| Date | November 2010 | Marks available | 3 | Reference code | 10N.3.SL.TZ0.B3 |
| Level | Standard level | Paper | Paper 3 | Time zone | Time zone 0 |
| Command term | Calculate and Determine | Question number | B3 | Adapted from | N/A |
Question
This question is about radioactive decay.
The half-life of Au-189 is 8.84 minutes. A freshly prepared sample of the isotope has an activity of 124Bq.
A nucleus of a radioactive isotope of gold (Au-189) emits a neutrino in the decay to a nucleus of an isotope of platinum (Pt).
In the nuclear reaction equation below, state the name of the particle X and identify the nucleon number \(A\) and proton number \(Z\) of the nucleus of the isotope of platinum.
\[_{\;79}^{189}Au \to _Z^APt + X + v\]
X:
A:
Z:
(i) Calculate the decay constant of Au-189.
(ii) Determine the activity of the sample after 12.0 min.
Markscheme
X: positron or \({\beta ^ + }\);
A: 189 and Z: 78; (both responses needed)
(i) \(0.0784{\text{ mi}}{{\text{n}}^{ - 1}}\);
(ii) recognize to use \(A = {A_0}{e^{ - \lambda t}}\);
\(A = 48.4{\text{ Bq}}\);
Award [2] for bald correct answer.
Examiners report
A surprisingly large number of candidates were unable to correctly identify the products of beta plus decay.
Unit errors were often made in (i) and in (ii) there was often some very strange arithmetic to be seen.
