8. Nuclear physics Flashcards
(32 cards)
What is the plum pudding model?
A model where atoms are spheres of positive charge, with small areas of negative charge evenly distributed throughout.
How did Rutherford scattering work?
An alpha source (e.g. lead) was aimed at a one atom thick layer of foil in a vacuum chamber of walls which are scintillators (fluorescent coating).
What were the Rutherford scattering findings?
Most alpha particles passed straight through the foil without being deflected, meaning the atom is mostly empty space.
Few alpha particles were deflected at large angles, meaning the centre of the atom is positively charged.
Very few alpha particles were deflected back by more than 90, meaning that the centre of the atom is very dense and very small.
What are the properties of different radiation?
Alpha: travels 2-10 cm in air, is highly ionising, is absorbed by paper.
Beta: travels 1 m in air, is moderately ionising, is absorbed by 3 mm of aluminium.
Gamma: infinite range (following the inverse square law), very weakly ionising, is absorbed by several meters of concrete or inches of lead
How can radiation be used in industry?
With a detector one side and a source the other, radiation can be used to monitor the thickness of certain materials when they are being produced.
How can gamma radiation be used in medicine?
As a detector: a radioactive source with a short half-life (to reduce exposure), which emits gamma radiation, can be injected into a patient and the gamma radiation can be detected using gamma cameras.
To sterilise surgical equipment : gamma radiation will kill any bacteria present on the equipment.
In radiation therapy: gamma radiation can be used to kill cancerous cells in a targeted region of the body such as a tumour, however it will also kill any healthy cells in that region.
The risks are reduced where possible: e.g. reduced exposure times, use of shielding.
Why does gamma radiation follow the inverse square law and how can this e proved?
Because it spreads out in all directions equally.
Intensity = k/distance^2.
This can be found experimentally by measuring the radioactivity of a source at different distances from a geiger muller tube, adjusting for the background radiation and then plotting count against 1/distance^2
What are the risks of radiation and how can they be minimised?
Any radiation can ionise body tissue, potentially causing damage to cells or cell mutations such as cancer.
The risks are minimised by: using long handled tongs to move the source. Storing the source in a lead-lined container when not in use. Keeping the source as far away as possible from yourself and others. Never pointing the source towards others.
What are the sources of background radiation?
Radon gas (released from rocks)
Artificial sources (from nuclear weapons testing etc.)
Cosmic rays entering the Earth’s atmosphere from space.
Why are some nuclei unstable?
Nuclei are held together by the strong nuclear force, however protons experience the repulsive electromagnetic force.
A nucleus could have:
(1) Too many neutrons (beta-minus decay)
(2) Too many protons (beta-plus decay or electron capture)
(3) Too many nucleons (alpha decay)
(4) Too much energy (gamma emission)
What is the pattern of stability?
It is not uniform because after about 20 protons the electromagnetic force of repulsion becomes larger so more neutrons are needed to increase the distance between the protons.
How can the nuclear radius of an atom be estimated using the distance of closest approach of a charged particle?
A charged particle approaching a nucleus will have an initial kinetic energy which can be measured, as it moves towards the positively charged nucleus it will experience an electrostatic force of repulsion and slow down as its kinetic energy is converted to electric potential energy. The point at which the particle stops and has no kinetic energy is its distance of closest approach.
How can the nuclear radius of an atom be estimated using electron diffraction?
Electrons are accelerated and fired through a very thin film so they go between the nuclei and form a diffraction pattern (a set of concentric circles), from which a graph of intensity against diffraction angle can be plot so that the diffraction angle of the first minimum can be found.
Then use the equation sinθ = 0.6λ/R.
How would you interpret a nuclear radius, R, - nucleon number, A, graph?
By taking the log of both sides of R = kA^n
n is 1/3 and k is approximately 1.4 fm and is renamed R naught.
How would you find the density of the nucleus?
ρ = m/V = A x m(nucleon) / (4/3)πR^3 where R = R(naught)A^1/3 => => => m(nucleon) / (4/3)πR^3 = a constant
What is the binding energy?
The energy required to separate the nucleus into its constituents. (or the energy released when a nucleus forms from its constituents i.e. the mass defect)
What is the mass defect?
The difference between the mass of a nucleus and the mass of its individual constituents.
What is one atomic mass unit, 1 u, defined as?
1/12th of the mass of a carbon 12 atom.
What is nuclear fission?
The splitting of a large nucleus into two daughter nuclei. Energy is released during fission because the smaller daughter nuclei have a higher binding energy per nucleon.
What is fusion?
Two smaller nuclei joining together to form one larger nucleus. Energy is released during fusion because the larger nucleus has a much higher binding energy per nucleon.
Fusion releases much more energy that fission, but can only occur at very high temperatures because a massive amount of energy is needed to overcome the electrostatic force of repulsion between nuclei. .
What is the binding energy per nucleon?
The binding energy of a nucleus divided by the number of nucleons.
How can fission be induced?
By firing a thermal neutron into a heavy nucleus, causing it to become extremely unstable.
Thermal neutrons have a low energy, as opposed to neutrons with a higher energy which would rebound away from the nucleus after a collision, which would not cause a fission reaction.
The products if fission are the two daughter nuclei and at least one neutron which can then cause more fission reactions forming a chain reaction.
What is the critical mass?
The minimum mass of fuel required to maintain a steady chain reaction.
(i.e. using less means the chain would eventually stop)
What is the function of a moderator?
To slow down the neutrons released in fission reaction to thermal speeds through elastic collisions between the nuclei of the moderator atoms and fission neutrons.
The closer the moderator atoms are in size to a neutron, the larger the proportion of momentum is transferred.
Water is often used as it contains hydrogen, is inexpensive, and not very reactive.
Graphite is also used upon occasion.
