A pure sample of a known element has a very long half-life. What measurement(s), together with the initial activity of the sample, must be made in order to measure the half-life of the element?

A. The activity of the sample after a given period of time.

B. The mass of the sample after a given period of time.

C. The activity and the mass of the sample after a given period of time.

D. The mass of the sample.

D

Well over 50% of the candidates chose A and seemed unaware that if we say that an element has a ‘very long half-life’ then we are, in all likelihood talking about timespans that far exceed the length of a human life. Hence it is not possible to come back later and hope to see a noticeable diminution of the activity of the sample. The syllabus guide clearly invites teachers to consider how to measure very long half-lives and this can only be done without reference to time. Hence it must be D. Note that the stem does specify that the sample is pure.