Radiation

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Radiation Radioactivity
Eugen Kvasnak, PhD. Department of Medical
Biophysics and Informatics 3rd Medical Faculty of
Charles University
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a little bit of history
1808
1897
1924
Solid Sphere Model orBilliard Ball
Modelproposed by John Dalton
Plum Pudding Model orRaisin Bun Modelproposed
by J.J. Thomson
1913
1909
Electron Cloud Model orQuantum Mechanical
Modelproposed by Louis de Broglie Erwin
Schrodinger
Planetary Model orNuclear Modelproposed by E.
Rutherford
Bohr Model orOrbit Modelproposed by Neils Bohr
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The Bohrs 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.
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• 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.

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• 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.

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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
eV1.6021012ergs. Masses are also given by their
"mass-equivalent" energy (Emc2). The mass of the
proton is 938.27231 MeV. Emc2 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.
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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
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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.
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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.
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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.
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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.
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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 higher-intensity 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.
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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. X-rays 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.
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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 CaptureCoulomb
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.
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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 (ee-) 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.
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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.
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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.
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• 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.

• __________________________________________________
  ________________________________
• 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

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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.
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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.
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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 gamma-rays from three radioactive
(Germanium-68/Gallium-68) rotating rod sources.
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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.
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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
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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.
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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.

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NUCLEAR MEDICINE

• highly specialized on detection and diagnosis of
  functional disturbances, the morphology is mostly
  secondary

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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 processes with amounts of
substances so small that they have no influence
on the life processes.
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In vivo methods

• labeled molecules and compounds, which behave
  virtually identically to the unlabelled ones in
  the various chemical, biochemical and biological
  processes
• radioactive isotopes form compounds in the same
  way like as the stable isotopes
• 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 dont
  emit any particle (a, b or neutron).

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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)

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• Thank you for your attention