Or a radioactive substance may be disintegrating at a known rate and we may be required to determine the amount of material present after a given time.

又如，某种放射性物质可能正在以已知的速度进行衰变，需要我们确定在给定的时间后遗留物质的总量。

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For no other new radioactive substance have such positive results been obtained.

其他任何一种新的放射性物质尚未取得如此积极的结果。

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Mr Litvinenko died in 2006 after he was apparently poisoned with the radioactive substance polonium-210.

BBC: Alexander Litvinenko murder was 'London nuclear terror'

Tritium is a radioactive substance produced by the reactors from nuclear-powered submarines based at the Devonport yard.

BBC: Tritium is released from submarine reactors

Meanwhile, Japan's recall of Meiji baby formula because of pollution by a radioactive substance has also raised a lot of concern.

BBC: China morning round-up: WTO membership marked