Nuclear recoil energy scale in liquid xenon with application to the direct detection of dark matter

We show for the first time that the quenching of electronic excitation from nuclear recoils in liquid xenon is well-described by Lindhard theory, if the nuclear recoil energy is reconstructed using the combined (scintillation and ionization) energy scale proposed by Shutt {\it et al.}. We argue for the adoption of this perspective in favor of the existing preference for reconstructing nuclear recoil energy solely from primary scintillation. We show that signal partitioning into scintillation and ionization is well-described by the Thomas-Imel box model. We discuss the implications for liquid xenon detectors aimed at the direct detection of dark matter

Evaluation of the performance of self-healing concrete at small and large scale under laboratory conditions

HEALCON is an EU-FP7 project which aims to develop self-healing concrete to create durable and sustainable concrete structures. While during the first years of the project the self-healing materials (including the healing agents and suitable encapsulation methodologies) and monitoring techniques were designed and tested at lab-scale, large scale elements have been tested near the end of the project to verify the feasibility and efficiency of the self-healing concrete under conditions closer to reality. For this paper, two types of healing agents were investigated for use in mortar and concrete. The first type of healing agent studied was a coated superabsorbent polymer (C-SAP). It is known that the autogenous healing capacity is increased by incorporation of superabsorbent polymers (SAPs) in mortar/concrete. The agents present in the crack can absorb intruding water, swell and block the crack, leading to immediate sealing, but can also exude moisture to the surrounding concrete environment stimulating healing of the concrete by hydration of unreacted cement particles or by CaCO3 precipitation. The disadvantage of these SAPs in the fresh mortar/concrete mix is however that they absorb large quantities of mixing water, leading to unwanted effects (e.g. loss of workability and macro-pore formation). By coating of the SAPs, we want to eliminate this disadvantage. The fluid bed spraying of the different layers was applied by VTT. A second healing agent studied, is a biogenic healing agent, namely a Mixed Ureolytic Culture (MUC). This type of healing agent was developed by Avecom in order to reduce the cost associated with the production of pure bacterial strains. This mixed ureolytic culture is moreover self-protecting and does not need any further encapsulation. At first, the performance of the healing agents itself was evaluated. For the coated SAPs, the swelling performance and swelling rate were determined, showing that the coating can limit the uptake of water during the first 10-15 minutes. For the MUC, the ureolytic and CaCO3 precipitating capacity was determined, immediately after production of the MUC and after 3 months of storage. The results show the potential of these mixed cultures to be used as self-healing agent in mortar/concrete, but also show a decrease of their effectiveness with time. Subsequently, the healing agents were incorporated in mortar mixes at UGent. A dosage of 1 wt% relative to the cement content caused a large reduction of the mechanical properties of the mortar (up to ~ 50%), except for the coated SAP. The sealing efficiency was evaluated with the water flow test, as designed by one of the project partners in HEALCON. The performance of reference mixes was compared to that of self-healing mixes with SAP, coated SAP or MUC (+ urea). Results showed that for cracks with a width less than 0.150 mm, all mortars were sealed (almost) completely after storage for 28 days in wet-dry environment (12 h wet – 12 h dry) after crack creation. For cracks with a larger width, differences were noticed between the different specimens. Moreover, also the immediate sealing effect induced by the presence of SAPs could be noticed. It has to be noted however that the crack width plays an important role but is varying along the crack length (within a specimen) and between specimens, making the analysis more difficult. In order to extend the application to concrete, self-healing and reference reinforced concrete beams (2500 x 400 x 200 mm) were produced at the Danish Technological Institute. The self-healing concretes contained coated SAPs or MUC. Moreover, the beams were equipped with corrosion sensors that are connected to a wireless monitoring system, developed by the Technology-Transfer- Initiative at the University of Stuttgart. The multi reference electrodes (MuRE) were installed alongside the reinforcements and measure the corrosion potential at certain positions. Data is collected in sufficiently dense intervals by battery powered nodes that transmit the data wirelessly to a base station and further on to a database where it can be accessed through a web based application for data analysis over the internet. At the age of 28 days, three-point bending cracks up to 0.6 mm were created in the beams. Subsequently, the beams were regularly sprayed with water (four times one hour per day) for 6 weeks and afterwards, the beams were, once a week, exposed to 3 wt% NaCl solution for 24 h. Evaluation of the self-healing performance by microscopic analysis (crack microscopy and analysis of thin sections) showed that for the reference beam and beam with MUC no significant healing could be noticed (probably because of insufficient supply of nutrients for the bacteria). For the beams with coated SAPs, the smaller cracks (0.1 and 0.2 mm) were partly closed. Continuous corrosion monitoring showed corrosion in the reference and MUC beams already after the first exposure to NaCl solution. Onset of corrosion was delayed in the case the beams contained coated SAPs

Adding some Dirt to Clean energy: Applying clay nanocomposites in solar cells

Polymer clay nanocomposite (PCN) thin films have found application across a number of applications, ranging from oxygen barriers to flame retardants, where their resistance to molecular gas diffusion has proven remarkably effective, even in films only a few hundred nanometers thick. Deposited using a layer-by-layer processing approach that takes advantage of self-assembly of the constituent components, these composite thin films comprise highly organized, alternating molecular layers of functional polymers and exfoliated clay platelets, commonly montmorillonite or vermiculite. Here, we explore the potential application and utility of PCN thin films in solar cells, where they serve as conformal, transparent barrier films with the potential to impact solar cell lifetime, reliability, and safety. Solar cell failures commonly result when environmental moisture and corrosive or reactive gases penetrate a cell’s encapsulant. Moreover, such cell degradation can manifest as a gradual decline in solar cell performance or, in the case when degradation leads to significantly damaged electrical elements, much more dramatic arc-faults that can lead to complete and dramatic module failure, even igniting module fires. Here, we describe how the unique nanostructure, materials chemistry, and gas barrier properties of PCNs offer promise toward addressing these challenges. Applying the PCN coatings to various elements of a solar cell module, we demonstrate the efficacy of PCNs as gas barriers, corrosion inhibitors, and arc-fault flammability mitigators. I will discuss here not only the results of our studies but also potential mechanisms for effective PCN function and present some apparent limitations of select approaches to PCN integration. These results reveal significant potential for PCNs to impact photovoltaic and other energy-related technologies, and our work highlights how these diverse, highly functional thin films may offer tremendous new opportunities for other next generation materials advances. Please click Additional Files below to see the full abstract

On the single mode approximation in spinor-1 atomic condensate

We investigate the validity conditions of the single mode approximation (SMA) in spinor-1 atomic condensate when effects due to residual magnetic fields are negligible. For atomic interactions of the ferromagnetic type, the SMA is shown to be exact, with a mode function different from what is commonly used. However, the quantitative deviation is small under current experimental conditions (for $^{87}$Rb atoms). For anti-ferromagnetic interactions, we find that the SMA becomes invalid in general. The differences among the mean field mode functions for the three spin components are shown to depend strongly on the system magnetization. Our results can be important for studies of beyond mean field quantum correlations, such as fragmentation, spin squeezing, and multi-partite entanglement.Comment: Revised, newly found analytic proof adde

Synchrotron X-Ray Study of the Thickness Dependence of the Phase Diagram of Thin Liquid-Crystal Films

The phase diagram of freely suspended thin films of heptyloxybenzylidene-heptylaniline shows dramatic changes for thicknesses below 22 layers. The most surprising feature of the phase diagram is the inclusion of two phases lacking long-range crystalline order (smectic-F and hexatic-B phases) between two crystalline phases (crystalline smectic B and smectic G). Neither the smectic F nor the hexatic B occurs in bulk samples. Between sixteen and ten layers the width, in temperature, of the hexatic-B phase increases.Engineering and Applied Science