Particles and Radiation Flashcards
Nucleons
Collective term for protons and neutrons
Atoms are made up of three types of particles:
Protons
Neutrons
Electrons
properties of each particle in SI units
Specific Charge
specific charge is defined as:
The ratio of the total charge of a particle to its mass
Specific charge is measured in units of coulombs per kilogram (C kg–1)
The specific charge of the electron = (e / me) = 1.76 × 10^11 C kg–1
The specific charge of the proton = (e / mp) = 9.58 × 10^7 C kg–1
An atom of mass: 24, atomic no: 12 Mg gains 2 electrons.
What is the specific charge of the ion?
Isotopes
Nuclei that have the same number of protons but different numbers of neutrons
Isotopic Data
The relative amounts of different isotopes of an element present within a substance
the strong nuclear force
- an attractive force acting between all nucleons which is stronger than the electrostatic force
- acts between particles called quarks
- Protons and neutrons are made up of quarks, so the interaction between the quarks in the nucleons keeps them bound within a nucleus
- In comparison to other fundamental forces, the strong nuclear force has a very small range (from 0.5 to 3.0 fm)
Properties of the Strong Nuclear Force
The strength of the strong nuclear force between two nucleons varies with the separation between them
This can be plotted on a graph which shows how the force changes with separation:
- The strong force is highly repulsive at separations below 0.5 fm
- The strong force is very attractive up to a nuclear separation of 3.0 fm
- The maximum attractive value occurs at around 1.0 fm, which is a typical value for nucleon separation
- The equilibrium position, where the resultant force is zero, occurs at a separation of about 0.5 fm
Comparison of Electrostatic and Strong Forces
The graph below shows how the strength of the electrostatic and strong forces between two nucleons vary with the separation between them:
- The repulsive electrostatic force between protons has a much larger range than the strong nuclear force - However, it only becomes significant when the proton separation is more than around 2.5 fm
- The electrostatic force is influenced by charge, whereas the strong nuclear force is not. This means the strength of the strong nuclear force is roughly the same between all types of nucleon (i.e. proton-proton, neutron-neutron and proton-neutron). This only applies for separations between 0.5 and 3.0 fm (where the electrostatic force between protons is insignificant)
- The equilibrium position for protons, where the electrostatic repulsive and strong attractive forces are equal, occurs at a separation slightly below 0.5 fm
radioactive decay
When nuclei are unstable, they can become more stable through the process of radioactive decay.
Three of the most common decay mechanisms are:
* Alpha decay
* Beta-minus decay
* Beta-plus decay
Alpha Decay
The decay involves a nucleus emitting an alpha particle and decaying into a different nucleus
An alpha particle consists of 2 protons and 2 neutrons
This is equivalent to a helium nucleus
Alpha decay equation
When an unstable nucleus (the parent nucleus) emits radiation, the constitution of its nucleus changes
As a result, the isotope will change into a different element (the daughter nucleus)
Beta-Minus Decay
A beta-minus, β-, particle is a high energy electron emitted from the nucleus
β- decay is when a neutron turns into a proton emitting an electron and an anti-electron neutrino
Beta-Minus Decay equation
When a β- particle is emitted from a nucleus:
The number of protons increases by 1: proton number increases by 1
The total number of nucleons stays the same: nucleon number remains the same
The new nucleus formed from the decay is called the “daughter” nucleus (nitrogen in the example above)
Beta-Plus Decay
A beta-plus, β+, particle is a high energy positron emitted from the nucleus
β+ decay is when a proton turns into a neutron emitting a positron (anti-electron) and an electron neutrino
Beta-Plus Decay equation
When a β+ particle is emitted from a nucleus:
The number of protons decreases by 1: proton number decreases by 1
The total number of nucleons stays the same: nucleon number remains the same
Neutrino Emission
An electron neutrino is a type of subatomic particle with no charge and negligible mass which is also emitted from the nucleus
The anti-neutrino is the antiparticle of a neutrino
alpha vs beta energy levels
Although the neutrino has no charge and negligible mass, its existence was hypothesised to account for the conservation of energy in beta decay
When the number of α particles is plotted against kinetic energy, there are clear spikes that appear on the graph
This demonstrates that α-particles have discrete energies (only certain values)
When the number of β particles is plotted against kinetic energy, the graph shows a curve
This demonstrates that beta particles (electrons or positrons) have a continuous range of energies
This is because the energy released in beta decay is shared between the beta particles (electrons or positrons) and neutrinos (or anti-neutrinos)
This was one of the first clues of the neutrino’s existence
The principle of conservation of momentum and energy applies in both alpha and beta emission
How did the evidence in the graph below lead Pauli to predict the existence of the (electron anti)neutrino? (image)
Each beta decay releases the same amount of energy ✔ When the beta particle has less than the maximum energy the missing energy cannot be accounted for by the recoil of the daughter nucleus ✔, so there must be another particle to carry away the missing energy ✔
Antimatter
- All particles of matter have an antimatter counterpart
Corresponding matter and antimatter particles have: - Opposite charges
- The same mass
- The same rest mass-energy
rest mass-energy
The rest mass-energy of a particle is the energy equivalent to the mass of the particle when it is at rest
The Photon Model
A massless “packet” or a “quantum” of electromagnetic energy
Each photon carries a specific amount of energy, or “quanta”, and transfers it all in one go, rather than supplying it consistently
Calculating Photon Energy
Planck’s Constant = h = 6.63×10^-34 Js
speed of light = c = 3.0 x 10^8 ms^-1
This equation tells us:
The higher the frequency of EM radiation, the higher the energy of the photon
The energy of a photon is inversely proportional to the wavelength
A long-wavelength photon of light has a lower energy than a shorter-wavelength photon
Annihilation
When a particle meets its corresponding antiparticle ✔, the mass of the two particles is converted to energy in gamma photons ✔, two photons are produced that travel in opposite directions so that momentum is conserved ✔
The two most common particle-antiparticle pairs that are seen are:
Proton-antiproton annihilation
Electron-positron annihilation
The minimum energy of one photon after annihilation is…
Pair Production
Photon (with sufficient energy) interacts with a nucleus ✔, energy of photon is used to create a particle-antiparticle pair ✔, excess photon energy is transferred to KE of particle-antiparticle pair ✔
total energy formula
E(total) = Erest +Ekinetic
The minimum energy required for a photon to undergo pair production is…
The minimum energy required for a photon to undergo pair production is equal to the total rest mass energy of the particles produced:
Worked example
Calculate the maximum wavelength of one of the photons produced when a proton and antiproton annihilate each other.
The Four Fundamental forces / Interactions
The Four Fundamental forces / Interactions - strongest to weakest:
* Strong Nuclear (or Strong Interaction)
* Electromagnetism
* Weak Nuclear
* Gravity
Range and particles affected by the 4 fundamental forces
gravitational:
* infinite range
* only affects particles with mass
electromagnetic:
* infinite range
* only affects particles with charge
weak:
* range of up to 10–18 m
* affects all particles
strong:
* range of ~ 10–15 m
* only affects hadrons
Exchange Particles
When two particles exert a force on each other, a virtual particle is created
Virtual particles only exist for a short amount of time and carry the fundamental force between each particle
repulsive exchange particles analogy
Two people are on skateboards and a ball is passed between them. Due to this, they start to move away from each other. The ball represents an exchange particle creating repulsion
attractive exchange particles analogy
However, if one person throws a boomerang to the other, they will start to move closer together. The boomerang represents an exchange particle creating attraction
Gauge Bosons
Exchange particles that mediate the
four fundamental interactions.
gravity exchange particle
Since gravity is so weak, it only has a noticeable effect on large masses, therefore, gravity does not play a part in particle interactions
The theorised exchange particle for the gravitational force is the graviton, however, this has not yet been discovered
The Electromagnetic Force exchange particle
The electromagnetic force is only between charged particles
The exchange particle that carries this force is the virtual photon, γ
The electromagnetic force is also responsible for binding electrons to atoms
This is due to the attractive force between the negative electrons and positive nucleus
Properties of the photon are:
It has no mass
It has no charge
It is its own antiparticle
Hadrons
Hadrons are the group of subatomic particles that are made up of quarks
Therefore, hadrons can feel the strong nuclear force
two classes of hadrons:
- Baryons (3 quarks) - protons uud and neutrons udd
- Mesons (quark and anti–quark pair) - pions and kaons
two classes of Anti–hadrons:
Anti–baryons (3 anti-quarks) - anti-protons and anti-neutrons
Anti–mesons (quark and anti–quark pair) - anti-pions and anti-kaons
Quarks
- The three most common flavours of quarks are: up, down and strange
- The majority of hadrons are made up of different combinations of these quarks
- Each quark has a charge, baryon number and strangeness
Anti-quarks
These are identical to quarks except with opposite relative charges, baryons numbers and strangeness
Worked example
A K- particle has a strangeness of –1. Determine the quark structure of this particle.
kaon = meson, so it is:
quark and anti-quark.
c = 0
s = -1
Worked example
In the nucleus of Iron 56, 26, Fe, how many ‘up’ quarks are there?
- The number of protons is from the proton number = 26 protons
- Number of neutrons = 56 - 26 = 30 neutrons
- Protons are made up of uud quarks = 2 up quarks
- Neutrons are made up of udd quarks = 1 up quark
- 6 protons × 2 up quarks = 52 up quarks
- 30 neutrons × 1 up quark = 30 up quarks
- Total number of up quarks = 52 + 30 = 82 up quarks
Baryon Number
- The baryon number, B, is the number of baryons in an interaction
- B depends on whether the particle is a baryon, anti-baryon or neither
- Baryons have a baryon number B = +1
- Anti-baryons have a baryon number B = –1
- Particles that are not baryons have a baryon number B = 0
The Proton as a Baryon
- The proton is the most stable baryon
- This means it has the longest half-life of any baryon and is the particle which other baryons eventually decay to
- It is the most stable baryon because it is also the lightest baryon
- Radioactive decay occurs when heavier particles decay into lighter particles
- A decay of the proton would therefore violate the conservation of baryon number
- It is theorised that the proton has a half-life of around 1032 years and research experiments are still underway that are designed to detect proton decay
Pions
- Pions (π–mesons) can be positive (π+), negative (π–) or neutral (π0)
- The anti–particle of the positive pion is the negative pion (and vice versa)
- The neutral pion is its own anti–particle
- Pions are the lightest mesons, making them more stable than other types of meson
- They were originally discovered in cosmic rays and can be observed in a cloud chamber
Pions & The Strong Nuclear Force
- The strong nuclear force keeps the protons and neutrons bound together in a nucleus and is one of the four fundamental interaction. Each of these interactions is caused by a particle exchange
- The pion is the exchange particle of the strong nuclear force
- This means that the strong force is transmitted between a proton and neutron by the exchange of a pion
- Pions are said to mediate (bring about) the strong nuclear force
- The pion created is a temporary violation of energy and mass conservation but since it is a virtual particle, it is not directly observed
gluon & The Strong Nuclear Force
The gluon is also an exchange particle of the strong force. The difference between pions and gluons as mediators of this force are:
* Gluons are responsible for binding quarks together. This is referred to as the strong interaction
* Pions are responsible for binding nucleons together. This is referred to as the strong nuclear force
Collectively, these are referred to as the strong force
Kaons
Kaons (K–mesons) can also be positive (K+), negative (K–) or neutral (K0)
The anti–particle of the positive kaon is the negative kaon (and vice versa)
Kaons can be produced by the strong interaction between pions and protons
Kaon Decay
Kaons are heavy and unstable and normally decay into pions
They are known to have unusually long lifetimes
This is because kaons contain a strange quark and longer lifetimes are characteristic of particles containing strange quarks
Kaons decay through the weak interaction
An example of a kaon decay would be a neutral kaon decaying into a positive pion and negative pion:
κ0 -> π+ + π-
Leptons
Leptons are a group of fundamental (elementary) particles - This means they are not made up of any other particles (no quarks)
Leptons interact with other particles via the weak, gravitational or electromagnetic interactions
They do not interact via the strong nuclear force
The most common leptons are:
- The electron, e–
- The electron neutrino, ve
- The muon, μ–
- The muon neutrino, vμ
er
comparison of masses and chharges of the most common Leptons
- The muon is similar to the electron but is slightly heavier
- The mass of an electron is about 0.0005u, whereas the mass of a muon is about 0.1u
- Electrons and muons both have a charge of -1e
- Neutrinos are the most abundant leptons in the universe and have no charge and negligible mass (almost 0)
Lepton Number
Similar to baryon number, the lepton number, L is the number of leptons in an interaction
* Leptons have a lepton number L = +1
* Anti-leptons have a lepton number L = –1
* Particles that are not leptons have a lepton number L = 0
Muon Decay
Muons (μ–) -> electron (e–) + anti-electron neutrino (ṽe) + muon neutrino (νμ):
Charge:
-1 -> -1 + 0 +0
Electron lepton number, Le
0 -> 1 - 1 + 0
muon lepton number, Le
1 -> 0 + 0 + 1
The numbers on each side of the equation are equal for charge, electron lepton number and muon lepton number, therefore, muon decay satisfies these conservation laws
Quantum Number
A value which describes physical properties of subatomic particles e.g. baryon number, lepton number, strangeness. Quantum numbers can only take a certain set of integer values and must be conserved in particle interactions.
Strangeness
Strangeness is conserved in every interaction except the weak interaction
This means that strange particles are always produced in pairs (e.g. K+ and K–)
exchange particle of the strong interaction
- The exchange particle of the strong interaction is either:
- The pion (between nucleons)
- The gluon (between quarks)
- This means that leptons cannot interact with the strong force, since they are not made up of quarks
The Weak Interaction
- The weak interaction is responsible for the radioactive decay of atoms
- The exchange particle that carries this force is the W–, W+ or Z0 boson
- The type of exchange particle depends on the type of interaction
The Weak Interaction - β– decay
In β– decay, a neutron turns into a proton emitting an electron and an anti-electron neutrino.
The Weak Interaction - β+ decay
In β+ decay, a proton turns into a neutron emitting a positron and an electron neutrino
Electron Capture & Electron–Proton Collisions
Electrons and protons are attracted to each other via the electromagnetic interaction
However, when they interact with each other, it is the weak interaction that facilitates the collision
Both electron capture and electron-proton collisions have the same decay equation
P + e- => n + Ve
Electron Capture
Electron capture is when an atomic electron is absorbed by a proton in the nucleus resulting in the release of a neutron and an electron neutrino
This decay is mediated by the W+ boson
Electron–Proton Collisions
Electron-proton collisions are similar; when an electron collides with a proton, a neutron and an electron neutrino are emitted
This decay is mediated by the W– boson
Feynman Diagrams
Particle interactions and decays can be represented using Feynman diagrams
They are a way of visualising particle equations and the exchange particles involved
Rules for Feynman diagrams
The y-axis represents time and the x-axis represents space
A vertex is where particles and exchange particles meet - these represent points of interaction (e.g. electromagnetic, weak or strong)
Incoming particles come in at the bottom, and outgoing particles leave at the top
Particles are represented by straight lines which have arrows
Each straight line must have an arrow with its direction forward in time
Exchange particles are represented by wavy lines which have no arrows
The transfer of exchange particles is from left to right unless indicated by an arrow above the wavy line
Hadrons/quarks are present on the left and leptons on the right, they must never meet at a vertex
Charge, baryon number and lepton number must be conserved at each vertex
Lines must not cross over
Electromagnetic Interactions
When two electrons approach each other, they experience repulsion due to the electromagnetic force
This can be represented on a Feynman diagram to show the exchange of a virtual photon
Quark Composition: β– decay
β– decay is when a neutron turns into a proton emitting an electron and anti-electron neutrino
More specifically, a neutron turns into a proton because a down quark turns into an up quark
Quark Composition: β+ decay
β+ decay is when a proton turns into a neutron emitting an positron and an electron neutrino
More specifically, a proton turns into a neutron because an up quark turns into a down quark
Application of Conservation Laws - things that need to be conserved
All particle interactions must obey a set of conservation laws. These are conservation of:
Charge, Q
Baryon number, B
Lepton Number, L
Strangeness, S
Energy (or mass-energy = rest energy)
Momentum
However, strangeness does not need to be conserved in weak interactions. It can change by either 0, +1 or –1
Quantum numbers such as Q, B, L and S can only take discrete values (ie. 0, +1, –1, 1/2)
To know whether a particle interaction can occur, check whether each quantum number is equal on both sides of the equation
If even one of them, apart from strangeness in weak interactions, is not conserved then the interaction cannot occur
particles with a strangeness of 1
particles with a strangeness of -1
Σ-, Σ0, Σ+
Particles with strangeness of 0
Pion-, 0, +
And others
k0 vs anti k0 and pi0 vs anti pi0
Wavelength of visible light
380-700nm
3 - 1.5eV
400 - 750 thz
