Sm (25). We prepared spectroscopic alpha sources (20-100 μg) from the three activations and counted them during several months using a silicon surface-barrier detector at Kanazawa University (Fig. 1, 25 Galactic-disk enrichment in low-metallicity gas (Fig. 3). This reduces the age of late events (≥ 50 Ma) (age is measured in this work relative to the birth of the solar system) but has a minor effect on earlier events (Table 1). Samarium-146 observations in terrestrial samples can be divided into two groups: (i) most terrestrial rocks display a 142 Nd/ 144 Nd ratio higher by ~18 parts per million (ppm) than CHUR (9) and (ii) anomalies in the 142 Nd/ 144 Nd ratio relative to the terrestrial standard, both positive, in rocks from Greenland and Australia (2,10), and negative, in rocks from Northern Quebec (2,11).

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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

▪ Abstract  Trace analysis of radionuclides is an essential and versatile tool in modern science and technology. Because of their ideal geophysical and geochemical properties, long-lived noble gas radionuclides—particularly 39Ar (t1/2 = 269 y), 81Kr (t1/2 = 2.3 × 105 y), and 85Kr (t1/2 = 10.8 y)—have long been recognized to have a wide range of important applications in Earth sciences. In recent years, significant progress in the development of practical analytical methods has led to applications of these isotopes in the hydrosphere (tracing the flow of groundwater and ocean water). In this article, we introduce the applications of these isotopes and review three leading analytical methods: low-level counting (LLC), accelerator mass spectrometry (AMS) and atom trap trace analysis (ATTA).

The excitation function for fusion evaporation in the (60)Ni+ (89)Y system was measured over a range in cross section covering 6 orders of magnitude. The cross section exhibits an abrupt decrease at extreme sub-barrier energies. This behavior, which is also present in a few other systems found in the literature, cannot be reproduced with present models, including those based on a coupled-channels approach. Possible causes are discussed, including a dependence on the intrinsic structure of the participants.