Physics Notes Part 5

  v A central nucleus made up of protons (+), and neutrons. Around this, electrons (-) orbit at high speed. Protons and neutrons are nucleons. Protons and neutrons are bonded tightly together in the nucleus by a different kind of force called strong nuclear force. Electrons are held in orbit by the force of attraction between opposite charges. Atom has the same number of electrons as protons, so its total charge is zero.

v All materials are made from about 100 basic substances called elements. The atomic number indicates the proton number in an atom. Isotopes are atoms of the same element with the same number of protons, but different numbers of neutrons. The total number of protons and neutrons in a nucleus is mass number (nucleon number). Each different type of atom. e.g. Lithium-7 is a nuclide. Electrons orbit the nucleus at certain fixed levels only, called shells. The periodic table is a chart of all elements. Unstable nuclei radiate particles and waves known as nuclear radiation. Materials which emit nuclear radiation are radioactive. The disintegration (break-up) of the nucleus is radioactive decay. (Carbon-14, Potassium-40, Uranium-234,-235,-238 are unstable) Radioactive isotopes are radioisotopes or radionuclides.

v Nuclear radiation can remove electrons from atoms in its path, so it has an ionizing effect.

v 3 Main types of nuclear radiation: Alpha particle, Beta Particle, and Gamma rays.

v Alpha particles: Each particle is 2 protons, and 2 neutrons (identical to the nucleus of helium-4). Its relative charge is +2 compared to the proton. Its mass is high, compared with betas. Its speed is up to 0.1 x the speed of light. The ionizing effect is strong. The penetrating effect is not very penetrating: stopped by thick sheets of paper, skin or a few centimetres of air. It is deflected by magnetic and electric fields.

v Beta particles: Each particle is an electron (created when the nucleus decays). Its relative charge is -1 compared to protons. Its mass is low. Its speed is up to 0.9 x the speed of light. The ionizing effect is weak. The penetrating effect is penetrating but stopped by aluminium or other metals. It is deflected by magnetic and electric fields.

v Gamma rays: Electromagnetic waves similar to x-rays. Its relative charge is 0 compared to the proton. Its speed is the speed of light. The ionizing effect is very weak. The penetrating effect is very penetrating: never completely stopped, through lead and thick concrete will reduce intensity. It is not deflected by magnetic or electric fields.

v Alpha has a greater charge and is slower, so exerts more force on electrons. So they are highly ionizing than Beta particles.

v A small amount of radiation around us all the time because of radioactive materials in the environment is background radiation.

v Nuclear radiation damages living cells, stops organs from working properly, and causes mutation (genetic changes) and cancer.

v Geiger – Müller (GM) tube: Detects nuclear radiation, using a ratemeter, scaler, amplifier and loudspeaker.

v Safety for radioactive sources: Store it in a lead container in a locked cabinet. Pick it up with tongs. Left out of the container for a short time. Kept away from the body and not point at other people.

v Cloud Chamber: Used for studying alpha particles. The chamber has cold alcohol vapour to see alpha’s tracks.

v The original nucleus is the parent nucleus. The nucleus formed is the daughter nucleus. The daughter nucleus and emitted particle are decay products.

v When Iodine -131 decays by beta, a neutron changes into a proton, an electron and an uncharged almost massless relative electron is antineutrino.

v Less common form of beta decay is positron. This is an antiparticle of an electron, it has a charge of +1.

v Half-life of a radioactive isotope is the time taken for half-life nuclei present in any given sample to decay. The half-life of radioactive is the time taken for the activity of any given sample to fall to half its original value.

v Average number of disintegrations per second is Activity. The SI unit is the becquerel (Bq).

v Whenever a particle penetrates and changes the nucleus is nuclear radiation. The splitting process is nuclear fission. It results in a chain reaction. To slow neutrons a moderator is needed. Energy is released by joining very light nuclei together to make heavier ones is called nuclear fusion.

v Radioactive isotopes used in tracers, cracks, radiotherapy, xrays, thickness monitoring, gamma irradiations and smoke detectors.

v Thomson's ‘plum pudding model’ shows atoms with electrons. Rutherford's ‘nuclear model’ shows an atom with a large space and a concentrated nucleus.