8.4 - Depth Study

COMPLETED - ALL OF 8.4

nucleon

collective name for protons and neutrons

atomic number

number of protons in an atom

mass number

number of nucleons in an atoms (protons+neutrons)

isotopic notation

Strong Nuclear Force

a force that keeps the nucleus together which only acts across small distances, occurs between nucleons

“nuclear glue”

• range: 2.5 to 3 femtometers, where 1 fm is 10^-15 meters

• note: the force closer than 1 × 10^-15 m is repulsive

nuclear stability

the tendency of a nucleus of an atom to decay

transmute

the process in which an atom decays and changes into something else (a different element)

radioactive

an unstable nuclide that undergoes radioactive decay

nuclide

Nuclides are a class of atoms characterized by their number of protons, Z, their number of neutrons, N, and their nuclear energy state — NOT SAME AS ISOTOPE, can be referring to same or different elements

predictions of nuclear stability

1. neutron to proton ratio

2. the band of stability

3. magic numbers

‘band of stability’

Found on graph of stability for elements of the periodic table. A nuclide falling in the band of stability is stable (not radioactive)

transmutation

the change of one chemical element into another by nuclear decay or radioactive bombardment

spontaneous radioactivity

radioactivity not caused by human intervention or accelerators

Ionisation

the removal of a bound electron from an atom to produce a free electron and a positive ion (THIS IS NOT RADIOACTIVE DECAY)

Radioisotopes

• unstable atoms that undergo nuclear reactions (decay) to become more stable by emitting particles

• version of a chemical element that has an unstable nucleus and emits radiation to become a more stable form

Decay series

Occurs when one radioactive isotope decays into another, but the daughter is also unstable and further decays occur until a stable nucleus is created

Radioactive Decay Law

N_t = N₀ e^-λt

where N₀ = initial number of radioactive particles (t=0)

N_t = number of radioactive particles remaining after time (t)

λ = decay constant (per time unit → determined by sample etc)

Nuclear Fission

a heavy nucleus splits to form two or more lighter nuclei, each of which is more stable than the original nucleus

Moderator

a substance that slows neutrons down to the required speed to cause a nuclear fission reaction

e.g. heavy water, deuterium oxide, graphite rods (most economical)

critical mass

the smallest amount of fissile material needed for a sustained nuclear chain reaction

nuclear reactor fuel

e.g. enriched uranium-235 (BE SPECIFIC) or plutonium-239

enriched uranium-235 has 5-20% of the uranium-235 isotopes and the rest is uranium-239

Control rods

control rate of nuclear reaction by absorbing neutrons (this rate can be adjusted)

e.g. made of steel with boron or cadmium

coolant

controls reaction temperature, can extract heat energy from reactor

can be: water, heavy water, air, helium or liquid sodium

Shielding

protects humans from gamma rays produced during nuclear reaction. shielded by:

• graphite and lead to reflect neutrons

• thick wall of concrete to absorb gamma radiation

equivalence

used to describe mass and energy and how they can be interchangable

Law of conservation of mass-energy (mass-energy equivalence)

E = mc² + kinetic energy

invariant mass

‘rest mass’ — total energy of an object is calculated by its rest mass and ‘increase of mass’ caused by an increase of kinetic energy when moving

relativistic mass

calculated by E = mc² + KE

special theory of relativity

certain types of matter can be created or destroyed, but the total mass and energy associated with such matter remains unchanged in quantity

mass defect

binding energy is proportional to this mass difference

binding energy

the energy required to separate an atomic nucleus completely into its constituent protons and neutrons

• when a nucleus breaks, binding energy is applied (to overcome the strong nuclear force holding it together)

• when components form a nucleus, binding energy is released (lower energy)

fusion

two light atomic nuclei combine to form a single, heavier nucleus (TO BECOME MORE STABLE), releasing immense energy because the resulting mass is less than the original total, converting mass to energy

nucleosynthesis

the creation of new atomic nuclei from existing protons, neutrons, and lighter nuclei, primarily through nuclear fusion — occurs in stars