Detector%20performance%20features

1
Detector performance features

• Matthew Whyndham
• MSSL

2
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

3
Basic operation
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)

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 _at_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

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

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

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)

9
High rate operation

• deadtimes in electronics
• gain depression
• image distortion

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

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

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

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