Nuclear Physics Flashcards
What is the Rutherford scattering experiment
alpha particles fired at thin gold foil and a detector on the other side to detect how many particles deflected at different angles
What were Rutherford’s results?
The majority of α-particles went straight through (A)
This suggested the atom is mainly empty space
Some α-particles deflected through small angles of < 10o (B)
This suggested there is a positive nucleus at the centre (since two positive charges would repel)
Only a small number of α-particles deflected straight back at angles of > 90o (C)
This suggested the nucleus is extremely small and this is where the mass and charge of the atom is concentrated
It was therefore concluded that atoms consist of small dense positively charged nuclei
Describe alpha radiation
Alpha is the most ionising type of radiation
-This is due to it having the highest charge of +2e
-This means it produces the greatest number of ion pairs per mm in air
-This also means it is able to do more damage to cells than the other types of radiation
Alpha is the least penetrating type of radiation
-This means it travels the shortest distance in air before being absorbed
-Alpha particles have a range of around 3-7 cm in air
-Alpha can be stopped by a single piece of paper
Describe beta particles
Beta (β−) particles are high energy electrons emitted from the nucleus
Beta (β+) particles are high energy positrons
(antimatter of electrons) also emitted from the nucleus
β− particles are emitted by nuclei that have too many neutrons
β+ particles are emitted by nuclei that have too many protons
Describe beta radiation
Beta is a moderately ionising type of radiation
This is due to it having a charge of +1e
This means it is able to do some slight damage to cells (less than alpha but more than gamma)
Beta is a moderately penetrating type of radiation
Beta particles have a range of around 20 cm - 3 m in air, depending on their energy
Beta can be stopped by a few millimetres of aluminium foil
Describe gamma rays
Gamma (γ) rays are high energy electromagnetic waves
They are emitted by nuclei that need to lose some energy
If these particles hit other atoms, they can knock out electrons, ionising the atom
This can cause chemical changes in materials and can damage or kill living cells
Describe gamma radiation
Gamma is the least ionising type of radiation
This is because it is an electromagnetic wave with no charge
This means it produces the least number of ion pairs per mm in air
It can still cause damage to cells, but not as much as alpha or beta radiation. This is why it is used for cancer radiotherapy
Gamma is the most penetrating type of radiation
This means it travels the furthest distance in air before being absorbed
Gamma radiation has an infinite range and follows an inverse square law
Gamma can be stopped by several metres of concrete or several centimetres of lead
Applications of alpha beta and gamma in smoke detectors
Smoke Detectors
Smoke detectors contain a small amount of Americium-241, which is a weak alpha source
Within the detector, alpha particles are emitted and cause the ionisation of nitrogen and oxygen molecules in the air
These ionised molecules enable the air to conduct electricity and hence a small current can flow
If smoke enters the alarm, it absorbs the alpha particles, hence reducing the current which causes the alarm to sound
Am-241 has a half-life of 460 years, meaning over the course of a lifetime, the activity of the source will not decrease significantly and it will not have to be replaced
Applications of alpha beta and gamma in thickness controls
Beta radiation can be used to determine the thickness of aluminium foil, paper, plastic, and steel
The thickness can be controlled by measuring how much beta radiation passes through the material to a Geiger counter
Beta radiation must be used, because:
Alpha particles would be absorbed by all the materials
Gamma radiation would pass through undetected through the materials
The Geiger counter controls the pressure of the rollers to maintain the correct thickness
A source with a long half-life must be chosen so that it does not need to be replaced often
What is the inverse-square law of gamma radiation
As an electromagnetic wave, gamma radiation shares many of the same wave properties as light
Light sources which are further away appear fainter because the light they emit is spread out over a greater area than a light source which is closer by
The moment the light leaves the source, it begins to spread out uniformly as a sphere, according to an inverse square law
Background radiation in natural sources
Radon gas from rocks and soil
Heavy radioactive elements, such as uranium and thorium, occur naturally in rocks in the ground
Uranium decays into radon gas, which is an alpha emitter
This is particularly dangerous if inhaled into the lungs in large quantities
Background radiation in cosmic rays from space
The sun emits an enormous number of protons every second
Some of these enter the Earth’s atmosphere at high speeds
When they collide with molecules in the air, this leads to the production of gamma radiation
Other sources of cosmic rays are supernovae and other high energy cosmic events
Background radiation in carbon-14 in biological material
All organic matter contains a tiny amount of carbon-14
Living plants and animals constantly replace the supply of carbon in their systems hence the amount of carbon-14 in the system stays almost constant
Background radiation in radioactive material in food and drink
Naturally occurring radioactive elements can get into food and water since they are in contact with rocks and soil containing these elements
Some foods contain higher amounts such as potassium-40 in bananas
However, the amount of radioactive material is minuscule and is not a cause for concern
Background radiation in medical sources
In medicine, radiation is utilised all the time
Uses include X-rays, CT scans, radioactive tracers, and radiation therapy
Background radiation in nuclear waste
While nuclear waste itself does not contribute much to background radiation, it can be dangerous for the people handling it
Background radiation in nuclear fallout from nuclear weapons
Fallout is the residue radioactive material that is thrown into the air after a nuclear explosion, such as the bomb that exploded at Hiroshima
While the amount of fallout in the environment is presently very low, it would increase significantly in areas where nuclear weapons are tested
Background radiation in nuclear accidents
Accidents such as that in Chernobyl contributed a large dose of radiation into the environment
While these accidents are now extremely rare, they can be catastrophic and render areas devastated for centuries
What characteristics are preferred when choosing a source to work with?
Short-lived isotopes
The smaller the amount of radioactive material, the better
What are the biggest risks when working with radioactive sources?
Exposure and contamination
When does contamination happen
when a piece of radioactive material is transferred onto a person, or a personal item, where it can then decay and cause damage
What precautions are taken to reduce the risk of harm when using radioactive sources
Keeping radioactive sources shielded when not in use, for example in a lead-lined box
Wearing protective clothing to prevent the body from becoming contaminated
Keeping personal items outside of the room to prevent these from becoming contaminated
Limiting exposure time so less time is spent with radioactive materials
Handling radioactive materials with long tongs to increase the distance from them
Monitoring the exposure of workers, such as radiographers, using detector badges
Describe radiation therapy
Gamma radiation can be used to destroy cancerous tumours
The gamma rays are concentrated on the tumour to protect the surrounding tissue
Less penetrating beta radiation can be used to treat skin cancer by direct application to the affected area
What are precautions for a patient in radiation therapy
The patient should be protected with lead to cover parts of the body not to be exposed to radiation
The exact dose should be calculated carefully
The dose should be directed very accurately at the cancerous tissue to minimise damage to healthy tissue
What are precautions for the radiographer during radiation therapy
The radiographer should handle the source remotely with tongs or a machine
The radiographer should be protected by a screen
The radiographer should be a long way from the source while the dose is given
The source should be immediately stored in its lead case once the dose is given
Why are radioactive tracers with a short half life preferred
Initially, the activity is very high, so only a small sample needed
The shorter the half-life, the faster the isotope decays
Isotopes with a shorter half-life pose a much lower risk to the patient
The medical test doesn‘t last long so a half-life of a few hours is enough
Sterilising medical equipment with…
Gamma radiation
Why is gamma most suited to Sterilising Medical Equipment
It is the most penetrating out of all the types of radiation
It is penetrating enough to irradiate all sides of the instruments
Instruments can be sterilised without removing the packaging
Why can’t equipment become radioactive when sterilising with gamma
In order for a substance to become radioactive, the nuclei have to be affected
Ionising radiation only affects the outer electrons and not the nucleus
The radioactive material is kept securely sealed away from the packaged equipment so there is no chance of contamination
Required Practical: Inverse Square-Law for Gamma Radiation aim of experiment
to verify the inverse square law for gamma radiation of a known gamma-emitting source
Required Practical: Inverse Square-Law for Gamma Radiation
variables
Independent variable = the distance between the source and detector, x (m)
Dependent variable = the count rate / activity of the source, C
Control variables
The time interval of each measurement
The same thickness of aluminium foil
The same gamma source
Required practical: Inverse Square-Law for Gamma Radiation
- Measure the background radiation using a Geiger Muller tube without the gamma source in the room, take several readings and find an average
- Next, put the gamma source at a set starting distance (e.g. 5 cm) from the GM tube and measure the number of counts in 60 seconds
- Record 3 measurements for each distance and take an average
- Repeat this for several distances going up in 5 cm intervals
8.1.7 Required Practical: Inverse Square-Law for Gamma Radiation
Analysis of results
According to the inverse square law, the intensity, I, of the γ radiation from a point source depends on the distance, x, from the source
- Square each of the distances and subtract the background radiation from each count rate reading
- Plot a graph of the corrected count rate per minute against 1/x2
- If it is a straight line graph through the origin, this shows they are directly proportional, and the inverse square relationship is confirmed
8.1.7 Required Practical: Inverse Square-Law for Gamma Radiation
Evaluating experiment
Systematic errors
The Geiger counter may suffer from an issue called “dead time”
- This is when multiple counts happen simultaneously within ~100 μs and the counter only registers one
-This is a more common problem in older detectors, so using a more modern Geiger counter should reduce this problem
The source may not be a pure gamma emitter
-To prevent any alpha or beta radiation being measured, the Geiger-Muller tube should be shielded with a sheet of 2–3 mm aluminium
8.1.7 Required Practical: Inverse Square-Law for Gamma Radiation
Evaluating experiment
Random errors
Radioactive decay is random, so repeat readings are vital in this experiment
Measure the count over the longest time span possible
-A larger count helps reduce the statistical percentage uncertainty inherent in smaller readings
-This is because the percentage error is proportional to the inverse-square root of the count
8.1.7 Required Practical: Inverse Square-Law for Gamma Radiation
Evaluating experiment
Safety conditions
For the gamma source:
- Reduce the exposure time by keeping it in a lead-lined box when not in use
- Handle with long tongs
- Do not point the source at anyone and keep a large distance (as activity reduces by an inverse square law)
Safety clothing such as a lab coat, gloves and goggles must be worn
How to calculate % uncertainty from graph
((Worst gradient — best gradient) / (best gradient)) x 100%
Define radioactive decay
The spontaneous disintegration of a nucleus to form a more stable nucleus, resulting in the emission of an alpha, beta or gamma particle
What does it mean if radioactive decay is a random process
There is an equal probability of any nucleus decaying
It cannot be known which particular nucleus will decay next
It cannot be known at what time a particular nucleus will decay
The rate of decay is unaffected by the surrounding conditions
It is only possible to estimate the proportion of nuclei decaying in a given time period
How can you demonstrate the random nature of radioactive decay
Using a Geiger-muller (GM) tube
When a GM tube is placed near a radioactive source, the counts are found to be irregular and cannot be predicted
Each count represents a decay of an unstable nucleus
These fluctuations in count rate on the GM tube provide evidence for the randomness of radioactive decay
What is the average decay rate
the average number of nuclei that are expected to decay per unit time
Define the decay constant (λ)
The probability that an individual nucleus will decay per unit of time
How to calculate the number of decays per unit time (activity)
Equation for radioactive decay
N = N0 e^–λt
N0 = the initial number of undecayed nuclei (when t = 0)
N = number of undecayed nuclei at a certain time t
λ = decay constant (s-1)
t = time interval (s)
Activity formula
A = A0 e–λt
Count rate formula
C = C0 e–λt
Formula for number of moles
Define avogadros constant (NA)
The number of atoms in one mole of a substance; equal to 6.02 × 1023 mol-1
Formula for number of nuclei (number of atoms)
Define half life
The time taken for the initial number of nuclei to halve for a particular isotope
What’s the half life formula
Define mass defect
The difference between an atom’s mass and the sum of the masses of its protons and neutrons
How to calculate the mass defect ( Δm) of a nucleus
Δm = Zmp + (A – Z)mn – mtotal
Where:
Z = proton number
A = nucleon number
mp = mass of a proton (kg)
mn = mass of a neutron (kg)
mtotal = measured mass of the nucleus (kg)
Why does it take energy (binding energy) to hold nucleons together as a nucleus
Since nuclei are made up of neutrons and protons, there are forces of repulsion between the positive protons
Define binding energy
The amount of energy required to separate a nucleus into its constituent protons and neutrons
As energy and mass are proportional, what does this mean for the total energy of a nucleus?
It is less than the sum of the energies of its constituent nucleons
The formation of a nucleus from a system of isolated protons and neutrons is…
An exothermic reaction
What is the atomic mass unit (u) equal to?
1.661 x 10^-27
Define the atomic mass unit (u)
The mass of exactly one-twelfth of an atom of carbon-12
Define nuclear fusion
The fusing together of two small nuclei to produce a larger nucleus
Define nuclear fission
The splitting of a large atomic nucleus into smaller nuclei
Describe nuclear fission
Fission must first be induced by firing neutrons at a nucleus
When the nucleus is struck by a neutron, it splits into two, or more, daughter nuclei
During fission, neutrons are ejected from the nucleus, which in turn, can collide with other nuclei which triggers a cascade effect
This leads to a chain reaction which lasts until all of the material has undergone fission, or the reaction is halted by a moderator
Describe nuclear fusion
Low mass nuclei (such as hydrogen and helium) can undergo fusion and release energy
When two protons fuse, the element deuterium is produced
In the centre of stars, the deuterium combines with a tritium nucleus to form a helium nucleus, plus the release of energy, which provides fuel for the star to continue burning
Formula for binding energy
Binding energy = Binding Energy per Nucleon × Mass Number
Convert MeV to Joules
1 MeV = 1.60 x 10^-13
Define binding energy per nucleon
The binding energy of a nucleus divided by the number of nucleons in the nucleus
Compare fission and fusion
Part 1
- In fusion, the mass of the nucleus that is created is slightly less than the total mass of the original nuclei
- The mass defect is equal to the binding energy that is released since the nucleus that is formed is more stable - Fission occurs at high values of A because:
- Repulsive electrostatic forces between protons begin to dominate, and these forces tend to break apart the nucleus rather than hold it together - In fission, an unstable nucleus is converted into more stable nuclei with a smaller total mass
- This difference in mass, the mass defect, is equal to the binding energy that is released
Compare fission and fusion
Part 2
- Fusion releases much more energy per kg than fission
- The energy released is the difference in binding energy caused by the difference in mass between the reactant and products
- Hence, the greater the increase in binding energy, the greater the energy released - At small values of A (fusion region), the gradient is much steeper compared to the gradient at large values of A (fission region)
- This corresponds to a larger binding energy per nucleon being released
Energy released formula
Energy released = Binding energy after – Binding energy before
Define induced fission
When a stable nucleus splits into small nuclei from the bombardment of a slow-moving neutron
What do we call neutrons involved in induced fission
Thermal neutrons
- low energy and speed, so can induce fission
- important as neutrons with too much energy will rebound away from uranium-235 nucleus and fission will not take place.
Define critical mass
The minimum mass of fuel required to maintain a steady chain reaction
What does it mean if you use the exact critical mass of fuel
A single fission reaction follows the last.
Using less than the critical mass (subcritical mass) would lead the reaction to eventually stop
Using more than the critical mass (supercritical mass) would lead to a runaway reaction and eventually an explosion
Purpose of moderator
To slow down neutrons
The fast-moving neutrons produced by the fission reactions slow down by colliding with the molecules of the moderator, causing them to lose some momentum
The neutrons are slowed down so that they are in thermal equilibrium with the moderator, hence the term ‘thermal neutron’
Purpose of control rods
To absorb neutrons
Lowering the rods further decreases the rate of fission, as more neutrons are absorbed
Raising the rods increases the rate of fission, as fewer neutrons are absorbed
Purpose of the coolant
To remove 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-generating turbines
