Chapter 1 - Matter and Radiation Flashcards
What is a nucleon?
A proton or neutron in the nucleus
Charge of a proton
+1.60 x 10^-19C
Charge of a Neutron
0C
Charge of an electron
-1.60 x 10^-19C
Mass of a proton
1.67 x 10^-27kg
Mass of a neutron
1.67 x 10^-27kg
Mass of an electron
9.11 x 10^-31kg
How does an uncharged atom become an ion?
If it gains or loses electrons
What is the proton/atomic number of an atom?
The number of protons in the nucleus
What are isotopes
Atoms of the same element (same number of protons) with a different number of neutrons
What is the mass/nucleon number of an atom
The total number of nucleons (protons or neutrons) in an atom
What is a nuclide
A type of nucleus (e.g. possible nuclei of isotopes of an element)
What is the specific charge of an atom?
Specific Charge (Ckg^-1) = Charge (C) / Mass (kg)
What is the strong nuclear force?
The force that overcomes the electrostatic force of attraction between protons in the nucleus
What does the strong nuclear force do?
Prevents the protons in the nucleus repelling eachother (as they have equal charges) and the nuclei disintegrating to keep the protons and neutrons together
What is the range of the strong nuclear force?
3-4 fm, 1 fm = 1.0 x 10^-15 m
Why is the range of the strong nuclear force significant?
It’s about the same as the diameter of a small nucleus
What is the range of the electrostatic force between two charged particles?
Infinite, although it decreases as the distance increases
What is the effect of the strong nuclear force on different particles?
It has the same effect between two protons as two neutrons or a proton and a neutron
How does the effect of the strong nuclear force change with range?
- Attractive force from 3-4 fm to about 0.5 fm.
- Repulsive force at separations less than 0.5 fm - prevents neutrons and protons being pushed into each other
Composition of an alpha particle
2 protons and 2 neutrons
- proton number 2, mass number 4
- identical to a helium nucleus
What happens to an unstable nucleus of an element when it emits an alpha particle?
- Nucleon number decreases by 4, atomic number decreases by 2
- Product nucleus belongs to a different element Y as the number of protons has changed
Composition of a beta particles
- Fast moving electrons
- Symbol 0,-1β (0 mass number, -1 proton number as charge equal and opposite to that of a proton with a much smaller mass)
- Can also be written as β-
What happens to an unstable nucleus of an element when it emits a beta-minus particle?
- A neutron in a neutron rich nucleus changes into a proton
- Beta particle created as a result of the change and emitted instantly (conservation of charge)
- Antineutrino also emitted to conserve the lepton number
- Atomic number increases by 1 (neutron changes into a proton) but mass number remains the same.
- Product nucleus therefore a different element
Why might beta decay happen?
When a nuclei has too may neutrons (beta-minus decay) or protons (beta-plus decay)
What is gamma radiation?
- Electromagnetic radiation emitted by an unstable nucleus
- Has no mass or charge
How were neutrinos discovered
- When the energy spectrum of beta particles was first measured
- Found that beta particles were released with kinetic energies up to a maximum that depended on the isotope
- Up to a maximum so each unstable nucleus lost a certain amount of energy
- Energy either not conserved or carried away by undiscovered particles, hypothesised to be neutrinos
Why might gamma radiation be emitted from an unstable nucleus?
When the nucleus has too much energy following alpha or beta emission
Speed of electromagnetic waves in a vacuum
3.00 x 10^8 ms^-1
- Speed of light, all em waves travel with the same speed in a vacuum
Wavelength equation
wavelength (m) = wavespeed (ms^-1)/frequency (Hz)
λ = c/f
Wavelength range of a radio wave
> 0.1 m
Wavelength range of a microwave
0.1 m to 1 mm
Wavelength range of an infrared wave
1 mm to 700 nm
Wavelength range of visible light waves
700 nm to 400 nm
Wavelength range of ultraviolet waves
400 nm to 1nm
Wavelength range of X-rays
10 nm to 0.001nm
Wavelength range of gamma rays
< 1 nm
Composition of an electromagnetic wave
An electric wave and a magnetic wave which travel together and vibrate:
- At right angles to each other and to the direction in which they are travelling
- In phase with each other
When are electromagnetic waves emitted
Emitted when a charged particle loses energy, for example:
- When a fast moving electron is stopped, slows down or changes direction
- An electron moves to a different shell of lower energy
What are photons?
- Electromagnetic waves emitted as short bursts of waves, each leaving the source in a different direction
- Each burst is a photon - a packet of electromagnetic waves
Photon energy equation
Photon energy, E = hf
- Energy depends on the frequency of the photon
- Planck’s constant quantises the energy so it can only take certain values
- f = c/ λ, E = hc/ λ
What is a laser beam?
A beam of photons of the same energy
Power of a laser beam
- Energy per second transferred by the photons
- Power of the beam = nhf
- n is the number of photons in the beam passing a fixed point each second
What is antimatter?
Matter consisting of antiparticles of the corresponding particles in ordinary matter
What is an antiparticle?
For every type of particle there is a corresponding antiparticle that:
- Annihilates the particle and itself if they meet, converting their total mass into photons
- Has exactly the same rest mass as the particle
- Has exactly opposite charge to the particle if the particle has charge
How are rest mass and rest energy linked?
Rest mass - The mass of a particle when it’s stationary (mass increases the faster it travels)
Rest mass is rest energy locked up as mass
What is annihilation
When a particle and its corresponding antiparticle meet, they destroy (annihilate) each other and their mass is converted into radiation energy
What happens in annihilation
Antiparticles unlock the rest energy stored as mass in the particles.
Radiation is released as 2 gamma photons to conserve energy - rest energy must be included in conservation of energy
What is pair production?
A photon with sufficient energy passing near a nucleus or electron can suddenly change into a particle-antiparticle pair, which then separate from each other and the photon vanishes.
What is an electron volt?
The energy transferred when an electron is moved through a potential difference of 1 volt.
1 eV = 1.60 x 10^-19 J
1 MeV = 1.60 x 10^-13 J
Why are two photons released in annihilation?
A single photon cannot ensure a total momentum of zero after the collision
What is the minimum energy of each photon produced in annihilation?
hf (min) = E (0)
- Energy of the two photons = 2hf (min)
- Rest energy of the particle and antiparticle = 2E (0), E (0) is the rest energy of the particle
- 2hf (min) = 2E (0)
What is the minimum energy a photon needs for pair production?
Min energy needed for pair production = hf (min) = 2E (0)
- Single photon of energy hf (min) creates particle-antiparticle pair, each of minimum energy E (0)
What is beta-plus decay/positron emission?
- A proton changes into a neutron in an unstable nucleus with too many protons.
- A positron (β+), the antiparticle of an electron, is emitted to conserve charge
- An electron neutrino (v), is emitted to conserve the lepton number
How are positron-emitting isotopes created?
- Manufactured by placing a stable isotope, in liquid or solid form, in the path of a beam of protons
- Some of the nuclei in the substance absorb extra protons and become unstable positron-emitters
- Do not occur naturally
What is a PET scanner?
- Positron Emitting Tomography (PET)
- Used for brain scans
- A positron emitting isotope is administered to the patient and some of it reaches the brain via the blood
- Each positron travels no further than a few millimeters before meeting an electron and annihilation occuring
- Two gamma photons produced as a result, which are sensed by detectors linked to computers
- An image is built up by the detector signals of where the positron-emitting nuclei are in the brain
What is a cloud chamber?
- A small transparent container containing air saturated with vapour and made very cold
- Mimics conditions high in the atmosphere
- Ionising particles condense the vapour and leave a visible trail of liquid droplets when they pass through the air
How were positrons discovered?
- Carl Anderson investigated if particles could pass through a lead plate in a cloud chamber by photographing trails
- With a magnetic field applied to the chamber, the trail of a charged particle would bend in the field
- A positive particle would be deflected by the magnetic field in the opposite direction to a negative particle travelling in the same direction
- The slower it went, the more it would bend
- If a particle went through the plate, he thought it would be slowed down so its trail would bend more afterwards
- However instead he discovered a beta particle that slowed down but bent in the opposite direction to all other beta particles photographed - a positron, the first antiparticle to be detected
What is momentum?
Mass x Velocity
What happens when a single force acts on an object?
It changes the momentum of the object
What happens when two objects interact?
- They exert equal and opposite forces on each other
- Momentum transferred between the objects by these forces if no other forces act on them
How do two like-charged particles interact with one another?
- As they approach each other, they repel and move away from each other
Why do two charged particles repel each other?
- The electromagnetic force between the charged objects
- Electromagnetic force due to the exchange of virtual photons
Why are virtual photons described as virtual?
- We cannot detect them directly
- If we intercepted them, for example with a detector, we would stop the force acting
How are particle interactions represented in a diagram
- Feynman diagram
- Lines do not represent the paths of the particles
- Force carriers represented as a wave between particles
What are the four fundamental forces
1: Strong Nuclear Force - holds protons and neutrons
together in stable nuclei
2: Weak Nuclear Force - Causes beta decay
3: Electromagnetic Force - Acts between objects due to their electric charge, explains all electrical and magnetic effects
4: Gravitational Force - Attraction between any two objects due to their mass, weakest of the fundamental forces
What is the weak nuclear force?
Causes some forms of radiation, decay of unstable particles, and nuclear fusion
What decays are the weak nuclear force responsible for?
- Beta-minus decay - Neutron changes into a proton, electron/beta-minus particle and an antineutrino emitted
- Beta-plus decay - Proton changes into a neutron, positron/beta-plus particle and a neutrino emitted
*New particle and antiparticle created but they’re not a corresponding particle-antiparticle pair
What are W bosons?
- Exchange particle responsible for the weak nuclear force
What are the properties of W bosons?
- Have non-zero rest mass
- Have a very short range at about <0.001 fm
- Can be positively charged (W+ bosons) or negatively charged (W- bosons) - charge conserved in weak interactions through W bosons
What role do W bosons play in beta-minus decay?
- Nuetron changes into a proton and emits a W- boson
- W- boson decays into a β- particle and an electron antinuetrino - conservation of charge and lepton number
What role do W bosons play in beta-plus decay?
- Proton changes into a neutron and emits a W+ boson
- W+ boson decays into a β+ particle and an electron neutrino - conservation of charge and lepton number
Neutrino-Neutron interaction
- A neutrino can interact with a neutron and make it change into a proton
- A β- particle (an electron) is created and emitted as a result of the change
Antineutrino-Proton interaction
- An antineutrino can interact with a proton and make it change into a neutron
- A β+ particle (a positron) is created and emitted as a result of the change
Why can the other fundamental forces not be responsible for beta decay?
- Cannot be due to electromagnetic force as neutrons are uncharged
- Must be weaker than the strong nuclear force, otherwise it would affect stable nuclei
How were W bosons first detected?
- Protons and antiprotons at very high energy were made to collide and annihilate each other at CERN
- At sufficiently high energies, annihilation produces W boson as well as photons
- β particles from the W boson decays detected as predicted
What is electron capture?
- A proton in a proton-rich nucleus interacts through the weak interaction with an inner-shell electron outside the nucleus
- The proton turns into a neutron and emits a W+ boson
- W+ boson changes the electron into an electron neutrino - conservation of charge
- Same change can happen when a proton and electron collide at very high speed
- For an electron with sufficient energy, the overall change could occur as a W- exchange from the electron to the proton
What are Force Carriers/Exchange Particles?
Moves between particles when they interact, carrying the force between them. Transfer:
- Momentum
- Energy
- Charge
Force Carrier for the Strong Nuclear Force and what particles they affect
Gluons (Pions between nucleons)
- Affects Quarks and Nucleons
Force carrier for the Weak Nuclear Force
W bosons
- Affects all particles
What is the force Carrier for the Electromagentic Force and what particles does this effect?
Virtual Photons
- Affects charged particles
Force carrier for Gravity
Gravitons
- Yet to be observed in nature, but must exist in order for gravity to exist
- Acts at infinite distances so must have no mass itself
- Affects all particles with mas
How do virtual photos transfer momentum between repelled particles?
- Particle emits a virtual photon with a force
- Equal and opposite force from the virtual photon pushes back on the particle
- Force of the virtual photon pushes on the second particle when they meet, away from the original particle
How do virtual photons transfer momentum between attracted particles?
- Particle emits a virtual photon with a force
- Equal and opposite force from the virtual photon pushes back on the particle in the direction of the second particle
- Virtual photon (theoretically) loops around like a boomerang to exert a force on the second particle and push it in the direction of the original force
