某一元素中质量较大的同位素，相对于质量较小的同位素而言，称为重同位素。在周期表后半部的同位素，质量数较大，相对于前半部质量数较小的同位素而言，也称为重同位素。
目前情况下值得忧虑的是电离辐射，它由重同位素——如碘131和铯137——自发衰变而生成。
Of concern in the current situation is ionizing radiation, which is produced by spontaneously decaying heavy isotopes, such as iodine 131 and cesium 137.
它们晶体里重轻同位素的比例并不像是从地底挖出来的东西那样。
The ratio between the heavy and light isotopes of the element in their crystals is not quite right for stuff that has come out of the ground.
科学家认为重氮来自一颗彗星，而重氧则是来自行星形成的自然过程，这一过程把更轻的氧同位素留在了星云中，形成了太阳。
Scientists say the excess heavy nitrogen could have come from a comet. And the heavier oxygen from a natural process that left more of the light isotope in the part of the nebula that made the sun.
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