E.3 Radioactive Decay Flashcards
What happens to unstable nuclei?
They spontaneously decay emitting particles and energy to transform into the nuclei of another element.
Alpha particle (what is it, symbol, range, penetration, ionisation)
Helium nucleus. Range 5cm in air. Penetration - stopped by paper. Very high ionisation.
Why is an alpha particle so good at ionising?
High kinetic energy and positive charge
Beta negative particle (what is it, symbol, range, penetration, ionisation)
An electron emitted from the nucleus (neutron to a proton), range is 30cm in air, stopped by 1mm of aluminium, low ionising ability, accompanied by an antineutrino
Beta positive particle (what is it, symbol, range, penetration, ionisation)
A positive electron or a positron (proton to a neutron), range is 30cm in air, stopped by 1mm of aluminium, low ionising ability, accompanied by a neutrino
Gamma radiation (what is it, range, penetration, ionisation)
Electromagnetic radiation, range is hundreds of metres, stopped by 10cm of lead, very low ionisation and always accompanies alpha and beta radiation
Can we predict radioactive decay?
No. It is random (we don’t know which nucleus will decay or when). It is spontaneous (not influenced by external factors)
Rate of radioactive decay
Exponentially decreases
Half life
Time taken for half of the nuclei in a radioactive sample to decay (or the activity to fall to half of the original level)
Bq
Becquerel, 1Bq means 1 radioactive nucleus decays per second
What does half life depend on?
Only the particular radioactive isotope - it does not depend on external factros.
What is the name of the new nuclei formed in radioactive decay?
Daughter nuclei
What rules must radioactive decay equations follow?
Conservation of mass - the total mass number must be the same. Conservation of charge - the total atomic number on each side must be the same
How do you account for background radiation?
Background radiation value must be subtracted from any measurements of radioactivity of an experimental sample
Natural background radiation
Cosmic (from space), terrestrial (rocks and soil), internal (food and air)
Artificial background radiation
Medical (xray, radiotherapy, PET scan), industrial (air travel, inside work), nuclear fall out (weapon testing)
Uses of medical tracers
Radioisotopes are used to trace the flow of blood. Short half life (a few hours), emit gamma radiation, less ionising than beta
Radiotherapy (external beam therapy)
Radioactive source is outside the body and radiation is directed towards the tumor. Gamma is used, penetrates deep, number of low intensity rays directed to overlap
Radiotherapy (brachytherapy)
Radioactive source can be put into the body in or near a tumor. Half life of a few days. Gamma or beta.
Checking for leaks in a water pipe
Gamma emitting contaminate water pipes, when there is a leak, water seeps into the ground and the build up of gamma sources. Gamma (penetrate the ground), half life of several days
Controlling thickness of paper or aluminium
Emitter placed on one side of a sheet and a detector on another. Change in thickness - squeeze harder/less. Beta source is used as can penetrate, long half life of a few years so count rate remains constant
What does radioactive dating do?
Activity levels of the radioactive isotope can be used to determine the age of the object
Carbon dating
Living organisms have C-14. During their lifetime, the level of C-14 remains constant as it exchanges carbon with the environment. After death the level decreases. By measuring the level, an estimate can be made.
Why can’t carbon-14 be used to date anything older than 60,000 years?
It has a half life of 5730 years so activity level will have fallen to a level too low to be accurately measured.
Nuclear fission
Large nuclei can break apart into smaller nuclei. Occur spontaneously or when nuclei are bombarded with neutrons.
Nuclear fusion
Small nuclei can be combined to form larger nuclei if temp and pressure is high enough to overcome electrostatic repulsion.
Why do fusion and fission release large amounts of energy?
Mass is destroyed and converted to energy
Mass and energy in a nuclear reaction
Mass is destroyed and converted to energy but total mass+energy remains constant.
Unifed atomic mass unit
Equivalent to the mass of 1/12 of the mass of a carbon-12 atom.
Binding energy
The amount of energy required to break apart a nucleus into seperate nucleons
Mass defect
The difference in mass between the total mass of individual nucleons and their total mass when bound in the nucleus
Why is there a mass defect?
Because protons and neutrons lose energy when they are bound in a nucleus and the loss of energy creates a loss of mass
BEPN
Binding energy per nucleon (larger the nucleus, the greater the binding energy)
