6 marksRadium is a radioactive element. The most common isotope of radium has a half-life of almost two thousand years. A sample of radium can remain at a higher temperature than its surroundings for a long period of time.
Explain how a sample of radium is able to release significant amounts of energy over a long period of time.
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3 marksA student used a Geiger-Müller (GM) tube to determine the activity of a radium source.
Radium emits α, β, and γ radiation.
He positioned the source 20 cm from the GM tube, as shown, and recorded the count for 1 minute. He repeated the measurement and calculated a mean count.
The student recorded the following results.
| Count 1 | Count 2 | Mean count |
| 183 | 178 | 181 |
Criticise the student’s method for determining the count at this position.
5 marksFrom his results the student determined that the activity of the source was 3.0 Bq.
Comment on his value for the activity of the source.
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1 markRadioactive decay is often described in textbooks as a spontaneous, random process.
State what is meant by spontaneous decay.
6 marksExplain why there is an exponential decrease in the rate of decay for a sample containing a large number of unstable nuclei.
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6 marksPhosphogypsum is a by-product in the manufacture of fertiliser. It is slightly radioactive because of the presence of radium-226, a radioisotope with a half-life of 1600 years.
It must be stored securely as long as the activity of the radium-226 it contains is greater than 0.4 Bq per gram of phosphogypsum.
Calculate the number of nuclei of radium-226 in this sample.
(3)
Number of nuclei = ...............................
(3)
Time = ..............................years
5 marksRadium-226 decays to radon-222 by alpha emission.
Determine the energy released in MeV in the decay of a single nucleus of radium-226.
mass of radium-226 nucleus = 225.97713 u
mass of radon-222 nucleus = 221.97040 u
mass of α particle = 4.00151 u
Energy released = .................................MeV
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2 marksAn old type of camping lamp used a ‘gas mantle’. The gas mantle is heated by the gas flame on the lamp and emits a bright white light. Gas mantles used to contain thorium-230.
Thorium-230 decays by alpha emission to form an isotope of radium. A student keeps a radioactive gas mantle in a sealed polythene bag. The student suggests that over a period of a year a significant volume of helium gas will be collected, since an alpha particle is a helium nucleus.
Give reasons why the sealed plastic bag is suitable for collecting the gas.
12 marksA particular gas mantle contains 5.18 x 10-5 g of thorium-230.
(4)
Volume = ................................................
(3)
Root mean square speed = .......................................
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10 marksThe photograph shows a vase made of uranium glass. Uranium glass is radioactive.
Uranium glass usually contains a maximum of 2% uranium. Uranium glass made in the early part of the 20th century can contain up to 25% uranium.
A student carried out an investigation to determine the percentage of uranium in the glass.
The student measured the count rate by placing a Geiger Muller (GM) tube against the vase at a single position. This value was used to calculate the decay rate for the whole vase.
(2)
(6)
Percentage of uranium = ....................................................................
5 marksA uranium nucleus decays to thorium by emission of an alpha particle.
It can be assumed that all the energy of the decay is transferred to kinetic energy of the alpha particle.
Calculate the speed of the emitted alpha particle.
mass of uranium nucleus = 238.0003 u
mass of thorium nucleus = 233.9942 u
mass of alpha particle = 4.0015 u
Speed of alpha particle = ....................................................................
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