Signal and charge collection efficiency of n-in-p strip detectors after mixed irradiation to HL-LHC fluences Susanne Kuehn a,n , Thomas Barber a , Gianluigi Casse b , Paul Dervan b , Adrian Driewer c,a , Dean Forshaw b , Torkjell Huse b , Karl Jakobs a , Ulrich Parzefall a a Institute of Physics, University of Freiburg, Hermann-Herder-Str. 3, 79104 Freiburg, Germany b Department of Physics, University of Liverpool, The Oliver Lodge Laboratory, Oxford Street L69 7ZE, United Kingdom c Fraunhofer Institute for Microelectronic Circuits and Systems, Finkenstr. 61, 47057 Duisburg, Germany article info Available online 30 April 2013 Keywords: Silicon particle detectors Radiation damage Mixed irradiation Charge collection efficiency abstract For the year 2020, an upgrade of the LHC with a factor ten increase in luminosity is planned. The resulting severe radiation doses for the ATLAS tracker demand extremely radiation tolerant detectors. In this study six planar n-in-p strip sensors produced by Hamamatsu Photonics were irradiated in consecutive irradiation steps with pions of 280 Mev/c, protons of 25 Mev/c and reactor neutrons resulting in a combined fluence of up to 3 Â 10 15 1 MeV neutron equivalent particles per square centimeter ðn eq =cm 2 Þ. This particle composition and fluence corresponds to the qualification limit specified by the ATLAS experiment for the outer pixel layers (assuming an integrated luminosity of 3000 fb -1 ). The 320 μm thick devices are investigated using electrons from a 90 Sr source. After each irradiation step both charge collection efficiency and noise measurements have been performed using the ALIBAVA readout system, which is based on analogue Beetle ASICs clocked at 40 MHz. Measurements of the signal and signal-to-noise ratio of detectors will be given after the sensors were exposed to radiation that both in fluence and composition are corresponding to the expectations for the HL-LHC trackers. Conclusions will be drawn on their operation in the ATLAS inner detector upgrade. & 2013 Elsevier B.V. All rights reserved. 1. Introduction It is foreseen to significantly increase the luminosity of the Large Hadron Collider (LHC) at CERN by upgrading it towards the HL-LHC (High Luminosity-LHC) in order to harvest the maximum physics potential of the machine. An integrated luminosity of 3000 fb -1 is planned to be achieved after 10–12 years of operation [1]. The resulting severe radiation doses for the ATLAS tracker demand extremely radiation tolerant tracking detectors. The inner most pixel detector layers will be exposed to fluences up to 2 Â 10 16 1 MeV neutron equivalent particles per square centimeter ðn eq =cm 2 Þ, while for the inner strip detector region fluences of 1 Â 10 15 n eq =cm 2 are expected [2]. The high radiation dose damages the crystal lattice of the silicon and causes a higher full depletion voltage and leakage current and degrades the collected charge due to trapping, thus reducing the signal-to-noise ratio. Several concepts are studied to cope with these disadvantages. The advantage of detectors based on p-doped bulk material is in the fact that the induced signal is dominated by electrons drifting to the readout side. Due to higher drift velocity of electrons and favorable weighting field the trapping affects less the induced charge for n-in-p than for p-in- n detectors. In addition, radiation increases trapping probabilities of electrons less than of holes [3]. And the effect of charge multiplication due to impact ionisation can occur for electrons at lower electric field strength compared to holes [4] and it has been observed in strip detectors by several groups [5,6]. A drawback of these detectors is the more complex processing since an additional inter-strip isolation has to be integrated into the design. To cope with these demands, Hamamatsu Photonics [7] has developed a prototype series of planar n-in-p strip sensors for the upgrade of the ATLAS inner tracker. In this study a set of six mini sensors was irradiated with three different particle compositions consisting of pions, protons and neutrons. Taking into account the NIEL scaling hypothesis [8], the particle compositions are close approximations to specific radii where the particular fluence is expected (see Fig. 1). Those predictions were the results of a FLUKA simulation for the proposed ATLAS upgrade (“strawman” layout v14-2009) [9]. Results are presented on the collected charge and noise, which are measured by using electrons from a 90 Sr source and connecting the sensors to fast analogue readout electronics. 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 & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nima.2013.04.068 n Corresponding author. Tel.: +49 7612035854 E-mail address: susanne.kuehn@cern.ch (S. Kuehn). Nuclear Instruments and Methods in Physics Research A 730 (2013) 58–61