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Physical, technological and
organizational bases of
radiation medicine.
Prof. Igor Y. Galaychuk, MD
Head, Department of Oncology & Radiology
Ternopil State Medical University
Physics of Radiation
Proton - positive
charge, in the
Neutron - neutral (no)
charge, in the
Electron - negative
charge, orbits the
Physics of Radiation
– Stable atoms do not contain excess energy.
– Unstable atoms contain excess energy. This is
caused by an imbalance in the ratio of protons
to neutrons in the nucleus of the atom. These
atoms release their excess energy during the
process known as radioactive decay. The
energy released in the process is called
ionizing radiation.
Physics of Radiation
Radiation - Radiation is energy in the form of waves or
particles given off during radioactive decay, or as a
consequence of certain physical processes that we can
control. Examples of these are x-ray machines and
particle accelerators.
Wave radiations include gamma and x-rays. A common term
used to describe this type of radiation is photon radiation.
Particle radiation can consist of charged or uncharged
particles which are emitted with very high velocity.
Physics of Radiation
Radioactive contamination - contamination is
radioactive material that is in a form or location which
may allow it to be spread to unwanted locations. Many
radioactive sources are sealed or are in a form that
isolates the material from potential spread.
Contamination may be Fixed, Transferable (loose), or
Physics of Radiation
Radioactivity - Radioactivity is the process of unstable (or
radioactive) atoms becoming stable by emitting
radiation. The radioactive decay process involves
fundamental physical constants which enable us to
characterize and measure radioactive materials very
• Radioactive half-life - Radioactive half-life is the time is
takes for one half of the radioactive atoms present in a
given sample to decay. The half-life of a particular isotope
is a constant, and depending on the isotope it may range
from a fraction of a second to millions of years. After seven
half-lives the activity will be less than 1% of the original
Physics of Radiation
• Ionization - The process of ionization is important in
understanding radiation, because it is this process that
differentiates ionizing radiation from other types. Ionization is
the process of removing electrons from atoms. If enough
energy is supplied to remove electrons from the atom the
remaining atom has a + charge. The positive charges atom
and the negatively charges electron are called an ion pair.
Ionization should not be confused with radiation. Ions (or ion
pairs) can be the result of radiation exposure and allow the
detection of radiation.
• Typically, we classify types of radiation as ionizing or non ionizing radiations depending on whether or not the radiation
can form ion pairs in common material such as air or tissue.
Physics of Radiation
– Ionizing radiation - Radiation which has enough
energy to ionize an atom is called ionizing radiation.
The four basic types of ionizing radiation that are of
primary concern to us are alpha particles, beta
particles, gamma rays (includes x -rays) and
neutron particles.
– Non-ionizing radiation - Radiation that doesn't have
the amount of energy needed to ionize an atom.
Examples of non-ionizing radiation are ultraviolet
rays, microwaves and visible light.
• Alpha particles are emitted during the decay of certain types
of radioactive materials. Compared to other types, the alpha
particle has a relatively large mass. It consists of two
protons and two neutrons. (Positive charge of plus two.)
• The alpha particle deposits a large amount of energy in a
short distance of travel. This large energy deposit limits the
penetrating ability of the alpha particle to a very short
distance. This range in air is about one to two inches.
– Shielding
• Most alpha particles are stopped by a few centimeters of air,
a sheet of paper, or the dead layer (outer layer) of skin on
our bodies.
– Biological hazard
• Alpha particles are not considered an external radiation
hazard. This is because they are easily stopped by the dead
layer of skin. If alpha emitting radioactive material is inhaled
to ingested, it becomes a source of internal exposure.
Internally, the source of the alpha radiation is in close
contact with body tissue and can deposit large amounts of
energy in a small volume of body tissue.
• Beta particles
• The beta particle is an energetic electron emitted during radioactive
decay. Compared to an alpha particle, a beta particle is nearly 8000
times less massive and has half the electrical charge. Beta radiation
causes ionization by the same forces at work with alpha radiation mainly electrical interactions with atoms which are encountered as it
travels. However, because it is not as highly charged, the beta particle is
not as effective at causing ionization. Therefore, it travels further before
giving up all its energy and finally coming to rest.
• The beta particle has a limited penetrating ability. Its typical range in air
is up to about 10 feet. In human tissue, the same beta particle would
travel only a few millimeters.
– Shielding
• Beta particles are easily shielded by relatively thin layers of plastic, glass,
aluminum, or wood. Dense materials such as leas should be avoided
when shielding beta radiation sue to the increase in production of x-rays
in the shield.
– Biological Hazard
• Externally, beta particles are potentially hazardous to the skin and eyes.
They cannot penetrate to deep tissue such as the bone marrow or other
internal organs. We call this type of external exposure shallow dose.
When taken into the body, materials that emit beta radiation can be a
hazard in a similar way to that described from alpha emitters - although
comparatively less damage is done in the tissue exposed to the beta
Gamma rays /x-rays
• Gamma /x-ray radiation is an electromagnetic wave or photon and has
no electrical charge. Gamma rays and x-rays can be thought of as
physically identical. The only difference is in the place of origin. These
photons have no mass or charge but can ionize matter as a result of
direct interactions with orbital electrons. Like all electromagnetic
radiations, gamma rays travel at the speed of light.
• Because gamma /x-ray radiation had no charge and no mass, it has a
very high penetrating power (said another way, the radiation has a low
probability of interacting in matter). Gamma rays have no specific
"range" but are characterized by their probability of interacting in a given
material. There is no distinct maximum range in matter, but the average
range in a given material can be used to compare materials for their
shielding ability.
• Gamma /x-ray radiation are best shielded by very dense materials, such
as lead, concrete, or steel. Shielding is often expressed by thicknesses
that provide a certain shielding factor, such as a "half-value layer"
(HVL). An HVL is the thickness of a given material required to reduce
the dose rate to one half the unshielded dose rate.
– Biological hazard
• Due to the high penetrating power, gamma /x-ray radiation can result in
radiation exposure to the whole body rather than a small area of tissue
near the source. Therefore, a photon radiation has the same ability to
cause dose to tissue whether the source is inside or outside the body.
This is in contrast to alpha radiation for example which must be received
internally to be a hazard. Gamma radiation is considered an external
hazard. Refer to the definition of "whole body" in the glossary.
• Neutron particles.
• Physical characteristics:
• Neutron radiation consists of neutrons that are ejected from
the nuclei of atoms. A neutron has no electrical charge. Due
to their charge, neutrons do not interact directly with
electrons in matter. A direct interaction occurs as the result
of a "collision" between a neutron and the nucleus of an
atom. A charges particle or other radiation which can cause
ionization may be emitted during these interactions. This is
called indirect ionization.
• Because neutrons do not experience electrostatic forces, they have
a relatively high penetrating ability and are difficult to stop. Like
gamma radiation, the range is not absolutely defined. The distance
they travel depends on the probability for interaction in a particular
material. You can think of neutrons as being "scattered" as they
travel through material, with some energy being lost with each
scattering event.
• Moderate to low energy neutron radiation is best shielded by
materials with a high hydrogen content, such as water (H2O) or
polyethylene plastic (CH2-CH2-X). High energy neutrons are best
shielded by more dense materials such as steel or lead. Sometimes
a multi-layered shield will be used to first slow down very 'fast'
neutrons, and then absorb the 'slow' neutrons.
– Biological hazard
• Like gamma radiation, neutrons are an external "whole body" hazard
due to their high penetrating ability.
… from history of ionizing radiation:
• Doctors and scientists paid a price or
even lost their lives in their early work
with x-rays and radionuclides. Marie
Curie, who discovered radium, died of
a malignant blood disease probably
because of the radiation exposure to
her bone marrow during her lengthy
research work with the radionuclide.
The widespread use of x-ray in medical
diagnosis and treatment for some
diseases in the early 30's without
realizing its harmful effects led to
cases of radiation dermatitis and
chronic ulceration, eventually resulting
in radiation induced cancers.
Relative penetrating power of different
Different ionization rate in tissue due to different
penetrating power of the radiations
External and internal modes of radiation
Radiation sources
• There are many different radiation exposure
scenarios that can be evaluated. Some examples
• External exposure from relatively distant radiation
sources (e.g., neutrons and/or gamma rays)
• External exposure from nearby radioactive soil
• External exposure from radioactive contamination on the
outside of the body
• Internal exposure from inhaled radioactive substances
• Internal exposure from ingested radioactive substances
• Combinations of the above
Human exposure pathways associated with
radioactive isotopes in the environment
Natural sources of radiation
• All humans were born slightly radioactive because all
living tissue contains radioactive substances. The
radioactive characteristic is maintained throughout life.
However, the normal radioactivity found in humans is
nothing to worry about. Furthermore, nothing can be done
to eliminate it.
• The main radioactive materials in rocks are potassium-40,
rubidium-87, and two series of radioactive elements
arising from the decay of uranium-238 and thorium-232.
Uranium-238 and thorium-232 are long-lived radioactive
isotopes that have remained on Earth since its origin. The
levels of terrestrial radiation differ from place to place
around the world because the concentrations of these
materials in the Earth’s crust vary.
• The naturally occurring isotopes carbon-14 and tritium are
produced by cosmic radiation. Other naturally occurring
isotopes of interest include potassium-40, lead-210,
polunium-210, radon-222, and radon-220.
Nuclear Reactor Accidents/Destruction
• Nuclear accidents, such as occurred in 1979 at Three Mile
Island in the U.S. and in 1986 at Chernobyl in Ukraine,
lead to public and worker exposures to radiation. Unlike the
Three Mile Island accident, the Chernobyl accident led to
the loss of many lives. Such accidents can lead to the
release of large amounts of alpha-, beta-, and gammaemitting radionuclides (i.e., radioactive isotopes) into the
• As with nuclear weapons, fallout can arise from nuclear
power plant accidents. The terms "cloud shine" and
"ground shine" are therefore also used in describing
radiation sources associated with nuclear accidents.
• Nuclear facilities can also be destroyed by weapons leading
to similar radiation sources such as those that arise from
nuclear accidents.
• The units often used follow:
• roentgen (R),
gray (Gy),
sievert (Sv)
• These units relate to radiation exposure, radiation dose, or
radiation protection.
• Other units (curie, becquerel) relate to radioactivity.
The International System of Units (or SI units) includes the
gray, sievert, and becquerel:
• The gray is the SI unit used for absorbed dose.
• The sievert is the SI unit used for the dose equivalent and
for the effective dose equivalent.
• The becquerel is the SI unit used for radioactivity.
Radiation exposure units
• The roentgen describes the amount of xrays or gamma rays to which a target (e.g.,
mouse, rat, human, cow, etc.) is exposed.
The roentgen relates to the ability of x-rays
and gamma rays to remove electrons from
atoms in air.
• One roentgen corresponds to 2.58 x 10-4
coulombs per kilogram of air.
Radiation absorbed dose units
• The absorbed dose relates to how much radiation energy gets put into a
given target mass (e.g., lung, eye, thyroid gland).
• Different absorbed doses can arise in different organs or tissue of
the body for the same exposure in R. Thus, if a person were exposed
to 10 R of gamma rays, the eye, the thyroid, and the lung would have
different absorbed doses. Special computer programs can calculate
such doses.
• Units of absorbed dose often used are the rad and gray (an SI unit).
• The rad is a relatively old unit of absorbed dose. One rad
corresponds to 100 ergs of radiation energy per gram of target
• The gray unit represents 1 joule of radiation energy put into a
kilogram mass. Thus, 1 gray equals 1 joule per kilogram.
• The gray and rad apply to all types of ionizing radiation, unlike the
roentgen unit, which only applies to x-rays and gamma rays.
• Some useful conversion factors that relate to absorbed dose follow:
1 gray (Gy) = 100 rad, 1 milligray (mGy) = 0.1 rad, 1 rad = 1 centigray (cGy)
Radioactivity Units
• Radioactivity arises from the disintegration of
unstable atoms and is expressed in units like the
becquerel (Bq) and curie (Ci).
• One becquerel (Bq) corresponds to 1
disintegration (transformation) per second.
• One curie represents 37,000,000,000 (i.e., 3.7 x
10x10) disintegrations per second. A curie is a
very large amount of radioactivity.
• Some useful conversions follow:
 1 microcurie (µCi) = 37,000 becquerels (Bq)
Radiation-Protection Units
• Special dosimetric units are used in radiation protection
to limit radiation exposure of nuclear workers and the public.
These units include the rem and sievert (SI unit), which
apply to single and mixed radiations and are measures of
potential harm to humans.
• 1 sievert (Sv) = 100 rem.
• One rem of alpha radiation would be expected to pose the
same risk of harm as 1 rem of gamma rays or as 1 rem of
combined exposure to neutrons and gamma rays.
• The rem and Sv were developed to account for different
efficiencies of different types of radiation in producing
harm. Because these units apply to single and mixed
radiations, it follows that:
• 1 rem of alpha radiation = 1 rem of gamma rays.
• 1 rem of gamma rays = 1 rem of neutrons + gamma rays.
Types of doses expressed in rem or Sv
• dose equivalent (applies to single organ)
• committed dose equivalent (applies to
single organ)
• effective dose equivalent (applies to total
• committed effective dose equivalent
(applies to total body)
Dose equivalent (applies to single organ)
• The dose equivalent is a quantity that
accounts for the different efficiencies of
different external radiations in causing harm
to a given organ or tissue.
• Special weighting factors (e.g., quality factor) are
used to account for differences in radiation
quality. The term "external radiation" simply
means that radiation originates from outside the
body. Examples are gamma rays from
contaminated soil or from a radioactive cloud.
Committed dose equivalent
(applies to single organ)
• The committed dose equivalent is similar
to the dose equivalent but applies to doses
from radionuclides taken inside the body.
• Quality factors are used to account for
different efficiencies of different radiations
in producing biological damage internally.
Committed doses are evaluated to some
future time (e.g., to 50 years) after intake
of radionuclides.
Effective dose equivalent
(applies to total body)
• Effective dose equivalents account for different
biological sensitivities of different organs and tissue
and apply to the total body. They also account for
different doses (dose equivalents) to different
organs. Effective dose equivalents specifically apply to
external radiation sources (i.e., sources outside the
• An effective dose equivalent of 1 Sv for a nonuniform,
combined exposure to neutrons and gamma rays would
represent the theoretical dose of gamma rays uniformly
distributed over the body that would incur the same risk
of harm from stochastic effects (mainly cancer) as for the
actual nonuniform exposure to neutrons and gamma
Committed effective dose equivalent
(applies to total body)
• Committed effective dose equivalents also
account for different biological sensitivities of
different organs and tissue but apply only to
radionuclides that enter the body (e.g., via
inhalation or ingestion).
• A committed dose equivalent of 1 Sv from
inhaled alpha particle emitters would represent
the theoretical committed dose of gamma rays to
the total body that would yield the same risk of
harm from stochastic effects such as cancer.
Committed dose equivalents are evaluated to a
fixed time in the future (e.g., 50 years) after
intake of radionuclides.
Radiation weighting factors
• The equivalent dose is obtained by multiplying the
absorbed dose by special factors called radiation
weighting factors (WR) that are intended to account for
different efficiencies of the different radiations in producing
biological damage.
• Like its predecessor the quality factor, the weighting factor
WR was designed to protect against radiation-induced
harm (mainly from cancer induction).
• Radiation weighting factors, WR, currently used for alpha
radiation, beta radiation, gamma rays, and x-rays follow:
• WR for alpha radiation = 20
• WR for beta radiation = 1
• WR for gamma radiation = 1
• WR for x-rays = 1
What is the equivalent dose for combined
exposure of the cornea (of the eye) to 0.1 Gy of
alpha radiation plus 0.2 Gy of gamma rays?
The answer follows:
Equivalent dose in Sv = WR (alpha radiation)*0.1 Gy
+ WR (gamma rays)*0.2 Gy.
Since WR (alpha radiation) equals 20 and WR
(gamma rays) equal 1,
you get
Equivalent dose in Sv = 20 x 0.1 + 1 x 0.2 = 2.2 Sv.
• Physical protection is used to prevent
radiation exposure of individuals from
• Chemical protection is provided to protect
when a possible radiation exposure is
anticipated or after an exposure has
• Physiological protection is provided after a
radiation incident has occurred to lessen the
harm from the radiation exposure.
The end?