Atomic, Nuclear and Particle Physics Flashcards
Rutherford’s (Geiger - Marsden) Experiment - aim & method
aim: to investigate the distribution of charge within the atom
Method: fired alpha particles (helium nuclei) at a thin gold foil. a fluorescent screen detected where they went.
Why gold? (GM experiment)
- can be hammered into very thin sheets (approx 1000 atoms thick)
- large atom
Why alpha particles? (GM experiment)
- large enough mass not to be deflected by electrons
- positive charge to investigate charge distribution
Expected Results vs actual (GM Experiment)
expected: if Thompson’s model was correct then the alpha particles would pass straight through undeflected (as charge evenly distributed)
actual: most alpha particles passed straight through as most of the atom is empty space
a significant number pf alpha particles had their path deflected as they passed through the foil as the positive charge of an atom is concentrated in its centre
some particles bounced straight back from gold foil (hit nucleus) therefore most of the mass of an atom is in the nucleus
What are atomic energy levels?
electrons can only exist within a series of discrete energy levels around a nucleus
electrons can move between energy levels by either absorbing or emitting photons of light (packets of energy)
Continuous Spectrum
from a light source producing photons of all wavelengths (and frequencies)
shine a light through slit to produce beam then through a prism
Emission Line Spectrum
only photons of specific wavelengths are emitted (black with coloured bands)
pass light through a hot gas then through a slit
then shine through a prism
Absorption Line Spectrum
only photons of specific wavelengths are absorbed (continuous with black bands)
shine a light through a cold gas and then through a slit
then shine through a prism
How are protons and neutrons held together?
a strong nuclear force which:
- acts at a short range
- is balanced by repulsion between protons that acts at a long range (Coulomb interaction)
if too many protons repulsion > strong nuclear force
if too many neutrons strong nuclear force is not strong enough to hold whole nucleus together
Nucleon
is any particle in the nucleus (e.g. proton or neutron)
Isotopes
are different nuclei of an element that have different number of neutrons
Alpha Radiation
helium nucleus He4/2
range of about 5cm
penetration is stopped by paper
very high ionising ability (high Ek and +ve charge)
Beta Radiation
Beta -ve = an electron emitted from the nucleus (neutron -> proton) 0/-1 accompanied by an antineutrino
Beta +ve = a positive electron or positron (proton -> neutron) 0/1 accompanied by neutrinos
range of 30 cm
stopped by aluminium foil
low ionising ability
Gamma Radiation
EM radiation (photon)
very large range
stopped by 10cm of lead
very low ionising ability
always accompanies alpha and beta radiation
Radioactive decay is:
- spontaneous (cannot be modified in any way)
- random (don’t know when a particular nucleus will decay or which particular nucleus will decay)
Half Life
the time taken for half of the nuclei in a sample to decay (or for the activity to fall to 50% of the initial value)
unit is the becquerel (Bq)
after n half lives, 1/2n of the original substance remains
Background Radiation
constantly exposed to radiation - must be subtracted when taking measurements
natural:
- cosmic, terrestrial and internal (food and air)
or artificial:
- medical, industrial / occupational, nuclear fall out
Rules of Nuclear Equations
- conservation of mass: the total mass number of each side of the equation must be the same
- conservation of charge: the total atomic number on each side of the equation must be the same
Alpha particle
helium nucleus emitted during radioactive decay
Antineutrino
particle emitted with a beta negative particle
Atomic number (Z)
number of protons in a nucleus
baryon
formed from three quarks (or antiquarks)
beta negative particle
an electron emitted from the nucleus
beta positive particle
a positive electron (positron) emitted from the nucleus
binding energy
the energy required to break apart a nucleus into separate nucleons
elementary (fundamental) particle
a particle with no internal structure (i.e. is not made of any smaller particles)
exchange particle (gauge boson)
transmit forces between particles
gamma particle
a photon emitted during radioactive decay
gluon
an exchange particle that holds a hadron (e.g. a proton or neutron) together
graviton
a yet to be discovered particle responsible for gravitational force
hadron
formed from a group of quarks and/or antiquarks
Higgs boson
particle responsible for mass
ionisation
removal of an electron from an atom or molecule to create a positive ion
lepton
any member of the electron family
mass defect
difference in mass between individual nucleons and their total mass when bound in a nucleus
mass number (A)
the total number of protons and neutrons within a nucleus
meson
formed from a quark-antiquark pair
neutrino
particle emitted with a beta positive particle
nuclear fission
a large nucleus splitting into smaller nuclei
nuclear fusion
smaller nuclei combining to form a larger nucleus
photon
a packet of energy that makes up light, and transmits the electromagnetic force
pion (or pi meson)
an exchange particle that holds a group of hadrons together (e.g. a nucleus) together
quark
a fundamental particle, makes up protons and neutrons
quark confinement
the reason quarks cannot exist in isolation, as they are moved apart the energy required forms another quark
standard model
the theory that describes the electromagnetic weak and strong interactions of particles
transmutation
changing a nucleus from one form to another by the addition of nucleons
unified atomic mass unit (u)
equivalent to the mass of one twelfth of the mass of a carbon-12 atom
virtual particle
a boson that cannot be detected (as detection means that it would no longer be acting as a boson)
the larger the rest mass of the boson the shorter the range of the force
Energy release from nuclear reactions
any nuclear reaction will release energy if the binding energy per nucleon of its products is greater than the binding energy per nucleon of its reactants
Fe-56 is the hardest element to break apart (most stable)
before this fusion will release energy and after this fission will release energy
Matter and Antimatter
when they combine, both are annihilated and their total mass is converted to energy in the form of a pair of photons (the reverse can also happen)
Gravitational force
weak, infinite range, acts on all particles, always attractive (weakest)
Electromagnetic force
infinite range, causes electric and magnetic effects, acts between all charged particles, can be attractive or repulsive, stronger at short distances (second strongest)
Strong nuclear force
holds nuclei together, very strong (strongest), very short range, acts between quarks and gluons (NOT leptons)
Weak nuclear force
responsible for radioactive decay and neutrino interactions, allows quarks to change flavours, very short range, acts between quarks and LEPTONS (second weakest)
W and Z bosons
W+ and W- transfer charge
Z transfers momentum and energy
Feynman Diagrams rules
solid straight line represents quarks and leptons
arrows go forward in time for particles and against time for antiparticles
exchange particles are represented by wavy lines (photons, W+, W- and Z) or curly (gluons)
a point where lines meet is called a vertex
nuclide
a particular form of a nucleus
