K. G. Prasad

Ion beam mixing studies in the Sn-Si system

Abstract The process of ion beam mixing in the Sn-Si system is investigated by varying both the incident Ar+ ion energy (100–220 keV) at a fixed dose (2 × 1016/cm2) and the fluence (5 × 1015/cm2− 5 × 1016/cm2) at a constant energy (130 keV). The analyses of the spectra. obtained from Rutherford backscattering (RBS) of 3 MeV alpha particles, have shown that, in these samples (i) the width of the intermixed Sn-Si region increases as the Ar+ energy or fluence is increased (ii) the number of Si atoms/cm2 increases linearly with Ar+ dose and (iii) no significant penetrating tail is present in the Sn profile. From conversion electron Mossbauer (CEM) studies of these systems, it is observed that, at the most, about 40% of Sn in the intermixed layer contributes to form the Sn(1−x)Six compound. It is deduced from these results that x is in the range 0.5 ≦ x ≦ 0.8. These observations also suggest a large transport of Si atoms into the Sn layer, possibly due to radiation damage effects caused by Ar+ ion bombardment.

Investigations of Ar+ implantation and proton irradiation of amorphous Fe40Ni40B20 samples by Mössbauer techniques

The effect of the implantation of 150 keV Ar+ ions at different doses on the surface of amorphous Fe40Ni40B20 and the changes in the bulk properties following proton irradiations are investigated by conversion electron and transmission Mössbauer spectroscopy respectively. In the former case a correlation between the total Mössbauer absorption and the total energy deposited by incident Ar+ ions is established, indicating the development of certain stresses in the material, affecting the inter and intra molecular bonding in the near surface region. On the other hand, the proton irradiation seems to cause a reorientation of the atomic spins and also the formation of an additional Fe or Fe−Ni rich phase in the sample. Also the low field A.C. susceptibility is found to decrease as a function of the dose of incident protons. Possible reasons for the above behaviour are discussed.

Investigation of ion beam induced mixing in PdSi, SnSi and WSi systems

Abstract Systematic studies of the ion beam mixing are carried out with 70–250 keV Ar+ ions in the dose range 5 × 1015 to 5 × 1016 ions/cm2 in Pd Si , Sn Si and W Si systems. Rutherford back scattering measurements with 2–3 MeV a-particles have shown that (i) the formation of Pd silicide is dominated by a cascade mixing process (ii) the Sn Si system is more complex and a composition like SixSn(1−x) is formed in the intermixed region with 0.5 Si system for room temperature Ar+-bombardments.

Estimation of oxygen content in high Tc superconductors: An accelerator based method☆

Abstract It is now well known that the role played by oxygen is very important for the occurrence of superconductivity in the 1-2-3 type YBa 2 Cu 3 O 7−x compounds. We have made an attempt to estimate the oxygen content relative to that of Ba in these compounds after various types of heat treatments, using the resonance elastic scattering and Rutherford backscattering of 3.01 MeV alpha particles. It is consistently observed that those samples, which show good superconducting properties, also contain relatively higher oxygen content (6–16%) as compared to non-superconducting ones. However, it is difficult to make absolute determinations of the oxygen content by this technique. The oxygen content in these samples has also been determined by a chemical method. The two results are compared and their relative merits and disadvantages are discussed.

Study of x-ray and L-REC photon emissions from highly ionized swift and projectiles passing through a thin carbon foil

X-rays and L-shell radiative electron capture (L-REC) photons have been detected from highly ionized swift projectiles of and passing through a thin carbon foil at two incident projectile energies. All the K and L peaks show strong shifts towards higher energy as a result of multiple ionization of L- and M-shells. The intensity ratios have been found to be almost half of the value for the singly ionized atoms. The measured positions of various L and K x-ray peaks and the measured intensity ratios have been attributed to certain types of electronic configurations based on the extensive multiconfiguration Dirac-Fock calculations. Generally, it has been found that the lowest-energy configuration plays a rather small role, but in contrast the excited configurations with holes in the L-shell and with occupied 3d, 4s, 4p, 4d or higher subshells seem to be very important. The relative L-REC cross section between higher and lower projectile energies shows excellent agreement with the results obtained under the non-relativistic dipole approximation.

Lattice location of Hf implanted in cubic Al, Cu and Ag host metals

Abstract Investigations regarding the location of implanted 178 Hf at 110 keV in Al, Cu and Ag single crystals by the channeling technique have revealed that in Al and Ag the Hf ions occupy substitutional sites. The effect of a large disparity in the masses of the impurity Hf and the host Al is reflected in a 15% lower minimum yield and a 5% larger width of the channeling dip for the Hf as compared to Al. In Cu the substitutionality is only 61%, at the same time the impurity ions are displaced by 0.05 to 0.07 A from a truly substitutional site. These results are discussed in terms of the differences in electronegativities and atomic radii of the impurity and the host atoms.

Electric quadrupole interaction at181Ta in RNi5 compounds

The time differential perturbed angular correlation measurements in PrNi5, NdNi5, GdNi5, DyNi5 and ErNi5 rare-earth intermetallics with181Ta as probe nuclei have revealed that these impurity nuclei experience a non-axial electric quadrupole interaction. In all the cases, except in GdNi5, the interaction frequency has a single value in the range 15–20 MHz with asymmetry parameterη ∼ 0.98. In GdNi5, however, there are two interaction frequencies (13.61 MHz and 26.89 MHz), the former being the dominant one (90%) withη ∼ 0.35 and the latter (10%) withη ∼ 0.98. It is concluded that in all cases the probe nuclei experiencing the highly asymmetric electric field gradient (efg) occupy the unique Ni(3g) site. Annealing studies have shown that in the case of GdNi5 the interaction gets highly damped, while there is negligible effect in the other cases. The temperature variation of the efg in all cases seems to follow the empiricalT3/2 relation.

Subshell-resolved L - K electron transfer and ionization cross sections for Au by bare and H-like sulphur ions

We have measured, for the first time, subshell-resolved L - K ( - K, - K and - K) electron transfer cross sections for electron transfer from the L-shell of the target to the vacant K-shell of the projectile. These cross sections are derived by measuring the charge-state dependence of the target L x-rays from a thin Au target using bare and H-like sulphur ions. In addition, the subshell-resolved ionization cross sections are also derived from the analysis of the L x-ray production cross sections. We found that the vacancy production cross section for the subshell is much smaller than that for strongly bound subshell and the - K transfer cross section is also much smaller than the - K transfer cross section. These observations are contrary to the prediction of first-order theories.

Instrumentation for beam-foil spectroscopic studies in the UV-visible region

A facility for carrying out beam-foil spectroscopic studies in the UV and visible region using the 400 kV electrostatic ion accelerator at Tata Institute of Fundamental Research, Bombay is described.

A versatile PC based control system for channeling experiments

Abstract A PC based control system for aligning the low index directions of a single crystal target with that of a charged particle beam from an accelerator has been designed. Two stepper motors are employed to control the tilt (θ) and rotation (φ) of the target mounted on a double axis high precision goniometer. A user friendly software using BASIC language is developed for controlling the motors and acquiring data with two detectors simultaneously. Many additional features are also included in the design of the system.