Grade 11 Nuclear Physics Overview

Yabello Ifa Boru Special Boarding Secondary School Physics Short Note for Grade 11

•  The course is for Grade 11 Physics, at Yabello Ifa Boru Special Boarding Secondary School.
•  The material was written by Wondimu Getachew (2016 EC).
•  Contact information for the school is available on Telegram: @samuelfromethiopia and @bluenileacademy.

Unit 7: Nuclear Physics - Introduction

•  Nuclear physics studies atomic nuclei and their components, interactions, radioactive decay, fission (splitting of nuclei), and fusion (merging of nuclei).
•  The strong nuclear force acts at a very short distance (femtometers, 10^-15 m) within atomic nuclei.
•  The strong nuclear force binds protons and neutrons to form atomic nuclei.
•  Nuclear processes can produce significant radiation.
•  Unstable nuclides spontaneously emit alpha (α), beta (β), and gamma (γ) radiation.

Unit 7: Nuclear Physics - Research

•  Nuclear physicists are researching atomic nuclei to understand the universe's structure.
•  Research reactors (RRs) and power reactors (PRs) are used globally to study nuclear processes.
•  Small research reactors mainly produce neutrons, unlike larger power reactors which generate electricity.
•  Nuclear reactions use heavy unstable nuclei, such as Uranium (U), Thorium (Th), Potassium (K), and Platinum (Pt) as fuel.

Unit 7: Nuclear Physics - Safety

•  Nuclear radiation can be hazardous to humans if precautions are not taken.
•  International Atomic Energy Agency (IAEA) safety rules must be followed for handling nuclear instruments and materials.
•  Independent inspection bodies are essential for RRs and PRs.

7.1 The Nucleus - Structure of the Atom

•  Atoms are the fundamental building blocks of matter, comprising a positively charged nucleus surrounded by negatively charged electrons.
•  The nucleus primarily consists of protons (positive charge) and neutrons (neutral charge), collectively known as nucleons.
•  Electrons are much lighter than protons or neutrons.
•  The table below summarizes the properties of sub-atomic particles:

•  The mass number (A) specifies the sum of protons and neutrons; the atomic number (Z) signifies the number of protons.
•  The number of neutrons is often different between atoms of the same element, leading to isotopes.

7.1 The Nucleus - Properties of Nuclei

•  Isotopes: atoms of the same element with different numbers of neutrons. The isotopes' chemical properties are largely similar.
•  The radius of a nucleus (R) is proportional to the cube root of the mass number (R∝A^1/3).
•  The mass of an atom is primarily determined by its nucleus.
•  Examples of some isotopes: Oxygen(160), Helium(4He), carbon(6C).

Isotopes

•  An element can consist of multiple isotope atoms.
•  Isotopes have the same number of protons, but different numbers of neutrons, affecting their mass and physical properties but not chemical ones.
•  Examples of hydrogen isotopes: protium (¹H), deuterium (²H), tritium (³H).
•  Examples of carbon isotopes: ¹²C, ¹³C, ¹⁴C.
•  Examples of uranium isotopes: ²³²U, ²³³U, ²³⁴U, ²³⁵U, ²³⁶U, ²³⁸U. 

7.1 The Nucleus - Historical Origins of the Nucleus

•  In 1909, Rutherford's students Geiger and Marsden aimed positively charged helium particles at a thin gold foil.
•  The experiment showed that: most particles passed through undeflected, fewer were deflected at small angles, and some were deflected back, indicating a tiny, dense, positive nucleus at the atom's center.

7.1 The Nucleus - Atomic Model

•  In 1911, Rutherford proposed the "solar system" model of the atom: electrons orbit a dense positive nucleus.
•  However, this model couldn't explain the stability of electrons, as accelerating charges lose energy and should spiral into the nucleus.

7.1 The Nucleus - Discovery of the Proton

•  Rutherford speculated about the existence of a fundamental building block for nuclei (similar to a hydrogen nucleus).
•  He coined the name proton (from the Greek “protos," meaning “first").

7.1 The Nucleus - Discovery of the Neutron

•  In 1932, James Chadwick discovered the neutron, a neutral particle with a mass similar to a proton.
•  Neutrons, present in the atom's nucleus are important in stabilizing against the positive charge repulsion protons experience.

What keeps the nucleus together?

•  Protons and neutrons are held together by the strong nuclear force, a short-range, attractive force more powerful than electromagnetism at short distances.
•  The strong nuclear force overcomes the electrostatic repulsion between positively charged protons.

Nuclear Binding Energy (BE)

•  The binding energy is the minimum energy needed to disassemble a nucleus into its constituent protons and neutrons.
•  The mass of a nucleus is always less than the sum of the individual masses of its constituent protons and neutrons, the difference in mass is the mass defect.
•  Einstein's equation (E=mc²) shows the equivalence of mass and energy, allowing calculation of nuclear binding energy from the mass defect.

Nuclear Binding Energy (BE) - Calculation

•  Given the mass defect (Δm), nuclear binding energy (BE= Δmc²) can be determined using Einstein's equation, where Δm is expressed in atomic mass units (amu) and c is the speed of light.
•  The atomic mass units (amu) are converted to mega-electron volts (MeV) -1 atomic mass unit (amu) = 931.5MeV.

Nuclear Stability

•  A nucleus’ stability depends on the binding energy per nucleon which is determined by the balance of the attractive nuclear force and disruptive force from the electrostatic repulsion of protons against each other.
•  Nuclei with a high binding energy per nucleon are more stable (up to Iron(Fe⁵⁶))
•  The stability of an isotope is related to its neutron-to-proton ratio (N/Z):
  • Nuclei with a high neutron-to-proton ratio tend towards instability.
  • The isotopes with a ratio of neutron-to-proton around 1 to 1 are mostly stable.

Radioactive Decay

•  Radioactivity is the spontaneous emission of particles from unstable atomic nuclei.
•  Elements with more than 84 protons are considered radioactive.
•  All nuclei with lower atomic numbers have both stable and unstable isotopes

Types of Nuclear Radiation

•  Alpha (α) particles: consists of two protons and two neutrons (essentially a helium nucleus).
•  Beta (β) particles: either an energetic electron (β−) or a positron (β+).
• -Beta minus emission (B−): a neutron decays into a proton, an electron, and an antineutrino.
• -Beta plus emission (B+): a proton converts into a neutron, a positron, and a neutrino.
•  Gamma (γ) rays: high energy electromagnetic radiation; typically accompanies α and β decays, helping the nucleus to reach a stable state.

Ionization and Penetration power

• Alpha particles have the highest ionization power but the lowest penetration power.
• Gamma rays have the lowest ionizing power but the highest penetration power.
• Beta particles have intermediate ionization and penetration power

Dangers of Ionization Radiation

•  Ionizing radiation can damage cellular tissue, particularly DNA, leading to cell death or mutations.
•  The extent of harm depends on the type of radiation, tissue exposed , and exposure duration

Effective Dose

•  The effective dose (expressed in sieverts, Sv) quantifies the biological effects of radiation exposure, considering the varied sensitivities of different tissues.

Safety Precautions when using Radioactive Sources

•  Radioactive sources used in schools should be handled with appropriate care by trained personnel using safety precautions.
•  The sources should be shielded, and handled using tongs or forceps to keep distance and avoid direct contact..
•  Handling should be limited to authorized teachers only, and performed in accordance to all necessary safety protocols.

Radiation Detectors

•  Scintillators convert radiation energy into light, which can be detected and measured.
• G-M tubes measure ionizing radiation by detecting the ions produced in the gas.
• A Geiger counter can also be used to measure radiation.

Half-Life

•  Half-life (t_1/2) is the time it takes for half of the radioactive nuclei in a sample to decay.
• It is a fixed constant for any given radioactive nuclide and unaffected by external conditions like temperature or pressure.

Radioactive Dating

•  Radioactive dating uses naturally occurring radioactivity to determine the age of materials like artifacts or biological samples.

7.4 Nuclear Reactions and Energy Production

• Nuclear fission is splitting heavy nuclei such as Uranium-235 (U-235) into lighter nuclei such as Barium-144 (Ba-144) or Krypton-89 (Kr-89) leading to large release in energy , a chain reaction occurs in nuclear reactors
• A fission reaction usually produces more neutrons than the ones that were used to initiate the reaction, enabling it to propagate the reaction further
• Nuclear fusion is the process of combining light nuclei such as hydrogen isotopes like deuterium and tritium to form a heavier nucleus (helium) with high energy release
• Fusion is the source of energy in the sun

Applications of Fission Reactions

• Power Reactors: used to generate electricity employing nuclear fission.
• Research Reactors: used mainly for producing neutrons for research purposes and also to produce radioisotopes

Radioisotopes

• Radioisotopes are produced by exposing target materials to neutron radiation within research reactors.

Chain Nuclear Reaction

• Neutrons released in fission start a chain reaction; this process can be controlled or uncontrolled, leading to either energy production or nuclear weapon explosions.

The Problems Posed by Nuclear Waste of Reactors

•  Nuclear waste from power plants, consisting of isotopes with long half-lives (e.g., plutonium-239), requires specialized containment procedures.

7.4.2 Nuclear Fusion Reaction and its Uses

• Nuclear fusion reactions involve the combination of lighter atomic nuclei into heavier nuclei, liberating significant energy.
• This is a crucial process for energy production in stars like the sun.