Particles and Radiation

Question 1

The Atom

Answer 1

The atom has a positively charged nucleus composed of protons and neutrons - the nucleus is surrounded by negatively charged electrons

• proton number (atomic number) - number of protons in nucleus
• nucleon number (mass number) - number of protons and neutrons
• isotope - same number of protons but different number of neutrons

Question 2

Forces in the Atom

Answer 2

Strong Nuclear Force

In order to prevent nuclei from breaking apart, there must be a force stronger than the electrical repulsion force, to hold the nucleus together.

• 100 times stronger than electrical force
• very short range of 3-4 fentometres
• if the electrical force overcomes the nuclear force then a state of matter may escape, or the nucleus may break into smaller nuclei “nuclear fission”

Electrical Force (short range attraction)

The positively charged protons exert an electrical force of repulsion on each other

• infinite range

Question 3

Unstable Nuclei

Answer 3

Any element past lead on the periodic table is an unstable nuclei as it is too big

• wrong ratio of protons to nuetrons, so the electrostatic force exceeds the range of the strong nuclear force

Unstable nuclei can undergo decay by

• Alpha decay
• Beta decay
• Gamma decay

Question 4

Alpha Decay

Answer 4

When a nucleus emits an alpha particle, it loses two neutrons and two protons, and due to changes to mass number and atomic number, it becomes a different element.

Question 5

Beta Decay

Answer 5

When an atom has too many neutrons, the atom will emit a beta particle

• its mass will be unchanged since the neutron has decayed and emitted an electron with negligable mass
• the proton number will increase by 1
• the nucleus has the same mass but becomes a new element

Question 6

Gamma Emission

Answer 6

Gamma rays are photons (fundamental particle of light) of short-wave electromagnetic radiation and are emitted when a nucleus has too much energy

• gamma rays don’t have mass or charge so the nuclide stays the same but in a more stable state

Question 7

The Neutrino

Answer 7

Neutrons were hypothesised to account for conservation of energy laws

• observations showed that the energy of particles was less after beta decay
• a neutrino is a neutral particle that carries away missing energy

Anti-neutrinos were detected as a result of their interaction with cadmium nuclei in a large tank of water, installed next to a nuclear reactor, as a controllable source of the particles

Question 8

Fundamental Interactions & Exchange Particles

Answer 8

Gravitational

Acts between all particles with mass and is responsible for holding planets in orbit around the sun - only responsible for very large masses

Electromagnetic (virtual photon)

Acts between all charged particles and is the binding force of atoms and molecules

Weak Force (W⁺, W^-, Z⁺, Z^-)

Responsible for radioactive decay and change in quark flavour, acts between all particles and seen in lepton reactions

Strong Force (pions, gluons)

Holds neutrons and protons together in a nucleus, only acts between hadrons since they contain quarks

Exchange Particles

These are particles that are passed between the two interacting particles and ‘carry’ the force between them

When an electron repels another electron, they both emit a photon, and this photon exchange is what carries the force to push them apart

Question 9

Antiparticles

Answer 9

For every type of particle, there is a corresponding antiparticle which has the same mass and rest energy but an opposite charge and spin

Antiparticles are created by pair production and destroyed by annihilation

Pair Production

Each particle-antiparticle pair is produced from a single photon

• it can only occur if one photon has enough energy to produce enough mass for a particle-antiparticle pair
• only gamma photons fulfill these requirements
• happens near the nucleus to conserve momentum
• when energy is converted into mass, you get equal amounts of matter and antimatter

minimum energy needed = total rest energy of particles produced

E_min = hf_min = 2E_o

Annihilation

When a particle meets its antiparticle, the result is annihilation, and all the mass is converted back into energy

• both photons need to have the minimum energy E_min which makes 2E_o for energy to be conserved
• momentum is conserved
• each photon has half the energy produced
• the positron is more stable

2E_min = 2E_o

E_min = hf_min = E_o

Question 10

Photons

Answer 10

Photons are packets of EM radiation

The energy of a photon depends on the frequency of the radiation

E = hf = hc/λ​

Photon Model

All objects at a temperature above zero emit a range of wavelengths, but the peak energy radiation curve moves towards the short-wavelength, high frequency end as the temperature is increased

• more energy is emitted as short-wave radiation
• two curves represent objects at two different temperatures

Laser Power

• laser beams are made of photons of the same frequency
• the power of the laser is the energy transferred per second by the photons

power of beam = n x hf

Question 11

Electromagnetic Force

Answer 11

When objects interact they exert an equal and opposite force on each other

e. g. Repulsion
1. each time the ball is thrown or caught, the people move further away due to momentum caused by the exchange of virtual photons (ball)
e. g. Attraction
1. each time the boomerang if thrown or caught, the people get closer

Question 12

Weak Interaction

Answer 12

In the weak interaction that governs β⁺ and β​^- decay, electron-proton collisions and electron capture, the exchange particles are the W⁺ and W^- bosons

In β^- decay, a W^- boson is the exchange particle, and an antineutrino is emitted to carry away charge and momentum

In β⁺ decay, a W⁺ boson is the exchange particle, and a neutrino is emitted to carry away charge and momentum

Question 13

Electron Capture and Electron Proton Collisions

Answer 13

Electrons and Protons are attracted by the electromagnetic interaction between them, but if a proton captures an electron, the weak interaction force causes this interaction to happen

p + e^- → n + V_e

• a W⁺ boson is the exchange particle used
• a neutrino is emitted

When an electron collides with a proton, a W^- boson goes from the electron to the proton instead

Question 14

Electromagnetic Repulsion

Answer 14

When two particles with equal charges get close to each other, they will repel

If an electron gets too close to an electron, they will repel

If a positron gets too close to a positron, they will repel

A virtual photon acts as the exchange particle

Question 15

Hadrons

Answer 15

Hadrons can either be baryons or mesons and they feel the strong nuclear force

• protons need the strong force to hold them together
• only hadrons feel the strong nuclear force
• they are made up of smaller particles called quarks
• they are classified into baryons and mesons by the number of quarks they have

Question 16

Baryons

Answer 16

Protons and neutrons are baryons - all baryons are unstable except for protons

• baryons are made up of 3 quarks
• all particles except protons decay into a proton
• n → p + e^- + ^_v
• the number of baryons in an interaction is the baryon number
• protons and neutrons have a baryon number of B =+1
• antibaryons have a baryon number of B = -1
• the baryon number is conserved in all reactions, if not conserved, the reaction can’t happen

Question 17

Mesons

Answer 17

All mesons are unstable and have baryon number B=0

• pions are the lightest mesons and come as π⁺, π^-, π⁰
• lots of pions are involved in high energy particle collisions
• kaons are heavier and more unstable than pions and come as K⁺, K^-
• they have a short lifetime and decay into pions

Question 18

Leptons

Answer 18

Leptons don’t feel the strong nuclear force and only interact via weak interaction

• if they become charged they might interact via the electromagnetic force
• electrons and muons are leptons
• electrons are stable
• muons are unstable and decay into electrons
• leptons each have their own neutrino which only take part in weak interaction
• they have a lepton number of +1 but electrons and muons have lepton numbers of L_e and L_μ​
• their antiparticles have an opposite charge to their lepton numbers
• lepton number is conserved

Question 19

Strange Particles

Answer 19

Strange Particles are produced through strong interaction and decay through the weak interaction e.g. Kaons

Strangeness is conserved in strong interaction but can change in weak interaction

• strange particles are always produced in pairs so that an overall strangeness of 0 is conserved

Question 20

Quarks and Antiquarks

Answer 20

Quarks are the building blocks for hadrons

• baryons have 3 quarks
• mesons have 2 quarks
• there are 3 types of quarks that all have a corresponding antiquark

proton: uud

neutron: udd

Question 21

Neutron Decay

Answer 21

Weak interaction changes quark flavour

• β^- decay changes a d quark into an u quark
• β⁺ decay changes an u quark into a d quark

Question 22

Conservation Laws

Answer 22

Charge is always conserved

Baryon number is always conserved

Strangeness is conserved in strong interactions - it only changes quark flavour in weak interaction

Lepton numbers have to be conserved separately

Question 23

The Photoelectric Effect

Answer 23

The threshold frequency is the minimum frequency that will cause electron emission from a given material

If UV light is shone onto the surface of a metal with a negative charge, electrons will be emitted

1. Free electrons on the surface of the metal absorb energy from the light - the free electrons are held in a ‘hole’ in an electric field called the potential well, energy releases them from this well
2. If the electron absorbs enough energy, the bonds holding it to the metal will break and the electron is released - emitted electrons are called photoelectrons

No photoelectrons will be emitted if the light has a frequency lower than the threshold frequency

Energy carried by photons = hf = hc/λ

• it is assumed that the photon would transfer all energy to one specific electron

1. when light hits the surface, the metal is bombarded with photons
2. if a photon collides with a free electron, the electron gains energy equal to hf

work function (Ø)= energy needed to break bonds to release photoelectron

hf = Ø + E_k(max)

Question 24

Stopping Potential

Answer 24

This is the potential difference needed to stop fast moving electrons with E_k(max)

E.g. V is increased so that no more electrons can reach the detector

• emitted electrons will have to do work against the applied pd

work done = energy carried

eVs = E_k(max)

Question 25

Photoelectric Equation

Answer 25

The kinetic energy of an electron when it leaves the metal is hf minus the energy lost on the way out

Electrons deeper in the material will lost more energy

hf = Ø + E_k(max)

E_k = 1/2 mv²_max

Question 26

Ionisation and Excitation

Answer 26

Electrons are spread through energy levels and can only exist in this way

1. if an electron absorbs a photon with the exact energy difference between two levels, it will move up an energy level

The movement of an electron to a higher energy level is called excitation

If an electron is removed from an atom it has been ionised

• the ionisation level is the level to which an electron will be raised if the collision with the incoming electron provides enough energy (ionisation energy)
• removal of electrons will leave atoms as positive ions

Question 27

Fluorescent Tube

Answer 27

1. the fluorescent tube contains mercury vapour, across which a high voltage is supplied
2. the voltage accelerates fast moving photoelectrons that ionise the mercury atoms, producing more free electrons
3. when the flow of free electrons collides with electrons in other mercury atoms, the electrons in the mercury atoms are excited to a higher level
4. when the electrons return to their ground state, they emit photons in the UV range
5. A phosphor coating absorbs the photons, exciting its electrons to a higher orbit which then emit lower energy photons in the form of visible light

Question 28

Electron Volt

Answer 28

An electron volt is the energy gained by an electron when it is accelerated through a potential difference of IV

1eV = 1.6 x 10^-19 J

Question 29

De Broglie Wavelength

Answer 29

If wave like light showed particle properties, particles like electrons should be expected to show wave-like properties

The De Broglie equation relates a wave property (wavelength) to a particle property (momentum)

It is the probability of finding a particle at a point that is directly proportional to the square of the amplitude of the wave at that point

λ​ = h/mv

Question 30

Wave Particle Duality

Answer 30

Wave

• light produces interference and diffraction patterns
• can only be explained by waves interfering destructively or constructively

Diffraction shows wave nature of electrons

• diffraction patterns can be seen when accelerated electrons in a glass tube interact with spaces in a graphite crystal
• the spread of lines in a pattern increases if the wavelength is greater
• in experiments, a smaller accelerating voltage gives more widely spaced rings
• the increase in electron speed + momentum causes the diffraction pattern circles to squash together towards the centre
• greater momentum = shorter wavelength and smaller spread of lines

Particle

• the photoelectric experiment showed light as a series of particle like photons
• if a photon of light is a bundle of energy, then it interacts with an electron in a one-to-one way
• all the energy in a photon is given to one electron

Acceptance of nature of matter

• the idea was first hypothesised to explain the observation of light acting as a particle + wave
• the DB theory had to be evaluated before it was published and tested
• understanding of wave particle duality has changed over time but the DB theory will be accepted until conflicting evidence comes along