Radiation

The effect of radiation on living organisms is 
understood by studying the interaction of this 
radiation with the atoms of this target organisms.
Radiation can burn or destroy tissues (cells). It 
can also involve illness, alterations of genetic 
material, or death.

Radiation includes radioactivity (&alpa; β 
γ and neutrons) and electromagnetic radiation 
suchas X-rays. These radiations pass through matter, 
lose and deposit their energy. They break molecular 
bonds or ionise atoms. In the latter case, they are 
called  ionizing radiation. 

Charged particles interact essentially with the 
electrons of the target atoms of a material. X-rays 
and γ-rays interact by photoelectric effect or 
compton scattering. Neutrons cause ionizations 
indirectly through collisions with nuclei, or 
absorption by nuclei that decay subsequently.

Radiation dosimetry  is the quantitative 
description of the effect of radiation on living 
tissue. The  absorbed dose of radiation is 
the energy delivered to the tissue per unit mass.
The SI unit of the absorbed dose is the Joule/kilogram
or the gray (Gy): 1 Gy = 1 J/kg. We use also other unit 
as the rad: 1 Gy = 100 rad.

The biological effect of the absorbed dose in tissue 
cells depends on the type of radiation. Equal energy 
deposited by different kind of radiation causes 
different biological effect. This variation is described 
by a numerical factor called the RBE: relative biological 
effectiveness, or the quality factor (QF) of 
each radiation.

X-rays 200 keV of energy are defined to have 
an RBE equal to unity. The effect of other 
radiation are compared experimentally. Here are 
some relative biological effectiveness (RBE) 
for some types of radiation:

The biological effect is defined as the product of the 
absorbed dose Dabs and the related RBE. It 
is called the equivalent dose Dequiv.

Dequiv = RBE x Dabs

We define the units Sievert (Sv), and 
rem: röntgen equivalent man: 

Dequiv(Sv) = RBE x Dabs (Gy)
Dequiv(rem) = RBE x Dabs (rad)

With 1 Gy = 100 rad, we have 1 Sv = 100 rem.
The unit of the RBE is Sv/Gy = rem/rad

Example:

A mass of 3 kiligrams of tissue receive un equivalent 
dose of 0.50 mSv by a beam fo X-rays of 60 KeV of 
energy. a) We will calculate the value of this dose 
equivalent in mrem and the corresponding value of 
the dose absorbed in mrad and Gy. b) We will also 
determine the number of X-rays received by this 
target tissue. The X-rays RBE is of 1 Sv/Gy = 
rem/rad.

Note: The energy of 60 KeV correspond to ONE 
ray. That is each X-ray is accelerated at 60 kilo volts 
(kV).

a)
0.50 mSv = 5.0 x 10-4 Sv = 5.0 x 10-4 x 100 rem = 
5.0 x 10-2 rem = 50.0 mrem

The related absorbed dose is Dabs = Dequiv(Sv)/RBE  =
5.0 x 10-4 Sv /(1 Sv/Gy) = 5.0 x 10-4 Gy (J/kg) = 
5.0 x 10-2 rem /(1 rem/rad) = 5.0 x 10-2 rad = 50.0 mrad

b)
The dose delivered by one (1) x-ray is: D1-del 
E1-X/related mass of tissue =  
60 keV/3 kg = 20 x 1000 x 1,6 x 10-19 (J/kg) = 
3.20 x 10-15 (J/kg)
This is the dose absorbed by the tissue due to one X-ray. 

There is NX rays in the beam X-rays of 
energy EX = 60 keV per each ray. 
The corresponding dose delivered by the beam is: 
Ddel = NX x D1-del = NX x 3.20 x 10-15 (J/kg).

The dose absorbed by the 3 Kg tissue due to the beam 
containing NX rays has the equivalent dose 
equal to 0.50 mSv, that corresponds to 5.0 x 10-4 Gy (J/kg).

We assume that all the dose delivered is absorbed, hence:
Ddel = Dabs, that is:

NX x 3.20 x 10-15 (J/kg) = 5.0 x 10-4 Gy (J/kg)
Therefore:
NX = 5.0 x 10-4/3.20 x 10-15 = 
1.65 x 1011 photons X-rays.