192
12 Health Hazards and Protection
C = source strength in curies,
E = energy of γ -rays,
N = number of radiation per disintegration, and
d = distance from the source in feet.
Likewise, a very simple equation is used for the calculation of the dose-rate for
β-particles:
Dose-rate = 300 C rad/hr at one foot
where, C is source strength in curies.
These calculations of dose-rate can be explained by considering an example of a
radioactive isotope Bromine-82, (see the decay scheme of
82 Br) which gives several
γ -rays and one β-particle of energy 0.46 MeV. From the decay scheme of Bromine82, it will be noticed that each γ -rays has its own % abundance and the total energy
of γ -rays considering their abundances comes to about 2.697 MeV. Though not a
single γ -rays of Bromine-82 has such high energy, the body will experience the
same effect as if one γ -rays of energy 2.697 MeV were causing the damage. Hence,
in the calculation of dose-rate, 2.697 MeV should be considered. Consequently, if
the source activity is 1.7 mCi (millicuries), the dose-rate due to γ -rays at one foot
would be
D = 6C E N
= 6 × 1.7 × 10
−3
× 2.697 × 1
= 27.5 × 10
−3 rad/hr.
Calculation for dose-rate for β-particles would require only the source
strength i.e., 1.7 mCi. Hence,
Dose-rate = 300C rad/h at one foot
= 300 × 1.7 × 10
−3
= 510 × 10
−3 rad/h
Thus total dose-rate received by the person working with
82 Br isotope of 1.7 mCi at
one foot distance is sum of these two dose rates i.e., 510 mrad/hr + 27.5 mrad/hr
= 537.5 mrad/hr.
It is noteworthy, that although the dose-rate due to β-particles in the calculation
seems to be the highest, it has very much less effect on the body. Since most of the
β-particles (E max = 0.46 MeV) will be absorbed by the glass container, the lead-pot
etc., very few fractions of β-particles would be able to reach the body at one foot
distance. On the other hand, the Bremsstrahlung radiation may perhaps be appreciable. Nevertheless, γ -rays will be received by the body without any considerable loss,
unless lead shielding has been used. In other words, in practice the dose-rate at one
foot will not be 537.5 mrad/hr, but definitely more than 27.5 mrad/hr. This calculation
also points out the difficulties in calculating theoretically the dose rate. Therefore,
12 Health Hazards and Protection
C = source strength in curies,
E = energy of γ -rays,
N = number of radiation per disintegration, and
d = distance from the source in feet.
Likewise, a very simple equation is used for the calculation of the dose-rate for
β-particles:
Dose-rate = 300 C rad/hr at one foot
where, C is source strength in curies.
These calculations of dose-rate can be explained by considering an example of a
radioactive isotope Bromine-82, (see the decay scheme of
82 Br) which gives several
γ -rays and one β-particle of energy 0.46 MeV. From the decay scheme of Bromine82, it will be noticed that each γ -rays has its own % abundance and the total energy
of γ -rays considering their abundances comes to about 2.697 MeV. Though not a
single γ -rays of Bromine-82 has such high energy, the body will experience the
same effect as if one γ -rays of energy 2.697 MeV were causing the damage. Hence,
in the calculation of dose-rate, 2.697 MeV should be considered. Consequently, if
the source activity is 1.7 mCi (millicuries), the dose-rate due to γ -rays at one foot
would be
D = 6C E N
= 6 × 1.7 × 10
−3
× 2.697 × 1
= 27.5 × 10
−3 rad/hr.
Calculation for dose-rate for β-particles would require only the source
strength i.e., 1.7 mCi. Hence,
Dose-rate = 300C rad/h at one foot
= 300 × 1.7 × 10
−3
= 510 × 10
−3 rad/h
Thus total dose-rate received by the person working with
82 Br isotope of 1.7 mCi at
one foot distance is sum of these two dose rates i.e., 510 mrad/hr + 27.5 mrad/hr
= 537.5 mrad/hr.
It is noteworthy, that although the dose-rate due to β-particles in the calculation
seems to be the highest, it has very much less effect on the body. Since most of the
β-particles (E max = 0.46 MeV) will be absorbed by the glass container, the lead-pot
etc., very few fractions of β-particles would be able to reach the body at one foot
distance. On the other hand, the Bremsstrahlung radiation may perhaps be appreciable. Nevertheless, γ -rays will be received by the body without any considerable loss,
unless lead shielding has been used. In other words, in practice the dose-rate at one
foot will not be 537.5 mrad/hr, but definitely more than 27.5 mrad/hr. This calculation
also points out the difficulties in calculating theoretically the dose rate. Therefore,