Gain Sag in the COS FUV
Detector
David Sahnow

Microchannel  Plates  101
Wiza, NIM 162 (1979)
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Pulse  Height  and  Gain
•   The  MCP  gain  is  the  number  of  electrons  output  for  each   input  photon.  Typically  ~10 7  for  Delay  Lines.
•   The  Pulse  Height  of  an  event  is  the  (5-­‐bit)  digiHzed  value  of   the  gain.  It  is  saved  with  the  photon  posiHon  in  TIME-­‐TAG   mode.
•   The  Pulse  Height  DistribuHon  (PHD)  is  the  distribuHon  of  pulse   heights,  and  is  typically  characterized  by  its  modal  gain  and   width.
•   Gain  is  a  funcHon  of  high  voltage  on  the  MCPs,  properHes  of   the  glass,  number  of  MCPs,  etc.
•   PHD  and  electronics  were  matched   before  launch  for  the  best  overall   performance.
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XDL  Anode
•   Not  a  CCD  (not  even  a  MAMA).
•   PosiHon  of  photon  event  is  determined  by  the  Hme  it  takes   for  the  event  to  propagate  along  the  anode,  which  means  it  is   dependent  on:
–   ProperHes  of  anode
–   Aging  of  electronic  components
–   Temperature
–   Size  of  charge  cloud
–   etc.
•   Requires  correcHons  for  geometric,  thermal  (and  other?)   distorHons.
•   Analog  process
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XDL  Anode
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Segment  A  Cumula=ve  Image
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Segment  B  Cumula=ve  Image
Back
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Segment  B  Cumula=ve  Image
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Cumula=ve  Counts
•   Ly-­‐α  airglow  can  appear  at  20  different  X   posiHons  on  Segment  B.
•   CumulaHve  exposure  at  any  given  posiHon   depends  on  graHng,  central  wavelength,  FP-­‐
POS,  exposure  Hme,  etc.
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Integrated  Counts
Back
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Types  of  Gain  Sag
•   Short-­‐term
–   When  counHng  at  high  rates,  electrons  cannot  be   replenished  fast  enough,  and  the  number  of   electrons  per  event  decreases.
•   Long-­‐term
–   Exposure  to  photons  leads  to  a  decrease  in  the     secondary  emission  coefficient  of  the  glass,  and  a   drop  in  the  gain.
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A  Tale  of  Two  (Super)pixels
CI
Int
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Modal  Gain
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Gain  vs.  Exposure
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FUSE  Gain  Sag
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Effects  on  the  Data
•   Loss  of  events
–   Loss  of  Photons
•   Recently  adjusted  lower  pulse  height  threshold  from  4  to  2
•   Plan  to  add  a  posiHon-­‐dependent  threshold
–   Detector  background
•   Add  a  Hme-­‐dependent  scale  factor  to  the  background  in  calcos
•   Y  Walk
–   Flat  Field
•   Add  a  Y  walk  correcHon  to  calcos
–   Spectroscopic  Target  AcquisiHon
•   Recommend  other  ACQ  types
•   Correct  on  board
•   X  Walk  (Maybe)
–   Decrease  in  Resolving  Power
•   Add  an  X  walk  correcHon  to  calcos
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Holes
Gain sag holes
Blue !
PHA=[2,30] Red !
PHA=[4,30]
G160M/1577 data from program 12424, obtained on
Dec 22 nd 2010.
Back
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Dark  Rate
Back
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Y  Walk
Back
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What  Can  We  Do  About  It?
•   Change  the  lower  pulse  height  threshold  (Done,  21  December
2010)
–   Advantages:
•   Quick  and  easy
–   Disadvantages:
•   Possible  change  in  flux  calibraHon
•   Increased  background
•   Live  with  the  holes
–   Advantages:
•   No  changes  to  operaHons,  etc.
–   Disadvantages:
•   CalibraHon  becomes  increasingly  difficult
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What  Can  We  Do  About  It?  (2)
•   Increase  the  High  Voltage  (Did  a  test  last  month;  did  this   regularly  on  FUSE)
–   Advantages:
•   RelaHvely  quick  to  change
•   Not  much  addiHonal  calibraHon  required
–   Disadvantages:
•   Very  likle  ground  tesHng  done  at  higher  voltage  levels
•   Possible  increase  in  HV  Transients
•   Move  to  another  lifeHme  posiHon  (Did  a  parHal  test  in  March)
–   Advantages:
•   Four  (or  so)  more  posiHons  available
–   Disadvantages:
•   Requires  a  test  to  determine  where  to  move
•   More  extensive  calibraHon  may  be  required
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HV  Test
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