与此同时，不知疲倦的卢瑟福这时候已经返回剑桥大学，接替J.J.汤姆逊担任卡文迪许实验室主任。他设计出了一种模型，说明原子核不会爆炸的原因。他认为，质子的正电荷一定已被某种起中和作用的粒子抵消，他把这种粒子叫做中子。这个想法简单而动人，但不容易证明。卢瑟福的同事詹姆斯·查德威克忙碌了整整11个年头寻找中子，终于在1932年获得成功。1935年，他也获得了诺贝尔物理学奖。

　　我国第一颗原子弹

　　As Boorse and his colleagues point out in their history of the subject, the delay in discovery was probably a very good thing as mastery of the neutron was essential to the development of the atomic bomb. (Because neutrons have no charge, they aren't repelled by the electrical fields at the heart of an atom and thus could be fired like tiny torpedoes into an atomic nucleus, setting off the destructive process known as fission.) Had the neutron been isolated in the 1920s, they note, it is "very likely the atomic bomb would have been developed first in Europe, undoubtedly by the Germans."

　　正如布尔斯及其同事在他们的物理学史中指出的，较晚发现中子或许是一件很好的事，因为发展原子弹必须掌握中子。（由于中子不带电荷，它们不会被原子中心的电场排斥，因此可以像小鱼雷那样被射进原子核，启动名叫裂变的破坏过程。）他们认为，要是在20世纪20年代就能分离中子，“原子弹很可能先在欧洲研制出来，毫无疑问是被德国人”。