Effect of sodium hydroxide pretreatment of NiOx cathodes on the performance of squaraine-sensitized p-type dye-sensitized solar cells

Squaraines are full-organic dyes employed as sensitizers in ptype dye-sensitized solar cells (p-DSSC). Their absorption spectrum shows a wide tunability that ranges from visible to NIR. Sensitization in the NIR region is crucial for exploiting a particularly intense portion of the solar spectrum. In this work three squaraines will be presented and tested as sensitizers in NiO-based p-type DSSC O4_C2, O4_C4 and O4_C12). The structures of the dyes differ for the length of the alkyl side chain (C2, C4 and C12). Alkyl side chains improve the solubility of the dye, influence the extent of dye loading on the electrode and affect the overall efficiency of devices. The generally low stability of squaraines represents a critical issue in view of their employment as sensitizers of p-DSSC. Such a problem becomes even more evident when this class of molecules is bound onto an acidic surface like the one of the photocathode here employed: non-stoichiometric nickel oxide (NiOx). NiOx possesses a quite acidic character because of the high surface concentration of Ni(III) sites. To buffer the surface acidity of NiOx due to the presence of high-valence states of nickel, we considered the electrode pretreatment with sodium hydroxide (NaOH) prior to sensitization. This assures a major stability of the solar cell. At the same time the chemisorbed hydroxyl moieties act as passivating agents of the Ni(III) sites thus diminishing the surface concentration of sites for dye anchoring. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Influence of the conditions of sensitization on the characteristics of p-DSCs sensitized with asymmetric squaraines

The effect of the conditions of sensitization on the photoelectrochemical performance of p-type dye-sensitized solar cells (p-DSCs) with screen-printed nickel oxide (NiO) photocathodes is analyzed. The dye-sensitizers employed in the present study are asymmetric squaraines. The conditions of sensitization differ for the relative concentration of the anti-aggregating agentCDCA(chenideoxycholic acid) with respect to the concentration of the dye-sensitizer. The co-adsorption of CDCA onto NiO electrode brings about a decrease in the surface concentration of the anchored dye as well as a blueshift of the characteristic wavelengths of optical absorption of the asymmetric squaraines considered here. Beside this, the employment of CDCA as co-adsorbent reduces the overall conversion performance of the resulting squaraine-sensitized p-DSCs with consequent diminution of the short-circuit current density. This result is ascribed to the acid action of CDCA toward the amminic nitrogen of the squaraines. Quantum efficiency spectra show that CDCA acts as a quencher of the intrinsic photoelectrochemical activity of NiO. Moreover, CDCA does not interfere with the mechanism of charge injection effectuated by the photoexcited squaraines. The photoelectrochemical impedance spectra was analyzed employing a model of equivalent circuit developed for semiconducting nanostructure electrodes

Statistical sampling applied to the radiological characterization of historical waste

International audienceThe evaluation of the activity of radionuclides in radioactive waste is required for its disposal in final repositories. Easy-to-measure nuclides, like g-emitters and high-energy X-rays, can be measured via non-destructive nuclear techniques from outside a waste package. Some radionuclides are difficult-to-measure (DTM) from outside a package because they are a-or b-emitters. The present article discusses the application of linear regression, scaling factors (SF) and the so-called "mean activity method" to estimate the activity of DTM nuclides on metallic waste produced at the European Organization for Nuclear Research (CERN). Various statistical sampling techniques including simple random sampling, systematic sampling, stratified and authoritative sampling are described and applied to 2 waste populations of activated copper cables. The bootstrap is introduced as a tool to estimate average activities and standard errors in waste characterization. The analysis of the DTM Ni-63 is used as an example. Experimental and theoretical values of SFs are calculated and compared. Guidelines for sampling historical waste using probabilistic and non-probabilistic sampling are finally given

Uncertainty quantification applied to the radiological characterization of radioactive waste

This paper describes the process adopted at the European Organization for Nuclear Research (CERN) to quantify uncertainties affecting the characterization of very-low-level radioactive waste. Radioactive waste is a by-product of the operation of high-energy particle accelerators. Radioactive waste must be characterized to ensure its safe disposal in final repositories. Characterizing radioactive waste means establishing the list of radionuclides together with their activities. The estimated activity levels are compared to the limits given by the national authority of the waste disposal. The quantification of the uncertainty affecting the concentration of the radionuclides is therefore essential to estimate the acceptability of the waste in the final repository but also to control the sorting, volume reduction and packaging phases of the characterization process. The characterization method consists of estimating the activity of produced radionuclides either by experimental methods or statistical approaches. The uncertainties are estimated using classical statistical methods and uncertainty propagation. A mixed multivariate random vector is built to generate random input parameters for the activity calculations. The random vector is a robust tool to account for the unknown radiological history of legacy waste. This analytical technique is also particularly useful to generate random chemical compositions of materials when the trace element concentrations are not available or cannot be measured. The methodology was validated using a waste population of legacy copper activated at CERN. The methodology introduced here represents a first approach for the uncertainty quantification (UQ) of the characterization process of waste produced at particle accelerators

A new approach to characterize very-low-level radioactive waste produced at hadron accelerators

Radioactive waste is produced as a consequence of preventive and corrective maintenance during the operation of high-energy particle accelerators or associated dismantling campaigns. Their radiological characterization must be performed to ensure an appropriate disposal in the disposal facilities. The radiological characterization of waste includes the establishment of the list of produced radionuclides, called “radionuclide inventory”, and the estimation of their activity. The present paper describes the process adopted at CERN to characterize very-low-level radioactive waste with a focus on activated metals. The characterization method consists of measuring and estimating the activity of produced radionuclides either by experimental methods or statistical and numerical approaches. We adapted the so-called Scaling Factor (SF) and Correlation Factor (CF) techniques to the needs of hadron accelerators, and applied them to very-low-level metallic waste produced at CERN. For each type of metal we calculated the radionuclide inventory and identified the radionuclides that most contribute to hazard factors. The methodology proposed is of general validity, can be extended to other activated materials and can be used for the characterization of waste produced in particle accelerators and research centres, where the activation mechanisms are comparable to the ones occurring at CERN

Collimation for the LHC high intensity beams

The unprecedented design intensities of the LHC require several important advances in beam collimation. With its more than 100 collimators, acting on various planes and beams, the LHC collimation system is the biggest and most performing such system ever designed and constructed. The solution for LHC collimation is explained, the technical components are introduced and the initial performance is presented. Residual beam leakage from the system is analysed. Measurements and simulations are presented which show that collimation efficiencies of better than 99.97 % have been measured with the 3.5 TeV proton beams of the LHC, in excellent agreement with expectations.peer-reviewe

Radiological characterization of radioactive waste at CERN

Zsfassung in dt. SpracheDie Europaeische Organisation fuer Teilchenphysik (CERN) betreibt seit 50 Jahren zahlreiche Teilchenbeschleuniger fuer Forschung im Bereich der Hochenergiephysik. Der Betrieb dieser Beschleuniger fuehrt zur Produktion radioaktiver Abfaelle. Fuer die Entsorgung dieser Abfaelle ist die Erfassung des Nuklidvektors - einer Liste aller Nuklide und deren spezifische Aktivitaet - erforderlich.Zur Zeit gibt es keine einzelne Methode zur Bestimmung des Nuklidvektors von allen radioaktiven Abfaellen aus beliebigen Beschleunigern. Die Entwicklung einer Methode zur radiologischen Charakterisierung von Beschleunigerkomponenten des CERN - des groessten Elementarteilchenphysiklabors Europas - ist das herausforderndes Ziel der vorliegenden Arbeit.Fuenf verschiedene Methoden werden hier vorgestellt, die die Eigenschaften der verschiedenen Kategorien von CERNs radioaktiven Abfaellen beruecksichtigen. Zu diesen Methoden gehoeren stationaere automatische Messanlagen und Monte Carlo Simulationen der erzeugten Radioaktivitaet. Wenn die chemische Zusammensetzung der Materialien und ihre radiologische Historie bekannt sind, kann die Aktivitaet durch die Matrix Methode berechnet werden. Gemischte Abfaelle und Gebinde mit unbekannter radiologischer Historie koennen mit der statistischen Methode charakterisiert werden, die auf Wahrscheinlichkeitsverteilungen und Fehlerfortpflanzung beruht. Die Fingerprint Methode schaetzt den Nuklidvektor durch eine Kombination von Berechnung und Probennahme ab. Durch die erfolgreiche Anwendung der Fingerprint Methode wurde der Nuklidvektor der CERN ISOLDE-Targets bestimmt.Die beschriebenen Methoden besitzen generelle Gueltigkeit und koennen auf die radiologische Charakterisierung von Komponenten jedes beliebigen Beschleunigers angewendet werden.The European Laboratory for Particle Physics (CERN) has been operating a number of particle accelerators for fundamental and applied research for more than 50 years. The operation of these accelerators has lead to the unavoidable production of radioactive waste. The elimination of an item of radioactive waste towards final repositories requires the radionuclide inventory, i.e. a list of nuclides with their specific activity. At present there is no single method for the radiological characterization which can be applied to all items of waste from any particle accelerator. The development of a characterization method to meet the needs of CERN - the largest accelerator's complex in Europe - is the challenging task of this thesis.Five different methods are proposed to address the specificities of the different categories of CERN waste. These methods range from an automated system for gamma-spectroscopy measurements to Monte Carlo predictions of induced radioactivity. If the material composition and radiological history are known, the activity can be calculated analytically with the Matrix method. Mixed waste and items with unknown radiological history can be characterized with the Statistical method, which is based on probability distributions and error propagation. The Fingerprint method combines Monte Carlo predictions with gamma-spectroscopy. This method has been successfully applied to the characterization of the targets irradiated in the ISOLDE experiment. The methods described in this thesis are of general validity and can be applied to any particle accelerator.13

Activation studies for a beta-beam Decay Ring (DR): residual dose rates during maintenance and airborne activity

In a future beta-beam facility radioactive ions (6He and 18Ne) are produced, accelerated and then stored in a large Decay Ring (DR), where they eventually produce anti-neutrino and neutrino beams through β± decay. CERN is one of the candidate sites for the beta-beam facility, as existing machines like the Proton Synchrotron (PS) and the Super Proton Synchrotron (SPS) could be used for the acceleration. The DR is composed of two long straight sections and two arcs. The straight sections host the collimators and two bumps. The beam is injected into the ring at nearly 92 GeV per nucleon. Beam losses occur in different sections of the machine: relevant losses include collimation losses in the collimation section and the bumps, and decay losses in the magnets in the arcs. This work focuses on two radiation protection aspects related to the operation of the DR, namely the induced radioactivity and the air activation generated by collimation and decay losses. All the calculations are performed with the Monte Carlo transport code FLUKA and are based on a continuous three-month operation. The induced radioactivity in the machine components and the expected residual dose rates inside the tunnel during maintenance are calculated for three different waiting times. Airborne activity is evaluated through the convolution of predicted particle spectra in the tunnel with isotope-production cross-sections. Using activity-to-dose coefficients, previously calculated for the ISOLDE facility at CERN, and a laminar flow model for the air diffusion, the airborne activity is converted into effective dose to the reference population group. The results show that residual dose rates during maintenance decrease significantly in a week after the shutdown of the machine, reaching values that correspond, according to CERN area classification, to a limited stay area. The effective dose given to the reference population in one year of operation is below the reference value for CERN emissions into the environment