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OVERVIEW OF SOLAR ENERGETIC PARTICLE EVENT HAZARDS TO HUMAN CREWS PowerPoint PPT Presentation


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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

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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

  • 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


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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


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ENVIRONMENT (cont.)

  • Galactic Cosmic Rays (GCR) important for missions outside Earth’s magnetosphere

    • Chronic exposures are at issue (unique effects?)

    • Acute effects not possible


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CORONAL MASS EJECTION (SOHO Image)


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Annual GCR Doses


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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


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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


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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)


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


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AUGUST 1972 SKIN DOSE RATE


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EFFECTIVE DOSE AUGUST 1972 SPE


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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


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ORGAN DOSE LIMITS (Gy-Eq)NCRP Report 132


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October 1989 SEP


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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


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Carrington Flare Dose Estimates


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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


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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


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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


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Mars Surface(mainly protons and neutrons)


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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


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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


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Dose Calculations-July 14, 2000


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November 4, 2001-Dose to Eye


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September 24, 2001 – Dose to BFO


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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


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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


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HETC-HEDS Results

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


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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)


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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


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1 A GeV iron ion beam validation

NSRL Test Rig


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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


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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


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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


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QUESTIONS?


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RBE VALUES FOR CONVERTING DOSE TO Gy-Eq (NCRP 132)


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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


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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


November 2001 spe bayesian methodology dose forecast at 2 hours into event l.jpg

NOVEMBER 2001 SPE Bayesian Methodology Dose Forecast at 2 hours into event


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NOVEMBER 2001 SPE Bayesian Methodology Dose Forecast at 6 hours into event


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