Improved performance of neutron activation analysis laboratories by feedback workshops following interlaboratory comparison rounds.

The International Atomic Energy Agency (IAEA) implemented an innovative project for assisting neutron activation analysis laboratories in improving the validity of their results by feedback workshops for discussion of results from participation in interlaboratory comparisons rounds in 2010. The participants learned during these meetings to identify the most probable sources of errors in their analytical procedures and how to implement corrective actions to prevent reoccurrence. The outcome of successive rounds between 2010 and 2018 is discussed and experiences during the feedback workshops are given. The quantitative evaluation of the results shows an overall improvement in satisfactory performance. Moreover, there is a clear indication that improvements are consolidated in most laboratories but also stimulate laboratories to develop to a higher level of excellence. Regional differences in performance are also analysed.

Quantum limit transport and destruction of the Weyl nodes in TaAs.

Weyl fermions are a recently discovered ingredient for correlated states of electronic matter. A key difficulty has been that real materials also contain non-Weyl quasiparticles, and disentangling the experimental signatures has proven challenging. Here we use magnetic fields up to 95 T to drive the Weyl semimetal TaAs far into its quantum limit, where only the purely chiral 0th Landau levels of the Weyl fermions are occupied. We find the electrical resistivity to be nearly independent of magnetic field up to 50 T: unusual for conventional metals but consistent with the chiral anomaly for Weyl fermions. Above 50 T we observe a two-order-of-magnitude increase in resistivity, indicating that a gap opens in the chiral Landau levels. Above 80 T we observe strong ultrasonic attenuation below 2 K, suggesting a mesoscopically textured state of matter. These results point the way to inducing new correlated states of matter in the quantum limit of Weyl semimetals.

Assessment of PM2.5 sources and their corresponding level of uncertainty in a coastal urban area using EPA PMF 5.0 enhanced diagnostics.

Datasets that include only the PM elemental composition and no other important constituents such as ions and OC, should be treated carefully when used for source apportionment. This work is demonstrating how a source apportionment study utilizing PMF 5.0 enhanced diagnostic tools can achieve an improved solution with documented levels of uncertainty for such a dataset. The uncertainty of the solution is rarely reported in source apportionment studies or it is reported partially. Reporting the uncertainty of the solution is very important especially in the case of small datasets. PM2.5 samples collected in Patras during the year 2011 were used. The concentrations of 22 elements (Z=11-33) were determined using PIXE. Source apportionment analysis revealed that PM2.5 emission sources were biomass burning (11%), sea salt (8%), shipping emissions (10%), vehicle emissions (33%), mineral dust (2%) and secondary sulfates (33%) while unaccounted mass was 3%. Although Patras city center is located in a very close proximity to the city's harbor, the contribution of shipping originating emissions was never before quantified. As rotational stability is hard to be achieved when a small dataset is used the rotational stability of the solution was thoroughly evaluated. A number of constraints were applied to the solution in order to reduce rotational ambiguity.

Assessment of measurement result uncertainty in determination of (210)Pb with the focus on matrix composition effect in gamma-ray spectrometry.

Reference materials were used to assess measurement result uncertainty in determination of (210)Pb by gamma-ray spectrometry, liquid scintillation counting, or indirectly by alpha-particle spectrometry, using its daughter (210)Po in radioactive equilibrium. Combined standard uncertainties of (210)Pb massic activities obtained by liquid scintillation counting are in the range 2-12%, depending on matrices and massic activity values. They are in the range 1-3% for the measurement of its daughter (210)Po using alpha-particle spectrometry. Three approaches (direct computation of counting efficiency and efficiency transfer approaches based on the computation and, respectively, experimental determination of the efficiency transfer factors) were applied for the evaluation of (210)Pb using gamma-ray spectrometry. Combined standard uncertainties of gamma-ray spectrometry results were found in the range 2-17%. The effect of matrix composition on self-attenuation was investigated and a detailed assessment of uncertainty components was performed.

Observation of a martensitic structural distortion in V, Nb, and Ta.

Thermal-expansion measurements of the Group 5 elements V, Nb, and Ta reveal a structural distortion below 300 K. Data for single-crystalline Nb and Ta display anisotropic thermal expansion, martensitic in character, that is inconsistent with cubic crystal structures at low temperature. Published results on V show similar behavior. Interstitial impurities suppress the transition.

CdSe spherical quantum dots stabilised by thiomalic acid: biphasic wet synthesis and characterisation.

CdSe quantum dots stabilised by thiomalic acid have been synthesised by an aqueous biphasic ligand exchange reaction in air. The materials are completely water-soluble and were found to be stable over a long time. X-ray diffraction and transmission electron microscopy reveal the formation of CdSe nanocrystals with cubic structure (a=0.6077 nm; spatial group: F-43m). The average particle size is about 5 nm. Energy dispersive X-ray analysis shows that the nanocrystals are nonstoichiometric, with a Cd/Se ratio varying between 60/40 and 70/30, and indicates the presence of Cd(2+) ions at the nanocrystal surface. Diffuse reflectance infrared Fourier transform measurements suggest that thiomalic acid chelates CdSe through the thiol group and one carboxylic function, while the second COOH group is semi-free. A complex-like structure is proposed, in which thiomalic acid forms a five-membered chelate ring with the Cd(2+) ions present on the nanocrystal surface. Chelate effect accounts for the easiness of ligand exchange and is expected to additionally stabilise the nanosystem.

Polycrystalline gamma-plutonium's elastic moduli versus temperature.

Resonant ultrasound spectroscopy was used to measure the elastic properties of pure polycrystalline (239)Pu in the gamma-phase. Shear and longitudinal elastic moduli were measured simultaneously and the bulk modulus was computed from them. A smooth, linear, and large decrease in all elastic moduli with increasing temperature was observed. The Poisson ratio was calculated and an increase from 0.242 at 519 K to 0.252 at 571 K was found. These measurements on extremely well-characterized pure Pu are in agreement with other reported results where overlap occurs. We calculated an approximate Debye temperature Theta(D)=144 K. Determined from the temperature variation in the bulk modulus, gamma-Pu shows the same Gruneisen parameter as copper.

An integrated approach to assess air pollution threats to cultural heritage in a semi-confined environment: the case study of Michelozzo's Courtyard in Florence (Italy).

An example of an integrated approach to assess air pollution threats to cultural heritage in a semi-confined environment is presented in this work, where the monitoring campaign carried out at the Michelozzo's Courtyard (in Palazzo Vecchio, Florence, Italy) is used as a case study. A wide research project was carried out, with the main aim of obtaining the first quantitative data on air quality and microclimate conditions inside the Courtyard, and, if possible, identifying the main causes of degradation and suggesting appropriate conservation strategies. The investigation adopted a holistic approach involving thermographic measurements on the wall paintings, microclimatic analysis, gaseous pollutant monitoring, atmospheric particles characterisation and dry deposition compositional analysis. Attention was focused on the wall painting depicting the city of Hall because of its anomalous and critical conservation conditions, which are visible at a glance, due to the contrast between a wide darker zone around the central subject of the painting and external lighter areas.

Quantum phase transition in the magnetic-field-induced normal state of optimum-doped high-Tc cuprate superconductors at low temperatures.

A 60 T magnetic field suppresses the superconducting transition temperature T_{c} in La_{2-p}Sr_{p}CuO_{4} to reveal a Hall number anomaly, which develops only at temperatures below zero-field T_{c} and peaks at the exact location of p that maximizes T_{c}. The anomaly bears a striking resemblance to observations in Bi_{2}Sr_{2-x}La_{x}CuO_{6+delta}, suggesting a normal-state phenomenology common to the cuprates that underlies the high-temperature superconducting phase. The peak is ascribed to a Fermi surface reconstruction at a quantum phase transition near optimum doping that is coincident with the collapse of the pseudogap state.

Doping dependent nonlinear Hall effect in SmFeAsO(1-x)F(x).

We report the Hall resistivity, ρ(xy), of polycrystalline SmFeAsO(1-x)F(x) for four different fluorine concentrations from the onset of superconductivity through the collapse of the structural phase transition. For the two more highly doped samples, ρ(xy) is linear in magnetic field up to 50 T with only weak temperature dependence, reminiscent of a simple Fermi liquid. For the lightly doped samples with x<0.15, we find a low temperature regime characterized as ρ(xy)(H) being both nonlinear in magnetic field and strongly temperature-dependent even though the Hall angle is small. The onset temperature for this nonlinear regime is in the vicinity of the structural phase (SPT)/magnetic ordering (MO) transitions. The temperature dependence of the Hall resistivity is consistent with a thermal activation of carriers across an energy gap. The evolution of the energy gap with doping is reported.

Sliding charge-density wave in manganites.

Stripe and chequerboard phases appear in many metal oxide compounds, and are thought to be linked to exotic behaviour such as high-temperature superconductivity and colossal magnetoresistance. It is therefore extremely important to understand the fundamental nature of such phases. The so-called stripe phase of the manganites has long been interpreted as the localization of charge at atomic sites. Here, we present resistance measurements on La(0.50)Ca(0.50)MnO(3) that strongly suggest that this state is in fact a prototypical charge-density wave (CDW) that undergoes collective transport. Dramatic resistance hysteresis effects and broadband noise properties are observed, both of which are typical of sliding CDW systems. Moreover, the high levels of disorder typical of manganites result in behaviour similar to that of well-known disordered CDW materials. The CDW-type behaviour of the manganite superstructure suggests that unusual transport and structural properties do not require exotic physics, but could emerge when a well-understood phase (the CDW) coexists with disorder.

Alpha-plutonium's polycrystalline elastic moduli over its full temperature range.

alpha-plutonium's volume-corrected polycrystal elastic moduli were measured between 18 K and the upper limit of its occurrence, near 400 K. The two independent moduli for a polycrystal-bulk and shear-behave smoothly, indicating no phase transition. Both moduli show the same 50% increase on cooling, an order of magnitude larger than in other metals. The Debye temperature obtained from low-temperature elastic moduli, 207 K, significantly exceeds most previous estimates. The Gruneisen parameter gamma=5.3, obtained from the temperature dependence of the bulk modulus, is intermediate among previous estimates using other approaches, alpha-plutonium's Poisson ratio nu is low: 0.18, nearly temperature independent, and its small decrease on warming opposes usual behavior. The high gamma, large but equal bulk modulus and shear modulus fractional stiffening on cooling, and near-temperature-invariant nu are attributed to a single mechanism: 5-f electron localization-delocalization.

Bulk vs nanoscale WS2: finite size effects and solid-state lubrication.

To investigate phonon confinement in nanoscale metal dichalcogenides, we measured the low-temperature specific heat of layered and nanoparticle WS2. Below 9 K, the specific heat of the nanoparticles deviates from that of the bulk counterpart. Further, it deviates from the usual T 3 dependence below 4 K due to finite size effects that eliminate long wavelength acoustic phonons and interparticle-motion entropy. This separation of nanoscale effects from T 3 dependence can be modeled by assuming that the phonon density of states is flexible, changing with size and shape. We invoke relationships between the low-temperature T 3 phonon term, Young's modulus, and friction coefficient to assess the difference in the tribological properties. On the basis of this analysis, we conclude that the improved lubrication properties of the nanoparticles are extrinsic.

Pinning frequencies of the collective modes in alpha-uranium.

Uranium is the only known element that features a charge-density wave (CDW) and superconductivity. We report a comparison of the specific heat of single-crystal and polycrystalline alpha-uranium. In the single crystal we find excess contributions to the heat capacity at 41 K, 38 K, and 23 K, with a Debye temperature ThetaD = 265 K. In the polycrystalline sample the heat capacity curve is thermally broadened (ThetaD = 184 K), but no excess heat capacity was observed. The excess heat capacity Cphi (taken as the difference between the single-crystal and polycrystal heat capacities) is well described in terms of collective-mode excitations above their respective pinning frequencies. This attribution is represented by a modified Debye spectrum with two cutoff frequencies, a pinning frequency V0 for the pinned CDW (due to grain boundaries in the polycrystal), and a normal Debye acoustic frequency occurring in the single crystal.

Complete elastic tensor through the first-order transformation in U2Rh3Si5.

The complete elastic tensor of U(2)Rh(3)Si(5) has been determined over the temperature range of 5-300 K, including the dramatic first-order transition to an antiferromagnetic state at 25.5 K. Sharp upward steps in the elastic moduli as the temperature is decreased through the transition reveal the first-order nature of the phase change. In the antiferromagnetic state the temperature dependence of the elastic moduli scales with the square of the ordered moment on the uranium ion, demonstrating strong spin-lattice coupling. The temperature dependence of the moduli well above the transition indicates coupling of the ultrasonic waves to the crystal electric field levels of the uranium ion where the lowest state is a singlet. The elastic constant data suggest that the first-order phase change is magnetically driven by a bootstrap mechanism involving the ground state singlet and a magnetically active crystal electric field level.

Monocrystal elastic constants of the negative-thermal-expansion compound zirconium tungstate (ZrW2O8).

We measured zirconium tungstate's elastic constants C(ij). This compound shows relatively soft, nearly isotropic elastic constants with normal Poisson ratios and no approach to Born instability. ZrW2O8 shows normal ambient-temperature elastic constants C(ij), but remarkable dC(ij)/dT that show dominant low-frequency acoustic-vibration modes. From the bulk modulus, we estimated the total ambient-temperature thermodynamic Grüneisen parameter as gamma = -1.2. The dB/dT slope gives a Grüneisen parameter gamma = -7. The 300-0 K bulk-modulus increase (40%) seems unprecedented and breaks Birch's law of corresponding states.

Unusual phonon softening in delta-phase plutonium.

The phonon density of states and adiabatic sound velocities were measured on fcc-stabilized 242Pu0.95Al0.05. The phonon frequencies and sound velocities decrease considerably (soften) with increasing temperature despite negligible thermal expansion. The frequency softening of the transverse branch along the [111] direction is anomalously large ( approximately 30%) and is very sensitive to alloy composition. The large magnitude of the phonon softening is not observed in any other fcc metals and may arise from an unusual temperature dependence of the electronic structure in this narrow 5f-band metal.

Experimental electronic heat capacities of alpha- and delta-plutonium: heavy-fermion physics in an element.

We have measured the heat capacities of delta-Pu0.95Al0.05 and alpha-Pu over the temperature range 2-303 K. The availability of data below 10 K plus an estimate of the phonon contribution to the heat capacity based on recent neutron-scattering experiments on the same sample enable us to make a reliable deduc-tion of the electronic contribution to the heat capacity of delta-Pu0.95Al0.05; we find gamma=64+/-3 mJ K(-2) mol(-1) as T-->0. This is larger than that of any element and large enough for delta-Pu0.95Al0.05 to be classed as a heavy-fermion system. By contrast, gamma=17+/-1 mJ K(-2) mol(-1) in alpha-Pu. Two distinct anomalies are seen in the electronic contribution to the heat capacity of delta-Pu0.95Al0.05, one or both of which may be associated with the formation of the alpha(')-martensitic phase. We suggest that the large gamma value of delta-Pu0.95Al0.05 may be caused by proximity to a quantum-critical point.

Calculated phonon spectra of plutonium at high temperatures.

We constructed computer-based simulations of the lattice dynamical properties of plutonium using an electronic structure method, which incorporates correlation effects among the f-shell electrons and calculates phonon spectra at arbitrary wavelengths. Our predicted spectrum for the face-centered cubic delta phase agrees well with experiments in the elastic limit and explains unusually large shear anisotropy of this material. The spectrum of the body-centered cubic phase shows an instability at zero temperature over a broad region of the wave vectors, indicating that this phase is highly anharmonic and can be stabilized at high temperatures by its phonon entropy.

Megagauss sensors.

Magnetic fields change the way that electrons move through solids. The nature of these changes reveals information about the electronic structure of a material and, in auspicious circumstances, can be harnessed for applications. The silver chalcogenides, Ag2Se and Ag2Te, are non-magnetic materials, but their electrical resistance can be made very sensitive to magnetic field by adding small amounts--just 1 part in 10,000--of excess silver. Here we show that the resistance of Ag2Se displays a large, nearly linear increase with applied magnetic field without saturation to the highest fields available, 600,000 gauss, more than a million times the Earth's magnetic field. These characteristics of large (thousands of per cent) and near-linear response over a large field range make the silver chalcogenides attractive as magnetic-field sensors, especially in physically tiny megagauss (10(6) G) pulsed magnets where large fields have been produced but accurate calibration has proved elusive. High-field studies at low temperatures reveal both oscillations in the magnetoresistance and a universal scaling form that point to a quantum origin for this material's unprecedented behaviour.