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Fast Timing with Diamond Detectors

Fast Timing with Diamond Detectors. Lianne Scruton. Outline. Why do we need fast timing? The LYCCA array What makes diamond an excellent timing detector? Constructing and testing the diamond detector The diamond detector with LYCCA Comparing diamond with plastic scintillator

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Fast Timing with Diamond Detectors

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  1. Fast Timing with Diamond Detectors LianneScruton

  2. Outline • Why do we need fast timing? • The LYCCA array • What makes diamond an excellent timing detector? • Constructing and testing the diamond detector • The diamond detector with LYCCA • Comparing diamond with plastic scintillator • Future plans for LYCCA

  3. Fast Timing: Why? • Quite simply: for identification • A better timing resolution makes the time-of-flight measurements more precise. • High rate measurements possible. Stop Signal Start Signal Target

  4. What are we using it for? • FAIR facility under construction at GSI July 2013

  5. What are we using it for? • FAIR facility under construction at GSI FRS

  6. What are we using it for? • The Super-FRS will allow for cleaner, more intense secondary beams. • The HISPEC (High-resolution In-flight SPECtroscopy) campaign will be located at the end of the Super-FRS. • HISPEC will focus on in-flight decays of exotic nuclei to study collective motion, position of neutron dripline and much more. • Need something to track and identify these exotic nuclei.

  7. Introducing: The LYCCA Array • LYCCA: Lund-York-Cologne Calorimeter • The design of LYCCA is based on CATE (CAlorimeterTElescope). CsI array for residual E Si array for ΔE (Lozeva 2006)

  8. Simulations • Simulations performed by M J Taylor showed that including Time-of-Flight (ToF) detectors improved identification Data from CATE (Taylor 2009)

  9. Simulations • Simulations performed by M J Taylor showed that including Time-of-Flight (ToF) detectors improved identification (FWHM) (Taylor 2009)

  10. Simulations • Simulations performed by M J Taylor showed that including Time-of-Flight (ToF) detectors improved identification (FWHM) (Lozeva 2006)

  11. The LYCCA Array Secondary Target Start Detector Stop Detector CsI scintillator for residual E 3.6 m Si DSSSD for tracking Si DSSSDs for ΔE and tracking

  12. The LYCCA Array • LYCCA energy detectors make up a modular wall that can be arranged into different configurations • Maximum number of modules is 26 which covers an area of over 1000 cm2

  13. The LYCCA Array

  14. Semiconductor Detectors • Heavy charged particles pass through the semiconductor and interact with electrons in the material via the Coulomb interaction. • Electrons are excited across the band gap into the conduction band creating electron-hole (e-h) pairs. • Applying a bias across the semiconductor allows electrons and holes to travel to opposite contacts, inducing charge on contacts. + 600 V Contact e- e- e- Semiconductor h+ h+ h+ 0 V Contact

  15. Signal Generation • Charge builds up on contacts until charge carrier motion ceases. Tstart Tstart

  16. Signal Generation • Rise time of current pulse is unaffected by the interaction point.

  17. Why Diamond? • Diamond has a number of properties that are advantageous for timing measurements: • Electrons and holes have high and similar mobilities • High optical phonon energies lead to high saturation velocity • Wide band gap – low dark current and noise • Diamond has a low dielectric constant – small capacitance • Resultant current pulse is short with a large amplitude and a rise time with a steep gradient.

  18. Polycrystalline Diamond • Large size of diamond start detector meant that polycrystalline diamond must be used which contains grain boundaries. • Other impurities (B, N etc.) can also act as traps. (Hammersberg 2001)

  19. Polarisation Fields • For the best timing, we need to limit the number of impurities and grain boundaries. • Trapped charge carriers can no longer contribute to the signal current. • Polarisation field set up by trapped holes and electrons • This field opposes the electric field, reducing velocity of charge carriers.

  20. Constructing the Diamond Detector • Diamond detector consists of a 300 μm-thick diamond wafer sandwiched between two metallic contacts (Pt/Au, Au or Al). • Top contacts are divided into four 18 x 4.5 mm2 strips to reduce the capacitance associated with the detector. • Contacts with different pad sizes were used to test the effects of different capacitance on pulse.

  21. B’ham Optimisation Test • 50-MeV He4 beam scattered from Pb target. 14.6 pF 8.11 pF 14.6 pF 1.95 pF

  22. B’ham Optimisation Test

  23. LYCCA and the Diamond detector • Diamond start detector placed ~5cm downstream of target. • Plastic scintillator used as second ToF option with start and stop scintillators upstream and downstream of target.

  24. Commissioning Experiment – Sep 2010 DSSD Wall DSSD DSSD 3.6 m Diamond ToF 4.3 m CsI Plastic ToF

  25. Identifying the Fragments

  26. Mass Measurements • Beam velocity, β, and energy used to calculate fragments on an event-by-event basis. Diamond ToF Plastic ToF Mass res = 1.27 u Mass res = 0.55 ± 0.02 u (FWHM) Time res = 50.8 ± 2.4 ps (FWHM)

  27. What Went Wrong? • Time resolution of diamond detector = 193.0 ± 25.6 ps • Resolution of 104 ps obtained at Texas A & M test…

  28. What Went Wrong? • Cables used between detector and preamp were ~2 m long. Added unwanted capacitance to detector circuit. • Gradient of signal is shallow and noisy. • Noise described as: • (Ciobanu 2011) • Require electronic development to overcome this issue.

  29. The New Plastic Scintillators • Decision made to use plastic scintillator for LYCCA ToF. • 1 PMT has poor timing resolution, but 56 measurements of same event improves resolution .

  30. Summary • Fast timing detectors require current signals that are large and short with a fast rise time. • Fast charge carrier mobilities, high saturation velocity and low dielectric constant make diamond ideal for fast timing. • Diamond detector and plastic scintillator timing options were compared in first LYCCA commissioning experiment. • Clear isotopic resolution obtained using plastic scintillator timing , but poor resolution for diamond ToF. • Poor performance of diamond attributed to the necessarily long cable lengths used between detector and preamplifier. • New plastic stop scintillator detector under development here at York for use with LYCCA at FAIR.

  31. References Thank you for Listening • R Lozevaet al.Nuclear Instruments and Methods A, 562, pg 298-305 (2006) • M J Taylor et al.Nuclear Instruments and Methods A, 606, pg589-597 (2009) • J Hammersberget al.Diamond and Related Material, 10, pg574-579 (2001) • M Ciobanuet al.IEEE Transactions on Nuclear Science, 58, pg2073-2083 (2011)

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