1.4 - PARTICLES AND ANTIPARTICLES Flashcards
What happens when antimatter and matter particles meet?
Annihilation.
All mass from the particle and antiparticle gets converted back to energy in the form of two gamma ray photons
What must the particles be for annihilation?
Corresponding particle and antiparticle
How long do antiparticles last before they are annihilated?
Only exist for a fraction of a second before this happens, so you don’t usually get them in ordinary matter.
How do we make use of the effect of Annihilation?
in a positron emitting tomography (PET) hospital scanner
What happens when a PET scanner is used for a brain scan?
a positron-emitting isotope is administered to the patient and some of it reaches the brain via the blood system
How long does each positron travel before it meets an electron?
each positron emitted travels no further than a few millimeters before it meets an electron
What happens when the positron and electron meet?
they annihilate each other
What is produced due to the annihilation?
two gamma photons
What senses the two gamma photons produced?
detectors linked to computers
What is gradually produced from the detector signals?
an image is built up from the detector signals of where the positron-emitting nuclei are inside the brain
How do you distinguish gamma rays due to annihilation compared to other gamma rays?
Always produced in pairs moving in the opposite directions
When does positron emission take place?
when a proton changes into a neutron in an unstable nucleus with too many protons
What charge does a positron carry?
positive charge
Why does the positron carry a positive charge?
as it’s the antiparticle of the electron
What is the symbol for the positron?
β⁺
What else is emitted during the positron emission?
a neutrino
What is a neutrino’s antiparticle?
antineutrino
What is the charge of the neutrino?
uncharged
What is the symbol of the neutrino?
v
Neutrino’s are so small, what can you assume about them?
zero mass
zero rest energy
Are positron-emitting isotopes naturally occurring?
no
Which one of Einstein’s equations relate the energy supplied to the particle to its increase in mass?
E = mc^2
What did Einstein most importantly say?
the rest mass (when stationary) of a particle corresponds to rest energy locked up as mass.
Energy can turn into mass, Mass can turn into energy.
For every type of particle with a corresponding antiparticle they (3 rules about what antiparticles and particles have/don’t have in common, 1 rule about what happens when they meet)
annihilate if they meet, converting their total mass into photons
have exactly the same rest mass
have exactly the same rest energy
have exactly opposite charges to the particle (if they have a charge)
What process is opposite to the annihilation process
pair production
Where does pair production happen?
When a high enough energy photon interacts with an orbital electron / nucleus / atom.
What is pair production?
Energy is converted to mass…
you get equal amounts of matter and antimatter
When does pair production only happen?
If there is enough energy to produce the masses of the particles.
Why can’t pair production occur if the photon’s frequency is too low
Energy of photon depends on frequency.
If energy/frequency is below a certain value…
…there is not enough energy to provide mass/rest energy of particles.
What must pair production produce in order to keep certain quantities conserved?
particle and it’s corresponding antiparticle
Why must a photon produce a particle and antiparticle pair?
To conserve momentum the photon needs to interact with interacting particles
What often happens when photons are passing near a nucleus?
a photon with sufficient energy passing near a nucleus or an electron can suddenly change into a particle-antiparticle pair, which would then separate from each other.
Usually electron - positron pair
How does the positron and the electron produced in a detector move away from each other after pair production?
curve away from each other due to the applied magnetic field they’re in and they have opposite charges
Where does excess energy from pair production go?
To the kinetic energy of the particle + antiparticle?
How is the energy of a particle or antiparticle often expressed?
in millions of electron-volts :
Mega Electron-volts
What is the unit for electron volts?
MeV
How many joules does 1 MeV equal to?
1 MeV = 1.60 * 10^-13 J
What is one electron volt defined as?
the energy transferred when an electron is moved through a potential difference of 1 volt
Why are two photons produced in annihilation
A single photon cannot ensure a total momentum of zero after the collision.
Two gamma ray photons going in opposite directions conserve momentum
What is the minimum energy of each photon from annihilation equated to?
The energy of the two photons equate to the rest energy of the particle and of the antiparticle
How do you calculate the minimum energy of a photon produced in annihilation?
Interaction = between a particle and an antiparticle.
Both have a rest energy of E0.
The two photons need to have a total energy of at least 2E0 …
… for energy to be conserved.
Emin = minimum energy of photons produced.
E0 = rest energy of particle type annihilated.
2Emin = 2E0
Emin = E0
What is E0 measured in?
MeV
What happens in pair production?
a photon creates a particle and a corresponding antiparticle, and vanishes in the process
What is the minimum energy needed for pair production?
Total rest energy of the particles that are produced.
What is the rest energy of the particle the same as in pair production?
The rest energy of the particle is just the amount of energy that would be produced if all of it’s mass was transformed into energy.
What is the minimum energy needed for pair production (explain the equation)
Produces an antiparticle and particle…
Both have rest energy, E0 …
Minimum energy needed is at least 2E0 …
…for there to be enough energy to produce the particles.
Emin = 2E0
Rest energy of electron = 0.510999 MeV.
what is the min energy needed to produce an electron positron pair?
2 x 0.5109999 = 1.021998 MeV
Why are electron - positron pairs more common?
relatively low mass = low rest energy = less energy needed for pair production.
How can the path of alpha and beta particles be seen?
using a cloud chamber
How did the speed affect the particle?
the slower it went, the more it would bend
At what speed do all electromagnetic waves travel in a vacuum?
at the speed of light
What is the speed of light?
3.00 * 10^8ms^-1
Equation for wavelength
λ = v/f
wavelength = velocity/frequency
In what measurement is light wavelengths often expressed in?
nanometres (nm)
How is an electromagnetic wave emitted?
as short bursts of waves
How does each burst of EM wave leave from the source?
each burst leaves the source in a different direction
What is each burst of EM wave?
a packet of electromagnetic waves
What is each burst of EM wave referred to as?
photon
What is the photoelectric effect?
emission of electrons from a metal surface when light is directed at the surface
What did Einstein assume the energy E of a photon depend on?
its frequency f in accordance with the equation:
photon energy E = hf
What is the h in the equation?
a constant referred to as the Planck constant
How else can you write E = hf?
E = hc/λ
What is the value of h?
6.63 *10^-34 Js
What does a laser beam consist of?
photons of the same frequency
