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Rutherford scattering and the nuclear atom questions
Most alpha particles fired at gold foil passed straight through, showing that most of an atom is empty space, and the rare ones that bounced back implied a nucleus ten thousand times smaller than the atom around it. The radiations that nucleus emits can each be identified with paper, aluminium and lead.
18 original questions · 54 marks · the rutherford scattering and the nuclear atom notes · Nuclear physics
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Describe the main observations of the Rutherford alpha-particle scattering experiment.
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Most alpha particles passed through the thin gold foil with little or no deflection (1); a small fraction were deflected through large angles, with a very few deflected back through more than 90° (1).In the Rutherford scattering experiment, most alpha particles passed straight through the gold foil, while a tiny fraction were deflected through large angles. State the conclusions about the structure of the atom drawn from these observations.
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The atom is mostly empty space, since most particles pass straight through (1); the positive charge and nearly all the mass are concentrated in a very small central nucleus, which repels the few particles that pass close to it (1).State what is meant by background radiation and give two of its origins.
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Ionising radiation that is always present, from sources other than the source under investigation (1). Any two origins: radon gas from the ground, rocks and building materials, cosmic rays, radioactive traces in food, medical procedures (1).State the nature of an alpha particle and of gamma radiation.
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An alpha particle is a helium nucleus: two protons and two neutrons (1). Gamma radiation is a high-frequency electromagnetic wave (a photon) (1).State a material that will absorb each of alpha, beta and gamma radiation.
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Alpha: a sheet of paper (or a few cm of air) (1). Beta: a few mm of aluminium (1). Gamma: several cm of lead reduces the intensity (1).A source emits three types of radiation. One is stopped by paper. A second passes through paper but is stopped by a few millimetres of aluminium. A third passes through both and is only reduced by lead. Identify each radiation.
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Stopped by paper: alpha (1). Stopped by aluminium but not paper: beta (1). Only reduced by lead: gamma (1).A gamma source produces an intensity of 800 arbitrary units at a distance of 1.0 m. Assuming an inverse-square law, calculate the intensity at 2.0 m.
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I ∝ 1/r2, so I = 800 × (1.0/2.0)2 (1)
I = 200 units (1)A detector reads 1600 counts per second at a distance of 0.50 m from a gamma source. Assuming an inverse-square law, calculate the expected reading at 2.0 m.
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I = 1600 × (0.50/2.0)2 = 1600 × 1/16 (1)
I = 100 counts per second (1)Explain why the great majority of alpha particles pass through a thin gold foil almost undeflected.
Before Rutherford's experiment, the atom was modelled as a sphere of positive charge with electrons embedded in it. Explain why the deflection of some alpha particles through more than 90° could not be explained by this model.
A detector placed 150 mm from a sealed gamma source records a corrected count rate of 3600 counts per second. Assuming the intensity obeys an inverse-square law, determine the distance from the source at which the corrected count rate falls to 400 counts per second. Give your answer in metres.
A domestic smoke detector contains a source that emits only alpha particles. Explain why the source presents very little hazard while sealed in the detector, but would be dangerous if the source material were swallowed.
With a gamma source in place, a detector at 0.10 m records 520 counts per second, including a background count of 20 counts per second. Use the inverse-square law to predict the measured count rate at 0.20 m.
Explain why a few alpha particles in the scattering experiment were deflected through very large angles, and state which particles are deflected the most.
Compare the ionising power and the penetrating power of alpha, beta and gamma radiation.
A factory rolls aluminium foil about 0.1 mm thick. The thickness is monitored continuously by a radioactive source above the moving foil and a detector below it. Four sources are available. W: alpha emitter, half-life 432 years; X: beta emitter, half-life 29 years; Y: beta emitter, half-life 15 hours; Z: gamma emitter, half-life 5.3 years. Suggest, with reasons, which source the factory should choose.
A student measures the count rate near a sealed gamma source. With the detector 0.20 m from the source the measured count rate is 340 counts per second; at 0.40 m it is 100 counts per second. Both readings include a constant background count rate. Assuming the gamma intensity obeys an inverse-square law, determine the background count rate.
Describe an experiment to test whether the intensity of the gamma radiation from a small sealed source obeys an inverse-square law. Include the measurements taken, how the data are analysed, and the precautions taken to keep the experimenter's dose low.
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