最为有名的就是铀- 238衰变为铅- 206的半衰期为45亿年，当然放射性铷和钐也同样对测算几十亿年前的物体的年龄很有用。
Most famously, uranium-238 decays into lead-206 with a half life of 4.5 billion years, though radioactive rubidium and samarium are also useful for dating things billions of years old.
很多情况下有必要，定义一个称为半衰期的量，这个量很有用。
In lots of cases, it's useful to define what's called a half life. It's just useful.
大众传播领域学术来源的扩散导致了信息爆炸，而信息爆炸简化了整个学科有用的半衰期知识。
The proliferation of scholarly sources in the mass communication field adds to the information explosion that routinely abbreviates the useful half-life of knowledge throughout the discipline.
一个很有用的量是反应的半衰期。
一个很有用的量是反应的半衰期。
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