Radioactivity

Radioactivity

1. Atomic structure

   • 1) Atoms have no charge overall
   • 2) the number of protons always equals the number of electrons
   • 3) if some electrons are added or removed, the atom becomes a charged particle called an ion
   • isotopes
     • atoms with the same number of protons but a different number of neutrons
       • same atomic number but different mass number
     • the mass number is the number of protons + the number of neutrons in an atom
     • most isotopes are unstable and radioactive, which means they decay into other elements and give out radiation

2. Radiation

   • radioactivity is a random process
   • radioactive substances spit out one or more of the three types of radiation
   • harms living cells
     • lower doses can cause minor damage without killing the cells this can give rise to mutant cells which divide uncontrollably. this is cancer
     • high doses tend to kill cells completely, which causes radiation sickness
     • alpha beta and gamma radiation will enter the living cells and cause ionisation which damages or destroys molecules
     • outside the body beta and gamma are the most dangerous because they can get inside the delicate organ spheres alpha can't because it can't penetrate through the skin
     • inside the body alpha sources are the most dangerous as they do all their damage in a very localised area whereas beta and gamma aren't as dangerous because they pass straight lout without doing much damage
   • background radiation
     • natural sources such as unstable isotopes found in rocks and radiation from space
     • man-made sources e.g nuclear weapons tests, nuclear accidents or nuclear waste

3. Ionising Radiation

   • Alpha Particles
     • 1) are helium nuclei with a mass of 4 and a charge of 2+
     • 2) they're made up of two protons and two neutrons
     • 3) so when a nucleus emits an alpha particle it forms a new element because the number of protons has changed
     • 4) they are relatively big and heavy and slow moving
     • 5) they therefore don't penetrate very far into materials and can be stopped easily
     • When a nucleus emits an alpha particle
       • the mass no decreases by 4
       • the atomic number decreases by 2
   • Beta particles
     • 1) are fast-moving electrons that come from nuclei
     • 2) beta particles are quite fast and quite small
     • 3) they penetrate moderately into materials before colliding
     • 4) for every B particle emitted, a neutron turns into a proton in the nucleus
     • 5) because a b particle is simply an electron it has virtually no mass and a charge of -1
     • When a nucleus emits a beta particle
       • the mass number doesn't change - because it has lost a neutron but gained a proton
       • the atomic number increases by 1
       • it forms a new element
   • Gamma rays
     • 1) are very short wavelength EM waves
     • 2) they penetrate far into materials
     • 3) this means they're are weakly ionising because they tend to pass through rather than collide with other atoms
     • 4) gamma rays are electromagnetic waves with no mass and no charge. As it is just energy it doesn't change the element of the nucleus

4. Half Life

   • radioactive isotopes have unstable nuclei which decay emitting ionising radiation
   • the older a sample becomes the less radiation it will emit
   • The average time it takes for the number of nuclei in a radioactive isotope sample to halve
   • the half life is found from the graph by finding the time interval on the bottom axis corresponding to a halving of the activity on the vertical axis

5. Uses of Radiation

   • Smoke detectors - use alpha radiation
     • 1) a weak source of alpha radiation is placed in the detector, close to two electrodes
     • 2) the source causes ionisation, and a current flows between the electrodes
     • 3) if there is a fire then smoke will absorb the radiation - so the current stops and the alarm sounds
   • Tracers in Medicine (always short half life) beta or gamma emitters
     • 1) certain radioactive isotopes can be injected into people and their progress around the body can be followed
     • 2) iodine 131 can be absorbed by the thyroid gland and gives out radiation which can be detected to indicate whether the thyroid gland is taking iodine as it should
     • 3) all isotopes which are taken into the body must be beta or gamma emitters so that radiation passes out the body
   • Radio Therapy use gamma rays
     • 1) since high doses of gamma rays will kill living cells it can be used to treat cancer
     • 2) they are directed carefully and at just the right dosage so they kill only cancer cells and not normal ones

6. Nuclear Fission and Fusion

   • Fission
     • the splitting up of big atomic Nuclei
     • 1) a slow moving neutron must be absorbed into a uranium or plutonian nucleus, this addition of a neutron makes the isotope unstable, causing it to split
     • 2) each time uranium or plutonium nucleus splits up, it spits out two or three neutrons
     • 3) when a large atom splits in two it will form two smaller nuclei. These new nuclei are usually radioactive because they have the 'wrong' number of neutrons in them
     • 4) a nucleus splitting gives out a lot of energy
   • Fusion
     • the joining of small atomic nuclei
     • 1) two nuclei can join to create a larger nucleus
     • 2) fusion releases a lot of energy - all the energy released in stars come from fusion
     • 3) but fusion requires really high temperatures and pressure (which are found inside stars)
     • 4) the high temperatures and pressures inside stars cause different nuclei to fuse together

7. The Life Cycle of Stars

   • 1) stars initially form from clouds of Dust and Gas. the force of gravity makes gas and dust spiral together to form a prostar
   • 2) gravitational energy is converted into heat energy, so the temperature rises, when it is hot enough for nuclear reactions to start energy is released which keeps the star hot.
     • hydrogen nuclei fuse together to make helium
     • smaller masses of gas and dust may also pull together to make planets
   • 3) the star is then stable the force of gravity holding the star together is balanced by the high pressure due to high temperatures
     • main sequence star - lasts for billions of years
   • 4) when all the hydrogen has been used up a larger nuclei begins to form and the star expands to form a red giant. It becomes red because the surface cools.
     • heavier elements such as iron are made by the nuclear fusion of helium
   • 5) a small star like the sun then becomes unstable and ejects its outer layer of dust and gas as a planetary nebula
     • this leaves behind a hot dense solid core ( a white dwarf)
   • 6) a larger star with more mass will go on making nuclear reactions, expanding and getting hotter until it explodes as a supernova
     • forming elements heavier than iron and ejecting them into the universe to form new planets and stars
   • 7) neutron star

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