Topic 6 Radioactivity

• Atom is a positively charged nucleus consisting of protons and neutrons surrounded by negatively charged electrons with the nuclear radius much smaller than that of the atom
• 1*10^-15 (radius of the nucleus) 1*10^-10 (radius of the atom)
• Isotopes have the same number of protons different number of neutrons
• Num of protons = electrons in atom so overall charge is 0
• In an atom its electrons orbit the nucleus at different set distances from the nucleus
• electrons change orbit when there is absorbtion or emission of electromagnetic radiation
• if an atom absorbs energy an electron can move to a high orbit, each different change produces a different wavelength of light
• atoms may form positive ions by losing outer electrons
• background radiation is radiation we are constantly exposed to from space and from naturally radioactive substances in the environment
• the radioactivity of a source can also be measured using a geiger muller tube, radiation passes through the tube and ionises gas inside it and allows a short pulse of current to flow,  and photographic film becomes darker and darker as more radiation reaches it
• an alpha particle is identical to a helium nucleus, a beta particle is an electron emitted  from the nucleus and a gamma ray is a form of electromagnetic radiation
• Most ionising:

  alpha

  beta

  gamma
• Penetration distance:

  Gamma

  Beta

  Alpha - transfer a lot of energy and are good at ionising by when they ionise an atom they lose energy
• alpha beta minus positron and gamma rays are ionising radiation
• alpha, beta minus, positron, gamma rays and neutron radiation are emitted from an unstable nucleus in a random process
• nuclei that have undergone radioactive decay often undergo nuclear rearrangement with a loss of energy as gamma radiation
• 1. the unit of activity of a radioactive isotope is the Becquerel, Bq
• the half-life of a radioactive isotope is the time taken for half the undecayed nuclei to decay or the activity of a source to decay by half
• it cannot be predicted when a particular nucleus will decay but half-life enables the activity of a very large number of nuclei to be predicted during the decay process
• the dangers of ionising radiation depend on half- life
• • At the end of the 19th Century, Physicist Joseph Jon Thompson discovered the existence of electrons
• • They were directing a beam of alpha particles (He2+ ions) at a thin gold foil
  • They expected the alpha particles to travel through the gold foil, and maybe change direction a small amount
  • Instead, they discovered that :
  • Most of the alpha particles passed straight through the foil
  • Some of the alpha particles changed direction but continued through the foil
  • A few of the alpha particles bounced back off the gold foil
  • The bouncing back could not be explained by the Plum Pudding model, so a new model had to be created
• • Rutherford proposed the nuclear model of the atom
  • In the nuclear model:
  • Nearly all of the mass of the atom is concentrated in the centre of the atom (in the nucleus)
  • The nucleus is positively charged
  • Negatively charged electrons orbit the nucleus at a distance
• The nucleus of each element has a characteristic positive charge but that isotopes of an element differ in mass by having different numbers of neutrons
• in each atom its electrons orbit the nucleus at different set distances from the nucleus
• The main source of background radiation is radon gas which is produced by rocks that contain uranium. some foods contribute to exposure of background radiation's well as hospital treatments such as xrays
• Beta minus decay is when a neutron becomes a proton and an electron. the atomic number increases by 1 and the mass number doesn't change
• beta plus decay: a proton is converted into a neutron and a positron. The atomic number goes down by 1 but the mass number stays unchanged
• The half life is the time taken for half the unstable nuclei in a sample of a radioactive isotope to decay. Radioactive decay is a random process. The half life allows us to predict the activity of a large number of nuclei
• Radioactivity can be used in household fire alarms, these usually contain a source of alpha particles. The detector has an electrical circuit with an air gap between two electrically charged plates. The alpha particles ionise molecules in the air gap, causing a current to flow through the circuit. When smoke gets into the air gap the smoke particles slow down the ions meaning the current flowing across the gap decreases
• Food can be irradiated with gamma rays to kill bacteria
• surgical instruments need to be sterilised to kill microorganisms. Some equipment cant be heated so they are irradiated with gamma rays
• Paper can be made in different thicknesses, when the paper is too thin more beta particles penetrate the paper and the detector detects this.
• radioactive materials are used to diagnose medical conditions without having to operate. a tracer is placed in the patient. tracers are detected using gamma cameras they are used to detect tumours by detecting how much glucose is being taken up (cancer takes up more than normal cells)
• Small amounts of ionising radiation over long periods of time can damage the DNA inside a cell called a mutation. Some mutations can cause the cell to malfunction and may cause cancer
• Ionising radiation can also cause tissue damage
• precautions with radiation include increasing their distance from the source, shielding the source and minimising the time they spend with it. Their exposure is measured with dosimeter badges
• Radioactive waste remains hazardous and radioactive for long periods of time and must be secured safely. The effects will last longer with longer half lives than materials with shorter half lives
• someone is irradiated when they are exposed to radiation from nearby materials when they move away this stops. if someone is contaminated this means that they will be exposed to the radiation as the unstable isotopes in the material decay and will continue until this is removed (not always possible)
• cancer cells divide more rapidly so are more susceptible to be killed by radiation. this can be done internally or externally. internally uses a beta emitter placed inside the body externally uses a gamma emitter with rays directed at the tumour