Module 6: Chapter 24 - Particle Physics Flashcards
How was the nucleus discovered?
Geiger and Mardsen’s Alpha-particle scattering
Explain the alpha particle scattering experiment:
- Geiger and Mardsen set up the apparatus with a thin layer of gold foil as the target layer and zinc sulfide as the detector. All the apparatus was then placed in a vacuum
- Positively charged alpha particles were fired towards a thin sheet of gold foil from a radioactive source. They were first passed through 2 thin lead slits in order to create an approximately parallel beam of particles
- Most of the alpha particles passed straight through the foil, however some were deflected as they passed through and very few were deflected by over 90 degrees.
- The alpha particles were detected by the zinc sulfide screen as when the alpha particle hit the screen, a tiny speck of light could be seen with a microscope
Explain the results of the alpha particle scattering experiment
- Most of the alpha particles passed straight through the foil, this indicates that the atom is mostly empty space
- About 1 in every 2000 alpha particles were scattered
- About 1 in every 10,000 alpha particles were deflected by more than 90 degrees. This could not be due to a collision between the alpha particle and the atom as the alpha particles had an energy of 6 million eV and the vibrational energy of a gold atom is only 0.02 eV, therefore if they did collide the gold atom would just be knocked out of the way. Therefore, the only way that the alpha particle could get this close to the gold atom to be deflected by this amount was if the positive charge was concentrated in a radius of 1x10⁻¹⁴m
What model of the atom was before the alpha scattering experiment?
JJ Thomson’s plum pudding model
What was the plum pudding model of the atom?
The atom consists of a sphere of positive charge with negative electrons embedded within it
Calculate the distance of closest approach to the centre of a gold nucleus (z = 79) if Rutherford used alpha particles of kinetic energy 1.2x10⁻¹² J in his scattering experiment
3x10⁻¹⁴m
What is the radius of an atom?
1x10⁻¹⁰m
How does the radius of the nucleus and atom differ for different elements?
- The radius of the nucleus differs depending on the nucleon number of the isotope of the element, however is around the order of 10⁻¹⁵m
- The radius of the atom is roughly the same for all elements (1x10⁻¹⁰m)
Why were only a small number of alpha particles scattered through large angles?
The chance of getting close to the tiny nuclei of atoms is very small, therefore fewer alpha particles are deflected through large angles
Approximately how large is an atomic nuclei (just to the rough order of magnitude)
1x10⁻¹⁵m
What is a nucleon?
A proton or a neutron
What is A?
Nucleon number
What is Z?
Proton Number
What is X?
Element symbol
What are the units for the masses of atoms and nuclear particles?
Atomic mass units (u)
What is one atomic mass unit (1u)?
One-twelth the mass of a neutrol atom of carbon-12
What is 1 atomic mass unit (1u) in kg?
1.661x10⁻²⁷ kg
What is the equation for the radius of a nucleus?
R = r₀∛A
r₀ = 1.2 fm, A = nucleon number
What is the value of r₀?
1.2 fm (1.2x10⁻¹⁵m)
What is r₀?
The average radius of a nucleon
What value is the prefix femto- (f) for?
1x10⁻¹⁵
Calculate the average density of the nucleus of an atom containing 28 protons and 36 neutrons
1u = 1.66x10⁻²⁷kg
2.29x10¹⁷ kg m⁻³
What is an assumption made when calculating the density of an atomic nucleus?
The nucleons are packed together with little or no empty space
Calculate the average density of the atom of an atom containing 28 protons and 36 neutrons
1u = 1.66x10⁻²⁷kg
2.54x10⁴ kg m⁻³
What is an assumption made when calculating the density of an atom?
- The entire mass of the atom is concentrated in the nucleus
- All atoms have the same approximate radius of 1x10⁻¹⁰m
In a helium-4 nucleus, calculate the electrostatic force of repulsion between the 2 protons at a separation of 10⁻¹⁵m
230N
Calculate the gravitational force of attraction between 2 protons in a helium nucleus at a separation of 10⁻¹⁵m
1.84x10⁻³⁴N
What are the 3 forces acting within the nucleus of an atom?
- Gravitational force
- Electrostatic force
- Strong nuclear force
Describe the nature and range of the gravitational force acting within the nucleus of an atom
- The gravitational force is an attractive force
- It exists between protons and neutrons due to the mass of nucleons in the nucleus.
- It is a long-ranged force and obeys 1/r² relationship
Describe the nature and range of the electrostatic force acting within the nucleus of an atom
- The electrostatic force is a repulsive force between protons in the nucleus due to the positive charge on a proton.
- However, it does not act between neutrons or neutrons and protons.
- It is a long-ranged force and obeys 1/r² relationship
Describe the nature and range of the strong nuclear force acting within the nucleus of an atom
- The strong nuclear force exists between all nucleons
- It is attractive at larger distances and repulsive at very short distances
- It is short ranged ~ 10⁻¹⁴m
What is the range of the strong nuclear force?
0 - 3fm
How does the density of a nucleus depend on the nucleon number?
The density of the nucleus is independent of the nucleon number, the density of all nuclei is roughly the same
Show that the density of the nucleus is independent of the nucleon number
What is the density of an atomic nucleus?
2.3x10¹⁷m
What is an isotope?
Isotopes are nuclei of the same element that have the same number of protons but different numbers of neutrons. All isotopes of an element undergo the same chemical reactions
What is the strong nuclear force?
One of the 4 fundamental forces in nature, acting on hadrons and holding nuclei together
Why does the nucleus not fly apart when the electrostatic force of repulsion is so much greater than the gravitational force of attraction acting within the nucleus?
Due to the strong nuclear force
Sketch a graph of the strength of the strong nuclear force against separation:
When is the strong nuclear force attractive?
0.5fm - 3fm
When is the strong nuclear force repulsive?
0fm - 0.5fm
What is a fundamental particle?
A fundamental particle is a particle which has no internal structure and hence cannot be divided into smaller pieces
Are protons and neutrons fundamental particles?
No, they are made up of quarks
What is the relative strength of the fundamental forces?
- Strong Nuclear
- Electromagnetic
- Weak nuclear
- Gravitational
What is the range of the weak nuclear force?
~10⁻¹⁸m
What is the theory of antimatter?
Every known particle has a corresponding antiparticle. An antiparticle has the same rest mass as it’s particle pair, but has an opposite charge (if the particle has charge) and an opposite baryon number
How do you symbolise an antiparticle?
With a bar over the top of the letter for the particle
EXCEPT THE ANTIPARTICLE OF AN ELECTRON
What is annihilation?
When a particle and its corresponding antiparticle meet, they completely destroy each other in a process called annihilation. The masses of both particle and antiparticle are converted into a high-energy pait of photons
What is the antiparticle of an electron?
Positron
What is the symbol for an electron and for a positron?
- Electron: e⁻
- Positron: e⁺
What is the symbol for a neutron and its antiparticle?
- Neutron: n
- Antineutron: see image below
What are the 2 families of subatomic particles?
Subatomic does not necessarily mean fundamental
- Hadrons
- Leptons
What is a hadron?
Hadrons are particles and antiparticles that are affected by the strong nuclear force. Hadrons decay by the weak nuclear force
What is a Lepton?
Leptons are particles and antiparticles that are not affected by the strong nuclear force.
What is an example of a hadron?
- Baryons (protons neutrons)
- Mesons
What is an example of a lepton?
- Electrons
- Neutrinos
- Muons
What are hadrons made of?
Quarks
What are leptons made of?
Nothing, they are fundamental particles
What are the 2 types of hadrons?
- Baryons
- Mesons
What is a Baryon?
A type of hadron which is made up of 3 quarks
What is a Meson?
A type of hadron which is made up of a quark and an anti-quark
What are the 6 types of quarks?
- Up
- Down
- Top
- Bottom
- Charm
- Strange
What are the 6 types of leptons?
- Electron
- Muon
- Tau
- Electron neutrino
- Muon neutrino
- Tau neutrino
What is the charge of an up quark?
+2/3 e
What is the charge of a down quark?
-1/3 e
What is the charge of a strange quark?
-1/3 e
What is the charge of a charm quark?
+2/3 e
What is the charge of a top quark?
+2/3 e
What is the charge of a bottom quark?
-1/3 e
What is the Baryon number of all quarks?
+1/3
All quarks have the same baryon number
What is the strangeness of quarks?
- All quarks have a strangeness value of 0, EXCEPT
- Strange quarks have a strangeness value of -1
What is the spin of all quarks?
+1/2
All quarks have the same spin
What are the 4 properties of quarks we are interested in?
- Charge
- Baryon Number
- Strangeness
- Spin
Describe what happens to the baryon number in a particle reaction
During a particle reaction, the total baryon number is conserved
What is the baryon number of a meson?
0, all Mesons are unstable
What is strangeness?
A quantum number assigned to particles
What is spin?
A property of particles that describes the angular momentum
What is the quark combination for a proton?
uud
Up Up Down
What is the quark combination for a neutron?
udd
Up Down Down
What is the quark combination of an antiproton?
What are the 2 types of Beta decay?
- Beta-minus (β⁻)
- Beta-plus (β⁺)
Describe what happens in a beta-minus (β⁻) decay
A neutron decays into a proton, emitting a high speed electron and an electron antineutrino
What is the equation for beta-minus (β⁻) decay?
Describe what happens in a beta-plus (β⁺) decay
A proton decays into a neutron, emitting a high speed positron and an electron neutrino
What is the equation for beta-plus (β⁺) decay?
Write the symbol equation for the beta-plus (β⁺) decay of carbon-10
Write the symbol equation for the beta-minus (β⁻) decay of carbon-14
How does a proton transform into a neutron or vice versa?
Through quark transformation
What is the quark transformation in Beta minus decay?
udd -> uud
What is the quark transformation in Beta plus decay?
uud -> udd
What is the force responsible for beta decay?
Weak nuclear force
Predict the quark composition of particle X
An up quark and an up antiquark annihilate each other, leaving udd
What is conserved in particle physics?
- Energy/mass
- Charge
- Momentum
- Baryon/Lepton number
- Strangeness
- Spin
What is the symbol for a neutrino?
ν
What is the weak nuclear force?
One of the four fundamental forces of nature, responsible for inducing beta-decay within unstable nuclei
What are neutrinos?
A lepton that carries no charge and has a tiny mass (less than 1 millionth the mass of an electron)
What does the weak nuclear force act between?
Between quarks and leptons
unlike the strong nuclear force which does NOT act between leptons
What holds quarks together?
The strong nuclear force
What is the baryon number of an antiquark?
-1/3
What are the similarities and differences between a particle and its antiparticle?
Similarities:
* Same rest mass/energy
* Same Spin
Differences:
* Opposite Charges
* Opposite Quantum numbers: Baryon number + Strangeness
Explain why it is not possible for a meson to have a charge of +2e:
A meson is comprised of a a quark and an antiquark. The charges on these particles are ±2/3 e and ±1/3e. Therefore the only possible charges for a meson are +1e, 0, and -1e. Therefore it isnnot possible to have a charge of +2e on a muon
Deduce the quark composition of X:
Strange quark + down antiquark
Why is an electron neutrino emitted in beta plus decay and an electron antineutrino emitted in beta minus decay?
to conserve the lepton number
