Mithlesh Kumar

Photoluminescence and EPR studies on Fe3+ doped ZnAl2O4: an evidence for local site swapping of Fe3+ and formation of inverse and normal phase

Considering that ZnAl2O4 spinel has two different sites (octahedral and tetrahedral) and its properties change with dopant ion distribution among these two sites; ZnAl2O4 doped with varied concentrations of Fe(3+) was synthesized by a low temperature sol-gel combustion method. Phase purity and structural investigations were carried out using Rietveld refined X-ray diffraction which shows a decrease in the value of cell parameters at higher doping levels. Photoluminescence (PL) and electron paramagnetic resonance (EPR) studies have shown that on doping, Fe(3+) ions were distributed in both tetrahedral and octahedral sites. At octahedral sites, Fe(3+) exhibited a broad red emission around 745 nm while at tetrahedral sites it exhibited well-defined vibronic sidebands at 665, 674, 684 and 693 nm along with a broad blue band with a maxima at 445 nm at room temperature. EPR studies have shown a broad spectrum at g ≈ 2.2 which corresponds to the Fe(3+) in octahedral sites, while the broad signal at g ≈ 4.2 belongs to Fe(3+) in tetrahedral sites. It was also inferred from these studies that Fe(3+) prefers to occupy octahedral sites at higher concentrations and at higher annealing temperatures. The PL decay behavior of Fe(3+) in ZnAl2O4 has also shown that two different types of Fe(3+) ions were present in this matrix. The first type was a long lived species (τ ≈ 170 μs) present at octahedral sites and the other was a short lived species (τ ≈ 40 μs) present at the tetrahedral sites; the fraction of the long lived species predominate at higher concentrations. Thus the present work is mainly focused on understanding the tuning of local site occupancy of the dopant ion among those sites with varying concentration and annealing temperature, using the dopant ion itself as a spectroscopic probe, which further helps in understanding the phase (inverse and normal) of the spinel.

Photoacoustic spectroscopic studies of Ho3+, Er3+, and Sm3+ doped polyvinyl alcohol films

Photoacoustic (PA) spectra of Ho 3+ , Er 3- , and Sm 3+ doped PVA films were obtained in 350-800 nm range. PA spectra were also obtained for the respective dopant oxides: Ho 2 O 3 , Er 2 O 3 , and Sm 2 O 3 for comparison. It was found that in PVA the PA sensitivity has increased considerably compared to pure rare earth oxides. The relative intensities of absorption bands at 540 and 637 nm of Ho 3+ : PVA have shown distinct enhancement, indicating the increase in nonradiative relaxation at these excitations. Furthermore, the PA signals at wavelengths for different PA absorption bands were monitored as a function of chopping frequency. These experiments have shown that PA signal varies ω -1 both for oxides and PVA samples, suggesting that they behave as thermally thin samples.

Preparation of alumina stabilized aqueous Pd colloid by reduction using gamma-radiolysis, UV-photolysis and H2

The techniques of gamma-radiolysis, UV photolysis and hydrogen-induced reduction of aqueous palladium perchlorate to ultrafine particles of Pd, in the presence of alumina sol, have been studied. As compared to H2-induced reduction, both UV photolytic and gamma-radiolytic reduction methods lead to a very stable, brown colored Pd colloid with relatively less absorption in the higher wavelength region and possessing much smaller volume average particle size (62 and 61 nm, respectively). Higher concentration of alumina sol and increase in pH from 1.8 to 7.2, favour the formation of smaller sized particles as determined by the dynamic light scattering technique.

Synthesis, characterization, photoluminescence and thermally stimulated luminescence investigations of orange red-emitting Sm3+-doped ZnAl2O4 phosphor

Sm3+-doped ZnAl2O4 phosphor was synthesized by citrate sol-gel method and characterized using X-ray diffraction and scanning electron microscopy to identify the crystalline phase and determine the particle size. Photoluminescence (PL) studies on the sample showed emission peaks at 563, 601, 646 and 707 nm with λex = 230 nm corresponding to the 4G5/2 → 6H5/2, 4G5/2 → 6H7/2, 4G5/2 → 6H9/2 and 4G5/2 → 6H11/2 transitions, respectively, due to Sm3+ ions. PL lifetime decay studies confirmed that Sm3+ ions partly entered into the lattice by replacing Al3+ ions and remaining located at the surface of ZnAl2O4 host matrix. Thermally stimulated luminescence (TSL) studies of γ-irradiated Sm3+-doped ZnAl2O4 sample showed two glow peaks at 440 and 495 K, the former being most intense than the latter. The trap parameters were determined using different heating rate methods. Spectral characteristics of the TSL glow showed emission around 565, 599 and 641 nm, indicating the role of Sm3+ ion as the luminescent centre. A probable mechanism for the prominent TSL glow peak, observed at 440 K, was proposed. CIE chromaticity coordinates for the system was evaluated, which suggested that Sm3+-doped ZnAl2O4 could be employed as a potential orange red-emitting phosphor.

Trap level spectroscopy of actinide-doped alkaline-earth sulphates. III. SrSO4:UO22+

For pt.II see ibid, vol.21, p.5913 (1988). Electron paramagnetic resonance (EPR) and thermally stimulated luminescence (TSL) studies were conducted on gamma -irradiated SrSO4:UO22+ to elucidate the role of electron and hole traps in thermally stimulated reactions and to obtain the trap parameters (the trap depth and frequency factor). For samples annealed at 970 K (samples A), intense hydrogen atom signals were seen in EPR, and it was found that the thermally stimulated reaction between H0 atoms and SO4- is responsible for the luminescence glow at 380 K. In the close vicinity of this temperature, the EPR spectrum of hydrogen exhibited features of chemically induced dynamic nuclear polarisation, suggesting that this reaction proceeds through pair formation. The hydrogen atom intensities were considerably smaller for samples quenched at 1170 K (samples B) but appeared to have participated in the formation of SH2. In both samples A and samples B, a trapped hole centre with g=1.995 formed owing to gamma -irradiation at 300 K. The temperature stability of this crucially depended on the presence of H0 and also SO4u-. The intense glow peak at 420 K present in both samples was identified as due to the reaction between SO4SuS- and UO2+.

Direct determination of metallic impurities in graphite by EDXRF

Energy dispersive X-ray fluorescence (EDXRF) methods have been developed for the direct determination of 14 trace metallic impurities in graphite powder without any need for sample dissolution. Using synthetic standards, calibration curves were established for different elements after optimizing the spectrometer parameters. Two synthetic samples were analyzed to evaluate the performance of the developed analytical methods. The estimates for most of the analytes were in good agreement with the added amounts. Three graphite powder samples were analyzed by the present method as well as by D.C. arc emission spectrometric technique for comparison and the agreement between the analyte values determined, using both methods was good. Samples in pellet form were analyzed using a separate calibration with standards in pellet form. The present method is rapid, as it alleviates the need for any chemical treatment and gives good precision.

A case study of energy transfer mechanism from uranium to europium in ZnAl2O4 spinel host by photoluminescence spectroscopy

Zinc aluminate (ZAO), a member of spinel class of inorganic compounds has been of much interest of late due to its wide range of use in catalysis, optical, electronic and ceramic industries. When doped with several lanthanides, this material has proved to be a potential host matrix for phosphors. As lanthanides suffer from poor (direct) excitation and emission cross sections, the use of a co-dopant ion can help to circumvent this and extract better emission from a lanthanide doped ZAO system. In this connection, energy transfer mechanism from uranium to europium in the ZAO host was investigated by photoluminescence spectroscopic technique. It was seen that uranium gets stabilized in the hexavalent state as UO6(6-) (octahedral uranate) where as the lanthanide ion, Eu is stabilized in its trivalent state in the ZAO host. In the co-doped system, an efficient energy transfer pathway from the uranate to europium ion was observed. Based upon emission and life time data a suitable mechanism was proposed for the energy transfer (quenching) process. It was proposed that after excitation by photons, the uranate ions transfer their energy to nearby (5)D1 level of Eu(3+) ions which non-radiatively de-excites to the corresponding lower levels of (5)D0. Further this (5)D0 level decays in a radiative mode to the (7)F manifold giving the characteristic emission profile of trivalent Eu. It was proposed that both static and dynamic types of energy transfer mechanism were responsible for this process.

Photoinduced changes in photorefractive PVA films doped with Cr3+ and VO2+: EPR and PAS investigations

In order to understand the mechanism of optical image storage in photorefractive polyvinyl alcohol (PVA) films, photo-EPR and photoacoustic spectral investigations were carried out on PVA films doped with Cr3+ and VO2+. The EPR spectrum of Cr3+ has shown reduction in intensity onin situ illumination with copper vapor laser (CVL). The decay and recovery of Cr3+ signal, with and without CVL illumination respectively, was monitored at different temperatures in 10–300 K region. These were found to obey a double exponential, with one time constant independent of temperature, and the other showing significant temperature dependence. From Τ(T), activation energy for the charge carrier transport in one of the processes was estimated to be 0.016 eV. The PA spectra showed shift towards lower wavelength side on consecutive runs. On the other hand, VO2+ doped PVA film has not shown any significant changes in intensity on laser illumination. These observations suggest (i) interaction of PVA matrix with excited Cr3+ and (ii) predominant non-radiative relaxation in VO2+: PVA system with no change in the oxidation state.

Photo-neutron cross-section of 96Zr using bremsstrahlung radiation with end point energies of 10 and 12.5 MeV

Abstract The 96Zr(γ, n)95Zr reaction cross-section was experimentally determined at end point bremsstrahlung energies of 10 and 12.5 MeV using off-line γ-ray spectrometric technique. The cross-section increases sharply from the end point energy of 10 to 12.5 MeV, which indicates the role of excitation energy. The reaction cross-section as a function of photon energy was also calculated theoretically using the TALYS 1.2 computer code. The flux weighted average value of the 96Zr(γ, n)95Zr reaction cross-section for the end-point bremsstrahlung energies of 10 and 12.5 MeV was also calculated using the experimental and theoretical data of mono-energetic photons to compare with the present data. The cross-section given in the present work at end- point bremsstrahlung energy of 10 MeV is closer to the flux-weighted value of TALYS but higher than the literature data. On the other hand, at 12.5 MeV, it is higher than the flux weighted value of TALYS and the literature data.

Glove Box Adaptation of a High-Resolution Inductively Coupled Plasma Optical Emission Spectrometer for Analysis of Radioactive Materials

A glove box adaptation of a high resolution atomic emission spectrometer (AES) is reported employing sequential radial viewing of an inductively coupled plasma (ICP) source for determination of trace metallic elements in plutonium bearing samples. The original compact ICP-AES unit was converted into separate units. All electronic and optical components were placed outside radioactive containment. The entire assembly of the ICP torch, radiofrequency coil, nebulizer, spray chamber, peristaltic pump, and drainage system were placed inside the glove box for the analysis of radioactive samples. All these modifications were achieved without any compromise on the analytical performance of the spectrometer. Operational experience of the instrument for analysis as well as additional information for the upkeep and maintenance of the same is described. Proper optical alignment of the ICP source and detection system was performed.