What’s the mass/nucleon number?
The the number of protons added to the number of neutrons.
What’s an isotope?
Atoms with the same number of protons but different number of neutrons (same atomic number, different mass number)
What’s radioactive decay?
The nuclei of unstable isotopes break down at random. Each nucleus decays spontaneously (unaffected by physical conditions like temperature or chemical bonding…)
When nucleus decays, emits ionising radiations - alpha, beta, gamma
Alpha and beta decay processes cause the original element to turn into another element.
Describe the nature of alpha particles;
2 PROTONS, 2 NEUTRONS.
(Alpha particles are helium nuclei.)
- big, heavy, slow moving, strongly ionising (because of size - bash into lots of atoms and knock electrons off before they slow down…creating ions)
- least penetrating - stopped quickly (heavy, slow)
- because they’re electrically charged (w a positive charge) they are deflected by electric and magnetic fields!
- emitting an alpha particle, decreases atomic number by 2, and mass number by 4.
What is deflection?
The process of changing direction.
Describe the nature of beta particles;
ONE ELECTRON
- quite fast, quite small
- penetrate moderately before colliding
- moderately ionising
- when a neutron turns into a proton and electron, the electron is emitted (beta). when emitted, atomic number increases by one (the proton), mass number stays the same.
- because they’re electrically charged (w a negative charge) they are deflected by electric and magnetic fields!
Describe the nature of gamma rays;
- no mass, just ENERGY (in the form of an EM wave)
- most penetrating
- weakly ionising (tend to pass through rather than collide with atoms - but eventually hit something and do damage)
- no charge, not deflected by electric/magnetic fields
- gamma emission always happens after beta or alpha decay - never just gamma rays emitted
- gamma ray emission has no effect on atomic or mass numbers of the isotope. (If a nucleus has excess energy, loses energy by a emitting gamma ray)
What are alpha particles blocked by? (penetrating power)
Alpha particles blocked by paper, skin or a few centimetres of air
What are beta particles blocked by? (penetrating power)
Beta particles stopped by thin metal (eg aluminium)
What are gamma rays blocked by? (penetrating power)
Gamma rays are blocked by thick lead (or very thick concrete)
How can ionising radiations be detected?
Radiation imprints on photographic/camera film.
Geiger-Muller detector (or GM counter) beeps in the presence of ionising radiations, the more radiation, the more frequent the beeps.
What are the sources of background radiation?
Concrete buildings
Food eg bananas
Cosmic rays from space
Why does the activity of a radioactive source decrease over a period of time?
Each time a decay happens and radiation is emitted, one more radioactive nucleus has disappeared, decreasing the reactivity. (The older a sample becomes, the less radiation it will emit)
➡️Activity of radioactive source is measured in becquerels
What’s half life?
Half life is the time taken for half of the radioactive atoms (currently) present to decay/ time taken for radiation emitted by a source to decrease by half.
How quickly activity drops off varies (some isotopes-hours, some -millions of years before all unstable nuclei have decayed)
What’s the problem with trying to measure how quickly activity drops off?
The activity never reaches zero (so we need to use the idea of half-life)
What do short and long half-lives mean?
Short half life- activity falls quickly, lots of nuclei decay quickly
Long half life - activity falls more slowly, most of nuclei don’t decay for a long time
What’s 1 becquerel? (Bq)
1 decay per second.
Example how to use a graph to measure half life;
Measure background radiation before, then subtract it from radiation of source + background radiation so graph is source radiation alone.
Describe the uses of radioactivity in medical tracers
Beta and gamma penetrate the skin, and other body tissues - suitable as medical tracers
- A source that emits beta or gamma radiation is injected into patient (or swallowed). Radiation penetrates the body tissues and can be detected externally…as source moves around body, radiographer uses a detector connected to computer screen display to see where it builds up (where is blockage)
- short half life - use less of radioactive source but still get a reading on detector
- alpha would be useless as a medical tracers because it would be stopped by body’s tissue - never be able to detect it externally. And strong ionising power makes it harmful inside the body
Describe the uses of radioactivity in industrial tracers
- looking for leak in an underground pipe
- squirt gamma ray source into pipe, let it flow along, go along the outside with a detector
- it will penetrate through a metal pipe but some of it gets absorbed (how much depends on the thickness of pipe and what it’s made of)
- if crack in pipe, gamma source will collect outside the pipe and defector will show extra high radioactivity at that point
- isotope must be gamma emitter-radiation detected through any earth or rocks surrounding pipe (alpha and beta would be blocked easily)
- short half life - not cause long term hazard if it collects somewhere
Describe the uses of radioactivity in radiotherapy
treats cancer -
- radiotherapy kills cancer cells and stops them dividing.
- high dose of gamma raps, carefully direction at the cells in the tumour, while minimising the dose to the rest of the body
Describe the uses of radioactivity in the radioactive dating of archaeological specimens and rocks:
- work out age of rocks, fossils, archaeological specimens
- by measuring amount of radioactive isotope in a sample and knowing its half life - can work out how long it’s been around
- igneous rocks contain radioactive uranium (v long half life) eventually decays ➡️ stable isotopes of lead. So to find age - relative proportions of uranium and lead isotopes. (Same with igneous rocks also containing potassium-40 which decays to stable argon- compare potassium-40 to trapped argon gas proportion)
Describe the uses of radioactivity in carbon dating;
Carbon-14 makes up about one ten-millionth or carbon in air. (Stays constant in atmosphere)
Same proportion of C-14 in living things. When they die, C-14 trapped in wood, bones, wool etc
C-14 is a beta-emitting radioactive isotope so the C-14 inside them decays over time, radioactivity decreases.
Comparing activity of sample to living tissue lets you make an estimate of how many half lives have passed. Idea of how long ago the animal or plant died.
The ratio of C-14 to C-12 in living materials is fixed, so comparing the ratio in sample can help estimate age.
Dangers of ionising radiations to cells and tissues:
- beta, gamma can penetrate skin, soft tissues to reach organs in body. so more hazardous than alpha when outside body but if they get in, the radiation mostly passes out w/o much damage.
- alpha can’t penetrate skin but danger of gets inside - do all the damage in a localised area. when radiation in body - collide with molecules in cells, collisions cause ionisation, damages or destroys molecules.
- lower doses - minor damage w/o killing cells. This can cause mutations in cells which divide uncontrollably which is cancer
- higher doses kill cells- causes radiation sickness of large part of body affected at same time
- extent of harmful effects depends on how much exposure, it’s energy, it’s penetration
