Experimental limit to interstellar ²⁴⁴pu abundance

Short-lived nuclides, now extinct in the solar system, are expected to be present in the interstellar medium (ISM). Grains of ISM origin were recently discovered in the inner solar system and at Earth orbit and may accrete onto Earth after ablation in the atmosphere. A favorable matrix for detection of such extraterrestrial material is presented by deep-sea sediments with very low sedimentation rates (0.8-3 mm kyr^-1). We report here on the measurement of Pu isotopic abundances in a 1 kg deep-sea dry sediment collected in 1992 in the North Pacific. Our estimate of (3 ± 3) × 10₅ ²⁴⁴Pu atoms in the Pu-separated fraction of the sample shows no excess over the expected stratospheric nuclear fallout content and, under reasonable assumptions, sets a limit of 0.2 ²⁴⁴Pu atoms cm^-2 yr^-1 for extraterrestrial deposition. Using the available data on the ISM steady-state flux on Earth, we derive a limit of 2 × 10^-11 g ²⁴⁴Pu (g ISM)^-1 for the abundance of ²⁴⁴Pu in the ISM.