Lecture 2 Flashcards
What is the optimal ratio of protons to neutrons for a stable nuclei
- around 1
- Goes up to 1.5 with increasing A
If there are too many protons what radiation occurs
- Positron emission
- Increase ratio n/p
- Proton–> neutron + Beta+ + momentum
If there are too many neutrons what radiation occurs
- Beta- decay
- Decreases ratio n/p
- Converts neutron to proton and e-
What is mass defect
- Mass defect is the difference between the actual atomic mass and the predicted mass calculated by adding the mass of protons and neutrons present in the nucleus.
- The actual atomic mass is slightly less than the predicted mass calculated by adding the masses of nucleons.
- The missing mass is called the mass defect
- 𝚫M = mass of nucleons – mass of nucleus
What is an explanation for the mass defect
- The additional mass is accounted for by binding energy that is released when a nucleus is formed.
- When a nucleus is formed, some of the mass is converted to energy and this results in the mass defect.
- Due to this reason, the actual mass of an atomic nucleus is less than the mass of particles it is made up of.
What is Einstein equation for nuclear binding energy
- 𝚫E = (m nucleus - Zmproton - (A-Z)mneutron)c^2
- (A-Z)mneutron) = number of neutrons * mass of neutrons
What is a simpler way of finding nuclear binding energy
- 𝚫E= 𝚫m(in a.m.u) * 931.5 MeV
- 𝚫E= 𝚫m(in a.m.u) * 8.987*10^10 kJmol^-1
What is it important to remember when calculating nuclear binding energy
- Ensure e- mass are included on both sides of the equation or on neither
What radiation leads to the most stable particle
- Alpha radiation leads to the most stable particle- alpha particle
- But it only changes n/p when n/p>1 and leads to an increase of n/p
- Typical for heavy elements - ideal ratio goes up to 1.5
What is the nuclear binding energy per nucleon
- 𝚫E/A
- 𝚫E = binding energy
- A = number of nucleons
- Allows the comparison of the stability of different nuclei
What element has the most binding energy/nucleon
- Iron
- Lighter nuclei have too few nucleons
- Heavier nuclei have too many
What is fission
- Breaking large atoms
- Releases energy as you increase binding energy
- Breaking atoms releases energy
- Binding energy released as thermal energy, electromagnetic radiation
What is fusion
- Combining small atoms
- Energy production in sun
- The process releases energy because the total mass of the resulting single nucleus is less than the mass of the two original nuclei
- Releases more energy than fission
Are even or odd numbers of neutrons and protons preferred
- Even numbers of protons and neutrons are preferred
- 60% of all stable nuclei have even/even P/N numbers
- 20% have even/odd or odd/even P/N numbers
- only 4 nuclei 1% have stable odd/odd combinations - small elements
Which 4 nuclei have stable odd/odd P/N numbers
- 2H
- 6Li
- 10B
- 14N
- after small elements odd/odd = radioactive
What are other point of increased stability
- Additional islands of increased stability around magic numbers of protons or neutrons
- 2 He
- 8 O
- 20 Ca
- 28 Ni
- 50 Sn
- 82 Pb
What is the last stable element
- Bi
- Are radioactive and decay becomes increasingly fast beyond uranium
What are the forces that hold the nucleus together
- Strong interaction
- Electromagnetic force
- Weak interaction
- Gravity
Describe strong interaction
- Strength- 1
- Range 10^-15 - diameter of medium sized nucleus
- particle- glucons, pi(nucleons)
- The strong force is ‘felt’ between nucleons (protons and neutrons) inside of the nucleus of an atom.
- This force is strong enough that it overcomes the repulsive force between the two positively charged protons, allowing protons and neutrons to stick together in an unimaginably small space.
Describe electromagnetic force
- Strength- 1/137
- Range - infinite
- particle- protons, mass=0, spin=1
Describe weak interactions
- Strength- 10^-6
- Range 10^-18m diameter of a proton
- particle- intermediate vector bosons W+, W-, Z0
Describe Gravity
1.Strength- 6*10^-39 (weakest interaction)
2. Range infinite
3. particle- graviton, mass=0, spin=2
What is a quark
- Protons and neutrons are made up of three quarks held together by the strong force
Describe the quarks in a neutron
- 1 up (+2/3e)
- 2 down (-1/3e *2)
When does a neutron decay
- Free neutrons are unstable and decay with a 15 min half-life
- When a down quark splits into an up quark and W- (then e-)
Describe protons in terms of quarks
- 2 up and 1 down
What can happen to a proton of an unstable nuclei
- Stable (half-life >10^33 years)
- Unstable nuclei with excess energy can convert an up quark plus electron to a down quark changing it to a neutron
- p+ e-+energy –>n
- The electron comes from inner shell (electron capture) - heavy atoms
- Or from pair formation with release of a positron- light elements
What do strong interactions determine
- The size of nuclei
Why is Helium-2 (diproton)
unstable
- Contains 2 protons and no neutrons
- Spin-spin interaction introduces a repulsive force especially between ‘like’ nucleons
What are the two nuclear bonding models
- Liquid drop model
- Nuclear shell model
What is the liquid drop model
- Homogenous ‘drop’ of pairs of neutrons and protons
- Electrostatic repulsion in increasing size requires more neutrons for heavier nuclei
- If given sufficient extra energy (as by the absorption of a neutron), the spherical nucleus may be distorted into a dumbbell shape and then split at the neck into two nearly equal fragments, releasing energy
What is the nuclear shell model
- Complex quantum-mechanical treatment of the nuclear particles and forces lead to a shell structure similar to atomic models
- Magic numbers for filled shells - most stable
- Even numbers of nuclei favoured
What is a decay scheme
- A graphical representation of radioactive decay
- Depicts the parent/daughter relationship
- Branching fractions and energy levels are shown
- Excited state is dictated by m next to atomic mass
When does electrostatic repulsion exceed strong force
- > 2 fm
- Residual strong force acts only over about 1fm
What are the two types of nuclear 1reactions
- Nuclear decay reactions
- Nuclear transmutation reactions
Define radioactive decay
- The spontaneous disintegration of a nucleus into a slightly lighter nucleus, accompanied by emission of particles, electromagnetic radiation or both
What is the parent nuclide
- The unstable nuclide
What is the daughter nuclide
- the nuclide resulting from the decay is known as the daughter nuclide which may be stable or may decay itself
What does the radiation produced during radioactive decay do
- The radiation produced during radioactive decay is such that the daughter nuclide lies closer to the band of stability than the parent nuclide
- So the location of a nuclide relative to the band of stability can serve as a guide to the kind of decay it will undergo
What is a nuclear transmutation reaction
- A nucleus reacts with subatomic particle or another nucleus to form a product nucleus that is more massive than the starting material
What is the difference between transmutation and nuclear decay
- Nuclear decay reactions occur spontaneously under all conditions
- Nuclear transmutation reactions occur only under very special conditions, such as the collision of a beam of highly energetic particles with a target nucleus or in the interior of stars - artificially induced
What happens when radiation interacts with matter
- Radiation deposits small amounts of energy ot heat in matter
- Alters atoms
- Changes molecules
- Damage cells and DNA
- Similar effects may occur from chemicals
- Much of the resulting damage is from the production of ion pairs
Define ionisation
- The process by which a neutral atom acquires a positive or negative charge
What is ionisation by an alpha particle
- Electron is stripped from atom by alpha particle (2+ charge)
- The neutral atom gains a + charge = an ion
What is ionisation by a beta particle
- The beta particle ejects an e-
- The neutral absorber atom acquires a positive charge
What is the compton effect
- An incident photon interacts with an orbital electron to produce a recoil electron and a scattered photon of energy less than the incident
When does alpha decay occur
- For very heavy nuclei with mass numbers greater than 200- where nuclei are unstable due to large numbers of nucleons
When does beta decay occur
- Nuclei above the band of stability are unstable as their neutron to proton ratio is too high
When does positron emission occur
- Nuclei below the band of stability are unstable because their neutron to proton ratio is too low
When does electron capture occur
- An alternate way for a neutron poor nuclide to increase its neutron to proton ratio- below the band of stability
When does gamma ray emission occur
- When nuclear particles undergo transitions between nuclear energy levels
When does spontaneous fission occur
- Very massive nuclei with high neutron-to-proton ratios can undergo spontaneous fission
What is the penetrating power of alpha-emission, beta-emission and gamma-emission
- Alpha- mean range approx few mm of air
- Beta- mean range 1mm of aluminium foil
- Gamma- mean range few cm of lead
What is mean range
- mean range = (thickness of material * density) needed to reduce intensity by half multiplied by the density (kg m^-2)
What are the hazards of alpha emissions
- Easily shielded
- Considered hazardous if alpha emitting material is ingested or inhaled
What are the hazards of beta emissions
- Shielded by thin layers of material
- Considered hazardous if a beta emitter is ingested or inhaled
What are the hazards of gamma emissions
- Need dense materials for shielding
- Considered hazardous when external to the body
What are radioelements
- Where only radioactive isotopes known
