N2D2  High Energy and Nuclear Physics Instrumentation: Silicon Detectors

Tuesday, Nov. 3  16:30-18:10  San Diego

Session Chair:  Gregor Kramberger, Jozef Stefan Institute, Slovenia; Susanne Kuehn, CERN, Germany

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(16:30) N2D2-1, Lessons Learned in the ATLAS Pixel IBL Project

M. Bindi

CERN, Geneva, Switzerland

On behalf of the ATLAS Collaboration
The ATLAS experiment is ready to face the Run-2 of the Large Hadron Collider at CERN with improved tracking performance thanks to the installation of a new Pixel layer, also called Insertable B-Layer (IBL). The IBL has been installed in May 2014 being placed at only 3.3 cm radius from the beam axis. The combination of the limited distance from the interaction point and the increase of Luminosity that LHC will face in Run-2 will require to cope both with higher radiation environment and pixel occupancy.Two different silicon sensor technologies (planar and 3D) have been developed and a new readout chip within CMOS 130nm technology with larger area, smaller pixel size and faster readout capability. Dedicated design features in combination with a new composite material were considered and used in order to reduce the material budget of the support structure while keeping the optimal thermo-mechanical performance. Due to the limited radial space about less than 1 cm, the IBL detector was a challenge in terms of design and mechanical integration. An overview of the lessons learned during the IBL project will be presented, focusing on the challenges and highlighting the issues met during the production, integration, installation and commissioning phases of the detector. Early performance tests using cosmic ray and beam data will also be presented.

(16:50) N2D2-2, The Phase-1 Upgrade of the CMS Vertex Detector

M. Menichelli

INFN Sezione di Perugia, Perugia, Italy

On behalf of the CMS Collaboration
The operation of the present pixel detector has started in 2010 with LHC operating at a center of mass (CM) energy of 7 TeV. At the beginning of 2012 CM energy was increased to 8 TeV and within December 2012 a total of 19 fb-1 integrated luminosity has been delivered, with instantaneous peak luminosities approaching 7 x 1033 cm-2s-1. The present pixel detector originally was designed for a luminosity of 1 x 1034 cm-2s-1 and a pileup (number of inelastic interaction per bunch crossing) of 25 in 25 ns bunch spacing. These beam parameters will be reached in the middle of the data taking period 2015-2018 (with an additional increase in the center of mass energy up to the value of 13-14 TeV) and then, peak luminosity will keep increasing until 2017 when it will reach the value of 1.5 x 1034 cm-2s-1. The present detector will remain operative until the end of 2016 and will be replaced with an upgraded detector that will be described in this presentation before Long Shutdown 2 (LS2). After LS2 the beam parameters will change again, around 2021 a peak luminosity reaching at least 2 x 1034 cm-2s-1 is foreseen, consequently pile-up will increase up to 50 if the bunch spacing will be kept at 25 ns, or to 100 if the bunch spacing will be brought to 50 ns. In this context the present pixel detector will be unable to perform adequately and this is the reason why new detector needs to be built and installed before LS2. The new upgraded detector will have higher tracking efficiency and lower mass with four barrel layer and three forward/backward disks to provide a hit pixel coverage up to absolute pseudorapidities of 2.5. In this presentation the new pixel detector will be described focusing mostly on the barrel detector design, construction and expected performances. Preliminary tests on detector module production will be also presented.

(17:10) N2D2-3, The LHCb VELO Upgrade

P. Rodriguez

University of Manchester, Manchester, UK

On behalf of the LHCb Collaboration
The upgrade of the LHCb experiment, planned for 2018, will transform the experiment to a trigger-less system reading out the full detector at 40 MHz event rate. All data reduction algorithms will be executed in a high-level software farm. This will enable the detector to run at luminosities of 2 x 10^33 /cm^2/s and probe physics beyond the Standard Model in the heavy flavour sector with unprecedented precision. The Vertex Locator (VELO) is the silicon vertex detector surrounding the interaction region. The current detector will be replaced with a hybrid pixel system equipped with electronics capable of reading out at 40 MHz, designed to accumulate an integrated luminosity of 50 fb-1 and beyond. The upgraded VELO will provide fast pattern recognition and track reconstruction to the software trigger. The detector comprises silicon pixel sensors with 55x55 um^2 pitch, read out by the VeloPix ASIC, from the TimePix/MediPix family. The hottest region will have pixel hit rates of 900 Mhits/s yielding a total data rate more than 3 Tbit/s for the upgraded VELO. The detector modules are located in a separate vacuum, separated from the beam vacuum by a thin custom made foil. The foil will be manufactured through milling and possibly thinned further by chemical etching. The detector halves are retracted when the beams are injected and closed at stable beams, positioning the first sensitive pixel at 5.1 mm from the beams. The material budget will be minimised by the use of evaporative CO_2 coolant circulating in microchannels within 400 um thick silicon substrates. Microchannel cooling brings many advantages: very efficient heat transfer with almost no temperature gradients across the module, no CTE mismatch with silicon components, and low material contribution. This breakthrough technology is being developed for LHCb. The current status of the VELO upgrade will be described and latest results from operation of irradiated sensor assemblies will be presented.

(17:30) N2D2-4, Small Pitch Pixel Sensors for the CMS Phase-II Upgrade

G. Steinbrueck

Hamburg University, Hamburg, Germany

On behalf of the CMS Tracker Collaboration
The CMS experiment intends to exchange the pixel detector for the high luminosity phase of the Large Hadron Collider (HL-LHC).
Therefore, a large R&D effort has been started in order to develop sensors capable of withstanding the expected extremely high radiation damage. The targeted integrated luminosity of 3000 fb-1, estimated for 10 years of operation, translates into an equivalent fluence of 2 � 1016 cm-2 and a dose in the SiO2 of 5 MGy at the expected position of the innermost pixel detector layer.
The CMS collaboration has undertaken two baseline sensor R&D programs on thin n-in-p planar and 3D silicon sensor technologies. Resulting from the increased instantaneous luminosity, the pixel area has to be minimized to approximately 2500 �m2 to keep the occupancy at the percent level. Suggested pixel cell geometries to match the requirement are 50 � 50 �m2 or 25 � 100 �m2, minimizing space for design choices and a possible biasing scheme.
CMS has initiated the production of test-sensors with the envisaged pixel geometries. Status, progress, and prospects of this effort will be discussed.

(17:50) N2D2-5, The Silicon Vertex Tracker for the Heavy Photon Search Experiment

P. O. Hansson Adrian

SLAC National Accelerator Laboratory, Menlo Park, CA, USA

On behalf of the HPS Collaboration
The Heavy Photon Search (HPS) is a new, dedicated experiment at Thomas Jefferson National Accelerator Facility (JLab) to search for a massive vector boson, the heavy photon (a.k.a. dark photon, A'), in the mass range 20-500~MeV/c$^{2}$ and with a weak coupling to ordinary matter. An A' can be radiated from an incoming electron as it interacts with a charged nucleus in the target, accessing a large open parameter space where the A' is relatively long-lived, leading to displaced vertices. HPS searches for these displaced A' to e$^+$e$^-$ decays using actively cooled silicon microstrip sensors with fast readout electronics placed immediately downstream of the target and inside a dipole magnet to instrument a large acceptance with a relatively small detector. With typical particle momenta of 0.5-2~GeV/c, the low material budget of 0.7\% $X_0$ per tracking layer is key to limiting the dominating multiple scattering uncertainty and allowing efficient separation of the decay vertex from the prompt QED trident background processes. Achieving the desired low-mass acceptance requires placing the edge of the silicon only 0.5~mm from the electron beam. This results in localized hit rates above 4~MHz/mm$^2$ and radiation levels above $10^{14}$ 1~MeV neutron equivalent dose. Hit timing at the ns level is crucial to reject out-of time hits not associated with the A' decay products from the almost continuous CEBAF accelerator beam. To avoid excessive beam-gas interactions the tracking detector is placed inside the accelerator beam vacuum envelope and is retractable to allow safe operation in case of beam motion. This contribution will discuss the design, construction and first results from the first data-taking period in the spring of 2015.