nuclear physics Flashcards
What did Rutherford scattering demonstrate
The existence of the nucleus
What did scientists believe before Rutherford scattering
Thomsons plum pudding
What is Thomson’s plum pudding model
A sphere of positive charge with small areas of negative charge evenly distributed throughout
What equipment is used in Rutherford scattering
Alpha source and gold foil in an evacuated chamber which was covered in a fluorescent coating
What did the fluorescent coating do in Rutherford scattering
You could see where the alpha particles hit inside Of the chamber
How was the path of the alpha particles observed in Rutherfords scattering
A microscope
What were the expected results if plum pudding model was true
They would be deflected by a small amount when passing through the foil
What were the 3 observations in scattering experiment
Most passed through the foil with no deflection
a small amount were deflected by a large angle
A few particles were deflected back by more than 90 degrees
What did the fact that a small amount of particles were deflected by a large angle tell us
That the centre of the atom is positively charged
What did the fact that most alpha particles passed straight through gold foil with no deflection tell us
That the atom was mostly empty space and not a uniform density
What did the fact that very few particles were deflected back by more than 90 degrees tell us
That the centre of the atom was very dense and very small
What was concluded from Rutherford experiments
The atom has a small dense positively charged nucleus at its centre
What is radiation
Where an unstable nucleus emits energy in the form of em waves or subatomic particles in order to become more stable.
What are the 3 types of radiation
Alpha beta gamma
What is alphas range in air
2-10cm
What is betas range in air
1m
What is gammas range in air
Infinite - follows inverse square law
How ionising is alpha
Highly
How ionising is beta
Weakly
How ionising is gamma
Very weakly
Are alpha particles deflected by electric and magnetic fields
yes
Are beta particles deflected by electric anf magnetic fields
yes
Are gamma rays deflected by electric and magnetic fields
no
what is alpha absorbed by
paper
what is beta absorbed by
approx 3mm of aluminium foil
what is gamma absorbed by
several meters of concrete or several inches of lead
how can the 3 types of radiation be differentiated
different penetrating powers
what experiment differentiates between the 3 radiations
A Geiger-muller tube and counter to find background count when source isn’t present.
Place the source of radiation close to the GM and measure count rate
place paper/aluminium/lead between the source and GM and measure again.
Depending on which decreases tells you what source it is
How can radiation be used in material production
To monitor the thickness of the materials
How does beta radiation help the production of aluminium foil
The beta source is on one side of the material, detector on the other
if too thick less passes through foil
if too thin more passes through foil
rollers adjust depending on reading
What are the main uses of gamma radiation
As a detector, sterilise surgical equipment, radiation therapy
How does gamma radiation work as a detector
A radioactive source with a short half life that emits gamma radiation which can be injected into a patient and then can be detected using gamma cameras
How does gamma radiation sterilise surgical equipment
It kills any bacteria present
How does gamma radiation work in radiation therapy
It kills cancerous cells in a targeted region of the body but also healthy cells
Methords to reduce risk in radiation therapy
Use of shielding, reduced exposure times
How does gamma disperse in the air
Spreads out equally in all directions following an inverse square law
How to investigate the inverse square law for gamma
Measure the count rate of a gamma source at different distances from the GM tube, adjusting for background radiation
What does it mean if alpha radiation is highly ionising
Incredibly dangerous if inhaled or ingested as it can ionise body tissue
Can beta cause damage to body tissue
Yes
What does prolonged exposure to gamma cause
Mutation and damage to cells
How to safely handle radioactive sources
- Use long handled tongs
- store source in a lead lined container
- keep source far away from people as possible
- never point source towards a person
What is background radiation
Radiation that is always present
How to adjust radiation readings to account for background radiation
Measure background radiation first then subtract the value to find the correct count
Corrected count =
Total count rate - background count
What are sources of background radiation
Radon gas(from rocks)
artificial sources (nuclear weapons testing)
cosmic rays
rocks (containing naturally occurring radioisotopes)
What type of process is radioactive decay
A random one
What is a decay constant
Probability of a nucleus decaying per unit time
How do you calculate decay constant
Change in number of nuclei over time over initial number of nuclei
How can you estimate the nuclear radius of an atom
Calculate the distance of closest approach of a charged particle
What happens as a charged particle moves towards a positively charged nucleus
Experiences an electrostatic force of repulsion and slows down since kinetic energy is converted to electric potential
What is the distance of closest approach
The point at which the particle stops and has no kinetic energy
What is the other method for calculating nuclear radius (Not distance of closest approach)
Electron diffraction
Why is distance of closest approach not an accurate estimate
It is an overestimate
Why do electrons not interact with nucleons in the nucleus through strong nuclear force
They are leptons
How does electron diffraction estimate the nuclear radius
They are accelerated to high speeds so that their de broglie wavelength is around 10^-15
they are directed at a thin film of material causing them to diffract between gaps between nuclei
What is the formula for the estimate of nuclear radius from electron diffraction
Sinx = [0.61(lambda)]/R
what is the approx size of a radius of a nuclei
1x10^-15
what is the graph of nuclear radius (y) and nucleon number (a)
how does nuclear density differ
it doesn’t, it is constant for all values
what is the approx nuclear density
1.45x10^17
what does the fact that nuclear density is so great suggest
the atom is mostly empty space with most of its mass concentrated in its centre
what is half life
time taken for the number of radioactive nuclei to half
why is the time taken for the number of nuclei to halve constant
the decay is exponential
what is a more accurate way to measure Half life
plotting a graph of ln(initial nuclei) against time as the modulus of the gradient of the line is the decay constant
what is the activity of a radioactive sample
number of nuclei that decay per second
what is the activity of a radioactive sample proportional to
number of nuclei where the decay constant is the constant of proportionality
what can the decay constant be used to model
the decay of a nuclei only when there is a large number of nuclei in a sample
why can you only use the decay constant to model decay if there is a large number of nuclei
its modelled by statistical means
what are two ways the half life of a radioactive nucleus can be used
dating of objects and medical diagnosis
how does carbon dating work
carbon-14 has a long half life so it can used to date organic objects by measuring the current amount of c14 and comparing to initial amount (initial amount is similar in all living things)
how does medical diagnosis with radioactive nuclei work
nuclei with a short half life can be used as radioactive tracers in medical diagnosis.
what specific element is used in medical diagnosis with radioactive nuclei and why
technetium 99m as it is a pure gamma emitter with a half life of 6 hours. short enough half life to limit exposure but long enough for tests and can be prepared on site
why do the activity and half life of radioactive nuclei affect the way they must be stored
to prevent nuclei with a long half life damaging the environment and people living around them for a long period of time
what are nuclei held together by
strong nuclear force
what force of repulsion do protons experience
electromagnetic force
what happens if the strong nuclear and the electromagnetic force are out of balance
the nuclei becomes unstable and will experience decay
what are the 4 reasons why a nucleus may become unstable
too many neutrons, too many protons, too many nucleons, too much energy
what does a nucleus with too many neutrons decay through
beta minus and one of the neutrons changes to a proton, beta minus and antineutrino is released
what does a nucleus with too many protons decay through
beta plus or electron capture
what is beta plus decay
proton changes into a neutron and a beta plus particle and neutrino is released
what is electron capture
an orbiting electron is taking in by the nucleus and combined with a proton causing formation of neutron and neutrino
what does a nucleus with too many nucleons decay through
alpha emission
what does a nucleus with too much energy decay through
gamma emission (usually occurs after another type of decay as nucleus becomes excited and has excess energy)
why doesn’t the number of neutrons and protons in a stable nucleus increase uniformly beyond 20 of each
the electromagnetic force of repulsion becomes larger than the strong nuclear force keeping the nucleus together
what happens when more neutrons are added to a nucleus
distance between protons increases decreasing magnitude of electromagnetic force
How can nuclear decay be represented other than equations
Energy level diagrams
How does an element in an excited nuclear state go to ground state
Emits a gamma photon
Why is technetium-99m good for medical diagnosis
Emits pure gamma source so easily detected by cameras, weakly ionising, short half life of 6 hours, easily prepared on site
graph of number of neutrons against number of protons and the areas in which what type of decay will occur
what are energy level diagrams
diagrams that show the differences in energy of particles
what state is technetium formed in
an excited state
what is shown on an energy level diagram is a source is formed in an excited state
ground state and gamma photon is drawn on
what is the mass defect/mass difference
the difference in mass between a nucleus and the mass of its constituents
what happens to the mass “lost” in mass defect
converted into energy and released when the nucleons fuse to form a nucleus
what is the binding energy of a nucleus
energy required to separate the nucleus into its constituents ( or energy released when a nucleus is formed from its constituents)
what is one atomic mass unit
1/12th the mass of a carbon-12 atom
what does a change of 1u in mass mean in terms of energy
931.5 MeV of energy is released
what is nuclear fission
splitting of a large nucleus into two daughter nuclei
where does nuclear fission occur
large unstable nuclei
how does nuclear fission occur
randomly but can also be induced
why is energy released during nuclear fission
the smaller daughter nuclei have a higher binding energy per nucleon
what is nuclear fusion
where two smaller nuclei join together to form one larger nucleus
why is energy released during fusion
the larger nucleus has a much higher binding energy per nucleon
does fusion or fission release more energy
fusion
why can fusion only occur at extremely high temperatures
a massive amount of energy is required to overcome electrostatic force of repulsion between nuclei
how can you identify if an element will undergo fission or fusion
plotting a graph of binding energy per nucleon against nucleon number
what element has the highest binding energy per nucleon
iron
what do nuclei smaller than iron undergo
fusion
what do nuclei greater than iron undergo
fission
where is nuclear fission used
nuclear power plants to create electricity without emission of greenhouse gasses
what is a risk of using nuclear fission in power plants
daughter nuclei are radioactive and need to be stored safely for thousands of years, meltdowns are also a possibility which cause harm to environment
what allows society to make informed decisions about how electricity should be generated
understanding nuclear physics behind production of nuclear power
how can fission be induced
by firing a thermal neutron into uranium nucleus causing it to become extremely unstable
why is the fact that thermal neutrons have a low energy useful
they can induce fission
why cant neutrons with a higher energy cause fission
they rebound away from uranium after a collision
what are the products of fission
2 daughter nuclei and at least one neutron
what does the neutron released in fission go onto cause
more fission causing a chain reaction where each fission causes at least one more fission
what is critical mass
minimum mass of fuel required to maintain a steady chain reaction
what does using exactly the critical mass of fuel mean
a single fusion reaction follows the last continuing indefinitely
what does using less than the critical mass of fuel mean
the reaction would eventually stop
what are the 3 key features of a nuclear reactor
moderator, control rods, coolant
what do moderators do
slow down the neutrons released in fission reactions to thermal speeds
how do moderators slow down neutrons
through elastic collisions between the nuclei of the moderator atoms and fission neutrons
the closer the moderator atoms are in size to a neutron the…….
large the proportion of momentum which is transferred so lower collisions to get neutrons to thermal speeds
why is water used as a moderator
it contains hydrogen, inexpensive, not very reactive
what is sometimes used as a moderator
graphite
what do control rods do
absorb neutrons in the reactor in order to control chain reactions
what are control rods made of
materials that absorb neutrons without undergoing fission e.g. boron and cadmium
why can the height of the control rods be controlled
to control the rate of fission reactions which controls amount of energy produced
what does coolant do
absorbs heat released during fission reactions in the core of the reactor
what is the heat absorbed by the coolant used for
to make steam which powers electricity generating turbines
why is water also used as a coolant
high specific heat capacity
what other materials can be used as a coolant
molten salt or a gas e.g. helium
what is the fuel used in nuclear reactors called
enriched uranium
how is enriched uranium formed
enrichment of mined uranium which is 99% U-238
what does the fact that U 238 doesn’t experience fission do
increases percentage of U 235 to 5%
what does u-238 do
absorbs fission neutrons so controls rate of fission reactions
how are the fuel rods inserted into the reactor
remotely to limit workers exposure
what is around a nuclear reactor
a thick concrete shielding which blocks radiation escaping the reactor and harming workers in power station
what may happen to the shielding after long term use
it may become radioactive
why may the shielding become radioactive
neutrons which escape the reactor may enter the shielding nuclei causing them to become unstable and experience beta minus decay
what happens in an emergency in nuclear reactors
control rods dropped into the reactor core entirely to stop fission reactions
how does dropping control rods stop fission entirely
absorbs all free neutrons in core
what is it called when control rods are dropped into the core entirely
emergency shut down
what is the diffraction pattern of electrons
concentric circles with a central bright stop that get dimmer from the centre
at the nuclear level, are mass and energy interchangeable
yes
what is lower when measuring mass of nucleus and mass of constituents
mass of nucleus is always lower
where is the mass lost when converting constituents to a nucleus
converted to energy and released when nucleons form
what are daughter nucleus stability and activity
extremely unstable and have a high activity
what are spent fuel rods
used up fuel rods
what type of waste are spent fuel rods
high level waste ( dangerous)
what is low level waste
contains short-lived radioactivity
examples of low level waste
tools and gloves
how can low level waste be disposed
close to surface
why can low level waste be disposed close to surface
won’t take long to stop being radioactive
how long can high level waste be radioactive for
thousands of years
how is high level waste removed and handled
remotely to reduce exposure
where are materials removed from the reactor placed
in a cooling pond for up to a year
why are materials removed from a reactor very hot
due to fission reactions
where are cooling ponds usually located and why
on same site or very close by so they don’t have to travel long distances to reduce risk of exposure
what is removed from spent fuel rods after cooling
plutonium or useable uranium to be recycled
what happens to the spent fuel rods after plutonium or useable uranium is removed
it is vitrified and placed in thick steel casks, stored in deep caverns in geologically stable locations
what is vitrification
encasement in glass
why is high level waste stored in geologically stable locations
so no chance of waste coming free of its casing
how are locations to store radioactive waste chosen
so they make minimal impact on environment
people living in the are are consulted beforehand
how much power does 1kg of uranium give in comparison to coal
25 tonnes of coal
pros of nuclear power stations
no polluting gas, reliable power, need far less fuel
cons of nuclear power stations
produce radioactive waste, nuclear meltdowns are catastrophic
how are risks in nuclear power stations minimised
via safety aspects
do benefits or risks outweigh each other for nuclear power stations
benefits outweigh the risk
