Transcript JWST OVerview - RIT - Center for Detectors Site

JWST Radiation Environment
Don Figer (STScI)
Janet Barth, Ray Ladbury,
Jim Pickel, Robert Reed (GSFC)
March 13, 2003
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Ionizing Particle Impacts to FPA
primary
Surrounding Material
deltas
FPA
(latent emission)
secondaries
induced radioactivity
natural radioactivity
 Note that secondaries and delta electrons are time coincident
with primary and have limited range
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Primaries
1. Barth, Isaacs, & Poviey (2000): “The Radiation Environment for
the NGST”
2. the transient particles (TeV GCRs and GeV solar particles)
a.
protons and
b.
heavier ions of all of the elements of the periodic table
3. the trapped particles
a.
which include protons (100s of MeV)
b.
electrons (10 MeV) and
c.
heavier ions (100s of MeV)
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Primaries – Sunspot Cycle
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Secondaries
1. Single Event Effects (excluding detectors)
2. Single Event Effects (detectors)
3. Total Ionizing Dose
4. Displacement Damage
5. Spacecraft Charging
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Single Event Effects (excluding
detectors)

These effects result from interaction between a single energetic
particle and electronics.

Contributors here are the galactic cosmic rays and solar protons.

These environments are the same as those given in Barth et al.
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Single Event Effects (detectors)

These effects produce transient charge in detector pixels.
Effects in the MUX might be included in this category.

The secondary environment (possibly including radioactivation) may
become primary in this case. Proton, electrons and just about
anything else can cause glitches in the detectors by direct
ionization.
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Total Ionizing Dose

This is the accumulated detector exposure to particles.

Main contributors here are primary solar protons and the
secondary environment. Electrons in the geotail (and maybe even
from the Jovian magnetosphere) may contribute somewhat, but
these are thought to be mostly low energy, and the environment
is not well characterized at L2 in any case.

Note that this affects detectors, optics (increased absorption or
phosphorescence) and electronics.

5 year dose is 18 krad-Si and 10 year dose is 24 krad-Si with
zero margin.
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Displacement Damage

This is long-term damage due to crystal lattice disruption.

The main contributors here are solar protons, although for some
applications, secondary neutrons may also be important.
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Spacecraft Charging

This is the charge that the spacecraft gradually accumulates.

While not normally covered in radiation analyses, but it is a
threat (or threats) that is caused by the radiation environment.

The important environment here is the flux electrons as a
function of electron energy.

This environment is not at all well understood.

We know the geotail contributes, but that's a rather dynamic
region that isn't particularly well modeled. And the Jovian
electrons? Who knows? The physics argues that the electrons
will be low energy, but L2 is really beyond the point where we
know "there be dragons".
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Primary GCR Hit Amplitude Distribution for Detectors
(NOVICE Calculations)
- No Secondary Particles or Delta Electrons Single Cell
1E-04
Si: 25x25x15
InSb: 25x25x8
HgCdTe: 18x18x10
Events/s > Q
1E-05
1E-06
1E-07
1E-08
1E+00
1E+01
1E+02
1E+03
1E+04
1E+05
1E+06
Q (e)
Rate will be higher when secondaries, diffusion and radioactivity are included
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Primary GCR Hit Amplitude Distribution for Array
(NOVICE Calculations)
2K by 2K Array
1E+03
Si: 25x25x15
InSb: 25x25x8
HgCdTe: 18x18x10
Events/s > Q
1E+02
1E+01
100 events/s x 1000 s = 1e5 contaminated pixels
1E+00
2kx2k = 4.19e6 pixels
1e5 / 4.19e6 = 2.4 %
1E-01
1E+00
1E+01
1E+02
1E+03
1E+04
1E+05
1E+06
Q (e)
There is considerable uncertainty in region below 1000 eRate will be higher when secondaries, diffusion and radioactivity is included
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Studies
 Rauscher et al. (2000)
– Used SIRTF/IRAC data to model effects of cosmic rays on
sensitivity for NIRCam
– Estimated that 20% of pixels will be affected by cosmic rays in
1000 seconds
– Found negligible impact by cosmic rays if data are sent to the ground
every 125 seconds
 Regan and Stockman (2001)
– Modeled S/N for three read modes in detector-limited case: long
integration, MULTIACCUM, short coadded integrations
– Found MUTLIACCUM with total exposure of several thousand
seconds to be most effective
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SIRTF/IRAC
 Rauscher et al. (2000)
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JWST/IDTL
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Ideal Readout Modes
 Regan et al. (2001)
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HST/NICMOS
 Persistence from cosmic ray hits (post-SAA passage)
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HST/NICMOS
 On-board cosmic-ray rejection has been “rejected” on NICMOS because:
1. The detector instabilities were such that in order to derive "good" slopes in the early
readouts, we needed to add in a delay between the flush and the initial (zeroth) read of on
the order of 30 seconds to let the detector stabilize. This greatly reduced the usefulness
of RAMP mode for extending the dynamic range in the final image, as any object that was
was relatively bright would be saturated before a good slope could be produced (at least
four, preferably more, readouts).
2. I believe there were (are) some problems still remaining with the slope calculation
and/or the variance calculation in the FSW, and there was no acceptable solution offered.
These are the reasons I can remember -- but I think there may be more reasons, too. The
obvious source for "definitive" information is Glenn Schneider at UofA.
Of course, the actual reason RAMP is not used now is that it has been removed from the
ground system (it is no longer supported by TRANS; we did remember to capture the
TRANS requirements before removing them, though!). :)
Courtesy – Wayne Baggett
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JWST Radiation Efforts
 Radiation Effects and Analysis Group (REAG, GSFC), Robert Reed
– Effects of materials on secondary environment
– Effects of secondaries and primaries on detector functions and
performance.
– Effects of radiation on electrical devices, i.e. RAM
 Sensors and Instrumentation Branch (ARC), Craig McCreight
– Effects of proton beam irradiation (UC, Davis) on JWST prototype
detectors
 Independent Detector Testing Lab (IDTL, STScI/JHU), Don Figer
– Effects of gamma source (Cf-252) irradiation on JWST NIR
prototype detectors
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Summary
 Primary radiation environment at L2 has ~5 ions/cm2/s
 Secondary radiation environment will be determined by hardware
designs
 Detector susceptibilities will be determined by ground tests
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