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OVERVIEW OF SOLAR ENERGETIC PARTICLE EVENT HAZARDS TO HUMAN CREWS Lawrence W. Townsend University of Tennessee.

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Presentation on theme: "OVERVIEW OF SOLAR ENERGETIC PARTICLE EVENT HAZARDS TO HUMAN CREWS Lawrence W. Townsend University of Tennessee."— Presentation transcript:

1 OVERVIEW OF SOLAR ENERGETIC PARTICLE EVENT HAZARDS TO HUMAN CREWS Lawrence W. Townsend University of Tennessee

2 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

3 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

4 ENVIRONMENT (cont.) Galactic Cosmic Rays (GCR) important for missions outside Earth’s magnetosphere –Chronic exposures are at issue (unique effects?) –Acute effects not possible

5 CORONAL MASS EJECTION (SOHO Image)

6 Annual GCR Doses Al Shield (g cm -2 ) SkinBone Marrow Annual Effective Dose (cSv) Annual Dose (cGy) Annual Dose Equiv. (cSv) Annual Dose (cGy) Annual Dose Equiv. (cSv) 1977 Solar Minimum

7 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 ~ cSv per annum At solar maximum these are ~ cSv Secondary neutrons and charged particles are the major sources of radiation exposure in an interplanetary spacecraft

8 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

9 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)

10 SKIN DOSE AUGUST 1972 SPE (1 g/cm 2 Al shielding)

11 AUGUST 1972 SKIN DOSE RATE

12 Shielding Effective Dose (cSv) Avg. BFO Dose Eq. (cSv) % Diff. 1 g/cm 2 Al % 2 g/cm 2 Al % 5 g/cm 2 Al % 10 g/cm 2 Al % EFFECTIVE DOSE AUGUST 1972 SPE

13 POSSIBLE ACUTE EFFECTS August 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

14 ORGAN DOSE LIMITS (Gy-Eq) NCRP Report 132 Bone Marrow EyeSkin Career y d

15 October 1989 SEP

16 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 fluence much larger than Aug 72 -actual spectrum energy dependence unavailable, assume both hard and soft spectra

17 Carrington Flare Dose Estimates Shielding (g/cm 2 ) Soft Spectrum (8/72) Hard Spectrum (9/89) Skin (cGy) BFO (cGy) Skin (cGy) BFO (cGy)

18 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

19 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

20 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

21 Mars Surface (mainly protons and neutrons) GCR Solar Minimum GCR Solar Maximum October 1989 SPE D skin 5.7 cGy/yr2.7 cGy/yr3.2 cGy H skin 13.2 cSv/yr6.7 cSv/yr4.8 cSv D BFO 5.5 cGy/yr2.6 cGy/yr1.7 cGy H BFO 11.9 cSv/yr6.1 cSv/yr2.7 cSv

22 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

23 Methodology - Data Used the 5 largest events, in terms of accumulated dose, from years (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

24 Dose Calculations-July 14, 2000 Al Shield Thickness (g/cm 2 ) Dose to Shock (cGy) Total Dose to BFO (cGy) Dose to Shock (cGy) Total Dose to Eye (cGy) Dose to Shock (cGy) Total Dose to Skin (cGy)

25 November 4, 2001-Dose to Eye

26 September 24, 2001 – Dose to BFO

27 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/cm 2 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

28 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

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

30 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)

31 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

32 1 A GeV iron ion beam validation NSRL Test Rig

33 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

34 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 cm Al 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

35 CONCLUDING REMARKS (cont.) Results presented only for aluminum Other materials with low atomic mass numbers are better  LH 2 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 ~ g cm -2 CO 2

36 QUESTIONS?

37 RBE VALUES FOR CONVERTING DOSE TO Gy-Eq (NCRP 132) Radiation TypeRBE 1-5 MeV Neutrons MeV Neutrons3.5 Heavy Ions (A  4) 2.5 Protons > 2 MeV1.5

38 LEO DOSES GCR and SAA protons dominate About half and half at ~ 400 km altitude Shuttle flights ( º; 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

39 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

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

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


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