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Detector performance features

Detector performance features. Matthew Whyndham MSSL. 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

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Detector performance features

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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

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