Browsing by Subject "Cesium"

Now showing 1 - 9 of 9

Open Access
33-fs Yb-fiber laser comb locked to Cs-atomic clock
(IEEE, 2013) Şenel, Çağrı; Hamid, R.; Erdoğan, C.; Çelik, M.; Kara, O.; İlday, Fatih Ömer
Despite the prevalence of fiber frequency combs around 1.5 μm, few fully stabilized frequency combs have been demonstrated around 1.0 μm, despite the generally superior performance of Yb-fiber lasers compared to Er-fiber lasers. Short pulses are to generate coherent supercontinuum using anomalous dispersion regime of microstructured fibers. Near-zero cavity dispersion is highly desirable for low-noise frequency comb performance. Here, we report a Yb-doped fiber laser that operates at net-zero group-velocity dispersion and produces pulses that can be compressed externally to 33-fs. The frequency comb generated by this system is repetition-and carrier-envelope-phase-locked to Cs atomic clocks. The laser oscillator design is based on a novel algorithmic methodology, which allows us to design cavities to meet specific requirements; in this case, there was the need to generate as short pulses as possible, while having several nJ of pulse energy and the cavity at strictly zero total dispersion.
Open Access
Adsorption site of alkali metal overlayers on Si(001) 2 × 1
(1992) Batra, I. P.; Çıracı, Salim
The alkali metal semiconductor interfaces are currently being investigated by a variety of tools. Most studies to date at half a monolayer coverage have shown preference for either a quasi-hexagonal (H) site or a long-bridge (B) site. At this coverage one-dimensional chain structure for K on Si(001) 2 × 1 have now been confirmed by scanning tunneling microscopy (STM). The data, however, is consistent with either of the two sites. STM investigations at low coverages suggested that alkali metals like K and Cs occupy a novel site, Y, which is a bridge site between two Si atoms belonging to different dimers along the dimer row [110] direction. The total energy calculations for this new Y site, discovered by STM, have shown that it is indeed a site of (local) energy minimum. The ability of the surface silicon atoms, which are not adjacent to the alkali metal atom, to buckle makes the Y site a competitive adsorption site. We deduce the nature of bonding between alkali metals and Si using the STM data. It is concluded that the bond is substantially ionic in nature. © 1992.
Open Access
Investigation of the adsorption behavior of cesium, barium and phenol onto modified humic acid and iron nanopraticles
(2007) Çelebi, Oğuzhan
Open Access
Radiochemical and spectroscopic studies of cesium, barium, and cobalt sorption on some natural clays
(2000-08) Shahwan, Talal
The wide growth in the nuclear activities results in an increasing subsequent influx of radioactive wastes into the environment. This problem has manifested a great deal of interest aiming at finding out ways through which those wastes can be harmlessly isolated from the human environment. Geological disposal is considered as one of the most promising solutions that ensures a safe storage of radioactive wastes as long as their activities are above the accepted levels. Clay minerals are proposed as backfill buffering materials in the geological repositories that can delay the migration of the radionuclides through sorption and thus decrease the contamination of underground waters. The extent of retardation of the radionuclide migration is dependent on factors like time of contact, pH and Eh of groundwater, concentration, temperature and grain size of the mineral particles. In this study radiochemical, spectroscopic (ToF-SIMS, XPS), and X-ray diffraction techniques were applied to examine different aspects of the sorption behavior of cesium, barium and cobalt on three natural clay minerals containing primarily kaolinite, illite-chlorite, and bentonite. The elements cesium (Z=55), barium (Z=56), and cobalt (Z=27) have the radioactive isotopes superscript 137 Cs (half-life=30.17 years), superscript 140 Ba (half-life=12.79 days), and superscript 60 Co (half-life=5.3 y) which are important in radioactive waste management. The first two radionuclides are produced in high yields in nuclear fission, whereas the third is an activation product. The natural clay samples that were used in this study originated from natural mineralogical beds at Sindırgı, Afyon, and Giresun regions in Turkey. The characterization of these clay samples showed that the primary clay minerals were kaolinite in Sındırgı clay, chlorite and illite in Afyon clay, and montmorillonite in Giresun clay. Each of these clays possesses different structural properties that result in different sorption capabilities. Radiochemical batch experiments were carried out to examine the effects of time, concentration, and temperature on the sorption of cesium, barium and cobalt on clays. Solutions of these cations spiked with several microliters of the radionuclides 137 CS (half-life=30.1 y), 133 Ba (half-life=10.7 y), and 60 Co (half-life=5.3 y) were monitored using gamma-ray spectroscopy prior to and after each sorption experiment. These results showed that equilibrium is achieved within two days in all cases. The sorption data was adequately described by Freundlich and Dubinin-Radushkevich isotherm models. Based on the parameters of those isotherm models, it was found that sorption was nonlinear, and that bentonite showed the highest sorption affinity and sorption capacity towards the sorbed ions. The thermodynamic parameters indicated that while sorption of cesium and barium on the three clays is exothermic that of cobalt is endothermic. The obtained values of Gibbs free energy change, Delta G degrees, were generally in the 8-16 (kJ/mol) energy range that corresponds to ion exchange type sorption mechanism. Since sorption is mainly a surface phenomenon, part of our sorption studies were carried out using the surface sensitive techniques; Time of Flight- Secondary Ion Mass Spectroscopy (ToF-SIMS) and X-ray Photoelectron Spectroscopy (XPS). In addition, depth profiling up to 70 angstroms was performed using ToF-SIMS to investigate cesium, barium and cobalt concentrations through the clay surface. ToF-SIMS and XPS studies were helpful in figuring out the surface composition of different clays prior to and after sorption. Quantification of the depletion of different alkali and alkaline-earth metals initially contained within the analyzed clay surface showed that ion exchange plays a primary role in the sorption process. In addition, X-Ray Diffraction (XRD) technique was applied to figure out the mineralogical composition of the clay minerals used and examine any structural change a accompanying the sorption process. XRD spectra of the clay samples after sorption showed that -apart from some intensity reductions in some clay features-, no primary changes were detected in the sorption cases of cesium and cobalt. In barium sorption, however, features belonging to barium carbonate were present in the spectra corresponding to sorption on chlorite-illite and bentonite.
Open Access
Sorption of Cs+ and Ba2+ on magnesite
(Cambridge University Press, 1998) Shahwan, Talal; Süzer, Şefik; Erten, Hasan N.
Sorption behavior of Cs and Ba ions on magnesite was studied using the radiotracer method complemented by X-ray photoelectron spectroscopy. Cs and Ba were used as radiotracers. The sorption of Cs is seen to be temperature dependent, whereas Ba sorption is not much affected by temperature changes. It was found that Freundlich type isotherms provide an adequate description of the sorption process. The magnitude of the free energy of sorption are seen to be within the 8-16 kj/mol range. It can be remarked that the sorption of Cs and Ba on magnesite is a fast process suggesting an ion exchange type mechanism, mainly taking place at the surface of mineral particles.
Open Access
Sorption studies of cesium and barium on magnesite using radiotracer and x-ray photoelectron spectroscopy
(1997-06) Shahwan, Talal
As the consumption of the radioactive materials is continuously increasing, the problem of disposing the resulting v^^astes safely is becoming more challenging. One way through which these radioactive wastes could be isolated from the biological environment is by disposing them in deep geological formations. Clay minerals are proposed as backfill buffering materials that can delay the migration of the radionuclides and thus decrease the contamination of underground waters. The extent of retardation of the radionuclide migration is dependent on factors like time of contact, pH and Eh of groundwater, concentration, temperature and grain size of the mineral particles. Up to now, several studies were carried out to examine the effect of such parameters on the sorption behavior of different radionuclides on various kinds of minerals. This study was conducted to investigate the effects of time, concentration and temperature on the sorption behavior of cesium and barium ions on magnesite. Cesium 137 140 and barium have the radioactive isotopes Cs (t^^^ = 30.1 y) and Ba (t^^^ = 12.8 d) 1/2 produced in high yields during the fission process which are important in radioactive waste considerations. Magnesite is a mineral composed mainly of magnesium carbonate 2+ together with minor amounts of quartz and has a single exchangeable cation. Mg . The radiotracer method and x-ray photoelectron spectroscopy, which is a powerful surface sensitive tool, were used in this study. The results obtained from both methods complemented each others and were in good agreement. Kinetic studies of the sorption process show that equilibrium was approached within one day of contact for both of cesium and barium ions. The data of the sorption of both cations using different concentrations at various temperatures were most adequately described by the Freundlich type isotherms which correspond to multilayer adsorption on heterogeneous surfaces. The values of the Freundlich constants k and n imply that barium ions have slightly larger adsorption affinity and adsorption intensity than cesium ions. The adsorption data at low concentrations were also observed to obey the Dubinin-Radushkevich type isotherms which describe monolayer adsorption on heterogeneous or homogeneous surfaces. The adsorption data were very poorly described by the Langmuir type isotherms. Thermodynamic parameters such as enthalpy change, AH°, entropy change, AS° and free energy change of adsorption, AG°, were calculated from the sorption data of cesium and barium ions at different temperatures. The values obtained for AH° and AS° were -37 kJ/moL, -0.09 kJ/moL.K and -13 kJ/moL, -0.009 kJ/moL.K for cesium and barium ions respectively. The negative AH° values indicate the exothermic nature of adsorption which means that low temperatures are favored. The decrease in entropy upon adsorption implied by the negative AS° values is indicative of the stability of adsorption for both cations. The values of AG° at different temperatures were all negative indicating the spontaneity of the adsorption process for both cesium and barium ions. The magnitudes of AG° were seen to be within the 8-16 kJ/moL range which is the energy range of ion- exchange type processes.
Open Access
Sorption studies of Cs+ and Ba2+ cations on magnesite
(Elsevier, 1998-05-11) Shahwan, T.; Süzer, Şefik; Erten, H. N.
The adsorption behavior of Cs+ and Ba2+ cations on magnesite has been studied as a function of time, cation concentration and temperature, utilizing both the radiotracer method and X-ray photoelectron spectroscopy (XPS). Saturation was approached in about I day for both cations. The sorption data were found to follow Freundlich type isotherms. Sorption of both Cs+ and Ba2+ cations were found to be exothermic in nature with ΔH0 (kJ/mol) of -37, -13 and ΔS0 (kJ/mol·K) of -0.09, -0.009, respectively. Negative ΔG0 values were obtained for both cations, indicating the spontaneity of their sorption on magnesite. The magnitude of ΔG0 suggest that ion exchange is the dominating sorption mechanism. The adsorption behavior of Cs+ and Ba2+ cations on magnesite has been studied as a function of time, cation concentration and temperature, utilizing both the radiotracer method and X-ray photoelectron spectroscopy (XPS). Saturation was approached in about 1 day for both cations. The sorption data were found to follow Freundlich type isotherms. Sorption of both Cs+ and Ba2+ cations were found to be exothermic in nature with ΔH0 (kJ/mol) of -37, -13 and ΔS0 (kJ/mol·K) of -0.09, -0.009, respectively. Negative ΔG0 values were obtained for both cations, indicating the spontaneity of their sorption on magnesite. The magnitude of ΔG0 suggest that ion exchange is the dominating sorption mechanism.
Open Access
Sorption studies of Cs+, Ba2+, and Co2+ ions on bentonite using radiotracer, ToF-SIMS, and XRD techniques
(De Gruyter Oldenbourg, 2001) Shahwan, T.; Erten H. N.
The sorption behaviour of Cs+, Ba2+, and Co2+ ions on bentonite were investigated using the radiotracer method, Time of Flight-Secondary Ion Mass Spectroscopy (ToF-SIMS), and X-Ray Diffraction (XRD). The sorption of Cs+ and Ba2+ were exothermic while sorption of Co2+ was endothermic. The sorption data were well described by Freundlich and Dubinin-Radushkevich isotherms. According to ToF-SIMS results Na+ and Mg2+ were the primary exchanging ions in bentonite. The XRD spectra showed that no structural changes were associated with the sorption of Cs+ and Co2+, and BaCO3 precipitate was formed upon the sorption of Ba2+ on bentonite.
Open Access
Surface spectroscopic studies of Cs+, and Ba2+ sorption on chlorite-illite mixed clay
(De Gruyter Oldenbourg, 2000) Shahwan, T.; Sayan, S.; Erten, H. N.; Black, L.; Hallam, K. R.; Allen, G. C.
The sorption behavior of Cs+, and Ba2+ on natural clay was investigated using ToF-SIMS, XPS, and XRD. The natural clay was composed mainly of chlorite and illite in addition to quartz and calcite. Depth profiling up to 70 Å was performed at 10 Å steps utilizing ToF-SIMS to study the amount of sorbed Cs+ and Ba2+ as a function of depth in the clay matrix. The results suggest that Cs+ and Ba2+ ions were sorbed primarily by ion exchange coupled with hydrolytic sorption. According to ToF-SIMS and XPS results, the total sorbed amount of Ba2+ was larger than that of Cs+. Quantitative determination of the primary cations within the analyzed clay before and after sorption indicated that for Ba2+ sorption, Ca2+, Mg2+ and for Cs+ sorption Ca2+, K+ were the major exchanging ions. The XRD spectra of Ba-sorbed clay contained new peaks that were identified as BaCO3.