single content Flashcards
What is an electric field?
The region where a charged object would experience a force
How is an electric field represented?
Using field lines
What does the direction of a field line show?
The direction of the force that would act on a small positive charge at that point
What does the spacing of field lines show?
The strength of the force.
The closer together the lines, the stronger the force
What is a uniform field?
A field for a force that has a constant size
What direction do field lines point when an object is positively charged?
Field lines point outwards because like charges repel
What direction do field lines point when an object is negatively charged?
Field lines point inwards because unlike charges attract
What happens to field lines as you move away from the charged object?
Field lines spread out as you move away, so the force becomes weaker
How does a ‘Van de Graff’ generator work?
A charge builds up on the dome due to electrons being rubbed off by the belt. If charge is big enough then the voltage comes high enough to ionise the air molecules so electrons jump down to earth = electric current
Dangers with static electricity when refuelling?
Fuel gains electrons from pipe so pipe is positive and fuel negative. Resulting voltage may cause a spark.
Solutions for static electricity when refuelling?
Earth the fuel tank with a copper rod or connect the tanker to a place by a copper conductor
How does paint spraying work?
Spray gun is charged and so are the paint particles, particles repel each other giving a fine spray. Object is charged opposite to paint so object attracts paint = even coat
What is an electrostatic dust precipitator?
A mechanism that remove smoke, dust particles etc. from chimneys
How do electrostatic dust precipitators work?
Dust particles gain charge as they pass through a grid
Dust particles are attracted to plates
Dust falls down the chimney when the particles are heavy enough or the plates are knocked
What does the pressure of a gas depend on?
mass of molecules
speed of molecules - hitting walls more often
how many molecules - more molecules to hit the walls
What happens when the volume of a gas decreases while the temperature stays the same?
The average speed stays the same but the molecules hit the walls more often
What does Boyles law state?
The pressure of a fixed mass of gas at a constant temperature is inversely proportional to its volume
pV=constant
pV=constant or…
p1V1=p2V2
How is gamma used for tracers in medicine?
Gamma radiation should be emitted from the source injected into the patient, so it can be detected outside the body
Why isn’t alpha used for tracers in medicine?
Would be too dangerous as the ionisation it causes could mutate cells and cause cancer
How is beta used for leak detection in pipes?
The radioactive isotope is injected into the pipe. The outside of the pipe is checked with a Geiger-Muller detector to find areas of high radioactivity - where the pipe is leaking
Why is beta used for leak detection in pipes?
It has a short half life so the material doesn’t become a long term problem. It has to be detected through the metal as well as the earth
Why is gamma used in radiotherapy?
Gamma kills cancer cells
Why is gamma used in food irradiation?
Because gamma keeps it fresh for longer as it kills bacteria or fungi
What happens when there are more nuclei in a sample in terms of half life?
When there are more nuclei, more are likely to decay in one half life, so the higher the count rate will be
What is the half life of a radioactive isotope?
• The average time it takes for the number of nuclei of the isotope in a sample to half
or
• The time it takes for the count rate from a sample containing the isotope to fall to half its initial level
What is the net decline of a radioactive sample?
The fraction of nuclei that have decayed in a certain time
How is the net decline of a radioactive sample portrayed?
Fraction, ratio, percentage
doesn’t have a unit
What is the half life required when sterilising medical instruments?
Long - years
What is the half life required when using medical tracers?
Short - days
What is the half life required when using a thickness monitor?
Long - years
What is the half life required when using industrial tracers?
Short - days
What is the half life required when using a smoke alarm?
Long - years
When does a nucleus become unstable?
When it possesses either too many or too few neutrons compared to the number of protons
What is radiation detected with ?
A Geiger counter
Define half life using the idea of activity
The time it takes for activity to half from its original value
Define half life using the idea of count rate
The time it takes for count rate to half from its original value
Define half life using the idea of undecayed atoms left in the source
The time it take for half of the nuclei present to decay
How is nuclear radiation used in medicine for imaging?
- radioactive tracers - imaging flows
* gamma cameras - imaging internal organs
What do radioactive tracers image?
Flows inside the body
What do gamma cameras image?
Internal organs
Why is imaging using radioactive radiation important?
To help with diagnosis
Why is nuclear radiation used in medicine?
For treatment; to destroy cancer cells
What processes using nuclear radiation help with diagnosis?
- radioactive tracers
* gamma cameras
What processes using nuclear radiation help with treatment?
- gamma radiotherapy
* radioactive implants
What are radioactive tracers used for?
To trace the flow of a substance through an organ
How do radioactive tracers work?
- a small amount of radioactive material is put into the patients body
- tracer is given time to move through body
- radioactive detector is positioned to detect radiation outside the body
- multiple images are taken to show progress of tracer over time
How is radioactive material put into a patients body to be used as a tracer?
By ingestion or injection
What type of radiation is used in medical tracing?
Gamma
Why is gamma used in medical tracing?
It is the least ionising and the most penetrating, meaning it can leave the body eventually
Why isn’t alpha used in medical tracing?
It can’t be detected outside the body and is very ionising so would damage cells as it moved through the bloody
What are the properties of gamma that ensure medical tracing is not harmful to the patient?
Gamma is not ionising, has a short half life and is not toxic
What does a gamma camera do?
Images internal organs
How does a gamma camera work?
- a radioactive isotope (gamma emitting) is objected into the patient
- the isotope concentrates in the organ
- a gamma camera is positioned over the area emitting gamma
What are gamma cameras made from?
A crystal scintillator (sodium iodide)
What happens when gamma hits a crystal scintillator?
It produces a burst of light
What do photo multiplier tubes do?
Convert energy in electrical signals - which are fed to a computer to produce an image
What does the lead grid do in a gamma camera?
Acts as a collinator - only allows gamma aligned with the holes to hit the crystal, making image sharper
What is a collinator used for in a gamma camera?
To make the image sharper, less blurred
What is the process happening inside a gamma camera to produce an image?
- gamma rays hit a crystal scintillator and crystal produces a burst of light
- bursts of light are picked up by a photo multiplier which convert energy to electrical signals
- the electrical signals are sent to a computer to produce an image
What can ionising radiation do to cells?
Low doses - damage cells
High doses - destroy cells
This can lead to cancer
How are cancer cells destroyed in a tumour?
Gamma radiation in narrow beams
Where is radiation emitted from in radiotherapy?
A radioactive isotope of cobalt
What is the half live of the radioactive isotope of cobalt?
Five years
Why are narrow beams used in radiotherapy?
So that no tissue before the area that is being treated becomes damaged
How do narrow beams of gamma destroy a tumour?
Lots of beams are shone from different angles to overlap on the problem area
Why is gamma used in radiotherapy?
It can damage deep inside the body - it is very penetrating - and can escape the body easily
Why is a half life of five years suitable for gamma radiotherapy?
It lasts for long enough so it doesn’t need to be replaced and the dose each patient receives doesn’t decrease
What are gamma knives used for?
To accurately focus many beams of gamma radiation on one or more brain tumours
What are the beams of radiation in gamma knives life?
- very low intensity
* approximately 200 beams - concentrated on a small volume very accurately
What are radioactive implants used for?
To attack the tumour from inside the body
How do radioactive implants work?
The sources of radiation are placed directly inside the tumour or nearly tissue
How long can radioactive implants stay in for?
Hours or days due to low dose rate sources
What type of radiation can radioactive implants emit?
Beta because they are more local to the tumour and are more ionising than gamma
What is a suitable half life for a radioactive implant?
A few weeks:
- enough time for radiation dose to be low for an extended period
- not long enough to stay active in patient after it’s needed
Advantages of radioactive implants?
- therapy over longer period compared to external beam therapy
- radiation more localised - especially with beta
- patient can spend less time in hospital
Disadvantages of radioactive implants?
- small risk of exposure of radiation to family and friends
* only early stage diseases and small tumours can be treated
What is nuclear fission?
The splitting of a large nucleus into two smaller nuclei (‘daughters’) while releasing energy
Which nuclei are split in fission reactions?
Usually uranium-235 but also plutonium-239
What is induced fission?
When a reaction occurs due to a neutron being fired at a uranium nucleus
What do fission reactions release?
- two or three neutrons at high speeds (these are used in further reactions)
- energy in the form of radiation and kinetic energy of the fission neutrons