Section 6 - Nuclear Physics Flashcards
What are isotopes?
Atoms with the same number of protons, but a different number of neutrons.
How do the mass number and atomic number of isotopes compare?
Mass number - Different
Atomic number - Same
What is the atomic number?
The number of protons in the atom.
What is the mass number?
The number of protons and the number of neutrons added together.
What is the difference between isotopes of an element?
- 1 or 2 are stable.
* The rest are unstable and radioactive, so they decay into other elements and give out radiation.
What factors affect when radioactive decay may happen?
None - it is entirely random.
Can you tell when radioactive decay may happen?
No, it is entirely random.
What are the 3 types of radiation?
- Alpha
- Beta
- Gamma
What is background radiation?
Radiation that is present at all times, all around us, wherever you go.
What are the two main sources of background radiation?
- Natural sources - Unstable isotopes found in rocks and radiation from space
- Man-made sources - From nuclear weapons tests, nuclear accidents, etc.
How does radiation cause damage?
- Low doses -> Ionise the cell without killing it, which can lead to cancer
- High doses -> Kill cells, which causes radiation sickness
What two factors affect the effects of radiation?
- Length and type of exposure
2. Energy and penetration -> Some types of radiation more harmful than others
Which type of radiation is most dangerous outside the body?
Beta and gamma radiation, because they can penetrate skin and reach delicate organs, unlike alpha radiation.
Which type of radiation is most dangerous inside the body?
Alpha,because they do all their damage in a localised area, while beta and gamma mostly pass through the body.
What precautions should be taken when handling radioactive material?
- Minimise exposure time
- Never allow skin contact + Always handle with tongs
- Hold the source at arm’s length
- Keep the source pointing away from the body and don’t look directly at it
- Store sources in a lead box
Describe the structure of an alpha particle.
Made up of two protons and two electrons
What can an alpha particle be described as?
A helium nucleus
Describe the size, movement and penetration of alpha particles?
Large and slow moving, so they don’t penetrate far.
What is the range of alpha particles in air?
About 5cm.
How ionising are alpha particles and why?
Very ionising, because they are large, so they hit many atoms and ionise them.
Describe the properties of alpha particles.
- Large
- Slow
- Not penetrating
- Very ionising
Describe the structure of a beta particle.
A single electron.
Where do beta particles come from?
The nucleus.
What is the mass and charge of an alpha particle?
Mass: 4
Charge: +2
Describe the size, movement and penetration of beta particles?
They are small and quite fast, so they are moderately penetrating.
What is the range of beta particles in air?
About 1m.
How ionising are beta particles and why?
Moderately ionising, because, although they are not as large as alpha particles, they still can collide with some atoms and ionise them.
Describe the properties of beta particles.
- Quite small
- Quite slow
- Moderately penetrating
- Moderately ionising
What is the mass and charge of a beta particle?
Mass: Almost none
Charge: -1
How is a beta particle released?
A neutron turns into a proton and a beta particle, which is emitted.
What are gamma rays?
Very short wavelength EM waves.
Describe the penetration of gamma rays.
Very penetrating.
How ionising are gamma rays and why?
Not very, because they tend to pass through rather than collide with atoms.
Where do gamma rays come from?
The nucleus.
What is the mass and charge of gamma rays?
It is an EM wave, so it has no charge or mass.
What blocks alpha radiation?
Paper
What blocks beta radiation?
Thin aluminium.
What blocks gamma radiation?
Thick lead.
What happens to an element when it emits a gamma ray?
Nothing, because the gamma ray is just energy.
What happens to an element when it emits an alpha particle?
- The mass number decreases by 4
- The atomic number decreases by 2
- It forms a new element
What happens to an element when it emits a beta particle?
- Mass number stays the same
- Atomic number increases by 1
- It forms a new element
What happens to alpha, beta and gamma particles in a magnetic or electric field and why?
- Alpha is deflected slightly, because it is charged, but heavy
- Beta is deflected more, because it is charged and light
- Gamma is not deflected, because it has no charge
What is the unit for radioactivity?
Becquerel (Bq)
What is 1 becquerel equal to?
1 nucleus decaying per second.
How does the radioactivity of a sample change over time?
It decreases.
What is the half-life of a sample?
The average time it takes for the number of nuclei in a radioactive isotope sample to halve.
OR
The average time it takes for the count rate to halve.
What does a short half-life mean?
The activity falls quickly.
What does a long half life mean?
The activity falls slowly.
How is the half-life of a sample found by looking at a graph?
You look at the amount of time it takes for the count rate to halve on the graph (e.g. look at the time it takes for the activity to go from 80 Bq to 40 Bq).
What are the uses of radiation?
- Smoke detectors (alpha)
- Tracers in medicine (beta and gamma)
- Radiotherapy (gamma)
- Sterilisation of food and surgical instruments (gamma)
What are some uses of alpha radiation?
• Smoke detectors
What are some uses of beta radiation?
• Tracers in medicine
What are some uses of gamma radiation?
- Tracers in medicine
- Radiotherapy
- Sterilisation of food and surgical instruments
How do smoke alarms work?
- Weak source of alpha radiation is placed in the detector, close to two electrodes.
- The source causes ionisation, and a current flows between the electrodes.
- If there is a fire, the smoke will disrupt the current, causing the alarm to sound.
How do tracers in medicine work?
- Beta or gamma emitters are injected or swallowed by a patient
- They can be traced using an external detector
- This can be used to see if substances are being transported correctly, etc.
What are the two factors which must be taken into account when picking a medical tracer?
- Must be a beta or gamma emitter
* Must have a very short half life, so the radioactivity disappears very quickly
How does radiotherapy work?
- The correct dosage of gamma radiation is directed carefully at the cancer
- This kills the cells, without damaging too many normal cells
How does sterilisation of food and surgical instruments work?
- Food and medical instruments are exposed to high doses of gamma rays, which will kill all microbes
- The food is not radioactive afterwards, so it is safe to eat
What are the advantages of sterilising using gamma rays instead of boiling?
It doesn’t involve high temperatures, so items like apples and plastic instruments are not damaged.
What are the two factors which must be taken into account when picking an emitter for sterilising food and surgical instruments?
- Strong emitter of gamma rays
* Quite long half-life, so it doesn’t need frequent replacing
What is nuclear fission?
The splitting of an atomic nuclei into two smaller nuclei and the release of two or three electrons.
What process is used in nuclear power stations?
Nuclear fission
How does a nuclear reactor work?
- A fuel (uranium-235 or plutonium-239) is split up using nuclear fission
- This creates a chain reaction, with each splitting releasing heat
- This heat is used to heat up water to make steam, which drives a steam turbine and generates electricity
What are control rods in a nuclear reactor used for?
Absorb surplus neutrons, which keeps the chain reaction under control.
What is the reactor core made from?
Thick steel, to withstand the temperature and pressure. The whole thing is enclosed by thick concrete walls to absorb radiation that escapes.
Remember to revise the structure of a nuclear reactor.
Pg 96 of revision guide
Describe the chain reaction in a nuclear reactor.
- Slow moving neutron is absorbed into a uranium or plutonium nucleus, causing it to split into 2 smaller nuclei and 2 or 3 neutrons.
- These neutrons can each collide with another nucleus, continuing the chain reaction.
What is nuclear fusion?
The process of forcing two atomic nuclei close enough together so they form a single nucleus.
Which isotopes are used in fission?
Uranium-235 and plutonium-239
Where does the energy in stars come from?
Nuclear fusion
How can nuclei be made to fuse?
Colliding them at very high speeds.
Why is making a nuclear fusion reactor difficult?
It requires very high temperatures before the fusion can take place and the nuclei fused are difficult to contain.
What factors allow nuclear fusion to happen inside stars?
- High temperature
* High pressure
Describe how all the different elements were formed in the universe.
- Early universe contained only hydrogen
- Helium and other small nuclei formed from this in early stars
- When a star became a red giant, the other elements formed by nuclear fusion
- These elements were distributed by supernovas, and heavier elements were also created
Where is the division between light elements formed in stars and the heavier ones formed in supernovas?
- Up to iron -> Fusion
* Heavier than iron -> Supernova
How are light elements formed in the universe?
By nuclear fusion in stars.
How are heavy elements formed in the universe?
In supernovas.
Describe the formation of a star.
- Particles in a cloud of dust and gas are pulled together by gravity
- When they become more concentrated, they form a protostar
- As the protostar gets denser, it gets hotter, until fusion can happen.
What prevents a main sequence star from collapsing?
The heat created by the fusion creates an outward pressure, which balanced the force of gravity pulling everything inward.
Where in a star is energy released from?
The core.
What causes a main sequence star to turn into a red giant?
It runs out of hydrogen to fuse.
Why is a red giant/supergiant red?
Because the surface has cooled slightly.
What causes a star to change from being a red giant or red supergiant?
It runs out of light elements to fuse, so it collapses.
Describe the life cycle of a star smaller than the Sun.
- Protostar
- Main sequence star
- Red giant
- White dwarf
- Black dwarf
Describe the life cycle of a star larger than the Sun.
- Protostar
- Main sequence star
- Red supergiant
- Supernova
- Neutron star OR Black hole
