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Model of an atom
- 1 An atom consists of a positive nucleus and negative electrons. In a neutral atom, there are the same number of protons (positive charge) as electrons.
- 2 The nucleus may also contain neutrons. For example, the element Carbon-12 has 6 protons and 6 neutrons in the nucleus. To balance the positive charge, it has 6 electrons.
- 3 Some elements have isotopes. These are atoms with the same number of protons but different numbers of neutrons. An isotope of Carbon is C-14. This is an atom with 6 protons and 8 neutrons. It still has 6 electrons.
- 4 If an atom has one or more electrons removed, it is called an ion. It has positive charge. Helium-4 has two protons and 2 neutrons in the nucleus. Plus two electrons. If the two electrons are removed, we end up with an ion of Helium. It is just the nucleus of Helium and it is also called an alpha particle.
- 5 Some atoms are unstable. These atoms may have extra neutrons. To become more stable, these atoms release energy from the nucleus and this is called radiation.
What is radiation?
- 1 Energy from the nucleus is released in one of three ways : alpha, beta or gamma radiation.
- 2 Alpha radiation is the emission of an alpha particle (a helium nucleus) from the nucleus of an atom. In the process, the nucleus loses 2 protons and so the smaller nucleus is now a different element. Alpha particles carry positive charge.
- 3 Beta radiation is when the nucleus emits a fast moving electron (beta particle). It is not one of the electrons from the atom. Beta particles carry negative charge.
- 4 Gamma radiation. Unlike alpha and beta radiation, gamma is an electromagnetic wave. It travels at the speed of light and is part of the electromagnetic spectrum. Gamma radiation carries no charge.
- 5 When a substance emits radiation, it is said to be radioactive. It is impossible to tell when a nucleus will emit radiation because it is a totally random process.
Testing for radiation
- 1 Test for the presence of radiation using a Geiger Muller (GM) tube. This doesn’t reveal which type of radiation is being detected. Measure the background count and then do these tests.
- 2 Let’s assume that the GM tube is detecting a count rate that is higher than the background from a radioactive sample. Alpha is stopped by 5cm of air or paper. So placing paper between the GM tube and sample will reduce the count rate if alpha is present. If there is no change, then beta or gamma is present. Test for beta using aluminium sheet. If no change then gamma is present.
Fundamentals of Spectroscopy Spectroscopy is the distribution of electromagnetic energy as a function of wavelength. Spectrum is basically white light dispersed by a prism to produce a rainbow of colours; the rainbow is the spectrum of sunlight refracted through raindrops. All objects with temperatures above absolute zero emit electromagnetic radiation by virtue of their warmth alone; this radiation is emitted at increasingly shorter wavelengths as temperature is increased. Individual atoms can emit and absorb radiation only at particular wavelengths equal to the changes between the energy levels in the atom. The spectrum of a given atom therefore consists of a series of emission or absorption lines.
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Beta (?) - this is an electron emitted from the nucleus when a neutron changes into a proton. It is smaller and less ionizing and can travel metres through the air before being stopped. It requires a more dense material such as aluminium to stop it. 3) Gamma - (?? - this is an electro-magnetic wave and an travel very large distances. It requires several metres of lead to stop it. Type Symbol Chemical symbol Range Charge Stopped by alpha ?
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Determine the penetrating power and the range in air of the three radioactive emissions (Plutonium 239 for alpha, Strontium 90 for beta and Cobalt 60 for gamma).4 star(s)
Safety precautions included removing the sources from a secure wooden box using thongs and tweezers and placed in plasticene, the set square was held using thongs and all those carrying out the experiment stood behind the source to minimise any direct radiation exposure. Method 2 The equipment was set up as shown below to measure the penetrating power of each radioactive source. Again the measurements were taken without the absorber to measure background radiation. The source was placed quite close to the counter (1cm)
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Convection- The convection of heat occurs in liquids and heat only. It is a much more effective process then conduction so I am going to concentrate on stopping convection more then stopping conduction. Convection is when heat from a hot region takes the heat and moves to a cooler region. Here is a picture of what happens in the convection process... I will therefore need an item which can stop convection from happening in the experiment, as this will reduce the loss of heat dramatically. Radiation- Heat radiation can travel in a few ways; here are some of them, 1.
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