embedded, hot pressed chips, glass capillaries
Special dosimeters Powder, teflon embedded
(From Cameron Jr, Suntharalingam N, Kenney GN. Thermoluminescent Dosimetry. Madison, WI: University of Wisconsin Press; 1968, with permission.)
Conduction band
ENERGY
Valence band
Electron trap
Irradiation
A B
Heating
TL photon
Ionizing
radiation Figure 8.11. a
simplified energy-level diagram to illustrate the thermoluminescence (tL) process.
In an individual atom, electrons occupy discrete energy levels. In a crystal lattice, on the other hand, electronic energy levels are perturbed by mutual interactions between atoms and give rise to energy bands: the “allowed” energy bands and the forbidden energy bands. In addition, the presence of impurities in the crystal creates energy traps in the forbidden region, providing metastable states for the electrons. When the material is irradiated, some of the electrons in the valence band (ground state) receive sufficient energy to be raised to the conduction band. The vacancy thus created in the valence band is called a positive hole. The electron and the hole move independently through their respective bands until they recombine (electron returning to the ground state) or until they fall into a trap (metastable state). If there is instantaneous emission of light owing to these transitions, the phenomenon is called fluorescence. If an electron in the trap requires energy to get out of the trap and fall to the valence band, the emission of light in this case is called phosphorescence (delayed fluorescence). If phosphorescence at room temperature is very slow, but can be speeded up significantly with a moderate amount of heating (~300°C), the phenomenon is called thermoluminescence.
A plot of TL against temperature is called a glow curve (Fig. 8.12). As the temperature of the TL material exposed to radiation is increased, the probability of releasing trapped electrons
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ChaPter 8 Measurement of absorbed Dose 121
increases. The light emitted (TL) first increases, reaches a maximum value, and falls again to zero. Because most phosphors contain a number of traps at various energy levels in the forbid- den band, the glow curve may consist of a number of glow peaks as shown in Figure 8.12. The different peaks correspond to different “trapped” energy levels.
C.3. Lithium Fluoride
The TL characteristics of LiF have been studied extensively. For details, the reader is referred to Cameron et al. (42).
Lithium fluoride has an effective atomic number of 8.2 compared with 7.4 for soft tissue. This makes this material very suitable for clinical dosimetry. Mass energy absorption coefficients for this material have been given by Greening et al. (43). The dose absorbed in LiF can be converted to the dose in muscle by considerations similar to those discussed earlier. For example, under electronic equilibrium conditions, the ratio of absorbed doses in the two media will be the same as the ratio of their mass energy absorption coefficients. If the dimensions of the dosimeter are smaller than the ranges of the electrons crossing the dosimeter, then the Bragg-Gray relationship can also be used. The ratio of absorbed doses in the two media then will be the same as the ratio of mass stopping powers. The applicability of the Bragg-Gray cavity theory to TLD has been discussed by several authors (44,45).
C.4. Practical Considerations
As stated previously, the TLD must be calibrated before it can be used for measuring an unknown dose. Because the response of the TLD materials is affected by their previous radiation history and thermal history, the material must be suitably annealed to remove residual effects. The stan- dard preirradiation annealing procedure for LiF is 1 hour of heating at 400°C and then 24 hours at 80°C. The slow heating, namely 24 hours at 80°C, removes peaks 1 and 2 of the glow curve (Fig. 8.12) by decreasing the “trapping efficiency.” Peaks 1 and 2 can also be eliminated by postir- radiation annealing for 10 minutes at 100°C. The need for eliminating peaks 1 and 2 arises from the fact that the magnitude of these peaks decreases relatively fast with time after irradiation. By removing these peaks by annealing, the glow curve becomes more stable and therefore predictable.
The dose–response curve for TLD-1002 is shown in Figure 8.13. The curve is generally linear up to 103 cGy, but beyond this it becomes supralinear. The response curve, however, depends on many conditions that have to be standardized to achieve reasonable accuracy with TLD. The calibration should be done with the same TLD reader, in approximately the same quality beam and to approximately the same absorbed dose level.
The TLD response is defined as TL output per unit absorbed dose in the phosphor.
Figure 8.14 gives the energy–response curve for LiF (TLD-100) for photon energies below mega- voltage range. The studies of energy response for photons above 60Co and high-energy electrons have yielded somewhat conflicting results. Whereas the data of Pinkerton et al. (46) and Crosby et al. (47) show some energy dependence, other studies (48) do not show this energy dependence.
When considerable care is used, precision of approximately 3% may be obtained using TLD powder or extruded material. Although not as precise as the ion chamber, TLD’s main
2TLD-100 (Harshaw Chemical Co.) contains 7.5% 6Li and 92.5% 7Li.
30
24
16
8 (5 min)
∼105°C (10 h)
∼190°C (80 y)
(0.5 y) (70 y) 5
4 3 2
1
00 8 16
Time (s)
Relative TL
24 30
Figure 8.12. an example of glow curve of LiF (tLD-100) after phosphor has been annealed at 400°C for 1 hour and read immediately after irradiation to 100 r. tL, thermoluminescence. (From Zimmerman DW, rhyner Cr, Cameron Jr. thermal annealing effects on thermoluminescence of LiF. Health Phys. 1966;12:525, with permission.)
122 Part I Basic Physics
advantage is in measuring doses in regions where ion chamber cannot be used. For example, TLD is extremely useful for patient dosimetry by direct insertion into tissues or body cavities.
Since TLD material is available in many forms and sizes, it can be used for special dosimetry situations such as for measuring dose distribution in the buildup region, around brachytherapy sources, and for personnel dose monitoring.