Biennial Report 2005-2007 - Saha Institute of Nuclear Physics

162 Biennial Report 2005-07cases, detection of MCs + n (n = 2,3,. . .) molecular ions offers a better sensitivity as the yieldsof such molecular ion complexes have often been found higher than that of MCs + ions. This istrue in most of the cases where the elements are strongly electronegative with respect to cesium.Several works have been reported on the emission of MCs + n molecular ions in the SIMS process, buta complete understanding on the formation mechanism of these ion complexes is still lacking. Thekinetic energy distributions of secondary MCs + n molecular ion complexes has been found to be aneffective approach to estimate the local instantaneous surface work function changes under varioussurface exposure conditions, thereby enabling one to elucidate on the probable formation mechanismsof these molecular complexes from their emission dynamics. The present paper addresses abrief review on the MCs + n molecular ion complexes that are emitted in the SIMS process, includingvarious phenomenological approaches on the subject.Biswajit Saha, Purushottam ChakrabortySP5.1.3.12 On the formation mechanism of MCs + 2environmentmolecular ions under varying oxygenFormation of MCs 2 + molecular ions under Cs + bombardment of silver surface has been investigatedin the SIMS process under varying oxygen environments. Energy distributions of MCs+ and MCs 2+ions have shown a remarkable dependence on the changing oxygen environment. The changes inthe intensities of the above molecular species are attributed to the changes in the resputtered Cs +intensity due to the decrease in the local surface work function. The MCs + molecular ion hasbeen found to form via the recombination of a neutral Mo atom with a Cs + ion and its formationprobability remains unaffected by changing oxygen environment. A systematic study on the kineticenergy distributions of secondary Cs + , Cs 2 + , MCs + and MCs 2 + ions and the estimation of themean emission energies of various constituents participating in the respective formation processesconfirm that the MCs 2 + molecular ion formation via recombination of a neutral MCs ◦ moleculewith a Cs + ion is the most probable one.Biswajit Saha, Purushottam ChakrabortySP5.1.3.13 Silver Diffusion in SiCThis is a work connected with PBMR (pebble bed modular reactor) project of Pretoria University,South Africa. It concerns the optimization in the design parameters of multilayer-coated HTGR(high temperature gas cooled reactor) nuclear fuel particles. The core of the particle, i.e. fuelkernel of 300-500 micrometer diameter, consists of UCO- a mixture of UO 2 (uranium dioxide) andUC 2 (uranium carbide) phases distributed throughout the kernels. The presence of UC 2 serves toreduce the oxygen potential within the kernel, thereby reducing the tendency for SiC damage dueto oxidation. UC 2 reduces the gas pressure generated as a result of oxygen release from UC 2 followingfission; therefore, the tendency for fuel failures due to kernel migration and excessive internal