Cosmogenic radionuclide production in NaI(Tl) crystals

arXiv:1411.0106v2.-- et al.The production of long-lived radioactive isotopes in materials due to the exposure to cosmic rays on Earth surface can be an hazard for experiments demanding ultra-low background conditions, typically performed deep underground. Production rates of cosmogenic isotopes in all the materials present in the experimental set-up, as well as the corresponding cosmic rays exposure history, must be both well known in order to assess the relevance of this effect in the achievable sensitivity of a given experiment. Although NaI(Tl) scintillators are being used in experiments aiming at the direct detection of dark matter since the first nineties of the last century, very few data about cosmogenic isotopes production rates have been published up to date. In this work we present data from two 12.5 kg NaI(Tl) detectors, developed in the frame of the ANAIS project, which were installed inside a convenient shielding at the Canfranc Underground Laboratory just after finishing surface exposure to cosmic rays. The very fast start of data taking allowed to identify and quantify isotopes with half-lives of the order of tens of days. Initial activities underground have been measured and then production rates at sea level have been estimated following the history of detectors; values of about a few tens of nuclei per kg and day for Te isotopes and 22Na and of a few hundreds for I isotopes have been found. These are the first direct estimates of production rates of cosmogenic nuclides in NaI crystals. A comparison of the so deduced rates with calculations using typical cosmic neutron flux at sea level and a carefully selected description of excitation functions will be also presented together with an estimate of the corresponding contribution to the background at low and high energies, which can be relevant for experiments aiming at rare events searches.This work has been supported by the Spanish Ministerio de Economía y Competitividad and the European Regional Development Fund (MINECO-FEDER) (FPA2011-23749), the Consolider-Ingenio 2010 Programme under grants MULTIDARK CSD2009-00064 and CPAN CSD2007-00042, and the Gobierno de Aragón (Group in Nuclear and Astroparticle Physics, ARAID Foundation and C. Cuesta predoctoral grant). C. Ginestra and P. Villar are supported by the MINECO Subprograma de Formación de Personal Investigador.Peer Reviewe

Design, scale-up and characterization of the data acquisition system for the ANAIS dark matter experiment

El proyecto ANAIS, iniciado en los noventa, se ha dedicado a desarrollar un experimento de materia oscura con ioduro de sodio en el Laboratorio Subterráneo de Canfranc (LSC) y podría confirmar el resultado positivo de DAMA/LIBRA usando la misma técnica y el mismo material blanco. Un experimento de estas características posee unos requisitos muy estrictos para tener sensibilidad suficiente a la modulación anual: tener el menor fondo radioactivo posible en la zona de interés, poseer un umbral energético muy bajo y tener suficiente masa. Además de estos requisitos fundamentales es necesaria una muy buena estabilidad en la adquisición de datos y un buen control de los parámetros ambientales para evitar que posibles efectos sistemáticos puedan ser tomados por modulación anual de materia oscura. El experimento ANAIS constará de más de cien kilogramos de ioduro de sodio ultrapuro en proceso de fabricación y que serán instalados en los próximos meses en el LSC. Esta tesis ha estado enfocada al diseño, implementación y caracterización de un sistema de adquisición de datos adecuado para el experimento ANAIS teniendo en cuenta los requisitos antes mencionados. Se han estudiado los fotomultiplicadores (PMTs) elegidos para la detección de la luz del centelleo del ioduro de sodio. Además se han desarrollado los algoritmos y protocolos necesarios para hacer el control de calidad de todas la unidades. Se ha descrito el diseño de la electrónica necesaria para la adquisición de datos de los módulos de ioduro y los centelladores plásticos usados como detector de muones junto con el software de adquisición y de análisis de datos. También se ha medido la recogida de luz de tres módulos de Alpha Spectra, los dos primeros de que constó el prototipo ANAIS-25 y un tercero que unido a los dos anteriores formaron ANAIS-37. Por último, se ha hecho un estudio de la estabilidad de los parámetros ambientales y de parámetros cruciales para la adquisición de datos.The ANAIS project has been a long time effort devoted to carry out an experiment to detect dark matter annual modulation with very low background NaI(Tl) detectors. This experiment could confirm the DAMA/LIBRA positive signal with the same target and technique. Such an experiment has a very stringent requirements in order to have enough sensitivity to an annual modulation at very low energy. These requirements are: very low energy threshold, a background as low as possible in the region of interest and a high enough target mass. In addition to these fundamental requirements, very good stability and control of environmental parameters have to be accomplished in order to avoid systematic effects to mimic the effect of the annual modulation. An experiment of more than one hundred kilograms of ultrapure NaI(Tl) has been conceived and it is being commissioned at the Canfranc Underground Laboratory (LSC). This work was devoted to the design, implementation and characterization of a data acquisition system suitable for the ANAIS experiment, having in mind the previously mentioned requirements. It has described the Photomultiplier Tubes (PMTs) used by the ANAIS modules and the algorithms and protocol developed in order to pass quality tests to all units. It has presented the design and implementation of the electronic front-end for the ANAIS modules and muon tagging system along with the data acquisition software. The analysis software was adapted from the software of the previous prototypes to allow easy scale-up to the full experiment. Finally, the test of the optical performance of the Alpha Spectra modules and the tests of data acquisition and monitoring of environmental parameters were performed

Temperature and Current Sensitivities of Bare Mo/Au Transition-Edge Sensors

We study the logarithmic transition sensitivities to temperature and current, α and β, of bare Mo/Au TES sensors without any normal metal structure, with and without relevant weak link effects. Both parameters display a smooth dependence with bias. We analyze them as a function of bias, bath temperature and TES size. We observe relevant differences in the behavior of α and β as the aspect ratio increases

Analysis of the 40K contamination in NaI(Tl) crystals from different providers in the frame of the ANAIS project

NaI(Tl) large crystals are applied in the search for galactic dark matter particles through their elastic scattering off the target nuclei in the detector by measuring the scintillation signal produced. However, energies deposited in the form of nuclear recoils are small, which added to the low efficiency to convert that energy into scintillation, makes that events at or very near the energy threshold, attributed either to radioactive backgrounds or to spurious noise (non-bulk NaI(Tl) scintillation events), can compromise the sensitivity goals of such an experiment. DAMA/LIBRA experiment, using 250 kg NaI(Tl) target, reported first evidence of the presence of an annual modulation in the detection rate compatible with that expected for a dark matter signal just in the region below 6 keVee (electron equivalent energy). In the frame of the ANAIS (Annual modulation with NaI Scintillators) dark matter search project a large and long effort has been carried out in order to understand the origin of events at very low energy in large sodium iodide detectors and develop convenient filters to reject those non attributable to scintillation in the bulk NaI(Tl) crystal. 40K is probably the most relevant radioactive contaminant in the bulk for NaI(Tl) detectors because of its important contribution to the background at very low energy. ANAIS goal is to achieve levels at or below 20 ppb natural potassium. In this paper we will report on our effort to determine the 40K contamination in several NaI(Tl) crystals, by measuring in coincidence between two (or more) of them. Results obtained for the 40K content of crystals from different providers will be compared and prospects of the ANAIS dark matter search experiment will be briefly reviewed.Comment: Submitted to International Journal of Modern Physics

Modelización de sistemas térmicos en experimentos criogénicos usando redes neuronales

En este Trabajo de Fin de Grado, se modeliza un refrigerador de dilución con una ecuación integro-diferencial de evolución, simulando el control PID de la temperatura. A continuación, se plantea un modelo de "transfer learning", entrenando una red neuronal con datos tanto reales como provenientes de las simulaciones para predecir el comportamiento del sistema en función de los valores de los parámetros del PID. Por último, se implementa un descenso de gradiente en el modelo integro-diferencial para hallar, dadas las temperaturas inicial y de referencia, los valores de los parámetros del PID que permiten que el refrigerador se estabilice en la temperatura de referencia en el menor tiempo posible, partiendo de la temperatura inicial.<br /

Sensores Superconductores Para Astrofísica e Información Cuántica

Un sensor de transición superconductora o TES es un termómetro que mide variaciones de temperatura trasformándolas en variaciones de la resistencia. Su alta sensibilidad lo hace un candidato ideal para la medida de radiación de baja energía. Dentro del proyecto astrofísico ATHENA, el equipo de bajas temperaturas de la universidad de Zaragoza trabaja en el desarrollo y caracterización de estos TES. Este trabajo estudia la respuesta de un TES a estímulos de corriente con el objetivo de aportar una caracterización alternativa a los métodos que se usan actualmente

Desarrollo de sensores superconductores de rayos X para la misión espacial ATHENA

La mayor parte de la materia bariónica en el universo se encuentra en forma de gas caliente, cuyo pico de emisión está en el espectro de los rayos X. Con el fin de estudiar esta radiación, la Agencia Espacial Europea está desarrollando la misión Athena, que hará uso de un telescopio con sensores de transición superconductora (TES) fabricados con materiales superconductores. Un equipo de investigación del Instituto de Ciencia de los Materiales de Aragón forma parte de este proyecto y está trabajando en la caracterización de estos sensores en el Laboratorio de Bajas Temperaturas de la Universidad de Zaragoza. Tras una introducción teórica, una explicación de su principio de funcionamiento y un resumen de su proceso de fabricación, se han realizado diversas caracterizaciones de estos sensores con un refrigerador de dilución y un equipo de medida automatizada denominado PPMS. Se ha tratado de relacionar la potencia de fabricación por sputtering y el espesor de las bicapas con la temperatura de transición del TES. Finalmente, al hacer el estudio de las curvas I(V) se descubrió un comportamiento anómalo asociado al proceso de medición, cuyo origen se ha logrado identificar

Present status of MI-BETA cryogenic experiment and preliminary results for CUORICINO

Present results on neutrinoless double beta decay of 130Te obtained with an array of 20 cryogenic detectors are presented. The setup consists of 20 crystals of tellurium oxide of 340 g each, corresponding to the largest presently operating cryogenic mass. Combining the results of the few runs obtained with the same array, corresponding to 1.5 kg yr, a limit on neutrinoless double beta decay half-life of 9.5]1022 yr (90% CL) has been obtained. On the basis of the results obtained with the MI-BETA experiment, we propose a construction of a 42 kg array of 56 TeO 2 bolometers (CUORICINO project) to extend the sensitivity of the present experiment, and as a "rst test for CUORE. Thanks to an innovative technique of vibration reduction and despite the high mass (750 g instead of the 340 g of the presently running detectors) we reached an energy resolution of 3.9 keV FWHM at 2615 keV decreasing to 1.4 keV at low energies. ( 2000 Elsevier Science B.V. All rights reserved

From ANAIS-25 towards ANAIS-250

The ANAIS (Annual modulation with NaI(Tl) Scintillators) experiment aims at the confirmation of the DAMA/LIBRA signal using the same target and technique at the Canfranc Underground Laboratory (LSC). 250 kg of ultra pure NaI(Tl) crystals will be used as target, divided into 20 modules, 12.5 kg mass each, and coupled to two high efficiency photomultiplier tubes from Hamamatsu. The ANAIS-25 set-up at the LSC consists of two prototypes, amounting 25 kg NaI(Tl), grown from a powder having a potassium level under the limit of our analytical techniques, and installed in a convenient shielding at the LSC. The background has been carefully analyzed and main results will be summarized in this paper, focusing on the alpha contamination identified in the prototypes and the related background contributions. Status of fulfillment of ANAIS experimental goals and prospects for the building of ANAIS-250 experiment will be also revised