Search for supermassive nuclei in nature

We report on a search for supermassive nuclei in nature with masses up to 10⁷ amu. Such exotic nuclei might consist, for example, of stable strange matter, which comprises a mixture of up, down, and strange quarks, or of relic particles from the early Universe. The experiments are based on Rutherford backscattering of heavy ions, preferably²³⁸U, from various target samples. The measured parameters of a detected particle are its time-of-flight, scattering angle, and specific ionization. From this information the mass of the target nucleus can be inferred. Upper limits for the abundance of strange supermassive nuclei with massesA?4·10² to 10⁷ amu relative to the number of nucleons were found to be in the range 10^?11 to 10^?15. For the narrower mass rangeA ?10³ to 10⁴ amu the limit is 2· 10^?17.