Predictions of induced radioactivity for spacecraft in low Earth orbit

ihe/. Tracks Rodit%Meas., Vol. 20, No. 1, pp. 101-130, 1992 ht. 3. Radial. Appl.Instnm.,Parr D Rinted in Great Britain

0735-245X/92 S5.00 + .OO

Pergamon Press plc

PREDICTIONS OF INDUCED RADIOACTIVITY SPACECRAFT IN LOW EARTH ORBIT

FOR

T. W. ARM.STRONO and B.L. COLB~RN

Science Applications International Corporation, Route 2, Prospect, TN 38477, U.S.A. (Received

Abstract-Predictions

15 November 1990; in revised form 13 December 1990)

of the induced radioactivity in representative structural materials (aluminum and

stainless steel) have been made for spacecraft in low Earth orbit (LEO) with emphasis on determining the importance of different space radiation sources, secondary particle contributions, and the spatial dependence expected within spacecraft. Orbit parameters corresponding to those of the recently recovered long duration exposure facility (LDEF) satellite are used, which are also very similar to the orbit parameters planned for Space Station Freedom.

1. INTRODUCTION NUCLEAR radiation emissions from the radioactivity induced in spacecraft and payload materials by the ionizing space radiation environment can be an important background source for certain types of sensors, particularly as more sensitive sensors are developed for future missions (e.g. Rester and Trombka, 1989). The induced radioactivity from space radiation is also of interest because for some recoverable spacecraft, namely for the recently recovered long duration exposure facility (LDEF) satellite and some Shuttle missions, the activation of certain materials is utilized as a passive space radiation dosimeter (Pamell and Fishman, 1992; Dyer et al., 1990). The purpose of the calculations reported here is to characterize the induced radioactivity produced in spacecraft materials in low Earth orbit (LEO) in terms of: (a) the importance of different space radiation sources (trapped protons, galactic protons, albedo protons, and albedo neutrons); (b) the importance of secondary particles, particularly secondary neutrons; and (c) the spatial dependence of the radiation environments and effects within spacecraft. The induced radioactivity predictions have been made for the LDEF satellite orbit parameters and mission duration as part of a calculational program under way at the NASA Marshall Space Flight Center to support the LDEF radiation dosimetry data analysis and assessments (Fig. 1). The results are particularly applicable to Space Station Freedom in that the LDEF orbits had a similar altitude range and the same inclination as planned for the space station. The basis of the calculational method (described in Section 2) is the use of the High-Energy Transport Code (HETC) code system (Armstrong and Colbom, 1980) to determine the radiation transport for the different sources, accounting for the production and transport of secondary particles. While the primary

interest is in the induced radioactivity, fluence spectra and absorbed dose estimates are readily obtainable for the calculational method used, and these results are also presented. Results are given (Section 3) for particle fluences, absorbed dose in tissue and silicon, and induced radioactivity as a function of penetration depth. The radioisotope production from aluminum and stainless steel is computed for varying aluminum shielding thicknesses. For the calculations performed to date, several simplifying approximations have been made; namely, a one-dimensional (aluminum slab) model of the spacecraft is used, and the angular variation of the incident radiation is not accurately simulated. In particular, the strong anisotropy of trapped proton spectra (Watts et al., 1989) is not taken into account in these one-dimensional calculations.

2. CALCULATTONAL

METHOD

Radiation transport calculations have been performed to obtain estimates of the depth dependence of fluence, dose, and induced radioactivity produced in spacecraft having the LDEF satellite orbit parameters. The radiation sources considered are geomagnetically trapped (Van Allen belt) protons, galactic protons, and “albedo” neutrons and protons emanating from the Earth’s atmosphere due to cosmic-ray bombardment. The source spectra used as input for the transport calculations are shown in Fig. 2, and the procedure used in estimating these spectra is described in Appendix A. (The trapped electron spectrum is shown in Fig. 2 for comparison only. Since the trapped electrons are of low energy and produce effects very near the spacecraft surface, they are not considered in the transport calculations here.) 101

T. W. ARMSTRONG

102

and B. L. COLBORN

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Spectra for the different sources were assumed incident isotropically on one side of a slab of aluminum 100 g cmw2 in thickness. This is, of course, an important approximation, not only because it neglects the three-dimensional shielding effects of the spacecraft but also because the actual angular distribution of the incident radiation is very different for the different sources, as illustrated in Fig. 3. The transport calculations were carried out using the SAIC version (Armstrong and Colbom, 1980) of the HETC code system (Armstrong and Chandler, 1972). This code uses Monte Carlo methods to obtain a detailed simulation of the radiation transport (Fig. 4). At each nuclear collision during the transport process, a calculation of particle transport inside the nucleus is performed using a highenergy intranuclear-cascade-evaporation (ICE) model (Bertini and Guthrie, 1971; Armstrong, 1980) to obtain the multiplicity, direction, and energy of all secondary particles (Fig. 5). For low-energy neutron (~20 MeV) transport, the high-energy ICE model is not applicable, but various experimental data libraries and transport codes are available in this low-energy region. For the calculations here, the lowenergy neutron source computed by HETC is coupled to the MORSE Monte Carlo code (Straker er al., 1970) for low-energy neutron transport (Fig. 6). The main output obtained from the transport calculations is depth dependent fluence spectra. These

spectra are folded with the response functions given in Appendix B to estimate the absorbed dose in tissue and silicon as a function of aluminum shielding thickness. While HETC provides radionuclide production directly as a natural outcome of the ICE model calculation, the statistical accuracy is generally poor when the product nucleus mass is far removed from the target nuclear mass. Since large targetproduct mass differences are of interest in the present problem (e.g. ‘Be from Fe in stainless steel), we have used an alternate procedure in which the HETC (and MORSE) computed fluence spectra are folded with available activation cross-sections to estimate radionuclide production. Radioisotope production from aluminum and stainless steel were calculated using the cross-sections given in Appendix C. 3. RESULTS Results from the transport calculations are arranged to show the contributions of different radiation sources and the contribution of secondary particles in terms of various effects. A summary of the results is given in this section, with additional results given in Appendices D-F. Table 1 is a guide to the location of various results. The spatial dependence of the results is in terms of the area1 density depth in aluminum from 0 to 100 gcrn-‘. To roughly relate these thicknesses to

PREDICTIONS

OF INDUCED

RADIOACTIVITY

IN LEO

103

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FIG. 2. LDEF exposure to ionizing radiation. Shown are predicted cumulative, orbit-average differential (top) and integral (bottom) Aucncc spectra over the 5.8 yr duration of the LDEF mission.

LDEF, the spacecraft diameter is 32 g cm-*, and the length is 68 gcrn-‘. This is based on an average density obtained from the overall dimensions of 14 ft diameter x 30 ft length, a spacecraft structure weight of 8000 lb, and a weight of 13,400 lb for the experiments (Clark er al., 1984). Fluence Figure 7 compares the proton and neutron fluences (over all energies) for all sources. For the trapped proton environment, the fluence from secondary neutrons exceeds the proton Auence for penetration depths L logcrn-*. The magnitude and spatial dependence of secondary neutrons from galactic protons is comparable to that of the secondary neutrons from trapped protons. NT 20/1-H

Dose Figure 8 compares the importance of different sources in terms of the absorbed dose in tissue and in silicon. The trapped proton source dominates for penetration depths Q 50 g cm-‘. The al&do sources contribute at most a few percent. Additional results for secondary particle contributions to the absorbed dose are given in Appendix E.

Activation Figure 9 compares the contribution of different sources to UNa and to ‘Be production from aluminum. The galactic source contribution exceeds the trapped source contribution for depths t 50 g cmm2 for %a production and 225 g cmm2 for ‘Be

and B. L. COLBORN

T. W. ARMSTRONG

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FIG. 3. Illustration of the nonuniform angular variation of LDEF exposure to ionizing radiation. Trapped proton exposure occurs in the South Atlantic Anomaly region where the flux is highly anisotropic at LDEF altitudes, with protons confined mainly in planes perpendicular to magnetic field lines and with in-plane asymmetry due to the east-west effect. Galactic protons are blocked out from below by the shielding effect of the earth. The angular distribution of albedo neutrons emanating from the Earth’s atmosphere is also geometrically constrained due to Earth shielding.

production. The relative importance of the galactic source, which has a harder spectrum, is expected to be higher for the higher threshold activation products, which is consistent with these ‘Be vs **Na results. Figure 10 shows that for the trapped proton source and the case of r*Na production the secondary neutron contribution becomes important at depths b30gcm-2.

lmut

Results from calculations of radioisotope production in stainless steel are given in Appendix F. 4. DISCUSSION In terms of the relative contribution of different space radiation environments to the induced radioactivity for the low-altitude, low-inclination LDEF

“Physics” Modulr

FIG. 4. Overiew of the High Energy Transport Code (HETC)

PREDICTIONS

OF INDUCED

RADIOACTIVITY

105

IN LEO

PROTON

FIG. 5. Illustration of the two-step intranuclear-cascadeaporation model used in the HETC for computing secondary particles from high-energy (3pallation”) nuclear collisions.

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FIG. 6. SAIC version of the HETC system, which includes high-energy proton and neutron transport,

low-energy neutron transport, and gamma-ray transport.

Table I

Radiation source contribution Secondary particle contribution

Flucncc

Fluence spectra

Tissue dose

Silicon dose

Aluminum activation

stainless steel activation

Fig. 7

Appendix D

Fig. 8

Fig. 8

Fig. 9

Appendix F

Appendix D

Appendix D

Appendix E

Fig. 10

Appendix F

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FIG. 8. Importance of various radiation sources in terms of absorbed dose in tissue and silicon for the total dose over the duration of the LDEF mission (top graph) and as a percent of the total dose at each depth from all sources (bottom graph).

FIG. 7. Depth dependence of proton (primary and secondary proton contributions) and neutron Ruences over all energies produced by trapped proton, galactic proton, albedo proton, and albedo neutron environments during the LDEF mission time. The different environments are all assumed incident isotropically on one side (0 depth) of an aluminum slab 100 g cnm2 in thickness.

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PREDICTIONS

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OF INDUCED RADIOACTIVITY IN LEO

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FIG. 9. Importance of different sources in terms of =Na and ‘Be production from aluminum. The production is normalized for the total lifetime of the LDEF mission.

type orbits considered, the results indicate that the radioisotope production near the spacecraft surface should be dominated by the geomagnetically trapped protons. However, for activation products having relatively high energy thresholds for production, such as ‘Be production from aluminum, production from the galactic proton source can become comparable to that from trapped protons at relatively small (- 25 g cm-~-~)shielding depths, as shown by Fig. 9. The albedo neutron and albedo proton sources are predicted to be relatively unimportant radioisotope producers. The rather complex angular variation of the spacecraft radiation exposure (as illustrated in

Fig. 3), together with the depth dependence obtained for the one-dimensional calculations performed here, suggests that a more realistic spacecraft geometry/mass model is needed for accurately predicting induced radioactivity at depths more than a few tens of g cm-*. In terms of the importance of secondary particles, the results show that the secondary particle contribution depends strongly upon the environment source, shielding depth, and the particular radiation effect of interest. Secondary neutrons dominate the energy-integrated particle fluence at depths L 10 gcme2 for the trapped proton source, and

T. W. ARMSTRONG

108

and B. L. COLBORN

40

20

60

100

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60% 50% 40% 30% 20% 10% 0% 0

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FIG. 10. Contribution of secondary particles in producing =Na from aluminum by incident trapped protons; top graph for production over LDEF mission, bottom graph as percentage of total production from primary and secondary particles by trapped protons.

neutrons dominate at all depths for the galactic proton source (Fig. D2 of Appendix D). For the trapped proton source, secondary neutrons and protons typically become significant at shielding depths beyond a few tens of g cm-*, as shown by the absorbed dose comparisons of Fig. El, the spectral comparisons of Figs D5 and D6, and the radioisotope production results of Fig. 10 and Appendix F. Results from the measurements of LDEF-induced radioactivity now in progress should allow a definitive evaluation of the accuracy of predictive methods and radiation environments models such as those employed here, which will in turn help establish the

accuracy of applying such methods for ionizing radiation assessments related to Space Station Freedom. Acknowledgemenfs-We wish to thank Dr T. A. Pamell of the NASA Marshall Space Flight Center for his numerous helpful suggestions. This work was supported by NASA Contract NAS8-38427. REFERENCES Adams J. H. Jr, Letaw J. R. and Smart D. F. (1983) Cosmic ray effects on microeletronics, Part II: the geomagnetic cutoff effects. NRL Memorandum Report 5099, May 1983.

PREDICTIONS

OF INDUCED

Adams J. H. Jr, Silberberg R. and Tsao C. H. (1981) Cosmic ray effects on microelectronics, Part I: the near Earth environment. NRL Memorandum Report 4506, August 1981. Alsmiller R. G. Jr, Armstrong T. W. and Coleman W. A. (1970) The absorbed dose and dose equivalent from neutrons in the energy range 60 to 3000 MeV and protons in the energy range 400 MeV to 3000 MeV. Nucl. Sci. Engr. 42, 367. Armstrong T. W. (1980) The intranuclear-cascade-evaporation model. In: Computer Techniques in Radiation Transport and Dosimetry (Edited by Nelson, W. R. and Jenkins, T. M.), Chapter 20. Plenum Press, New York. Armstrong T. W. and Chandler K. C. (1972) The highenergy transport code HETC. Nucl. Sci. Engr. 49, 110. Armstrong T. W. and Chandler K. C. (1973) Calculation of the absorbed dose and dose equivalent from neutrons and protons in the energy range from 3.5 GeV to l.OTeV. Health Phys. 24, 277. Armstrong T. W., Chandler K. C. and Barish J. (1973) Calculations of neutron flux spectra induced in the Earth’s atmosphere by galactic cosmic rays. J. Geophys. Res. 78, 2715.

Armstrong T. W. and Colborn B. L. (1980) A thick-target radiation transport code for low mass heavy ion beams. HETCLHI. Nucl. Instrum. Mesh. 169. 161. Aylmer D.,‘Begemann F., Cloth P., Dragovitsch P., Englert P., Filges D., Herpers U., Hertzog G. F., Jermaikan A., Klein J., Kruse T. H., Michel R., Moniot R. K., Middleton R., Peiffer F., Signer P., Stuck R., Theis S., Tuniz C., Vadja S., Weber H. and Weiler R. (1987) Monte Carlo modelling and comparsion with experiment of the nuclide production in thick stony targets isotrooicallv irradiated with 600 MeV urotons. CERN SC96 - Experiment, Kernforschungsanlage Julich GMBH, Report Jul-2 130. Bertini H. V. and Guthrie M. P. (1971) Results from medium-energy intranuclear-cascade calculations. Nucl. Phys. A169, 670. Bum11 M. 0. (1964) The calculation of proton penetration and dose rates. NASA TM X-53063, August 1964. Clark L. G., Kinard W. H., Carter D. J. Jr and Jones J. L. Jr (1984) The long duration exposure facility (LDEF), mission 1 experiments. NASA SP-473, NASA Langley Research Center, Langley, Virginia. DLC-31 (1976) 37 Neutron, 21 Gamma Ray Coupled Multigroup Library. Radiation Shielding Information Center, Data Library Collection DLC3l/(DPLl/FEWGl), Oak Ridge National Laboratory, 1976. Dyer C. S., Truscott P. R., Sims A. J., Comber C., Hammond N. D. A. and Haskins P. (1990) Calculation and observation of radioactive backgrounds in spaceborne science payloads. ConJ ESA Space Envir. Analysis Workshop, ESTEC, October 1990. ESA Report WPP-23. Harmon A. (1990) Private communication. NASA Marshall Space Flight Center. Huntsville, Alabama. Heinrich W. and Spill A. (1979) Geomagnetic shielding of cosmic rays for different satellite orbits. J. Geophys. Res. 84, 440

1.

Irving D. C., Alsmiller R. G. Jr and Moran H. S. (1967) Tissue current-to-dose conversion factors for neutrons from 0.5 to 60 MeV. ORNL-4432. Oak Ridae National Laboratory, Oak Ridge. Tennessee. Kuznetsov S. N., Logachev Yu. I., Ryumin S. P. and Trebuckhovskaya G. A. (1980) Albedo of galactic cosmic rays from the COSMOS-721 data, MG-28, p. 161. Michel R. and Stuck R. (1984) On the production of cosmogenic nuclides in meteorites by primary galactic particles: cross sections and model calculations. J. Geophys. Res. 89, B673. Pamell T. A. and Fishman G. J. (1992) Induced radio-

RADIOACTIVITY

IN LEO

activity in LDEF. Section VI in Ionizing radiation exposure of LDEF (Edited by Benton, E. V. and Heinrich, W.). Nucl. Tracks Radial. Meas. 20, 95. Pennypacker C. R., Smoot G. F., ButTmgton A., Mu R. A. and Smith H. (1973) Measurement of geomagnetic cutoff near Palestine, Texas. J. Geophys. Res. 78.1515. Prcszler A. M., Simnett G. M. and White R. S. (1972) Earth albedo neutrons from 10 to 100 MeV. Phys. Rev. few. 28,982.

Rester A. C. Jr and Trombka J. I. (1989) (Editors) HighEnergy Radiation Background in Space, AIP Conf. Proc. 186. AIP, New York. Straker E. A., Stevens P. N., Irving D. C. and Cain V. R. (1970) The MORSE cod-a multigroup neutron and gamma-ray Monte Carlo transport code. ORNL-4585, bak Ridge National Lab., Oak Ridge, Tennessee. Watts J. W. Jr 119921Predictions of LDEF fluxes and dose due to geomag&cally trapped protons and electrons. Section III in Ionizing radiation exposure of LDEF (Edited by Benton, E. V. and Heinrich, W.). Nucl. Tracks Radiat. Meas. 20, 85.

Watts J. W. Jr, Pamell T. A. and Heckman H. H. (1989) Approximate angular distribution and spectra for geomagnetically trapped protons in low-Earth orbit. In: High-Energy Radiation Backgrounds in Space, AIP Conf. Proc. 186 (Edited by Rester, A. C. Jr and Trombka, J. I.), pp. 75-85, November 1987, Sanibel Island, Florida. AIP, New York. Wenzel K. P., Stone E. C. and Vogt R. E. (1975) Splash albedo protons between 4 and 315 MeV at high and low geomagnetic latitudes. J. Geophys. Res. 80, 3580. White R. S., Moon S., Preszler A. M. and Simnett G. M. (1972) Earth albedo and solar neutrons. Report IGPPUCR-72-16, U. C. Riverside, 1972. Zazula J. M., Cloth P., Filges D. and Sterzenbach G. (1986) Secondary particle yield and energy release data from intranuclear-cascade-evaporation model calculations of high energy (2&11OOMeV) neutron interaction with elements of shielding and biological importance. Nucl. Instrwn. Meth. Bl6, 506. Zerby C. D. and Kinney W. E. (1965) Calculated tissue current-to-dose conversion factors for nucleons below 4OOMeV. Nucl. Insrrum. Meth. 36, 125.

APPENDIX A SOURCES OF LDEF IONIZING RADIATION EXPOSURE Trapped protons (omnidirectional) The trapped proton spectrum for the LDEF mission calculated by Watts (1992) was used as the trapped proton source for the HETC transoort calculations. The omnidimctional, altitude-average digerential and integral cumulative trapped proton flux spectra over the duration of the LDEF mission are shown in Fig. Al. For the one-dimensional transport calculation, one-half of this fluence was assumed to be incident isotropically on one side of the slab of material. While isotropy is a reasonable compromise for use in a one-dimensional approximation, the actual angular distribution is, as shown by Watts et al. (1989). highly anisotropic. Trapped electrons The trapped electrons are of such low energy that they contribute significantly to the dose only at small penetration depths ($0.5 g cmm2) (Watts, 1992) and do not contribute at all to radionuclide production. Thus, transport calculations for trapped electrons have not been made here, but the trapped electron spectra (from Watts, 1992) are given in Fig. A2 for comparison with trapped protons.

T. W. ARMSTRONG

110

and B. L. COLBORN

TmPpod Proton Sputm

102

Proton Energy (MeV)

FIG. Al. LDEF exposure to trapped protons, averaged over LDEF attitudes and cumulative over the LDEF mission duration, calculated by Watts (1992). Shown here are omnidirecfionol spectra; the trapped proton spectra at the LDEF altitude and orbit inclination are actually highly anisotropic (Watts et al., 1989). Galactic protons For the galactic proton spectrum we start with the analytic fit given by Adams er al. (1981) for the exomagnetime-dependent spectrum, which at solar tospheric, minimum and solar maximum reduces to F(E) = 10” (E/l 17500) where M = 6.52 exp{ -0.8(log,,E)‘j

-4

a = -2.2{ 1- exp[-b(log,,E)“‘]} b = 0.117, at solar minimum and b = 0.079, at solar maximum.

Here F has units of protons m-r steradian-’ MeV-’ and E is in MeV. This fit to the galactic spectrum (multiplied by 4x steradians, converted to cme2, and multiplied by the LDEF mission duration of 2114 days) is shown as the “outside Earth’s magnetosphere” spectrum in Fig. A3. To take into account the effect of geomagnetic shielding at the LDEF orbit, we have used the geomagnetic field “transmittance fraction” given in Adams et al. (1983) for 30” inclination at 400km altitude, which is based on the cosmic ray trajectory tracing calculations of Shea and Smart for effective geomagnetic cutoffs over a world-wide, longitudtlatitude grid at 400 km altitude and the orbit averaging method of Heinrich and Spill (1979). This fraction of the exoatmospheric galactic protons transmitted through the

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A2. LDEF exposure to trapped electrons, averaged over LDEF alitiudes and cumulative over the LDEF mission duration, calculated by Watts (1992).

PREDICTIONS

OF INDUCED

RADIOACTIVITY

IN LEO

Ill

octccttaPNtcn spcctrc -ldrmodnun ___.. *-

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FIG. A3. Galactic proton spectra in interplanetary space (from Adams er al., 1981) and at LDEF orbit after attenuation by geomagnetic field. 1

1 0.9

0.9

0.8

0.8

0.7

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0.6

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A4. Transmission factor for galactic proton penetration of geomagnetic field; averaged over 30” inclination, circular orbit at 400 km altitude (adapted from Adams er al., 1983).

geomagnetic field is shown in Fig. A4, and the result of applying this transmission factor to the exomagnetospheric spectrum gives the curves labeled “LDEF orbit” in Fig. A3. Another factor influencing LDEF’s exposure to galactic protons is the shielding effect of the Earth’s “shadow”. The solid angle occulation is

f 110” about the zenith direction. In the transport calculations, the incident galactic proton flux was assumed incident isotropically over &90” about the target surface normal. The galactic proton integral and differential energy spectra over the LDEF mission duration are given in Fig. AS.

AG = 2n {1 - [(I?, + h)2 - R:]“r/(R, + h)} where

R, is the Earth’s radius (6371 km) and h is the orbit altitude. For an average LDEF altitude of about 450 km, AS2/4r = 0.32. Thus, 32% of the 4x solid angle is blocked by the Earth, and the incident proton directions are within

A&t-do protons’ 9econdary protons produced in the Earth’s atmosphere by cosmic rays can escape upward as “splash albedo” and become trapped in the Earth’s magnetic field when the

*We wish to thank J. Adams, Naval Research Laboratory, measurements.

for providing background

material on albedo proton

T. W. ARMSTRONG

112

and B. L. COLBORN

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E.Energy (MN) FIG. A5 Cumulative galactic proton spectra over the duration of the LDEF mission. proton energy is below the geomagnetic cut off. These protons are guided by the field to impact with the atmosphere in the hemisphere opposite to their formation, providing a “reentrant” albedo. The splash albedo spectrum has been measured by several balloon flights at different latitudes. In particular, Wenml et al. (1975) and Pennypacker er al. (1973) measured the albedo spectrum in the 4 MeV-1 GeV energy range at about 4 gcme2 residual atmosphere over Palestine, TX (42”N geomagnetic latitude, 4.5GV geomagnetic cut off). Measurements of the proton albedo by the Cosmos-721

satellite (polar orbit, 210-240 km) have been reported by Kuznetsov ef al. (1980). For a 4.5 GV cut off they find a similar spectral shape as for the balloon tlights but a factor of four higher intensity, which Kuznetsov et al. attribute as possibly due to the different angular distribution of albedo protons at satellite vs balloon altitudes. For the splash albedo calculations in the present work, we have used a fit to these satellite and balloon measurements, with the magnitude of the balloon data increased by a factor of four and the reported measurements per steradian multiplied by 271to obtain an omnidirectional flux. These

Albedo Proton Data

1

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FIG A6 Measurements of the “splash” proton albedo spectrum at balloon (Wenzel et al., 1975; Pennypacker el al.. 1973) and satellite (Kuznetsov et al.. 1980) altitudes. (The balloon data have been multiplied by a factor of four to get agreement with the magnitude of the satellite data.) Also shown is a fit to the data used here as input for the transport calculations.

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A7. Cumulative aknxlo proton spectra over the duration of the LDEF mission.

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Measured upward Moving Flux, Preszbr et al. (1972)

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FIG. AS. HETC code calculations of the neutron albedo spectra from cosmic-ray bombardment of the Earth’s atmosphere (Armstrong er al., 1973). Shown are flux (4) and current (J) spectra for the upward moving (2s) and omnidirectional (4x) neutrons at 5 g cm-* residual atmosphere, 42”N geomagnetic latitude, and solar minimum conditions. Also shown are data from balloon flight measurements by Preszler er al. (1972) and by White er al. (1972).

T. W. ARMSTRONG

114

and B. L. COLBORN

FIG. A9. Cumulative neutron albedo spectra over the duration of the LDEF mission. The points (open circles) are from HETC code calculations of the differential spectrum at the top of the atmosphere (from Fig. A8), which have been renormalized for the LDEF orbit. data and the fit used are shown in Fig. A6, with the fit being 4 = 0.00113 exp( - O.O095E), IO
llS
where $I has units cm - 2 s- ’ MeV - ’ This differential flux multiplied by the LDEF mission duration, together with the corresponding integral fluence, are shown in Fig. A7. Neutron albedo

Some of the neutrons produced in the Earth’s atmosphere by cosmic-ray bombardment escape the top of the atmosphere to constitute a neutron albedo. Several measurements and calculations of the neutron albedo near the top of the atmosphere have been made-e.g. Fig. A8. The results of Fig. A8 are for upward moving flux at 45 km altitude, 42”N geomagnetic longitude. and solar minimum. We have fit the calculated spectrum as. !#I(E) = 0.047E

: h’.

10-5
= 0.40 expt - 0 97E). = 0.15 E ’ “,

0.1
= 0.0086 exp( - 0.045E) + 0.0021 exp( -0.0085E). = 1.95P’h’.

106E<200 200gE~3000

where r#~has umts cm -: SK’ MeV-’ and E is m MeV.

The analytic fit of Fig. A7 is scaled as follows to obtain an estimate of LDEF exposure to albedo neutrons. The maximum geomagnetic latitude reached by the LDEF orbit is &, 2 40”. which is approximately the latitude corresponding to the spectrum of Fig. A7. From measurements of the l-10 MeV albedo flux dependence on magnetic latitude, the variation of the albedo flux over LDEF orbits (ratio of flux at I, = 40” to flux at & = 0”) is about a factor of three, and the ratio of the maximum albedo flux to the 28” inclination orbit-average flux is estimated to be a factor of ~2. Thus, while a detailed orbit integration has not been carried out to obtain the average LDEF exposure to albedo neutrons, the &,, = 42” spectrum of Fig. A7 is multiplied by 0.5 as an estimate of the orbit-average exposure. To take into account altitude differences, I/r’ scaling is assumed and the 45 km spectrum of Fig. A7 is multiplied by 0.88 to obtain the spectrum at 450 km, which is approximately the average LDEF altitude. Finally, the flux of Fig. A7 is multiplied by the LDEF on-orbit time (2114 days) to obtain the albedo neutron fluence over the duration of the LDEF mission. The product of these scale factors (8 x 10’) times the analytic fit curve of Fig. A7 gives the estimate used for LDEF exposure to albedo neutrons (Fig. A9). At altitudes of about 450 km the albedo neutron directions are restricted within a cone half-angle of 70 about the zenith because the Earth shields neutrons of other directions. Thus, in the transport calculations, only neutron directions within & 70” about the slab normal were allowed.

Table A I. Source oarameters used for transoort calculations

Source

Mimmum Incident energy

Maximum incident energy

Trapped protons Galactic protons Albedo protons Albedo neutrons

I5 MeV 3.2 GeV 15 MeV 1 keV

600 MeV IOOGeV 3.5 GeV 3.0 GeV

Omnidirection integral fluence above E,,,,, (cm-*, over LDEF mission) 4.3 2.8 2.3 7.4

x x x x

IO9 10’ 10’ 10’

Range of angular distribution (steradians) 4x 2n 4rr 1.3rr

PREDICTIONS

104

10-Z

OF INDUCED

10-I

10'

IO0

RADIOACTIVITY

If

18

10'

115

IN LEO

18

lo1

E~WIY (Me’4

FIG. B 1. Flux-to-dose conversion factors used for absorbed dose in tissue. between 60MeV and 3 GeV, and from Armstrong and Chandler (1973) between 3 and 100 GeV. For protons incident on silicon, the Burrell (1964) stop ping power approximation was used below 200 MeV with the HETC code system kerma factor calculations of Zaxula et al. (1986) used above 200 MeV. The response to neutrons is based on the DLC31 data library (1976) below 20 MeV and the Zaaula et al. (1986) calculations at higher energies.

Summary

Table A 1 summarizes the energy range and notmalixation for the different sources used as input for the transport calculations. Also indicated is the angular distribution range assumed in computing the source spectra per unit solid angle.

APPENDIX B DOSE RESPONSE FUNCTIONS To estimate depth dependent doses the ilux spectra at various depths from the transport calculations were folded with dose response functions. The response functions used (Figs Bl and B2) are “surface doses” for protons or neutrons incident normally on a slab of tissue or silicon. For protons incident on tissue, the response function was estimated using the stopping power approximation of Burrell(1964) below 60 MeV and the transport calculation results of Zerby and Kinney (1965) in the range from 60 to 400 MeV, Alsmiller et al. (1970) from 400 MeV to 3 GeV, and Armstrong and Chandler (1973) from 3 to 1OOGeV. For neutrons incident on tissue, results from Irving et al. (1967) were used below 60 MeV, from Alsmiller et al. (1970)

103

10-l

IO0

IO'

APPENDIX C ACTIVATION CRO!3S-SECTIONS Michel and co-workers (e.g. Michel and Stilck, 1984) have developed a set of activation cross-sections for neutrons and protons incident on various elements by using a combination of experimental data, semi-empirical methods, and nuclear models. Predictions of the spatial dependence of radioisotope production in thick composite targets using the Michel et cl. cross-section set folded with flux spectra calculated by the HETC transport code are in very good agreement with experimental data for high-energy proton irradiations (Ayhner er cl., 1987). Thus, in these initial calculations we have used the Michel et al. activation

102

IO3

10'

IO8

EnWY (MeV)

FIG. B2. Flux-to-dose conversion factors used for absorbed dose in silicon.

T. W. ARMSTRONG

116

and B. L. COLBORN

cross-sections reported in Aylmer et al. (1987) and shown in Figs Cl-C3.

APPENDIX F RESULTS FOR RADIOlsOTOPE PRODUCTION FROM STAINLESS STEEL

APPENDIX D ADDITIONAL FLUENCE RESULTS Figures Dl-D4 compare the depth dependent fluences (over all energies) for primary and secondary particles for LDEF exposure to trapped proton, galactic proton, albedo proton, and albedo neutron sources. Fluence spectra of protons and neutrons from all sources are compared for 10 and 5Og crns2 aluminum shielding depths in Figs D5 and D6.

APPENDIX E ADDITIONAL DOSE RESULTS Figures El-E4 compare primary vs secondary particle contributions to the absorbed dose in tissue for each of the LDEF exposure sources considered.

zZNafrom

Al

,

,s-*~

Induced radioactivity measurements are being made for several LDEF components which are made of stainless steel, and calculated results are given here for several radioisotopes produced in thin stainless steel samples behind varying thicknesses of aluminum shielding. The stainless steel composition used in the calculations (75.3% Fe, 15.4% Cr. and 4.3% Ni, by weight) is based on post-flight X-ray fluorescence measurements [made at the NASA Marshall Space Flight Center, and provided by Harmon (1990)] of segments of the LDEF trunnion. Figures Fl-F3 compare the contributions from different sources to each of the radioisotopes considered. Figures F4-F8 compare the primary vs secondary particle contributions to the production of each isotope for trapped proton and galactic sources.

\

ld

10'

Energy (Mew

10'

“7

I

i i-

L t

‘Be

from

Al

/

I

to* EW&’ (MeV) FIG. Cl. Cross-sections for =Na and ‘Be production from aluminum; solid line by protons, dashed line by neutrons, from Michel er al., as reported in Aylmer er al. (1987).

117

PREDICTIONS OF INDUCED RADIOACIWITY IN LEO

lo.2

I 102

IO3

EWY WV) FIG.C2. Cross-sections for J7Co, “Mn and ‘Be production from iron; solid tine by protons, dashed tine by neutrons, from Michcl et CJ., as reported in Aylmcr cr 1 (1987).

T. W. ARMSTRONG

118

‘tin

from

1

NI

and B. L. COLBORN

1“Co

from

NI

looL ,

f102

10'

1OJ

10'

102

Energy (Mob')

EnergyWV)

3

NI -_ .

:.

EWJY

WV)

‘t

Energy (MeV)

FIG. C3. Cross-sections for “Mn, %Co, 57Co and yCo production from nickel; solid line by protons, dashed line by neutrons, from Michel et al., as reported in Alymer et al. (1987).

PREDICTIONS

0

OF INDUCED

20

40

RADIOACI’M’IY

60

IN LEO

80

119

100

Depth in Aluminum (g/cd) FIG. Dl.

!kcondary particle production for trapped proton source.

1 O’O

105 0

20

40

60

80

Depth in Aluminum (gbn2) FIG. D2. Secondary partkk

NT 20/l-I

production for galactic proton &wrce.

100

T. W. ARMSTRONG

120

and B. L. COLBORN

~‘,““,‘,“‘~“““~

Albedo

Proton

Souroo

secondary protons

0

20

40

60

100

60

Depth in Aluminum (g/cm*) FIG. D3. Secondary particle production for ah-do

1 oQ

E

n

1

1

’

t

a

1

”

1

”

r

proton source.

-

Albado

‘I”

Neutron

N

Source

i

primary + secondary neutmnr

o-

108

‘6 j 8

10’

.g s :: 9

secondary protons

106

105

0

20

40

60

60

Depth in Aluminum (g/cm*) FIG. D4. Secondary particle production

for albedo neutron source.

100

PREDICTIONS

1

OF INDUCED

.

“.‘.“I

.

RADIOACTIVITY

“.‘,‘I

IN LEO

121

...“..1

.

mmco spoctmCompwbon.

-cPmmlnlmaawlb-Pfalca~m-

10' 10.'

10'

100

D5.

fluencc sptara

.I,,..’

10' 10.'

100

at a d&h

s l,l..a’

10'

ld

ld

Em

10 gcxx~-~ radiation

,.cc,a’

w

from LDEF exposure to

* s .,,,,,’

103

ld

IO'

10s

WV1

.*.u

h. 10'

105

WV)

FIG. D6. Comparison of fluence spectra at a depth of 50 g cm-’ in aluminum from LDEF exposure to different ionizing radiation sources.

T. W. ARMSTRONG

122

and B. L. COLBORN

t.,,,..,,,.,,,,,,...t 0

20

40

60

80

100

Depth In Ahlnum (gkn?) FIG. El. Secondary particle contribution

to absorbed dose in tissue for trapped proton source

10'

3

E

H if5

‘2 3

k 9

100

0

20

40

60

80

100

Depth In Aluminum(@cm~ FIG. EZ. Secondary particle contribution

to absorbed dose in tissue for galactic proton source

PREDICTIONS

OF INDUCED RADIOACTIVITY IN LEO

123

10'

AJbmdoPIam6ouro

t

FIG. E3. Secondary particle

100

1

0

Ahdo

20

40

60

t4eutm Sourca

1

80

100

DephinMmlhJm(g/cm2)

FIG. E4. Secondary particle contribution

to absorbed dose in tissue for albedo neutron source.

T. W. ARMSTRONG

124

and B. L. COLBORN

%J

from S Stool

1

60

40

Dspth in Aluminum (g/cm?

/ 0

/ 20

/,

a

I

40

I

I

I

*

60

I

-1

* 80

2

’

100

Depth in Aluminum (g/cm’) FIG. Fl.

Contribution (bottom

of various space radiation sources to the production of 58Co (top graph) and “Co graph) from stainless steel, normalized for LDEF mission duration.

60

40

Depth in Aluminum(gkn?)

103’’ 0

’

’

’

’

’

’

’

s

’

’

’

40

20

’

’

’

’

’

’

’

60

60

100

Depth in Aluminum(g/cm2) FIG. F2. Contribution of various space radiation sources to the production of Wo (top graph) and %Mn (bottom graph) from stainless steel, normalized for LDEF mission duration.

I,.

10'

0

20

I

I

40

I1

*

h

*

60

I

*

I

60

*

.

I

1oc

Depth in Aluminum (g/cm2) FIG. F3. Contribution

of various space radiation sources to the production of ‘Be from stainless steel, normalized for LDEF mission duration.

T. W. ARMSTRONG

126

and B. L. COLBORN

Id

10'

g -8

2 to3

f

il loz

Trappad Proton Sourca

10'

ld

10'

P

P _ ld 5

1 ld

Galactic Proton Sourca

10'

0

40

DepthIn Atumirum

60 (gkn?)

FIG. F4. Secondary particle contribution to J’Co production from stainless steel by trapped proton (top) and galactic proton (bottom) sources, normalized for LDEF mission duration.

PREDICTIONS

I

lo2

I

I

0

I

I

OF INDUCED

I,

I

*

40

20

.

RADIOACTIVITY

.

.

.

60

.

.

IN LEO

.

.

80

.

127

..I 100

Depthin Aluminum(+n2)

Gala& 0

Pmmn Sourea 20

40 De&

60

80

100

In Aluminum WcmS

FIG. F5. Secondary particle contribution to J7Co production from stainless steel by trapped proton (top) and galactic proton (bottom) sources, normalized for LDEF mission duration.

T. W. ARMSTRONG

28

and B. L. COLBORN

Trapped Pmlon Source 0

40

20

60

60

100

60

100

Depthin Aluminum(ghn2)

0

40

20

60

Depth in Aluminum (g/cm21

FIG. F6.

“Co production from stainless steel by trapped proton (top) and galactic proton (bottom) sources, normalized for LDEF mission duration.

Secondary

particle contribution

to

PREDICTIONS

0

20

OF INDUCED

40

RADIOACTIVITY

60

129

IN LEO

60

100

Cbpth In Aluminum (@an2)

20

40

60

60

100

DepthIn Aluminum(g/cm2)

FIG. F7. Secondary particle contribution to YMn production from stainless steel by trapped proton (top) and galactic proton (bottom) sources, normalized for LDEF mission duration.

T. W. ARMSTRONG

130

and B. L. COLBORN

10'

lo3

E

3

z

Id

i 10'

Trapped Proton Source

1o”

0

20

40

60

Depth In Aluminum (gkm2)

lo3

F d

3 to2 i

Galactic Proton Source

10' 0

20

40

60

60

100

Depth in Aluminum(e/cm*) FIG. F8.

Secondary particle contribution to ‘Be production from stainless steel by trapped proton and galactic proton (bottom) sources, normalized for LDEF mission duration.

(top)