2. Particles & Radiation Flashcards
Relative mass of a proton?
1
Relative mass of an electron?
0
Relative mass of a neutron?
1
Relative charge on a proton?
1
Relative charge on a neutron?
0
Relative charge on an electron?
-1
What is the nucleon number?
Number of protons + number of neutrons
What letter can be used to represent the nucleon number?
A
What are isotopes?
Atoms with a different number of neutrons, but the same number of protons
What is a radioisotope?
An isotope that is radioactive
What is carbon 14 used in?
Carbon dating
What is the specific charge of a nucleus or ion?
Its charge per unit mass
What is specific charge used in?
Mass spectrometry to identify nuclei
How to calculate specific charge?
Charge / mass
Units for specific charge?
Ckg⁻¹
What is each type of nucleus called?
A nuclide
What is the range of the strong force?
3-4 fm (small)
What is 1 fm in m?
10⁻¹⁵ m
What does the strong force act between?
Nucleons (e.g. protons and neutrons)
Is the strong force attractive or repulsive?
Both
Why is the strong force both attractive and repulsive?
Otherwise the nucleus would collapse or explode
When is the strong force attractive?
> 0.5 fm
When is the strong force repulsive?
< 0.5 fm
For light nuclei, what is the ratio of neutrons to protons?
Proton number = neutron number → the two particles must exist together
For heavy nuclei, what is the ratio of neutrons to protons?
More neutrons than protons (and very large nuclei and radioactive)
What is equilibrium separation?
A point when the resultant force is zero and the attractive and repulsive forces balance
What is the decay of americium-241 used for?
Smoke alarms
What is the decay of polonium-210 used for?
Ionisers
What force is responsible for beta decay?
The weak force
How strong is the weak force?
1 millionth the value of the strong force
How does the range of the weak force compare to that of the strong force?
It has a smaller range
What does the weak force act on?
Leptons and hadrons
What are the types of beta decay?
β+ and β-
When does beta decay occur?
When the nucleus emits an electron or a positron
What does a free neutron decay into in beta decay?
A proton, an electron and an anti-neutrino
What does a free proton decay into in beta decay?
A neutron, a positron and a neutrino
What type of beta decay is it when a free neutron decays into a proton?
β-
What type of beta decay is it when a free proton decays into a neutron?
β+
Why it called β- decay when a neutron decays into a proton?
An electron is produced
Why is it called β+ decay when a proton decays into a neutron?
A positron is produced
What are the energies of the particles emitted in beta and alpha decay?
- beta decay - beta particles emitted have a range of energies
- alpha decay - monoenergetic
What happens to the unaccounted-for energy in beta decay?
It is carried away by the neutrinos
What happens if the nucleus is still unstable after emitting alpha or beta radiation?
It is in an excited state, and gives off gamma radiation
What type of wave is gamma?
Electromagnetic
What is the mass and charge of gamma?
Has no mass or charge
What does the strong force overcome?
The electrostatic forces of repulsion between protons in the nucleus
Why was the existence of the neutrino hypothesised?
To account for conservation of energy in beta decay
What type of particle are neutrinos?
Leptons
What does an electromagnetic wave consist of?
An electric wave and a magnetic wave which travel together in phase
When are electromagnetic waves emitted?
When a charged particle loses energy
When can a charged particle lose energy (and an electromagnetic wave emitted as a result)?
- when a fast moving electron is stopped, slows down or changes direction
- when electrons move to a lower energy shell
In what form is electromagnetic radiation emitted?
Photons - bursts or packets of energy
How do photons travel?
In one direction only in a straight line
What happens to an atom’s energy when it emits a photon?
Its energy changes by an amount equal to the photon energy
What is the amount of energy contained in each quantum proportional to?
The frequency of the radiation
What is the energy of a photon given by?
E = hf
What is ‘h’ in E=hf?
The Planck constant
What is the Planck constant measured in?
joule-seconds, Js
What is photon energy usually given in?
Electron-volts (eV)
What is one electron volt defined as?
The energy transferred when an electron is moved through a p.d. of 1V
What is the value of 1 eV?
1.6 x 10⁻¹⁹ J
What was Dirac’s theory about particles and antiparticles?
For every type of particle, there is a corresponding antiparticle that:
- annihilates the particle and itself if they meet, converting total mass to photons
- has same rest mass and opposite charge
When does annihilation occur?
When a particle and its corresponding antiparticle meet and their mass is converted into radiation energy
What can the rest energy of an antiparticle be calculated from?
By using the rest mass of the colliding particles and E=mc²
What is pair production?
When a photon with enough energy can change into a particle antiparticle pair
What is minimum energy required by the photon in pair production?
The rest energy of the particle pair
What are the four fundamental interactions?
- strong
- electromagnetic
- weak
- graviational
What is the exchange particle for strong interaction?
- gluon (for quarks)
* pion (for nucleons)
What is the exchange particle for electromagnetic interaction?
Photon
What is the exchange particle for weak interaction?
W boson
What is the exchange particle for gravitational interaction?
Graviton
Generally, what happens when two particles interact?
They exert equal and opposite forces on each other
What happens if two protons approach each other?
They repel and move away
Why do protons repel when they approach each other?
Due to the exchange of a virtual photon
What would happen if we tried to intercept virtual photons?
We would stop the exchange from happening
What is the interaction model of repulsive forces?
Two people on skateboards facing each other - throwing a ball between them causes them to move away from each other
What is the interaction model of attractive forces?
Two people on skateboards - throw a boomerang and momentum causes them to move towards each other
In interaction diagrams, what do the straight and wavy lines represent?
- the lines do NOT represent the paths of the particles
* the wavy line shows the exchange particle
What must be conserved in interaction diagrams?
Charge, lepton and baryon number
Describe the interaction diagram between two protons.
- lines show protons approaching
- wavy line shows virtual photon as the exchange particle
- then shows that protons move away
Describe the interaction diagram between a neutron and a neutrino.
- lines show neutron and neutrino approaching
- wavy line shows W⁻ boson as the exchange particle
- then shows that a proton and an electron move away
Describe the interaction diagram between a proton and an anti-neutrino.
- lines show proton approaching an anti-neutrino
- wavy line shows W⁺ boson as the exchange particle
- then shows that a neutron and a positron move away
Describe the interaction diagram for electron capture.
- lines show a proton and electron approaching each other
- wavy line shows W⁺ boson as the exchange particle
- then shows that a neutron and a neutrino move away
Describe the interaction diagram for β- decay.
- line shows neutron
- wavy line shows W⁻ boson as the exchange particle
- then shows that a proton, electron and anti-neutrino move away
Describe the interaction diagram for β+ decay.
- line shows proton
- wavy line shows W⁺ boson as the exchange particle
- then shows that a neutron, positron and neutrino move away
What is the process of electron capture?
When a proton in a proton-rich nucleus turns into a neutron, as a result of interacting with an inner shell electron from outside the nucleus
Why can’t strong or electromagnetic interaction be responsible for beta decay?
- strong force holds neutrons and protons in a nucleus together, but doesn’t cause neutron to change into proton
- electromagnetic force only when a charged particle loses energy → neutron not charged
What do leptons exist as?
Particles on their own
What do quarks exist as?
Only exist bound together
Examples of leptons?
- electron
- electron neutrino
- muon
- tau
Which leptons are constituents of ordinary matter (1st family)?
- electons
* electron neutrinos
Which leptons are only found in cosmic rays and particle accelerators?
- muon
- muon neutrino
- tau
- tau neutrino
Charge on an electron neutrino?
0
Which quarks are part of the 1st family?
- up
* down
Which quarks are part of the 2nd family?
- charm
* strange
Which quarks are part of the 3rd family?
- top
* bottom
Which quarks are protons made up of?
Two up quarks and one down quark
Which quarks are neutrons made up of?
One up quark and two down quarks
Which quarks are antiprotons made up of?
Two antiup quarks and one antidown quark
What is the charge on an up quark?
+ 2/3
What is the charge on a down quark?
- 1/3
What is the charge on an antiup quark?
- 2/3
What is the charge on an antidown quark?
+ 1/3
What is a muon?
A heavier relative of the electron
What is the charge on a muon?
-1
What is a tau?
A heavier relative of the electron and muon
What is the charge on a tau?
-1
What is a strange particle?
A heavier relative of the down quark
What are hadrons?
Particles that feel the strong force
How do hadrons decay?
Weak interaction
What groups are hadrons split into?
- baryons - 3 quarks
* mesons - 2 quarks
Are protons and neutrons fundamental? Why is this?
No, they are made up of quarks
Are protons and neutrons mesons or baryons? Why is this?
Baryons - they are made up of three quarks
What is the only stable baryon?
Protons
What is the pion?
The exchange particle of the strong nuclear force
What do strange particles contain?
A strange quark
How are strange particles produced?
Strong interaction
How do strange particles decay?
Weak interaction
When is strangeness conserved?
Only in strong interactions
What do Kaons decay into?
Pions
Do leptons feel the strong force?
No
What force are leptons affected by?
Weak interaction
What do muons decay into?
Electrons
What can leptons and antileptons interact to produce?
Hadrons
How fast do neutrinos travel?
Almost as fast as light
How can leptons change into other leptons?
Weak interaction
Baryon number on up quarks?
1/3
Baryon number on down quarks?
1/3
Baryon number on strange quarks?
1/3
Baryon number on anti-up quarks?
-1/3
Baryon number on anti-down quarks?
-1/3
Baryon number on anti-strange quarks?
-1/3
Strangeness of an up quark?
0
Strangeness of a down quark?
0
Strangeness of a strange quark?
-1
Strangeness of an anti-strange quark?
1
What do mesons consist of?
A quark and an antiquark
What do pions consist of?
Up and down quarks
What do kaons consist of?
A strange quark and either an up or down quark
What charge can pions have?
Zero charge, or positively/negatively charged
What are the kaon combinations?
- strange-antiup (-1)
- strange-antidown (0)
- antistrange-up (1)
- antistrange-down (0)
When does beta decay occur?
When the nucleus emits an electron or a positron
What is conserved in beta decay?
Charge, spin, baryon number and lepton number
What else needs to be conserved (along with charge, spin, baryon and lepton no.) in particle reactions?
Energy and momentum
What is photoelectric emission?
The emission of electrons from the surface of a metal when it is exposed to electromagnetic radiation of sufficiently high frequency
In photoelectric emission, what is the rate of electrons emitted proportional to?
Rate of electrons is directly proportional to the intensity of the radiation
What kinetic energy are photoelectrons emitted with?
A range of kinetic energies (the max. ↑ with f, and is independent of intensity)
What is threshold frequency in photoelectric emission?
The minimum frequency to produce emission
Why, when a photon causes an electron to be ejected from the surface of the metal, is the energy of the electron always less than that of the incident photon?
Energy from the photon is used to remove the electron
What is the work function?
The minimum energy needed by an electron to escape from the metal surface
Symbol for work function?
Φ
Equation for work function?
Φ = hf₀ (where f₀=threshold frequency)
What type of energy does a photo-electron have when it has absorbed a photon and escaped from the metal?
Kinetic
Equation to calculate maximum KE of photo-electrons?
KEₘₐₓ = hf - Φ
What is stopping potential?
The minimum potential needed to stop photoelectric emission
At the stopping potential, what is the maximum KE of the emitted electrons?
0
How are line spectra formed?
- atoms excited by heating or electrical discharge
- energy raises electrons to higher energy levels
- when electrons fall back to a lower level there is an energy output
Which elements have a line spectra?
All of them
What happens to the energy of electrons in shells as they get further from the nucleus?
Further away from nucleus = higher energy
What is the ground state?
The lowest energy state of an atom
What are excitation energies?
The energies at which an atom absorbs energy
How can electrons be excited?
- electron collisions - doesn’t have to have exact value to move an electron to the next level
- photon absorption - only photons with exactly enough energy between energy levels can be absorbed
How does a fluorescent tube emit light?
- ionisation and excitation of mercury atoms as they collide with each other and with electrons in the tube
- mercury atoms emit UV photons
- UV photons absorbed by fluorescent coating, causing excitation of atoms
- coating atoms de-excite in steps and emit visible photons
What is a fluorescent tube?
A glass tube with a fluorescent coating on its inner surface
What does a fluorescent tube contain?
Mercury vapour at a low pressure
Why do energy levels have negative energy values?
0 is considered to be that of a free electron just outside the atom, so all the energy states below this are negative
How to calculate the increase in energy difference of a transition?
hf = E₁ - E₂
What is the difference between line spectra and band spectra?
- line spectra - formed by hot gases as atoms in gas isolated and so have minimal interaction
- band spectra - solids and liquids have atoms close together so interaction between neighbouring atoms means there’s a large number of lines close together so appear to be bands
What did De Broglie propose?
That electrons, which we thought were particles, can be waves also
How does electron diffraction occur?
- beam of electrons strike thin layers of graphite carbon - most pass through, others pass through at certain angles only, giving rings
- rings like interference maxima (when light waves pass through diffraction grating)
- this shows electrons are diffracted by the gaps between atoms, and give maxima on the screen when they’re in phase
What size did De Broglie suggest that the wavelength of electron waves are?
Very small, about the size of an atom
How big do the separation of the slits in a diffraction grating for electrons have to be?
Very small - about the size of an atom
What is De Broglie’s equation?
mv = h / λ
What is the setup for electron diffraction?
- beam of electrons produced by attracting them from a heated wire filament to a positively charged metal plate
- plate has a hole in, electrons pass through
What changes the size of diffraction rings electron diffraction?
An increase in velocity of electrons will decrease de Broglie wavelength and so make the rings smaller
