Particle & Nuclear Physics
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109 Terms
What was the Plum Pudding Model of the atom?
A model where electrons were embedded in a uniform sphere of positive charge
Who proposed the Plum Pudding Model?
J. J. Thomson
What experiment disproved the Plum Pudding Model?
Rutherford’s alpha-particle scattering experiment
What particles were used in Rutherford’s experiment?
Alpha particles
Why was the gold foil very thin in Rutherford’s experiment?
To allow alpha particles to pass through individual atoms
Why was the experiment carried out in a vacuum?
To prevent alpha particles being absorbed by air
What detected the scattered alpha particles?
A ring of scintillation detectors
What observation showed atoms are mostly empty space?
Most alpha particles passed straight through undeflected
What caused small-angle deflections in Rutherford’s experiment?
Repulsion between alpha particles and the nucleus
What observation showed the nucleus is small and dense?
A few alpha particles were deflected by large angles or backscattered
What conclusion did Rutherford make about nuclear charge?
The nucleus is positively charged
What conclusion did Rutherford make about nuclear mass?
Most of the atom’s mass is concentrated in the nucleus
What model replaced the Plum Pudding Model?
The Rutherford (planetary) model
What particles orbit the nucleus in the Rutherford model?
Electrons
What is an isotope?
Atoms of the same element with different numbers of neutrons
Why do isotopes have the same chemical properties?
They have the same electronic configuration
What is a nucleon?
A proton or a neutron
What is the charge of a proton?
+e
Why is an atom electrically neutral?
It has equal numbers of protons and electrons
What is the atomic number Z?
The number of protons in the nucleus
What is the mass number A?
The total number of nucleons
What is the approximate mass of a nucleon?
1 atomic mass unit
What is 1 atomic mass unit?
One twelfth of the mass of a carbon-12 atom
What is the approximate radius of an atom?
~10⁻¹⁰ m
What is the approximate radius of a nucleus?
~10⁻¹⁴ m
How much smaller is the nucleus than the atom?
About 10,000 times smaller
Why is nuclear density very high?
Large mass is contained in a very small volume
What is the approximate nuclear density?
~10¹⁷ kg m⁻³
What relationship describes nuclear radius?
R = r₀A¹ᐟ³
What is the value of r₀?
~1.2 fm
What force causes alpha particle deflection in Rutherford scattering?
Electrostatic repulsion
How were nuclear radii determined experimentally?
Using scattering and diffraction experiments
How many fundamental forces exist?
Four
What are the four fundamental forces?
Gravitational, electromagnetic, weak nuclear, strong nuclear
Which force holds nucleons together in the nucleus?
The strong nuclear force
Why do protons not repel the nucleus apart?
The strong nuclear force overcomes electrostatic repulsion
What is the range of the strong nuclear force?
~10⁻¹⁵ m
What force causes beta decay?
The weak nuclear force
Which force acts between charged particles?
The electromagnetic force
Which force acts between masses?
The gravitational force
State Einstein’s mass–energy equivalence equation
ΔE = Δmc²
What does mass–energy equivalence imply?
Mass and energy are interchangeable
What is binding energy?
The energy required to separate a nucleus into its nucleons
Why is nuclear mass less than the sum of nucleon masses?
Due to binding energy
What is mass defect?
The difference between nucleon mass sum and nuclear mass
Why is binding energy often quoted per nucleon?
To compare nuclear stability
Which nucleus has maximum binding energy per nucleon?
Iron-56
Why does fusion release energy for light nuclei?
Binding energy per nucleon increases
Why does fission release energy for heavy nuclei?
Binding energy per nucleon increases for smaller fragments
What is antimatter?
Particles with same mass but opposite charge to matter
What happens when matter meets antimatter?
Annihilation producing energy
What is pair production?
A high-energy photon creating a particle–antiparticle pair
What minimum energy is required for electron–positron pair production?
1.02 MeV
What are hadrons?
Particles made of quarks affected by the strong force
What are leptons?
Fundamental particles not affected by the strong force
Give examples of leptons
Electrons, neutrinos, muons
What is baryon number conservation?
Total baryon number remains constant in interactions
What is lepton number conservation?
Total lepton number remains constant in interactions
What is radioactivity?
Spontaneous random decay of unstable nuclei
Why is radioactive decay unpredictable?
It is a random process
What is half-life?
Time for half the nuclei in a sample to decay
What are the three types of radiation?
Alpha, beta, gamma
What is an alpha particle?
A helium nucleus
What is the charge of an alpha particle?
+2
Which radiation is most ionising?
Alpha
Which radiation is most penetrating?
Gamma
What stops alpha radiation?
Paper
What stops beta radiation?
Aluminium
What stops gamma radiation?
Lead
What detector measures activity?
Geiger–Müller tube
What is activity?
The rate of decay of nuclei
What is the unit of activity?
Becquerel
What is the decay constant?
Probability of decay per unit time
State the activity equation
A = λN
State the exponential decay equation
N = N₀e⁻ˡᵗ
How is half-life related to decay constant?
t₁ᐟ₂ = ln2 / λ
Why must background radiation be subtracted?
To obtain true source activity
What is alpha decay?
Emission of an alpha particle from a nucleus
How does alpha decay affect A and Z?
A decreases by 4, Z decreases by 2
What is beta-minus decay?
Neutron changes into a proton, electron and antineutrino
What changes occur in beta-minus decay?
Z increases by 1, A unchanged
What is beta-plus decay?
Proton changes into a neutron, positron and neutrino
What changes occur in beta-plus decay?
Z decreases by 1, A unchanged
What causes gamma decay?
Excess nuclear energy
Does gamma decay change A or Z?
No
What is the belt of stability?
Region of stable nuclei on an N–Z graph
Which nuclei are neutron-rich?
Left of the belt of stability
Which nuclei are proton-rich?
Right of the belt of stability
What decay occurs for neutron-rich nuclei?
Beta-minus decay
What decay occurs for proton-rich nuclei?
Beta-plus decay
Which nuclei commonly undergo alpha decay?
Very heavy nuclei (Z > 82)
What is radiocarbon dating used for?
Estimating age of organic materials
Which isotope is used in radiocarbon dating?
Carbon-14
What is the half-life of carbon-14?
~5700 years
Why does C-14 decrease after death?
No further intake and radioactive decay continues
What limits radiocarbon dating accuracy?
Background radiation and small C-14 amounts
What is nuclear fusion?
The joining of small nuclei releasing energy
Where does fusion occur naturally?
In stars
Why are high temperatures needed for fusion?
To overcome electrostatic repulsion
What fusion process occurs in the Sun?
Proton–proton chain