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Radiation Backgrounds in the ATLAS New Small Wheel

This report discusses the radiation backgrounds in the ATLAS New Small Wheel (NSW) and the effects of added shielding on rates and doses. The study examines the highest background rates and doses in the NSW for the initial design and an improved design. Conclusions are drawn on the reductions achieved with added shielding and further areas of study are suggested.

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Radiation Backgrounds in the ATLAS New Small Wheel

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  1. Radiation Backgrounds in the ATLAS New Small Wheel Erich Varnes and Michael Shupe, University of Arizona July 17, 2016 The ATLAS Radiation Backgrounds Working Group is focusing on detector subsystem upgrades for high luminosity operation of the LHC in Phase 2. The Arizona group has long experience with the FCal region, the Small Wheel, and ATLAS radiation shielding. In tandem with other radiation backgrounds groups at Sheffield, SLAC, and CERN, we are updating our simulation geometries to match the detector revisions for Phase 2. At this point, we have updated the beamline (matching Sheffield), implemented a simple model of the New Small Wheel (NSW), and modified the JD shielding. We have also implemented an improved version of the JD shield, with additional shielding disks, moderators, and Pb, following the latest SLAC geometry. In this report, we study the highest background rates and doses in the NSW for the initial JD design, and for the improved design from SLAC.

  2. ATLAS Baseline Geometry vs JD Shield upgrade Phase2 Baseline SLAC JD shields added NSW NSW Added Shielding FCal JD FCal JD The SLAC shielding includes (1) additional rings of material on the front of the flux return plate, (2) BPE and Pb layers at the inner front corner of the NSW, and (3) BPE layers at the back of the JD hub and on the JD core outer surface.

  3. NIEL Dose [1 MeV equiv N/cm^2/3000/fb]: Base Geom vs JD Shield upgrade Phase2 Baseline NSW SLAC JD shields added JD JD FCal FCal The additional SLAC shielding pieces reduce the maximum rates and doses in the small wheels. The next slide shows the reduction of the dose.

  4. NIEL Dose For 3000/fb In First Wheel At Inner Radius 94 cm Base JD: 1.95E14/cm^2 SLAC JD: 5.94E13/cm^2 Black – Baseline Red – SLAC JD shielding

  5. SEU Rates: Ppi>10MeV + N>2MeV [/cm^2/3000/fb]_In the NSW ATLAS 2013 ATLAS Baseline NSW SLAC JD shields added NSW JD JD FCal FCal SEU rates in the NSW are not very sensitive to changes in the JD shield. The NSW rates themselves are of interest for operation of the electronics and detectors.

  6. SEU 2MeV Dose For 3000/fb In First Wheel At Inner Radius 94 cm Base JD: 2.33E13/cm^2 SLAC JD: 2.23E13/cm^2 Black – Baseline Red – SLAC JD shielding

  7. Approximate Single Plane MM Counting Rates [kHz/cm^2] _In the NSW ATLAS 2013 ATLAS Baseline NSW SLAC JD shields added NSW JD JD FCal FCal The highest single plane counting rates in the NSW would be at the front, and near the inner radius, of the MicroMega (MM) detectors in the first wheel.

  8. Single Plane Hit Rate In First Wheel At Inner Radius 94cm From Run 1 data, the expected maximum hit rate is 15 kHz/cm^2. For Phase 2, our hit estimator will need to be retuned. Base JD: 58.6 kHz/cm^2 SLAC JD: 15.2 kHz/cm^2 Black – Baseline Red – SLAC JD shielding

  9. Conclusions from Studies of Rates and Doses in the New Small Wheel, and Their Reduction with Added JD Shielding Max dose reductions with SLAC geometry volumes added to the JD: * NIEL dose in the NSW is reduced by a factor of 3.3. Dose looks OK. * SEU dose is little reduced (1.04) since low energy particles are ignored. * Single-plane hit rates are reduced by 3.9. But at max they are close to the design limit anticipated in the NSW TDR. Further study is needed. Future studies: -> Detector responses to incoming particles need to be updated by simulation or test beam measurements, to refine the estimation of hit rates in new Phase 2 detectors. -> The VI beampipe will need to be updated again if the 3-segment scheme is adopted. -> Final studies cannot go forward until the ITk design is finalized, and translated into simulation geometries.

  10. Radiation Background Normalizations In AZ Phase 2 Studies As in past radiation background studies, the CM energy is 14 TeV. But we have changed the normalization of the flux and dose maps and histograms to match Phase 2 conditions. Up to 2015 studies used the luminosity 10^34 [cm^2/s]. But we now use the Phase 2 levelled luminosity of 5 X 10^34. This factor of 5 affects all rate calculations. In the past, doses were for one running year of 10^7 s, leading to an integrated luminosity of 100/fb. But now we are reporting doses for the full Phase 2 expectation of 3000/fb, increasing the doses by a factor of 30.

  11. Files Used For Studies Presented Here This report contains only a small fraction of the information coming from these simulations. All studied rates and doses are available at the site below, in the subdirectories listed on the next slide. Location of files: http://atlas.physics.arizona.edu/~shupe/Cavern_Backgrounds_Phase2/ Select Study: Phase2_NBP_NSWA_Baseline_14TeV, or Phase2_NBP_NSWA_SLACshie_14TeV Then select: FLUXPLOTS or FLUXTEXTS Available doses and particle fluxes: Detector impact: energy deposition, NIEL dose, ionizing dose, hadrons > 20 MeV, SEU rates, star densities (for activation). Particles: total neutrons, neutrons > 100 keV, thermal neutrons, photons, electrons, protons, charged pions, muon single particle detector rates. 11

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