[osake]

One of the answers in the mark scheme is "Half-life long enough to emit over a few years" - but the americium sample has a half life of 432 years so how can it only emit over a few years?

[SDA]

It means you don't want to use a sample that decays quickly as pretty soon it will be useless, by using a sample with a longer half-life the sample remains useful for longer. Obviously 432 years is much longer than any detector is required to last and so there will be no worries about the sample decaying away significantly during the detectors lifetime.