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Radiation Safety Training Module
Radiation Safety Training
Study Guide
ISU Technical Safety Office, Campus Box 8106
Pocatello, ID 83209
(208) 282-2311/2310

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Radioactivity and its units


Activity is the rate of disintegration (transformation) or decay of radioactive material.

1)   Becquerel (Bq) = one disintegration per second
2)     Curie (Ci) = 3.7 x 1010 Bq
Because the Ci is so large and the Bq is so small, we often use prefixes to define
levels of activity. Examples of these prefixes follow:

(m) milli (10-3)

(K) kilo (103)

(µ) micro (10-6)

(M) mega (106)

(n) nano (10-9)

(G) giga (109)

(p) pico (10-12)

(T) terra (1012)


Half Life (t1/2)

The amount of time that it takes for a radionuclide to be reduced by one half of its value through the process of radioactive decay.
The radioactivity decay equation is expressed as follows:
 A = Ai exp (- λT) = Ai exp (- 0.693T / t1/2)
Ai, is the initial activity of the source; 
λ, is the decay constant (λ= 0.693/t1/2);
T, is the elapsed time from the initial decay; and
t1/2, is the half-life of the radioactive material.


Dose and dose rate

a.   dose is the amount of energy per unit of mass.

b.   dose rate is the dose divided by the time in which the dose is received.
Roentgen (R)

1)   Is the unit of exposure to ionizing radiation and applies only to gamma and x -rays.

2)   Corresponds to the generation of approximately 2.58×10-4 C/kg in dry air at standard temperature
       and pressure (STP).

3)   Does not relate biological effects of radiation to the human body.

4)   1R = 1,000 milliroengten (mR)
Rad (unit of absorbed dose)

1)   Is the amount of energy from any type of radiation deposited in the unit mass of any material.

2)   Measures absorbed dose on different types of material but does not take into account the effect that                 different types of radiation have on the body.
Rem (unit of dose equivalent)

1)   Takes into account the energy absorbed in tissue as well as the biological effect on the body that                    different types of radiation have (dose equivalent).

2)   1 rem = 1,000 millirem (mrem)

These are the SI units equivalent to rad and rem.

1 Gy= 100 rad
1 Sv = 100 rem
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