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12 Health Hazards and Protection
the limit based upon the number of particles being emitted by the isotopes. Thus, in
either of these two cases, we need to know the number of particles being emitted by
the isotopes per unit time. Therefore, the unit to measure the radiation dose must be
related to the number of particles emitted by the isotopes per unit time. Moreover,
for defining the limit for external hazard though, we need to measure the number of
radiations emitted per unit time, but for internal hazards, we need to define the unit
of radiation, which is related to the amount of material and the number of radiation
emitted by the isotopes. Considering these two aspects of the requirements, the unit
of radiations are classified broadly into two categories (i) unit of rad (or rem)/unit
time and (ii) curie.
12.5.1 Curie
When we wish to know the amount of radioactive material present in the sample so
that we can get some idea about the number of radiations, which could be emitted
by the isotope per unit time, the unit for such purpose is Curie, the name given in
the honor of Curie family who discovered the application of radioactivity.
This unit was established by following the decay of Radium-226 isotope. The
atomic weight of radium is 226.03. Therefore, if pure Radium-226 is separated
then at zero time of its formation (i.e., No), 1 gram of Radium-226 will contain
(6.022 × 10
23
/226.03) = 2.664 × 10
21 atoms. The half-life of Radium-226 being
1600 yr, hence its decay constant (λ) is equal to [0.69315/(1600 × 3.156 × 10
7
)] =
0.1373 × 10
−10 per second, 3.156 × 10
7 being the number of seconds in a year.
Thus, disintegration rate of 1 gram of Radium-226 is given by d N 0 /dt = 0.1373 ×
10
−10
× 2.664 × 10
21
= 3.66 × 10
10
α-particles (or radon atoms) per seconds, αparticle being the type of radiation emitted by
226 Ra and radon being the decay
product of this isotope. Thus curie (Ci) is defined as the amount of radon (the decay
product of Radium-226) in equilibrium with 1 gram of Radium-226. Based on this
calculation, a general definition of one curie (1 Ci) is the activity of a radioactive
substance which decays (i.e., emits radiations) at a rate of 3.66 × 10
10 disintegration
per second. Curie is thus the quantity of radioactive material which emits 3.66 × 10
10
disintegration per second. The advantage of this unit is that we can use this quantity to
establish the efficiency of counting process, or the efficiency of chemical separation
techniques etc.
12.5.2 Rad
When our interest is in knowing the amount of radiations received externally by the
person per unit time, the unit of this amount is referred to as rad. It is defined as the
dosage of radiation that will impart 100 ergs of energy to each gram of matter
through which the radiation passes. The rad is being replaced by its SI equivalent,
12 Health Hazards and Protection
the limit based upon the number of particles being emitted by the isotopes. Thus, in
either of these two cases, we need to know the number of particles being emitted by
the isotopes per unit time. Therefore, the unit to measure the radiation dose must be
related to the number of particles emitted by the isotopes per unit time. Moreover,
for defining the limit for external hazard though, we need to measure the number of
radiations emitted per unit time, but for internal hazards, we need to define the unit
of radiation, which is related to the amount of material and the number of radiation
emitted by the isotopes. Considering these two aspects of the requirements, the unit
of radiations are classified broadly into two categories (i) unit of rad (or rem)/unit
time and (ii) curie.
12.5.1 Curie
When we wish to know the amount of radioactive material present in the sample so
that we can get some idea about the number of radiations, which could be emitted
by the isotope per unit time, the unit for such purpose is Curie, the name given in
the honor of Curie family who discovered the application of radioactivity.
This unit was established by following the decay of Radium-226 isotope. The
atomic weight of radium is 226.03. Therefore, if pure Radium-226 is separated
then at zero time of its formation (i.e., No), 1 gram of Radium-226 will contain
(6.022 × 10
23
/226.03) = 2.664 × 10
21 atoms. The half-life of Radium-226 being
1600 yr, hence its decay constant (λ) is equal to [0.69315/(1600 × 3.156 × 10
7
)] =
0.1373 × 10
−10 per second, 3.156 × 10
7 being the number of seconds in a year.
Thus, disintegration rate of 1 gram of Radium-226 is given by d N 0 /dt = 0.1373 ×
10
−10
× 2.664 × 10
21
= 3.66 × 10
10
α-particles (or radon atoms) per seconds, αparticle being the type of radiation emitted by
226 Ra and radon being the decay
product of this isotope. Thus curie (Ci) is defined as the amount of radon (the decay
product of Radium-226) in equilibrium with 1 gram of Radium-226. Based on this
calculation, a general definition of one curie (1 Ci) is the activity of a radioactive
substance which decays (i.e., emits radiations) at a rate of 3.66 × 10
10 disintegration
per second. Curie is thus the quantity of radioactive material which emits 3.66 × 10
10
disintegration per second. The advantage of this unit is that we can use this quantity to
establish the efficiency of counting process, or the efficiency of chemical separation
techniques etc.
12.5.2 Rad
When our interest is in knowing the amount of radiations received externally by the
person per unit time, the unit of this amount is referred to as rad. It is defined as the
dosage of radiation that will impart 100 ergs of energy to each gram of matter
through which the radiation passes. The rad is being replaced by its SI equivalent,