Particles & Radiation A level Physics AQA

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- AQA A Level Physics Notes Electromagnetic radiation with no mass or charge Emitted after alpha or beta emission when the nucleus has too much energy Photons are "packets" of EM radiation and can be emitted when: ● ● ● ● ● ● ● ● ● ● O A fast-moving electron is stopped or slows down or changes direction O An electron in a shell of an atom moves to a different shell of lower energy Photon energy E = hf A laser beam consists of photons of the same frequency, and have a power equal to the energy per second transferred by the photons Power = nhf where n is the number of photons in the beam passing a fixed point each second When antimatter and matter particles meet, they destroy each other and radiation is released A positron is the antiparticle of an electron Positron emission is another type of decay where a proton changes into a neutron in an unstable nucleus with too many protons AY+ B+v Positron-emitting isotopes do not occur naturally - they have to be artificially manufactured by placing a stable isotope in the path of a beam of protons Paul Dirac theorised the existence of antimatter in 1928 His theory predicted that for every type of particle there is a corresponding antiparticle that: O O O Annihilates the particle and itself if they meet converting their total mass into photons (E = mc²) Has exactly the same rest mass as the particle Has exactly the opposite charge to the particle (if the particle has charge) The opposite process to annihilation is pair production Pair production is when a photon with sufficient energy passing near a nucleus or an electron can suddenly change into a particle-antiparticles pair, which would then separate from each other O antiparticle photon. particle photon wwwwwww Charlotte Pincher - AQA A Level Physics Notes particle antiparticle a annihilation photon b pair production The minimum energy of each photon produced during annihilation is equal to the rest energy of one of the particles: hfmin = Eo The minimum energy of a photon needed for pair production is equal to twice the rest energy of one the of the particles: hfmin = 2E0 When particles interact, momentum is transferred between them - this is due to the exchange of virtual photons between the particles They are "virtual" photons because they cannot be detected directly - using a detector would stop the force Two protons approaching each other will repel due to electromagnetic force, and this force is due to the exchange of virtual photons ● ● ● ● Р Y n A Figure 1 Diagram for the electromagnetic force between two protons The weak nuclear force affects unstable nuclei and causes decays Some interactions are due to the exchange of particles referred to as W bosons W bosons have a non-zero rest mass, a range of no more than 0.001 fm and have a charge either W or W+ boson W bosons also play a part in beta decay: W time Yvirtual photon p = proton B- p = proton n = neutron PA W+ Charlotte Pincher - AQA A Level Physics Notes B+ a B decay b B* decay Sometimes a proton in a proton-rich nucleus turns into a neutron as a result of interacting through the weak interaction with an inner-shell electron from outside the nucleus - this is electron capture Photons and W bosons are known as force carriers because they are exchanged when the electromagnetic force and the weak nuclear force act respectively W hu a A neutron-neutrino interaction n Р p== proton n=neutron W+ b A proton-antineutrino interaction A Figure 3 The weak interaction B W+ ▲ Figure 5 Electron capture Quarks and Leptons ● Short-lived particles and antiparticles have been discovered using cloud-chambers and other detectors: O Muon (negatively charged particle with a rest mass over 200 times heavier than an electron) Pion (can be negative, positive or neutral and has a rest mass greater than a muon but less than a proton) ● ● ● ● ● ● ● ● O ● O Each type of particle has different properties: Antiparticle and symbol antiproton p antineutron n positron et antineutrino V antimuon u π for a π for a t Particle and symbol proton p Kaon (can be negative, positive or neutral and has a rest mass greater than a pion but less than a proton) neutron n electron e neutrino V muon U" pions π',π,π kaons K*, Kº, K Charge / proton charge +1 0 -1 0 -1 +1, 0,-1 º for a tº +1,0,-1 Charge / proton charge -1 0 +1 0 Charlotte Pincher - AQA A Level Physics Notes +1 -1,0, +1 -1,0, +1 Rest energy/ MeV Neutrinos and antineutrinos produced in beta decays are different from those produced in muon decays There are muon neutrinos and electron neutrinos 938 939 0.511 0 106 140, 135, 140 see Topic 2.4, Quarks and antiquarks Particles can be classified into two groups: hadrons and leptons Hadrons are particles and antiparticles that can interact through the strong interaction (eg. protons, neutrons, pions and kaons) Leptons are particles and antiparticles that do not interact through the strong interaction (eg. electrons, muons and neutrinos) Leptons interact through the weak interaction, the gravitational interaction, and through the electromagnetic interaction (if they have charge) ● Hadrons can interact through all 4 fundamental interactions Apart from the proton, hadrons tend to decay through the weak interaction Hadrons can also be split into baryons and mesons Baryons are protons and all other hadrons (including neutrons) that decay into protons, either directly or indirectly Mesons are hadrons that do not include protons in their decay products, eg. kaons and pions Baryons and mesons are composed of smaller particles called quarks and antiquarks Leptons and antileptons can interact to produce hadrons ● Neutrinos travel almost as fast as light and there are billions that pass through the Earth with almost no interaction 494,498,494 Interaction muons, antimuons, neutrinos, and antineutrinos from pion decay strong, weak, electromagnetic incoming pions decay strong, weak weak, electromagnetic weak weak, electromagnetic strong, electromagnetic (π, π) strong, electromagnetic (K", K-) neutrinos and antineutrinos only absorber (muons and antimuons target absorbed) ▲ Figure 2 Muon and electron neutrinos muons and antimuons created in the target from interacting neutrinos and antineutrinos muons and antimuons and neutrinos and antineutrinos that did not interact ● ● ● ● ● ● ● ● ● Leptons can change into other leptons through the weak interaction and can be produced or annihilated in particle-antiparticle interactions The lepton number is +1 for any lepton, -1 for any antilepton, and 0 for any non-lepton In any interaction, lepton number must be conserved Strangeness is always conserved in a strong interaction, whereas strangeness can change by 0, +1, or -1 in weak interaction The properties of the hadrons (charge, strangeness and rest mass) can be explained by assuming they are composed of smaller particles - quarks and antiquarks There are 6 different types of quarks, but only 3 are needed for now: up, down and strange (u, d and s respectively) charge O strangeness S baryon number B ملنڈ d, W OOO O up u a B decay 2 3 0 Quarks down d - 0 Mesons are hadrons that consist of a quark and an antiquark Baryons and antibaryons are hadrons that consist of 3 quarks and 3 antiquarks respectively Proton: uud Neutron: udd Antiproton: uud A Σ particle is a baryon containing a strange quark The proton is the only stable baryon In ß decay, a down quark changes to an up quark In ß+ decay, an up quark changes to a down quark strange up S u 2 - -²3³3 +1/12 -1 0 0 +1/3 -- - 12 W+ u b B decay Antiquarks Ve down strange d S 4+1/323 +1 o Conservation of baryon number Charlotte Pincher - AQA A Level Physics Notes ▲ Figure 3 Quark changes in beta decay Particles obey certain conservation rules when they interact: O Conservation of energy Conservation of charge Conservation of lepton number Conservation of strangeness - (du) (ds) KO K+ (us) (uu and dd combinations) (ud) (su) (sd) ▲ Figure 2 Quark combinations for the mesons Quantum Phenomena ● ● ● ● ● ● ● When electromagnetic radiation is directed at metals, electrons are emitted from the surface if the EM radiation is above a certain frequency - this is known as the photoelectric effect The minimum frequency of the radiation required to produce the photoelectric effect depends on the metal ;; Therefore, the wavelength of the incident light must be less than a maximum value: λ = The number of electrons emitted per second is proportional to the intensity of the radiation, however the radiation must still be above the threshold frequency in order for the photoelectric effect to occur There is no delay between the radiation hitting the metal and electrons being given off - it is instant regardless of intensity, as long as it is above the threshold frequency Thinking of light as a wave cannot explain the photoelectric effect, as this would imply that each wave hitting the electrons would give them energy and eventually the photoelectric effect would occur regardless of the frequency Einstein put forward a theory that light is actually composed of wave packets of energy, called photons Each photon has energy E = hf where f is the frequency of the light and h is Planck's constant (6.63 x 10-34 Js) The photoelectric effect can thus be explained by saying that an electron on the surface of the metal absorbs a single photon from the incident light and therefore gains the energy that the photon had If, by absorbing this energy, the photon then has more energy than (or the same amount as) the work function of the metal, it can escape the surface of the metal The work function is the minimum energy needed by an electron to escape from the metal surface If the electron has more energy than the work function, the excess energy becomes kinetic energy The maximum kinetic energy of an emitted electron is therefore: Ek max = hf - where o is the work function of the metal Rearranging this equation, the threshold frequency for a metal can be calculated: fmin =&4 The minimum potential needed to stop photoelectric emission is called the stopping potential Vs Robert Millikan confirmed Einstein's photon theory by measuring the stopping potential for a range of metals using light of different frequencies In the 19th century, Max Planck suggested that the energy of each vibrating atom is quantised The energy of each atom has to be a multiple of hf, and they can move up and down levels Conduction electrons in a metal move around at random, and the average kinetic energy of a conduction electron depends on the temperature of the metal When the conduction electron absorbs a photon, its kinetic energy increases by the energy of the photon. If its energy exceeds the work function than the conduction electron can leave the metal, if not it collides repeatedly with other electrons and quickly loses its extra kinetic energy Charlotte Pincher-AQA A Level Physics Notes ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● A vacuum photocell is a glass tube that contains a metal plate (the photocathode), and a smaller metal electrode (the anode) ● Excitation is when an electron in the atom moves from an inner shell to an outer shell Excitation energy is less than ionisation energy as the electron is still remaining in the atom Electrons in an atom are kept there by the electrostatic force of attraction of the nucleus Electrons orbit the nucleus in shells, and the energy of the electrons in each shell is constant ● When light of a certain frequency (or above) is shone at the photocathode, electrons are emitted from the cathode and attracted to the anode This allows the current to be measured and thus the number of electrons per second For a photoelectric current I, the number of photoelectrons per second transferred from the cathode to the anode is equal to where e is the charge of the electron e The current is proportional to the intensity of the incident light Plotting a graph of frequency vs max kinetic energy gives a straight line The gradient of the line will be equal to h and the y-intercept equal to negative work function The x-intercept is equal to the threshold frequency An ion is a charged atom- it has a different number of electrons and protons Adding electrons makes a negative ion and removing electrons makes a positive ion The process of creating ions is called ionisation Alpha, beta and gamma radiation create ions when they pass through substances and collide with the atoms of the substance Electrons passing through a fluorescent tube create ions when they collide with the atoms of the gas or vapour in the tube An electron volt is a unit of energy equal to the work done when an electron is moved through a potential difference of 1 volt Work done qV where q is charge in Coulombs and V is potential difference in Volts 1 eV = 1.6 × 10-1⁹ J Gas atoms can absorb energy from colliding electrons without being ionised - this is known as excitation It only happens at certain energies which depend on the atoms of the gas These energies are called excitation energies The further away from the nucleus that the shell is, the higher the energy is Each shell holds a certain number of electrons: the first 3 are 2, 8, 8 The lowest energy state of an atom is its ground state When one or more electrons are excited, the atom is said to be in an excited state An energy level diagram shows the allowed energy values for the atom When an electron excites and moves to a higher energy level, it leaves behind a gap in that shell which is then eventually filled by another outer electron When the outer electron moves to the lower shell, it must lose energy - it does this by emitting a photon which has energy of the difference between the excited state and the ground state The electron can de-excite either all in one step, or in a series of steps Charlotte Pincher - AQA A Level Physics Notes 10.4, 10- 6- 2 lonisation -0 -2 energy level/ev relative to ionisation level) 0Bround state -10.4 ▲ Figure 1 The energy levels of the mercury atom ● ● ● ● ● ● ● ● ● An electron in an atom can absorb a photon and move to an outer shell where a vacancy exists, but only if the photon has exactly the required energy The photon energy must be equal to the difference in energies between the two levels If the photon has too much or too little energy, it will not be absorbed by the electron The photons emitted during de-excitation sometimes have energies that mean they have a frequency of visible light - this is why some substances "glow" with visible light when they absorb ultraviolet radiation ● The fluorescent tube is a glass tube with a fluorescent coating on its inner surface, and contains mercury vapour at low pressure When switched on, it emits light because: O The mercury atoms ionise and excite as they collide with each other and the electrons in the tube When they then de-excite, they emit UV photons, visible light photons and photons of much less energy The UV photons are absorbed by the atoms in the fluorescent coating, causing excitation of these atoms ● Energy of photon hf = E₁-E₂ Light behaves as both a particle and a wave The photoelectric effect is proof that light has particle-like properties Diffraction of light is proof that light has wave-like properties As light can behave as both a wave and a particle, Louis de Broglie considered that other particles may also be able to behave as waves His hypothesis stated that: O Matter particles have a dual wave-particle nature O The wave-like behaviour of a matter particle is characterised by its de Broglie wavelength, which is related to the momentum of the particle: λ = = h h р mv Evidence to support this theory came when they successfully diffracted a beam of electrons through a thin metal foil They observed a pattern of rings on a screen after the electron beam passed through the foil O O O When these atoms de-excite, they emit visible light photons White light can be split into a continuous spectrum of light of different wavelengths Visible light emitted from elements don't produce a continuous spectrum - only certain wavelengths of light are emitted and by studying these, the elements can be identified If the velocity of the electrons was increased, the diffraction rings got smaller, proving that the wavelength is inversely proportional to the velocity of the particles Charlotte Pincher - AQA A Level Physics Notes