A-level Physics (Advancing Physics)/Radioactive Emissions

right|thumb|400px|Table of the number of neutrons and protons in the nuclei of isotopes, showing their most common mode of decay. ‘Radioactivity’ is a catch‐all term for several different emissions from the nuclei of ‘radioactive’ atoms. There are three main types of radiation: alpha (α), beta (β) and gamma (γ). When radiation occurs, four things must be conserved: Mass number Electric charge Lepton number Baryon number In formulae, mass and charge are shown next to the symbol of the particle. For example, a neutron with mass 1u unified atomic mass unit and no charge is . The charge on an ordinary non‐exotic: the matter we are made of nucleus is equal to the number of protons in the nucleus (electrons can be ignored). The lepton and baryon numbers may be obtained by counting the number of leptons and baryons on either side of the equation, remembering that antiparticles have negative lepton and baryon numbers. α Radiation Unstable nuclei with a mass greater than 82u unified atomic mass units emit α radiation. This consists of a helium nucleus (). The alpha particle simply splits off from the nucleus. Since the particle has no electrons, it has a charge of +2e elementary charges. This, combined with its relatively large mass, means that it reacts easily with other particles, ionising them, meaning that it cannot penetrate more than a few centimetres of standard 101.325kPa beside other parameters (differs a little bit among standards) air. β Radiation Unstable nuclei with a mass below 82u unified atomic mass units emit β radiation. There are two types of β radiation. β− radiation consists of an electron (). This is produced by nuclei with many more neutrons than protons. A neutron changes into a proton, emitting an electron and an antineutrino in order to balance the lepton number. β+ radiation consists of a positron (). This is produced by nuclei with roughly the same number of neutrons as protons. A proton changes into a neutron, emitting a positron and a neutrino. β particles also ionise particles, but since they have less charge and mass, they do this less easily, and so they travel further (on average). Both α and β radiation result in the nucleus which emitted them being changed into another element (transmutation). γ Radiation The binding energies of nuclei are quantized – they can only take on certain values. When an electron jumps down an energy level, this energy has to go somewhere – it takes the form of a photon (γ). The structure of the nucleus is not changed by γ radiation. γ radiation is ionising, but only at the right frequency – the resonant frequency of the things it ionises. γ radiation travels very far, and only a good thick layer of lead can stop it. Questions { You will need a periodic table. Do not use a reference showing decay modes. } { Americium‐241 is an α emitter. What element, and what isotope, is produced by this decay? | type="{}" } { neptunium‐237 | neptunium‑237 | neptunium-237 | Np‑237 | Np‐237 | Np-237 | ²³⁷Np } || So, (neptunium‐237) is produced. { Iodine‑129 is a β− emitter. What element, and what isotope, is produced by this decay? | type="{}" } { xenon‐129 | xenon‑129 | xenon-129 | Xe‑129 | Xe‐129 | Xe-129 | ¹²⁹Xe } || So, (xenon‐129) is produced. { Gamma rays are used to kill microbes in food. Why doesn’t the food become radioactive? | type="()" coef="2" } + (Free‐form text answer: Marking needs to be done manually.) || Gamma radiation only interacts with the electrons of the atom, not with the nucleus. Radioactivity is due to nuclear properties, not chemical properties. { Plutonium‐244 decays by emitting an α particle. It does this twice, emits a β− particle, and then emits a further two α particles. The nucleus becomes a different element each time. What element is produced at the end? | type="{}" } { francium‐228 | francium‑228 | francium-228 | Fr‑228 | Fr‐228 | Fr-228 | ²²⁸Fr } || So, (francium‐228) is produced. { Carbon‐11 changes into boron‐11 by a radioactive emission. What was emitted? | type="{}" coef="2" } { electron neutrino and positron | electron neutrino, positron | antielectron and electron neutrino | antielectron, electron neutrino | e⁺, νₑ | β⁺, νₑ } || So, a positron ( was emitted. However, in order for the lepton numbers to balance, an electron neutrino () must also be emitted. { Uranium‐236 decays, following the equation:Identify the particle in this equation. | type="{}" } { α | alpha particle | α particle | helium-4 nucleus } || Solve these type of problems by applying the conservation laws: e. g. the decay must balance electric charge, nucleon number, proton number, lepton number etc. Nucleon number doesn’t balance (), so must have a nucleon number of .The proton number does not balance either () therefore must have protons.Therefore it can be concluded that is an alpha particle.