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OVERVIEW OF SOLAR ENERGETIC PARTICLE EVENT HAZARDS TO HUMAN CREWS. Lawrence W. Townsend University of Tennessee. OUTLINE. Environment GCR Doses and Effects Solar Particle Event Doses and Effects - Carrington Flare as a Worst Case Event Mars and Lunar Surface Doses

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overview of solar energetic particle event hazards to human crews

OVERVIEW OF SOLAR ENERGETIC PARTICLE EVENT HAZARDS TO HUMAN CREWS

Lawrence W. Townsend

University of Tennessee

outline
OUTLINE
  • Environment
  • GCR Doses and Effects
  • Solar Particle Event Doses and Effects

- Carrington Flare as a Worst Case Event

  • Mars and Lunar Surface Doses
  • Space Radiation Transport Code Development
  • Concluding Remarks
environment
ENVIRONMENT
  • Space environment is complex
  • Van Allen belts important for LEO; also GCR important for high-latitudes (ISS)
  • Solar Energetic Particles (SEP) important for missions outside Earth’s magnetosphere
    • Acute and chronic exposures possible
environment cont
ENVIRONMENT (cont.)
  • Galactic Cosmic Rays (GCR) important for missions outside Earth’s magnetosphere
    • Chronic exposures are at issue (unique effects?)
    • Acute effects not possible
deep space gcr doses
DEEP SPACE GCR DOSES
  • Annual bone marrow GCR doses will range up to ~ 15 cGy at solar minimum (~ 40 cSv) behind ~ 2cm Al shielding
  • Effective dose at solar minimum is ~ 45-50 cSv per annum
  • At solar maximum these are ~ 15-18 cSv
  • Secondary neutrons and charged particles are the major sources of radiation exposure in an interplanetary spacecraft
gcr risks
GCR Risks
  • Clearly, annual doses < 20cGy present no acute health hazard to crews on deep space missions
  • Hence only stochastic effects such as cancer induction and mortality or late deterministic effects, such as cataracts or damage to the central nervous system are of concern.
  • Unfortunately, there are no data for human exposures from these radiations that can be used to estimate risks to crews
  • In fact, it is not clear that the usual methods of estimating risk by calculating dose equivalent are even appropriate for these particles
solar particle event doses
SOLAR PARTICLE EVENT DOSES
  • Doses can be large in deep space but shielding is possible
  • August 1972 was largest dose event of space era (occurred between two Apollo missions)
possible acute effects august 1972 spe
POSSIBLE ACUTE EFFECTSAugust 1972 SPE
  • Bone marrow doses ~ 1 Gy delivered in a day may produce hematological responses and vomiting (not good in a space suit)
  • Skin doses ~15-20 Gy could result in skin erythema and moist desquamation (in some cases)

- doses inside nominal spacecraft might limit effects to mild erythema

solar particle event doses cont
SOLAR PARTICLE EVENT DOSES (cont.)
  • Ice core data from the Antarctic indicate that the largest event in past ~ 500 years was probably the Carrington Flare of 1859

- fluence much larger than Aug 72

- actual spectrum energy dependence unavailable, assume both hard and soft spectra

carrington flare doses 9 89 spectrum
CARRINGTON FLARE DOSES(9/89 Spectrum)
  • Bone marrow doses ~ 1-3 Gy possible inside a spacecraft (life threatening)
  • “Storm” shelter of about 18 cm Al needed to shield to the applicable deterministic limits (30 d limits of 0.25 Gy-Eq)
  • Major problem for non radiation hardened electronics built with COTS components

- up to 50 krads or more of total ionizing dose

carrington flare doses 8 72 spectrum
CARRINGTON FLARE DOSES(8/72 Spectrum)
  • Bone marrow doses in spacesuit up to ~1.5 Gy; much lower inside a spacecraft ( not life threatening)
  • “Storm” shelter of about 10 g cm-2 Al needed to shield to the applicable deterministic limits (30 d limits of 0.25 Gy-Eq)
  • Major problem for non radiation hardened electronics built with COTS components unless they are shielded by at least 1 g cm-2 Al

- up to 15 krads total ionizing dose for 15mils

lunar surface
Lunar Surface
  • Organ Doses and Dose Equivalents are ~ half those in deep space

- 2 shadow shielding provided

- Some neutron albedo from Lunar Surface

  • Inside a habitat the exposure is nearly all due to neutrons
hypothesis
Hypothesis

It has been proposed that

  • proton intensities on the stream-limited plateau present a minimal radiation hazard to astronauts
  • hazardous intensities occur upon CME-driven shock arrival at the spacecraft
methodology data
Methodology - Data
  • Used the 5 largest events, in terms of accumulated dose, from years 1996-2001(July 14, 2000; November 8, 2000; September 24, 2001; November 4, 2001; November 22, 2001)
  • Differential and integral flux and fluence spectra measured on GOES-8
  • Shock arrival times
    • ACE list of disturbances/transients (MAG and SWEPAM instruments)
    • SOHO/CELIAS solar wind data site
    • Discussions with NOAA SEC researchers
  • Stream Limited Intensities from Don Reames
implications for event triggered forecasting
Implications for Event-Triggered Forecasting
  • Hazardous radiation levels do occur prior to shock arrival for large events for shielding thicknesses on the order of 3 g/cm2 of Al
  • This suggests that we should attempt to predict the temporal evolution of dose for the SEP event prior to shock arrival
  • The temporal evolution of the SEP event determines the available time for making decisions
hetc heds code development
HETC-HEDS Code Development
  • HETC has been extended to include the transport of high-energy heavy ions (HZE particles) in a new version now named HETC-HEDS
  • HZE particle event generator has been developed and incorporated into the code to provide nuclear interaction data
  • Minor revisions to the models and techniques used in the event generator are performed as needed based upon comparisons with laboratory beam data
hetc heds results
HETC-HEDS Results

Fragment Fluence for 2A GeV 56Fe on 10 g/cm2 of Polyethylene (HETC-HEDS vs. PHITS)

status of fluka development
Status of FLUKA Development
  • Current version has the embedded event generators DPMJET
  • Four separate efforts on improvements to the event generators:

- rQMD approach based on the constrained Hamiltonian formalism of Dirac (E. N. Zapp)

- G. Xu is revisiting the original rQMD code

- "after-burner" to the rQMD codes to reassemble the fragments (M. –V. Garzelli)

- "Master Boltzmann Equation" approach (<100 MeV/A) (F. Cerutti)

hzetrn code development
HZETRN Code Development
  • Publicly-released version improved by incorporating better low energy treatment of interaction cross sections and better neutron transport
  • Meson and muon transport being incorporated
  • Green’s function techniques being developed for 3D transport
concluding remarks
CONCLUDING REMARKS
  • GCR exposures will be a problem for Mars missions due to large effective doses
  • Organ doses received from large SPEs can be hazardous to crews of vehicles in deep space

- exposures that are survivable with proper medical treatment on Earth may not be survivable in space

concluding remarks cont
CONCLUDING REMARKS (cont.)
  • Aside from acute effects, a single large SPE can expose a crewmember to an effective dose that exceeds their career limit
  • Due to their relatively soft energy spectra, most SPE doses can be substantially reduced with adequate shielding (several cmAl or equivalent)
  • A worst case event similar to the assumed Carrington Flare of 1859 could be catastrophic in deep space depending on spectral hardness and available shielding
concluding remarks cont35
CONCLUDING REMARKS (cont.)
  • Results presented only for aluminum
  • Other materials with low atomic mass numbers are better  LH2 reduces GCR dose equivalent by ~ one-half
  • In situ materials on lunar or Martian surface can be used to provide shielding (similar to Al in shielding characteristics)
  • Martian atmosphere is a relatively thick shield for operations on Mars surface

~ 16-20 g cm-2 CO2

leo doses
LEO DOSES
  • GCR and SAA protons dominate
  • About half and half at ~ 400 km altitude
  • Shuttle flights (28.5-62º; 220-615 km)

- crew doses : 0.02 – 3.2 cGy

  • MIR (51.6º; ~ 400 km)

- crew doses: 2.3 – 8.2 cGy

  • ISS (51.6º; ~ 400 km)

- crew doses: ~ 5 cGy (solar max)

  • Rapid transits limit doses for deep space
spe dose forecasting
SPE DOSE FORECASTING
  • At present it is not possible to forecast SPE fluences/doses before they occur
  • We are developing methods to forecast dose buildup over time based on the doses measured early in an SPE – “Nowcast” (supported by NASA LWS program)

- Artificial Intelligence: Sliding Time Delay Neural Network

- Locally-Weighted Learning

- Bayesian Inference

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