Unlike the positively-charged alpha particles , which are repelled by the electrical forces present in the nuclei of other atoms , neutrons do not need to overcome any Coulomb barrier and can therefore penetrate and enter the nuclei of even the heaviest elements such as uranium .
In contrast with the helium nuclei ( alpha particles ) which are positively charged , and therefore repelled by the strong electrical forces in the nuclei of heavy atoms , this new tool in atomic disintegration need not overcome any electric barrier and is capable of penetrating and splitting the nuclei of even the heaviest elements .