Home >> IGCSE Physics >> Nuclear Physics
We start off by exploring how Rutherford discovered that the atom has a central core - known as the nucleus - by analyzing the results from the famous Gold Foil experiment. Then we look at nuclear structure and how an unstable nucleus releases energy in the form of ionising radiation to become more stable. The practical uses and dangers of this radiation are considered, before we finish with an in-depth look at nuclear fission and how nuclear power stations work. The pros and cons of nuclear power are considered.
Student booklet (updated: Sept 2021)
8.1 - Discovery of the Nucleus
Objectives:
- Describe the results of Geiger and Marsden’s experiments with gold foil and alpha particles.
- Describe Rutherford’s nuclear model of the atom and how it accounts for the results of Geiger and Marsden’s experiment and understand the factors (charge and speed) which affect the deflection of alpha particles by a nucleus.
(Background reading - the Geiger-Marsden experiment is no longer on the syllabus)
Before the discovery of the nucleus, it was assumed that the atom was a solid(ish) ball of positive charge with negative electrons embedded within it. With the benefit of hindsight, this so-called 'Plum Pudding Model' was a bit daft. As positive and negative attract each other, then how are the electrons removed and if the electrons are embedded, how do chemical bonds work? Anyhow, this model was very short lived as in 1910 Rutherford and his two research assistants blew it out of the water when they scattered alpha particles (heavy positively charged particles) off the gold foil, which established the existence of a tiny, dense nucleus.
The experiment was set up to fire a narrow beam of alpha particles at a very thin gold foil. Gold was chosen as it is dense, malleable and able to be rolled into a foil of only a few atoms thick. The idea was to measure how much the alpha particles were scattered for a PhD thesis. The experiment was performed in a vacuum chamber to avoid the particles being scattered by the air molecules. They expected, and saw, that the vast majority of the radiation went through the gold foil undeflected. The surprise was that they noticed a few being deflected very very large angles, almost \(180^{\circ}\). This phenomenon is now known as Rutherford Scattering. As Rutherford knew the average speed of the alpha particles, the atomic mass of both the particles and the gold atom, and their electric charges he figured out (using some complicated maths) that the maximum possible radius for the nucleus was approx \(10^{-14} \,\text{m} \), which was \(10,000\,\times\) smaller than the radius of the atom itself! This means that the atom is mostly empty space - a lot of empty space.
Before the discovery of the nucleus, it was assumed that the atom was a solid(ish) ball of positive charge with negative electrons embedded within it. With the benefit of hindsight, this so-called 'Plum Pudding Model' was a bit daft. As positive and negative attract each other, then how are the electrons removed and if the electrons are embedded, how do chemical bonds work? Anyhow, this model was very short lived as in 1910 Rutherford and his two research assistants blew it out of the water when they scattered alpha particles (heavy positively charged particles) off the gold foil, which established the existence of a tiny, dense nucleus.
The experiment was set up to fire a narrow beam of alpha particles at a very thin gold foil. Gold was chosen as it is dense, malleable and able to be rolled into a foil of only a few atoms thick. The idea was to measure how much the alpha particles were scattered for a PhD thesis. The experiment was performed in a vacuum chamber to avoid the particles being scattered by the air molecules. They expected, and saw, that the vast majority of the radiation went through the gold foil undeflected. The surprise was that they noticed a few being deflected very very large angles, almost \(180^{\circ}\). This phenomenon is now known as Rutherford Scattering. As Rutherford knew the average speed of the alpha particles, the atomic mass of both the particles and the gold atom, and their electric charges he figured out (using some complicated maths) that the maximum possible radius for the nucleus was approx \(10^{-14} \,\text{m} \), which was \(10,000\,\times\) smaller than the radius of the atom itself! This means that the atom is mostly empty space - a lot of empty space.
- the nucleus is tiny - based on the high percentage of alpha particles undeflected.
- the nucleus is positively charged - based on the repulsion of the alpha particles.
- the nucleus is very dense - based on the large angles of deflection and the small radius.
The set up of the Geiger-Marsden experiment. It was performed in a vacuum chamber. (Image: Socratic)
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The previous model predicted that no alpha particles would be deflected by large angles. The image on the right shows how a small, dense central nucleus could scatter the incoming alpha particles.
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Rutherford's scattering experiment. This is really good interactive tutorial on the famous experiment.
CK12 - Interactive. Excellent simulation.
8.2 - Nuclear Structure
Objectives:
- Describe the structure of an atom in terms of protons, neutrons and electrons and use symbols such as C-14 to describe particular nuclei.
- Understand the terms atomic (proton) number, mass (nucleon) number and isotope.
- Understand that alpha and beta particles and gamma rays are ionising radiations emitted from unstable nuclei in a random process.
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PhET - Build an Atom, excellent for learning how the nucleus is made up and for getting the hang of atomic and mass numbers. Works on an iPad as well as a computer.
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8.3 - Nuclear Radiation
Objectives:
- Describe the nature of alpha and beta particles and gamma rays and recall that they may be distinguished in terms of penetrating power.
- Understand that ionising radiations can be detected using a photographic film or a Geiger-Muller detector.
- Describe the dangers of ionising radiations, including radiation can cause mutations in living organisms and radiation can damage cells and tissue.
Radioactive Decay. Nice one that shows the three types of radioactive decay, alpha, beta and gamma.
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8.4 - Nuclear Equations
Objectives:
- Recall the sources of background radiation.
- Describe how the atomic and mass numbers of a nucleus change following a radiation emission.
- Understand how to complete balanced nuclear equations.
Alpha Decay
The alpha particle consists of 2 protons and 2 neutrons. So, the atomic and mass numbers are 2 and 4. If a uranium nucleus emits an alpha particle, it loses 2 of its protons and two neutrons.
The alpha particle consists of 2 protons and 2 neutrons. So, the atomic and mass numbers are 2 and 4. If a uranium nucleus emits an alpha particle, it loses 2 of its protons and two neutrons.
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\[_{92}^{238}\text{U}\rightarrow _{90}^{234}\text{Th}+_{2}^{4}\alpha\]
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Beta Decay
The beta particle is a fast moving electron. Effectively a neutron turns into a proton, which remains in the nucleus, and an electron, which is ejected. This means that the atomic number increases by 1, and the mass number remains unchanged. Also emitted is a particle called a neutrino. You are not required to know anything about this at IGCSE. They are rather enigmatic thingies.
The beta particle is a fast moving electron. Effectively a neutron turns into a proton, which remains in the nucleus, and an electron, which is ejected. This means that the atomic number increases by 1, and the mass number remains unchanged. Also emitted is a particle called a neutrino. You are not required to know anything about this at IGCSE. They are rather enigmatic thingies.
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\[_{6}^{14}\text{C}\rightarrow _{7}^{14}\text{N}+_{-1}^{0}e\]
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8.5 - Half Life
Objectives:
- Understand that the activity of a radioactive source decreases over a period of time and is measured in becquerels.
- Recall the term ‘half-life’ and understand that it is different for different radioactive isotopes.
- Use the concept of half-life to carry out simple calculations on activity.
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PhET - Alpha Decay. Really good simulation for a) appreciating the random nature of radioactive decay and b) the concept of half-life. Only works on a computer. It is useful to pick on a single atom out of a multitude and see when it decays. There is a similar simulation for beta-decay.
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Balonium Half-life. This animation shows the randomness of decay and the half-life really well. It uses a mythical element called balonium.
8.6 - Applications of Nuclear Radiation
Objectives:
- Describe the uses of radioactivity in medical and non-medical tracers, in radiotherapy and in the radioactive dating of archaeological specimens and rocks.
Essentially the uses for radioactivity are based on one of the following properties of radiation:
- The ability to penetrate a material.
- The ability to kill living cells.
- The radioactivity level decreases with time.
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Thickness control. Beta particles are stopped by aluminium and other materials. If a machine is calibrated correctly it can detect changes of thickness of a foil (or such like) by the variations in the amount of radiation transmitted. Neither alpha or gamma are much use in this context.
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Leak detection. Leaks in underground pipes can be costly and hard to find without digging up the whole route. Adding a radioactive isotope to the fluid enables the engineers to detect where the leak is. This usually requires a gamma emitter.
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Gamma Camera Imaging works in a similar way but is based on the absorption of radioisotopes by certain organs - usually in the hunt for a tumour. E.g fast growing thyroid cells rapidly absorb iodine, so if a radioactive isotope of iodine was injected, it would end up in the thyroid and could be detected from outside the body. Similarly, calcium is used if bone cancer is suspected. A short half life isotope is preferred as there is an inherent risk associated with injecting radiosisotopes.
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Radiotherapy is one of the tools that can be used to kill cancerous cells. The idea is that gamma radiation is focused at the tumour to kill it and stop the uncontrolled mitosis. in order to reduce the dosage to healthy tissue, multiple beams are fired in intersecting at the tumour. The side effects can be pretty brutal.
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8.7 - Nuclear Fission
Objectives:
- understand that a nucleus of U-235 can be split (the process of fission) by collision with a neutron, and that this process releases energy in the form of kinetic energy of the fission products.
- recall that the fission of U-235 produces two daughter nuclei and a small number of neutrons.
- understand that a chain reaction can be set up if the neutrons produced by one fission strike other U-235 nuclei.
- understand the role played by the control rods and moderator when the fission process is used as an energy source to generate electricity.
- describe the dangers of ionising radiations, including the problems arising in the disposal of radioactive waste.
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A nuclear fission chain reaction. This is simplified to only 2 neutrons released per fission reaction.
If the reaction is controlled --> nuclear power If the reaction is uncontrolled --> nuclear bomb |
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PhET - Nuclear Fission. Reasonably good simulation of nuclear fission, where a neutron can split a uranium nucleus to release energy.
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The peaceful use of nuclear fission is in the generation of electricity. In the 1970s nuclear power was seen as the great hope for the future. Three Mile Island (1979), Chernobyl (1986) and Fukushima (2011) have put paid to that idea. The concept is simple. Nuclear fission produces heat. This heat turns water into steam, which can be used to drive a turbine that is connected to a generator. Control rods made from a neutron absorbing metal called boron control the rate of the reaction. The graphite core is used to moderate the speed of the ejected neutrons, which improves the efficiency of the reaction. A containment building is to prevent the accidental release of radioisotopes to the surrounding area. Nuclear power generates a fantastic amount of electricity from tiny amounts of uranium "fuel". The devil is in the detail though. The flow of cool water through the reactor is critical. Failure of this cooling system leads to runaway temperatures, leading to the melting of the reactor fuel - a meltdown. This has occurred more often than it should. In each of the above accidents the cooling system was compromised for some reason. The other major issues are: what to do with the waste which will remain highly radioactive for thousands of years and how to decommission the reactors once it has aged to the point of no longer being cost effective.
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Nuclear Bombs
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The Gadget - This was the first nuclear bomb made by the Manhattan Project during the Second World War. It was an implosion device. The sub-critical masses of plutonium were surrounded by TNT 'lenses'. The idea was that the TNT would explode evenly around the sphere, forcing the plutonium together and holding them together by the implosion shockwave for long enough for a sizable proportion of the plutonium to fission. The test was carried out in Alamogordo, New Mexico and was called Trinity.
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A nuclear explosion creates a characteristic mushroom cloud due to the huge amount of convection driven by the heat of the explosion. This has the unfortunate side effect of lifting vast quantities of the radioactive isotopes (remains of the fissile material and the fission products) high up into the stratosphere. They are then carried for hundreds of miles before they are rained out far from the original explosion. This is known as 'fall out".
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Youtube videos of dramatisations of a) the Trinity Test, where Oppenheimer blew up the Gadget and b) the first city destroyed by a nuclear bomb, Hiroshima.
8.8 - Nuclear Fusion (new for 2019)
Objectives:
- Explain the difference between nuclear fusion and nuclear fission.
- Describe nuclear fusion as the creation of larger nuclei resulting in a loss of mass from smaller nuclei, accompanied by a release of energy.
- Know that fusion is the energy source for stars.
- Explain why nuclear fusion does not happen at low temperatures and pressures, due to electrostatic repulsion of protons.
Other Resources
Nuclear Waste - John Oliver
Flash Animations/Tutorials
- Inside the Atom Smasher. BBC news interactive about the CERN Large Hadron Collider