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

• Space Radiation Transport Code Development

• Concluding Remarks

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.)

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

– Chronic exposures are at issue (unique effects?)

– Acute effects not possible

CORONAL MASS EJECTION (SOHO Image)

Annual GCR Doses

Al Shield

(g cm-2)

Skin Bone Marrow

Annual Effective

Dose (cSv)

Annual Dose

(cGy)

Annual Dose

Equiv.

(cSv)

Annual Dose

(cGy)

Annual Dose

Equiv.

(cSv)

1977 Solar Minimum

1 18.4 79.8 16.4 44.5 48.8

5 18.3 66.9 16.3 40.5 43.7

10 18.0 56.2 16.1 37.0 39.3

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• 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

• 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)

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

AUGUST 1972 SKIN DOSE RATE

0

20

40

60

80

100

120

140

0 5 10 15 20 25 30

Time (hours)

Ski

n D

ose

Rat

e (c

Gy/

hr)

1 g/cm2 Al

2 g/cm2 Al

5 g/cm2 Al

ShieldingEffective

Dose

(cSv)

Avg. BFO Dose Eq.

(cSv)% Diff.

1 g/cm2 Al 337.5 111.0 203.9%

2 g/cm2 Al 200.2 91.3 119.3%

5 g/cm2 Al 88.5 56.3 57.3%

10 g/cm2 Al 40.2 30.5 31.7%

EFFECTIVE DOSE 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

ORGAN DOSE LIMITS (Gy-Eq)NCRP Report 132

Bone Marrow

Eye Skin

Career --- 4.0 6.0

1 y 0.50 2.0 3.0

30 d 0.25 1.0 1.5

October 1989 SEP

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 Dose Estimates

 

Shielding

(g/cm2)

Soft Spectrum

(8/72)

Hard Spectrum

(9/89)

Skin

(cGy)

BFO

(cGy)

Skin

(cGy)

BFO

(cGy)

1 3426 141 3539 281

2 1905 105 1801 244

5 556 47 665 171

10 123 15 282 109

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)

• 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

• 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

Mars Surface(mainly protons and neutrons)

GCR Solar Minimum

GCR Solar Maximum

October 1989 SPE

Dskin 5.7 cGy/yr 2.7 cGy/yr 3.2 cGy

Hskin 13.2 cSv/yr 6.7 cSv/yr 4.8 cSv

DBFO 5.5 cGy/yr 2.6 cGy/yr 1.7 cGy

HBFO 11.9 cSv/yr 6.1 cSv/yr 2.7 cSv

HypothesisIt 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

8.7-14.5 MeV Protons

1.E-02

1.E-01

1.E+00

1.E+01

1.E+02

1.E+03

1.E+04

0 10 20 30 40 50 60 70 80 90

Time (h)

Prot

ons/

(cm

2 sr

s M

eV)

July 14 2000November 8 2000September 24 2001November 4 2001November 22 2001

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

Dose Calculations-July 14, 2000Al Shield Thickness

(g/cm2)

Dose to BFO @ Shock (cGy)

Total Dose

to BFO (cGy)

Dose to Eye @ Shock (cGy)

Total Dose

to Eye (cGy)

Dose to Skin @ Shock (cGy)

Total Dose

to Skin (cGy)

1 34 35 400 434 563 617

2 24 25 185 197 226 242

3 18 18 107 112 123 130

5 11 11 48 49 52 54

7 7 7 26 26 27 28

10 4 4 12 12 13 13

November 4, 2001-Dose to Eye

0

50

100

150

200

250

300

350

400

450

500

0 10 20 30 40 50 60

Time (h)

Do

se t

o E

ye (

cGy-

eq)

1 g/cm23 g/cm25 g/cm210 g/cm2

ShockArrival

30 days limitfor eye

September 24, 2001 – Dose to BFO

0

5

10

15

20

25

30

0 20 40 60

Time (h)

Do

se t

o B

FO

(cG

y-eq

)

1 g/cm23 g/cm25 g/cm210 g/cm2 Shock

Arrival

30 days limitfor BFO

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 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

0.001

0.01

0.1

1

10

1 3 5 7 9

11

13

15

17

19

21

23

25

27

Charge (Source Particle 56Fe listed at 27)

Nu

clid

e Y

ield

/56F

e io

n

HETC-HEDS

PHITS

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

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

• 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

1 A GeV iron ion beam validation

NSRL Test Rig

Material

Depth (g/cm^2)

<LET>trk model*

<LET>trk experiments

Carbon 3.9 125.3 127.0 Aluminum 7 127.3 125.4 Lead 3.6 148.2 145.8 Polyethylene 10 91.3 91.4 Graphite-Epoxy

5 116.3 121.3

Graphite-Epoxy

10 94.8 98.5

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.)

• 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

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

QUESTIONS?

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

Radiation Type RBE

1-5 MeV Neutrons 6.0

5-50 MeV Neutrons 3.5

Heavy Ions (A 4) 2.5

Protons > 2 MeV 1.5

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

• 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

0

50

100

150

200

250

300

350

400

0 5 10 15 20 25

Time (h)

Dos

e (c

Gy)

Actual Profile

Predicted Profile

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

0

50

100

150

200

250

300

350

400

0 5 10 15 20 25

Time (h)

Dos

e (c

Gy)

Actual Profile

Predicted Profile