Isopiestic Investigation of the Osmotic and Activity Coefficients of {yMgCl2 + (1 - y)MgSO4}(aq) and the Osmotic Coefficients of Na2SO4.MgSO4(aq) at 298.15 K

Isopiestic vapor pressure measurements were made for {l_brace}yMgCl{sub 2} + (1-y)MgSO{sub 4}{r_brace}(aq) solutions with MgCl{sub 2} ionic strength fractions of y = 0, 0.1997, 0.3989, 0.5992, 0.8008, and (1) at the temperature 298.15 K, using KCl(aq) as the reference standard. These measurements for the mixtures cover the ionic strength range I = 0.9794 to 9.4318 mol {center_dot} kg{sup -1}. In addition, isopiestic measurements were made with NaCl(aq) as reference standard for mixtures of {l_brace}xNa{sub 2}SO{sub 4} + (1-x)MgSO{sub 4}{r_brace}(aq) with the molality fraction x = 0.50000 that correspond to solutions of the evaporite mineral bloedite (astrakanite), Na{sub 2}Mg(SO{sub 4}){sub 2} {center_dot} 4H{sub 2}O(cr). The total molalities, m{sub T} = m(Na{sub 2}SO{sub 4}) + m(MgSO{sub 4}), range from m{sub T} = 1.4479 to 4.4312 mol {center_dot} kg{sup -1} (I = 5.0677 to 15.509 mol {center_dot} kg{sup -1}), where the uppermost concentration is the highest oversaturation molality that could be achieved by isothermal evaporation of the solvent at 298.15 K. The parameters of an extended ion-interaction (Pitzer) model for MgCl2(aq) at 298.15 K, which were required for an analysis of the {l_brace}yMgCl{sub 2} + (1-y)MgSO{sub 4}{r_brace}(aq) mixture results, were evaluated up to I = 12.025 mol {center_dot} kg{sup -1} from published isopiestic data together with the six new osmotic coefficients obtained in this study. Osmotic coefficients of {l_brace}yMgCl{sub 2} + (1-y)MgSO{sub 4}{r_brace}(aq) solutions from the present study, along with critically-assessed values from previous studies, were used to evaluate the mixing parameters of the extended ion-interaction model

Osmotic and Activity Coefficients of the {xZnCl2 + (1 - x)ZnSO4}(aq) System at 298.15 K

Isopiestic vapor pressure measurements were made for (xZnCl{sub 2} + (1 - x)ZnSO{sub 4})(aq) solutions with ZnCl{sub 2} molality fractions of x = (0, 0.3062, 0.5730, 0.7969, and 1) at the temperature 298.15 K, using KCl(aq) as the reference standard. These measurements cover the water activity range 0.901-0.919 {le} a{sub w} {le} 0.978. The experimental osmotic coefficients were used to evaluate the parameters of an extended ion-interaction (Pitzer) model for these mixed electrolyte solutions. A similar analysis was made of the available activity data for ZnCl{sub 2}(aq) at 298.15 K, while assuming the presence of equilibrium amounts of ZnCl{sup +}(aq) ion-pairs, to derive the ion-interaction parameters for the hypothetical pure binary electrolytes (Zn{sup 2+}, 2Cl{sup -}) and (ZnCl{sup +},Cl{sup -}). These parameters are required for the analysis of the mixture results. Although significant concentrations of higher-order zinc chloride complexes may also be present in these solutions, it was possible to represent the osmotic coefficients accurately by explicitly including only the predominant complex ZnCl{sup +}(aq) and the completely dissociated ions. The ionic activity coefficients and osmotic coefficients were calculated over the investigated molality range using the evaluated extended Pitzer model parameters

Influence of Annealing on the Optical and Scintillation Properties of CaWO4_4 Single Crystals

We investigate the influence of oxygen annealing on the room temperature optical and scintillation properties of CaWO$_4$ single crystals that are being produced for direct Dark Matter search experiments. The applied annealing procedure reduces the absorption coefficient at the peak position of the scintillation spectrum ($\sim430$ nm) by a factor of $\sim6$ and leads to an even larger reduction of the scattering coefficient. Furthermore, the annealing has no significant influence on the \emph{intrinsic} light yield. An additional absorption occurring at $\sim400$ nm suggests the formation of O$^-$ hole centers. Light-yield measurements at room temperature where one crystal surface was mechanically roughened showed an increase of the \emph{measured} light yield by $\sim40$ and an improvement of the energy resolution at 59.5 keV by $\sim12$ for the annealed crystal. We ascribe this result to the reduction of the absorption coefficient while the surface roughening is needed to compensate for the also observed reduction of the scattering coefficient after annealing

The CRESST Dark Matter Search

We present first competitive results on WIMP dark matter using the phonon-light-detection technique. A particularly strong limit for WIMPs with coherent scattering results from selecting a region of the phonon-light plane corresponding to tungsten recoils. The observed count rate in the neutron band is compatible with the rate expected from neutron background. CRESST is presently being upgraded with a 66 channel SQUID readout system, a neutron shield and a muon veto system. This results in a significant improvement in sensitivity.Comment: 6 pages, 3 figures, to be published in the proceedings of the 5th International Workshop on the Identification and Detection of Dark Matter IDM 2004, Edinburgh, Sept. 2004, World Scientifi

Effect of within-species plant genotype mixing on habitat preference of a polyphagous insect predator

The effects of within-species plant genotype mixing on the habitat preference of a polyphagous ladybird were studied. Plant species diversity is often claimed to positively affect habitat preferences of insect predators, but the effects of within-species genotype diversity have not been extensively studied. In a field experiment with different barley (Hordeum vulgare) genotypes in mixed and pure stands, adult seven-spot ladybird Coccinella septempunctata, a polyphagous predator, preferred a specific combination of genotypes over the single genotypes alone before aphids had arrived in the crop, and again when aphids were emigrating. In laboratory experiments on adult ladybird orientation to odour from barley, ladybirds were attracted/arrested by the mixed odour of the same barley genotype mixture that was preferred in the field. Exposure of one barley genotype to volatiles from the other also caused the odour of the exposed plants to become more attractive to ladybirds. The results support the hypothesis that plant volatiles may attract or arrest foraging adult ladybirds, contributing to the selection of favourable habitats, and they show that within-species plant genotype mixing can shape interactions within multitrophic communities

Detection of the Natural Alpha Decay of Tungsten

The natural alpha decay of 180W has been unambiguously detected for the first time. The alpha peak is found in a (gamma,beta and neutron)-free background spectrum. This has been achieved by the simultaneous measurement of phonon and light signals with the CRESST cryogenic detectors. A half-life of T1/2 = (1.8 +- 0.2) x 10^18 y and an energy release of Q = (2516.4 +- 1.1 (stat.) +- 1.2 (sys.)) keV have been measured. New limits are also set on the half-lives of the other naturally occurring tungsten isotopes.Comment: Submitted to Physical Review C Revised versio