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 coinprises 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, preferably 238U, from various target samples. The measured parameters of a detected particle are its timeof-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 masses A ~-4-102 to 107 amu relative to the number of nucleons were found to be in the range 10 -11 to 10 -is. For the narrower mass range A ,-~103 to i04 amu the limit is 2.10 -17. 14.80.Pb; 29.40.Jz In a previous letter [1] we reported on the results of a search for supermassive nuclei in iron meteorites and some terrestrial samples by Rutherford backscattering of uranium and lead nuclei. No indication of the presence of supermassive isotopes was,observed. Based on these data upper limits for the abundance of two potential heavy-mass candidates were inferred, viz. for strange matter [2-5] consisting of up-, down-, and strangequarks and for relic particles from the early Universe [6-8]: For strange matter with masses 103 to 107 amu the abundance (relative to the number of nucleons) is less than 10-lO to 10-14, respectively; for relic particles within the same interval of masses the upper limit is 10-21. The limits for the abundance of strange matter are close to its expected concentration in the Earth's crust which was estimated by De Rfijula and Glashow [5] who assumed that strange nuggets produced in collisions of neutron stars can reach the Earth.
PACS:In order to improve the experimental limits one can think of at least two possibilities: an increase of the beam intensity used for irradiation of the samples and, more importantly, the use of target samples chemically enriched in strange-matter content. In our more recent experiments, the results of which we present here, both possibilities were realized.The most efficient way of obtaining an enriched sample is to use a target composed of unstable elements (for example technetium, praseodymium, or transuranic elements). There is no reason to expect the stability of strange matter to be correlated with the stability of normal nuclei. Thus, presuming strange matter to be stable, it should exist with all atomic numbers of the periodic table (except possibly for the smallest ones [3~) and notably with those of unstable elements. In this case, strange matter can be chemically extracted from a large amount of bulk material that contains or did contain unstable elements.In 1971, D.C. Hoffman and coworkers [9] published the detection of primordial plutonium in nature. From an initial mineral sample of 85 kg containing about 8.5 kg of bastnasite, a rare earth fluocarbonate mineral, the authors reported to have extracted ,~+0.3 107 atoms "r 0.7 " of ...
