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RADIATION PROTECTION IN NUCLEAR MEDICINE
PART 2. RADIATION PHYSICS
1.
ATOMIC STRUCTURE
The atom consists of a central nucleus around which electrons rotate in fixed orbits.
The nucleus contains two kinds of particles, protons and neutrons, which together
are called nucleons. Both particles have nearly the same mass but the proton carries
a positive electric charge. Hence, the whole nucleus is positively charged. This
charge is balanced by the negative charges of the orbital electrons, so from outside,
the atom appears electrically neutral.
The different natural elements ranging from hydrogen to uranium are
built of increasing numbers of nucleons. The hydrogen nucleus has one proton and
the uranium nucleus has 92 protons and 146 neutrons. It is the number of protons
and hence the number of electrons that define the element and its chemical
characteristics. The number of protons is called the atomic number (Z) and the total
number of nucleons is called the mass number of the nucleus. All elements have
different isotopes, which will have the same number of protons but different numbers
of neutrons and thus different mass numbers. For instance, for the element carbon
there exist eight different isotopes with mass numbers between 9 and 16. The
atomic number for carbon is 6 so the number of neutrons ranges from 3 to 10. It
should be stressed that an isotope of an element is not necessarily radioactive.
Among the isotopes of carbon both carbon-12 and carbon-13 are stable nuclides and
the others are unstable and hence are radioactive.
The electrons are bound to the nucleus by electrostatic forces. The binding
energy of an electron is defined as the work necessary to release the electron from
its orbit. The binding energy depends upon both the element and the position of the
orbit. The different orbits or shells are named K, L, M, N, etc. where the K-shell is the
shell closest to the nucleus. The electrostatic force is dependent on the distance
between the charges which means that the binding energy will decrease from the
inner shell outwards. The electrostatic force will also be dependent on the size of the
charge, which means that the binding energy of electrons in a specified shell is
higher in an element with a high atomic number than in an element with a low atomic
number. For instance, the binding energy for an electron in the K-shell is 1.56 keV in
aluminum (Z=13) and 88 keV in lead (Z=82).
In an energetic stable atom the shells are filled by electrons from the inner
shell and outwards. This structure can be changed by adding energy to the atom.
The result will be either ionization or excitation. Ionization means that the energy
added is high enough to release an orbital electron from the atom. Excitation means
that an electron will be lifted to a shell further out. This results in a vacancy in the
shell originally occupied by the electron, a vacancy which the atom tries to fill with an
electron from an outer shell. When the vacancy is filled the released energy is
emitted as electromagnetic radiation (characteristic X-rays) or is transferred to
another electron which can then leave the atom (Auger electron).
The
electromagnetic radiation is called characteristic X-ray because its energy is
characteristic for the element. This means that it is possible to identify an element by
its characteristic X-rays.
It is known that the nucleons, just like the electrons, can also occupy different
energy levels and that the nucleus can be present in a ground state or in an excited
state. Just as in the case of the atom, an excited state can be reached by adding
energy to the nucleus. At deexcitation the nucleus will emit the excess of energy as
electromagnetic radiation. In this case the electromagnetic radiation is called a
gamma ray. The energy can also be transferred to one of the electrons in the inner
shells of the atom, which then have high enough energy to leave the atom. This
process is called internal conversion (IC). The energy of the gamma ray will be the
difference in energies between the different energy levels in the nucleus. In a sense
we can say that the energy of the gamma ray is characteristic for the nucleus and
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that the nucleus can be identified by its gamma rays in the same way as the
characteristic X- ray identifies the atom.
Normally the excited nucleus will undergo deexcitation within picoseconds. In
some cases, however, a mean residence time for the excited level can be measured.
The deexcitation of such a level is then called isomeric transition (IT). This property
of a nucleus is noted in the label of a nuclide by adding the letter m in the following
way: technetium-99m, Tc-99m or 99mTc.
2.
RADIOACTIVE DECAY
Not all combinations of protons and neutrons in the nucleus are stable. For light
elements (mass number <20) stability is achieved if the number of protons and the
number of neutrons are about the same. Because of the positive charge of the
protons there is a repelling electrostatic force between them which is balanced by a
different attractive short ranging nuclear force between the nucleons. In a heavy
element the electrostatic force will be considerable due to the large number of
protons. Thus, to reach stability, the number of neutrons must be relatively larger
because the neutron increases the nuclear force without increasing the electrostatic
force. If the relation between protons and neutrons is altered from the stable
condition the equilibrium between the forces will be disturbed and the nucleus
becomes energetically unstable. Such an unstable nucleus is called a radioactive
nucleus or a radionuclide.
In the transformation, which we generally call radioactive decay, the nucleus
loses its excess of energy by fission or by emission of charged particles (alpha
particles and beta particles). The result of the radioactive decay is a new nucleus
with a different atomic number and in some cases a different mass number. The new
nucleus will in many cases be excited and at deexcitation the energy emitted as one
or several gamma rays.
Fission is the process in which the unstable nucleus divides into two fragments of
about equal size. This process is only possible in heavy nuclides.
Radioactive decay by emission of an alpha () particle can also occur only in
heavy elements because the alpha particle itself is a comparatively heavy particle
consisting of two protons and two neutrons (a helium nucleus). In alpha decay the
daughter nuclide will have an atomic number that is two units less and a mass
number that is four units less. Examples of radionuclides with alpha decay are
radium-226 and radon-222. The alpha particle energy from different radionuclides is
usually in the range of 4-8 MeV. The medical use of alpha-emitting radionuclides is
very limited.
Beta () decay can be one of three different kinds, -, + and electron capture
(EC). The - particle is an electron which is released in the transformation of a
neutron to a proton. In - decay the daughter nuclide will have an atomic number
one unit greater than the mother nuclide but the same mass number.
The + particle is an `electron' with a positive charge. It is called a positron
and is released in the transformation of a proton to a neutron. In + decay the
daughter nuclide will have an atomic number one unit less than the mother nuclide
but the same mass number.
Electron capture is an alternative to + decay. In the process one of the
electrons in the inner shell of the atom is captured by the nucleus. No particle is
emitted in the decay but due to the vacancy in the inner shell of the atom, a
characteristic X-ray will be emitted.
The transition energy released in beta-decay is divided between the beta
particle and a particle called a neutrino. This means that the kinetic energy of the
beta particles from a certain radionuclide will show a spectral distribution where the
energies range from zero to a maximum which equals the transition energy. The
mean energy of the particles is roughly 1/3 of the maximum energy.
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When considering a radioactive nucleus it can never be known when it is
going to decay. It can only be stated that there is a certain probability that it decays
within a certain period of time. For instance, a nucleus of iodine-131 has a probability
of decaying of 0.086 (8.6 percent) per day. This is called the decay constant of the
radionuclide which is different for different radionuclides. The number of decaying
nuclei per unit time in a radioactive sample is called the activity of the radionuclide.
The unit of activity is 1 Becquerel (Bq) which is the number of decaying nuclei
per second.
One Becquerel is a very small activity. The natural body content of
potassium-40 is about 4000 Bq (4 kBq). In some nuclear medicine examinations the
patients can receive 500-1000 million Becquerel (500 MBq - 1 GBq) of
technetium-99m.
The activity of a sample containing a certain radionuclide will continuously
decrease with a speed determined by the decay constant. Mathematically it is a
monoexponential decrease.
As an alternative to the decay constant the half-life of the radionuclide can be
defined as the time needed to reduce the activity by 50 percent.
3.
PRODUCTION OF RADIONUCLIDES
In order to produce an artificial radionuclide it is necessary to change the nucleus
structure in a stable target nucleus e.g. by adding a proton or a neutron. The source
of neutrons is generally fission in a nuclear reactor and the source of protons is a
cyclotron.
The cyclotron consists of a magnet and two dee-shaped electrodes, which
are placed perpendicular to the magnetic field. A proton emitted from a central ionsource will be accelerated toward the electrode with positive charge and at the same
time bended in the magnetic field. When entering the gap between the electrodes the
charge of the electrodes will be switched and the particle accelerated. This will be
repeated for each turn of the proton. The resulting path of the proton will be spiralshaped. Due to the increasing radius of the path and hence the increasing time for
one turn, the oscillator frequency for the electrodes can be constant. After reaching
the intended energy, the protons are extracted from the cyclotron using an extraction
electrode. For production of a radionuclide a suitable target material is placed in the
beam of protons and irradiated for a certain time.
4.
INTERACTION OF IONIZING RADIATION WITH MATTER
The different types of radiation emitted in the radioactive decay are examples of
ionizing radiation, which means that the kinetic energy of the single particle or photon
is high enough to cause ionization, which is the process of removing an electron from
the atom. The theoretical energy limit is of the order of 100 eV. If the energy
transferred is less it will not cause ionization. Other types of ionizing radiation are
charged particles from accelerators and cosmic radiation. Also X-rays are ionizing.
Charged particles such as electrons, protons, alpha particles, etc. are called directly
ionizing radiation while neutrons and photons are called indirectly ionizing radiation.
This means that the ionization will take place in two steps, the first step being the
release of a charged particle such as an electron which is then directly ionizing.
When ionizing radiation passes through matter, it loses energy and will finally be
absorbed completely. The energy lost by the radiation is absorbed by the material,
e.g., the body. The processes involved are different for directly and indirectly ionizing
radiation and also different for heavy and light charged particles as well as for
photons.
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Charged particles
When a charged particle, such as an alpha particle, proton or electron, penetrates
matter it will lose energy by interaction with orbital electrons. The mode of interaction
is called collision although no true collision takes place between particles but rather
collisions between the electric fields surrounding the particles involved. The energy
transferred to the orbital electron in the process can be high enough for the electron
to leave the atom. In fact, the kinetic energy of the ejected electron can be so high
that it will act as an ionizing particle. It is then called a delta-particle.
A heavy, charged particle such as the alpha particle, which has a mass about
7300 times the mass of the electron, will lose only a small fraction of its energy in
each collision. Due to its mass it will not change direction in the collision with a light
electron. Changes in direction can only happen in a collision with a heavy nucleus.
On the one hand, such a collision is very rare because the volume of the nucleus is
so much less than the volume of the atom, but on the other hand, due to its extensive
electric field the heavy charged particle will literally strike every atom it passes. In
conclusion, this means that the path of an alpha particle is straight and that the range
in a material is quite small and well defined.
A light, charged particle (electron or positron) can lose up to half its energy in
each collision with an orbital electron. The possible high energy transfer in each
collision means that more energetic delta-rays can be produced than in the case of
heavy charged particles. In the collision, the incident particle will also change its
direction which means that its path will be irregular and curved. Due to its small size,
an electron can also pass several atoms without losing any energy or only a small
fraction of its energy. Therefore, the electron has the ability to penetrate deeper into
matter than a heavy charged particle. Because of the irregular path the range in
matter will not be so well defined as that of a heavy particle.
If the light particle is a positron it will annihilate when it stops. It will
recombine with an electron and their masses will be transformed to energy and
emitted as two photons in opposite directions, each having an energy of 511 keV.
This annihilation radiation is the one used in positron emission tomography (PET),
which means that the only radionuclides used in PET-studies are those decaying by
+.
When an electron comes close to the nucleus it will change direction due to
the electrostatic forces acting upon it. In each such event of deflection the energy
lost by the particle can be emitted as electromagnetic radiation called
bremsstrahlung. The energy of the emitted photon can be between zero and the
whole kinetic energy of the incident electron depending on the distance between the
electron and the nucleus. The energy distribution of bremsstrahlung emitted in the
process will continuously decrease from zero energy to the incident electron energy.
The energy loss of a charged particle passing through matter is described by
the mass stopping power which is the energy loss per unit length divided by the
density of the absorber. The unit is thus MeV·cm2/g. Its value depends on the type
of particle and the particle energy . The stopping power due to collisions is called non
restricted linear energy transfer, LET. Heavy charged particles are usually called
high LET radiation and light charged particles are called low LET radiation.
Photons
For photons (X-rays, gamma rays) there are three main types of interaction with
matter: the photoelectric effect, the Compton process and pair production. In all
three processes directly ionizing electrons or positrons are released or created.
The photoelectric effect is that in which the incident photon transfers all its
energy to a tightly bound orbital electron in one of the inner shells of the atom. This
electron will leave the atom carrying kinetic energy which equals the photon energy
minus the binding energy of the electron. The vacancy in the electron shell is filled
and characteristic radiation is emitted as this occurs.
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In the Compton process, the incident photon collides with a loosely bound
electron in the outer shell of the atom. In the collision, the incident photon transfers
some of its energy to the electron which then leaves the atom. The photon changes
its direction of movement after the collision so the result of the interaction will be a
scattered photon with reduced energy and a recoil electron.
Pair production only occurs if the incident photon has a very high energy.
When the photon encounters the strong field around the nucleus it disappears and its
energy transforms into an electron-positron pair. Since the sum of their masses is
equivalent to an energy of 1022 keV, pair production is limited to photons whose
energies equal or exceed 1022 keV.
Theoretically, a photon can penetrate an absorber with no interactions at all.
We can only define a probability that a photon will interact by some of the three
described processes. This probability per unit length is called the linear attenuation
coefficient. It is commonly divided by the density of the absorber. This gives the
mass attenuation coefficient which has the unit cm2/g. The total mass attenuation
coefficient is the sum of the single coefficients for each of the three modes of
interactions. The dominating process depends on the energy of the photon and the
atomic number of the absorber. Note that for human soft tissues with a mean atomic
number of 7.8, the dominating interaction process is the Compton process for all
photon energies used in medical applications (25 keV - 25 MeV).
5.
RADIATION QUANTITIES AND UNITS
Absorbed dose
The fundamental dosimetric quantity D, defined as:
D=
d
dm
where d is the mean energy imparted by ionizing radiation to matter in a volume
element and dm is the mass of matter in the volume element. The energy can be
averaged over any defined volume, the average dose being equal to the total energy
imparted in the volume divided by the mass in the volume. The SI unit of absorbed
dose is the joule per kilogram (J.kg-1), termed the gray (Gy).
Collective dose
An expression for the total radiation dose incurred by a population, defined as the
product of number of individuals exposed to a source and their average radiation
dose. The collective dose is expressed in man-sieverts (manSv).
Effective dose
The quantity E, defined as a summation of the tissue equivalent doses, each
multiplied by the appropriate tissue weighting factor:
E = W T . HT
T
where HT is the equivalent dose in tissue T and W T is the tissue weighting factor for
tissue T. From the definition of equivalent dose, it follows that:
E=
WT  W R  DT,R = W R  W T  DT,R
T
R
R
T
where W R is the radiation weighting factor for radiation R and DT,R the average
absorbed dose in the organ or tissue T. The unit of effective dose is J.kg -1, termed
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the sievert (Sv).
Equivalent dose
The quantity H, defined as:
H = DT . W R
where DT is the absorbed dose delivered by radiation type R averaged over a tissue
or organ T and W R is the radiation weighting factor for radiation type R.
When the radiation field is composed of different radiation types with
different values of W R, the equivalent dose is:
H = W R . DT
R
The unit of equivalent dose is J.kg-1, termed the sievert (Sv).
6.
RADIATION DETECTORS

As a detector of ionizing radiation any substance may be used that produces a
measurable signal as a result of energy deposition in the material. The signal can be
electrical charge, light, chemically changed molecules, etc. Some materials will emit
the signal during the exposure to ionizing radiation, others can retain the changes
and be measured a long time after the exposure. According to their uses detectors
for ionizing radiation are divided into counters, dosimeters and spectrometers.
A counter is a device that will only count the number of particles and photons
interacting with the detector. It will not provide information about the type and energy
of the radiation. This type of detector is generally used as a survey meter to
determine if radiation is present or not and to check for contamination of
radionuclides.
A dosimeter is a device used to measure absorbed dose and absorbed dose
rate, so the signal from such a detector must be proportional to the absorbed energy
in the detector over a period of time. Dosimeters are important devices in medical
applications and in radiation protection. Their uses range from measuring the
radiation output from therapy machines to personnel monitoring.
In a spectrometer the signal is proportional to the energy of the photon or
particle interacting with the detector. This property is used in many applications in
nuclear medicine. The gamma camera has spectrometric properties used to reduce
the influence of scattered radiation on the image. The energy of the scattered photon
is lower than that of the primary photon and thus it can be sorted out by special
electronics. A spectrometer can also be used to identify radionuclides from the
energy of the gamma-rays.
Gas-filled detectors
In the ionization process an ion-pair will be produced consisting of a negative
electron and a positive atom (ion). If an electric field is applied between two
electrodes then the electrons will move towards the positive electrode and the
positive ions towards the negative electrode. A current will appear in the outer circuit
which is proportional to the number of ion pairs produced per second. Depending on
the strength of the electric field (high voltage) and the design of the detector the
properties of the gas detector will be different. Usually a distinction is made between
the ionization chamber, a proportional counter and a Geiger-Müller counter (GM-
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counter). The ionization chamber can be used as a dosimeter while the GM-counter
is used as a survey meter.
Gas filled detectors are used as activity meters in nuclear medicine. In this case the
gas used is under high pressure and has a high atomic number in order to increase
the probability of gamma-ray absorption. The number of detected gamma-rays per
second is proportional to the number interacting with the detector per second and
thus the number emitted per second, which is proportional to the activity.
Luminescence detectors
Upon deexcitation some organic molecules and inorganic crystals can emit visible
light. This phenomenon is called radioluminescence. This property is used in
scintillations detectors which are commonly used in medical applications. The
gamma camera detector is a scintillation detector made of a crystal of sodium iodide
doped with thallium (NaI(Tl)). Liquid scintillators based on certain organic scintillation
molecules are frequently used in radioimmunoassay and biological research. The
liquid scintillation detector has advantages in detecting low energy beta-emitters such
as tritium and carbon-14. The sodium iodide crystal is mostly used to detect gammaradiation.
The number of light photons emitted upon absorption of a gamma-ray or a
charged particle depends on the energy transferred to the detector. The light
photons will be converted into an electrical signal in a device called a photomultiplier.
The magnitude of this signal will depend on the number of light photons and thus the
energy transferred to the detector by the photon or the particle. The size of the
signal is electronically analyzed (pulse height analyzer). If the pulse height
distribution from a certain gamma-emitting radionuclide is observed, peaks which
represent completely absorbed photons will be seen as well as a continuous
distribution representing scattered photons.
In certain materials, there exist energy levels from which deexcitation is a socalled forbidden process. The material stays in an excited mode for a long time and
the electrons are trapped. They can be released from these traps and the atom can
deexcitate emitting the energy as light photons if energy is added to the material. If
the necessary energy for releasing the electrons from the traps can be added by
heating the material it is called a thermoluminescent material. Such materials are,
for instance, doped crystals of lithium-fluoride and calcium-sulphate. The amount of
light emitted during heating is proportional to the number of trapped electrons. This
number, in turn, is dependent on the energy absorbed in the material. The main use
of thermoluminescent detectors are as dosimeters. They can be produced in a
variety of shapes and can be used as dosimeters in personnel monitoring.
7.
REFERENCES
1. WORLD HEALTH ORGANIZATION and INTERNATIONAL ATOMIC ENERGY
AGENCY. Manual on Radiation Protection in Hospital and General Practice. Vol.
1. Basic requirements (in press)
2. INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION. 1990
Recommendations of the International Commission on Radiological Protection,
ICRP Publication No. 60. Oxford, Pergamon Press, 1991 (Annals of the ICRP 21,
1-3).
3. KNOLL GF. Radiation detection and measurements 3rd edition. John Wiley and
Sons, 1999
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4. SORENSEN JA, PHELPS ME. Physics in Nuclear Medicine. Grune & Stratton,
1987.
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