M. Paul

A Shorter 146Sm Half-Life Measured and Implications for 146Sm-142Nd Chronology in the Solar System

A New Lease on Half-Life Radiometric dating relies on measuring the abundance of long-lived radionuclides relative to the abundance of their radiogenic decay products—a process determined by the original radionuclides half-life. For primordial radionuclides that decay slowly, such as 146Sm decaying to 142Nd, this method provides the timing of some of the earliest processes in solar system history. Using accelerator mass spectrometry, Kinoshita et al. (p. 1614) provide a revised estimate for the 146Sm half-life of ∼68.7 million years, which is 30% shorter than the previously accepted value. This shorter half-life suggests that reductions need to be made in the estimated ages for differentiation of Earths mantle and the solidification of the Moons magma ocean and for other more recent processes. Mantle differentiation of Earth, the Moon, and Mars occurred earlier and over a shorter time scale than previously estimated. The extinct p-process nuclide 146Sm serves as an astrophysical and geochemical chronometer through measurements of isotopic anomalies of its α-decay daughter 142Nd. Based on analyses of 146Sm/147Sm α-activity and atom ratios, we determined the half-life of 146Sm to be 68 ± 7 (1σ) million years, which is shorter than the currently used value of 103 ± 5 million years. This half-life value implies a higher initial 146Sm abundance in the early solar system, (146Sm/144Sm)0 = 0.0094 ± 0.0005 (2σ), than previously estimated. Terrestrial, lunar, and martian planetary silicate mantle differentiation events dated with 146Sm-142Nd converge to a shorter time span and in general to earlier times, due to the combined effect of the new 146Sm half-life and (146Sm/144Sm)0 values.

in uranium minerals and standards

Abstract We have recently extended our accelerator mass spectrometry (AMS) system to the detection of actinides for geophysical applications, taking 236 U as an interesting test case for this class of nuclides. We report on the development of the experimental setup and on preliminary measurements of 236 U in minerals and uranium standards. The isotopic abundance sensitivity is 1×10−11. The natural 236 U abundance of five U-rich minerals were measured to be between 1 and 33×10−11. 236 U is expected to be produced in situ by neutron capture on 235 U . Three uranium standards material were measured and showed a large 236 U contamination.

81Kr in the Great Artesian Basin, Australia: a new method for dating very old groundwater

Abstract The measurement of cosmogenic 81Kr (t1/2=(2.29±0.11)×105 yr) has been proposed for many years as a reliable tool for groundwater dating in the range from 105 to 106 yr. In this paper, we report on the first use of 81Kr to determine the age of groundwater from four wells in the Great Artesian Basin in Australia. As the concentration of 81Kr in old groundwater is only a few hundred atoms per liter, krypton was extracted from large (16 000 l) groundwater samples and was analyzed for the isotopic abundance of 81Kr by accelerator mass spectrometry (AMS) with a cyclotron. 81Kr/Kr isotope ratios of (1.54±0.22)×10−13, (1.78±0.26)×10−13, (2.19±0.28)×10−13 and (2.63±0.32)×10−13, respectively, were measured for these samples. It is reasonable to assume that krypton dissolved in surface water in contact with the atmosphere has the known atmospheric 81Kr/Kr ratio of (5.20±0.40)×10−13. The observed reduction of isotope ratios in the groundwater samples can then be interpreted as being due to radioactive decay since recharge. This results in respective groundwater ages of: (4.02±0.51)×105 yr, (3.54±0.50)×105 yr, (2.87±0.38)×105 yr and (2.25±0.42)×105 yr. The main emphasis of this paper lies on the description of the analytic procedure to extract a reliable 81Kr signal from large groundwater samples. Although the uncertainties are still relatively large (primarily due to counting statistics caused by the low cyclotron AMS efficiency), the new technique enabled for the first time a definite determination of residence times for old groundwater with 81Kr. It thus confirms the hope that this radionuclide may become a very valuable tool for groundwater dating.

Calcium-41 concentration in terrestrial materials: prospects for dating of pleistocene samples

Calcium-41 has been suggested as a new tool for radiometric dating in the range of 105 to 106 years. The concentration of cosmogenic calcium-41 in natural samples of terrestrial origin has now been determined by high-sensitivity accelerator mass spectrometry after pre-enrichment in calcium-41 with an isotope separator. Ratios of calcium-41 to total calcium between 2 x 10-14 and 3 x 10-15 were measured for samples of contemporary bovine bone and from limestone deposits. Some prospects for the use of calcium-41 for dating Middle and Late Pleistocene bone and for other geophysical applications are discussed.

Recent near-Earth supernovae probed by global deposition of interstellar radioactive 60 Fe

The rate of supernovae in our local Galactic neighbourhood within a distance of about 100 parsecs from Earth is estimated to be one every 2–4 million years, based on the total rate in the Milky Way (2.0 ± 0.7 per century). Recent massive-star and supernova activity in Earth’s vicinity may be traced by radionuclides with half-lives of up to 100 million years, if trapped in interstellar dust grains that penetrate the Solar System. One such radionuclide is 60Fe (with a half-life of 2.6 million years), which is ejected in supernova explosions and winds from massive stars. Here we report that the 60Fe signal observed previously in deep-sea crusts is global, extended in time and of interstellar origin from multiple events. We analysed deep-sea archives from all major oceans for 60Fe deposition via the accretion of interstellar dust particles. Our results reveal 60Fe interstellar influxes onto Earth at 1.5–3.2 million years ago and at 6.5–8.7 million years ago. The signal measured implies that a few per cent of fresh 60Fe was captured in dust and deposited on Earth. Our findings indicate multiple supernova and massive-star events during the last ten million years at distances of up to 100 parsecs.

Influence of Nuclear Structure on Sub-Barrier Hindrance in Ni + Ni Fusion

Fusion-evaporation cross sections for

Production of radioactive ion beams using the in-flight technique

^{64}

Measurement of 81Kr in the atmosphere

Ni+

Half-life of 60Fe

^{64}

Experimental Limit to Interstellar 244Pu Abundance

Ni have been measured down to the 10 nb level. For fusion between two open-shell nuclei, this is the first observation of a maximum in the