同时还需要足够多的、可裂变的原子核用于中子进行撞击，换句话说，需要一个临界质量。
There also need to be enough fissionable atoms about for the neutrons to bumpinto—in other words, a critical mass.
科学家们知道：只有在中子撞击原子的原子核时，中子才可能处于游离状态。这种碰撞会改变原子核的运动轨迹。
Scientists know that a neutron is loose only when the neutron hits the nucleus of an atom. The collision changes the path of the nucleus.
这四种新元素都是人造的，是由火花与原子核相互撞击并且循着放射性超重元素衰变的轨迹而发现的。
The four new elements, all of which are synthetic, were discovered by slamming lighter -nuclei into each other and tracking the following decay of the radioactive superheavy elements.
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