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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

Detector performance features

Matthew Whyndham

MSSL

detector effects
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

basic operation
Basic operation

window

anodes

Readout cathode

ReSIK Eng/Sci Meeting SRC Wroclaw

papers
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

detection of photons
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

position sensitivity
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

energy sensitivity
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

calibration data
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

high rate operation
High rate operation
  • deadtimes in electronics
  • gain depression
  • image distortion

ReSIK Eng/Sci Meeting SRC Wroclaw

dead times in electronics
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

gain depression
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

image distortion
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

summary
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