DSpace Collection:

A method for accurate zero calibration of asymmetric jaws in single-isocenter half-beam techniques

Tue, 09 Apr 2013 10:55:46 GMT

Title: A method for accurate zero calibration of asymmetric jaws in single-isocenter half-beam techniques Authors: Hernandez Espinosa, Victor; Sempau Roma, Josep; Abella, R.; Romero Lopez, Matilde; Lopez Perez, Marcedes; Artigues, Manel; Arenas, M. Abstract: Purpose: To present a practical method for calibrating the zero position of asymmetric jaws that provides higher accuracy at the central axis and improves dose homogeneity in the abutting region of half-beams. Methods: Junction doses were measured for each asymmetric jaw using the double-exposure technique and electronic portal imaging devices. The junction dose was determined as a function of jaw position. The shift in the zero jaw position (or in its corresponding potentiometer readout) required to correct for the measured junction dose could thus be obtained. The jaw calibration was then modified to introduce the calculated shift and therefore achieve an accurate zero position in order to provide a relative junction dose that was as close to zero as possible. Results: All the asymmetric jaws from four medical linear accelerators were calibrated with the new calibration procedure. Measured relative junction doses at gantry 0° were reduced from a maximum of ±40% to a maximum of ±8% for all the jaws in the four considered accelerators. These results were valid for 6 MV and 18 MV photon beams and for any combination of asymmetric jaws set to zero. The calibration was stable over a long period of time; therefore, the need for recalibrating is seldom necessary. Conclusions: Accurate calibration of the zero position of the jaws is feasible in current medical linear accelerators. The proposed procedure is fast and it improves dose homogeneity at the junction of half-beams, thus, allowing a more accurate and safer use of these techniques. © 2013 American Association of Physicists in Medicine.

Performance of the neutron time-of-flight facility n TOF at CERN

Wed, 27 Feb 2013 10:50:01 GMT

Title: Performance of the neutron time-of-flight facility n TOF at CERN Authors: Guerrero, C; Tsinganis, A; Calviño Tavares, Francisco; Cortés Rossell, Guillem Pere; Gómez Hornillos, María Belén; Hernández Prieto, Álvaro; Riego Pérez, Albert Abstract: The neutron time-of-flight facility n TOF features a white neutron source produced by spallation through 20 GeV/c protons impinging on a lead target. The facility, aiming primarily at the measurement of neutron-induced reaction cross sections, was operating at CERN between 2001 and 2004, and then underwent a major upgrade in 2008. This paper presents in detail all the characteristics of the new neutron beam in the currently available configurations, which correspond to two different collimation systems and two choices of neutron moderator. The characteristics discussed include the intensity and energy dependence of the neutron flux, the spatial profile of the beam, the in-beam background components and the energy resolution/broadening. The discussion of these features is based on dedicated measurements and Monte Carlo simulations, and includes estimations of the systematic uncertainties of the mentioned quantities.

The 93-Zr(n,gamma) reaction up to 8 keV neutron energy

Tue, 19 Feb 2013 11:09:27 GMT

Title: The 93-Zr(n,gamma) reaction up to 8 keV neutron energy Authors: Tagliente, G.; Milazzo, P.M.; Fujiii, K.; Abbondanno, U.; Aerts, G.; Calviño Tavares, Francisco; Pretel Sánchez, Carme; Cortés Rossell, Guillem Pere Abstract: he (n,gamma) reaction of the radioactive isotope 93Zr has been measured at the n_TOF high-resolution time-of-flight facility at CERN. Resonance parameters have been extracted in the neutron energy range up to 8 keV,yielding capture widths smaller (14%) than reported in an earlier experiment. These results are important for detailed nucleosynthesis calculations and for refined studies of waste transmutation concepts.

Neutron capture cross section of unstable 63-Ni: implications for stellar nucleosynthesis

Tue, 22 Jan 2013 10:39:04 GMT

Title: Neutron capture cross section of unstable 63-Ni: implications for stellar nucleosynthesis Authors: Lederer, C.; Massimi, C.; Calviño Tavares, Francisco; Cortés Rossell, Guillem Pere; Gómez Hornillos, María Belén Abstract: The 63Ni(n,γ) cross section has been measured for the first time at the neutron time-of-flight facility n_TOF at CERN from thermal neutron energies up to 200 keV. In total, capture kernels of 12 (new) resonances were determined. Maxwellian averaged cross sections were calculated for thermal energies from kT=5–100 keV with uncertainties around 20%. Stellar model calculations for a 25M⊙ star show that the new data have a significant effect on the s-process production of 63Cu, 64Ni, and 64Zn in massive stars, allowing stronger constraints on the Cu yields from explosive nucleosynthesis in the subsequent supernova.

Experimental analysis of the mass eficiency curve for gross alphya activity and morphological study of the residue obtained by the co-precipitation method

Thu, 08 Nov 2012 11:13:39 GMT

Title: Experimental analysis of the mass eficiency curve for gross alphya activity and morphological study of the residue obtained by the co-precipitation method Authors: Montaña Gurrera, Montserrat; Camacho García, Antonia; Valles Murciano, M. Isabel; Serrano Carreño, M. Isabel Abstract: Gross alpha activity determination in drinking water is one of the radiological parameters that must be determined in order to decide whether further specific analyses of radionuclides in samples is required. This parameter can be determined by the co-precipitation method using a ZnS(Ag) scintillation detector or a proportional counter. The method consists of collecting all alpha emitters of interest using barium and iron as carriers as they provide more uniform and homogeneous residues than the evaporation method, especially for highly saline waters. In the course of our work, the efficiencies of the gas proportional and ZnS(Ag) detectors for alpha particle detection in water residues were studied in function of sample mass (precipitate) with residues ranging between 16.0 and 35.0mg. In addition, the structure and morphological composition of a residue collected by the co-precipitation method was studied with the aim of verifying whether the residue is a homogeneous mixture. The effect of alpha energy on the efficiency and stability of the calibration samples (mass and efficiency) was studied using (230)Th, natural uranium ((nat)U) and (241)Am radionuclides.

Neutron-induced fission cross section of 245-Cm: New results from data taken at the time-of-flight facility n_TOF

Mon, 21 May 2012 10:11:30 GMT

Title: Neutron-induced fission cross section of 245-Cm: New results from data taken at the time-of-flight facility n_TOF Authors: Calviño Tavares, Francisco; Cortés Rossell, Guillem Pere; Poch Parés, Agustí; Pretel Sánchez, Carme Abstract: The neutron-induced fission cross section of 245Cm was measured at n_TOF in a wide energy range and with high resolution. The energy dependence, measured in a single measurement from 30 meV to 1 MeV neutron energy, has been determined with 5% accuracy relative to the 235U(n,f) cross section. In order to reduce the uncertainty on the absolute value, the data have been normalized at thermal energy to recent measurements performed at ILL and BR1. In the energy range of overlap, the results are in fair agreement with some previous measurements and confirm, on average, the evaluated cross section in the ENDF/B-VII.0 database, although sizable differences are observed for some important resonances below 20 eV. A similar behavior is observed relative to JENDL/AC-2008, a reactor-oriented database for actinides. The new results contribute to the overall improvement of the databases needed for the design of advanced reactor systems and may lead to refinements of fission models for the actinides.

Implementation of two-phase tritium models for helium bubbles in HCLL breeding blanket modules

Wed, 08 Feb 2012 12:10:25 GMT

Title: Implementation of two-phase tritium models for helium bubbles in HCLL breeding blanket modules Authors: Fradera, Jordi; Sedano, L.A.; Mas de les Valls Ortiz, Elisabet; Batet Miracle, Lluís Abstract: Tritium self-sufficiency requirement of future DT fusion reactors involves large helium production rates in the breeding blankets; this might impact on the conceptual design of diverse fusion power reactor units, such as Liquid Metal (LM) blankets. Low solubility, long residence-times and high production rates create the conditions for Helium nucleation, which could mean effective T sinks in LM channels. A model for helium nano-bubble formation and tritium conjugate transport phenomena in liquid Pb17.5Li and EUROFER is proposed. In a first approximation, it has been considered that He bubbles can be represented as a passive scalar. The nucleation model is based on the classical theory and includes a simplified bubble growth model. The model captures the interaction of tritium with bubbles and tritium diffusion through walls. Results show the influence of helium cavitation on tritium inventory and the importance of simulating the system walls instead of imposing fixed boundary conditions.

Controllability study of an ethanol reforming process for hydrogen production

Wed, 25 Jan 2012 12:31:05 GMT

Title: Controllability study of an ethanol reforming process for hydrogen production Authors: García, Vanesa Mariel; Serra, Maria; Llorca Piqué, Jordi Abstract: A system for ethanol steam reforming and purification of carbon monoxide (CO) designed to feed a PEM fuel cell has been modelled. From the model, we study the sensitivity and controllability emphasizing the study of the influence of the temperature on the output variables of interest. The results of the study of controllability are used for the identification of the best control structures

Energy from hydrogen. Hydrogen from renewable fuels for portable applications

Wed, 25 Jan 2012 10:27:04 GMT

Title: Energy from hydrogen. Hydrogen from renewable fuels for portable applications Authors: Llorca Piqué, Jordi Abstract: Molecular hydrogen is an environmentally clean source of energy, but it is not available on Earth. Steam reforming of bio-derived compounds represents a valuable route for the generation of molecular hydrogen and has the advantage that it is CO2-neutral and it requires a limited amount of additional infrastructure for implementation. At present, suitable catalysts for selective bio-alcohol and dimethyl ether reforming into hydrogen and carbon dioxide are being developed, but their use on structured wall reactors for practical application is still under way. Among them, aerogel-based coated structures appear very promising due to their very high mass transfer rates and their ability to disperse highly active metal nanoparticles. The performance of these systems improves considerably by using microreaction technologies. Microreactors based on silicon micromonoliths together with integrated downstream carbon monoxide selective oxidation hold a promising futurefor the effective on-site and on-demand generation of hydrogen from renewable fuels in portable fuel cell applications.

Ethanol steam reforming over cobalt talc in a plate microreactor

Mon, 09 Jan 2012 15:50:49 GMT

Title: Ethanol steam reforming over cobalt talc in a plate microreactor Authors: Dominguez, M.; Cristiano, G.; López, E.; Llorca Piqué, Jordi Abstract: Stainless steel plates were coated with cobalt talc (Co3[Si2O5]2(OH)2) and used in a catalytic plate microreactor for producing hydrogen from ethanol steam reforming at low temperature. Cobalt talc was deposited successfully by one-step hydrothermal synthesis from Co(NO3)2·6H2O and Na2SiO3 at 463 K for 60 h and characterized by scanning electron and confocal microscopy, X-ray diffraction, infrared spectroscopy, and mechanical stability tests. Catalytic plates did not require any activation treatment and showed fast start-up. At 648 K and W/FEtOH = 0.5 g min mol−1 complete ethanol conversion was attained and the yield to the reforming products, H2 and CO2, was 97 and 90%, respectively, whereas CO was detected at a trace level (<0.1%).

96Zr(n,gamma ) measurement at the n TOF facility at CERN

Fri, 25 Nov 2011 08:14:46 GMT

Title: 96Zr(n,gamma ) measurement at the n TOF facility at CERN Authors: Calviño Tavares, Francisco; Cortés Rossell, Guillem Pere; Pretel Sánchez, Carme; Poch Parés, Agustí Abstract: The (n,γ ) cross section of 96Zr has been investigated at the CERN n TOF spallation neutron source. Highresolution time-of-flight measurements using an enriched ZrO2 sample allowed us to analyze 15 resonances below 40 keV with improved accuracy. On average, the capture widths were found to be 25% smaller than reported in earlier experiments. If complemented with the contribution by direct radiative capture, the derived Maxwellian averaged cross sections are consistent with activation data at kT = 25 keV. The present results confirm the astrophysical implications for the s-process branching at 95Zr.

Measurement of the 236U(n, f ) cross section from 170 meV to 2 MeV at the CERN n TOF facility

Fri, 25 Nov 2011 07:57:26 GMT

Title: Measurement of the 236U(n, f ) cross section from 170 meV to 2 MeV at the CERN n TOF facility Authors: Calviño Tavares, Francisco; Cortés Rossell, Guillem Pere; Poch Parés, Agustí; Pretel Sánchez, Carme Abstract: The neutron-induced fission cross section of 236U was measured at the neutron Time-of-Flight (n TOF) facility at CERN relative to the standard 235U(n,f ) cross section for neutron energies ranging from above thermal to several MeV. The measurement, covering the full range simultaneously, was performed with a fast ionization chamber, taking advantage of the high resolution of the n TOF spectrometer. The n TOF results confirm that the first resonance at 5.45 eV is largely overestimated in some nuclear data libraries. The resonance triplet around 1.2 keV was measured with high resolution and resonance parameters were determined with good accuracy. Resonances at high energy have also been observed and characterized and different values for the cross section are provided for the region between 10 keV and the fission threshold. The present work indicates various shortcomings of the current nuclear data libraries in the subthreshold region and provides the basis for an accurate re-evaluation of the 236U(n,f ) cross section, which is of great relevance for the development of emerging or innovative nuclear reactor technologies.

Neutron measurements for advanced nuclear systems: The n_TOF project at CERN

Thu, 24 Nov 2011 13:06:39 GMT

Title: Neutron measurements for advanced nuclear systems: The n_TOF project at CERN Authors: Calviño Tavares, Francisco; Cortés Rossell, Guillem Pere; Gómez Hornillos, María Belén Abstract: A few years ago, the neutron time-of-flight facility n_TOF was built at CERN to address some of the urgent needs of high-accuracy nuclear data for Accelerator Driven Systems and other advanced nuclear energy systems, as well as for nuclear astrophysics and fundamental nuclear physics. Thanks to the characteristics of the neutron beam, and to state-of-the-art detection and acquisition systems, high quality neutron cross-section data have been obtained for a variety of isotopes, many of which radioactive. Following an important upgrade of the spallation target and of the experimental area, a new measurement campaign has started last year. After a brief review of the most important results obtained so far at n_TOF, the new features of the facility are presented, together with the first results on the commissioning of the neutron beam. The plans for future measurements, in particular related to nuclear technology are finally discussed.

Beta decay studies of neutron rich nuclei using total absorption gamma-ray spectroscopy and delayed neutron measurements

Thu, 29 Sep 2011 11:17:29 GMT

Title: Beta decay studies of neutron rich nuclei using total absorption gamma-ray spectroscopy and delayed neutron measurements Authors: Jokinen, Ari; Tain, J.L.; Algora, A.; Estevez, E.; Rubio, B.; Valencia, E; Jordan, D.; Äystö, J.; Eronen, T.; Gómez Hornillos, María Belén; Cortés Rossell, Guillem Pere; Caballero Folch, Roger; Gelletly, W.; Bunce, M.; Bowry, M.; Batist, L.; Riisanen, J.; Pentilla, H.; Moore, I.; Gorlychev, V.; Porta, A.; Van Vinh, B.; Fallot, M.; Giot, L.; Martínez, T.; Dominco Pardo, C.; Cano-Ott, D.; Garcia, A. Description: A complete characterisation of the β-decay of neutron-rich nuclei can be obtained from the measurement of β -delayed gamma rays and, whenever the process is energetically possible, β -delayed neutrons. The accurate determination of the β-intensity distribution and the β -delayed neutron emission probability is of great relevance in the elds of reactor technology and nuclear astrophysics. A programme for combined measurements using the total absorption gamma-ray spectroscopy technique and both neutron counters and neutron time-of-flight spectrometers is presented.

Improved neutron capture cross section measurements with the n_TOF total absorption Cclorimeter

Thu, 22 Sep 2011 14:19:59 GMT

Title: Improved neutron capture cross section measurements with the n_TOF total absorption Cclorimeter Authors: Calviño Tavares, Francisco; Cortés Rossell, Guillem Pere; Gómez Hornillos, María Belén Abstract: The n TOF collaboration operates a Total Absorption Calorimeter (TAC) [1] for measuring neu- tron capture cross-sections of low-mass and/or radioactive samples. The results obtained with the TAC have led to a substantial improvement of the capture cross sections of 237Np and 240Pu [2]. The experience acquired during the rst measurements has allowed us to optimize the performance of the TAC and to improve the capture signal to background ratio, thus opening the way to more complex and demanding measurements on rare radioactive materials. The new design has been reached by a series of detailed Monte Carlo simulations of complete experiments and dedicated test measurements. The new capture setup will be presented and the main achievements highlighted.