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
How to calculate the number of decays per unit time (activity)
Formula for number of moles
Formula for number of nuclei (number of atoms)
What’s the half life formula
