The Long Duration Exposure Facility (LDEF) spacecraft flew in a 28.5 deg inclination circular orbit with an altitude in the range from 172 to 258.5 nautical miles. For this orbital altitude and inclination two components contribute most of the penetrating charge particle radiation encountered - the galactic cosmic rays and the geomagnetically trapped Van Allen protons. Where shielding is less than 1.0 g/sq cm geomagnetically trapped electrons make a significant contribution. The 'Vette' models together with the associated magnetic filed models were used to obtain the trapped electron and proton fluences. The mission proton doses were obtained from the fluence using the Burrell proton dose program. For the electron and bremsstrahlung dose we used the Marshall Space Flight Center (MSFC) electron dose program. The predicted doses were in general agreement with those measured with on-board thermoluminescent detector (TLD) dosimeters. The NRL package of programs, Cosmic Ray Effects on MicroElectronics (CREME), was used to calculate the linear energy transfer (LET) spectrum due to galactic cosmic rays (GCR) and trapped protons for comparison with LDEF measurements

Armstrong, T. W.

Benton, E. V.

Derrickson, James H.

Parnell, T. A.

Watts, John W.

English

NASA Technical Reports Server

PREDICTION OF LDEF IONIZING RADIATION ENVIRONMENT
N92-
John W. Watts
ES62, NASA/Marshal! Space Flight Center
AL 35812
Phone: 205/544-7696, Fax: 205/544-7754
23292
T. A. Parnell
ES62, NASA/Marshall Space Flight Center
AL 35812
Phone: 205/544-7690, Fax: 205/544-7754
James H. Derrickson
ES62, NASA/Marshall S1)ace Flight Center
AL 35812
Phone: 205/544-7698, Fax: 205/544- 7754
T. W. Armstrong
Science Applications International Corporation
Route 2, Prospect, TN 38477
Phone:615/468-2603, Fax:615/268-2676
E. V. Benton
Physics Department, University of San Francisco
2130 Fulton St., San Francisco, CA 94117-1080
Phone:415/666-6281, Fax:415/386-1074
SUMMARY
The LDEF spacecraft flew in a. 28.5 ° inclina.tion circular orbit with an altitude in the range
fl'om 172 to 258.5 nautical miles. For this orbital altitude and inclination two components con-
tribute most of the penetrating charge particle radiation encountered--the gala.ctic cosnfic rays
and the geonmgnetically trapped Van Allen protons. Where shielding is less than 1.0 g/cm '2
geomagneticMly trapped electrons make a significant contribution. The "Vette" models (ref. 1-3)
together with the associated magnetic field models (ref. 4) were used to obtained the trapped
electron and proton fluences. The mission proton doses were obtained from the fluence using
the Bm'rell proton dose program (ref. 5). For the electron and bremsstrahhmg close we used the
MSFC electron dose program (ref. 6,7) The predicted doses (ref. 8) were in general agreement
with those measured with on-board thermohmfinescent detector (TLD) dosimeters (ref. 9). The
NRL package of programs, CREME, (ref. 10) was used to calculate the linear energy transfi_r
(LET) spectrum due to galactic cosmic rays (GCtl) and trapped protons (ref. 8) for comparison
with LDEF measurements (ref. 11).
213
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INTRODUCTION
The LDEF spacecraftflew in a 28.5° inclination circular orbit with an altitude in the range
fl'om 172 to 258.5 nautical miles. It wasgravity-gradient stabilized alld oriented so that one side
alwayspointed along the velocity vector. For this orbital altitude and inclination two coml)o-
nents c(mtribute most of the penetrating charge particle radiation encountered--the galactic cos-
mic rays and the geonmgnetically tra.pped Van Allen 1)rotons. Where shielding is less than 1.0
g/cm 2 geomagnetically trapl)ed electrons make a significant contribution. All three som'ces are
strongly modulated by the Earth's magnetic field. The trapped particles follow a helical path
al)out a magnetic field line as shown in figure 1. As the field intensity increases, both the diam-
(,ter and the pitch c,f the helix decrease until the pitch becomes zero. The point with zero pitch
angle is called the mirror point and the center of the helical path is called the guiding center.
From here the helix reverses direction and particles travel up the field line toward decreasing
field intensity and away fi'om the Earth. Almost all the trapped flux at LDEF altitudes will be
encountered in the region called the South Atlantic Anomaly (SAA) shown in figure 2, which is
produced because the Earth's magnetic field, though approximately dipolar, is not centered on
the Ea.rth. In the South Atlantic Anomaly ahnost all the particles observed are near their mir-
ror points. Any trapped particle there which is not nearly mirroring will travel deep into the at-
mosphere and be scattered or stopped by atmospheric interactions. Thus the flux is anisotropic
with most of the ftttx arriving from a. narrow band perpendicular to the local geomagnetic field
direction. At nxospheric interactions also affect the trapped proton angular distribution in another
fashion as shown in figure 3. Trapped protons that are observed traveling eastward are following
guiding centers above the observation point mad protons traveling westward are following guiding
centers below the observation point. The gyroradius (the radius of the helical path) for energetic
protons in the SAA is on the same order as the atnmspheric density scale height. Thus west-
ward traveling protons encounter a significantly nlore (h_nse atmosphere and are more likely t(J
suffer atmosl)heric interactions and be lost. The resulting energy-dependent anisotropy is called
the east-west effect. Galactic cosmic rays experience a similar effect. A model for 1)redicting the
trapped proton angular distribution has been developed (ref. 12) recently. A large part of the
calculational effort (ref. 13) of the LDEF Ionizing Radiation Special Interest Groul) has been di-
rected toward testing the prediction of this model against LDEF measurenxents (ref. 9, 14).
GEOMAGNETICALLY TRAPPED PROTON AND ELECTRON FLUXES
To predict the trapped fluxes the current environment model in use is the "Vette" model
(ref. 1-3) together with the associated magnetic field models (ref. 4). To obtain the LDEF mis-
si()n l_hlences we calculated long-term average fluxes for five circular orbits at 258.5, 255.0, 249.9,
230.0, and 172.0 nautical mile altitudes which occurred on mission days 0, 550, 1450, 1950, and
2105, respectively, and did a numerical integration over time assuming a straight line between
time 1)oints. The solar F10.7 cm radio flux which characterizes solar activity exceeded 150 al)out
mission day 1540 (June 27, 1988). Thus the last 565 days or 27 (?_ of the mission was sl)ent un-
der solar maximum conditions. The environment models used for solar mininmm (the first three
times) were APSMIN (ref. 2) for protons and AE8MIN (ref. 2,3) for electrons and the magnetic
field model was the IGRF 1965.0 80-term model (ref. 4) projected to 1964, the epoch of the en-
vironmental model. The enviromnent models used for solar maximum (the last two times) were
214
AP8MAX (ref. 2) for protons and AE8MAX (ref. 2,3) for electrons and tile magnetic field model
was the Hurwitz USCS1970168-term model (ref. 4) for 1970,the epochof the emdromnental
model. (The referencesprovided for the electron environment document the previousmodels
to AE8MIN and AESMAX which remain undocumented.) SinceLDEF was at a lower altitude
during the last part of the mission about 15% of the proton fluence and 24(Z,, of the electron flu-
ence was received under solar maximmn conditions. In figure 4 the t rapl)ed proton fluence is
ccmlpared to the galactic proton fluence and the atmospheric albedo fluences due t.o protons and
neutrons produced by GCR interactions in the atmosphere. The galactic proton fluence was pro-
duced by the CREME code (ref. 10) which modified the free space spectrmn external to the ge-
omagnetosphere based on the vertical rigidity cutoff at points along the LDEF orbit. The albedo
fluence was calculated from atmospheric transport of GCR (ref. 1,5). Figure ,5 shows the 1)re -
dieted electron fluence.
TOTAL MISSION DOSE
The mission proton doses were obtained from the fluence using the BurreU proton dose
progrmn (ref. 5) which is based on the "straight-ahead" and "continuous-slowing-down" ap-
proximatic,ns fi:,r transporting the protons. Two simple geometries were used-a, point tissue re-
ceiver material at the center of a spherical aluminum shell and a point tissue receiver material
behind a plane aluminunl slab with infinite shielding behind the receiver. N_r the electron and
1)remsstrahhmg dose we used the MSFC electron dose program (ref. 6). The electron dose is
based cm fits to data from the ETRAN electron Monte Carlo program (ref. 7). Bremsstrahhutg
dose is based on exl)onential attenuation with buildup factors from an al)l)roximated scmrcc. It.
yields fair agreement with more complicated trausports. It only 1)erforms the slab geometry cal-
culation. As an estimate for the spherical shell geometry we doubled the slab results which un-
derestimates the actual result. The close clue to trapped protcms plus secondary 1)article, the dose
due to electrons plus bremsstrahhmg and the total of the two are shown in figures 6 and 7 fii_r the
two geometries. A compariscm between the pre(licted total closes and doses measured with on-
board TLD dosinleters (ref. 9) is shown in figure 8. Although there is general agreement between
the measurement and the simple geometry calculation the planned three-dinlensic>nal geometry
calculation (ref. 16) will better clarify the spatial variations about LDEF due t.o shielding config-
uraticms and proton angular distributions.
MISSION LINEAR ENERGY TRANSFER (LET) SPECTRUM
The LET of a charged particle specifies how much energy is del)osited per unit length along
its path in passing through material. Particles with higher LETs are more likely to 1)r_duce sin-
gle event upsets (SEUs) in electronic devices and their lfiological effects are larger compared to
low LET particles. The NRL package of programs, CREME, (ref. 10) was used to calculate the
LET sl)ectrmn due to GCtt, the singly-charged anomalous cosmic ray component, and trapped
protons for comparison with LDEF measurements. The CREME package calculates the LET
spectra, at LEO by attenuating the GCR and anomalous flux to the orbital position based on
a magnetic rigidity cutoff model and material shielding transport, and then combining this re-
sult with the contribution due to trapped protons, also modified by material shielding transport.
215
Seccmdariesat(' nc_thandled. Tile CREME results (ref. 8) for LDEF are shownin figure 9. B('-
cause of the long mission time, exl)erimentally measured LET spectra from the LDEF data (ref.
11) will hay,- greatly improved statistical accuracy at high LET compared to previous measure-
llltqltS.
CONCLUSIONS
Predictions of the LDEF mission's ionizing radiation exposure have been made using the
currently accepted models. The LDEF experimental measurements are providing an opportmfity
to validate the model predicti_ns. Preliminary results fi_r the measured dose are in general agree-
ment with predictions, suggesting that the Vette AP8 model, although more than 20 years ohl, is
still valid, at least for predictions of long-term average dose. The observed variation in dose and
activation about the spacecraft shows that the angular distribution of the trapped pr_tons must
l)e cc_nsidered where more accurate predictions are needed. Because no dose measurenlents were
at thil,ly shiehled locations where the electron contribution to the dose is dominant, the LDEF
results will provide little infl)rmation about the trapped electron environment. The measured
LET spectra fi'om LDEF will provide a test of the CREME model with the best xnea.surements
at high LET to date.
216
REFERENCES
1. Sawyer,Donald hi. and Vette, James I.: AP-8 Tral,ped Proton Enviromnent for Solar Max-
ilmun and Solar Minimum. National Science Data Center, Goddard Space Flight Center,
NSSDC./WDC-A-R& S 76-06, 1976.
2. Teague, Michael J. and Vette, James I.: A Model of the Tral)ped Electron Polmlation fin.
Solar Minimunx. National Science Data Center, Goddard Space Flight Center, NSSDC 74-
03, 1974.
3. Teague, Michael J.; Chan, King W. and Vette, James I.: AE6: A Model Envi,'omnent of
the Trapped Electrons for Solar hlaximmn. National Science Data Center, Goddard Space
t eFlight Cent_r, NSSDC/WDC-A-R&S 76-04, 1976.
4. Stassinol)oulos, E. G. and Mead, Gilbe,'t D,: ALLMAG, GDALMG, LINTRA: (Omlmtel
Progra, ns for Geomagnetic Fiehl and Field-Line Calculations. National Space Science Data
' eCent r, Goddard Space Flight Cente,', NSSDC 72-12, 1972.
5. Burrell, Martin O.: The Calculation of Proton Penetration and Dose Rates. Marshall Space
Flight Center, NASA TM X-53063, 1964.
6. Watts, John W. and Bu,'rell, M. O.: Electron and Bremsstrahhmg penetration and Dose
Calculation. National Aeronautics and Space Administration, NASA TN D-6385, 1971.
7. Berger, Martin and Seltzer, Steve: Penetration of Electrons and Ass()ciated Bremsstrahhmg
through Ahuninunl Targets. Protection Against Space Radiation, National Aeronautics and
Space Administration, NASA SP-169, 1968.
8. Benton, E. V. and Heinich, W. (editors): Ionizing Radiation Exposm'e (,f LDEF (LDEF Pre-
Recovery Estinlates). USF-TR-77, August 1990.
9. Benton, E. V.: Frank, A. L.; Benton, E. R.; Csige, I.; Pa,'nell, T. A. and Watts, J. W.: Ra-
diation Exposure of LDEF: Initial Results. First LDEF Post-Retrieva.1 Symposium, NASA
CP-.3134, 1992.
10. Adants, James: Cosmic Ray Effects on MicroElectronic, Part IV. NRL Memorandunl Rep(n't
5901, December 31, 1986.
11. Benton, E. V.: C:lge, I.; Frank, A. L.; Frigo, L. A.; Benton, E. R.; Parnell, T. A. and Watts,
J. W.: Charged Particle LET-Spect,'a Measurements Aboard LDEF. First LDEF
Post-Retrieval Sy, nposium, NASA C,P- 3134, 1992.
tion. )
12. Watts, .J.W.; Parnell, T. A.; and Heckman, H. H.: Approxinmte Angula," Dist,'ibuti,m and
S1)ectra for Geomagnetically Tral)ped Protons in L(,w-earth Orbit. High Energy Ra.diati,,n
' eBackground in Space, Proceedings of AIP Confer.nee, Sanibel Isla,ld, Florida, vol. 18G,
1989, pp. 75-85.
13. Armstrong, T. W.,; C.olborn, B. L, and Watts, J. W.: Radiation Calculatl ms and Compar-
isons with LDEF Data.. First LDEF Post-Retrieval Syml)osiunt , NASA CP-3134, 1992.
217
14. Harmon. B. A.; Fislnnan, G. J.; Parnell, T. A.: and Lair, C. E.: Imluc_'d Radioa.ct.ivity in
LDEF Collll)OllelltS. First LDEF Post-Retrieval Syml_osiunl. NASA CP-3134, 1992.
15. Armstrong, T. W.; Chandler, K. C. and Barish, J.: Calculation of Neutron Flux Spectra In-
duced in the Earth's Atmosphere by Galactic Coslnic Rays. J. Geopliysical Besearch, v_l.
78, 1973, p. 2715.
16. (!_lborn, B. L.and Arlnstrong, T. \¥.: LDEF Geolnetry/Mass Model for Radiation Analyses.
First LDEF Post-tletrieval Symposium. NASA CP-3134, 1992.
218
Figure 1. Pathof trappedchargedparticlesin the geomagneticfield.
0o
10°
!00
GO0
30 o
1000
0
Figure 2.
50 °
|00 n 3000 310 ° 320 n 330 n 34() n :t_)n_ 0• 10° 20° 30°
Proton isoflux contours for energies above 34 MeV in the South Atlantic Anomaly at 440 km
(240 nautical mi.) altitude.
219
SPACECRAFT _._
^
JE
h2-hl
220
1010
1O9
081
g
10 7
LL
e"
-- 10 6
Figure 3.
NEAR MIRROR POINTS THE PROTONS CIRCLE NEARLY
A
PERPENDICULAR TO THE MAGNETIC FIELD LINES (B).
THE MAGNETIC GYRORADII OF THE TRAPPED PROTONS
ARE CLOSE TO THE ATMOSPHERIC SCALE HEIGHT.
Charged particle path near the mirror point.
10 5
10 4
INTEGRAL Spectra
s (Cummulative over LDEF Mission)
Ga/actic Protons
Albedo Protons
i
L _ L±JJ_J _ _. L JJJ J j ,,=,1,
101 10 2 10 3 1 04 10 5
Energy (MeV)
Figure 4. LDEF integral fluences from various sources (ref. 8).
i0_2,
i 101t
101o
g 1o'
I0 e
10 7
10 _ I J I i I J I , I , I
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Energy (MeV)
3.5 4.0
Figure 5. LDEF integral electron fluences (ref. 8).
Figure 6.
10 4
10 o
10-1
10 .2 10 1 10 0 10 t 10 2
Thickness (glcm 2)
The calculated LDEF mission absorbed dose from trapped protons and electrons (ref. 8). The
geometry consists of a point tissue receiver at the center of a spherical aluminum shell of the
given thickness.
221
10 s
10 4
Electron
Proton
Total
10 0
Figure 7.
10 4
10 .2 10-1
10° 10 i 10 2
Thickness (g/cm2)
The calculated LDEF mission absorbed dose from trapped protons and electrons (ref. 8). The
geometry consists of a tissue receiver behind a plane aluminum slab of given thickness with
the receiver completely shielded from behind.
222
Figure 8.
oooI!'
,
i o PO006 points (West side)
_1 _ PO004 points (West side)$00 l _ AO015 points (West side)
• A0015 points (Earth side)
o M0004 points (East side)
GO0
DOSE 500(rads)
\
SLAB
0 ) ' h _ l , . ; J L i , L , , "_
, 5 10 15 20
SHIELD THICKNESS (g/cm: Aluminunl)
Comparison of the predicted LDEF total mission dose (ref. 8) with on-board TLD dosimeter
measurements (mr. 9).
223
E÷5
10(3
u/
t.
D
1;
D
O
;3
Z
0.1
E-4 1
Figure 9.
Integral LET Spectra
(..t cmauEcam
for LDEF
I0 10(3 I000 10000 IE-F5
MeV cm'/g
Predicted LET spectrum at the LDEF orbit (ref. 8) from the CREME code (ref. 10).
224


Prediction of LDEF ionizing radiation environment

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