Lin, Z W

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29th International Cosmic Ray Conference Pune (2005) 2, 429432How Space Radiation Risk from Galactic Cosmic Rays at theInternational Space Station Relates to Nuclear Cross SectionsZi-Wei Lin and J.H. Adams, Jr.Mail Code XD12, NSSTC, 320 Sparkman Drive, Huntsville, AL 35805,USAPresenter: J.H. Adams, Jr. (James.H.Adams@nasa.gov), usa-lin-ZW-abs1-sh35-oralSpace radiation risk to astronauts is a major obstacle for long term human space explorations. Space radia-tion transport codes have thus been developed to evaluate radiation effects at the International Space Station(ISS) and in missions to the Moon or Mars. We study how nuclear fragmentation processes in such radiationtransport affect predictions on the radiation risk from galactic cosmic rays. Taking into account effects of thegeomagnetic eld on the cosmic ray spectra, we investigate the effects of fragmentation cross sections at dif-ferent energies on the radiation risk (represented by dose-equivalent) from galactic cosmic rays behind typicalspacecraft materials. These results tell us how the radiation risk at the ISS is related to nuclear cross sections atdifferent energies, and consequently how to most efciently reduce the physical uncertainty in our predictionson the radiation risk at the ISS.1. IntroductionSpace radiation to astronauts is a major hazard for long-duration human space explorations. Two major sourcesof space radiation are the galactic cosmic rays (GCR) and solar energetic particles (SEP). GCR are ever-presentand more energetic, thus they are able to penetrate much thicker materials than SEP. In order to evaluatethe space radiation risk and design the spacecraft and habitat for better radiation protection, space radiationtransport codes, which depends on the input physics of nuclear interactions, have been developed. Both ground-based and airborne experiments have been and will be used to improve the theoretical modeling of nuclearfragmentations as a function of energy and thus improve the accuracy of these transport codes. This studyfocus on the propagation of space radiation particles in materials, and the deterministic code HZETRN [1] isused in this study. A detailed sensitivity analysis to energy in deep-space GCR environments can be found inRef. [2]. At the International Space Station, however, the geomagnetic eld from the Earth affects the ux ofcharged particles at different energies differently. Fig. 1 shows the geomagnetic transmission function at theISS (including the shadowing effect from the Earth) as a function of magnetic rigidity, while in deep space thefunction is unity by denition as there are no geomagnetic or shadowing effects. Therefore we expect differentsensitivities to the energy in fragmentation cross sections, and these results tell us at what energies nuclearfragmentation most affects the GCR radiation risk at the ISS.2. ResultsA cross section as a function of energy is represented by its values at the following 18 discreet energy points 	(equally separated logarithmically by a factor of   ): 0.053, 0.075, 0.106, 0.15, 0.212, 0.3,0.424, 0.6, 0.849, 1.2, 1.697, 2.4, 3.394, 4.8, 6.788, 9.6, 13.58, and 19.2 GeV/u. For collisions at energiesbelow (or above ), the cross section is taken to be the same as that at the energy (or at ), while aninterpolated cross section is used for collisions at energies between two adjacent energies. In order to study thesensitivity to cross sections at energy , we change the values of cross sections at ﬀ	ﬂﬁﬃ, including allpartial cross sections for all projectiles, by the same percentage  , while cross sections at all other energy pointsare unchanged. The total inelastic cross sections for all projectiles at this energy also need to be increased430 Zi-Wei Lin and J.H. Adams, Jr.0 5 10 15 20Magnetic Rigidity (GV)10−410−310−210−1100Geomagnetic Transmission FunctionISS at 450km altitude and 51.6° inclinationIn deep spaceFigure 1. The geomagnetic transmission function at theISS, with the function in deep space being unity (by deni-tion).0.1 1 10Energy for change in cross sections (GeV/u)00.511.5−S(E) (cSv/year)behind 1g/cm2 water5g/cm210g/cm220g/cm230g/cm240g/cm250g/cm2Figure 2. Sensitivity functions  ﬀ!#"$ at the ISS for the 1977solar minimum GCR environment, showing the response ofdose-equivalent behind target to changes in cross sectionsvia Eq. (1).by the same percentage in order to satisfy baryon number conservation, %&'(& )*,+.-%-'-& , where %/&and %-represent the atomic weight of projectile 0 and its fragment 1 , respectively, '2& )represents the totalinelastic cross section of projectile 0 at energy , and '-& represents the partial cross section of producingfragment 1 from the fragmentation of 0 at energy . This relation is exact when the projectile fragmentationis separated from the target fragmentation and when baryon-antibaryon productions can be neglected 1. Dose-equivalent is used in this study as the measure of radiation risk, and we use the latest denition of the qualityfactor from ICRP60 [3].A normalized sensitivity function 3 for dose-equivalent 4 can be dened, at any depth in a target material,as54.63 5' ' 7#89 );:(1)This sensitivity function is then independent of the value of  [2] or the spacings between adjacent energypoints (unless the effect is strong enough to be non-linear), and can be used to estimate the change of dose-equivalent from an energy-dependent relative change of cross sections,5'< )=' ). If the interpolation ofcross sections is linear in8>9? , from calculations of54at discreet energy points we can extract 3< )as3 ?@BA5489 DC#EﬃFﬀGHIﬂﬁﬃJ(2)where the factor of8>9  is due to the equal energy spacing in logarithmic scale by factor of   . The 3 )KNote that baryon-antibaryon productions have a threshold of about 6 GeV/u and are not present in the space radiation transport codewe use in this study.How Space Radiation Risk ... 431curves thus obtained for water target in the 1977 solar minimum GCR environment are shown in Fig. 2. Wesee that they are broad Gaussian-like curves and peak at around 0.85 to 1.2 GeV/u.0.1 1 10Energy for change in cross sections (GeV/u)00.511.5−S(E) (cSv/year)behind 1g/cm2 water10g/cm250g/cm2GCR in deep space (/5)GCR at ISSFigure 3. Sensitivity functions in the 1977 solar minimumGCR environment for water target at the ISS (solid) and indeep space (dashed, scaled down by a factor of 5).0.1 1 10Energy for change in cross sections (GeV/u)00.511.5−S(E) (cSv/year)behind 10g/cm250g/cm21977 Solar Min., water1977 Solar Min., Al1989 Solar Max., waterFigure 4. Sensitivity functions at the ISS behind 10 and 50g/cm L water and aluminum targets in the 1977 solar mini-mum GCR environment and behind water target in the 1989solar maximum GCR environment.Fig. 3 shows that the sensitivity functions at the ISS are very different from those in deep space. At the ISS,the energies corresponding to the 3< )peaks are higher (sometimes by as much as a factor of 4) than thosein deep space as a result of the geomagnetic cutoff effect on low energy GCR particles. Also, the peak valuesof the 3< )functions for the ISS at a given depth are lower roughly a factor of 5 to 7, where the shadowingeffect from the Earth has also contributed. Note that the total dose-equivalent from the 1977 solar minimumGCR without shielding materials is 120 cSv/year in deep space and 15.5 cSv/year at the ISS (about a factor of8 lower).Fig. 4 shows the sensitivity functions for aluminum target in the 1977 solar minimum GCR environment andfor water target in the 1989 solar maximum GCR environment, in addition to those for water target in the1977 solar minimum GCR environment. We see that, for the same GCR environment, the shapes of 3< )functions are similar for aluminum and water targets although the overall magnitudes at a given depth aredifferent due to their different shielding effectivenesses. For different GCR environments, however, both theoverall magnitudes and the shapes of the 3 )curves are different because of changes in the abundance andaverage energy of the GCR particles. For the 1989 solar maximum GCR environment, the energy where crosssections have the largest effects is around 2.4 GeV/u, while for the 1977 solar minimum GCR environment itis around 0.85 to 1.2 GeV/u. Considering the nite energy resolution of this study due to the discreet energypoints, we conclude that cross sections around these peak values, say from 0.6 to 3.4 GeV/u, most affect theGCR dose-equivalent behind water or aluminum targets at the ISS. Note that, for GCR in deep space, crosssections from 0.2 to 1.7 GeV/u have the largest effects [2].432 Zi-Wei Lin and J.H. Adams, Jr.In this study we have only considered the case when all cross sections in a specic energy range are changed bythe same percentage, equivalent to assuming full positive correlations among different cross sections. In reality,however, fragmentation cross sections to different fragments can be unrelated or anti-related with differentmagnitudes, which can lead to much lower sensitivities in the dose-equivalent behind targets to cross sections.Thus this study addresses the sensitivity to the energy of fragmentation cross sections regardless of projectileand fragment combinations involved, and can be considered as a worst-case scenario or an upper limit onthe sensitivity to energy. On the other hand, sensitivity studies on projectile and fragment combinations offragmentation cross sections have been performed [4, 5], but regardless of the energy scale involved. Thereforea further study is needed to simultaneously address the sensitivity to both projectile and fragment combinationsand to energy.3. ConclusionsWe have studied the sensitivity of the total dose-equivalent to fragmentation cross sections in different energyranges at the International Space Station. Both water and aluminum targets with areal densities from 1 to50 g/cm M have been evaluated. Our results show that, for solar minimum GCR environments, cross sectionsbetween 0.85 and 1.2 GeV/u usually have the largest effects on dose-equivalent behind materials. For solarmaximum GCR environments, cross sections at higher energies (around 2.4 GeV/u for the 1989 GCR environ-ment) have the largest effects; however, both the total dose-equivalent and the sensitivity to changes in crosssections are smaller for solar maximum GCR environments. As a comparison, fragmentation cross sectionsbetween 0.3 and 1.2 GeV/u usually have the largest effects in deep space GCR environments. Considering thenite energy resolution in this study, in order to reduce the uncertainty on dose-equivalent behind targets atthe ISS, more emphasis can be put on fragmentation cross sections between about 0.6 and 3.4 GeV/u. Crosssections at both lower and higher energies have smaller effects on the dose-equivalent behind targets, althoughthe sensitivity functions cover quite a broad energy range.4. AcknowledgementsWe thank A.F. Barghouty and M. Bhattacharya for discussions.References[1] J. W. Wilson et al., NASA TP-3495 (1995).[2] Zi-Wei Lin, Proceedings of the Space Nuclear Conference 2005, Paper 1093 (2005).[3] The 1990 Recommendations of the International Commission on Radiological Protection, ICRP Publica-tion 60, Ann. ICRP 21 (1-3) (1991).[4] J. H. Heinbockel et al., Rad. Meas. in press (2005).[5] Zi-Wei Lin, presented at the Space Radiation Shielding Project Principle Investigators' Meeting,Huntsville, AL (2004); to be published.

How Space Radiation Risk from Galactic Cosmic Rays at the International Space Station Relates to Nuclear Cross Sections

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