Lectures 1, 2 and 3 Flashcards
What does the nucleus contain
Protons and Neutrons
What defines the chemical element
Number of protons
What defines the isotope of the element
Number of neutrons
What determines the chemical bonding behaviour of the element
Electrons
What determines whether an element can be radioactive
The relative number of protons and neutrons, which determines the stabiity of the nucleus
What forces create nucleus instability
Coulombic repulsions and short-range attractions
What occurs when the attractive forces outweigh the repulsive forces
Stability
What happens when the repulsive forces outweigh the attractive forces
nucleus loses stability and spontaneously disintegrates, emitting particles and/or elecromagnetic radiation
what is the term that describes the quantitative measure of nuclear stability
nuclear binding energy
what is the principal factor for determining nucleus stability and what is it
neutron to proton ratio
the energy required to split a nucleus into its componenet protons and neutrons
What ratios of n:p create instability
ratios greater than 1
Why does the band of stability veer off of the N=Z line
At higher atomic number a larger number of neutrons is needed to counteract the strong repulsions between protons
What is the process of rearranging in the structure of the nucleus commonly referred to
Radioactive decay
What are the 5 types of ionising radiation
- Alpha
- Beta
- Gamma
- Positron emission
- Electron capture
What does alpha decay emit
A helium nucleus is emitted from the nucleus
What does beta decay emit
a high speed electron/positron as well as a neutrino
What does the conservation of mass and charge mean
The sum of the mass numbers and charges before and after the radioactive decay must be the same
What dies a becquerel (Bq) describe
Radioactive decay - one bequerel is equal to one decay per second
What is half life
The time taken for half of the initial number of nuclei to disintegrate in a radioactive substance
t1/2 = 0.693/k
k - decay rate
What is decay rate (k)
The speed at chich a substance disintegrates
ln(N/N0) = -kt
N - nuclei remaining
N0 - nuclei initially
k - decay rate
t - time
How can you use half lives to measure the age of rocks, and therefore the earth
By measuring the ration of daughter to parent isotopes with in the rock, this tells you how man half lives have passed and then if you know how long a half life is you can date the rock
How long is the U-238 half-life
Approximately the age of the Earth
What is the final, and stable, daughter of U-238
Pb-206
What is ‘Blocking Temperature’
The temperature at which the atomic clock is ‘reset’
How does ‘Blocking Temperature’ work
If Igneous and Metamorphic rocks are heated to high enough temperatures, they no longer act as ‘closed systems’. Some of the daughter products ‘leak’ out of the primary mineral via diffusional migration. This changes the ratio of daughter to parent and gives an incorrect age for the rock.
What are discordant dates
The incorrect dating of rocks due to blocking temperatures resetting isotopic clocks
How is Lead used in dating
Ratios of the three radiogenic lead isotopes to the non-radiogenic lead-204 all change BUT at DIFFERENT RATES. These ratios can be used to date rocks
How does Carbon-14 dating work
In the upper atmosphere Nitrogen is cosmically bombarded so that it emits a proton and becomes C-14. C-14 is radioactive with a half life of 5,730 years.
Plants and animals ingest C-14 while they are alive. When they die they stop taking it in. The C-14 clock then begins to count down and by looking at ratios of C-14 in plants and animals, their age can be discerned.
What is fission track analysis
When an atom of U-238 disintegrates and emits an alpha particle (He), the massive Helium nucleus causes structural damage to the crystal which can be revealed by chemical etching
How does the dating of metamorphic events work
During a metamorphic event there is some redistribution of the daughter atoms out of the crystal into adjacent rock. Dating the crystal would then reveal the age of the metamorphic event, wheras dating the whole rock would provide the original age of the crystal and rock.
What is nuclear binding energy (in terms of how can you find it)
It is the difference between the sum of the masses of protons and neutrons in the nucleus - it in the energy that holds the nucleus together
What equation do you use to find nuclear binding energy
E = mc2
What is fission
The splitting of a large nucleus into smaller peices
What is fusion
The joining of two nuclei at extrememly high temperatures and pressures
Do we do fusion or fission right now
Fission
Does the sun do fusion or fission
Fusion
What is MeV
Megaelectron volts
What is a nucleon
A proton or neutron - for example if you were to count all of the nucleons in a Helium, there would be 4 (2 protons and 2 neutrons)
How to calculate the energy given from fission or fusion
The binding energy multiplied by the number of nucleons (things in the nucleus)
What is the most likely cause of fission
The absorption of a neutron which disturbs the nuleus structure.
What does fission produce
two fission nuclei (smaller), 2-3 free neutrons and energy (e.i. heat)
What are the factors that are important in inducing fission
Speed of the impacting neutron
Structure of the nucleas being impacted
What can a fast neutron do to U-238
Transmutation - it absorbs it, beta decays and becomes neptunium-239
What happens if a fission reaction is uncontrolled
It becomes a ‘runaway nuclear reaction’ as the split neutrons continue to hit other nuclei - the basis for atomic weapons
What are used as moderators in nuclear reactors and why
Water, D2O (heavy water) and Graphite
They slow the released neutrons from fission via collisions, so that the neutrons travel at a speed that allows for the continuation of fission (instead of the neutrons wizzing around and passing through everything)
What are ‘thermal neutrons’
Moderated neutrons
Can nuclear fuel be none fissile
Yes BUT it must be transmuted via the gain of a neutron to become a fissile isotope
Why does fission produce energy
The mass of the two fission fragments is less than the mass of the reactant, this mass difference is converted to energy
Can be calculated by E = mc2
Why does fusion produce energy
The mass of an atomic nucleus is always smaller than the sum of all of its protons and neutrons (binding energy) - so the mass difference (or mass defect) is the energy released
What is the ‘Rest Mass’
the ‘rest mass’ of an object is the inertial mass that an object has wehn it is at rest (perfectally stationary) - the MINIMUM MASS
What is sum for mass defect
= the sum of the masses of protons and neutrons - the rest mass
What are the best fuels for fusion and what is the problem with them
Heavy isotopes of Hydrogen - deuterium and tritium
There is a limiting supply of tritium currently and new ways of making much more need to be developed
What are the 4 benifits of fusion over fission and why
U-238 is not used and since it is only a fertile material and not a fissile material, this reduces costs as well as the number of radiactive isotopes produced its transmutation
Pu–244 - a very long half-life radioactive isotope and which can be used in the production of nuclear weapons
He is produced which is inert and not radioactive compared to the wide range of radioactive isotopes created via fission
A loss of coolent (LOCL) accident is much less likely to occur due to the strict conditions that fusion requires are not maintained, meaning chance of meltdown is much less likely
What happens if fusion occurs with elements heavier than Iron
Energy is consumed
Consumption (lifespan) stages of the Sun (the elements combined or created)
H - He - C - O - Si
How do we get elements heavier than Iron
Supernovae
At the end of the lifespan of a very large star (Supergiant), when there is nothing less to fuse a supanova will typically occur causing a very large explosion
What happens during a Supernova
Flood of neutrons is created and elements heavier than Fe are formed via the rapid capture of neutrons on seed nuclei at rates greater than disintegration through radiactivity
