C. Bordeanu

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.

A window on nucleosynthesis through detection of short-lived radionuclides

Abstract The detection of cosmic γ radioactivity of live nuclides from supernovae remnants during the last decade has had an immense impact on our understanding of stellar nucleosynthesis. It is expected that, similarly, nuclides such as 146 Sm, 244 Pu, 182 Hf, 247 Cm with halflives of the order of a few 107 years be present in the Interstellar Medium (ISM) as freshly synthesized matter; they are however not detectable by γ-ray astronomy because of their lower activity. Depending on their steady-state abundance and the ISM influx rate in the solar system, deposition on Earth of these nuclides through cosmic dust and their detection may provide a complementary approach. A search for 244 Pu in a deep-sea sediment sample, acting here as repository archive for cosmic dust, was performed by accelerator mass, spectrometry (AMS) and an upper limit for deposition of interstellar 244 Pu of 0.2 atoms/cm 2 /yr obtained. Derived constraints on 244 Pu abundance in the ISM are discussed. Further development of detection techniques for 244 Pu and other nuclides is under way.

Destruction of 7Li and 7Be in astrophysical environments

Abstract The destruction of 7 Li and 7 Be in astrophysical environments is essential for understanding several stellar and cosmological processes and is not well understood, though earlier 7 Li + 3 He experiments have been performed [1]. The primordial abundance of 7 Li after Big Bang Nucleosynthesis (BBN) plays a major role in our understanding of the early universe [2]. The value of the baryon to photon ratio (η) deduced from BBN combined with measurements of the cosmic microwave background provide some of the strongest and earliest evidence for the existance of non-baryonic dark matter [2]. The destruction of 7 Be during the hot-pp cycle may alter our conclusions on the production of carbon in this process, which is thought to compete with the triple-α process for the production of 12 C, as the reaction 7 Be( 3 He, 2α)2p competes with 7 Be( 4 He, γ) 11 C and may reduce carbon production [3]. These stellar and cosmological environments involve high temperatures, and thus, effective burning energies (Gamow windows) that are quite high. Experiments using 7 Be targets inevitably involve interactions with 7 Li as background due to the 7 Li daughters from the beta decay of 7 Be. The experiments were performed at the Weizmann Institute VDG Laboratory using 3 He beams from 390 keV to 1130 keV on 7 LiF foil targets and 7 Be implanted targets. Results from measurements using 10 μg 7 LiF foil targets will be discussed.

A new measurement of the proton capture rate on 7Be

Abstract We report on a new measurement of the cross section of this reaction, following our previous experiment with an implanted 7 Be target, a raster scanned beam and the elimination of the backscattering loss. Measurements were done at energies above and below the resonance as well as a detailed measurement of the resonance. We obtain an extrapolated value of S 17 (0) = 21.2 ± 0.7 from the entire set of measurements.

Counting 44Ti nuclei from the 40Ca(α,γ)44Ti reaction

Abstract Among the many production and destruction nuclear reactions that bear on the net 44 Ti yield in the supernova environment, recently highlighted by γ-ray astronomy observations, the α capture on 40 Ca stands out in the α-rich freezeout phase conjectured by astrophysical models. We activate a He-gas target with a 40 Ca beam and implant recoiling 44 Ti nuclei (∼10 6 ) in a Cu catcher. The 44 Ti atoms chemicallyextracted with a nat Ti carrier are counted by accelerator mass spectrometry (AMS). The measurement of the 44 Ti/Ti ratio, together with the amount of Ti carrier added, determine the number of 44 Ti nuclei produced in the activation, independent of chemical yield or transmission efficiency. The resonance strength (7.4±2.5 eV) at E cm ∼1.1 MeV/n determined for two close-by levels of 44 Ti is in excellent agreement with previous prompt-γ measurements. We plan to extend our experiments to the lower energy range.

Towards a measurement of σ[3He(4He,γ)7Be] with accelerator mass spectrometry (AMS)

Abstract The aim of this research is to determine the reaction cross-section for one of the most important fusion reactions in the sun that is directly connected to the solar neutrino problem: 3He(4He,γ)7Be. For this, we use the Accelerator Mass Spectrometry (AMS) technique at the Koffler accelerator of the Weizmann Institute. As a first step, we have tested our set-up by measuring the number of atoms of 7Be obtained from the 7Li(p,n)7Be reaction. The sensitivity proved to be better than 10−14, allowing us to proceed towards an independent measurement with a different approach of this astrophysical important cross section.

Magnetic moment of 17Ne using β-NMR and tilted foil polarization

We report on the measurement of the magnetic moment of the ground state of 17Ne. Radioactive 17Ne nuclei were delivered from the high resolution mass separator at ISOLDE onto a high voltage platform at −200 kV and were polarized using the tilted foil polarization method. The polarized nuclei were implanted into a Pt stopper situated in a liquid-helium cooled β-NMR apparatus and the asymmetry destruction of the ensuing β rays was monitored as a function of the rf frequency applied to the polarized nuclei. The measured value of μ = 0.74 ± 0.03 affirms the nature of the ground state of 17Ne and is compared to shell model calculations.

Direct determination of radioactive nuclide production in astrophysical reactions by Accelerator Mass Spectrometry (AMS)

We report on novel measurements of the total production cross section of long-lived nuclei produced in nuclear reactions relevant to astrophysical processes, using the Accelerator Mass spectrometry (AMS) technique at the Koffler accelerator of the Weizmann Institute as a direct “atom-counting” method. First results of these experiments for two such nuclides, 44Ti and 7Be, produced in nucleosynthesis sites and in solar fusion reactions, serve as a demonstration of the present approach and of its high sensitivity and very low background. The methodology and parameters of the present method, together with recent results, are presented.

Counting44Ti nuclei from the40Ca(α,γ )44Ti reaction

Abstract Among the many production and destruction nuclear reactions that bear on the net 44 Ti yield in the supernova environment, recently highlighted by γ-ray astronomy observations, the α capture on 40 Ca stands out in the α-rich freezeout phase conjectured by astrophysical models. We activate a He-gas target with a 40 Ca beam and implant recoiling 44 Ti nuclei (∼10 6 ) in a Cu catcher. The 44 Ti atoms chemicallyextracted with a nat Ti carrier are counted by accelerator mass spectrometry (AMS). The measurement of the 44 Ti/Ti ratio, together with the amount of Ti carrier added, determine the number of 44 Ti nuclei produced in the activation, independent of chemical yield or transmission efficiency. The resonance strength (7.4±2.5 eV) at E cm ∼1.1 MeV/n determined for two close-by levels of 44 Ti is in excellent agreement with previous prompt-γ measurements. We plan to extend our experiments to the lower energy range.