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The 62Ni(n,gamma)63Ni(t(1/2)=100+/-2 yr) reaction plays an important role in the control of the flow path of the slow neutron-capture (s) nucleosynthesis process. We have measured for the first time the total cross section of this reaction for a quasi-Maxwellian (kT=25 keV) neutron flux. The measurement was performed by fast-neutron activation, combined with accelerator mass spectrometry to detect directly the 63Ni product nuclei. The experimental value of 28.4+/-2.8 mb, fairly consistent with a recent calculation, affects the calculated net yield of 62Ni itself and the whole distribution of nuclei with 62

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The 44 Ti(t 1/2 = 59 y) nuclide, an important signature of supernova nucleosynthesis, has recently been observed as live radioactivity by γ-ray astronomy from the Cas A remnant. We investigate in the laboratory the major 44 Ti production reaction, 40 Ca(α, γ) 44 Ti (Ecm ∼ 0.6-1.2 MeV/u), by direct off-line counting of 44 Ti nuclei. The yield, significantly higher than inferred from previous experiments, is analyzed in terms of a statistical model using microscopic nuclear inputs. The associated stellar rate has important astrophysical consequences, increasing the calculated supernova 44 Ti yield by a factor ∼ 2 over previous estimates and bringing it closer to Cas A observations. PACS numbers: 26.30.+k,97.60.Bw,95.85.Pw,24.60Dr The radionuclide 44 Ti(t 1/2 = 59 y) is considered an important signature of explosive nucleosynthesis in corecollapse supernovae (SN) [1], where multiple α capture is the path for SN nucleosynthesis from 28 Si to 56 Ni(Fe). 44 Ti is mainly produced during an α-rich freeze-out phase, the ratio 44 Ti/ 56 Ni being sensitive to the explosion conditions. Stellar production of 44 Ti determines the abundance of stable 44 Ca and contributes to that of 48 Ti (fed by 48 Cr on the α-chain). Live 44 Ti has been directly observed from a point source identified as Cassiopeia A (Cas A) by γ-and X-ray telescopes (CGRO, RXTE, BeppoSAX) and very recently by the INTEGRAL mission (see [2,3]). Cas A is believed to be the remnant of a core-collapse SN whose progenitor mass was in the range 22-25 M ⊙ (M ⊙ denotes a solar mass) [4]. Using known values of the distance and age of the remnant, half-life of 44 Ti and the combined γ flux from all observations, an initial 44 Ti yield of 160± 60 µM ⊙ is implied [3]. This value is larger by a factor of 2-10 than 44 Ti yields calculated in current models (e.g. [5,6]) and various explanations have been proposed [4,7,8,9]. 44 Ti γ-ray emission from SN1987A in the near Large Magellanic Cloud galaxy, the closest known SN remnant in the last two centuries, is below detection limits. But its present lightcurve is believed to be powered by 44 Ti radioactivity and the inferred initial 44 Ti yield is estimated to be 100-200 µM ⊙ (see [2]), similar to that of Cas A. Using the 56 Ni yield of SN1987A directly measured by γ-ray astronomy, the implied 44 Ti/ 56 Ni ratio is larger by * To whom correspondence should be addressed, email address: paul@vms.huji.ac.il a factor ∼3 than estimated by stellar calculations [2]. No other source of 44 Ti activity has been confirmed so far, despite a number of candidates and the improved sensitivity of the INTEGRAL γ-ray telescope [2]. Although many nuclear reactions play roles in determining the SN yield of 44 Ti [9, 10], the major production reaction is 40 Ca(α, γ) 44 Ti and its importance has been emphasized [11]. Experimental information about this reaction is incomplete and theoretical estimates are made less reliable by the suppression of dipolar T = 0 → 0 transitions in selfconjugate (N =Z) nuclei. The reaction was studied in the 70's b...

Short-lived nuclides, now extinct in the solar system, are expected to be present in interstellar matter (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). 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) × 10 5 244 Pu 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 0.2 244 Pu atoms/cm 2 yr for extra-terrestrial deposition. Using available data on ISM steady-state flux on Earth, we derive a limit of 2 × 10 −11 g-244 Pu/g-ISM for the abundance of 244 Pu in ISM.