Re and Al 2 O 3 were heated with laser beams from both sides. Acting like planar heat sources, the two`hot plates' eliminate the axial temperature gradient in the sample between the plates. Temperature variation is less than 3% within roughly 30 mm diameter at 2,500 K. Before the melting experiments, the sample was scanned with a laser beam and heated to about 2,000 K to reduce the pressure gradient and to produce a high-pressure solid-phase assemblage. For stable and smooth temperature control, temperatures were increased by adjusting an aperture placed near the beam exit, stepwise, instead of by adjusting power. Each step corresponds to a 50±100 K increase. A 30-mm spot was homogeneously heated by opening the aperture (increasing the step). At the onset of melting, temperature remains constant or drops slightly with the step increment, and then drastically increases (.400 K) within one step. To ensure the reliability of the melting criteria used in this study, we conducted melting experiments at pressures (16±27 GPa) overlapped by the multi-anvil apparatus and the diamond-anvil cell, using the same starting material, and obtained consistent melting temperatures (Fig. 3). We also used the same melting criteria to determine the melting temperature of MgSiO 3 ±perovskite previously studied by other investigators, and our results agree with these recent determinations 13,14 (Fig. 3). The temperature runaway phenomena near the onset of melting observed in simple and complex samples were probably a result of the latent heat of melting, followed by melt migrating away from the heated spot because of the large thermal pressure and, ®nally, the Re foils would have been heated without sample in between. No chemical reaction between Re and sample was observed in the multi-anvil experiments on a scale of 1 mm. The melting temperatures reported here are the last temperatures before melting sets in. Pressures were measured using a ruby-¯uorescence technique after each measurement of melting temperature.

The half‐life of 26Al has been redetermined because of suggestions of an error in the accepted value based on its use in calculating 21Ne production rates from cosmic rays in meteorites. Two solutions of 26Al were analyzed for the specific radioactivity and mass spectrometric determination of the 26Al concentration. The half‐life obtained for 26Al was 7.05×105 years ±3.4%. This is identical to the accepted value of 7.16×105 years and indicates that problems with the 21Ne production rate is not due to an erroneous half‐life.

Radionuclide migration / Technetium-99 / Relative migration rates SummaryThe Cambric Experiment, under the auspices of the Hydrology/ Radionuclide Migration Project, is measuring the migration of radionuclides from the site of an old underground nuclear test. Ten years after the 1965 test, a re-entry well (RNM-1) was drilled into the remnant of the explosion cavity to obtain core and water samples. Pumping water from a satellite well (RNM-2S) 91 m from the cavity subsequently induced an artificial gradient that has allowed soluble radionuclides to migrate from the cavity. Tritium (HTO) has been observed in the RNM-2S water; its elution has been well characterized. Other radionuclides have also been monitored in water from RNM-2S: 36 C1, 85 Kr, l29 I, and 106 Ru. We have recently measured "Tc at the 10-20 fg/1 level in RNM-2S water. In contrast to the 3 H source term, which is essentially entirely available to transport, these results indicate that only ~0.01 % of the "Tc has escaped from the vitrified rock matrix into the water. The technetium in solution appears to be migrating more slowly than the 36 C1 is. Although "Tc's initial breakthrough is similar to that for 3 H, the migration rate of the "Tc center of mass appears to slightly exceed that of 3 H, perhaps as a result of anion exclusion effects. All measured "Tc concentrations are considerably below limits established for public drinking water.

In uncontaminated natural materials, plutonium and technetium exist exclusively as products (daughters) of nuclear reactions in which uranium is the principal reactant (parent). Under conditions of chemical stability over geologic periods of time, the relative abundances of daughter and parent elements are fixed by the rates of nuclear reactions and the decay of the daughter radionuclide. The state of this nuclear secular equilibrium condition is the primary basis of the geochemical study of these elements in nature. Thus, it is critical that nuclear parent and daughter abundances are measured in the same sample. We have developed a quantitative procedure for measuring subpicogram quantities of plutonium and technetium in gram quantities of geologic matrices such as uranium ores. The procedure takes advantage of the aggressive properties of sodium peroxide/hydroxide fusion to ensure complete dissolution and homogenization of complex materials, the precision provided by isotope dilution techniques, and the extreme sensitivity offered by thermal ionization mass spectrometry. Using this technique, a quantitative aliquot can be removed for uranium analysis by isotope dilution thermal ionization mass spectrometry or α spectrometry. Although the application of the procedure is unique, the analytical concepts may find more general application in studies of environmental contamination by nuclear materials. To assess the precision and accuracy of the analytical results, blanks and standards were analyzed routinely for a 1-year period to ensure quality control of our sample analyses. The average technetium blank is 5 ± 4 fg (n = 8), and that for plutonium is 0.17 ± 0.15 pg (n = 7). Thus, the detection limit for technetium (defined as 3 times the standard deviation of the average blank) is 11 fg, and that for plutonium is 0.44 pg. To assess the procedural precision, Canadian Reference Material BL-5 was analyzed routinely with samples. The results of seven replicate analyses for technetium in this standard reference material yield a technetium concentration of 59.0 fg/g, with a remarkably small standard deviation of 0.6 fg, 1.0% of the average value. The results of six replicate analyses for the concentration of plutonium in BL-5 give 1.012 pg/g, with an equally small standard deviation of 0.016, 1.6% of the average value. No direct measure of accuracy can be done on the technetium or plutonium analyses, because no standard reference material exists for these elements. To help constrain the accuracy of our measurements, equilibrium technetium/uranium and plutonium/uranium abundances were calculated using the nuclear reaction code MCNP. For technetium, such calculations are relatively insensitive to variations in model parameters, and measurements fall within a 21% high/low bias. For plutonium, the calculations are very sensitive to model parameters and hence inherently less precise. Indirectly, spike and isotope mix calibrations made from weighted quantities of certified isotopes (both technetium and plutonium) can be used t...