Atomic and nuclear physics are related but distinct fields that describe the structure and behavior of atoms and their nuclei. Atomic physics deals with atoms as systems of electrons and an atomic nucleus, while nuclear physics focuses on the nucleus as a system of nucleons (protons and neutrons). A knowledge of these fields is important for nuclear engineers working with nuclear reactors. The document then provides details on the key topics in atomic and nuclear physics, including fundamental particles, atomic and nuclear structure, mass and energy, radiation, nuclear stability, radioactive decay, and nuclear reactions.

1. Atomic and Nuclear Physics
A knowledge of atomic and nuclear physics is essential to nuclear engineers, who deal
with nuclear reactors. It should be noted that atomic and nuclear physics is very extensive
branch of science. Nuclear reactor physics belongs to an applied physics. Reactor physics,
particle physics or other branches of modern physics have common fundamentals. Atomic
and nuclear physics describes fundamental particles (i.e. electrons, protons, neutrons), their
structure, properties and behavior.
Atomic and nuclear physics are not the same. The term atomic physics is often associated
with nuclear power, due to the synonymous use of atomic and nuclear in Standard English.
However, physicists distinguish between atomic and nuclear physics. The atomic
physics deals with the atom as a system consisting of a nucleus and electrons. The
nuclear physics deals with the nucleus as a system consisting of a nucleons (protons and
neutrons). Main difference is in the scale. While the term atomic deals with 1Å = 10-10m,
where Å is an ångström (according to Anders Jonas Ångström), the term nuclear deals with
1femtometre = 1fermi = 10-15m.
Atomic Physics
Atomic physics is the field of physics that studies atoms as an isolated system of electrons
and an atomic nucleus. It is primarily concerned with the arrangement of electrons around
the nucleus and the processes by which these arrangements change. This includes ions as
well as neutral atoms and, unless otherwise stated, for the purposes of this discussion it
should be assumed that the term atom includes ions. Atomic physics also help to
understand the physics of molecules, but there is also molecular physics, which describes
physical properties of molecules.
Nuclear Physics
Nuclear physics is the field of physics that studies the constituents (protons and neutrons)
and interactions of atomic nuclei. The most commonly known applications of nuclear
physics are nuclear power generation, but the modern nuclear physics contains also particle

2. physics, which is taught in close association with nuclear physics. The nuclear physics has
provided application in many fields, including those in nuclear medicine (Positron Emission
Tomography, isotopes production, etc.) and magnetic resonance imaging, ion implantation
in materials engineering, and radiocarbon dating in geology and archaeology.
These physical fundamentals consist of following topics:
 Fundamental Particles
 Atomic and Nuclear Structure
 Mass and Energy
 Radiation
 Nuclear Stability
 Radioactive Decay
 Nuclear Reactions
 Binding Energy
1. Fundamental Particles
The physical world is composed of combinations of various subatomic or fundamental
particles. These are the smallest building blocks of matter. All matter except dark matter
is made of molecules, which are themselves made of atoms. The atoms consist of two
parts. An atomic nucleus and an electron cloud. The electrons are spinning around the
atomic nucleus. The nucleus itself is generally made of protons and neutrons but even
these are composite objects. Inside the protons and neutrons, we find the quarks.

3. Quarks and electrons are some of the elementary particles. A number of fundamental
particles have been discovered in various experiments. So many, that researchers had to
organize them, just like Mendeleev did with his periodic table. This is summarized in a
theoretical model (concerning the electromagnetic, weak, and strong nuclear interactions)
called the Standard Model. In particle physics, an elementary particle or fundamental
particle is a particle whose substructure is unknown, thus it is unknown whether it is
composed of other particles. Known elementary particles include the
fundamental fermions and the fundamental bosons. The fermions are generally “matter
particles” and “antimatter particles”.
 Quarks. The quarks combine to form composite particles called hadrons, the best
known and most stable are protons and neutrons
 Antiquarks. For every quark there is a corresponding type of antiparticle. The
antiquarks have the same mass, mean lifetime, and spin as their respective quarks,
but the electric charge and other charges have the opposite sign.
 Leptons. The best known of all leptons are the electrons and the neutrinos.
 Antileptons. For every lepton there is a corresponding type of antiparticle. The best
known of all antileptons are the positrons and the antineutrinos.
The bosons are generally “force particles” that mediate interactions among fermions.
 Gauge bosons. The gauge boson is a force carrier of the fundamental interactions of
nature.
 Higgs boson. The Higgs bosons give other particles mass via the Higgs mechanism.
Their existence was confirmed by CERN on 14 March 2013.
However, only a few of these fundamental particles (in fact, some of these are not
fundamental particles) are very important in nuclear engineering. Nuclear engineering or
theory of nuclear reactors operates with much better known subatomic particles such as:

4.  Electrons. The electrons are negatively charged, almost massless particles that
nevertheless account for most of the size of the atom. Electrons were discovered by
Sir John Joseph Thomson in 1897. Electrons are located in an electron cloud, which is
the area surrounding the nucleus of the atom. The electron is only one member of a
class of elementary particles, which forms an atom.
 Protons. The protons are positively charged, massive particles that are located inside
the atomic nucleus. Protons were discovered by Ernest Rutherford in the year 1919,
when he performed his gold foil experiment.
 Neutron. Neutrons are located in the nucleus with the protons. Along with protons,
they make up almost all of the mass of the atom. Neutrons were discovered by
James Chadwick in 1932, when he demonstrated that penetrating radiation
incorporated beams of neutral particles.
 Photon. A photon is an elementary particle, the force carrier for the electromagnetic
force. The photon is the quantum of light (discrete bundle of electromagnetic
energy). Photons are always in motion and, in a vacuum, have a constant speed of
light to all observers (c = 2.998 x 108 m/s).
 Neutrino. A neutrino is an elementary particle, one of particles which make up the
universe. Neutrinos are electrically neutral, weakly interacting and therefore able to
pass through great distances in matter without being affected by it.
 Positron. Positron is an antiparticle of a negative electron. Positrons, also called
positive electron, have a positive electric charge and have the same mass and
magnitude of charge as the electron. An annihilation occurs, when a low-energy
positron collides with a low-energy electron.
2. Atomic and Nuclear Structure
Notation of nuclei

5.  The atom consist of a small but massive nucleus surrounded by a cloud of rapidly
moving electrons. The nucleus is composed of protons and neutrons. Total number
of protons in the nucleus is called the atomic number of the atom and is given
the symbol Z. The total electrical charge of the nucleus is therefore +Ze, where e
(elementary charge) equals to 1,602 x 10-19 coulombs. In a neutral atom there are as
many electrons as protons moving about nucleus. It is the electrons that are
responsible for the chemical bavavior of atoms, and which identify the various
chemical elements.
 Hydrogen (H), for example , consist of one electron and one proton. The number of
neutrons in a nucleus is known as the neutron number and is given the symbol N.
The total number of nucleons, that is, protons and neutrons in a nucleus, is equal
to Z + N = A, where A is called the atomic mass number. The various species of
atoms whose nuclei contain particular numbers of protons and neutrons are
called nuclides. Each nuclide is denoted by chemical symbol of the element (this
specifies Z) with tha atomic mass number as supescript.
 Thus the symbol 1H refers to the nuclide of hydrogen with a single proton as
nucleus. 2H is the hydrogen nuclide with a neutron as well as a proton in the nucleus
(2H is also called deuterium or heavy hydrogen). Atoms such as 1H, 2H whose nuclei
contain the same number of protons but different number of neutrons (different A)
are known as isotopes. Uranium, for instance, has three isotopes occuring in nature –
238U, 235U and 234U. The stable isotopes (plus a few of the unstable isotopes) are the
atoms that are found in the naturally occuring elements in nature. However, they are
not found in equal amounts. Some isotopes of a given element are more abundant
than others. For example 99,27% of naturally occuring uranium atoms are the
isotope 238U, 0,72% are the isotope 235U and 0,0055% are the isotope 234U. Exact
structure of atoms is described by Atomic Theory and Theory of Nuclear
Structure.
 Atomic Theory. Atomic theory is a scientific theory of the nature of matter, which
states that matter is composed of discrete units called atoms. The word atom comes
from the Ancient Greek adjective atomos, meaning “uncuttable”. Today it is known
that also atoms are divisible. Atomic Theory consist of many models and discoveries,
which gradually formed this theory.
 Theory of Nuclear Structure. Understanding the structure of the atomic nucleus is
one of the central challenges in modern nuclear physics.

6. 3. Mass and Energy
Nuclear energy comes either from spontaneous nuclei conversions or induced nuclei
conversions. Among these conversions (nuclear reactions) belong for example nuclear
fission, nuclear decay and nuclear fusion. Conversions are associated with mass and energy
changes. One of the striking results of Einstein’s theory of relativity is that mass and energy
are equivalent and convertible, one into the other. Equivalence of the mass and energy is
described by Einstein’s famous formula:

7. where M is the small amount of mass and C is the speed of light.
What that means? If the nuclear energy is generated (splitting atoms, nuclear fussion), a
small amount of mass transforms into the pure energy (such as kinetic energy, thermal
energy, or radiant energy).
4. Radiation
Most general definition is that radiation is energy that comes from a source and travels
through some material or through space. Light, heat and sound are types of radiation. This
is very general definition, the kind of radiation discussed in this article is called ionizing
radiation. Most people connect the term radiation only with ionizing radiation, but it is not
correct. Radiation is all around us. In, around, and above the world we live in. It is a natural
energy force that surrounds us. It is a part of our natural world that has been here since the
birth of our planet. We should distinguish between:
 Non-ionizing radiation. The kinetic energy of particles (photons, electrons, etc.) of
non-ionizing radiation is too small to produce charged ions when passing through
matter. The particles (photons) have only sufficient energy to change the rotational,
vibrational or electronic valence configurations of target molecules and atoms.
Sunlight, radio waves, and cell phone signals are examples of non-ionizing (photon)
radiation. However, it can still cause harm, like when you get a sunburn.
 Ionizing radiation. The kinetic energy of particles (photons, electrons, etc.) of
ionizing radiation is sufficient and the particle can ionize (to form ion by losing
electrons) target atoms to form ions. Simply ionizing radiation can knock electrons
from an atom.
The boundary is not sharply defined, since different molecules and atoms ionize at different
energies. This is typical for electromagnetic waves. Among electromagnetic waves belong, in
order of increasing frequency (energy) and decreasing wavelength: radio waves,
microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma rays.
Gamma rays, X-rays, and the higher ultraviolet part of the spectrum are ionizing, whereas
the lower ultraviolet, visible light (including laser light), infrared, microwaves, and radio
waves are considered non-ionizing radiation.

8. Forms of ionizing radiation
Interaction of Radiation with Matter
Ionizing radiation is categorized by the nature of the particles or electromagnetic waves
that create the ionizing effect. These particles/waveshave different ionization mechanisms,
and may be grouped as:
 Directly ionizing. Charged particles (atomic nuclei, electrons, positrons, protons,
muons, etc.) can ionize atoms directly by fundamental interaction through the
Coulomb force if it carries sufficient kinetic energy. These particles must be moving
at relativistic speeds to reach the required kinetic energy. Even photons (gamma rays
and X-rays) can ionize atoms directly (despite they are electrically neutral) through

9. the Photoelectric effect and the Compton effect, but secondary (indirect) ionization is
much more significant.
 Alpha radiations. Alpha radiation consist of alpha particles at high
energy/speed. The production of alpha particles is termed alpha decay. Alpha
particles consist of two protons and two neutrons bound together into a particle
identical to a helium nucleus. Alpha particles are relatively large and carry a
double positive charge. They are not very penetrating and a piece of paper can
stop them. They travel only a few centimeters but deposit all their energies along
their short paths.
 Beta radiation. Beta radiation consist of free electrons or positrons at relativistic
speeds. Beta particles (electrons) are much smaller than alpha particles. They
carry a single negative charge. They are more penetrating than alpha particles,
but thin aluminum metal can stop them. They can travel several meters but
deposit less energy at any one point along their paths than alpha particles.
 Indirectly ionizing. Indirect ionizing radiation is electrically neutral particles and
therefore does not interact strongly with matter. The bulk of the ionization effects
are due to secondary ionizations.
 Photon radiation (Gamma or X-rays). Photon radiation consist of high energy
photons. These photons are particles/waves (Wave-Particle Duality) without rest
mass or electrical charge. They can travel 10 meters or more in air. This is a long
distance compared to alpha or beta particles. However, gamma rays deposit less
energy along their paths. Lead, water, and concrete stop gamma radiation.
Photons (gamma rays and X-rays) can ionize atoms directly through the
Photoelectric effect and the Compton effect, where the relatively energetic
electron is produced. The secondary electron will go on to produce multiple
ionization events, therefore the secondary (indirect) ionization is much more
significant.
 Neutron radiation. Neutron radiation consist of free neutrons at any
energies/speeds. Neutrons can be emitted by nuclear fission or by the decay of
some radioactive atoms. Neutrons have zero electrical charge and cannot directly
cause ionization. Neutrons ionize matter only indirectly. For example, when
neutrons strike the hydrogen nuclei, proton radiation (fast protons) results.
Neutrons can range from high speed, high energy particles to low speed, low
energy particles (called thermal neutrons). Neutrons can travel hundreds of feet
in air without any interaction.
5. Nuclear Stability
is a concept that helps to identify the stability of an isotope. To identify the stability of an
isotope it is needed to find the ratio of neutrons to protons. To determine the stability of an
isotope you can use the ratio neutron/proton (N/Z). Also to help understand this concept
there is a chart of the nuclides, known as a Segre chart. This chart shows a plot of the known

10. nuclides as a function of their atomic and neutron numbers. It can be observed from the
chart that there are more neutrons than protons in nuclides with Z greater than about 20
(Calcium). These extra neutrons are necessary for stability of the heavier nuclei. The excess
neutrons act somewhat like nuclear glue.
Connection between Nuclear Stability and Radioactive
Decay
The nuclei of radio isotopes are unstable. In an attempt to reach a more stable arrangement
of its neutrons and protons, the unstable nucleus will spontaneously decay to form a
different nucleus. If the number of neutrons changes in the process (number of protons
remains), a different isotopes is formed and an element remains (e.g. neutron emission). If
the number of protons changes (different atomic number) in the process, then an atom of a
different element is formed. This decomposition of the nucleus is referred to as radioactive
decay. During radioactive decay an unstable nucleus spontaneosly and randomly
decomposes to form a different nucleus (or a different energy state – gamma decay), giving
off radiation in the form of atomic partices or high energy rays. This decay occurs at a
constant, predictable rate that is referred to as half-life. A stable nucleus will not undergo
this kind of decay and is thus non-radioactive.
Radioactive Decay
Notation of nuclear reactions – radioactive decays

11. Nuclear decay (Radioactive decay) occurs when an unstable atom loses energy by
emitting ionizing radiation. Radioactive decay is a random process at the level of single
atoms, in that, according to quantum theory, it is impossible to predict when a particular
atom will decay. There are many types of radioactive decay:
 Alpha radioactivity. Alpha particles consist of two protons and two neutrons bound
together into a particle identical to a helium nucleus. Because of its very large mass
(more than 7000 times the mass of the beta particle) and its charge, it heavy
ionizesmaterial and has a very short range.
 Beta radioactivity. Consist of beta particles. Beta particles are high-energy, high-
speed electrons or positrons emitted by certain types of radioactive nuclei such as
potassium-40. The beta particles have greater range of penetration than alpha
particles, but still much less than gamma rays.The beta particles emitted are a form
of ionizing radiation also known as beta rays. The production of beta particles is
termed beta decay.
 Gamma radioactivity. Gamma radioactivity consist of gamma rays. Gamma rays are
electromagnetic radiation (high energy photons) of an very high frequency and of a
high energy. They are produced by the decay of nuclei as they transition from a high
energy state to a lower state known as gamma decay. Most of nuclear reactions are
accompanied by gamma emission.
 Neutron emission. Neutron emission is a type of radioactive decay of nuclei
containing excess neutrons (especially fission products), in which a neutron is simply
ejected from the nucleus. This type of radiation plays key role in nuclear reactor
control, because these neutrons are delayed neutrons.
6. Nuclear Reactions
A nuclear reaction is considered to be the process in which two nuclear particles (two
nuclei or a nucleus and a nucleon) interact to produce two or more nuclear particles or ˠ-
rays (gamma rays). Thus, a nuclear reaction must cause a transformation of at least one
nuclide to another. Sometimes if a nucleus interacts with another nucleus or particle without
changing the nature of any nuclide, the process is referred to a nuclear scattering, rather
than a nuclear reaction. Perhaps the most notable nuclear reactions are the nuclear fusion
reactions of light elements that power the energy production of stars and the Sun. Natural
nuclear reactions occur also in the interaction between cosmic rays and matter.
Nuclear reactors are devices to initiate and control a chain nuclear reaction, but there are
not only manmade devices. The world’s first nuclear reactor operated about two billion
years ago. The natural nuclear reactorformed at Oklo in Gabon, Africa, when a uranium-
rich mineral deposit became flooded with groundwater that acted as a neutron moderator,

12. and a nuclear chain reaction started. These fission reactions were sustained for hundreds of
thousands of years, until a chain reaction could no longer be supported. This was confirmed
by existence of isotopes of the fission-product gas xenon and by different ratio of U-
235/U238 (enrichment of natural uranium).
7. Binding Energy
Nuclear binding energy curve
A binding energy is generally the energy required to disassemble a whole system into
separate parts. It is known the sum of separate parts has typically a higher potential energy
than a bound system, therefore the bound system is more stable. A creation of bound
system is often accompanied by subsequent energy release. We usually distinguish the
binding energy according to these levels:
At atomic level the atomic binding energy of the atom derives from electromagnetic
interaction of electrons in the atomic cloud and nucleons (protons) in the nucleus. The
atomic binding energy is the energy required to disassemble an atom into free electrons
and a nucleus. This is more commonly known as ionization energy.
At molecular level the molecular binding energy of the molecule derives from bond-
dissociation energy of atoms in a chemical bond.
At nuclear level the nuclear binding energy is the energy required to disassemble (to
overcome the strong nuclear force) a nucleus of an atom into its component parts (protons
and neutrons).
Nuclear binding energy
The component parts of nuclei are neutrons and protons, which are collectively called
nucleons. The mass of a nucleus is always less than the sum masses of the constituent
protons and neutrons when separated. The difference is a measure of the nuclear binding

13. energy which holds the nucleus together. According to the Einstein relationship (E=m.c2) this
binding energy is proportional to this mass difference and it is known as the mass defect.
During the nuclear splitting or nuclear fusion, some of the mass of the nucleus gets
converted into huge amounts of energy and thus this mass is removed from the total mass
of the original particles, and the mass is missing in the resulting nucleus. The nuclear
binding energies are enormous, they are on the order of a million times greater than the
electron binding energies of atoms.
Nuclear Binding Curve
If the splitting releases energy and the fusion releases the energy, so where is the breaking
point? For understanding this issue it is better to relate the binding energy to one nucleon,
to obtain nuclear binding curve. The binding energy per one nucleon is not linear. There is
a peak in the binding energy curve in the region of stability near iron and this means that
either the breakup of heavier nuclei than iron or the combining of lighter nuclei than iron
will yield energy.
The reason the trend reverses after iron peak is the growing positive charge of the nuclei.
The electric force has greater range than strong nuclear force. While the strong nuclear
force binds only close neighbors the electric force of each proton repels the other protons.