Introduction to Health Physics: Fourth Edition - Ruang Baca FMIPA UB

456 CHAPTER 9
Figure 9-23. Glow curve for LiF that had been dosed with
100 rems (1 Sv) X-rays. The area under the curve is proportional
to the total dose. (Reproduced with permission from
Cameron JR, Zimmerman D, Kenney G, Buch R, Bland R,
Grant R. Thermoluminescent dosimetry utilizing LiF. Health
Phys. January 1964;10:25–29.)
electrons at readout, and consequently to a smaller dose at readout than originally
recorded, as shown in the following example.
W Example 9.4
In a laboratory where TLD badges are changed every 4 weeks, and are read 2 days
later, a radiation worker is exposed on the day that he received the dosimeter, and
has no further exposure. What fraction of the actual dose will be reported if the
spontaneous fading rate is 1.35 × 10 −8 s −1 ?
Solution
Since the dose is directly proportional to the number of trapped electrons, the
fraction of the original measured dose remaining 30 days later is
D
D0
= e −kt = e
1
s
−1.35×10−8 s ×8.64×104 d ×30 d = 0.97 = 97%.
Figure 9-23 shows a characteristic glow curve for LiF, which is obtained by heating
the irradiated crystal at a uniform rate and measuring the emitted light as the
temperature increases. The temperature at which the maximum light output occurs
is a measure of the binding energy of the electron or the hole in the trap. More
than one peak on a glow curve indicates different trapping sites, each with its own
binding energy. The total light output is proportional to the number of trapped,
excited electrons, which, in turn, is proportional to the amount of energy absorbed
from the radiation. Thus, the light output is directly proportional to the radiation
absorbed dose.