Modern Physics

The One Thing

Nuclear processes involve changes in atomic nuclei, and the associated energy changes arise from the conversion of mass defect into energy. Radioactive decay is spontaneous and random, whereas fission and fusion are nuclear reactions that can release very large amounts of energy.

Reactions, Processes and Experiments

Key Terms

• Radioactivity: The spontaneous disintegration of unstable atomic nuclei accompanied by the emission of alpha particles, beta particles or gamma rays.
• Nucleus: The small, dense central part of an atom containing protons and neutrons, where almost all nuclear mass is concentrated.
• Parent nucleus: The original unstable nucleus that undergoes radioactive decay.
• Daughter nucleus: The new nucleus formed after the parent nucleus undergoes radioactive decay.
• Alpha particle: A helium nucleus containing two protons and two neutrons, represented as 4/2 He or 4/2 α; it has a positive charge of +2e.
• Beta particle: A high-speed electron emitted from an unstable nucleus during beta decay; it has a charge of -e and a very small mass.
• Gamma ray: A highly energetic electromagnetic radiation emitted by an excited nucleus; it has no mass and no charge.
• Half-life: The time required for half the radioactive nuclei in a sample to decay.
• Activity: The number of radioactive disintegrations occurring per second in a sample; its SI unit is the becquerel (Bq).
• Nuclear equation: An equation showing the conservation of mass number and atomic number during a nuclear reaction.
• Mass defect: The difference between the mass of separate nucleons and the actual mass of the nucleus formed from them.
• Binding energy: The energy required to completely separate a nucleus into its individual protons and neutrons.
• Mass-energy equivalence: The principle that mass and energy are interconvertible, expressed by E = mc².
• Nuclear fission: The splitting of a heavy nucleus into two or more medium-sized nuclei, usually after absorbing a neutron, with the release of energy and additional neutrons.
• Chain reaction: A self-sustaining sequence in which neutrons released in one fission event cause further fission reactions.
• Critical mass: The minimum amount of fissile material needed to maintain a self-sustaining chain reaction.
• Nuclear reactor: A device in which a controlled nuclear fission chain reaction produces useful energy.
• Moderator: A material such as water or graphite that slows down fast neutrons so that they can efficiently cause further fission.
• Control rods: Rods made of neutron-absorbing materials, such as cadmium or boron, used to regulate the rate of a chain reaction.
• Coolant: A substance that carries heat away from the reactor core.
• Nuclear fusion: The process in which two light nuclei combine at extremely high temperature and pressure to form a heavier nucleus and release energy.
• Natural radioactivity: Radioactivity shown by naturally occurring unstable elements such as uranium and radium.
• Artificial radioactivity: Radioactivity produced when stable nuclei are made radioactive by bombardment with suitable particles.
• Ionising radiation: Radiation with enough energy to remove electrons from atoms and form ions.

Easily Confused

• Alpha, beta and gamma radiation: Alpha has the greatest ionising power and least penetrating power; beta has intermediate values; gamma has the least ionising power and greatest penetrating power.
• Alpha and beta deflection: Alpha particles bend towards the negative plate, whereas beta particles bend towards the positive plate; beta is deflected more because its mass is much smaller.
• Natural and artificial radioactivity: Natural radioactivity occurs in naturally unstable elements such as uranium and radium, whereas artificial radioactivity is produced by bombardment of stable nuclei.
• Radioactive decay and nuclear fission: Radioactive decay is spontaneous emission from an unstable nucleus; fission is the splitting of a heavy nucleus, usually after neutron absorption.
• Nuclear fission and nuclear fusion: Fission splits a heavy nucleus into medium-sized nuclei, whereas fusion joins light nuclei to form a heavier nucleus.
• Mass defect and binding energy: Mass defect is a mass difference; binding energy is the energy equivalent of that mass defect and is the energy required to separate the nucleus.
• Activity, absorbed dose and equivalent biological effect: Activity is measured in becquerels, absorbed dose in grays, and equivalent biological effect in sieverts.
• Moderator and control rods: A moderator slows neutrons, whereas control rods absorb neutrons.
• Controlled and uncontrolled chain reactions: A reactor controls the chain reaction to produce useful energy; an uncontrolled reaction can become extremely rapid, as in a nuclear weapon.
• Fission and fusion energy conditions: Fission commonly involves heavy fissile materials such as uranium-235 and plutonium-239; fusion requires extremely high temperature and pressure to overcome electrostatic repulsion.

What Gets Asked

• Complete or balance alpha, beta-minus and gamma nuclear equations. Marks are lost by changing the mass number during beta-minus decay or changing either number during gamma emission.
• Compare alpha, beta and gamma radiation. Answers must distinguish their nature, charge, mass, ionising power, penetrating power, shielding and deflection; assigning gamma the greatest ionising power is incorrect.
• Calculate decay quantities using N = N0(1/2)^(t/T), A = λN or T = 0.693/λ. Marks are lost by confusing half-life with activity or omitting the meaning of the variables.
• Use E = mc² or the conversion 1 atomic mass unit ≈ 931 MeV. The specific value c = 3.0 × 10^8 m/s and the idea of mass defect conversion must be applied correctly.
• Explain nuclear reactors and chain reactions. Answers should identify the moderator, control rods, coolant and the conversion from nuclear energy to heat, mechanical and electrical energy.
• Compare fission and fusion and explain their energy release. Marks are lost by reversing which process splits heavy nuclei and which joins light nuclei, or by omitting the need for extremely high temperature and pressure in fusion.
• Describe radiation applications and safety. Relevant examples include medical diagnosis, cancer treatment, industrial thickness measurement, leak detection and archaeological dating; safety answers should include reduced exposure time, increased distance and suitable shielding.