Atomic and Nuclear Physics

Nuclear Physics

What is nuclear physics?

Nuclear physics is the field of physics that studies the constituents of matter (protons and neutrons) and interactions between them. Modern nuclear physics contains especially particle physics, which is taught in close association with nuclear physics.

Key Facts

What is nuclear physics used for?
What is nuclear physics used for?

Knowledge of nuclear physics is essential in many fields in our lives. The most commonly known applications of nuclear physics are nuclear power generation. Modern nuclear physics also contains particle physics, which is taught in close association with nuclear physics. Many of today’s most important advancements in medicine, materials, energy, security, climatology and dozens of other sciences emanate from the wellspring of basic research and development in nuclear physics.

How hard is nuclear physics?
How hard is nuclear physics?

What kind of human knowledge is hard, and what kind is not? Nuclear physics isn’t hard to learn. It isn’t that hard if you want to understand the basic reactions like nuclear fusion and fission. Maybe the problem is that you cannot use most of the lessons in everyday life to understand nuclear physics. Mathematics is, and exact physics is better for this purpose. Only this may seem hard.

What are main types of nuclear reactions?
What are the main types of nuclear reactions?

Although the number of possible nuclear reactions is enormous, nuclear reactions can be sorted by types:

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 a very extensive branch of science. Nuclear reactor physics belongs to applied physics. Reactor physics, particle physics, or other branches of modern physics have common fundamentals. Atomic and nuclear physics describe fundamental particles (i.e., electrons, protons, neutrons), 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. Atomic physics deals with the atom as a system consisting of a nucleus and electrons. Nuclear physics deals with the nucleus as a system consisting of nucleons (protons and neutrons). The 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 and neutral atoms and, unless otherwise stated, it should be assumed that the term atom includes ions for this discussion. Atomic physics also helps to understand the physics of molecules, but molecular physics also describes the physical properties of molecules.

 
Volume of an Atom and Nucleus
Structure of Matter.
Structure of Matter.

The atom consists of a small but massive nucleus surrounded by a cloud of rapidly moving electrons. The nucleus is composed of protons and neutrons. Typical nuclear radii are of the order 10−14 m.  Nuclear radii can be calculated according to the following formula assuming a spherical shape:

r = r0 . A1/3

where r0 = 1.2 x 10-15 m = 1.2 fm

If we use this approximation, we, therefore, expect the volume of the nucleus to be of the order of 4/3πr3 or 7,23 ×10−45 m3 for hydrogen nuclei or 1721×10−45 m3 for 238U nuclei. These are volumes of nuclei, and atomic nuclei (protons and neutrons) contain about 99.95% of the atom’s mass.

Is an atom an empty space?
atomic-nucleus-volume-min
A figurative depiction of the helium-4 atom with the electron cloud in shades of gray. Protons and neutrons are most likely found in exactly the same space, at the central point. Source wikipedia.org License CC BY-SA 3.0

The volume of an atom is about 15 orders of magnitude larger than the volume of a nucleus. For uranium atom, the Van der Waals radius is about 186 pm = 1.86 ×10−10 m. The Van der Waals radius, rw, of an atom is the radius of an imaginary hard-sphere representing the distance of closest approach for another atom. The uranium atom has a volume of about  26.9 ×10−30 m3, assuming a spherical shape. But this “huge” space is occupied primarily by electrons because the nucleus occupies only about 1721×10−45 m3 of space. These electrons together weigh only a fraction (let say 0.05%) of the entire atom.

It may seem that the space and the matter are empty, but it is not. Due to the quantum nature of electrons, the electrons are not pointing particles, and they are smeared out over the whole atom. The classical description cannot be used to describe things on the atomic scale. On the atomic scale, physicists have found that quantum mechanics describes things very well on that scale. Particle locations in quantum mechanics are not at an exact position. A probability density function describes them. Therefore the space in an atom (between electrons and an atomic nucleus) is not empty. Still, it is filled by a probability density function of electrons (usually known as  “electron cloud“).

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 modern nuclear physics also contains particle physics, which is taught in close association with nuclear physics. Nuclear physics has provided application in many fields, including nuclear medicine (Positron Emission Tomography, isotopes production, etc.) and magnetic resonance imaging, ion implantation in materials engineering, and radiocarbon dating in geology and archaeology.

Our World

Fundamental Particles
Three generations of matter.
Three generations of matter.

See also: 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.

Quarks and electrons are some of the elementary particles. Several 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 is the electrons and the neutrinos.
  • Antileptons. For every lepton, there is a corresponding type of antiparticle. The best known of all antileptons is 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.

Atom StructureHowever, only a few of these fundamental particles (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:

  • Electrons. The electrons are negatively charged, almost massless particles that nevertheless account for most of the size of the atom. Sir John Joseph Thomson discovered electrons 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. Ernest Rutherford discovered protons in the year 1919 when he performed his gold foil experiment.
  • Neutron. Neutrons are located in the nucleus with 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 the 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. A positron is an antiparticle of a negative electron. Positrons, also called positive electrons,  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.

See also: Fundamental Forces.