ISOTOPIC FRACTIONATION IN LOW-ENERGY ION-IMPLANTATION

The evolutions of planetary atmospheres and other solar system reservo irs have been affected by a variety of fractionating mechanisms. It ha s been suggested that one of these mechanisms could be low-energy ion implantation. Bernatowicz and Hagee [1987] showed that Kr and Xe impla nted at low energy onto tungsten are fractionated by approximately 1% per amu, favoring the heavy isotopes; we confirm these effects. We hav e extended these studies to Ar and Ne, using a modified Bayard-Alpert type implanter design of cylindrical symmetry with collector potential s of -40 to -100 V, and observe systematically larger mass dependent i sotopic fractionation for argon and neon, greater than or equal to 3% per amu and greater than or equal to 4% per amu, respectively. These f ractionations scale approximately as Delta m/m for all of the noble ga ses measured, consistent with the findings of Bernatowicz and coworker s. Experimental data at higher energies and predictions by TRIM (Trans port of Ions in Matter) code simulations indicate that sticking probab ilities may depend upon the mass ratios of projectile and target. Many natural environments for low-energy ion implantation existed in the e arly solar nebula, such as in dusty plasmas or in the interaction of t he bipolar outflow with small grains or in the wind of the early activ e Sun with accreting planetesimals. Low-energy ions provide viable sou rces for gas loading onto nebular dust grains; the result is isotopic and elemental fractionation of the projectiles.