S Kasthurirangan

Absolute total ionization cross sections of uracil (C4H4N2O2) in collisions with MeV energy highly charged carbon, oxygen and fluorine ions

Ionization and fragmentation of uracil molecules (C4H4N2O2, m = 112 amu) in collisions with fast highly charged C, O and F ions have been investigated using a time-of-flight mass spectrometer. The measurement of total ionization cross sections (TCS) is reported for different charge states (q), such as Fq + with q = 5?8; Oq + with q = 5,7; Cq + with q = 5 and 6. These studies reveal a (q/v)?1.5 dependence of TCS, in contrast, to the well-known q2-dependence in ion?atom collisions. Scaling properties of the TCS with projectile energy and charge states are obtained. The experimental results for TCS measurements are compared with the theoretical calculations performed within classical and quantum mechanical frameworks. The trends in energy dependence of the TCSs is qualitatively well reproduced by the different models and more specifically by the classical description, which provides the best agreement with measurements.

Temperature diagnostics of ECR plasma by measurement of electron bremsstrahlung.

The x-ray bremsstrahlung spectrum emitted by the electron population in a 14.5 GHz ECR plasma source has been measured using a NaI(Tl) detector, and hence the electron temperature of the higher energy electron population in the plasma has been determined. The x-ray spectra for Ne and Ar gases have been systematically studied as a function of inlet gas pressure from 7 × 10(-7) mbar to 7 × 10(-5) mbar and for input microwave power ∼1 W to ∼300 W. At the highest input power and optimum pressure conditions, the end point bremsstrahlung energies are seen to reach ∼700 keV. The estimated electron temperatures (T(e)) were found to be in the range 20 keV-80 keV. The T(e) is found to be peaking at a pressure of 1 × 10(-5) mbar for both gases. The T(e) is seen to increase with increasing input power in the intermediate power region, i.e., between 100 and 200 W, but shows different behaviour for different gases in the low and high power regions. Both gases show very weak dependence of electron temperature on inlet gas pressure, but the trends in each gas are different.

Angular dependence of L x-ray emission in high-Z atoms bombarded by 18–60 MeV fluorine ions

The L x-ray intensities of 79Au, 82Pb and 83Bi have been measured in collisions with 18–60 MeV fluorine ions in an angular range 20°–75°. The Ll x-ray intensity, normalized to the intensity of other L x-ray lines in the same spectrum, shows an anisotropic emission pattern and the degree of anisotropy depends on the impact energy of the projectile. The alignment parameter values for the L3 subshell, deduced from the measured anisotropy parameter for the Ll line, have been compared with the theoretical ones.

L3 subshell alignment in bismuth induced by swift silicon ions

Angular- and impact energy- dependence of L x-rays of bismuth in collisions with silicon ions has been measured. Unlike isotropic emission of the -group and the -group x-rays, the Ll x-ray yield was observed to have impact energy dependent anisotropy emission. The anisotropy parameter for the Ll x-ray line was obtained by using the intensity ratio of the Ll-to-other L x-rays in the same spectrum. The alignment parameter of the L3 subshell was deduced from the measured anisotropy parameter of the Ll x-ray and has been compared with those obtained from the collisional theoretical models based on the plane-wave Born approximation and its extension.

High-resolution study of x-rays emitted from highly charged S ions using a bent-crystal spectrometer

We have measured the projectile x-rays from 56 MeV sulphur ions in collision with thin carbon foils using a high-resolution bent-crystal spectrometer. The resolution of the spectrometer is good enough to resolve lines arising from transitions in H-, He- and Li-like ions. The line energies are compared with values from the NIST x-ray database. The data have also been used to generate the approximate equilibrium charge state distribution, which is compared with semi-empirical model predictions.

Projectile q-dependence of electron DDCS in fast ion-He atom collision

We have measured the electron double differential cross section (DDCS) for 3.75MeV/u O8+ and O5+ colliding on He atom. The projectile q-dependence on electron emission for different angles is investigated. The DDCS ratios for two charge states are found to be dependent on emission angle and energy of the electrons. A good agreement is found between the experimental data of DDCS ratios and the continuum distorted wave-eikonal initial state (CDW-EIS) predictions.

Electron DDCS in ionization of H2O by 72 MeV O8+ ions: comparison with CDW-EIS post and prior forms

We have measured energy and angular distributions of double differential cross-sections (DDCS) for electron emission in ionization of water molecule under the impact of 72-MeV O8+ ions. The DDCS values have been compared with a state-of-the-art CDW-EIS calculation for both the prior and post forms of scattering matrix. The prior form of the CDW-EIS model produces better quantitative agreement with the experimental data.

Highly charged ion impact on uracil: Cross sections measurements and scaling

Absolute total ionization cross sections (TCS) of uracil in collisions with highly charge C, O and F ions are measured. The scaling properties of cross sections are obtained as a function of projectile charge state and energy. The measurements are compared with the CDW-EIS, CB1 and CTMC calculations. The absolute double differential cross sections (DDCS) of secondary electron emission from uracil in collisions with bare MeV energy C and O ions are also measured. Large enhancement in forward emission is observed.

Temperature diagnostics of ECR plasma by measurement of electron bremsstrahlung spectra

The electron temperature of a 14.5 GHz ECR plasma is determined by measuring the electron bremsstrahlung spectra. The dependence of this temperature is studied as a function of the inlet gas pressure and the input microwave power for different gases, such as Ne and Ar. The measurements were made along the axis of the ECR plasma using a NaI(Tl) detector with suitable collimation to cut down on the high count rate.