J. Hasi

The FP420 R&D project: Higgs and New Physics with forward protons at the LHC

We present the FP420 R&D project, which has been studying the key aspects of the development and installation of a silicon tracker and fast-timing detectors in the LHC tunnel at 420 m from the interaction points of the ATLAS and CMS experiments. These detectors would measure precisely very forward protons in conjunction with the corresponding central detectors as a means to study Standard Model (SM) physics, and to search for and characterise new physics signals. This report includes a detailed description of the physics case for the detector and, in particular, for the measurement of Central Exclusive Production, pp?p++p, in which the outgoing protons remain intact and the central system may be a single particle such as a SM or MSSM Higgs boson. Other physics topics discussed are ?? and ?p interactions, and diffractive processes. The report includes a detailed study of the trigger strategy, acceptance, reconstruction efficiencies, and expected yields for a particular pp?pHp measurement with Higgs boson decay in the b mode. The document also describes the detector acceptance as given by the LHC beam optics between the interaction points and the FP420 location, the machine backgrounds, the new proposed connection cryostat and the moving (``Hamburg) beam-pipe at 420 m, and the radio-frequency impact of the design on the LHC. The last part of the document is devoted to a description of the 3D silicon sensors and associated tracking performances, the design of two fast-timing detectors capable of accurate vertex reconstruction for background rejection at high-luminosities, and the detector alignment and calibration strategy.

First fabrication of full 3D-detectors at SINTEF

3D-detectors, with electrodes penetrating through the entire substrates have drawn great interests for high energy physics and medical imaging applications. Since its introduction by C. Kenney et al in 1995, many laboratories have begun research on different 3D-detector structures to simplify and industrialise the fabrication process. SINTEF MiNaLab joined the 3D collaboration in 2006 and started the first 3D fabrication run in 2007. This is the first step in an effort to fabricate affordable 3D-detectors in small to medium size production volumes. The first run was fully completed in February 2008 and preliminary results are promising. Good p-n junction characteristics have been shown on selected devices at the chip level with a leakage current of less than 0.5 nA per pixel. Thus SINTEF is the second laboratory in the world after the Stanford Nanofabrication Facility that has succeeded in demonstrating full 3D-detectors with active edge. A full 3D-stacked detector system were formed by bump-bonding the detectors to the ATLAS readout electronics, and successful particle hit maps using an Am-241 source were recorded. Most modules, however, showed largely increased leakage currents after assembly, which is due to the active edge and p-spray acting as part of the total chip pn-junction and not as a depletion stop. This paper describes the first fabrication and the encountered processing issues. The preliminary measurements on both the individual detector chips and the integrated 3D-stacked modules are discussed. A new lot has now been started on p-type wafers, which offers a more robust configuration with the active edge acting as depletion stop instead of part of the pn-junction.

Test Beam Characterization of 3-D Silicon Pixel Detectors

Three-dimensional (3-D) silicon detectors are characterized by cylindrical electrodes perpendicular to the surface and penetrate into the bulk material in contrast to standard Si detectors with planar electrodes on the top and bottom. This geometry renders them particularly interesting to be used in environments where standard silicon detectors have limitations, such as, for example, the radiation environment expected in an upgrade to the Large Hadron Collider at CERN. For the first time, several 3-D sensors were assembled as hybrid pixel detectors using the ATLAS-pixel front-end chip and readout electronics. Devices with different electrode configurations have been characterized in a 100 GeV pion beam at the CERN SPS. Here, we report results on unirradiated devices with three 3-D electrodes per 50times400 mum2 pixel area. Full charge collection is obtained already with comparatively low bias voltages around 10 V. Spatial resolution with binary readout is obtained as expected from the cell dimensions. Efficiencies of 95.9%plusmn0.1% for tracks incident parallel to the electrodes and of 99.9%plusmn0.1% for tracks incident at 15deg are measured. The homogeneity and charge sharing of the efficiency over the pixel area are measured.

3D Active Edge Silicon Detector Tests With 120 GeV Muons

3D detectors with electrodes penetrating through the silicon wafer and covering the edges were tested in the SPS beam line X5 at CERN in autumn 2003. Detector parameters including efficiency, signal-to-noise ratio, and edge sensitivity were measured using a silicon telescope as a reference system. The measured sensitive width and the known silicon width were equal within less than 10 mum.

Results from the first prototype of large 3D active edge sensors

3D active edge sensors have advantages such as radiation hardness and edgeless capability. With the use of deep reactive ion etching and wafer bonding, 18.5 by 20.5 mm2 3D detectors with active edges have been successfully fabricated at SINTEF MiNaLab. These sensors are compatible with the ATLAS FE-I4 readout electronics. Fabrication process and difficulties are presented and the preliminary electrical measurements are also discussed.

Active beam attenuators for synchrotron radiation

Attenuating materials are employed on synchrotron beam lines to achieve photon flux modulation as well as beam hardening of the x-ray spectrum. In some experiments it is desirable to maintain the intensity of the beam on the sample at a constant value as the current in the synchrotron decays during a fill cycle. This is often done by attenuating the incident beam with a set of discrete, thin foils. To change the amount of material in the beam path typically involves inserting or redrawing a number of foils. The use of discrete foils imposes practical limits on the number of thickness values available as well as the smallest thickness increment. Micro-machined attenuators can avoid these restrictions by offering either continuous variation of the material thickness or through the implementation of a large number of small thickness steps. Through a combination of photolithography and appropriate device geometry sub-micron thickness increments can be fabricated. Device geometries such as staircase, low-angle triangle, and overlapping triangles will be described. Fabrication of these devices by direct micro-machining of materials such as silicon as well as micro-molding of various polymers can be done relatively easily. In addition, by manufacturing a silicon diode into the attenuator the absorbed fraction of the beam can be continuously monitored. By using a feedback loop where the attenuator thickness is varied based on a downstream beam monitor, it should be possible to maintain the photon flux on a sample to vary by less than 0.5%. The performance of a variety of these devices at the Advanced Light Source is presented.

3D silicon sensors - Large area production, QA and development for the CERN ATLAS experiment pixel sensor upgrade

3D silicon sensors, where electrodes penetrate fully or partially through the silicon substrate, have been successfully fabricated in different processing facilities in Europe and the USA. They key to 3D fabrication is the use of plasma micromachining to etch narrow deep vertical openings which allow dopants to be diffused in and form the electrodes of the p-i-n junctions. Similar openings can be used at the sensors edge to reduce the perimeters dead area to be as narrow as 4 μm. Since 2009, four fabrication facilities of the 3D ATLAS R&D Collaboration started a joint effort aimed at one common design and compatible processing strategy for the production of 3D sensors for the LHC Upgrade and in particular for the ATLAS pixel Insertable B-Layer (IBL). In this project where the installation is aimed for 2013, a new layer will be inserted as close as 3.4 cm from the proton beams inside the existing pixel layers of the ATLAS experiment. The detector proximity to the interaction point will therefore require new radiation hard technologies for both sensors and front-end electronics. The latter, called FE-I4 is processed at IBM and is the biggest front end of its kind, with a surface area of about 4 cm2. This paper will discuss some design aspects, and the different approaches taken by the facilities. Results from both the qualification runs and the current production runs for the IBL are also reported.