© The scientific sentence. 2010
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
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:
| 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 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
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 =
Note: The energy of 60 KeV correspond to ONE
ray. That is each X-ray is accelerated at 60 kilo volts
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
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)
NX = 5.0 x 10-4/3.20 x 10-15 =
1.65 x 1011 photons X-rays.