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Opposite Polarity Signals in Wide Pitch Sensors Jim Libby (CERN/SLAC)

A detailed exploration of opposite polarity signals in wide pitch sensors, focusing on R&D in LHCb, charge sharing, signal behaviors, and their implications. The study includes simulations, conclusions, and cross-check analysis.

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Opposite Polarity Signals in Wide Pitch Sensors Jim Libby (CERN/SLAC)

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  1. Opposite Polarity Signals in Wide Pitch SensorsJim Libby (CERN/SLAC) Introduction to the R&D in LHCb The test-beam and the data Opposite polarity signals Other detectors Simulation and conclusions Jim Libby (CERN/SLAC)

  2. Introduction to the R&D • Floating strip R&D has begun for the beyond the baseline VELO, which will be a replacement after 2-3 years • Will improve resolution of the outer region of the Φ-sensor • VELO important for tracking: will aid extrapolation to the rest of the spectrometer • Wanted to study wide pitch n-on-n detectors read out at 40 MHz • After irradiation needs to be studied as well • For more LHCb details see presentation of Juan Palacios Jim Libby (CERN/SLAC)

  3. Detector and data-set • Tested detector was a double-sided silicon strip manufactured by Canberra • ALICE prototype donated by P. Kuijer at NIKHEF • 128 n-strips with a pitch of 95 m • FOXFET biasing, with Vdep ~60-70V • Regions bonded with and without floating strips to a SCT128A hybrid • Operated at SPS in 120 GeV/ beam • Track extrapolation error 24 m • Sample of 90k events at Vbias=100 V Jim Libby (CERN/SLAC)

  4. Charge sharing: I Floating strip region • Measure: =Qleft/ Qleft+ Qrightfrom track intercept • No clustering • Clear charge sharing in region with floating strips compared to region with adjacent strips • For floating strips peaks below 0 and above 1 when charge collected on a single strip Adjacent strip region Jim Libby (CERN/SLAC)

  5. Charge sharing: II • The charge collected on the floating strip is ~60% of that collected on a connected strip • Corresponds to a Cb/Cinter-implant ~ 2.65 • In an LHCb design we would aim to reduce this • Pitch is wider in ALICE prototype than LHCb scenario (20-60 m) Jim Libby (CERN/SLAC)

  6. Opposite polarity signals: I • Look at relative charge on neighbouring connected strips • Large coherent deviations over many strips, and significant distances • Signals integrate to 0 within errors • As noted  biased to higher and lower values, when charge is collected mainly on left or right strip respectively • Correlation of opposite polarity signal on neighbouring strip 6% 1 mm Jim Libby (CERN/SLAC)

  7. Opposite polarity signals: II • Looked at adjacent strip region – a hint but limited statistics • Otherwise: • Looked at O(1 ms) integration times • Looked at 40 mm pitch • Electronics No indication of any effect See some dynamics as a function of sampling (TDC) time and voltage (Asynchronous trigger used) Jim Libby (CERN/SLAC)

  8. Other detectors: I • LHCb testing a GLAST detector with SCT128A readout for all Si tracking station (see F. Lehner) • 400 mm thick, p-on-n and 208 mm pitch • Strips studied are bonded consecutively • Approx 1 million triggers collected with tracks in the acceptance • Results from D. Eckstein • Analysis of neighbouring strips repeated • Interesting results as a function of sampling time Jim Libby (CERN/SLAC)

  9. Other detectors: II Central • Relative signals vary between ±13% • Near maximum sampling time effect is signal on neighbouring strips zero • No large distance effects Signal [ADC counts] Neighbour 80ns to 85ns 90ns to 95ns Signal/Signal Strip 0 Ratio Signal/Signal Strip 0 115ns to 120ns 125ns to 130ns Relative strip number TDC time [ns] Jim Libby (CERN/SLAC)

  10. Conclusions and simulation • Significant opposite polarity signals on neighbouring strips have been observed in a wide pitch n-on-n sensor with floating strips • Signals around -7% and over significant distances • Behaviour dependent on sampling time • Varying capacitances at LHCb make us sensitive to these effects • Unique to n-on-n: in a wide pitch p-on-n detector signals on neighbouring strips vary with time. No long distance effect. • An explanation with Ramo’s theorem(Brodbeck and Chilangarov NIM A395 (1997) 29) • Simulation (Univ. of Glasgow) underway Jim Libby (CERN/SLAC)

  11. Cross-checks • Integration time: LHCb n-on-n prototypes with ~90 m pitch readout with VA2 electronics 2.2 s shaping time • Fine pitch: LHCb n-on-n prototype with 40 m pitch readout with SCT128A No effect • Electronics: across chips and between chips • Analysis: without common-mode correction Effect remains Jim Libby (CERN/SLAC)

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