نتایج اخیر از پروژه "تحقیق و توسعه" ارتقاء ATLAS سنسورهای پیکسلی دو وجهی

To extend the physics reach of the LHC experiments, several upgrades to the accelerator complex are planned, culminating in the HL-LHC, which eventually leads to an increase of the peak luminosity by a factor of five to ten compared to the LHC design value. To cope with the higher occupancy and radiation damage also the LHC experiments will be upgraded. The ATLAS Planar Pixel Sensor R&D Project is an international collaboration of 17 institutions and more than 80 scientists, exploring the feasibility of employing planar pixel sensors for this scenario. Depending on the radius, different pixel concepts are investigated using laboratory and beam test measurements. At small radii the extreme radiation environment and strong space constraints are addressed with very thin pixel sensors active thickness in the range of View the MathML source(75–150)μm, and the development of slim as well as active edges. At larger radii the main challenge is the cost reduction to allow for instrumenting the large area of (7–10) m2. To reach this goal the pixel productions are being transferred to 6 in production lines and more cost-efficient and industrialised interconnection techniques are investigated. Additionally, the n-in-p technology is employed, which requires less production steps since it relies on a single-sided process. An overview of the recent accomplishments obtained within the ATLAS Planar Pixel Sensor R&D Project is given. The performance in terms of charge collection and tracking efficiency, obtained with radioactive sources in the laboratory and at beam tests, is presented for devices built from sensors of different vendors connected to either the present ATLAS read-out chip FE-I3 or the new Insertable B-Layer read-out chip FE-I4. The devices, with a thickness varying between View the MathML source75μm and View the MathML source300μm, were irradiated to several fluences up to View the MathML source2×1016neq/cm2. Finally, the different approaches followed inside the collaboration to achieve slim or active edges for planar pixel sensors are presented.

Presently, the ATLAS pixel detector [1] comprises three barrel layers located at radii between 50.5 mm and 122.5 mm as well as three end-cap discs on each side of the detector. In total about 80 million read-out channels are distributed on 1744 pixel modules. Each module is composed of a View the MathML source250μm thick n-in-n planar silicon sensor interconnected via the solder bump bonding technique [2] to 16 FE-I3 read-out chips [3], featuring pixel pitches of View the MathML source50μm×400μm. Sensors and read-out chips are specified up to a fluence of View the MathML source1015neq/cm2 (1 MeV neutrons) or a dose of 500 kGy. To increase the physics reach of the LHC programme, it is foreseen to upgrade the accelerator chain in three dedicated long shutdowns (LS), followed by longer data-taking phases, called phase 0, I, and II. While increasing the beam energy to its design value, the peak luminosity will increase eventually up to (5–8)×1034 cm−2 s−1[4]. Each LS will be mirrored by upgrades to the ATLAS detector to cope with the increased luminosity. This paper will focus on the upgrades of the pixel detector, only. The first LS starts beginning of 2013 and lasts until the end of 2014; it will lead to an approximately fourfold increase in luminosity. In the ATLAS detector a new fourth pixel layer will be mounted on a new smaller beam pipe at a radius of 32 mm. This is referred to as the Insertable B-Layer (IBL) [5]. The smaller radius inhibits overlapping modules in z as employed in the present ATLAS pixel detector. Thus, the active fraction had to be increased, using a new design of the n-in-n sensors discussed in Section 3.2.1. Given the harsher radiation environment and the higher occupancy a new read-out chip, the FE-I4 [6], was developed, which is specified up to a received fluence of View the MathML source5×1015neq/cm2. The pixel cell size was reduced to View the MathML source50μm×250μm and the number of pixel cells increased from 2880 to 26,880. While the upgraded pixel detector is believed to retain sufficient tracking capabilities after the second LS, which starts around 2017, during the third LS from 2021 to 2022 a major upgrade of the entire inner tracking system is planned. The replacement of the tracking detector is required given the foreseen fluences in phase II of up to View the MathML source2×1016neq/cm2 in the innermost layer, along with the very high occupancies, calling for higher granularity and a new generation of read-out chips for the inner layers. The current baseline layout planned is depicted in Fig. 1. The barrel consists of four pixel layers, with a minimal radius around 39 mm and a maximal radius around 250 mm. Six pixel discs are foreseen for the forward region, i.e. at a pseudorapidity of about 1.8≤|η|≤2.81.8≤|η|≤2.8. Depending on performance simulations, it is planned to increase the radius even further, or to add an additional fifth pixel layer.

Planar pixel sensors are a well understood and established technology, which exhibits excellent performance within the present tracking detectors of the ATLAS and CMS experiments. The latest results obtained by the ATLAS Planar Pixel Sensor R&D Project and presented here imply a good performance also after the high irradiation levels expected after the upgrades of the LHC accelerator complex. Furthermore, they offer the cost-effectiveness needed for the large instrumented areas foreseen in the upgrades of the pixel systems of the LHC experiments.