Richard J. Borg

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.

Diffusion of Nonvolatile Metallic Elements in Graphite

The diffusion coefficients for Ag, Ni, U, Th, and Ra diffusing in various types of graphite have been determined as a function of temperature. Good precision is obtained in all cases and the corresponding activation energies are reported. These values, when comparison is possible, do not agree with values previously reported. An explanation is given for the lack of agreement based upon the relative insolubility of metals in graphite. Experimental evidence is presented which indicates that substitutional, pore, and interstitial diffusion are inoperative and that all mass transport proceeds solely via grain and subgrain boundaries. The activation energies and frequency factors are explained by a diffusion model involving widely spaced traps which are situated at imperfections.

An Explanation for the Effect of Ferromagnetism upon Atomic Diffusion

An explanation for the effect of ferromagnetism upon atomic diffusion is based upon the change in elastic properties corresponding to the degree of magnetic order. The unusual temperature dependence of D, the diffusion coefficient, in the vicinity of Tc, the Curie temperature, can be reconciled with recent measurements of the shear moduli by using the Zener model for diffusion.

Hyperfine Field of 197Au in Au–Fe Alloys

We have investigated the magnetic hyperfine splitting of 197Au by means of the Mossbauer effect in Au‐rich Au–Fe alloys. The hyperfine field appears with the onset of the magnetic order and the magnitude increases with concentration of iron. The spectra are broadened and unsymmetric due to the variety of compositional environments of the Au atoms in the alloys.

III – Mechanisms of Diffusion

hydrogen separation membrane x Diffusional Processes B A B A t = 0 C A C B

Short−range order in Au−Fe radiation−enhanced diffusion and the effectiveness of 14−MeV neutrons

Solute clustering in a Au−Fe (17% Fe) alloy has been demonstrated by Mossbauer effect measurements of the magnetic ordering temperature. Neutron irradiation at room temperature produces clustering by means of radiation−enhanced diffusion, while annealing at high temperatures is required to produce clustering thermally. The radiation−enhanced diffusion effect was used to compare directly the efficiency of mobile defect production by 14−MeV fusion neutrons and reactor neutrons. The 14−MeV neutrons are more effective by about a factor of 10 than the reactor neutrons (E≳0.1 MeV) in producing mobile lattice defects. Other compositions failed to respond to radiation−induced clustering, suggesting that Au−17−Fe is in a critical composition region regarding the response of the magnetic ordering temperature to variations in short−range order.

Some Observations on the Effect of Surface Roughness upon Diffusion

The effect of surface roughness has been investigated experimentally and found to lead to spuriously high values of the diffusion coefficient. The magnitude of the error has been estimated as a function of Dt and the degree of roughness. The results indicate that surface roughness may contribute the major source of error in the determination of very small diffusion coefficients. The presence of an uneven source surface may also account for the anomalous tracer penetration profiles which are observed in the vicinity of the surface and, corresponding to which, anomalous values of D have been frequently reported.

Comments on the Article ``Diffusion in Cobalt‐Nickel Alloys''

The magnetic effect on atomic diffusion is discussed briefly. The results of Hirano et al. on the diffusion of Co and Ni in pure Co and several Co-- Ni alloys (ibid., 33; 3049) were examined and found to contradict their claims. For the diffusion of Co into pure Co, if all the data points for the ferro- and paramagnetic regions are treated together rather than separately, one diffusion equation is obtained. Further, in the case of the magnetic effect found in some Co-- Ni alloys, the effect actually decreases with the Curie temperature, and the data do not support the vacancy decrease model. (D.L.C.)

Metallurgical explanation of ‘‘double magnetic transitions’’ and allied phenomena in AuFe alloys

Evidence for two magnetic ordering temperatures in AuFe alloys (0.1<XFe<0.28) is given by ac and static susceptibility measurements and Mossbauer experiments. These data have given rise to a variety of qualitative to semiquantitative theories based upon fluctuations in the random distribution of Fe atoms. Our explanation of the two ordering temperatures is based upon the precipitation of metastable Fe‐enriched regions which arise via a decomposition and which magnetically couple to the spin‐glass matrix. The high temperature transition denotes the onset of ferromagnetic order within the ‘‘precipitate’’ whose field is then coupled to the paramagnetic matrix. When the temperature of the system is decreased below the spin‐glass ordering temperature, the apparent high temperature ferromagnetic character is destroyed, attesting to the dominance of the random spin‐glass structure and the strength of the coupling between the two phases. Hence, it is the temperature dependence of the coupling between distinguisha...

Clustering and Giant Moments in Cu–Ni Alloys

There are several studies of paramagnetic susceptibility and neutron diffraction revealing the presence of giant moments associated with chemical clustering. However, in contrast to the paramagnetic state, there are but few analogous studies extending below Tc. We prepared Cu–Ni alloys of about 50–50 composition as Mossbauer sources containing 57Co. Internal fields, Tc, and isomer shifts were determined as functions of composition and heat treatment. The temperature dependence of the hyperfine spectrum changes very little as a result of annealing at low temperature. Nevertheless, the value of Tc itself dramatically increases in response to clustering, and the isomer shift and internal field at ∼0°K are singificantly altered in response to the change in chemical short‐range order.