radioactivity

• the Rutherford scattering experiment, a thin stream of alpha particles (from a radioactive source) were fired at very thin gold foil
• the following conclusions can be drawn from the results of Rutherford's experiment:

  most of the atom must be empty space, because most alpha particles simply pass straight through

  The nucleus must be (significantly) positively charged, as some of the positively charged alpha particles are repelled and deflected through a large angle

  the nucleus must be very small as very few alpha particles are deflected by an angle greater than 90°

  the mass must be concentrated in the nucleus as the fast-moving alpha particles (with high momentum) are deflected by the nucleus.
• an unstable nucleus becomes stable by a random process known as radioactive decay
• the intensity of radiation is defined as the radiation energy per second passing normally through a unit area
• the intensity of gamma radiation follows an inverse-square law

  $I=$$k/x^2$
• radioactive contamination is the unwanted presence of materials containing radioactive atoms on other materials
• irradiation is the process of exposing an object to nuclear radiation. the irradiated object does not become radioactive
• all deliberate exposure to radiation must have a benefit that outweighs the risk
• background radiation is that which is present in a particular location due to naturally occuring radioactive substances, it is there all the time
• there are many sources of background radiation:

  radon gas - which is released from rocks

  artificial sources - caused by nuclear weapons testing and nuclear meltdowns

  cosmic rays - enter the earth's atmosphere from space

  rocks containing naturally occurring radioactive isotopes
• radioactive decay is a random process
• the half-life of a radioactive sample is the time it takes for the number of nuclei of the isotope to half
• the activity (A) of a radioactive sample is the number of nuclei that decay each second, measured in becquerels, where 1bq = 1 decay per second
• the decay constant (λ) is the constant of proportionality that links the rate of decay to the number of undecayed nuclei
• $E=$$mc^2$

  applies to all energy changes
• the atomic mass unit (u) is 1/12th the mass of a neutral carbon-12 atom
• 1u = 1.661x10^-27kg = 931.5MeV
• the mass of a nucleus is less than the mass of its constituent nucleons. this difference in mass in known as the mass defect

  Δm = [Zmproton + (A-Z)mneutron] - mnucleus
• the binding energy of a nucleus is the work that must be done to separate the nucleus into its individual nucleons
• the binding energy of a nucleus is equivalent to its mass defect

  $ΔE=$$Δm*$$931.5$
• average binding energy per nucleon = B/A

  where b is binding energy and A is nucleon number. measured in MeV nucleon^-1
• iron is the most stable element
• nuclear fission is the splitting of a large and unstable nucleus (e.g uranium) to form two smaller daughter nuclei
• a thermal neutron is a neutron of low energy
• energy is released during nuclear fission because the new, smaller nuclei have a higher average binding energy per nucleon
• nuclear fusion is the joining of two light nuclei to form a heavier nucleus
• as they are much heavier, the new nuclei formed from nuclear fusion have a much greater average binding energy binding energy per nucleon. hence, nuclear fusion releases vast amounts of energy
• induced fission is the splitting of a large and unstable nucleus to form two smaller daughter nuclei following the bombardment of thermal neutrons
• the neutrons produced from nuclear fission can induce other nuclei to undergo further fission. this is called a chain reaction
• the mass of fuel required to maintain a steady rate of fission is known as the critical mass
• neutrons in a nuclear reactor undergo elastic collisions with the moderator, causing them to slow down. this increases the probability of the neutron being absorbed by the fuel isotopes and fission taking place
• commonly, water is used as a moderator

  this is because it contains hydrogen, which is of a similar mass to a neutron
• control rods absorb incident neutrons. hence, they are used to limit the number of neutrons in the reactor and control the rate of fission
• commonly, control rods are made of boron

  able to absorb neutrons without undergoing nuclear fission
• coolant transfers heat produce by fission away from the reactor core
• common coolants include water and gases (such as carbon dioxide and helium)

  have high specific heat capacities