7. Radioactivity & Particles

• alpha decay is the decay of a radioactive nucleus by the emission of an alpha particle
• an alpha particle is a particle of two protons and two neutrons emitted by an atomic nucleus during radioactive decay
• background radiation is the radiation from the environment to which we are exposed all the time
• a becquerel is the SI unit of activity; 1 Bq = 1 decay per second
• beta decay is the decay of a radioactive nucleus by emission of a beta particle
• a beta particle is a particle (an electron) emitted by an atomic nucleus during radioactive decay
• contamination is when an object has acquired some unwanted radioactive substance
• a radioactive half-life is the time taken for the number of radioactive nuclei to halve
• an electron is a negatively charges particle which is smaller than an atom
• ionisation is the removal of electron from an atom
• Alpha particles cannot penetrate the skin as it is stopped by a few cm of air. Therefore alpha radiation is not a risk to people
• Half-life is the time taken for half the radioactive nuclei to decay
• A nuclear reactor uses controlled fission reactions to produce heat energy. It consists of fuel rods containing enriched uranium surrounded by control rods made of cadmium. When the reactor is switched on, the control rods are withdrawn allowing the chain reaction to begin. Neutrons produced during the fissions collide with other atoms causing them to split too. This releases further neutrons and the process continues until the reactor is shut down by inserting the control rods back into the reactor
• the higher the frequency, the higher the energy of the radiation
• Radiation with higher energy is, highly ionising and harmful to cells and tissues causing cancer (e.g. UV, X-rays, Gamma rays)
• Radiation with lower energy is useful for communications and less harmful to humans
• Atoms have a tiny, dense nucleus at their centre, with electrons orbiting around the nucleus
• The nucleus contains: Protons - positively charged particles with a relative atomic mass of one unit. Neutrons – no charge, and also with a relative atomic mass of one unit
• Although atoms contain particles of different charge, the total charge within an atom is zero. This is because the number of electrons is equal to the number of protons
• Isotopes are atoms of the same element that have an equal number of protons but a different number of neutrons
• The number of neutrons in an atom does not affect the chemical properties of an atom, such as its charge, but only its mass. This is because neutrons have no charge but do have mass
• Some atomic nuclei are unstable this is because of an imbalance in the forces within the nucleus. Forces exist between the particles in the nucleus
• Unstable nuclei can emit radiation to become more stable. Radiation can be in the form of a high energy particle or wave
• As the radiation moves away from the nucleus, it takes some energy with it. This reduces the overall energy of the nucleus. This makes the nucleus more stable
• there are different types of radiation that can be emitted: Alpha, Beta and Gamma
• Alpha Particles- the symbol for alpha is α. An alpha particle is the same as a helium nucleus. This is because they consist of two neutrons and two protons. Alpha particles have a charge of +2. This means they can be affected by an electric field
• Beta Particles- The symbol for beta is β-. Beta particles are fast-moving electrons. They are produced in nuclei when a neutron changes into a proton and an electron. Beta particles have a charge of -1. This means they can be affected by an electric field
• Gamma Rays- The symbol for gamma is γ. Gamma rays are electromagnetic waves. They have the highest energy of the different types of electromagnetic waves. Gamma rays have no charge
• Penetrating Power- Alpha, beta and gamma have different properties. They penetrate materials in different ways. This means they are stopped by different materials
• Alpha is stopped by paper, whereas beta and gamma pass through it. Beta is stopped by a few millimetres of aluminium. Gamma can pass through aluminium. Gamma rays are only partially stopped by thick lead
• Ionising Power- All nuclear radiation is capable of ionising atoms that it hits. When an atom is ionised, the number of electrons it has changes. This gives it a non-zero charge
• During alpha decay an alpha particle is emitted from an unstable nucleus. A completely new element is formed in the process
• During beta decay, a neutron changes into a proton and an electron. The electron is emitted and the proton remains in the nuclei
• During gamma decay, a gamma ray is emitted from an unstable nucleus. The process that makes the nucleus less energetic but does not change its structure
• A small number of isotopes can decay by emitting neutrons. When a nucleus emits a neutron: The number of protons does not change:The atomic (proton) number does not change. The total number of particles (nucleons) in the nucleus decreases by 1: The mass (nucleon) number decreases by 1
• Photographic films detect radiation by becoming darker when it absorbs radiation, just like it does when it absorbs visible light. The more radiation the film absorbs, the darker it is when it is developed
• The Geiger-Müller tube is the most common device used to measure and detect radiation. Each time it absorbs radiation, it transmits an electrical pulse to a counting machine. This makes a clicking sound or displays the count rate. The greater the frequency of clicks, or the higher the count rate, the more radiation the Geiger-Müller tube is absorbing. Therefore, it matters how close the tube is to the radiation source. The further away from the source, the lower the count rate detected
• Background radiation is defined as: The radiation that exists around us all the time
• There are two types of background radiation: Natural sources and Man-made sources
• Every second of the day there is some radiation emanating from natural sources such as: Rocks, Cosmic rays from space and foods