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

214 CHAPTER 6
absorption, in joules per kilogram of tissue, corresponding to an exposure of 1 C/kg
air is, therefore,
3.28 J J
× 34 air = 37
3.01 kg kg tissue.
This value agrees very well with calorimetric measurements of energy absorption by
soft tissue from an exposure of 1 C/kg air. By analogy, an exposure of 1 R, which
corresponds to 87.7 ergs/g of air, leads to an absorption of 95 ergs/g muscle tissue.
This tissue dose from a 1-R exposure is very close to the tissue dose of 100 ergs/g,
which corresponds to 1 rad. For this reason, an exposure of1Risfrequently considered
approximately equivalent to an absorbed dose of 1 rad, and the unit “roentgen”
is loosely (but incorrectly) used to mean “rad.” Because of this simple approximate
one-to-one relationship of the roentgen to the rad, the roentgen continues to be
used. To be up to date vis-à-vis measurement units, an exposure of 1 R is often called
a dose of 1 centigray (cGy).
The exposure unit bears a simple quantitative relationship to the dosimetric
unit (the gray or the rad) that permits the calculation of absorbed dose in any
medium whose exposure (in coulombs per kilogram or statcoulombs per cubic
centimeter air) is known. This relationship may be illustrated by the following
example.
W Example 6.5
Consider a gamma-ray beam of quantum energy 0.3 MeV. If the photon flux is 1000
quanta/cm 2 /s and the air temperature is 20 ◦ C, what is the exposure rate at a point
in this beam and what is the absorbed dose rate for soft tissue at this point?
Solution
From Figure 5-20, the linear energy absorption coefficient for air, μa, at STP, for
300-keV photons is found to be 3.46 × 10−5 cm−1 . The exposure rate, ˙X, in C/kg/s
in a photon flux φ is given by
φ
X ˙ =
photons
cm2 MeV
× E
/s photon × 1.6 × 10−13 J
MeV × μair / cm
,
kg J/kg
ρair × 34
cm3 C/kg
where
(6.9)
μa is the linear energy absorption coefficient for air for the photon energy and
ρa is the density of air.