Nuclear Physics
The nucleus
Radioactivity
Applications
Particle accelerators
© The scientific sentence. 2010
 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 Xrays. 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. Xrays
and γrays 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.
Xrays 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:
Radiation  RBE (Sv/Gy = rem/rad) 
X rays and γ rays  1 
Electrons  1.0  1.5 
Slow neutrons  3  5 
protons  10 
α particles  20 
Heavy ions  20 
The biological effect is defined as the product of the
absorbed dose D_{abs} and the related RBE. It
is called the equivalent dose D_{equiv}.
D_{equiv} = RBE x D_{abs}
We define the units Sievert (Sv), and
rem: röntgen equivalent man:
D_{equiv}(Sv) = RBE x D_{abs} (Gy)
D_{equiv}(rem) = RBE x D_{abs} (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 Xrays 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 Xrays received by this
target tissue. The Xrays RBE is of 1 Sv/Gy =
rem/rad.
Note: The energy of 60 KeV correspond to ONE
ray. That is each Xray 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 D_{abs} = D_{equiv}(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) xray is: D_{1del}
E_{1X}/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 Xray.
There is N_{X} rays in the beam Xrays of
energy E_{X} = 60 keV per each ray.
The corresponding dose delivered by the beam is:
D_{del} = N_{X} x D_{1del} = N_{X} x 3.20 x 10^{15} (J/kg).
The dose absorbed by the 3 Kg tissue due to the beam
containing N_{X} 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:
D_{del} = D_{abs}, that is:
N_{X} x 3.20 x 10^{15} (J/kg) = 5.0 x 10^{4} Gy (J/kg)
Therefore:
N_{X} = 5.0 x 10^{4}/3.20 x 10^{15} =
1.65 x 10^{11} photons Xrays.

