Status of the BELLE II Pixel Detector

The Belle II experiment at the super KEK B-factory (SuperKEKB) in Tsukuba, Japan, has been collecting $e^+e^−$ collision data since March 2019. Operating at a record-breaking luminosity of up to $4.7×10^{34} cm^{−2}s^{−1}$, data corresponding to $424 fb^{−1}$ has since been recorded. The Belle II VerteX Detector (VXD) is central to the Belle II detector and its physics program and plays a crucial role in reconstructing precise primary and decay vertices. It consists of the outer 4-layer Silicon Vertex Detector (SVD) using double sided silicon strips and the inner two-layer PiXel Detector (PXD) based on the Depleted P-channel Field Effect Transistor (DePFET) technology. The PXD DePFET structure combines signal generation and amplification within pixels with a minimum pitch of $(50×55) μm^2$. A high gain and a high signal-to-noise ratio allow thinning the pixels to $75 μm$ while retaining a high pixel hit efficiency of about $99$. As a consequence, also the material budget of the full detector is kept low at $≈0.21$$\frac{X}{X_0}$ per layer in the acceptance region. This also includes contributions from the control, Analog-to-Digital Converter (ADC), and data processing Application Specific Integrated Circuits (ASICs) as well as from cooling and support structures. This article will present the experience gained from four years of operating PXD; the first full scale detector employing the DePFET technology in High Energy Physics. Overall, the PXD has met the expectations. Operating in the intense SuperKEKB environment poses many challenges that will also be discussed. The current PXD system remains incomplete with only 20 out of 40 modules having been installed. A full replacement has been constructed and is currently in its final testing stage before it will be installed into Belle II during the ongoing long shutdown that will last throughout 2023

Thermal mock-up studies of the Belle II vertex detector

The ongoing upgrade of the asymmetric electron–positron collider SuperKEKB at the KEK laboratory, Japan aims at a 40-fold increase of the peak luminosity to $8  ×  10^{35}  cm^ {−2} s ^{−1}$ . At the same time the complex Belle II detector is being significantly upgraded to be able to cope with the higher background level and trigger rates and to improve overall performance. The goal of the experiment is to explore physics beyond the standard model with a target integrated luminosity of 50 ab$^{−1}$ in the next decade. The new vertex detector (VXD), comprising two layers of DEPFET pixel detectors (PXD) surrounded by 4 layers of double sided silicon strip detectors (SVD), is indispensable for vertex determination as well as for reconstruction of low momentum tracks that do not reach the central drift chamber (CDC). Within the confined VXD volume the front-end electronics of the two detectors will dissipate about 1 kW of heat. The VXD cooling system has been designed to remove this heat with the constraint to minimize extra dead material in the physics acceptance region. Taking into account additional heat intake from the environment the cooling system must have a cooling capacity of 2–3 kW. To achieve this goal evaporative two-phase CO$_2$ cooling in combination with forced $N_2$ flow is used in the VXD cooling system. In order to verify and optimize the cooling concept and to demonstrate that acceptable operation conditions for the VXD system as well as the surrounding CDC can be obtained, a full size VXD thermal mock-up has been built at DESY. Various thermal and mechanical tests carried out with this mock-up are reported

A custom setup for thermal conductivity measurements

Future detector systems have increasing demands on the performance of their mechanical support structures and cooling systems. Novel materials and cooling technics are developed and continuously improved in order to fulfil these requirements. To quantify the thermal performance of these materials, a custom thermal conductivity measurement setup was developed.The setup consists of two heat flux meter blocks between which the samples are clamped. Each of the blocks has six temperature sensors embedded at equally spaced positions that allow to measure the heat flux through as well as the temperature gradient across the sample. A resistive load on top of the upper block acts as a heat source whereas the bottom block is thermally coupled to a cooling plate which acts as the heat sink. In order to minimize heat exchange between the heat flux blocks and the ambient via convection and radiation, the setup is covered with a radiation shield and measurements are carried out in a vacuum.The contribution will describe the setup in detail, motivate its design aspects and highlight the commissioning and calibration procedure. The analysis method as well as selected results from the currently ongoing measurement campaigns will be presented

Radiation qualification of thermal interface materials for detector cooling

Silicon sensor based particle detectors operated in an hadronic radiation environment need to be cooled to counteract the radiation induced leakage current and prevent thermal runaway. To achieve this most efficiently, a low thermal resistance is required between the detector modules and the cooling structures. In many cases dry thermal contacts are sufficient, but especially for large area contact so-called thermal interface materials (TIM) - of which many products are available on the market - are the preferred choice. However, in the use case for detector cooling there are many requirements, such as no liquid, no heat cure, low thermal impedance, no compression force, radiation hardness, making it more difficult to find a suitable TIM. An example use case is the cooling of the CMS Phase-2 Outer Tracker PS modules. Its entire underside of 5 x 13 cm must be thermally coupled to the mechanics. The current candidate materials are room temperature curing two component thermal gap fillers.The contribution will outline the measurements and highlight the results to qualify gap filler materials to the radiation dose expected for the lifetime of the CMS Outer Tracker. Three different types have been tested thermally and mechanically in this campaign.The thermal test setup determines the thermal conductivity of a test sample by measuring the temperature gradient with a controlled amount of heat flow through a sample. The development and calibration of this custom thermal conductivity measurement setup is detailed in a separate contribution to this conference.Mechanical tests are needed to ensure structural integrity of the thermal interface even when under some extent of thermal stress. Since the gap fillers can not be considered glues in classical sense, the standard lap shear and peel tests can't be used for qualification. Resembling the style of an ISO 4587 lap shear test, and an ISO 25217 mode-1 fracture test, test samples were made with a large 5 x 5 cm adhesion overlap using plasma cleaned carbon fibre plates to have a surface comparable to its intended use case. The testing method developed for this study will be presented and motivated.After testing of unirradiated samples, they have been irradiated to 600 kGy. The measured mechanical and thermal properties will be presented and the results before and after irradiation will be compared. We found that the gap filler material hardens significantly, however its thermal and adhesive properties are maintained. The hardening reduces the cohesion failure, leading to an increased mechanical strength

Thermal qualification of TEDD dees using infrared measurements

The instantaneous luminosity at LHC will increase by 2.5-4 times during Run-4 and Run-5 (HL-LHC). This poses new challenges for the tracker, such as higher pileup and track multiplicity. A new silicon tracking system designed to cope with the increased luminosity will be installed during the Phase-2 upgrade of the CMS detector. The new tracker has five double-disks (TEDD) in each endcap and each double-disk comprises four half disks (dees). A dee consists of two carbon fiber facings with carbon foam blocks sandwiched in between. The pixel strip (PS) and 2-strip (2S) silicon modules are mounted onto the surface of the dees.The PS and 2S modules need to be cooled efficiently to avoid thermal runaway. The carbon fiber facings function as cooling surfaces for the PS modules. Cooling pipes are embedded in the carbon foam blocks (which provide thermal contact between the facings and the pipes) to cool the dee facing. The 2S modules are cooled via aluminium inserts which connect the module to the cooling pipes. These inserts are embedded in the dee and protrude the facing.The integrity of the carbon foam blocks and the proper gluing of the carbon foam blocks and the 2S inserts with the carbon fiber facings play a crucial role in cooling the modules. Hence this needs to be validated during the reception test of the dees. A novel testing procedure using infrared imaging has been developed for this purpose and its capability has been demonstrated by thoroughly testing on prototype dees. The details of this procedure and measurement results of the prototype dees will be discussed in this poster

CMS Phase-2 Tracker endcap module cooling

For the high-luminosity LHC (HL-LHC), CMS will install a completely new silicon tracker. The future outer tracker will consist of two barrel parts and two endcaps (TEDD), one on each side. One endcap is made of five double-disks. One double disk is assembled from four half disks (Dees) on which the detector modules are mounted. The Dees are a highly embedded carbon fiber and foam sandwich with integrated cooling pipes and module positioning inserts.Due to its large and homogeneous power density, the PS detector modules need to be cooled from their entire underside of about 5 x 13 cm2 area. The carbon fiber facings of the Dees act as cooling surface. Carbon foam blocks are glued to the embedded cooling pipes and to the facing to facilitate the cooling of the Dee surface. The integrity of the carbon foam blocks and the proper gluing to the facing is important to establish the necessary cooling contact and needs to be validated during the Dee reception testing. A test system using infrared imaging has been built to discover non-conformities that would lead to a deteriorated cooling performance. The capabilities of this system has been demonstrated by extensively studying the Dee prototype. The infrared measurement setup will be presented and results obtained from measurements of prototypes will be discussed.One challenge is the identification of a thermal interface material (TIM) conforming to the requirements. The TIM must have a low thermal resistance even when used without pressure, re-workable in a potential module exchange, be radiation hard to the expected dose levels and an application technique has to be found, respecting the constrains of the handling of the fragile modules. Several candidate materials are being studied, with a focus on a two component self-curing thermal gap filler. The thermal performance and the mechanical properties of the TIM is being studied in preparation of an irradiation campaign to verify the material parameters at the end of life of the experiment. In this context a new thermal conductivity measurement setup has been built, commissioned and used to quantify the thermal performance of the candidate materials. The thermal conductivity measurement setup will be presented and the results will be discussed. The results of the mechanical testing will be presented as well as the plans and tests for the application of the material when integrating detector modules

Operational Experience and Performance of the Belle II Pixel Detector

The Belle II experiment at the super KEK B factory (SuperKEKB) started its physics operation with the full detector setup in March 2019, and it aims at collecting 50 ab$^{−1}$ of $e^+e^−$ collision data. The vertex detector (VXD) of Belle II contains a 4-layer silicon vertex detector (SVD) using double sided silicon strips and an inner 2-layer pixel detector (PXD) that is based on the depleted P-channel Field Effect Transistor (DEPFET) technology. The signal generation and amplification are combined in pixels with a minimum pitch of 55 × 50 µm$^2$. The sensors are thinned down to 75 µm, and each module has interconnects and ASICs integrated on the sensor with silicon frames for mechanical support. This approach led to a material budget of around 0.21% X$_0$ per layer including the cooling structure in the acceptance region. The PXD has an integration time of around 20 µs, a signal-to-noise ratio of around 50 and a detecting efficiency of better than 99%. Its two layers are arranged at the radii of 14 and 22 mm around the interaction point, and an impact parameter resolution of better than 15 µm has been achieved. Due to its close proximity to the beam line and its sensitivity to few-keV photons, the PXD also plays an important role in background studies

Data quality monitors of vertex detectors at the start of the Belle II experiment

The Belle II experiment features a substantial upgrade of the Belle detector and will operate at the SuperKEKB energy-asymmetric e+e− collider at KEK in Tsukuba, Japan. The accelerator completed its first phase of commissioning in 2016, and the Belle II detector saw its first electron-positron collisions in April 2018. Belle II features a newly designed silicon vertex detector based on double-sided strip layers and DEPFET pixel layers. A subset of the vertex detector was operated in 2018 to determine background conditions (Phase 2 operation). The collaboration completed full detector installation in January 2019, and the experiment started full data taking. This paper will report on the final arrangement of the silicon vertex detector part of Belle II with a focus on online monitoring of detector conditions and data quality, on the design and use of diagnostic and reference plots, and on integration with the software framework of Belle II. Data quality monitoring plots will be discussed with a focus on simulation and acquired cosmic and collision data

Alignment for the first precision measurements at Belle II

International audienceOn March 25th 2019, the Belle II detector recorded the first collisions delivered by the SuperKEKB accelerator. This marked the beginning of the physics run with vertex detector.The vertex detector was aligned initially with cosmic ray tracks without magnetic field simultaneously with the drift chamber. The alignment method is based on Millepede II and the General Broken Lines track model and includes also the muon system or primary vertex position alignment. To control weak modes, we employ sensitive validation tools and various track samples can be used as alignment input, from straight cosmic tracks to mass-constrained decays.With increasing luminosity and experience, the alignment is approaching the target performance, crucial for the first physics analyses in the era of Super-BFactories. We will present the software framework for the detector calibration and alignment, the results from the first physics run and the prospects in view of the experience with the first data