Distance
The intensity of a radiation source, and hence the radiation exposure, varies
inversely as the square of the distance from the source to the point of expo-
sure. It is recommended that an individual should keep as far away as prac-
tically possible from the radiation source. Procedures and radiation areas
should be designed so that individuals conducting the procedures or staying
in or near the radiation areas receive only minimum exposure.
The radiation exposure from g-ray and x-ray emitting radionuclides can
be estimated from the exposure rate constant,G, which is defined as the
exposure from g-rays and x-rays in R/hr from 1 mCi (37 MBq) of a radionu-
clide at a distance of 1 cm. Each g- and x-ray emitter has a specific value of
G, which has the unit of R · cm^2 /mCi · hr at 1 cm or, in System Internationale
(SI) units,mGy·m^2 /GBq · hr at 1 m. The Gvalues are derived from the
number of g-ray and x-ray emissions from the radionuclide, their energies,
and their mass absorption coefficients in air.aBecause g-rays or x-rays below
some 10 or 20 keV are absorbed by the container and thus do not contribute
significantly to radiation exposure, often g-rays and x-rays above these ener-
gies only are included in the calculation of G. In these instances, they are
denoted by G 10 or G 20. The values of G 20 for different radionuclides are given
in Table 16.2.
The exposure rate Xfrom an n-mCi radionuclide source at a distance d
cm is given by
Radiation Protection 275aThe Gvalue of photon-emitting radionuclides can be calculated from the expres-
sion G= 199 ΣNiEimi, where Niis the fractional abundance of photons of energy Eiin
MeV, and miis the mass absorption coefficient (cm^2 /g) of photons of energy Eiin air.
Table16.2. Exposure rate constants of commonly used radionuclides.
Radionuclides Γ 20 (R · cm^2 /mCi · hr at 1 cm) Γ 20 (mGy · m^2 /GBq · hr at 1 m)*
(^137) Cs 3.26 88.11
99mTc 0.59 15.95
(^201) Tl 0.45 12.16
(^99) Mo 1.46 39.46
(^67) Ga 0.76 20.54
(^123) I 1.55 41.89
(^111) In 2.05 55.41
(^125) I 1.37 37.03
(^57) Co 0.56 15.16
(^131) I 2.17 58.65
(^18) F† 5.70 154.05
*R·cm^2 /mCi · hr is equal to 27.027 mGy · m^2 /GBq · hr.
†Personal communication with Dr. M. Stabin, Oak Ridge Associated Universities, Inc., Oak
Ridge, Tennessee.
Adapted from Goodwin PN: Radiation safety for patients and personnel. In: Freeman LM, ed.
Freeman and Johnson’s Clinical Radionuclide Imaging.3rd ed. Philadelphia: WB Saunders Co;
1984: 320.