No abstract available

[en] We report on the measurement of optical isotope shifts for ^32-40Ar and for ⁴⁶Ar from which the changes in mean square nuclear charge radii across the N=20 neutron shell closure are deduced. The investigations were carried out by collinear laser spectroscopy in fast beams of neutral argon atoms. The ultra-sensitive detection combines optical pumping, state-selective collisional ionization and counting of β-radioactivity. By reaching far into the sd shell, the results add new information to the systematics of radii in the calcium region (Z ∼20). Contrary to all major neutron shell closures with N ≥28, the N=20 shell closure causes no significant slope change in the development of the radii. Information from the hyperfine structure of the odd-A isotopes includes the magnetic moments of ³³Ar (I=1/2) and ³⁹Ar (I=7/2), and the quadrupole moments of ³⁵Ar, ³⁷Ar (I=3/2) and ³⁹Ar. The electromagnetic moments are compared to shell-model predictions for the sd and fp shells. Even far from stability a very good agreement between experiment and theory is found for these quantities. The mean square charge radii are discussed in the framework of spherical SGII Skyrme-type Hartree-Fock calculations. (orig.)

[en] Doppler-free optogalvanic measurements of the 667.7 nm 4s[3/2]₁ to 4p[1/2]₀ transition in sup(36,38,40)Ar have yielded values for the isotope shifts, field shifts and delta< r²>. The field shift, calibrated with muonic X-ray data, confirm a significantly larger delta< r²> for ^38-40Ar (0.17 fm²) than for ^36-38Ar (0.07 fm²) consistent with similar data for K and Ca crossing the 1fsub(7/2) magic neutron number N=20. (orig.)

[en] A large volume, in-core gas irradiation technique was developed at The University of Texas at Austin MARK II TRIGA reactor to produce a sample of ⁴²Ar with an activity above 1 mBq. The method requires a large volume, 1.4 L, of natural Ar gas (99.6003 % ⁴⁰Ar) at about 1 atm and three 12 h irradiation periods. The production of ⁴²Ar requires a double capture to be produced from the stable ⁴⁰Ar isotope. This method produced 940 kBq of ³⁹Ar, 3.08 MBq ³⁷Ar, 114 GBq ⁴¹Ar, and 0.311 Bq ⁴²Ar at the end of the final irradiation period. (author)

[en] This is a report on the isotopic composition of argon found in neutron-irradiated quartz ampoules which contain samples of Allende coarse-grained inclusions or aluminium wrap only, and of argon released from one of these inclusions by stepwise heating. Besides atmospheric argon all ampoules contain, with varying concentrations but in constant proportions, ³⁸Ar from Cl and ³⁹Ar from K corresponding to Cl/K approx.= 1. No ³⁹Ar which has recoiled out of the samples and no argon which has diffused out of the samples has been observed in the ampoule gas. The analysis of the isotope variations of argon released by stepwise heating from a sample of inclusion 18 yields an apparent ⁴⁰Ar-³⁹Ar isochron age of 4.85 AE. A re-analysis of argon from another sample of the same inclusion dated earlier yields the same result. Thus inclusion 18 contains more radiogenic ⁴⁰Ar than can be accounted for by ⁴⁰K decay during 4.53 AE. (orig.)

[en] The separation of isotopes in opposed jet flows from slits was examined in Monte Carlo simulations of these flows at several density levels. The basic Monte Carlo method of Bird was employed to determine the flow fields and provide preliminary estimates of separation factors. Separation factors of high accuracy were obtained using a test-particle method in which single particles were followed through the flow fields established by the Bird method. Results were obtained for the system ⁴He/³⁶Ar, ⁴⁰Ar, with the argon isotopes seeded at low mole fraction into the helium carrier gas. Species densities, velocities and velocity distributions, as well as separation factors, are reported. For mixtures of 10% Ar in He, the optimum separation effect is observed at inverse Kundsen numbers, based on slit widths, in the range of 10 to 20

[en] ₄₀Ar-₃₉Ar-dating of individual chrondrules, which were petrographically and chemically characterized, was continued. ₄₀Ar-₃₉Ar-ages range from 4.4 to 4.6 billion years and thus cover a time interval, which cannot be reconciled with current theoretical calculations predicting a comparatively narrow time span needed for the formation of mm-sized particles in the early solar system. Surface-correlated argon is indicated in the younger chondrules which would exclude thermal stress of those chondrules and ₄₀Ar-diffusion-losses as an explanation for their relatively low ages. The higher ages are obtained for structurally and chemically peculiar chondrules. The interference, however, that these ages are due to the admixture of presolar ₄₀Ar from the present data is premature. (orig.)

[en] K/Ar and ⁴⁰Ar/³⁹Ar geochronology of minerals from the Stadlandet region, Norway, is complicated by the presence of excess ⁴⁰Ar in many samples. There is a correlation between excess ⁴⁰Ar concentration and textural occurrence of samples. Amphiboles and micas closely accociated with eclogite pods are likely to contain significant concentrations of excess ⁴⁰Ar, while samples from the country rocks are not. K/Ar and ⁴⁰Ar/³⁹Ar ages for hornblende from the country rocks suggest that post-metamorphic cooling through 500⁰C occured about 410 Ma ago, shortly after eclogite formation. Biotites containing no excess ⁴⁰Ar cooled through 300⁰C about 370 Ma ago. A post-metamorphic cooling rate of about 5⁰C/Ma can be inferred

No abstract available

[en] We have measured the isotope shifts between ^36,38,40Ar I using Doppler-free intermodulated optogalvanic spectroscopy on the 4p-5d transition at 604.3 nm. A field shift which is 60% greater than that of a 4s-4p transition previously measured was observed. The large relative field shift in the case of an atom of such low Z is surprising and not yet explained by any calculation. (author)