Juliet started working with a radioactive substance. In 1997, she started with 200 g. When she measured again in 2002, she had 100 g left. In 2007, she had 50 g.

Question

Which substance is she most likely measuring?

rubidium-91
cobalt-60
carbon-14

Answer 1

The decay pattern of the radioactive substance Juliet is working with suggests a half-life of about 5 years, given that she started with 200 g in 1997 and had 100 g in 2002 (5 years later), and then 50 g in 2007 (another 5 years later).

Now looking at the half-lives of the substances in question:

1. Rubidium-91: Rubidium-91 has a very short half-life of about 18.6 minutes, making it unlikely to be the substance she's measuring, as it would have decayed much more quickly than observed.

2. Cobalt-60: Cobalt-60 has a half-life of about 5.27 years, which is consistent with the data observed (200 g to 100 g in 5 years, and 100 g to 50 g in 5 years).

3. Carbon-14: Carbon-14 has a half-life of about 5,730 years, which is also inconsistent with the rapid decay observed.

Based on this analysis, the most likely substance Juliet is measuring is cobalt-60.