JWST Radiation Environment Don Figer (STScI) Janet Barth, Ray Ladbury, Jim Pickel, Robert Reed (GSFC) March 13, 2003

Ionizing Particle Impacts to FPA primary Surrounding Material deltas FPA (latent emission) induced radioactivity secondaries natural radioactivity Note that secondaries and delta electrons are time coincident with primary and have limited range

Primaries Barth, Isaacs, & Poviey (2000): “The Radiation Environment for the NGST” the transient particles (TeV GCRs and GeV solar particles) protons and heavier ions of all of the elements of the periodic table the trapped particles which include protons (100s of MeV) electrons (10 MeV) and heavier ions (100s of MeV)

Primaries – Sunspot Cycle

Secondaries Single Event Effects (excluding detectors) Single Event Effects (detectors) Total Ionizing Dose Displacement Damage Spacecraft Charging

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. 

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.

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.

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. 

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".

Post-Barth et al. (from Ladbury) Barth et al. is structured to be "general" regardless of launch date and even mission duration.  Uncertainties in this document are still uncertain. Main advances are with respect to defining the secondary environments. There really is no single JWST radiation environment. the environment appropriate for each instrument is the sum of the transported Primary Environment and the Secondary Environment for JWST, the secondary particles may dominate the corruption of detector pixels, etc with regard to the primary environment, we tend to "construct" the environment appropriate for the threat we are dealing with.  Since there are really 4 threats (spacecraft charging, single-event effects, total-ionizing dose and displacement damage) we are looking at 4 different environments

Primary GCR Hit Amplitude Distribution for Detectors (NOVICE Calculations) - No Secondary Particles or Delta Electrons - Rate will be higher when secondaries, diffusion and radioactivity are included

Primary GCR Hit Amplitude Distribution for Array (NOVICE Calculations) 100 events/s x 1000 s = 1e5 contaminated pixels 2kx2k = 4.19e6 pixels 1e5 / 4.19e6 = 2.4 % There is considerable uncertainty in region below 1000 e- Rate will be higher when secondaries, diffusion and radioactivity is included

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

SIRTF/IRAC Rauscher et al. (2000)

JWST/IDTL

Ideal Readout Modes Regan et al. (2001)

HST/NICMOS Persistence from cosmic ray hits (post-SAA passage)

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

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

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