Manfred Lindner

Direct determination of the half-life of 187Re

Abstract We have determined the beta-decay half-life of 187Re to be (4.23 ± 0.13) · 1010 y, stated at the 95% confidence level. The method consisted of separating the daughter 187Os atoms from large sources of initially osmium-free rhenium. Yields were measured by the isotope dilution method using inductively coupled plasma mass spectrometry. Precision was gained through the use of two independent spike isotopes on each of five separate, large sources of rhenium. Because of the present lack of knowledge of other unrelated but equally important parameters, our value does not substantially narrow the range of uncertainty for the duration of galactic nucleosynthesis. However, the use of our half-life value to reinterpret the measurements of others on iron meteorites and chondrites suggests that iron meteorites might be 380 ± 160 million years younger than chondrites. Our half-life result may therefore have more profound implications for solar-system chronology than for galactic nucleosynthesis.

Lifetime of the long-lived isomer of 236Np from α-, β- and electron-capture decay measurements

Abstract The half-life of long-lived 236Np, due to α, β and electron-capture decay, was found to be 1.55 × 105 yr. Of all decays, 88% populate excited states in 236U and 12% populate levels in 236Pu. Lifetimes measured by growth of the ground states of 236U and 236Pu agree with values from corresponding γ de-excitations in these daughter nuclei. Therefore, nearly all the electron-capture decays populate the 6+ level of the ground-state band in 236U. Similarily, essentially all the β− decay populates an analogous 6+ level in 236Pu, which de-excites through a previously unreported transition of 158.3 keV. If a very weak γ-ray at 894 keV can be ascribed to a level in 232U populated by β decay of 232Pa, its existence establishes a 0.2% α-branching decay in 236Np.

Precise determination of the natural abundance of 237Np and 239Pu in Katanga pitchblende

Abstract Improvements in instrumentation and techniques have made possible the accurate determination of the natural abundances of 237 Np and 239 Pu from the same sample of pitchblende ore weighing no more than 10g. It was found that the abundance of 237 Np in our sample of Katanga pitchblende was 2·16 × 10 −12 atoms per atom of 238 U, while that of 239 Pu was 3·10 × 10 −12 atoms per atom of 238 U. The uncertainties of the determinations were less than 5 per cent.

The isomer ratio 236Np(l)236Np(s) in the reaction 237Np(n, 2n)236Np from neutrons produced in thermonuclear devices

Abstract The ratio of the long-lived to the 22-hr isomer of 236Np was measured for the reaction 237Np(n, 2n)236Np by exposing 237Np to the neutron spectra produced in underground thermonuclear tests. The average value of this ratio was found to be 0·35 ± 0·05.

Separation of Carrier-free 185Os, 183Re and 184Re from Irradiated Tungsten Targets

We have separated carrier-free osmium and rhenium chemical fractions from metallic tungsten targets irradiated with 65-MeV helium ions in the Berkeley 88-inch cyclotron. Gamma-ray spectral analyses showed only the presence of 1 * Os in the purified osmium fraction and only 1 * Re and 1 , 4 Re (both isomers) in the rhenium fraction. The mass spectrum of a repurified 3-year-old osmium fraction showed no detectable levels of isotopically normal osmium but did yield a mass distribution of the light stable osmium isotopes produced in the irradiation.

Reactor Neutron Fission Characteristics of the Long-Lived Isomer of Neptunium 236

The fission-product distribution was determined for the longlived isomer of 2 3 6 Np irradiated with neutrons in the core of the Livermore pool-type reactor. Yields were measured by comparison with the yield of the same fission product produced simultaneously in 2 3 5 U and, independently, by absolute gamma-ray counting of the fission products. The heavy mass peak of the asymétrie distribution is very similar to the heavy mass peak for 2 3 Pu thermal-neutron fission. The average number of neutrons emitted per fission was 2.3 ± 0.8. The fission cross section was 2.14 + 0.16 kb when measured by comparison with the observed 2 3 5 U fissions and a 2 3 5 U fission cross-section of 0.54 kb for reactor spectrum neutrons. Comparison of isobar yields in 2 3 6 Np with those in 2 3 5 U were made for seven mass chains, including the shielded nuclide Cs.

Laboratory Determination of the Half-Life of 187Re, a Nuclide of Cosmological Interest

In recent years there has been great interest among cosmologists and geochemists in an accurate laboratory’-measured value for the half-life of 187Re. The reason is that the 187Re-187Os isobaric couple has potential use as a terrestrial and cosmic chronometer. We have recently completed experiments which have yielded a value of (4.35 ± 0.13)1010 y for this half-life. We used micro and radiochemical techniques combined with mass spectrometry to observe the growth of 187Os into a large source of osmium-free rhenium. When our value is compared with the 187Re lifetime recently determined from meteorite analysis, we can set limits on the possible variation of the fine-structure “constant”, a, over the past 4.5 billion years.