Particle & Nuclear Physics Flashcards
How did Rutherfords alpha scattering experiment work?
- A narrow beam of alpha particles (all of the same kinetic energy) from a radioactive source was targeted at a thin piece of gold, only a few atoms thick
- The alpha particles were scattered by the foil and detected on a zinc sulfide screen mounted in front of a microscope
- Each alpha particles hitting the fluorescent screen produced a tiny speck of light
- The microscope was moved around in order to count the number of alpha particles scattered through different angles per minute, for 0 to almost 180 degrees
What were the observations of Rutherfords alpha scattering experiment?
- Most of the alpha particles passed straight through the thin gold foil with very little scattering
- Very few of the alpha particles (about 1 in 10,000) were defelected though angles of more than 10 degrees
What were the conclusions for Rutherfords alpha scattering experiment?
- Most of the atom was empty space, with most of the mass concentrated in a small region
- The nucleus has a positive charge because it repelled the few positive alpha charges that came near it
What is the charge of a proton?
1.6 x 10^-19 C
What is the charge of an electron?
- 1.6 x 10^-19 C
What is the charge of a neutron
0 C
What is the mass of a proton?
1.67 x 10^-27 kg
What is the mass of a neutron?
1.67 x 10^-27 kg
What is the mass of an electron?
9.11 x 10^-31 kg
What is the relative charge of a proton?
+1
What is the relative charge of a neutron?
0
What is the relative charge of a neutron?
-1
What is the relative mass of a proton?
1
What is the relative mass of a neutron?
1
What is the relative mass of an electron?
1/2000
What is the diameter of an atom?
1 x 10^-10 m
What is the diameter of a nucleus?
1 x 10^-15 m
What is the closest approach method?
- In the rutherford scattering experiment, alpha particles are fired at a thin gold foil
- Some of the alpha particles are found to come straight back from the gold foil
- This indicates that there is electrostatic repulsion between the alpha particles and the gold nucleus : KE = eV = 1/2 mv^2
- At the point of closest approach, r, the repulsive force reduces the speed of the alpha particles to 0 momentarily
- At this point : initial KE of alpha particle = electric potential energy
What is the closest approach equation?
KE = Qq / 4 pi ε r
What is the nucleon number?
Total number of protons & neutrons in the nucleus
What is the proton number?
Total number of protons in the nucleus
What is the notation showing nucleon and proton number of an element?
What is an isotope?
An atom (of the same element) that has an equal number of protons but a different number of neutrons. The proton number is the same but the nucleon number is different?
What can the nucleon number also be called?
Mass number
What type of properties do neutrons not affect?
The chemical properties eg. charge
What properties do isotopes have different to the normal atoms?
Physical properties eg. mass
Why do isotopes tend to be more unstable?
Due to the imbalance of protons and neutrons
What 3 forces are there in the nucleus?
- Electrostatic forces (repulsion)
- Gravitational forces (attraction)
- The strong nuclear force
Why are there electrostatic forces in the nucleus?
Due to the charge of the protons
Why are there gravitational forces in the nucleus?
Because of the masses of the nucleons
What is the strong nuclear force?
The force that holds the nucleus together - it acts between all nucleons. It is one of the 4 fundamental forces
What is the range of the strong nuclear force?
Very short range, effective over just a few fentometres
What is the nature of the strong nuclear force (inc. ranges) ?
- Repulsive below 0.5 fm
- Attractive up to 3fm
- 0 after 3fm
What is the equation for radius of a nuclei?
R = radius of nuclei
r0 = constant
A = nucleon number
What is the value of constant r0?
1.2 x 10^-15 m
How do you show the density of a nucleus is constant and independent of the radius?
What is the universe made up of?
Matter particles
What is antimatter?
Matter counter particles - they are identical to their matter counterparts but have opposite charge
What is the antiparticle of the electron?
Positron
What is the antiparticle of the proton?
Anti-proton
What is a neutral particles anti particle?
Itself - because it has a neutral charge anyways
What do matter and anti-matter particles have identical to one another?
Their mass and rest-mass energy
What is a hadron?
Particles and anti-particles that are affected by the strong nuclear force
What are some examples of hadrons?
Protons
Neutrons
Mesons
What force are hadrons subject to when charged?
Electromagnetic charge
What force does decay occur by for hadrons?
The weak nuclear force
What is a lepton?
Particles and anti-particles that are not affected by the strong nuclear force
What forces are leptons subject to when charged?
Electromagnetic force
What other force are leptons subject to?
The weak nuclear force
What are some examples of leptons?
- Electrons
- Neutrinos
- Muons
What are neutrinos?
The most abundant leptons in the universe - they have no charge and negligible mass
What are the 3 flavours of neutrino?
What are the other 3 leptons (other than the neutrinos)?
- Electron
- Muon
- Tau
What are the 2 types of hadrons?
Baryon and meson
What is baryon?
A hadron from 3 quarks
What is a meson?
A hadron made from a quark and anti-quark pair
What is an anti-baryon?
An anti-hadron made from 3 anti-quarks
What is an anti-meson?
An anti hadron made from a quark and anti-quark pair
In refernce to charge, what do all baryons and mesons have?
Integer charges, eg +1e, -2e
This means quarks in a baryon are either all quarks or all anti-quarks
Why are protons and neutrons not fundamental particles?
Because they are each made up of 3 quarks
What is the charge of an up, charm or top quark?
+ 2/3
What is the charge of a down, strange or bottom quark?
- 1/3
What is the charge of an anti-up, anti-charm or anti-top anti-quark?
-2/3
What is the charge of an anti-down, anti-strange or anti-bottom anti-quark?
+ 1/3
What is a proton made up of?
2 up quarks and 1 down
uud
What is a neutron made up of?
2 down quarks and 1 up
udd
What is beta-minus decay?
Neutron -> Proton + electron (beta particle) + anti electron neutrino
What is beta-plus decay?
Proton -> Neutron + positron (beta particle) + electron neutrino
Why does beta-minus decay occur?
Because the down quark turns into an up quark
Why does beta-plus decay occur?
Because the up quark turns into a down quark
What is radioactive decay described as?
Random and spontaneous
Why is radioactive decay random?
- We cannot predict when a particular nucleus in a sample will decay or which one will decay next
- Each nucleus within a sample has the same chance of decaying per unit
What is radioactive decay spontaneous?
Because the decay of nuclei is not affected by:
- the presence of other nuclei in the sample
- external factors such as pressure
How can the random nature of radioactive decay be demonstrated?
Using a Geiger-Muller Tube
When a GM tube is placed near a radioactive source, the counts are found to be irregular and cannot be counted - each count represents a decay of an unstable nucleus. The fluctuations in count rate on the GM tube provide evidence for the randomness of radioactive decay
How can the random nature of unstable nuclei be simulated?
Rolling a large number of dice repetitively and removing all of the same number each time
What are alpha particles?
High energy particles made up of 2 protons and 2 neutrons - the same as a helium nucleus. They are usually emitted from nuclei that are too large
What are alpha particles stopped by?
Paper - they only have a range of a few cm in air
What are beta particles?
High energy electrons emitted from the nucleus. They are emitted by nuclei that have too many neutrons
What are beta particles stopped by?
A few mm of aluminium foil - they have a range of 20cm to 3m in air
What are gamma rays?
High energy electromagnetic waves - they are emitted by nuclei that need to lose some energy
What are gamma rays stopped by?
1-10 cm of lead or a few metres in concrete
What is the least ionising type of raidation?
Gamma
What is the moderate ionising type of radiation?
Beta
What is the most ionising type of radiation?
Alpha
What is the least penetrating type of radiation?
Alpha
What is the moderate penetrating type of radiation?
Beta
What is the most penetrating type of radiation?
Gamma
How can you investigate the absorption of alpha, beta and gamma radiation?
What are the results from investigating the absorption of alpha, beta and gamma radiation?
- Alpha will be absorbed by the paper
- Beta will be absorbed by the aluminium foil
- Some gamma will be absorbed by the thick lead
What is the alpha decay equation?
What are the beta decay equations?
What is the activity of a source (A)?
The rate at which nuclei decays, in a given isotope
What is the decay constant (λ)?
The probability, per second, that a given nucleus will decay
What is the activity of a source equation?
What does the activity of a source equation show?
- The greater the decay constant, the greater the activity of the sample
- The activity depends on the number of undecayed nuclei remaining in the sample
- The minus sign indicates that the number of nuclei remaining decreases with time - for calculations it can be omitted
What is half-life?
The time taken for the initial number of nuclei to reduce by half
What is the decay constant and half-life equation?
How do you determine the half-life of an isotope (protactinium-234)?
- Protactinium-234 isotope is often used in experiments to measure the half-life because its half-life is short
- The protactinium-234 isotope is produced from the decay of thorium-234
- A sealed plastic bottle containing an organic solvent and a solution of uranyl (VI) nitrate in water is used to separate the protactinium from the thorium. This works because the compound of the protactinium daughter isotope is soluble in the organic solvent, whereas the parent thorium is not
- The background count is firstly determined in the absence of the source (take 3 repeats and find the source)
- The plastic bottle is shaken for 15s to dissolve the protactinium in the organic solvent - this will all sit on the top
- The end-window of the GM tube is placed opposite the organic layer - in order to avoid contamination, the GM tube tube must not touch the bottle
- Counts of the decaying protactinium should be recorded for a 10s count every 30s
- The corrected count rate is directly proportional to the activity of the source
- The half-life of protactinium-234 can be determined by plotting a graph of corrected count rate against time
What are the exponential equations for radioactivity?
How do you model radioactive decay?
1) Start with a given number of undecayed nuclei, 0N, is the sample
2) Choose a very small interval of time t - this should be significantly shorter than the half-life of the chosen isotope
3) Calculate the number of decaying nuclei, ΔN during the time period. ΔN = λ x t x N
4) Calculate the number of undecayed nuclei, N now left at the end of the time period t N0 - ΔN = N
5) Repeat this process by iterating your value for N as your new N0 for many values of t
6) Plot a graph of N against t
What does radioactive/ carbon dating work on?
The fact that all living things on earth contain carbon
How does carbon/radioactive dating work?
It has been shown that the production of C-14 (isotope of carbon) is constant and ratio of C-14 to C-12 has a constant value that is the same in the atmosphere, as well as all living things on earth.
Once an organism dies it stops taking in carbon, this means that the amount of cabron-14 it has decreases over time. If we were then to compare a modern day sample of something, such as a tree, to an ancient sample, we are able to determine its age
What is the half life of C-14?
5730 years
How is C-14 produced?
What range is carbon dating highly reliable for?
From around 1000 years old up to a limit of around 40,000 years old
Why is carbon dating unreliable if the sample is less than 1000 years old?
- The activity of the sample will be too high
- It is difficult to accurately measure the small change in activity
- Therefore the ratio of C-14 to C-12 will be too high to determine an accurate age
Why is carbon dating unreliable if the sample is more than 40,000 years ago?
- The activity will be too small and have a count rate similar to that of background radiation
- So, there will be very few C-14 atoms remaining, hence very few decays will occur
- Therefore, the ration of C-14 to C-12 will be too small to determine an accurate age
What is the energy, mass, speed equation?
E = mc^2
E = energy J
m = mass kg
c = speed of light ms^-1
What does the energy mass speed equation show?
Mass can be converted into energy and energy can be converted into mass
What is binding energy equal to?
The amount of energy released in forming a nucleus
What is the binding energy, mass defect and speed equation?
E = (Δm) c^2
E = binding energy J
Δm = mass defect kg
c = speed of light
What is significant about the daughter nuclei produced as both a result of fusion and fission?
It has a higher binding energy per nucleon than the parent nucleus. Energy is released as a results of the mass difference between the parent nuclei and the daughter nuclei
What is annihilation?
When a particles meets its equivalent anti-particle they both are destroyed and their mass is converted into energy in the form of 2 gamma ray photons
What is the opposite of annhihilation?
Pair production
What is pair production?
When a photon interacts with a nucleus or atom and the energy of the photon is used to create a particle antiparticle pair
What is essential for pair-production to occur?
The presence of a nearby neutron - this is so that the process conserves both and energy and momentum. A single photon alone cannot produce a particle anti-particle pair because the coservation laws would be broken. Pair creation is a case of energy being converted into matter
In pair production, what is the energy of a photon equation?
E photon = hf = hc / λ = c^2 Δm
What is the typical energy of 2 photons emitted in annhihilation?
1.02 MeV ; found from E = mc^2 where m = 2m rest photon (because 2 photons are emitted)
What did experiments into nuclear structure find?
The total mass of a nucleus is less than the sum of the masses of its constituent nucleons
What is the mass defect?
The difference between the measured mass of a nucleus and the sum total of the masses of its constituent nucleons
What does the existence of binding energy imply?
Due to the equivalence of mass and energy, this decrease implies that energy is released in the process. Since nuclei are made up on neutrons and protons there are forces of repulsion between the positive protons; it therefore takes energy to hold nucleons together as a nucleus
What is binding energy?
The energy required to break a nucleus into its constituent protons and neutrons
What type of reaction is the formation of a nucleus from a system of isolated protons and neutrons?
An exothermic reaction - it releases energy
What is binding energy per nucleon?
The binding energy of a nucleus divided by the number of nucleons in the nucleus
What does a higher binding energy per nucleon indicate?
A higher stability - it requires more energy to pull the nucleus apart
What element has the highest binding energy per nucleon, and therefore the highest stability?
Iron-56
What is the binding energy and mass number equation?
Binding energy = binding energy per nucleon x mass number
What does a lower number of nucleons in a nucleus typically mean in reference to binding energy?
- Nuclei tend to have a lower binding energy per nucleon, they are generally less stable
- This means the lightest elements have weaker electrostatic forces and are the most likely to undergo fusion
What elements do not fit the binding energy-number of nucleons trend and why?
HELIUM-4 : it is a particularly stable nucleus and hence it has a high binding energy per nucleon
CARBON-12 & OXYGEN-16 : can be considered yo be 3 or 4 helium nuclei respectively bound together and hence also stable
What does a higher number of nucleons in a nucleus typically mean in reference to binding energy?
- The general binding energy per nucleon is high and gradually decreases with the number of nucleons
- This means that the heaviest elements are the most unstable and likely to undergo fission
What is the mass defect equation?
Δm = Z mp + (A - Z) mn - m total
Δm = mass defect kg
Z = proton number
mp = mass of proton kg
A = nucleon/ mass number
mn = mass of neutron
m total = measured mass of the nucleus kg
What is nuclear fission?
The splitting of a large, unstable nuclei into 2 smaller nuclei
How does nuclear fission occur?
- A neutron collides with an unstable nucleus
- The nucleus splits into 2 smaller nuclei (daughter nuclei) as well as 2 or 3 neutrons
- Gamma rays are also emitted
- The products of fission move away very quickly
- Energy is transferred from nuclear potential energy to kinetic energy
What is spontaneous fission?
Where nuclei undergo fission without additional energy being put into the nucleus (this is rare)
What is induced fission?
A neutron is fired at and absorbed by the nuclei to begin the fission
What is induced fission used for?
To speed up the fission of elements that have a long half-life - this way they become more suitable for producing energy in nuclear power stations
What is a chain reaction?
During fission, 2 or 3 neutrons are produced and these move away at a high speed. Each of these neutrons can start another fission reaction, which again creates further excess neutrons. This occurrence can keep repeating and this is a chain reaction
What is the purpose of a moderator?
To slow down neutrons
- The moderator is a material that surrounds fuel rods and control rods inside the reactor core
- The fast-moving neutrons produced by the fission reactions slow down by colliding with the molecules of the moderator causing them to lose momentum
- The neutrons are slowed down so they are in thermal equilibrium with the moderator - this ensures neutrons can react efficiently with the uranium fuel
What is the purpose of a control rod?
To absorb neutrons
- The number of neutrons absorbed is controlled by varying the depth of the control rods in the fuel roads
- Lowering the rods means more neutrons are absorbed - lowering the rate of fission
- Raising the rods means fewer neutrons are absorbed - increasing the rate of fusion
- This is adjusted automatically so that exactly one fission neutron produced by each fission goes onto the next
- In the event that the nuclear reactor needs to be shut down, the control rods can be lowered all the way so no reaction can take place
What is the purpose of a coolant?
To remove the heat released by the fission reactions
- The coolant carries the heat to an external boiler to produce steam
- This steam then goes on to power electricity turbines
What does a nuclear reactor look like?
What are the 3 types of radioatcive waste?
- Low level
- Intermediate
- High level
What is low level waste?
Waste such as clothing, tools and gloves which may be lightly contaminated. This type of waste will be radioactive for a few years, so must be encased in concrete and stored a few metres underground until it can be disposed of with regular waste
What is intermediate level waste?
Everything between daily used items and the fuel rods themselves. Usually, this is the waste produced when a nuclear power station is decommissioned and taken apart. This waste has a longer half-life than the low level waste, so it must be encased in cement in steel drums and stored safely underground
What is high level waste?
This waste compromises of the unusable fission products from the fission of U-235 or from spent fuel rods. This is the most dangerous waste and will remain radioactive for thousands of years. Additionally the spent rods are extremely hot and must be handled and stored very carefully
How is high-level waste treated?
- The waste is initially placed in cooling ponds of water close to the reactor for a number of years
- Isotopes of plutonium and uranium are harvested to be used again
- Waste is mixed with molten glass and made solid (vitrification)
- It is then encased in containers made from steel, lead or concrete
- This type of waste must be stored very deep underground
What are the environmental considerations for disposing radioactive waste?
- Isotopes with long half-lives must not enter our water and food supplies
- Burial locations must be geologically stable, secure from attack and designed for safety
- Space for such locations is limited
What is nuclear fusion?
Small nuclides that combine together to make larger nuclei, releasing energy
What type of nuclei can undergo fusion?
Low mass nuclei such as hydrogen & helium
What is required for fusion to occur?
The 2 nuclei must have high kinetic energy. This is because the protons inside the nuclei are positively charged, which means they repel one another.
Why can fusion only occur in high energy environments, such as stars?
Because it takes a great deal of energy to overcome the electrostatic forces
