section 7: radioactivity Flashcards
an atom:
-the nucleus of an atom contains protons and neutrons, it makes up most of the mass of an atom, but takes up virtually no space
-radius of 1 x 10^-10 metres
-atomic number = number of protons
-mass number = number of protons + neutrons
-electrons are negatively charged and really small, their paths take up a lot of space giving the atom its overall size
-atoms are neutral, number of protons = number of electrons
isotopes:
-> isotopes are atoms with the same number protons and electrons but a different number of neutrons
-there are two common isotopes of carbon, Carbon-14 has 2 more enutrons than ‘normal carbon’ (Carbon-12)
-usually each element has only rwo stable isotopes, he other isotopes tend to be radioactive (the nucleus is unstable, so it decays and emits radiation)
radioactive decay:
-the nuclei of unstable isotopes break down at random
-each nucleus just decays (breaks down) quite sponteneously in its own good time and is compltetly unnaffected by physical condiions like temperature or any sor of chemical bonding…
-when the nucleus does decay it spits out one or more types of radiation- alpha, beta, gamma or neutrons
-in the process, he nucleus ofen changes into a new element
background radiation:
-there’s (low-level) background nuclear radiation all around us all the time, it comes from:
-substances here on earth-some radioactivity comes from air, food, building materials, soil, rocks…
-radiation from space (cosmic rays)- mostly from the Sun
-living things- there’s a litle bi of radioactive materials in all living things
-radiation due o human activity-e.g. fallout from nuclear explosions, or nuclear waste
ionisation:
-nuclear radiation causes ionisation into atoms and knocking electrons off them, atoms (no charge) are turned into ions (which are charged)-hence the term “ionisation”
-the further the radiation can pentrate before hitting an atom and getting stopped, the less damage it will do along the way and so the less ionising it is
-ionising radiation can be detected using a Geiger Müller (GM) detectoror photographic film
ionisation: alpha particles
-(helium nuclei) 42,He
-aplha (α) particles are made up of 2 protons and 2 neutrons- they are big, heavy and slow-moving
-they therefore don’t penetrate far into materials but are stopped quickly
-because of their size they’re strongly ionising, which means they bash into a lot of atoms and knock electrons off them before they slow down, which creates a lot of ions
-because they’re electrically charged (positive charge), aplha particles are deflected (direction changes) by electric and magnetic fields
-emitting an alpha particle decreases the atomic number of the nucleus by 2 and the mass number by 4
ionisation: beta particles
-(electrons) 0,-1 e-(on top)
-a beta (β) particle is an electron which has been emitted from the nucleus of an atom when a neutron turns into a proton and an electron
-when a beta particle is emitted, the number of protons in the nucleus increases y 1, so the atomic number increases by 1 but the mass number stays the same
-they move quite fast and they are quite small
-they penetrate moderately before colliding and are moderately ionising too
-because they’re charged (negative), beta particles are deflected by an electric and magnetic fields
ionisation: gamma rays
-gamma (γ) rays are the opposite of alpha particles, they have no mass-they’re just energy
-they can penetrate a long way into materials without veing stopped
-this means they are weakly ionising because they tend to pass through rather than collide, but eventually they hit something and do damage
-gamma rays have no charge, so they’re not deflected by electric or magnetic fields
-gamma emission always happend after beta or alpha decay, you never get just gamma rays emitted
-gamma ray emission has no effect on he atomic or mass numbers of he isotope, if a nucleus has excess energy, it loses this energy by emitting a gamma ray
ionisation: how it is blocked
-alpha particles: are blocked by paper, skin or a few cm of air
-beta particles: are blocked by thin metal
-gamma rays: are blocked by thick lead or very thick concrete
ionisation: uses
-alpha emission: smoke alarms
-beta emission: measuring thickness of metal foil+ detecting leaks in pipes + medical
-gamma emission: irradiating food + to kill bacteria -> sterilising + cancer treatment -> medical
balancing nuclear equations:
-alpha emission: mass number decreases by 4, atomic number decreases by 2
-beta emission: mass number stays the same, atomic number increases by 1
-gamma emission: mass number stays the same, atomic number stays the same
-neutron emission: mass number decreases by 1, atomic number stays the same
half-life:
-half-life is the time ime for half of the radioactive atoms now present to decay
-a short half-life means the activity falls quickly, because of lots of the nuclei decay quickly
-a long half-life means the activity falls more slowly because most of the nuclei don’t decay for a long time-they just sit there, basically unstable, but kind of biding their time
irradiation:
-objects near a radioactive source are irradiated byt it, this means they’re exposed o it
-irradiating something does not make it radioactive
-keeping sources in lead-lined boxes, standing behind barriers or being in a different room and using a remote-controlled arms are all ways of reducing the risk of irradiation
nuclear fission:
-nuclear fission is the splitting of an atom, which releases energy, it can be spontaneous but in a nuclear reactor it’s made to happen-e.g. to uranium-235
-if a slow-moving neutron is absorbed by a uranium-235 nucleus, the nucleus can split
-each time it spits out a small number of neutrons, these might go on to hit other uranium-235 nuclei, causing them to split also.. and so on and so on = chain reaction
-when uranium-235 splits into two it will form two daughter nuclei, both lighter elements than uranium
-these nuclei are usually radioacive, this is a big problem with nuclear power-radioactive waste
-each nucleus splitting gives out a lot of energy-this energy is in he kinetic energy stores of the fission products
-in a reactor, this energy is transferred to thermal energy stores to produce steam to drive a turbine
nuclear reactors:
-the neutrons released by fission reactions in a nuclear reactor have a lot of energy. In order to be absorbed by uranium nuclei and sustain the chain reaction, they need to be slowed down
-the moderator, usually graphite or water, slows down neutrons
-control rods, often made of boron, limit the rate of fission by absorbing excess neutrons
-the high-energy neutrons and gamma rays (energy) released in fission are highly penetrating ionising radiation. Shielding has to be used to absorb the ionising radiation, it’s usually a thick concrete structure, which may also contain lead or other metals
-a substance pumped round the reactor transfers the energy (by heating) to the water in the heat exchanger. The water turns into steam, which turns a turbine, which turns a generator and generates electricity
