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  1. Detector performance features Matthew Whyndham MSSL

  2. Basic operation quantum efficiency pulse height distribution gain vs HV Position sensitivity linearity digitisation effects Energy sensitivity linearity use of discriminators RESIK high-order operation High rate operation gain depression image distortion Detector effects ReSIK Eng/Sci Meeting SRC Wroclaw

  3. Basic operation window anodes Readout cathode ReSIK Eng/Sci Meeting SRC Wroclaw

  4. papers • [1] M. W. Trow, ESA-SP 356, 1992 • [2] Trow/Bento/Smith, Rate dependant image distortions in proportional counters, Nucl. Instr. Meth. A 328 (1994) (W. = Whyndham = Trow) ReSIK Eng/Sci Meeting SRC Wroclaw

  5. Detection of photons • Quantum efficiency peaks ~ 7 keV • low energy / long wavelength - drop due to window absorption • high energy - long range in gas • pulse height distribution • FWHM ~14% @6 keV • Ar-K and Xe-L escape peaks possible • Gain • logarithmic gain law log G = A + B.V • one HV step (30V) results in factor ~1.3 gain change • eight steps available in RAL HVU ReSIK Eng/Sci Meeting SRC Wroclaw

  6. Position sensitivity • 8-bit charge measurement • ~ 256 bins in optimal case • but SNR determines real performance • position determination • done by lookup table • stretched bins 45 10 • digitisation spikes near 1/2, 1/4, 3/4 • more prominent if PHD restricted or low ReSIK Eng/Sci Meeting SRC Wroclaw

  7. Energy sensitivity • initial number of ion/electron pairs strictly proportional to energy • but gain not equal for all energies • hence non-linear relation • fig 4 in PSD paper • 2/3 order - reduce HV • order 1 still present! • study PHA • adjust discriminators ReSIK Eng/Sci Meeting SRC Wroclaw

  8. Calibration data • latest data taken at RAL summer 2000 • scans of beam over detector length • slot-mask - 300 m slots - defines positions • beam normal to detector - minimal parallax • all energies used by RESIK • mostly low count rate • ADCs in appropriate analog-card • “best HV” in each case (Fe55 cal - lower HV) • RESIK-A = BCS-B card (3,4) • RESIK-B = BCS-A card (1,2) ReSIK Eng/Sci Meeting SRC Wroclaw

  9. High rate operation • deadtimes in electronics • gain depression • image distortion ReSIK Eng/Sci Meeting SRC Wroclaw

  10. dead times in electronics • anti-pileup deadtime of position-encoding logic of 32 s • extendable (paralysable) • affects entire detector (1,2) (3,4) • accounted for in Yohkoh software ReSIK Eng/Sci Meeting SRC Wroclaw

  11. gain depression • reduction in gain at high rate • effects pulse height distribution markedly- and therefore validity of discriminator settings • localised effect - wherever the intense line happens to be • function of charge (rate × gain × ph. Energy × conversion-constant) • operates at all rates but significant over 1000 c/s • compensate by raising HV or adjusting discriminators ReSIK Eng/Sci Meeting SRC Wroclaw

  12. image distortion • “attraction” of counts towards areas of highest rate • caused by viz [2] fig 1. • build-up of slow ions • establishment of axial field • distortion of electron path • total number of counts conserved • function of charge • probably detectable at all rates • significant over 103 c/s ReSIK Eng/Sci Meeting SRC Wroclaw

  13. Summary • Detector health • build plot of Fe55 cal fit params during pre-launch and mission • position-calibration • data analysis required • rate effects • deadtime : understood • gain depression / image distortion : complex picture • high order operation • observe PHA carefully • build experience progressively ReSIK Eng/Sci Meeting SRC Wroclaw