Particle physics paper 2 Flashcards
thompson plum pudding model
neutral atom is made up of a uniform sphere of positive charge with tiny electrons embedded
Rutherfords alpha scattering experiment
disproved plum pudding model. alpha particles fired at thin sheet of gold foil under a vacuum. deflected particles were detected on all sides by a ring of scintillators.
scintillators
materials that release photons when a particle hits them
rutherford prediction
expected most alpha particles to travel through with deflection and rebound off charged wall.Majority of particles passed straight through with only a slight deflection of on average only a degree. Therefore atom is mostly empty, mass is concentrated in centre
Rutherfords development of the nucleus
small portion of alpha particles were deflected by more than 90, this is only possible if the charge on the nucleus was the same as the alpha particle (positive). Hence the concentrated mass was positively charged with electrons orbiting
isotopes
atoms of the same element with differing numbers of neutrons so they can undergo the same chemical reactions but will undergo different nuclear reactions
nucleon
subatomic particle that resides in the nucleus so either a proton or neutron
atomic or proton number
z
mass number
a
weak nuclear force
force responsible for beta decay acts to change quark types over very small distances
strong nuclear force
acts between all nucleons and all quarks, counteracts the repulsive electrostatic forces, attractive at small distances >3fm and repulsive at small 0.5>fm
concept of mass as a form of energy
demonstrated by annihilation of matter and antimatter where the combined mass of the two particles is related by the mass energy equivalence
binding energy
minimum energy required to break a nucleus into its constituent parts
this would include the thermal energy and electrostatic potential energy that arise from strong nuclear forces
mass defect of a reaction
difference in the mass of the constituent nucleons against the mass of the nucleus due to the potential of the electrostatic and strong forces
radiation is emitted in the form of high energy particles or photons. this energy must come from a change in mass. when a parent nucleus emits a daughter nucleus and a high energy particle there is a mass difference. this is known as the mass defect.
binding energy per nucleon
most stable isotope is iron 56, max binding energy per nucleon. for low nucleon numbers A<56 Binding energy per nucleon increases otherwise decreases
anti particles
every particle has a corresponding antiparticle which have equal mass but opposite charge, they annihilate to produce energy in the form of photons in space
pair production
occurs when a high energy photon spontaneously creates a matter-anti matter pair. the photon must have an energy greater than combined rest masses of the two particles. ev=mc^2
fundamental particles
consist of two main classes, hadrons and leptons
hadrons
made up of fundamental particles called quarks and are acted on by Bothe strong and weak nuclear force, can only exist in quark-antiquark pairs which make a class of hadrons called the mesons. protons and neutrons are baryons whereas particles such as pions are mesons.all baryons have number of 1 anti baryons -1
how do we know quarks exist
from particle collisions, some of the kinetic energy and mass energy of the particles can be transferred into other forms and particles created or destroyed by the mass energy equivalence.
strong interactions conserves
the strangeness (as
well as top-ness, charm etc.). If a hadron contains a strange quark it will have a
strangeness of -1 and if it has an anti-strange quark it has a strangeness of 1
Leptons
Leptons are also fundamental particles but unlike quarks they are not affected by the
strong force. They are subject to the weak nuclear force however and so are created in
nuclear decays such as beta decay to conserve the charge and mass-energy of the
interaction. Leptons have a lepton number of 1 whereas their antiparticles have a lepton
number of -1.e electrons, positrons (antielectrons),
neutrinos and muons
Radioactive decay
spontaneous breakdown of an atomic nucleus resulting in the
release of energy and matter from the nucleus
random process
that it is
impossible to predict which of a number of identical nuclei will decay next. decay follows a defined pattern and given a large enough number of nuclei this yields
predictable results
types of radiation can be easily distinguished
different
penetrating powers so simply screening the source with different materials and
measuring the drop in activity should allow for the type of source to be determined. In
addition to this, all three have different responses to magnetic fields and electric fields
due to their differing charges making specific radiation detectors easy to design.
bubble chambers
use tanks of water that from tracks of bubbes when ionising particles pass
through them and Geiger-Muller tubes (Geiger counters) which form cascades of
electrons when an ionising particle hits the detector.
Beta Decay and the Weak Nuclear Forc
weak nuclear force causes the transformation of quarks via emission of leptons. In
beta minus decay, the weak force mutates a down quark into an up quark within a
neutron transforming it into a proton. This process releases energy in the form of a high
speed electron or beta particle which is also needed to conserve charge. However, an
antineutrino is also created in order to conserve lepton number.
beta minus decay formula
Beta plus decay
also occurs when an up quark transforms into a down quark in a proton
changing it to a neutron. Again, energy is produced creating a positron and neutrino
Beta plus decay formula
Alpha decay
occurs in very unstable nuclei and sees the loss of an alpha particle or
helium nucleus
Alpha emission
can be thought of as spontaneous fission of unstable
nuclei in which the strong nuclear force is not great enough to overcome the electrostatic
repulsion between protons in the nucleus. The high binding energy per nucleon of the
helium nucleus makes this daughter nucleus a favourable choice as total binding energy
will increase vastly as the parent nuclei moves closer to the middle of the binding
energy per nucleon graph.
Alpha decay FORMULA
decay chain
A common occurrence in nuclear decays is many will happen in succession until a stable
atom is reached,
Nuclei with greater than 82 protons
likely to decay via alpha radiation
Nuclei to the right of the belt of stability
have too many protons and therefore are
proton-rich meaning that they are more likely to decay via beta-plus decay
Nuclei to the left of the belt of stability
have too many neutrons and therefore are
neutron-rich meaning that they are more likely to decay via beta-minus decay.
Gamma decay
s caused when a nucleus has surplus energy following alpha or beta
emission. There is no change to nucleon composition, but energy is released in the form
of a gamma photon
activity
the rate at which nuclei decay, or number of decays per
second, measured in Becquerel (s-1)
decay constant,
probability that an individual nucleus
will decay per unit time.
background radiation
must be recorded before the expirement then measure the sources radiation then subtract the difference to find the true activity
Radiocarbon Dating
ratio of C-14 to C-12 in the organism will match the atmospheric ratio yet at the point of
death the number of C-14 atoms will be capped. C-14 is a radioactive isotope that decays
via beta emission with a half-life of ~5700 years. Hence, by measuring the ratio of C-14 to
C-12 in the dead tissue and comparing this to the atmospheric composition an estimation
for the time since the organismβs death can be calculated
CNO cycle
in which carbon, nitrogen
and oxygen are synthesised.
proton-proton chain
which lone protons in the starβs plasma fuse to form an unstable π»π»ππ2
2 nuclei. This
particle then undergoes beta decay forming deuterium nuclei ( π»π»1
2 , an isotope of
hydrogen). The next reaction in the chain fuses deuterium with a further proton to form
π»π»ππ2
3 and so the process continues forming nuclei up to π»π»ππ2
4
Artificial fusion reactors
alternative power sources as
they have no radioactive by-products. However, the extremely high temperatures and
pressures needed to maintain fusion and can only be operated for short periods of time
Nuclear Fission
Fission is the breaking apart of large nuclei into small nuclei causing a reduction in the
total binding energy, a mass defect and energy to be released
Nuclear Fission process
Typically, uranium-235 is
used the fissile material as it easily undergoes fission and is relatively abundant. A low
speed, thermal neutron is fired at the U-235 nuclei which absorbs the extra nucleon to
become the unstable U-236 isotope. This isotope then either decays via fission (about
85% of the time), splitting into two smaller daughter nuclei and more fast neutrons or
decays via gamma emission.
fissile materials sit inside a
reactor core which is surrouned by a thermal coolant which absorbs the thermal energy
from the fission and transforms this thermal energy into kinetic energy in the turbines
which then turn the generator transferring this energy into electrical energy i.e.
alternating current
fuel rods
within the core contain
the fissile material and are surrounded by the moderator and control rods.
moderator
The absorption of neutrons and consequent fission is more likely to occur with slower
neutrons, which means the neutrons released by the fission process are too high energy
to continue the reaction. They are slowed by a moderator
rate of the reaction is
controlled by
control rods (e.g. boron, cadmium) which absorb thermal neutrons to
prevent these neutrons from causing further fission reactions.
