Nuclear Physics Flashcards
What is the experiment to show the existence of a small charged nucleus in an atom?
- positively charged alpha particles are incident on a thin gold foil in a vacuum chamber
- majority went straight through or deviated by small angles, indicating that the nucleus is very small compared to the size of an atom and the atom is mainly made of empty spaces
- some deflected at more than 10°, indicating that the nucleus is positively charged
- a very small proportion of alpha particles are deviated at large angles and came straight back, indicating that the mass of nucleus is massive and highly concentrated in a very small charged volume
Why are beta particles not used in Rutherford’s scattering experiment?
- beta-particles have a range of energies and are easily deviated by the orbital electrons of a gold nucleus
- hence unable to interact with nucleus and deduce that nucleus is positively charged
- beta particles are too light so unable to interact with nucleus and deduce that nucleus is massive
How to ensure the accuracy of Rutherford’s experiment?
- ensure that gold foil is as thin as possible to limit the number of collisions between alpha particles and gold nuclei
- experiment to be done in vacuum to avoid collisions between alpha particles and air molecules
- use target element of high atomic number to ensure large scattering of alpha particles
What affects the degree of scattering of alpha particles?
The greater the charge of the target nuclei, the stronger the repulsive electrostatic force exerted on the approaching alpha particle and the greater the degree of scattering
Wht is very thin gold foil used in Rutherford’s experiment?
- prevent too many alpha particles from being absorbed
- allow alpha particles to be scattered only once
Is the interaction between alpha particle and gold nucleus considered an elastic collision?
- as alpha particle apporaches the nucleus, kinetic energy of the alpha particle is converted to electrical potential energy
- when they separate, potential energy is converted back to kinetic energy of the alpha particle and the nucleus
- kinetic energy of the system remains constant before and after the experiment
- thus the interaction is considered elastic
What is an atomic mass unit?
1/12 of the mass of a Carbon-12 atom
What is nuclear force?
Strong attractive force that binds protons and neutrons together in nucleus
What are the characteristics of nuclear force?
- identical between proton-proton, proton-neutron, neutron-neutron
- extremely short range, 1 to 3fm
Why is the density of the nucleus independent of atomic number?
Nuclear attractive force is much larger than the electrostatic forces of repulsion between protons in the nucleus
What is mass defect?
The difference between the total mass of separate nucleons and the mass of nucleus
What is binding energy?
The energy to separate nucleons in a nucleus completely to infinity
Why is nuclear reactions being able to produce large amounts of energy not a violation of conservation of energy?
- some mass from reactant is converted into energy and released
- total mass-equivalent energy in the system is conserved
What is binding energy per nucleon?
total energy needed to completely separate all the nucleons in a nucleus divided by the number of nucleons in the nucleus
What is the relationship between binding energy per nucleon and nucleus stability?
The higher the binding energy per nucleon, the greater the nucleus stability
What is the trend of BE per nucleon for light elements?
- increasing binding energy per nucleon
- the increase in attractive nuclear forces per nucleon outweights the increase in electrostatic forces of repulsion between protons
What is the trend of BE per nucleon for large elements?
- decreasing BE per nucleon
- electrostatic forces of repulsion between protons are beginning to overcome attractive nuclear forces between nucleons
What is conserved in nuclear reactions?
- proton number
- mass number
- mass-energy
- momentum
What is nuclear fission?
Splitting of heavy nucleus into two or more light nuclei of approximately the same mass, with emission of a few neutrons and/or other radiations
What is a nuclear chain reaction?
a series of similar reactions in which one of the products of a nuclear reaction causes further reactions to occur
How can fission be controlled?
absorbing excess neutrons using water/boron/cadmium sheets
What is nuclear fussion?
combination of two light nuclei to form a more massive nucleus, with possible emission of other radiations
What are the conditions required for fusion and fission to occur?
- fusion: high temperature
- fission: bombardment of small electrons (minimal KE)
What are the advantage/disadvantage of fusion/fission?
- fusion produces radioactive waste that has very short half life, less hazardous
- fission produces radioactive waste, hence hazardous
- fusion release more energy than fission
- fusion energy released is not controllable while fission energy can be controlled by absorbing neutrons away using control rod
- fusion is more efficient than fission
What is the energy conversion that takes place during fusion?
- when fusion occurs, mass of products is less than the mass of reactants
- difference in mass, the mass defect in the form of binding energy is converted and released as KE of products and gamma radiations
What is radioactivity?
Process in which a radionuclide spontaneously decompose to form an atom with a more stable nucleus by emitting particles and EM radiation
What happens to the right of the stability region?
neutron-proton graph
- nuclides have too many protons relative to neutrons
- repulsion is greater than nuclear attractive force, hence nucleus tear apart by emitting positron
What happens to the left of stability region?
neutron-proton graph
- nuclides have too many neutrons relative to protons
- nuclides decay to convert neutrons to protons via beta- decay
What is radioactive decay?
A spontaneous and random process where an unstable nucleus emits alpha or beta particles and gamma radiations, forming a more stable nucleus
What is random decay?
- Inability to predict which and when a nucleus will decay next
- nucleus have a constant probability of decay per unit time
What is spontaneous emission?
Rate of decay of nuclei which is unaffected by external factors such as temperature and pressure
basically it occur on its own
What are the energy changes that takes place in a nucleus undergoing radioactive decay?
- an unstable nucleus decays by emitting alpha, beta and possibly gamma radiations
- difference in rest mass is converted to energy release
- total binding energy of products is larger than the parent nucleus, releasing more energy during the formation of daughter nuclei than it absorbs during the decay of nucleus
- energy is released to the surroundings in the form of KE of products and possible gamma rays
What is ionising power?
Ability of radiation to remove electrons from atoms
What is pentrating power?
Ability of the radiation to pass through a material before being completely absorbed
What is the relationship between mass, ionisation energy and penetrating power?
- the greater the mass
- the lower the penetrating power
- the greater the ionising power
What can alpha particles be stopped by?
A piece of paper
What can beta particles be stopped by?
A few mm of aluminium
What is gamma particles stopped by?
10cm of lead
What is the path of deflection of alpha and beta in a magnetic field?
- alpha: slight deflections in a circular path
- beta: strong deflections in circular path
What is the path of deflection of alpha and beta particles in e-field?
- alpha particles have slight deflection in parabolic path
- beta particles have strong deflections in parabolic path
What is decay constant, λ and units?
- probability of decay of nucleus per unit time
- units: s⁻¹
What is activity, A and units?
- rate of decay of radioactive nuclei
- units: Bq
Waht is half life?
Average time taken for inital activity to reduce to half of its original value
What does the spontaneous decay of nuclear decay mean?
- the process cannot be speeded by or slowed down by physical means such as changes in temperature or pressure
- decay is not affected by any chemical condition or chemical compound it exists in
What does the random nature of nuclear decay mean?
impossible to predict which nucleus and when any particular nucleus will disintegrate
What is background radiation?
radiation detected by a radiation counter when no radiaoactive sources are nearby
What are alpha particles?
helium 4 nuclei
What are beta particles?
energetic electrons
What are gamma rays?
Electromagnetic waves with wavelengths shorter than those of x-rays
What are the biological effects of ionising radiation?
- ionising radiation can ionise atoms by knocking out electrons, its structure could be altered such that it does not perform a normal function or performs harmful functions
- For proteins, large doses of radiation may damage so many cells that new copies cannot be made quickly enough and the cell dies
- damage to DNA is more serious as a cell may only have one copy
What is the difference between direct and indirect cell damage?
- direct cell damage is when radiation interacts with the DNA of a cell directly causing it to disintegrate
- Indirect cell damage occurs when radiation strikes the cytoplasm surrounding the nucleus, this causes the formation of free radicals which ultimately cause cell damage
