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Radioactivity and its units 
Activity
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
10^{10} 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 (10^{3})

(µ) micro (10^{6})

(M)
mega (10^{6})

(n)
nano (10^{9})

(G)
giga (10^{9})

(p)
pico (10^{12})

(T)
terra (10^{12})

Half Life (t_{1/2})
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 = A_{i} exp ( λT)
= A_{i} exp ( 0.693T / t_{1/2})
where,
A_{i,} is the initial activity of the source;
λ, is the
decay constant (λ= 0.693/t_{1/2});
T, is the
elapsed time from the initial decay; and
t_{1/2},_{ }is the halflife of the
radioactive material.

Dosimetry 
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)
Gray/Sievert
These are the SI units equivalent to rad and rem.
1 Gy= 100 rad
1 Sv = 100 rem

