particles and radiation Flashcards
Approximate size of an atom
radius of 0.1nm
define ‘isotope’
atoms of an element with the same number of protons but different number of neutrons
what is isotopic data? what is its use?
The relative amounts of different isotopes of an element found within a substance. Can be used to approximate the age of archaeological finds
How was it deduced that the strong force must exist?
The electromagnetic force acts within the nucleus causing repulsion between the similarly charged protons. This is much stronger than the gravitational force acting between all nucleons (due to their mass). If no other forces were present, nucleons would fly apart
Range of the strong force?
repulsive at very short range <0.5fm
attractive up to 3fm
negligible afterwards
(very short range)
1fm
1 * 10^-15m
What particles experience the electromagnetic force?
charged particles
What particles experience the strong nuclear force?
hadrons
range of the electromagnetic force
infinite
what is nuclear decay
unstable nuclei emitting particles to become more stable
describe alpha decay
When the nuclei of an atom is too big for the strong nuclear force to keep them stable
they emit an alpha particle (He)
range of an alpha particle
short range - few cm in air
How can we observe the range of an alpha particle?
Geiger counter - bring it close to a source of alpha radiation and move away. watch count rate drop after a few cm
cloud chamber - alpha particles leave tracks
describe beta minus decay
occurs in neutron rich isotopes
the nucleus ejects a beta particle ( fast-moving electron ) and in turn a neutron in the nucleus becomes a proton
an antineutrino is released and carries away some momentum and energy
range of beta particle
can travel up to a several metres through air
What led to the hypothesis of neutrinos?
to account for conservation of energy
kinetic energies of beta particles varied up to a maximum
had to be neutral so charge was conserved
had to have almost zero mass as it hadn’t been detected
Electromagnetic spectrum
radio, micro, IR, visible, UV, x-rays, gamma
frequency
number of complete waves passing a point per second
wavelength
distance between adjacent crests of wave
the higher the frequency of electromagnetic radiation…
the greater its energy
photon energy {}
E = hf E = hc/λ
define work function [2]
Energy required to remove an electron
Minimum energy required to remove an electron from a (metal) surface
What is meant by an antiparticle [2]
Particle with equal (rest) mass/energy
but opposite charge/baryon number/lepton number
how does an antiparticle compare with its corresponding particle?
They have the same mass and rest energy
opposite charge
What does the formula E=mc^2 refer to?
pair production
energy converted into mass, equal amount of matter and antimatter are produced
what are photons?
packets of EM radiation
When in an applied magnetic field why do corresponding particles of matter and antimatter curve ?
They have opposite charges hence they curve away from each other
How do you calculate the minimum energy needed for pair production?
total rest energy of the particles produced
Emin = 2E0
1ev in J
- 1.6 * 10^-19
1Mev in J
- 1.6 * 10^-13
Why are electron-positron pairs more likely to be produced than proton antiproton pairs?
They have a relatively low mass
low mass -> low rest energy
so less energy required for pair production
Mev -> ev
- 10^6
what is annihilation
when a particle meets its corresponding antiparticle
all mass of the particle and the antiparticles are converted back to energy ( 2 gamma ray photons )
How do you calculate the energy of the gamma ray photons produced in annihilation?
energy of the pair = 2E0
produces two gamma ray photons
so each gamma ray photon will have energy E0
What is an application of annihilation?
PET scanners
positron emitting isotope in bloodstream
positrons will annihilate with electrons giving off gamma rays which can easily be detected (they also move in opposite directions so can be distinguished)
What are the four fundamental forces?
weak
strong nuclear force
electromagnetic
gravitational
what is a hadron
a particle that experiences the strong nuclear force
( subject to the strong interaction )
made up of quarks
what are the two types of hadrons and what are they made up of
baryons
3 quarks
mesons
quark antiquark pair
What do all baryons eventually decay to and why?
All baryons can be unstable except protons, so they eventually decay to become a proton which is stable
what is baryon number?
a quantum number that must be conserved
it is the number of baryons
protons, neutrons have baryon number +1
antibaryons -1
other particles 0
neutron decay
n –> p + e~ + V~
weak interaction
what are mesons?
are all unstable
have baryon number 0 , quark anti-quark pair
they interact with baryons via the strong force
what are the lightest mesons
pions
exchange particle of the strong nuclear force
π±, π0
What are the heavier mesons
kaons
have a strange quark
have a short lifetime and decay into lighter pions
K±, K0
waffle about cosmic rays detecting mesons
high energy particles from space known as cosmic rays interact with molecules in the atmosphere and produce mesons
these leave tracks in a cloud chamber
what are leptons
fundamental particles
don’t feel the strong nuclear force, interact via the weak interaction
( and em and g a bit, em if charged )
which leptons are stable and unstable
electrons are stable leptons
unstable leptons include muons, tauons, . they are essentially heavy electrons
they eventually decay into electrons
describe neutrinos
both electrons and muons have their constituent neutrinos,
and antineutrinos
they have virtually zero mass
no charge
neutrino - lepton number +1
antineutrino - lepton number -1
lepton electron number and lepton muon number
lepton number of a muon antineutrino
electron lepton number 0
muon lepton number -1
what are strange particles?
how are they created
have a property called strangeness
created via the strong interaction in pairs
when is strangeness conserved
not in the weak interaction
strangeness of K+ K-
+1 -1
via which interaction do strange particles decay
weak ( strangeness isn’t conserved )
what properties need to be conserved in particle interactions?
charge
energy
momentum
baryon/lepton number
strangeness ( not in weak )
what are quarks
fundamental particles
they are the building blocks for hadrons
baryon number of the quarks
up 1/3
down 1/3
strange 1/3
everything else 0
strange number of the quarks
up 0
down 0
strange -1
everything else 0
charge of the quarks
up +2/3
down -1/3
strange -1/3
which quarks make up pions
[π0] up, anti-up
[π0] down, anti-down
[π+1] up, anti-down
[π-1] down, anti-up
what are the four possible kaons
[K-] anti up strange
[K0] antidown strange
[K+] up antistrange
[K0] down antistrange
is it possible to have just 1 quark
quark confinement is not possible
pair production occurs instead
describe the process of beta minus and plus decay
udd -> uud + e~ + v~
neutron -> proton
change of quark character
uud -> udd + e+ + v
proton -> neutron
what can forces also be described as
particle exchange
electrostatic repulsion is caused by what?
the exchange of virtual photons
transfer energy, momentum
they are short lived
how is the range of a force determined
the size of the exchange particle
heavier exchange particles have shorter range
virtual photon has virtual zero mass, infinite range of em
w bosons have a mass 100 times as big as a proton’s so weak force very short range
what is electron capture and how does it differ to electron-proton collision
when a proton rich nuclei captures an electron from the inner shells of an atom and use it to become a neutron
electron capture has w+ boson going right ->
electron proton collision has w- boson going left <-
what is the exchange particle of electromagnetic repulsion
virtual photon
