Biennial Report 2005-2007 - Saha Institute of Nuclear Physics

Nuclear Sciences 57length of positrons. In large nanocones, crystal vacancies within the cones also trapped positronsand the effects of agglomeration of such vacancy clusters due to increased temperatures of synthesiswere reflected in the variation of the annihilation parameters with cone sizes. The sizes of thenanoparticles used were all in the limit of thermal diffusion length of positrons and the annihilationcharacteristics were in accordance with the decreasing contribution from surfaces with increasingparticle size. Photoluminescence studies were also carried out and characteristic emission bandscorresponding to Zn vacancies acting as luminescence centers were identified.Tandra Ghoshal†, Subhajit Biswas†, Soumitra Kar†, PMG Nambissan, Subhadra Chaudhuri† NAP2.4.2 Perturbed Angular Correlation Studies2.4.2.1 Perturbed angular correlation (PAC) studies in Hf-Zr alloyPAC studies have been performed in Hf-Zr alloy with 3 at% Zr using the PAC spectrometer with fourBaF 2 detectors. PAC measurements were performed at 77K and from RT to 700 ◦ . Two quadrupoleinteraction frequencies have been found at all temperatures. At RT, the parameters were found tobe ω Q = 49.6(3) Mrad/s, η = 0.24(2), δ = 6.9(7) and ω Q = 70(1) Mrad/s, η = 0.25(8), δ = 3(4)forthe strong (90%) and weak component (10%), respectively. Assuming a random distribution ofZr atoms following Poission statistics, populations of 70, 25 and 5 % for 12, 11 and 10 nearestneighbours (NN) of Hf in the first coordination shell are expected for the above Zr concentration.If we consider the two components to be due to 12 and 11 NN Hf, the experimental results showhigher percentages than the probabilities obtained by statistical distributions. At 600 ◦ , the sitepopulations for both the components have been found to be almost equal (50%).CC DeyNAP2.4.2.2 Temperature dependence of quadrupole interactions in HfO 2 studied by perturbedangular correlation (PAC)Temperature dependence of electric field gradient (efg) in HfO 2 has been studied by PAC wherethe sample has been produced for the first time from Hf metal by heating in air at 900 ◦ . Startingwith Hf metal and heating it in a resistive furnace up to 600 ◦ , no indication of HfO 2 was observedfrom the PAC spectrum. At 700 ◦ , only a small frcation (17%) was identified as due to HfO 2 . Afterheating at 900 ◦ for 24 hrs. and annealing at 600 ◦ , a complete conversion of HfO 2 from Hf metalwas obtained as characterised by PAC spectroscopy. PAC measurements in this sample were carriedout in the temperature range from 77K to 873K. A T 3/2 relation with temperature for the efg wasobtained.CC DeyNAP