Simulation of Radiation Effects on NGST Bryan Fodness, Thomas Jordan, Jim Pickel, Robert Reed, Paul Marshall, Ray Ladbury 1
Outline Introduction to Project Goals and Challenges Approach Preliminary Results Current and Future Efforts Summary * Currently we are still implementing GEANT 2
NGST Science Goals The main goal of the Next Generation Space Telescope is to observe the first stars and galaxies in the Universe to answer questions like: What is the shape of the universe? How do galaxies evolve? How did the universe build up its present elemental/chemical composition? 3
NGST Science Goals (cont.) Because of the remoteness of the earliest objects in the universe, these questions are easiest to answer by observing the universe in the infrared portion of the spectrum. NGST will be capable of detecting radiation whose wavelength lies in the range 0.6 to 20 microns (optimized for the 1 to 5 micron region). 4
Ionizing Particle Impacts to FPA primary Surrounding Material deltas secondaries FPA (latent emission) induced radioactivity natural radioactivity Note that secondaries and delta electrons are time coincident with primary and have limited range 5
Small Transients are Problematic Read noise goal is 3 electrons (requirement is 10) Ionization energies for 5µm HgCdTe and InSb detector material is on order of 1 ev/e - Need to deposit only 3 to 10 ev Characteristic pathlengths are on order of 10µm Need LET of only 0.3 ev/µm to 1 ev/µm Note that 1 GeV proton in HgCdTe has LET of 1.16 kev/µm Every primary and every secondary particle is problematic 6
IRFPA Transient Response Model Goal of Radiation Effects Group is to predict FPA response to incident particles (protons, heavy ions, electrons) with high fidelity Charge contamination on pixel-by-pixel basis Source term is external radiation environment and transport through material surrounding FPA Includes primary and secondary environment Includes decay of activated material and inherent radioactivity 7
Estimate Transient Rate, Pulse Height Distribution, Temporal Behavior Use detailed FPA model combined with environment transport, secondary production, and radioactive decay models to predict transient effects in IRFPA. Provide guidance on material selection and operational choices that affect transient rates 8
Block Diagram Showing Approach Space Environment Spacecraft Model and Materials Transport Calculations Secondary and Transported Primary Environment protons, neutrons, photons, electrons, etc. Activation Studies Transient Analysis 9
Model Space Radiation Environment External to Spacecraft Transient Particles Galactic Cosmic Rays Solar Particle Events Trapped Particles Electrons < 10 MeV Protons and heavier ions 100s MeV-10s GeV 10
Importance of Secondary Environment for NGST Emphasis of transport studies for NGST is different from other programs Most transport calculations for space seek to reduce dose levels for components by considering realistic shielding representations For NGST, need to develop a realistic model of secondary environment to assess its contribution to detector transients 11
Spacecraft Modeling Secondary Environment depends on spacecraft structure and materials Spacecraft Modeling Use CAD files where available to model spacecraft and instruments 12
Spacecraft Generated Environment Prompt secondary particle concerns Electrons generated near sensitive volumes of focal plane array (FPA) Neutrons causing activation of spacecraft materials 13
Important Source Terms Potential Sources include: δ ray production Activation and subsequent decay of spacecraft materials (mainly β and γ) may be important Bremstrahlung photons Neutron production Importance of each term must be assessed 14
Material Studies Preliminary Results Perform simulations for secondary environment Material Region Void Region 1.E+00 GCR Min Proton Spectrum 1.E-01 Flux (#/cm 2 MeV) 1.E-02 2.5 cm Hollow Sphere (R in =1.25cm, R out =2.5cm) 1.E-03 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 Energy (MeV) 15
Moving Toward Real Geometries Detector Geometries Perform simulations for secondary environment 100 mil Al shielding Detector array assembly 1.E+00 GCR Min Proton Spectrum 1.E-01 Flux (#/cm 2 MeV) 1.E-02 1.E-03 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 Energy (MeV) 16
Sample MCNPX Results 2.5cm Hollow Beryllium Sphere (Rin=1.25cm, Rout=2.5cm) GCR Minimum Proton Source 1E+01 Void Region Material Region Group Integrated Fluence (particles/region) 1E+00 1E-01 1E-02 1E-03 1E-04 17% 8% primary protons secondary neutrons secondary photons secondary electrons secondary alphas proton-deltas 1E-05 0.0 0.5 1.0 1.5 2.0 2.5 Thickness (cm) 17
Sample MCNPX Results Average Number of Particles Produced Normalized to One Source Proton 1E+01 1E+00 Average Particles (/cm2) 1E-01 1E-02 1E-03 Neutrons Photons Electrons Alphas Proton Deltas Be Si Cu Mo W 1E-04 1E-05 1 10 100 Z 18
Current and Future Efforts Purpose of transport studies is to provide reliable estimates of the transported primary and secondary environments as input to: Contamination and activation analysis Fluxes of protons (mostly primary) and neutrons (secondary), along with reaction cross-sections for spacecraft materials determine the numbers of activated nuclei Decay of these activated nuclei is, in turn, a source term for subsequent transport and transient studies Transient Analysis Supply location, trajectory, particle type, energy, and temporal information for every particle incident on the IRFPA IRFPA model will use these inputs to calculate the transient rate for relevant environments, configurations and conditions 19
Radioactive Contamination Review known contaminated materials e.g. Uranium in Be and alpha emitters in optics Assess FPA for possible concerns Emphasis on short range particles (alphas) in FPA and with line of sight locations Define source terms for transport calculations 20
Material Activation Transport calculations determine: Production term for all unstable nuclei (primarily proton driven) from GCR and SPE Neutron transport details to various materials to quantify neutron activation 21
Summary We are currently focused on determining the frequency of occurrence, magnitude and pulse width distributions of radiation induced transients in IRFPAs used on NGST We are using a first principles approach based on NGST requirements and NGST detector systems to estimate the rate, magnitude and duration distributions We would like to implement GEANT as a tool to assist in this study as well as future studies 22