BIOPHYSICS
Radiation & Radioactivity
E.Kvasnak, 3rd Medical Faculty, Charles University, Prague
…a little bit of history…
…1808
…1897
…1924
Solid Sphere Model or
Billiard Ball Model
proposed by John Dalton
…1909
Planetary Model or
Nuclear Model
proposed by E. Rutherford
Plum Pudding Model or
Raisin Bun Model
proposed by J.J. Thomson
…1913
Bohr Model or
Orbit Model
proposed by Neils Bohr
Electron Cloud Model or
Quantum Mechanical Model
proposed by Louis de Broglie
& Erwin Schrodinger
The Bohr’s atomic model consists of a central nucleus composed of neutrons and protons,
which is surrounded by electrons which “orbit” around the nucleus. By means of Quantum
Mechanical Model, proposed by Louis de Broglie and Erwin Schrodinger, the Electron
Cloud has been postulated.
Protons carry a positive charge, Neutrons are electrically “neutral”, Electrons carry a
negative charge. Atoms in nature are electrically neutral so the number of electrons orbiting
the nucleus equals the number of protons in the nucleus.
Without neutrons, the nucleus would split apart because the positive protons would repel
each other. Elements can have nucleii with different numbers of neutrons in them.
For example hydrogen, which normally only has one proton in the nucleus, can have a
neutron added to its nucleus to from deuterium, or have two neutrons added to create
tritium, which is radioactive. Atoms of the same element which vary in neutron
number are called isotopes.
Radiation
is energy in transit in the form of high speed particles and electromagnetic
waves.
Ionizing radiation
is radiation with enough energy so that during an interaction with an atom, it
can remove tightly bound electrons from their orbits, causing the atom to
become charged or ionized (examples: gamma rays, neutrons)
Non-ionizing radiation
is radiation without enough energy to to separate molecules or remove
electrons from atoms. Examples are visible light, radio and television waves,
ultra violet (UV), and microwaves with a large spectrum of energies.
Radioactivity
is the spontaneous transformation of an unstable atom and often results in the
emission of radiation. This process is referred to as a transformation, a decay or
a disintegrations of an atom. These emissions are collectively called ionizing
radiations. Depending on how the nucleus loses this excess energy either a
lower energy atom of the same form will result, or a completely different nucleus
and atom can be formed.
Ionization
is a particular characteristic of the radiation produced when radioactive
elements decay. These radiations are of such high energy that when they interact
with materials, they can remove electrons from the atoms in the material. This
effect is the reason why ionizing radiation is hazardous to health.
Radioactive Material
is any material that contains radioactive atoms.
Radioactive Contamination
is radioactive material distributed over some area, equipment or person.
Energy Scale
The energy scale used by most nuclear scientists is electron volts (eV), thousands of electron
volts (keV), and millions of electron volts (MeV). An electron volt is the energy acquired when
an electron falls through a potential difference of 1 volt. 1 eV=1.602*10 12ergs. Masses are also
given by their "mass-equivalent" energy (E=mc2). The mass of the proton is 938.27231 MeV.
E=mc2
Where: e is energy, m is mass, and c is the speed of light. Einstein's famous equation describes
how energy and mass are related. In our animated decays, mass is lost. That mass is converted
into energy in the form of electromagnetic waves. Because the speed of light is so great, a little
matter can transform into large amount of energy.
Common Types of Radiation
Alphas
An alpha is a particle emitted from the nucleus of an atom, that contains 2 protons and 2 neutrons.
It is identical to the nucleus of a Helium atom, without the electrons.
Betas
A beta is a high speed particle, identical to an electron, that is emitted from the nucleus of an atom
Gamma Rays
Gamma rays are electromagnetic waves / photons emitted from the nucleus (center) of an atom.
X rays
X Rays are electromagnetic waves / photons emitted not from the nucleus, but normally emitted
by energy changes in electrons. These energy changes are either in electron orbital shells that
surround an atom or in the process of slowing down such as in an X-ray machine.
Neutrons
Neutrons are neutral particles that are normally contained in the nucleus of all atoms and may be
removed by various interactions or processes like collision and fission
Alpha decay is a radioactive process in which a particle with two neutrons and two protons is
ejected from the nucleus of a radioactive atom. The particle is identical to the nucleus of a helium
atom.
Alpha decay only occurs in very heavy elements such as uranium, thorium and radium. The
nuclei of these atoms are very “neutron rich” (i.e. have a lot more neutrons in their nucleus than
they do protons) which makes emission of the alpha particle possible.
After an atom ejects an alpha particle, a new parent atom is formed which has two less neutrons and two less protons. Thus, when
uranium-238 (which has a Z of 92) decays by alpha emission, thorium-234 is created (which has a Z of 90).
Because alpha particles contain two protons, they have a positive charge of two. Further, alpha
particles are very heavy and very energetic compared to other common types of radiation. Typical
alpha particles will travel no more than a few centimeters in air and are stopped by a sheet of
paper.
Beta decay is a radioactive process in which an electron is emitted from the nucleus of a
radioactive atom, along with an unusual particle called an antineutrino (almost massless particle that
carries away some of the energy).
Like alpha decay, beta decay occurs in isotopes which are “neutron rich” .
When a nucleus ejects a beta particle, one of the neutrons in the nucleus is transformed into a
proton.
Since the number of protons in the nucleus has changed, a new daughter atom is formed which has one less neutron but one more
proton than the parent. For example, when rhenium-187 decays (which has a Z of 75) by beta decay, osmium-187 is created (which has
a Z of 76).
Beta particles have a single negative charge and weigh only a small fraction of a neutron or proton.
As a result, beta particles interact less readily with material than alpha particles. Beta particles will
travel up to several meters in air, and are stopped by thin layers of metal or plastic.
Gamma decay
After a decay reaction, the nucleus is often in an
“excited” state. This means that the decay has
resulted in producing a nucleus which still has
excess energy to get rid of. Rather than emitting
another beta or alpha particle, this energy is lost
by emitting a pulse of electromagnetic
radiation called a gamma ray. The gamma ray
is identical in nature to light or microwaves,
but of very high energy.
Like all forms of electromagnetic radiation, the
gamma ray has no mass and no charge. Gamma
rays interact with material by colliding with the
electrons in the shells of atoms. They lose their
energy slowly in material, being able to travel
significant distances before stopping. Depending
on their initial energy, gamma rays can travel
from 1 to hundreds of meters in air and can
easily go right through people.
It is important to note that most alpha and
beta emitters also emit gamma rays as part of
their decay process. However, there is no such
thing as a “pure” gamma emitter.
Over a century ago in 1895, Roentgen discovered the first example of ionizing radiation, x-rays.
Device: a glass envelope under high vacuum, with a wire element at one end forming the cathode, and a
heavy copper target at the other end forming the anode. When a high voltage was applied to the electrodes,
electrons formed at the cathode would be pulled towards the anode and strike the copper with very high
energy. Roentgen discovered that very penetrating radiations were produced from the anode, which he
called x-rays.
X-ray production whenever electrons of high energy strike a heavy metal target, like tungsten or
copper. When electrons hit this material, some of the electrons will approach the nucleus of the metal
atoms where they are deflected because of there opposite charges (electrons are negative and the nucleus
is positive, so the electrons are attracted to the nucleus). This deflection causes the energy of the electron to
decrease, and this decrease in energy then results in forming an x-ray.
Making X-rays
Where do x-rays come from?
An x-ray machine, like that used in a doctor's or a dentist's office, is really very simple. Inside the machine is an x-ray tube.
An electron gun inside the tube shoots high energy electrons at a target made of heavy atoms, such as tungsten. X-rays come
out because of atomic processes induced by the energetic electrons shot at the target.
X-rays are just like any other kind of electromagnetic radiation. They can be produced in parcels of energy called photons,
just like light. There are two different atomic processes that can produce x-ray photons. One is called Bremsstrahlung,
which is a fancy German name meaning "braking radiation." The other is called K-shell emission. They can both occur in
heavy atoms like tungsten.
So do both ways of making x-rays involve a change in the state of electrons?
That's right. But Bremsstrahlung is easier to understand using the classical idea that radiation is emitted when the velocity of
the electron shot at the tungsten changes. This electron slows down after swinging around the nucleus of a tungsten atom and
loses energy by radiating x-rays. In the quantum picture, a lot of photons of different wavelengths are produced, but none of
the photons has more energy than the electron had to begin with. After emitting the spectrum of x-ray radiation the original
electron is slowed down or stopped.
What is the "K-shell" in the other way of making x-rays?
Do you remember that atoms have their electrons arranged in closed "shells" of different energies? Well, the K-shell is the
lowest energy state of an atom.
What can the incoming electron from the electron gun do to a K-shell electron in a tungsten target atom?
It can give it enough energy to knock it out of its energy state. Then, a tungsten electron of higher energy (from an outer
shell) can fall into the K-shell. The energy lost by the falling electron shows up in an emitted x-ray photon. Meanwhile,
higher energy electrons fall into the vacated energy state in the outer shell, and so on. K-shell emission produces higherintensity x-rays than Bremsstrahlung, and the x-ray photon comes out at a single wavelength. Have a look at both
mechanisms in the experiment below.
Properties of Radiation
Alpha particles are heavy and doubly charged
which cause them to lose their energy very
quickly in matter. They can be shielded by a
sheet of paper or the surface layer of our skin.
Alpha particles are considered hazardous only to
a persons health if an alpha emitting material
is ingested or inhaled.
Beta and positron particles are much smaller and
only have one charge, which cause them to
interact more slowly with material. They are
effectively shielded by thin layers of metal or
plastic and are again considered hazardous only
if a beta emitter is ingested or inhaled.
Gamma emitters are associated with alpha, beta, and positron decay. X-Rays are produced either when
electrons change orbits within an atom, or electrons from an external source are deflected around the nucleus
of an atom. Both are forms of high energy electromagnetic radiation which interact lightly with matter. Xrays and gamma rays are best shielded by thick layers of lead or other dense material and are hazardous to
people when they are external to the body.
Neutrons are neutral particles with approximately the same mass as a proton. Because they are neutral they
react only weakly with material. They are an external hazard best shielded by thick layers of concrete.
Neutron-Induced Fission
Bombardment with a neutron resulting in splitting
the nucleus into two parts (fission fragments),
neutrons, and gamma rays.
Fusion
Cold Fusion
Neutron Capture
Coulomb Excitation
Particle Transfer
Pair Production
A collision process for gamma rays with
energies greater than 1022-keV (two electron
masses) where an electron /positron pair is
produced. A heavy nucleus must be present for
pair production.
Photoelectric effect
Collision process between an x-ray or gamma rays
and a bound atomic electron where the photon
disappears, the bound electron is ejected, and the
incident energy is shared between the ejected electron
and the remaining atom. The photon energy must be
greater than the atomic binding energy.
Positron Annihilation
Positron decay in matter by annihilation with
an electron. Usually and "atom" of
positronium (e+e-) forms which annihilates to
produce two 511-keV photons. Occasionally,
the positron will annihilate in flight to produce
on or more photons sharing the total rest mass
and kinetic energy of the positron and electron.
Half-life is the time required for the quantity of a radioactive material to be reduced to one-half its
original value.
All radionuclides have a particular half-life, some of which a very long, while other are extremely short.
For example, uranium-238 has such a long half life, 4.5x109 years, that only a small fraction has decayed
since the earth was formed. In contrast, carbon-11 has a half-life of only 20 minutes. Since this nuclide has
medical applications, it has to be created where it is being used so that enough will be present to conduct
medical studies.
When given a certain amount of radioactive material, it is customary to refer to the quantity based on its
activity rather than its mass. The activity is simply the number of disintegrations or transformations the
quantity of material undergoes in a given period of time.
The two most common units of activity are the Curie and the Becquerel.
The Curie is named after Pierre Curie for his and his wife Marie's discovery of radium. One Curie is equal
to 3.7x1010 disintegrations per second.
A newer unit of activity if the Becquerel named for Henry Becquerel who is credited with the discovery of
radioactivity. One Becquerel is equal to one disintegration per second.
It is obvious that the Curie is a very large amount of activity and the Becquerel is a very small amount. To
make discussion of common amounts of radioactivity more convenient, we often talk in terms of milli and
microCuries or kilo and MegaBecquerels.
Common Radiation Units – SI
Gray (Gy) - to measure absorbed dose ... the amount of energy actually absorbed in some
material, and is used for any type of radiation and any material (does not't describe the
biological effects of the different radiations)
Gy = J / kg (one joule of energy deposited in one kg of a material)
Sievert (Sv) - to derive equivalent dose ... the absorbed dose in human tissue to the effective
biological damage of the radiation
Sv = Gy x Q (Q = quality factor unique to the type of incident radiation)
Becquerel (Bq) - to measure a radioactivity … the quantity of a radioactive material that have 1
transformations /1s
Bq = one transformation per second, there are 3.7 x 1010 Bq in one curie.
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Roentgen (R) - to measure exposure but only to describe for gamma and X-rays, and only in air.
R = depositing in dry air enough energy to cause 2.58E-4 coulombs per kg
Rad (radiation absorbed dose) - to measure absorbed dose
Rem (roentgen equivalent man) - to derive equivalent dose related the absorbed dose in
human tissue to the effective biological damage of the radiation.
Curie (Ci) - to measure radioactivity. One curie is that quantity of a radioactive material that
will have 37,000,000,000 transformations in one second. 3.7 x 1010 Bq
Since we cannot see, smell or taste radiation, we are dependent on instruments to indicate the presence of ionizing radiation.
The most common type of instrument is a gas filled radiation detector. This instrument works on the principle that as radiation passes through air or a specific gas, ionization of
the molecules in the air occur. When a high voltage is placed between two areas of the gas filled space, the positive ions will be attracted to the negative side of the detector
(the cathode) and the free electrons will travel to the positive side (the anode). These charges are collected by the anode and cathode which then form a very small current in
the wires going to the detector. By placing a very sensitive current measuring device between the wires from the cathode and anode, the small current measured and displayed
as a signal. The more radiation which enters the chamber, the more current displayed by the instrument.
Many types of gas-filled detectors exist, but the two most common are the ion chamber used for measuring large amounts of radiation and the Geiger-Muller or GM detector
used to measure very small amounts of radiation.
The second most common type of radiation detecting instrument is the scintillation detector. The basic principle behind this instrument is the use of a special material which
glows or “scintillates” when radiation interacts with it. The most common type of material is a type of salt called sodium-iodide. The light produced from the scintillation
process is reflected through a clear window where it interacts with device called a photomultiplier tube.
The first part of the photomultiplier tube is made of another special material called a photocathode. The photocathode has the unique characteristic of producing electrons
when light strikes its surface. These electrons are then pulled towards a series of plates called dynodes through the application of a positive high voltage. When electrons from
the photocathode hit the first dynode, several electrons are produced for each initial electron hitting its surface. This “bunch” of electrons is then pulled towards the next
dynode, where more electron “multiplication” occurs. The sequence continues until the last dynode is reached, where the electron pulse is now millions of times larger then it
was at the beginning of the tube. At this point the electrons are collected by an anode at the end of the tube forming an electronic pulse. The pulse is then detected and
displayed by a special instrument.
Scintillation detectors are very sensitive radiation instruments and are used for special environmental surveys and as laboratory instruments.
Terms Related to Radiation Dose
Chronic dose … means a person received a radiation dose over a long period of time.
Acute dose … means a person received a radiation dose over a short period of time.
Somatic effects … are effects from some agent, like radiation that are seen in the individual who receives
the agent.
Genetic effects … are effects from some agent, that are seen in the offspring of the individual who
received the agent. The agent must be encountered pre-conception.
Teratogenic effects … are effects from some agent, that are seen in the offspring of the individual who
received the agent. The agent must be encountered during the gestation period.
Stochastic effects … are effects that occur on a random basis with its effect being independent of the size
of dose. The effect typically has no threshold and is based on probabilities, with the chances of seeing the
effect increasing with dose. Cancer is a stochastic effect.
Non-stochastic effect … are effects that can be related directly to the dose received. The effect is more
severe with a higher dose, i.e., the burn gets worse as dose increases. It typically has a threshold, below
which the effect will not occur. A skin burn from radiation is a non-stochastic effect.
PET
In clinical applications, a very small amount of labelled compound (called
radiopharmaceutical or radiotracer) is introduced into the patient usually by
intravenous injection and after an appropriate uptake period, the concentration of
tracer in tissue is measured by the scanner. During its decay process, the radionuclide
emits a positron which, after travelling a short distance (3-5 mm), encounters an
electron from the surrounding environment. The two particles combine and
"annihilate" each other resulting in the emission in opposite directions of two
gamma rays of 511 keV each.
The image acquisition is based on the external detection in coincidence of the emitted
gamma-rays, and a valid annihilation event requires a coincidence within 12
nanoseconds between two detectors on opposite sides of the scanner. For accepted
coincidences, lines of response connecting the coincidence detectors are drawn
through the object and used in the image reconstruction. Any scanner requires that
the radioisotope, in the field of view, does not redistribute during the scan. A tissue
attenuation correction is performed by recording a short transmission scan using gammarays from three radioactive (Germanium-68/Gallium-68) rotating rod sources.
NMR (MRI)
Nuclear Magnetic Resonance (NMR) Spectroscopy
In NMR, EM radiation is used to "flip" the alignment of nuclear spins from the low energy spin aligned state to the
higher energy spin opposed state. The energy required for this transition depends on the strength of the applied
magnetic field (see below) but in is small and corresponds to the radio frequency range of the EM spectrum.
Nuclei with an odd mass or odd atomic number have "nuclear spin" (in a similar fashion to the spin of
electrons). This includes 1H and 13C (but not 12C). The spins of nuclei are sufficiently different that NMR
experiments can be sensitive for only one particular isotope of one particular element. The NMR behaviour
of 1H and 13C nuclei has been exploited by organic chemist since they provide valuable information that can
be used to deduce the structure of organic compounds. These will be the focus of our attention.
Since a nucleus is a charged particle in motion, it will develop a magnetic field. 1H and 13C have nuclear
spins of 1/2 and so they behave in a similar fashion to a simple, tiny bar magnet. In the absence of a
magnetic field, these are randomly oriented but when a field is applied they line up parallel to the applied
field, either spin aligned or spin opposed. The more highly populated state is the lower energy spin state
spin aligned situation. Two schematic representations of these arrangements are shown below:
Computed Tomography Imaging (CT Scan, CAT Scan)
Computed Tomography is based on the x-ray principal: as x-rays pass through the body they are absorbed
or attenuated (weakened) at differing levels creating a matrix or profile of x-ray beams of different strength.
This x-ray profile is registered on film, thus creating an image. In the case of CT, the film is replaced by a
banana shaped detector which measures the x-ray profile.
Computed Tomography (CT) imaging, also known as "CAT scanning" (Computed Axial Tomography),
combines the use of a digital computer together with a rotating x-ray device to create detailed cross sectional
images or "slices" of the different organs and body parts such as the lungs, liver, kidneys, pancreas, pelvis,
extremities, brain, spine, and blood vessels. For many patients, CT can be performed on an outpatient basis
without requiring admittance to the hospital.
High resolution axial CT image of the inner ears and
sinuses. A large polyp in the right sinus (arrow) can be seen
Among the various imaging techniques such as MR and x-ray, CT has the unique ability to image a
combination of soft tissue, bone, and blood vessels. For example, conventional x-ray imaging of the
head can only show the dense bone structures of the skull. X-ray angiography of the head only
depicts the blood vessels of the head and neck and not the soft brain-tissue. Magnetic resonance
(MR) imaging does an excellent job of showing soft tissue and blood vessels, but MR does not give
as much detail of bony structures such as the skull. CT images of the head allow physicians to see
soft-tissue anatomic structures like the brain's ventricles or gray and white matter. Physician then can
selectively "window" the digital CT images on the computer monitor to look at the soft tissue, then
the bone and then the blood vessels, as needed.
Radiopharmaceuticals

the most widely used radioisotope is Tc, with
a half-life of six hours.

activity in the organ can then be studied
either as a two dimensional picture or, with a
special technique called tomography, as a
three dimensional picture (SPECT, PET)
NUCLEAR MEDICINE
- highly specialized on detection and diagnosis
of functional disturbances, the morphology is
mostly secondary
“In vivo methods”
1)
labeled molecules and compounds, which behave virtually
identically to the unlabelled ones in the various chemical,
biochemical and biological processes
2)
radioactive isotopes form compounds in the same way like as
the stable isotopes
3)
isotopes disclose their presence by their radiation, and thus
their movement and fate can be traced
For these purposes are used radionuclides that emit
electromagnetic waves (g rays) but don’t emit any particle (a,
b or neutron).
4)
Main advantages ot this method are :
Radioactive isotopes introduced into an
organism are distinguishable by their
radiation from the atoms already present.
This permits the relatively simple acquisition
of information about the dynamics of
processes of uptake, incorporation, exchange,
secretion, etc.
The tracer method is extremely sensitive. In
principle even the presence of only one atom
can be detected.
The high sensitivity allows the study of various