New micromegas for axion searches in CAST T. Dafni a,Ã , S. Aune b , G. Fanourakis c , E. Ferrer-Ribas b , J. Gala ´n a , A. Gardikiotis d , T. Geralis c , I. Giomataris b , H. Go ´ mez a , F.J. Iguaz a , I.G. Irastorza a , G. Luzo ´n a , J. Morales a,1 , T. Papaevangelou b , A. Rodrı ´guez a , J. Ruz a,2 , A. Toma ´s a , T. Vafeiadis e , S.C. Yildiz f a Laboratorio de Fı ´sica Nuclear y Astropartı ´culas, University of Zaragoza, Zaragoza, Spain b IRFU, Centre d’ E ´ tudes de Saclay, CEA, Gif-sur-Yvette, France c Institute of Nuclear Physics, NCSR Demokritos, Athens, Greece d University of Patras, Patras, Greece e Aristotle University of Thessaloniki, Thessaloniki, Greece f Do˘ gus - University, Istanbul, Turkey article info Available online 7 July 2010 Keywords: Gas micropattern detectors Micromegas X-ray detectors Cast Axions Low background abstract Micromegas detectors have been taking data in the CAST experiment since 2002, occupying one opening (out of the two looking for sunrise axions) of the magnet and showing good performance and stability. Currently, three of the four X-ray detectors used in the experiment are micromegas. The new detectors are of the Microbulk technology, which have attracted a lot of attention because of the advantages they present, among them the low-material construction, high radiopurity and good energy resolution. Here, their performance during the last year will be commented. In particular, the low background levels reached in some detectors have triggered a set of studies in order to understand the effect. & 2010 Elsevier B.V. All rights reserved. 1. Cast Axions are hypothetical particles which could explain the strong CP problem. They could be produced in the core of stars like the Sun via the Primakoff effect. The CAST experiment (CERN Axion Solar Telescope) has been looking for solar axions since 2002 [1]. It is using a decommissioned LHC-prototype magnet which is 10 m long and can reach a 9 T magnetic field as a converter of axions coming from the Sun into detectable X-rays (Fig. 1). The energy range of the expected signal is between 1 and 10 keV, and its rate is dependent on the very weak axion–photon coupling (Fig. 2). Therefore, low background X-ray detectors are necessary in order to have a high sensitivity. The requirements for such detectors include radiopure components, shielding, good energy and spatial resolution to perform powerful off-line rejection conditions and reduce backgrounds, as well as stability over long periods of operation. In the first phase of the experiment, the magnet bores were kept in vacuum, allowing CAST to explore the axion mass range up to 0.02 eV/c 2 [2,3]. The three different detectors of the experiment at the time (an X-ray telescope coupled to a CCD [4], a Time Projection Chamber [5] and a micromegas [6]) registered no signal of axions, and for this range of masses, CAST has given the most stringent limit on the axion-to-photon coupling constant. In order to extend the search to a wider range of axion rest masses, data have also been acquired by introducing inside the magnet bores a buffer gas (first 4 He and then 3 He). Using 4 He allowed CAST in the years 2005–2006 to scan axion masses up to 0.4 eV giving an improved limit for the new scanned region [7,8]. In 2007 the experiment was upgraded to the second part of Phase II, where 3 He was to be used as a buffer gas inside the magnet bores. This change gave the opportunity for several upgrades regarding the detector systems. On the sunset end, a new system was designed for the installation of two new micromegas detectors (Fig. 3). These detectors replaced the Time Projection Chamber that was sitting on that end since 2002. The decision on the replacement of the TPC was taken considering the very good results obtained with the micromegas detector through the CAST data-taking period. On the sunrise end, the system was re-designed in order to implement a new shielding of the detector as well as new controls for several monitoring parameters [9]. The new design has taken into account the possibility to install in the future an X-ray focusing device in front of the detector, which would increase the signal-to-background ratio significantly. Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/nima Nuclear Instruments and Methods in Physics Research A 0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.06.310 Ã Corresponding author. E-mail address: Theopisti.Dafni@cern.ch (T. Dafni). 1 Deceased. 2 Present address: CERN, European Organization for Particle Physics and Nuclear Research. Nuclear Instruments and Methods in Physics Research A 628 (2011) 172–176