- Email: [email protected]
Interplanetary crew dose estimates for worst case solar particle events based on historical data for the Carrington flare of 1859
Acta Astronautica 56 (2005) 969 – 974 www.elsevier.com/locate/actaastro
Interplanetary crew dose estimates for worst case solar particle events based on historical data for the Carrington flare of 1859 Daniel L. Stephens Jr., Lawrence W. Townsend∗ , Jennifer L. Hoff Department of Nuclear Engineering, The University of Tennessee, 1004 Estabrook Road, Knoxville, TN 37996-2300, USA Available online 7 March 2005
Abstract Over the past two decades, hypothetical models of “worst-case” solar particle event (SPE) spectra have been proposed in order to place an upper bound on radiation doses to critical body organs of interplanetary crews on deep space missions. These event spectra are usually formulated using hypothetical extrapolations of space measurements for previous large events. Here we take a different approach. Recently reported analyses of ice core samples indicate that the Carrington flare of 1859 is the largest event observed in the past 500 years. These ice core data yield estimates of the proton fluence for energies greater than 30 MeV, but provide no other spectrum information. Assuming that the proton energy distribution for such an event is similar to that measured for other recent, large events, interplanetary crew doses are estimated for these hypothetical worst case SPE spectra. These estimated doses are life threatening unless substantial shielding is provided. © 2005 Elsevier Ltd. All rights reserved.
1. Introduction Solar particle events (SPE) have historically been of concern for crewed space missions due to the potential for exposing crews to large radiation doses that may be mission—or life threatening. During the space era many SPEs have been observed. Most have too few energetic protons to be a concern to interplanetary crews. Very large events that pose significant health risks to crews typically occur once or twice during an 11-year solar cycle. For mission planning purposes a
∗ Corresponding author. Tel.: +1 423 974 5048;
fax: +1 423 974 0668. E-mail address: [email protected] (L.W. Townsend). 0094-5765/$ - see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.actaastro.2005.01.024
realistic, hypothetical worst-case solar particle event spectrum can provide a reasonable upper bound on radiation doses for these events. In this work, estimates of interplanetary crew organ doses for several plausible worst-case solar particle events are made. Previous analyses of hypothetical worst-case events are summarized elsewhere [1] and typically involved various combinations of events, or arbitrary scaling of measurements of large events, that previously occurred during the four decades of the space era. For this work, we take a different approach and develop plausible worst-case SPE spectra based on recently reported SPE fluence estimates obtained from the concentration of nitrates found in ice core samples spanning approximately the last 500 years [2]. These nitrates are produced in
970
D.L. Stephens Jr. et al. / Acta Astronautica 56 (2005) 969 – 974
the upper atmosphere by several different physical and chemical mechanisms. A particularly important mechanism is ionization resulting from the impacts of energetic protons with the atmospheric constituents. The nitrates are quickly deposited in the snow pack in the polar regions by water droplets and ice crystals falling as precipitation. Over time the snow consolidates to high-density firn. The thin nitrate layer concentration from each large event can be extracted from ice core samples taken from the firn in the polar regions (Greenland and the Ross Ice Shelf). Annual variations can be deduced along with impulsive increases that are correlated, with a high degree of reliability, with major solar particle events that have occurred over time. A conversion factor to relate solar proton fluences above 30 MeV to the measured nitrate concentration in the ice core has been extracted and gives predictions that compare favorably with measurements of solar proton fluences from satellites during the space era. These nitrate measurements can then be used to provide estimates of the integral fluence of protons above 30 MeV for such events. Details of the methodology are available in McCracken et al. [2] and references therein. For this 500 year period the “Carrington” solar flare of 1859 had the largest estimated integral fluence of protons > 30 MeV with a value of 18.8 × 109 cm−2 . This value was at the top of the polar atmosphere (exoatmospheric) and is assumed to be that which would have been found in space at the location of Earth’s orbit. Hence, it is an excellent candidate for a plausible worst-case event. Unfortunately, one fluence datum at a single energy does not constitute a spectrum. Therefore, to generate plausible spectra, the Carrington flare fluence for > 30 MeV protons, reported by McCracken et al. [2] is used as an overall normalization or scaling point in combination with the measured spectral shapes of several large solar particle events from the space era to create hypothetical worst-case solar particle event spectra.
2. Methodology Five worst-case SPE proton spectra, based on the spectral shapes of large space era events with the total fluence above 30 MeV datum normalized to the Carrington flare fluence value of 18.8 × 109 cm−2 , are assumed. Specifically the spectral shapes of the 4
August 1972, 12 August 1989, 29 September 1989, 19 October 1989 and 23 March 1991 events are used. Both exponential in rigidity (momentum per unit charge) [3] and energy (Weibull) [4] parameterizations of the input spectra of these events are used, yielding 10 input proton spectra for analysis. The exponential in rigidity form is J = J0 exp(−R/R0 ),
(1)
where J is the integral fluence (protons/cm2 ) exceeding some energy E and R is the proton rigidity in MV (momentum per unit charge). The J0 and R0 parameters are determined from the proton spectral data using a least squares analysis. Values are listed in Table 1. This parameterization is the one most commonly used for previous analyses [1]. Recent studies indicate that the exponential in energy form more accurately represents the measured proton spectra for these events [4]. The exponential in energy form is J = J0 exp(−kE ),
(2)
where J is as before, E is the proton energy in MeV, and k and are parameters used to fit the spectrum. They are listed in Table 2. Table 1 Spectral parameters for Carrington flare event in the rigidity parameterization form Spectra shape used
J0 (protons cm−2 )
R0 (MV)
Aug-72 Aug-89 Sep-89 Oct-89 Mar-91
2.87E+11 1.13E+12 1.91E+11 2.45E+11 4.29E+12
87.78 58.42 103.05 93.15 44.03
Table 2 Spectral parameters for Carrington flare event in power law parameterization form Spectrum shape used
J0 (protons cm−2 )
Aug-72 Aug-89 Sep-89 Oct-89 Mar-91
5.23E+10 1.81E+12 4.79E+11 4.64E+12 1.47E+12
k
0.0236 1.166 0.877 2.115 0.972
1.108 0.4015 0.3841 0.2815 0.441
D.L. Stephens Jr. et al. / Acta Astronautica 56 (2005) 969 – 974
Doses are estimated for these assumed worst-case events using the BRYNTRN space radiation transport code [5] and the computerized anatomical man (CAM) human geometry model [6] behind four representative thicknesses of aluminum shielding in free space. These thicknessses are 1, 2, 5 and 10 g/cm2 , which are representative of a spacesuit, a thin spacecraft, a nominal spacecraft and a SPE “storm shelter”. In all discussions in this work, “dose” refers to absorbed dose in tissue unless otherwise indicated. The BRYNTRN space radiation transport code is used to transport incident SPE protons (and their secondary protons, neutrons, deuterons, tritons, 3 He and alpha particles generated by nuclear interactions in the shield and overlying tissue) through the aluminum shielding and overlying body tissue (simulated with water). The CAM model body self-shielding distributions in combination with the BRYNTRN results are then used to calculate the dose estimates to critical organs. Doses are calculated for the ocular lens, skin and bone marrow (BFO) since these are the major critical organs of interest for radiation protection purposes [7]. Since the skin and BFO are distributed throughout the body, doses for them are obtained by averaging over 33 anatomical locations for each organ. Since the eye is localized, a single point in the right ocular lens is used for estimating the eye dose.
3. Results Tables 3–7, shown below, present the organ doses calculated for each thickness of aluminum shielding for the 10 assumed spectral shapes. Reviewing the dose estimates presented in Tables 3–7, it is apparent that, except for the skin and eye doses shielded by 1– 2 g cm−2 of aluminum, the largest organ doses result from the Carrington flare with the assumed proton spectral shape of the September 1989 event. Since the September 1989 spectrum is the “hardest”, i.e. has the slowest decrease in fluence with increasing proton energies, this result is not unexpected. A plot of the Carrington flare spectra for the September 1989 shape, compared with the measured August 1972 spectrum, which usually yields the highest organ dose estimates for large events of this type, is shown in Fig. 1. Note that the assumed Carrington flare spectrum is much harder and has significantly
971
higher proton fluence. We also note that the energy parameterization for the input proton spectrum, given by Eq. (2), typically yields larger estimated doses than the rigidity parameterization, given by Eq. (1), indicating that using the latter parameterization may result in dose underestimates and is not conservative. In order to compare these dose estimates to the organ doses necessary to induce acute radiation syndrome responses in the crew, which are based upon acute exposures to gammas and not protons, a multiplicative relative biological effectiveness (RBE) factor should be applied [7]. The resulting dose in units of Gray-Equivalent (Gy-Eq) is obtained from dose (Gy-Eq) = dose (Gy) × RBE,
(3)
where an RBE = 1.5 is assumed [7]. Hence, whenever “dose” is presented in units of Gy-Eq, the absorbed dose (in Gy) has been multiplied by the RBE using Eq. (3). The organ doses in Gy-Eq for the Carrington flare with a September 1989 spectral shape, using an energy parameterization, are displayed in Fig. 2 for thicknesses of aluminum shielding as large as 125 g cm−2 . The whole body dose, as approximated by the BFO dose, indicates that significant acute radiation syndrome effects are possible. With a dose of 2.8 Gy (4.2 Gy-Eq) resulting from typical space suit shielding thicknesses (1 g cm−2 aluminum) symptoms will be severe and could include death. At typical spacecraft shielding thicknesses (5– 10 g cm−2 aluminum) a BFO dose in the range of 1.6–2.6 Gy-Eq is calculated. Expected symptoms include nausea and emesis, malaise, hematologic damage, and possibly death [7]. To reduce the BFO dose below the 0.25 Gy-Eq level (current 30 d limit for missions in low-Earth orbit [7]) a ‘storm shelter’ of 45– 50 g cm−2 (about 18 cm) of aluminum would be required. From the tables, eye and skin doses for space suit shielding (1 g cm−2 ) for an event of this type could be as large as 26.3 Gy (39.5 Gy-Eq) and 56.2 Gy (84.3 Gy-Eq) respectively. Expected symptoms include lens cataracts, keratitis, erythema, epilation and moist desquamation [7]. For typical space craft shielding thicknesses (∼ 10 g cm−2 ) the calculated doses are 2.8 Gy (4.2 Gy-Eq) for the skin and 2.7 Gy (4.1 Gy-Eq) for the eye. The doses are reduced to 0.33 Gy-Eq for both organs behind a storm shelter thickness of 50 g cm−2 of aluminum. Such doses are below the currently
972
D.L. Stephens Jr. et al. / Acta Astronautica 56 (2005) 969 – 974
Table 3 Organ doses in cGy for the Carrington flare assuming an August 1972 event spectral shape Al shield (g cm−2 )
1 2 5 10
Skin dose (cGy)
Eye dose (cGy)
BFO dose (cGy)
R
E
R
E
R
E
3746 1804 566 191
3426 1905 556 123
2390 1349 495 180
2383 1439 461 108
196 162 99 53
141 105 47 15
Columns labeled R and E refer to the spectra given by Eqs. (1) and (2), respectively. Aluminum shield thicknesses are given in g cm−2 .
Table 4 Organ doses in cGy for the Carrington flare assuming an August 1989 event spectral shape Al shield (g cm−2 )
1 2 5 10
Skin dose (cGy)
Eye dose (cGy)
BFO dose (cGy)
R
E
R
E
R
E
4697 1665 312 64
4362 1710 414 119
2517 1081 249 57
2471 1180 350 110
78 57 27 10
129 102 59 30
Columns labeled R and E refer to the spectra given by Eqs. (1) and (2), respectively. Aluminum shield thicknesses are given in g cm−2 .
Table 5 Organ doses in cGy for the Carrington flare assuming a September 1989 event spectral shape Al shield (g cm−2 )
1 2 5 10
Skin dose (cGy)
Eye dose (cGy)
BFO dose (cGy)
R
E
R
E
R
E
3458 1828 671 264
3539 1801 665 282
2339 1430 602 252
2337 1400 602 273
263 223 148 86
281 244 171 109
Columns labeled R and E refer to the spectra given by Eqs. (1) and (2), respectively. Aluminum shield thicknesses are given in g cm−2 .
Table 6 Organ doses in cGy for the Carrington flare assuming an October1989 event spectral shape Al shield (g cm−2 )
1 2 5 10
Skin dose (cGy)
Eye dose (cGy)
BFO dose (cGy)
R
E
R
E
R
E
3635 1815 605 217
3967 1749 546 208
2371 1381 535 206
2404 1288 483 200
220 183 116 64
212 180 122 75
Columns labeled R and E refer to the spectra given by Eqs. (1) and (2), respectively. Aluminum shield thicknesses are given in g cm−2 .
D.L. Stephens Jr. et al. / Acta Astronautica 56 (2005) 969 – 974
973
Table 7 Organ doses in cGy for the Carrington flare assuming a March 1991 event spectral shape Al shield (g cm−2 )
Skin dose (cGy)
1 2 5 10
Eye dose (cGy)
BFO dose (cGy)
R
E
R
E
R
E
5621 1488 171 22
4480 1694 378 98
2625 856 126 18
2488 1145 313 90
35 23 8 2
109 85 46 22
Columns labeled R and E refer to the spectra given by Eqs. (1) and (2), respectively. Aluminum shield thicknesses are given in g cm−2 .
Carrington Flare Spectra with September1989 Spectral Shapes Compared with the August 1972 SPE Fluence (protons/sq. cm)
1.E+12 1.E+11 1.E+10 1.E+09 1.E+08
Carrington - Rigidty
1.E+07
Carrington - Energy
1.E+06 Aug 1972 - Energy
1.E+05 1.E+04 1
10
100
1000
10000
Energy (MeV) Fig. 1. Proton fluence spectrum for the Carrington flare of 1859 using an assumed September 1989 event shape. Plotted are spectra obtained using the rigidity and energy parameterizations for this hypothetical spectrum. For comparison, the energy parameterization of the proton spectrum for the August 1972 solar particle event is also displayed.
100
Organ Dose (Gy-Eq)
Skin Eye Bone Marrow 10
1
0 0
25
50
75
100
125
Al Thickness (g/sq. cm) Fig. 2. Estimated organ doses in Gy-Eq for the skin, eye and bone marrow versus the thickness of aluminum shielding in g cm −2 for the Carrington flare of 1859 with a September 1989 event spectral shape. Note that the skin and eye curves are nearly identical for thicknesses of aluminum greater than 10 g cm−2 .
974
D.L. Stephens Jr. et al. / Acta Astronautica 56 (2005) 969 – 974
recommended 30 d limits for crews on missions in low-Earth orbit [7]. 4. Conclusions Recent measurements of nitrates in ice core samples make available a history of integral fluences of large SPEs over a period covering the last 500 years. This information is used to estimate a plausible, hypothetical worst case SPE for use in interplanetary mission planning. Given the lack of spectral shape information in the ice core methods spectral shapes from large space era SPEs are used in combination with the 30 MeV fluence measurement for the Carrington flare of 1859 to estimate critical organ doses from such an event. The largest doses were typically received for the September 1989 spectral form. Hence, it is recommended that this spectral form, normalized to the Carrington flare fluence at 30 MeV, be used as the “worst case” spectrum for mission planning purposes. The whole body dose, as approximated by the BFO dose, is 2.8 Gy resulting from typical space suit shielding thicknesses (1 g cm−2 ) for the September 1989 spectral shape. In contrast the August 1972 SPE is generally considered to be the largest event with respect to dose observed during the space era [8]. The worst-case Carrington event, however, is approximately a factor of 4 larger for the BFO dose in a space suit and is a factor of 15 larger for the estimated BFO dose inside a typical spacecraft. An event this large presents a significant risk to mission success and crew survival. Substantial shielding will be required to insure crew survival.
References [1] L.W. Townsend, D.L. Stephens, J.L. Hoff, G.S. Braley, H.M. Moussa, Worst case solar energetic particle events for deep space missions, Presented at the 2001 International Conference on Environmental Systems, Orlando, FL, July 9–12, 2001, Paper No. 01ICES-292. [2] K.G. McCracken, G.A.M. Dreschhoff, E.J. Zeller, D.F. Smart, M.A. Shea, Solar cosmic ray events for the period 1561–1994, 1. Identification in polar ice, 1561–1950, Journal of Geophysical Research 106 (2001) 21585–21598. [3] E.N. Zapp, C.R. Ramsey, L.W. Townsend, G.D. Badhwar, Solar particle event dose distributions: parameterizations of dosetime profiles, Acta Astronautica 46 (1998) 249–259. [4] M.A. Xapsos, J.L. Barth, E.G. Stassinopoulos, S.R. Messenger, R.J. Walters, G.P. Summers, E.A. Burke, Characterizing solar proton energy spectra for radiation effects applications, IEEE Transactions on Nuclear Science 47 (2000) 2218–2223. [5] J.W. Wilson, F.F. Badavi, F.A. Cucinotta, J.L. Shinn, G.D. Badhwar, R. Silberberg, C.H. Tsao, L.W. Townsend, R.K. Tripathi, HZETRN: description of a free-space ion and nucleon transport and shielding computer program, NASA Technical Paper No. TP 3495, May 1995. [6] M.P. Billings, W.R. Yucker, The computerized anatomical man (CAM) model, NASA Contractor Report No. CR-134043, 1973. [7] NCRP, Radiation protection guidance for activities in lowearth orbit, National Council on Radiation Protection and Measurements, Report 132, 2000. [8] J.L. Parsons, L.W. Townsend, Interplanetary crew dose rates for the August 1972 solar particle event, Radiation Research 153 (2000) 729–733.
Interplanetary crew dose estimates for worst case solar particle events based on historical data for the Carrington flare of 1859 Daniel L. Stephens Jr., Lawrence W. Townsend∗ , Jennifer L. Hoff Department of Nuclear Engineering, The University of Tennessee, 1004 Estabrook Road, Knoxville, TN 37996-2300, USA Available online 7 March 2005
Abstract Over the past two decades, hypothetical models of “worst-case” solar particle event (SPE) spectra have been proposed in order to place an upper bound on radiation doses to critical body organs of interplanetary crews on deep space missions. These event spectra are usually formulated using hypothetical extrapolations of space measurements for previous large events. Here we take a different approach. Recently reported analyses of ice core samples indicate that the Carrington flare of 1859 is the largest event observed in the past 500 years. These ice core data yield estimates of the proton fluence for energies greater than 30 MeV, but provide no other spectrum information. Assuming that the proton energy distribution for such an event is similar to that measured for other recent, large events, interplanetary crew doses are estimated for these hypothetical worst case SPE spectra. These estimated doses are life threatening unless substantial shielding is provided. © 2005 Elsevier Ltd. All rights reserved.
1. Introduction Solar particle events (SPE) have historically been of concern for crewed space missions due to the potential for exposing crews to large radiation doses that may be mission—or life threatening. During the space era many SPEs have been observed. Most have too few energetic protons to be a concern to interplanetary crews. Very large events that pose significant health risks to crews typically occur once or twice during an 11-year solar cycle. For mission planning purposes a
∗ Corresponding author. Tel.: +1 423 974 5048;
fax: +1 423 974 0668. E-mail address: [email protected] (L.W. Townsend). 0094-5765/$ - see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.actaastro.2005.01.024
realistic, hypothetical worst-case solar particle event spectrum can provide a reasonable upper bound on radiation doses for these events. In this work, estimates of interplanetary crew organ doses for several plausible worst-case solar particle events are made. Previous analyses of hypothetical worst-case events are summarized elsewhere [1] and typically involved various combinations of events, or arbitrary scaling of measurements of large events, that previously occurred during the four decades of the space era. For this work, we take a different approach and develop plausible worst-case SPE spectra based on recently reported SPE fluence estimates obtained from the concentration of nitrates found in ice core samples spanning approximately the last 500 years [2]. These nitrates are produced in
970
D.L. Stephens Jr. et al. / Acta Astronautica 56 (2005) 969 – 974
the upper atmosphere by several different physical and chemical mechanisms. A particularly important mechanism is ionization resulting from the impacts of energetic protons with the atmospheric constituents. The nitrates are quickly deposited in the snow pack in the polar regions by water droplets and ice crystals falling as precipitation. Over time the snow consolidates to high-density firn. The thin nitrate layer concentration from each large event can be extracted from ice core samples taken from the firn in the polar regions (Greenland and the Ross Ice Shelf). Annual variations can be deduced along with impulsive increases that are correlated, with a high degree of reliability, with major solar particle events that have occurred over time. A conversion factor to relate solar proton fluences above 30 MeV to the measured nitrate concentration in the ice core has been extracted and gives predictions that compare favorably with measurements of solar proton fluences from satellites during the space era. These nitrate measurements can then be used to provide estimates of the integral fluence of protons above 30 MeV for such events. Details of the methodology are available in McCracken et al. [2] and references therein. For this 500 year period the “Carrington” solar flare of 1859 had the largest estimated integral fluence of protons > 30 MeV with a value of 18.8 × 109 cm−2 . This value was at the top of the polar atmosphere (exoatmospheric) and is assumed to be that which would have been found in space at the location of Earth’s orbit. Hence, it is an excellent candidate for a plausible worst-case event. Unfortunately, one fluence datum at a single energy does not constitute a spectrum. Therefore, to generate plausible spectra, the Carrington flare fluence for > 30 MeV protons, reported by McCracken et al. [2] is used as an overall normalization or scaling point in combination with the measured spectral shapes of several large solar particle events from the space era to create hypothetical worst-case solar particle event spectra.
2. Methodology Five worst-case SPE proton spectra, based on the spectral shapes of large space era events with the total fluence above 30 MeV datum normalized to the Carrington flare fluence value of 18.8 × 109 cm−2 , are assumed. Specifically the spectral shapes of the 4
August 1972, 12 August 1989, 29 September 1989, 19 October 1989 and 23 March 1991 events are used. Both exponential in rigidity (momentum per unit charge) [3] and energy (Weibull) [4] parameterizations of the input spectra of these events are used, yielding 10 input proton spectra for analysis. The exponential in rigidity form is J = J0 exp(−R/R0 ),
(1)
where J is the integral fluence (protons/cm2 ) exceeding some energy E and R is the proton rigidity in MV (momentum per unit charge). The J0 and R0 parameters are determined from the proton spectral data using a least squares analysis. Values are listed in Table 1. This parameterization is the one most commonly used for previous analyses [1]. Recent studies indicate that the exponential in energy form more accurately represents the measured proton spectra for these events [4]. The exponential in energy form is J = J0 exp(−kE ),
(2)
where J is as before, E is the proton energy in MeV, and k and are parameters used to fit the spectrum. They are listed in Table 2. Table 1 Spectral parameters for Carrington flare event in the rigidity parameterization form Spectra shape used
J0 (protons cm−2 )
R0 (MV)
Aug-72 Aug-89 Sep-89 Oct-89 Mar-91
2.87E+11 1.13E+12 1.91E+11 2.45E+11 4.29E+12
87.78 58.42 103.05 93.15 44.03
Table 2 Spectral parameters for Carrington flare event in power law parameterization form Spectrum shape used
J0 (protons cm−2 )
Aug-72 Aug-89 Sep-89 Oct-89 Mar-91
5.23E+10 1.81E+12 4.79E+11 4.64E+12 1.47E+12
k
0.0236 1.166 0.877 2.115 0.972
1.108 0.4015 0.3841 0.2815 0.441
D.L. Stephens Jr. et al. / Acta Astronautica 56 (2005) 969 – 974
Doses are estimated for these assumed worst-case events using the BRYNTRN space radiation transport code [5] and the computerized anatomical man (CAM) human geometry model [6] behind four representative thicknesses of aluminum shielding in free space. These thicknessses are 1, 2, 5 and 10 g/cm2 , which are representative of a spacesuit, a thin spacecraft, a nominal spacecraft and a SPE “storm shelter”. In all discussions in this work, “dose” refers to absorbed dose in tissue unless otherwise indicated. The BRYNTRN space radiation transport code is used to transport incident SPE protons (and their secondary protons, neutrons, deuterons, tritons, 3 He and alpha particles generated by nuclear interactions in the shield and overlying tissue) through the aluminum shielding and overlying body tissue (simulated with water). The CAM model body self-shielding distributions in combination with the BRYNTRN results are then used to calculate the dose estimates to critical organs. Doses are calculated for the ocular lens, skin and bone marrow (BFO) since these are the major critical organs of interest for radiation protection purposes [7]. Since the skin and BFO are distributed throughout the body, doses for them are obtained by averaging over 33 anatomical locations for each organ. Since the eye is localized, a single point in the right ocular lens is used for estimating the eye dose.
3. Results Tables 3–7, shown below, present the organ doses calculated for each thickness of aluminum shielding for the 10 assumed spectral shapes. Reviewing the dose estimates presented in Tables 3–7, it is apparent that, except for the skin and eye doses shielded by 1– 2 g cm−2 of aluminum, the largest organ doses result from the Carrington flare with the assumed proton spectral shape of the September 1989 event. Since the September 1989 spectrum is the “hardest”, i.e. has the slowest decrease in fluence with increasing proton energies, this result is not unexpected. A plot of the Carrington flare spectra for the September 1989 shape, compared with the measured August 1972 spectrum, which usually yields the highest organ dose estimates for large events of this type, is shown in Fig. 1. Note that the assumed Carrington flare spectrum is much harder and has significantly
971
higher proton fluence. We also note that the energy parameterization for the input proton spectrum, given by Eq. (2), typically yields larger estimated doses than the rigidity parameterization, given by Eq. (1), indicating that using the latter parameterization may result in dose underestimates and is not conservative. In order to compare these dose estimates to the organ doses necessary to induce acute radiation syndrome responses in the crew, which are based upon acute exposures to gammas and not protons, a multiplicative relative biological effectiveness (RBE) factor should be applied [7]. The resulting dose in units of Gray-Equivalent (Gy-Eq) is obtained from dose (Gy-Eq) = dose (Gy) × RBE,
(3)
where an RBE = 1.5 is assumed [7]. Hence, whenever “dose” is presented in units of Gy-Eq, the absorbed dose (in Gy) has been multiplied by the RBE using Eq. (3). The organ doses in Gy-Eq for the Carrington flare with a September 1989 spectral shape, using an energy parameterization, are displayed in Fig. 2 for thicknesses of aluminum shielding as large as 125 g cm−2 . The whole body dose, as approximated by the BFO dose, indicates that significant acute radiation syndrome effects are possible. With a dose of 2.8 Gy (4.2 Gy-Eq) resulting from typical space suit shielding thicknesses (1 g cm−2 aluminum) symptoms will be severe and could include death. At typical spacecraft shielding thicknesses (5– 10 g cm−2 aluminum) a BFO dose in the range of 1.6–2.6 Gy-Eq is calculated. Expected symptoms include nausea and emesis, malaise, hematologic damage, and possibly death [7]. To reduce the BFO dose below the 0.25 Gy-Eq level (current 30 d limit for missions in low-Earth orbit [7]) a ‘storm shelter’ of 45– 50 g cm−2 (about 18 cm) of aluminum would be required. From the tables, eye and skin doses for space suit shielding (1 g cm−2 ) for an event of this type could be as large as 26.3 Gy (39.5 Gy-Eq) and 56.2 Gy (84.3 Gy-Eq) respectively. Expected symptoms include lens cataracts, keratitis, erythema, epilation and moist desquamation [7]. For typical space craft shielding thicknesses (∼ 10 g cm−2 ) the calculated doses are 2.8 Gy (4.2 Gy-Eq) for the skin and 2.7 Gy (4.1 Gy-Eq) for the eye. The doses are reduced to 0.33 Gy-Eq for both organs behind a storm shelter thickness of 50 g cm−2 of aluminum. Such doses are below the currently
972
D.L. Stephens Jr. et al. / Acta Astronautica 56 (2005) 969 – 974
Table 3 Organ doses in cGy for the Carrington flare assuming an August 1972 event spectral shape Al shield (g cm−2 )
1 2 5 10
Skin dose (cGy)
Eye dose (cGy)
BFO dose (cGy)
R
E
R
E
R
E
3746 1804 566 191
3426 1905 556 123
2390 1349 495 180
2383 1439 461 108
196 162 99 53
141 105 47 15
Columns labeled R and E refer to the spectra given by Eqs. (1) and (2), respectively. Aluminum shield thicknesses are given in g cm−2 .
Table 4 Organ doses in cGy for the Carrington flare assuming an August 1989 event spectral shape Al shield (g cm−2 )
1 2 5 10
Skin dose (cGy)
Eye dose (cGy)
BFO dose (cGy)
R
E
R
E
R
E
4697 1665 312 64
4362 1710 414 119
2517 1081 249 57
2471 1180 350 110
78 57 27 10
129 102 59 30
Columns labeled R and E refer to the spectra given by Eqs. (1) and (2), respectively. Aluminum shield thicknesses are given in g cm−2 .
Table 5 Organ doses in cGy for the Carrington flare assuming a September 1989 event spectral shape Al shield (g cm−2 )
1 2 5 10
Skin dose (cGy)
Eye dose (cGy)
BFO dose (cGy)
R
E
R
E
R
E
3458 1828 671 264
3539 1801 665 282
2339 1430 602 252
2337 1400 602 273
263 223 148 86
281 244 171 109
Columns labeled R and E refer to the spectra given by Eqs. (1) and (2), respectively. Aluminum shield thicknesses are given in g cm−2 .
Table 6 Organ doses in cGy for the Carrington flare assuming an October1989 event spectral shape Al shield (g cm−2 )
1 2 5 10
Skin dose (cGy)
Eye dose (cGy)
BFO dose (cGy)
R
E
R
E
R
E
3635 1815 605 217
3967 1749 546 208
2371 1381 535 206
2404 1288 483 200
220 183 116 64
212 180 122 75
Columns labeled R and E refer to the spectra given by Eqs. (1) and (2), respectively. Aluminum shield thicknesses are given in g cm−2 .
D.L. Stephens Jr. et al. / Acta Astronautica 56 (2005) 969 – 974
973
Table 7 Organ doses in cGy for the Carrington flare assuming a March 1991 event spectral shape Al shield (g cm−2 )
Skin dose (cGy)
1 2 5 10
Eye dose (cGy)
BFO dose (cGy)
R
E
R
E
R
E
5621 1488 171 22
4480 1694 378 98
2625 856 126 18
2488 1145 313 90
35 23 8 2
109 85 46 22
Columns labeled R and E refer to the spectra given by Eqs. (1) and (2), respectively. Aluminum shield thicknesses are given in g cm−2 .
Carrington Flare Spectra with September1989 Spectral Shapes Compared with the August 1972 SPE Fluence (protons/sq. cm)
1.E+12 1.E+11 1.E+10 1.E+09 1.E+08
Carrington - Rigidty
1.E+07
Carrington - Energy
1.E+06 Aug 1972 - Energy
1.E+05 1.E+04 1
10
100
1000
10000
Energy (MeV) Fig. 1. Proton fluence spectrum for the Carrington flare of 1859 using an assumed September 1989 event shape. Plotted are spectra obtained using the rigidity and energy parameterizations for this hypothetical spectrum. For comparison, the energy parameterization of the proton spectrum for the August 1972 solar particle event is also displayed.
100
Organ Dose (Gy-Eq)
Skin Eye Bone Marrow 10
1
0 0
25
50
75
100
125
Al Thickness (g/sq. cm) Fig. 2. Estimated organ doses in Gy-Eq for the skin, eye and bone marrow versus the thickness of aluminum shielding in g cm −2 for the Carrington flare of 1859 with a September 1989 event spectral shape. Note that the skin and eye curves are nearly identical for thicknesses of aluminum greater than 10 g cm−2 .
974
D.L. Stephens Jr. et al. / Acta Astronautica 56 (2005) 969 – 974
recommended 30 d limits for crews on missions in low-Earth orbit [7]. 4. Conclusions Recent measurements of nitrates in ice core samples make available a history of integral fluences of large SPEs over a period covering the last 500 years. This information is used to estimate a plausible, hypothetical worst case SPE for use in interplanetary mission planning. Given the lack of spectral shape information in the ice core methods spectral shapes from large space era SPEs are used in combination with the 30 MeV fluence measurement for the Carrington flare of 1859 to estimate critical organ doses from such an event. The largest doses were typically received for the September 1989 spectral form. Hence, it is recommended that this spectral form, normalized to the Carrington flare fluence at 30 MeV, be used as the “worst case” spectrum for mission planning purposes. The whole body dose, as approximated by the BFO dose, is 2.8 Gy resulting from typical space suit shielding thicknesses (1 g cm−2 ) for the September 1989 spectral shape. In contrast the August 1972 SPE is generally considered to be the largest event with respect to dose observed during the space era [8]. The worst-case Carrington event, however, is approximately a factor of 4 larger for the BFO dose in a space suit and is a factor of 15 larger for the estimated BFO dose inside a typical spacecraft. An event this large presents a significant risk to mission success and crew survival. Substantial shielding will be required to insure crew survival.
References [1] L.W. Townsend, D.L. Stephens, J.L. Hoff, G.S. Braley, H.M. Moussa, Worst case solar energetic particle events for deep space missions, Presented at the 2001 International Conference on Environmental Systems, Orlando, FL, July 9–12, 2001, Paper No. 01ICES-292. [2] K.G. McCracken, G.A.M. Dreschhoff, E.J. Zeller, D.F. Smart, M.A. Shea, Solar cosmic ray events for the period 1561–1994, 1. Identification in polar ice, 1561–1950, Journal of Geophysical Research 106 (2001) 21585–21598. [3] E.N. Zapp, C.R. Ramsey, L.W. Townsend, G.D. Badhwar, Solar particle event dose distributions: parameterizations of dosetime profiles, Acta Astronautica 46 (1998) 249–259. [4] M.A. Xapsos, J.L. Barth, E.G. Stassinopoulos, S.R. Messenger, R.J. Walters, G.P. Summers, E.A. Burke, Characterizing solar proton energy spectra for radiation effects applications, IEEE Transactions on Nuclear Science 47 (2000) 2218–2223. [5] J.W. Wilson, F.F. Badavi, F.A. Cucinotta, J.L. Shinn, G.D. Badhwar, R. Silberberg, C.H. Tsao, L.W. Townsend, R.K. Tripathi, HZETRN: description of a free-space ion and nucleon transport and shielding computer program, NASA Technical Paper No. TP 3495, May 1995. [6] M.P. Billings, W.R. Yucker, The computerized anatomical man (CAM) model, NASA Contractor Report No. CR-134043, 1973. [7] NCRP, Radiation protection guidance for activities in lowearth orbit, National Council on Radiation Protection and Measurements, Report 132, 2000. [8] J.L. Parsons, L.W. Townsend, Interplanetary crew dose rates for the August 1972 solar particle event, Radiation Research 153 (2000) 729–733.