Also according to Wikipedia the mass of neutronium is only 4x10^17/m3 and based on what the article states about its half life i'd be using it as an energy core rather than a mantle otherwise the whole earth would be a radioactive ball of molten rock. The electroweak material doesn't really make sense unless it refers to the healing process? Or the method by which the traveller creates life ( I only say this due to the lack of knowledge about electroweak materials) The lead is more likely the mantle material this is based on the colour ( lead oxide) and its radiation protection, necessary for space fairing organisms Just my theory feel free to correct anything :)