Ionising radiation, its
measurement and interactions
The Foundation Course on Physics
Professor Vojtěch Mornstein
Glencoe pp. about 752-775, 802 - 833
Revisiting Basic Knowledge – Atomic
Nucleus
Composition of the atomic nucleus:
Particles inside the atomic nucleus – neutrons and protons = nucleons.
Attracted by the „strong interaction“.
Nuclei are characterised mainly by:
Atomic (or proton) number Z – number of protons inside a nucleus.
Neutron number N – number of neutrons inside a nucleus.
Atomic mass (or nucleon) number A – number of nucleons inside a nucleus.
A = Z + N
Example:
The electric charge of a nucleus is:
Qnuc = Z·e,
where e is the elementary charge (electric charge of one electron or proton,
1.602×10-19 C).
U235
92
Revisiting Basic Knowledge – Atomic
Nucleus
An element is a substance the nuclei of which are made up of the
same number of protons (number Z is the same in all the nuclei).
A nuclide is a substance the nuclei of which are of identical
composition (numbers Z and N have the same values in all the
nuclei).
Isotopes are nuclides the nuclei of which have the same number
Z but they differ in number of neutrons.
Isotopes have very similar chemical properties because they
possess identical electron shells.
Isotopes are often denoted by the name of the chemical element
followed by the respective nucleon number A. For example:
carbon-12, carbon-14 or uranium-235, uranium-238 etc.
Revisiting Basic Knowledge – Atomic
Nucleus
The nucleons are bound by the “strong interaction”. The
binding energy Enuc is the energy just necessary to disintegrate
the nucleus into individual nucleons. During synthesis of nuclei
the same amount of energy is liberated.
The greater the binding energy the more stable is the
nucleus.
This energy can be calculated from the mass difference
(defect or excess):
Dm = Zmp + Nmn - mnuc and
Einstein’s formula of mass – energy equivalence:
Eb = Dmc2
Revisiting the Basic Knowledge –
Atomic Nucleus
Nuclear fission – chain reaction – nuclear reactor
Importance for production of energy and also some radionuclides
used in medicine
Natural and artificial radioactivity:
Radioactivity is the ability of a nucleus to emit particles or quanta of
energy. This process is also called radioactive decay or
disintegration.
Radioactive nuclides are called radionuclides (about 2000
radionuclides have been identified in nature or prepared artificially).
The radioactivity is accompanied by nuclear transformation. It
means that during the radioactive decay new (daughter) nuclei arise.
Stability of the Nucleus
Stable nuclides
are green
Revisiting the Basic Knowledge –
Atomic Nucleus
We distinguish several types of radioactive decay according to the particles produced:
a (alpha)-radiation: Helium nuclei. They are electrically charged (+2e charge) and cause direct
ionization of ambient atoms, and have very short track. It means that this radiation is little
penetrating.
b- (beta-minus)-radiation: Electrons. One neutron is transformed into a proton, an electron and a very
light particle “electron antineutrino”. Interaction of b- particles with matter is like origin of X-rays.
They have relatively low ionization ability hence they penetrate easier into matter in comparison with
a-particles.
b+ (beta-plus)-radiation: Positrons - antiparticles of the electrons. One proton is transformed into a
neutron, a positron and “positron neutrino”
g (gamma)-radiation: High energy photons. A nucleus with some excess energy emits a photon. The gemission
often accompanies other types of radioactive decay. Its ionization ability is relatively low
hence it penetrates deep into matter. Remember photoelectric effect and Compton scatter.
Neutron radiation: A neutron is emitted from a radionuclide or can be liberated after collision of two
accelerated nuclei. Neutrons ionize matter indirectly. Fast neutrons transmit their energy by impacts.
This process is highly effective in collisions with light nuclei. Moderated (slow, thermal) neutrons can
enter some heavy nuclei and cause their disintegration – fission.
Proton radiation: Protons are emitted from a radionuclide, or liberated after collision of two
accelerated nuclei.
8
Laws valid for radioactive decay
➢Law of mass-energy conservation
➢Law of electric charge conservation
➢Law of nucleon number conservation
➢Law of momentum conservation
Revisiting the Basic Knowledge –
Atomic Nucleus
9
Law of Radioactive Decay
The activity A of a radioactive sample at a given time (A = DN/Dt)
is proportional to the total number of undecayed nuclei present
in the sample. Then we can obtain by integration:
 is the decay or transformation constant, A is activity. The negative sign
indicates that the number of undecayed nuclei is decreasing.
Unit of A is becquerel (Bq) [1 disintegration per second, s-1]
(in the past: curie, 1 Ci = 3.7 x 1010 Bq)
Nt = N0.e-.t
A more useful equation for medicine is (obtained by dividing the
above equation by Dt on both sides):
At = A0.e-.t
10
Physical Half-Life
Tf – time in which the sample activity At decreases to
one half of the initial value A0. Derivation:
A0/2 = A0.e-.Tf thus ½ = e-. Tf
➢ taking logarithm of both sides of the equation and
rewriting:
Tf = ln2/f thus Tf = 0,693/f
Detection and Measurement of
Ionizing Radiation
• We will briefly mention some of the most
important dosimetric methods and detectors
of ionizing radiation. All of them are based on
the ability of radiation to ionize or to form free
radicals.
• Medicine, industry, environmental
applications …………
Geiger-Müller Counter
K - cylindrical cathode, A - anode
central wire, O - window, I insulator,
R - working resistor, C capacitor
of the capacity coupling,
Co - counter connectors.
The Geiger-Müller (GM) counter consists of a GM tube, a source of high
direct voltage, and an electronic counter of pulses. The GM tube is a
metallic hollow cylinder which acts as a cathode. The central wire is the
positively charged anode. The GM tube is usually filled by argon and 10 %
of the quenching agent. This agent stops (quenches) the ion multiplication
process, and so prevents the formation of an electric discharge between
the anode and cathode. The duration of ion avalanche is short, about 5 ms.
However, during this time the tube is not able to detect another particle of
ionising radiation. This dead time is an important characteristic of GM
tubes.
Scintillation Counter
The scintillation counter consists of a scintillator – a crystal which produces light after irradiation by
ionizing radiation. The scintillator is in a light-proof housing. One side of the housing is transparent,
hence the originating photons can come to a photomultiplier, which measures low-intensity light.
Fig. 38. The scintillation detector. I - ionising radiation particle (photon), S - scintillator, FK photocathode,
D - dynodes, A - anode, O - light-proof housing.
The photons hit the photocathode – a thin layer of a metal like caesium. Ejected electrons are
accelerated towards the closest positively charged electrode, the first dynode. There are some more
electrons ejected. These secondary electrons are attracted to the next dynode, where the process is
repeated. Resulting voltage pulses are counted in the electronic part. The magnitude of the pulse
depends on by the energy of the ionising particle!
Cloud and Bubble Chambers
• Wilson cloud chamber (or fog trail apparatus) is used for
visualization of the trajectory of ionizing particles. These
chambers are filled with an oversaturated vapour (water or
alcohol). The ions formed along the particle trajectory
serve as centres of condensation. So the particle trail can
be seen as a series of small droplets – the fog trail. It is
possible to follow collisions of particles with neutral atoms,
their disintegration etc. The magnetic or electric field in the
chamber enables to determine the charge and velocity of
the particle.
• In the bubble chamber, a similar principle is exploited. The
particles of ionizing radiation move in a vessel filled by an
overheated liquid. The ions formed serve as centres of
evaporation. It is possible to see the trajectory of the
particle as a series of small bubbles.
Cloud and Bubble
Chambers
Hydrogen bubble chamber, diameter 1 meter,
National Laboratory for High Energy Physics
(KEK), built 1971-1975 - National Museum of
Nature and Science, Tokyo - DSC07799.JPG
Video:
https://www.youtube.com/watch?v=NeydrHKvpYM
Personal Dosimeters
• Photographic films are sensitive to the action of
ionizing radiation which causes a local blackening of
the film. These (thick-layered) films were also used in
scientific research to follow trajectories of ionizing
particles. Blackening of the film can be used also for
determination of the dose of radiation absorbed by
people working with sources of ionizing radiation.
• Some contemporary personal dosimeters are based on
thermoluminescence principle. The irradiated material
is heated and light is produced which amount depends
on the dose of radiation absorbed in the material.
Personal Dosimeters - Badges
18
Interaction of Ionising Radiation with
Matter
➢ The interaction of radiation with matter is usually
accompanied by the formation of secondary radiation
which differs from the primary radiation in lower energy
and often also in kind of particles.
➢ Primary or secondary radiation directly or indirectly
ionises the medium, and creates also free radicals.
➢ A big portion of the radiation energy is always transformed
into heat.
➢ The energy loss of the particles of primary radiation is
characterised by means of LET, linear energy transfer, i.e.
energy loss of the particle in given medium per unit length
of its trajectory. The higher the LET the more damaging is
the radiation to humans.
19
Attenuation of X / Gamma Radiation
When a beam of X or gamma radiation passes through a substance:
absorption + scattering = attenuation
I = I0.e-m.x
I is intensity of radiation passed through the layer of thickness x, I0 is
the intensity of incident radiation, m is linear coefficient of
attenuation [m-1] (depending on photon energy, atomic number of
medium and its density).
Electron - Positron Pair Production
➢ WE KNOW ALREADY: Photoelectric effect and Compton scatter!!!
➢ For very high energy photons only: The energy of the photon is transformed into mass
and kinetic energy of an electron and positron. The mass-energy E in each particle:
E = m0 c2 (= 0,51 MeV),
m0 is rest mass of an electron or positron, c is speed of light in vacuum. Energy of the
photon must be higher than twice the energy calculated above (1.02 MeV). Thus:
E = h.f = (m0.c2 + Ek1) + (m0.c2 + Ek2)
➢ Terms in brackets: mass-energies of created particles, Ek1 a Ek2 kinetic energies of
these particles.
➢ The positron quickly interacts (annihilates) with any nearby electron, and two photons
originate, each with energy of 0.51 MeV.
21
Electron - Positron Pair Production
22
Interaction of Corpuscular Radiation with
Matter
➢b radiation = fast electrons or positrons – ionise the medium as in
X-ray production. Trajectory of a b particle is some mm or cm in
water.
➢a radiation ionises directly by impacts. There are formed many
ions along its very short trajectory in medium (mm) – it loses energy
very quickly (= very high LET) .
➢Neutrons ionise by elastic and non-elastic impacts (scatter) with
atom nuclei. When a fast neutron hits the nucleus of a heavy
element, it bounces off almost without energy loss. Collisions with
light nuclei lead to big energy losses. In non-elastic scatter, the slow
(moderated, thermal) neutrons penetrate into the nucleus. Among
others, they can lead to the emission of other particles or fission of
heavy nuclei.
23
Main Quantities and Units for
Measurement of Ionising Radiation
A single particle energy is very small → the electron volt (eV) was introduced.
1 eV is the kinetic energy of an electron accelerated from rest by electrostatic
field of the potential difference of 1 volt.
1 eV = 1.602×10-19 J.
Energy absorbed by the medium is described by absorbed dose (D) - unit gray,
Gy). It is the amount of energy absorbed per 1 kg of matter. Gray = J.kg-1
Dose rate is the absorbed dose in unit time [J.kg-1.s-1]. The same absorbed dose
can be reached at different dose rates during in different times.
The biological radiation hazard depends mainly on the absorbed dose and kind
of radiation. The radiation weighting factor is a number indicating how
hazardous a type of radiation is (the higher the LET the higher the radiation
weighting factor).
Equivalent dose De is defined as the product of the absorbed dose and the
radiation weighting factor. The unit of Equivalent dose is the sievert (Sv).
Biological Effects of Ionising Radiation
• All kinds of nuclear radiation,
together with X-rays, are
called ionizing radiation
because they ionize atoms
along their tracks
(trajectories) through matter.
Even more dangerous
products of ionising radiation
are the free radicals which
can cause chemical changes
of various substances
including DNA.
25
Biological Effects of Ionising Radiation
➢ Direct action (hits) – physical and physical-chemical process
of radiation energy absorption, leading directly to changes in
important cellular structures. It is the most important action
mechanism in cells with low water content. Theory of direct
action is called target theory. It is based on physical energy
transfer.
➢ Indirect effects are mediated by water radiolysis products,
namely by free radicals H* a OH*. It is most important in cells
with high water content. The free radicals have free unpaired
electrons which cause their high chemical reactivity. They
attack chemical bonds in biomolecules and degrade their
structure. Theory of indirect action – radical theory – is based
on chemical energy transfer.
http://www.frontiersin.org/files/Articles/111103/fonc-04-00381-HTML-r1/image_m/fonc-04-00381-g001.jpg
Indirect
Mechanism of
DNA Damage
27
Effects on the Cell
In proliferating cells we find these levels of radiation damage:
➢ Transient stopping of proliferation
➢ Reproductive death of cells (vital functions are kept but proliferation ability is lost)
➢ Instantaneous death of cells
Cell sensitivity to ionising radiation (radiosensitivity), or its resistance (radioresistance)
depends mainly on the repair ability.
Main factors influencing biological effects:
➢ Physical and chemical: equivalent dose, dose rate, temperature, spatial distribution of
absorbed dose, presence of water and oxygen.
➢ Biological: species, organ or tissue, degree of cell differentiation, physiological state,
spontaneous ability of repair, repopulation and regeneration.
Sensitivity of cells is influenced by:
➢ Cell cycle phase (S-phase!)
➢ Differentiation degree. Differentiated cells are less sensitive.
➢ Water and oxygen content. Direct proportionality (+,+)
Very sensitive are e.g. embryonic, generative, epidermal, bone marrow and tumour cells.
28
Tissue Sensitivity and Radiation Sickness
lymphatic
spermatogenic epithelium of testis
bone marrow
gastrointestinal epithelium
ovaries
cells of skin cancer
connective tissue
liver
pancreas
kidneys
nerve tissue
brain
muscle
Arranged according to the
decreasing radiosensitivity:
Typical symptoms of radiation
sickness:
1. Non-lethal – damage to the
erythropoiesis (bone marrow),
effects on gonads
2. Lethal – gastrointestinal
syndrome (damaged epithelium),
skin burning, damage to suprarenal
glands, damaged vision, nerve
syndrome (nerve death)
Late sequels – cumulative – genetic
damage, cancer
It is not radiation sickness but some other effect of
irradiation or nuclear bomb explosions!!!
Accelerators
• Accelerators are electronic devices by means of
which some charged particles (electrons, protons,
ions) are accelerated to reach energies
comparable or higher than the energies of
particles emitted during natural radioactivity
processes.
• Linear accelerators are used for electrons.
• Cyclotrons can accelerate protons or ions.
• Synchrotrons accelerate mainly electrons to very
high energies.
31
The Linear Accelerator
CLINAC 2100C in Masaryk Memorial Institute of Oncology in Brno
32
The Linear Accelerator - Scheme
http://www.cs.nsw.gov.au/rpa/pet/RadTraining/MedicalLinacs.htm
33
The Cyclotron
Z – source of the accelerated
particles (protons),
D1 and D2 – duants or dees,
G - generator of highfrequency
voltage.
m
qB
f
.2
.

=
Cyclotron frequency
– condition for
circular trajectory of
accelerated
particles.
34
The Cyclotron in
Oncology – Proton
(Hadron) Therapy
The Sumitomo cyclotron
Radiotherapy
Synchrotron Research Facility
If we change the direction of a beam of very
fast electrons, intense electromagnetic
radiation (VIS, UV, X-RAY) can be produced.
Many experimental labs are connected to a
single synchrotron.
CERN Collider
the Strongest Accelerator with Collision
Energy of Two Particles of about 10 TeV
A very small part of the 27 km long
underground device
By alpinethread - Flickr, CC BY-SA 2.0,
https://commons.wikimedia.org/w/in
dex.php?curid=4538468
CERN Hadron Collider
(one of the collision product detectors)
Nuclear Reactor and Power Plant
NuclearPowerPlantsinEurope
Nuclear Bomb
See you next year in Medical Biophysics!!!!
Version 02 – March 2020
It is not
English but
I hope you
understand
it!!