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We have developed a simple precipitation route to synthesize magnetite (Fe 3 O 4 ) nanoparticles with controlled size without any requirement of calcination step at high temperatures. The study of these nano-particles indicates an enhancement in saturation magnetization with reduction in size down to ~10 nm beyond which the magnetization reduces. The latter is attributed to surface effects becoming predominant as surface to core volume ratio increases. From the view -point of applications, 10 nm size of magnetite particles seems to be the optimum.

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The saturation effect in the excitation cross sections has been investigated for Si 12ϩ ions colliding with gas target atoms ͑atomic number Z t between 2 and 54͒ in the ͑8-12͒-a.u. velocity range, both experimentally and theoretically. The experimental excitation cross sections have been deduced from the measured Ly x rays following the decay of the excited atoms. Theoretical calculations have been performed using the first Born approximation, the symmetric eikonal continuum distorted wave ͑SECDW͒ approximation, and the Schwinger variational principle ͑SVP͒. Similar to the earlier observations on the heavier ion species, the saturation has been found in both the experimental and the SVP cross sections in the entire velocity range. The SECDW approximation, however, has been found to underestimate the cross sections at large values of Z t in the presently investigated systems. In the light of these findings, the role of higher-order effects within different theoretical models has been reviewed and the possible interaction mechanisms have been discussed.

Lifetimes and excitation behaviour of a number of core excited quartet levels in neutral Na, produced by beam foil excitation, have been established. The present results have helped to resolve the discrepancy seen in the published lifetime values of the 3s4s 4P5/2 and 3s3d 4P5/2 levels. Further, a new line having a wavelength of 4343 AA displaying similar excitation behaviour to the known transitions between the quartet states has been observed. Its lifetime is measured to be 0.91+or-0.13 ns. It is suggested that this line is most probably due to a transition between the 3s(1P)3p 2/2P3/2 and 3s2(1S) 2P3/2 states.