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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 l.jpg

OVERVIEW OF SOLAR ENERGETIC PARTICLE EVENT HAZARDS TO HUMAN CREWS

Lawrence W. Townsend

University of Tennessee


Outline l.jpg
OUTLINE CREWS

  • 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 l.jpg
ENVIRONMENT CREWS

  • 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 l.jpg
ENVIRONMENT (cont.) CREWS

  • 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 l.jpg
DEEP SPACE GCR DOSES CREWS

  • 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


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GCR Risks CREWS

  • 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


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SOLAR PARTICLE EVENT DOSES CREWS

  • 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)


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SKIN DOSE AUGUST 1972 SPE CREWS(1 g/cm2 Al shielding)




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POSSIBLE ACUTE EFFECTS CREWSAugust 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


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ORGAN DOSE LIMITS (Gy-Eq) CREWSNCRP Report 132



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SOLAR PARTICLE EVENT DOSES (cont.) CREWS

  • 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



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CARRINGTON FLARE DOSES CREWS(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


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CARRINGTON FLARE DOSES CREWS(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 l.jpg
Lunar Surface CREWS

  • 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


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Mars Surface CREWS(mainly protons and neutrons)


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Hypothesis CREWS

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


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Methodology - Data CREWS

  • 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




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September CREWS 24, 2001 – Dose to BFO


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Implications for Event-Triggered Forecasting CREWS

  • 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


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HETC-HEDS Code Development CREWS

  • 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


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HETC-HEDS Results CREWS

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


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Status of FLUKA Development CREWS

  • 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 l.jpg
HZETRN Code Development CREWS

  • 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 l.jpg
CONCLUDING REMARKS CREWS

  • 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


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CONCLUDING REMARKS (cont.) CREWS

  • 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


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CONCLUDING REMARKS (cont.) CREWS

  • 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


Questions l.jpg
QUESTIONS? CREWS



Leo doses l.jpg
LEO DOSES CREWS

  • 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


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SPE DOSE FORECASTING CREWS

  • 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


November 2001 spe bayesian methodology dose forecast at 2 hours into event l.jpg
NOVEMBER 2001 SPE CREWSBayesian Methodology Dose Forecast at 2 hours into event


November 2001 spe bayesian methodology dose forecast at 6 hours into event l.jpg
NOVEMBER 2001 SPE CREWSBayesian Methodology Dose Forecast at 6 hours into event


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