3.8 Nuclear physics Flashcards
Describe Thomson’s plum pudding model of the atom.
The atom was made up of a sphere a positive charge, with small areas of negative charge evenly distributed throughout.
What experiment disproved Thomson’s plum pudding model of the atom?
The Rutherford scattering experiment
Which model replaced Thomson’s plum pudding model of the atom?
The nuclear model
Describe the setup of the Rutherford scattering experiment.
- An alpha source aimed at gold foil in an evacuated chamber covered in a fluorescent coating
- A microscope that could be moved around the outside of the chamber to observe the path of the alpha particles.
What observations were made during the Rutherford scattering, and what was their significance?
- Most alpha particles passed straight through the foil with no deflection, suggesting the atom is mostly empty space
- A small amount of particles were deflected by a large angle, suggesting the centre of the atom is positively charged
- Very few particles were deflected back by more than 90˚, suggesting the centre of an atom is very small and very dense
What is radiation?
Where an unstable nucleus emits energy in the form of EM waves or subatomic particles in order to become more stable
What are the three types of radiation?
Alpha, beta and gamma
What is the range of alpha radiation in air?
2-10 cm
What is the range of beta radiation in air?
Around 1m
What is the range of gamma radiation in air?
Infinite, following an inverse square law
How ionising is alpha radiation?
Highly
How ionising is beta radiation?
Weakly
How ionising is gamma radiation?
Very weakly
Is alpha radiation deflected by electric and magnetic fields?
Yes
Is beta radiation deflected by electric and magnetic fields?
Yes
Is gamma radiation deflected by electric and magnetic fields?
No
What is alpha radiation absorbed/stopped by?
Paper
What is beta radiation absorbed/stopped by?
Aluminium foil (3mm)
What is gamma radiation absorbed/stopped by?
Several metres of concrete or several inches of lead
Why can the types of radiation emitted from a source be easily identified?
Due to their differing penetrating powers, by measuring the count rate with a GM tube with different barriers in between it and the source, the each type can be identified.
What must be done before measuring the count rate of a source in an experiment?
The background count must be measured before the source is present, then taken away from measured values with the source present.
What is an application of alpha radiation?
Smoke detectors
Explain how smoke detectors work.
- An alpha source is emitted across a gap in the smoke detector circuit
- The air particles are ionised by the alpha particles colliding with them across the gap
- The electrons move to one plate and the positive ions move to the other, creating an electric current
- When there is smoke in the way, it attracts ions and reduces the current, setting the alarm off
Why is alpha radiation used in smoke detectors?
- Alpha radiation does not penetrate very far in air, and therefore in the detector, so is safe to use at home
- The source has a long half life so it does not have to be changed frequently
What are applications of beta radiation?
- Measuring paper/foil thickness
- Radioactive tracers
Explain how beta radiation can be used to measure paper/foil thickness.
- A beta source is placed on one side of the material and a detector on the other
- The thicker it is, the less radiation will get through and so be detected
- The thinner the paper/foil, the more radiation will be let through
- A computer will detect these changes and adjust the roller to change the thickness
Why is beta radiation used to measure paper/foil thickness?
Beta radiation can penetrate paper or thin aluminium, but the amount of penetrating will vary sufficiently as thickness changes.
How do radioactive tracers work?
- A radioactive chemical in injected into someone’s body
- This decays by beta plus decay, emitting a positron
- When this meets electrons in the body they annihilate to produce 2 identical gamma photons
- Detectors around the body can pick this radiation up and produce an image
Do radioactive chemicals used as radioactive tracers have short or long half-lives?
Short, to reduce exposure
What is the purpose of radioactive tracers?
To create an image of a patient’s body to help diagnose them.
What are applications of gamma radiation?
- Radiotherapy
- Sterilising equipment
How does radiotherapy use gamma radiation?
- Ionising radiation kills living cells
- Gamma radiation can be used to kill cancerous cells in a targeted region of the body
- But it will also kills any healthy cells in that region
Why does radiotherapy use gamma radiation?
As gamma radiation is highly penetrating and is weakly ionising, it is the safest type of radiation to use in a patient’s body.
Why is gamma radiation used in sterilising equipment?
Gamma radiation will kill any microbes and bacteria present on the equipment.
What are examples of safety measures used to reduce risk of gamma radiation to medical staff and patients?
- Reduced exposure times
- Shielding
Why does the intensity of gamma radiation follow an inverse square law?
As it moves through air it spreads out in all directions equally.
How can the inverse square law for gamma radiation be investigated?
- Measure the count rate of a gamma source at different distances from the GM tube, making sure to adjust for the background radiation
- Plotting a graph of corrected count against 1/x² will form a straight line verifying the relationship
How must radioactive sources be handled safely?
- Using long handled tongs to move the source
- Storing the source in a lead-lined container when not in use
- Keeping the source as far away as possible from yourself and others
- Never pointing the source towards others
How is the corrected count rate found?
Total count rate - background count rate
What are sources of background radiation?
- Cosmic rays - enter the Earth’s atmosphere from space
- Artificial sources - caused by nuclear weapons testing and meltdowns
- Radon gas - released from rocks
- Rocks containing naturally occurring radioactive isotopes
What kind of process is radioactive decay?
Random
What does radioactive decay being a random process mean?
You can’t predict when the next decay will occur.
What does λ represent for a given radioactive nucleus?
The decay constant
What is the decay constant of a radioactive nucleus?
The probability of a nucleus decaying per unit time.
What is the half life of a radioactive substance?
The time taken for the number of nuclei to halve.
What kind of decay is radioactive decay?
Exponential
How can the half-life of a radioactive substance be determined graphically?
- For a graph of the number of nuclei against time, measure the time taken for the sample size to half
- The modulus of the gradient of the graph of ln(N0) against time
What is activity?
The number of nuclei that decay per second
How is activity related to half-life?
The time taken for activity to half is equal to the half-life
When can the decay constant be used to model the decay of nuclei?
When there is a large number of nuclei in a sample
How does the half-life of a radioactive nucleus affect the way it can be used?
- Nuclei with a long half-life, such as carbon-14, which has a half-life of 5730 years, can be used to date organic objects
- Nuclei with relatively short half-lives are used as radioactive tracers in medical diagnosis, such as technetium-99, as it is a pure gamma emitter with a half-life of 6 hours
How can carbon-14 be used to date organic objects?
By measuring the current amount of carbon-14 and comparing it to the initial amount, the percentage of which is approximately equal in all living things.
How and why must radioactive nuclei with an extremely long half-life be stored?
For example, in steel casks underground, to prevent these nuclei from damaging the environment and the people that may be living around them hundreds of years into the future.
Why does a nucleus experience radioactive decay?
- Nuclei are held together by the strong nuclear force
- But protons experience a force of electromagnetic repulsion
- If these forces are out of balance the nucleus will become unstable and experience radioactive decay
What are the four reasons why a nucleus might become unstable?
- Too many neutrons
- Too many protons
- Too many nucleons
- Too much energy
How does a nucleus with too many neutrons decay?
- Through beta-minus emission
- One of the neutrons in the nucleus changes to a proton and a beta-minus particle and antineutrino is released
- The nucleon number is constant, while the proton number increases by 1
How does a nucleus decay if it has too many protons?
- Through beta-plus emission or electron capture
- In beta-plus decay, a proton changes into a neutron and a beta-plus particle and neutrino is released
- In electron capture, an orbiting electron is taken in by the nucleus and combined with a proton forming a neutron and neutrino
- The nucleon number stays constant, while the proton number decreases by 1
How does a nucleus decay if it has too many nucleons?
- Through alpha emission
- A helium nucleus is released from the nucleus
- The nucleon number decreases by 4 and the proton number decreases by 2
How does a nucleus decay if it has too much energy?
Through gamma emission.
When does gamma emission usually occur and why?
After a different type of decay, such as alpha or beta decay because the nucleus becomes excited and has excess energy.
What does N represent in nuclear physics?
Number of neutrons
What does Z represent in nuclear physics?
Number of protons
What does A represent in nuclear physics?
Number of nucleons
On an N-Z graph, where does beta minus decay take place?
Above the stable nuclides
On an N-Z graph, where does beta plus decay take place?
Below the stable nuclides
On an N-Z graph, where does alpha decay take place?
Below the stable nuclides, from around the middle to the very top.
Why does the number of neutrons and protons in a stable nucleus not increase uniformly beyond around 20 of each nucleon?
- Beyond this amount the electromagnetic force of repulsion becomes larger than the strong nuclear force keeping the nucleus together
- So more neutrons are needed to increase the distance between protons in order to decrease the magnitude of the electromagnetic force to keep the nucleus stable
How is technetium-99m formed?
Beta minus decay
What state is technetium-99m formed in?
An excited nuclear state
What happens because technetium-99m is formed in an excited nuclear state?
It will emit a gamma photon to reach the ground state.
Why is technetium-99m used in medical diagnosis?
- It is a pure gamma emitter
- Gamma rays can be easily detected by a gamma camera
- Gamma rays are very weakly ionising
- It has a half-life of 6 hours
- It can be easily prepared on site
How can the nuclear radius of an atom be estimated?
By calculating the distance of closest approach of a charged particle.
What is the distance of closest approach of an alpha particle fired at a gold nucleus?
- It will have an initial kinetic energy
- As it moves towards the positively charged nucleus, it will experience an electrostatic force of repulsion and slow down as its kinetic energy is converted to electric potential energy
- The point at which the particle stops and has no kinetic energy is its distance of closest approach
- It’s electric potential energy is equal to its initial kinetic energy due to conservation of energy
Why is the distance of closest approach not a very accurate estimate of nuclear radius?
It will always be an overestimate.
What is a more accurate method for calculating nuclear radius?
Electron diffraction
