树木年轮的碳同位素和冰核中钚同位素的检测都显示出在上述阶段的大多数时间内太阳辐射能量出现下降。
Isotopes of carbon in tree rings and beryllium in ice cores show a drop-off in solar radiation during much of the period.
铀或钚的同位素被称为裂变材料，即我们可以利用自由中子去轰击它们来产生裂变反应。
The uranium or plutonium isotopes are referred to as "fissile," which means we can use them to induce fission by bombarding them with free neutrons.
铀或钚的同位素被称为裂变材料，即我们可以利用自由中子去轰击它们来产生裂变反应。
"The uranium or plutonium isotopes are referred to as" fissile, "which means we can use them to induce fission by bombarding them with free neutrons."
铀或钚的同位素被称为裂变材料，即我们可以利用自由中子去轰击它们来产生裂变反应。
"The uranium or plutonium isotopes are referred to as" fissile, "which means we can use them to induce fission by bombarding them with free neutrons."
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