| Date | November 2015 | Marks available | 2 | Reference code | 15N.3.SL.TZ0.7 |
| Level | Standard level | Paper | Paper 3 | Time zone | Time zone 0 |
| Command term | Explain | Question number | 7 | Adapted from | N/A |
Question
This question is about radioactive decay.
Meteorites contain a small proportion of radioactive aluminium-26 \(\left( {_{{\text{13}}}^{{\text{26}}}{\text{Al}}} \right)\) in the rock.
The amount of \(_{{\text{13}}}^{{\text{26}}}{\text{Al}}\) is constant while the meteorite is in space due to bombardment with cosmic rays.
After reaching Earth, the number of radioactive decays per unit time in a meteorite sample begins to diminish with time. The half-life of aluminium-26 is \(7.2 \times {10^5}\) years.
Aluminium-26 decays into an isotope of magnesium (Mg) by \({\beta ^ + }\) decay.
\[_{{\text{13}}}^{{\text{26}}}{\text{Al}} \to _{\text{Y}}^{\text{X}}{\text{Mg}} + {\beta ^ + } + {\text{Z}}\]
Identify X, Y and Z in this nuclear decay process.
X:
Y:
Z:
Explain why the beta particles emitted from the aluminium-26 have a continuous range of energies.
State what is meant by half-life.
A meteorite which has just fallen to Earth has an activity of 36.8 Bq. A second meteorite of the same mass, which arrived some time ago, has an activity of 11.2 Bq. Determine, in years, the time since the second meteorite arrived on Earth.
Markscheme
X: 26 and Y: 12; (both needed for [1])
Z: v/neutrino;
Do not allow the antineutrino.
total energy released is fixed;
neutrino carries some of this energy;
(leaving the beta particle with a range of energies)
the time taken for half the radioactive nuclides to decay / the time taken for the activity to decrease to half its initial value;
Do not allow reference to change in weight.
\(\lambda = \left( {\frac{{\ln 2}}{{7.2 \times {{10}^5}}} = } \right){\text{ }}9.63 \times {10^{ - 7}}\);
\(11.2 = 36.8{e^{ - (9.63 \times {{10}^{ - 7}})t}}\);
\(t = 1.24 \times {10^6}{\text{ (yr)}}\);
Examiners report
This was generally well answered, although a significant minority insisted that nuclear half-life is defined by a loss of mass.
This was generally well answered, although a significant minority insisted that nuclear half-life is defined by a loss of mass.
This was generally well answered, although a significant minority insisted that nuclear half-life is defined by a loss of mass.
This was generally well answered, although a significant minority insisted that nuclear half-life is defined by a loss of mass.
