FLUX Particle Environment Models

At present, the CREME FLUX routine offers a choice of two different kinds of (non-trapped) ionizing-radiation models:

Solar Quiet (no "flare") Model: This model is appropriate for evaluating typical and long-term average particle fluxes and SEE rates.

Solar Energetic Particle ("flare") Model: This model is appropriate for evaluating "worst case" and "peak" particle fluxes and SEE rates.

As far as possible, these models are based on particle measurements at energies relevant to SEE effects, with only minimal use of theoretical descriptions. It is hoped that this approach will make these models significantly more reliable than previous efforts.

Some limitations of these environment models should be kept in mind:

• These models are based on measurements near Earth. They can probably be used in orbits out to Mars (~1.5 AU) but are not applicable to inner heliosphere or deep-space missions.
• Electrons, neutrons, and gamma-rays are not included in these models.

In general, electrons and gamma rays are not expected to be a significant cause of single event effects. Neutrons are absent from galactic cosmic rays and relatively rare in solar particle events.

However, it should be noted that a significant flux of neutrons can be locally produced by nuclear interactions of cosmic rays in thick shielding.

The present version of TRANS (which transports particle fluxes through shielding) does not track neutrons.

Like protons, neutrons can cause single event upsets via target-nuclei fragments and recoils. As a result, the omission of neutrons may lead to a significant underestimate of SEE rates in thickly-shielded proton-sensitive devices.

Similarly, these locally-produced neutrons (and other secondary particles) can dominate the total dose accumulated by crews living under thick-shielding during a long-duration mission (to Mars, for example).