Three-dimensional shielding effects on cosmic ray charged particle fluences were measured with plastic nuclear track detectors in the P0006 experiment on Long Duration Exposure Facility (LDEF). The azimuthal and polar angle distributions of the galactic cosmic ray particles (mostly relativistic iron) were measured in the main stack and in four side stacks of the P0006 experiment, located on the west end of the LDEF satellite. A shadowing effect of the shielding of the LDEF satellite is found. Total fluence of stopping protons was measured as a function of the position in the main and side stacks of the P0006 experiment. Location dependence of total track density is explained by the three-dimensional shielding model of the P0006 stack. These results can be used to validate 3D mass model and transport code calculations and also for predictions of the outer radiation environment for the Space Station Freedom

Armstrong, T. W.

Benton, E. V.

Colborn, B. L.

Csige, I.

Frigo, L.

Parnell, T. A.

Watts, J. W., Jr.

English

NASA Technical Reports Server

N93-29639
THREE-DIMENSIONAL SHIELDING EFFECTS ON CHARGED PARTICLE FLUENCES
MEASURED IN THE P0006 EXPERIMENT OF LDEF *
I. Csige, E. V. Benton and L. Frigo
Physics Department, University of San Francisco, 2130 Fulton St.
San Francisco, CA 94117-1080, USA
Phone: 415-666-2333, Fax: 415-666-2346
T. A. Parnell and J. W. Watts, Jr.
ES-62, NASA-Marshall Space Flight Center
George C. Marshall Space Flight Center, A1 35812, USA
Phone: 205-544-7690, Fax: 205-544-7754
T. W. Armstrong and B. L. Colborn
Science Applications International Corporation, Route 2
Prospect, TN 38477, USA
Phone: 615-68-2603, Fax: 15-468-2676
SUMMARY
Three-dimensional shielding effects on cosmic ray charged particle fluences were measured with
plastic nuclear track detectors in the P0006 experiment on LDEF. The azimuthal and polar angle
distributions of the galactic cosmic ray particles (mostly relativistic iron) were measured in the main stack
and in four side stacks of the P0006 experiment, located on the west end of the LDEF satellite. A
shadowing effect of the shielding of the LDEF satellite is found. Total fluence of stopping protons was
measured as a function of the position in the main and side stacks of the P0006 experiment. Location
dependence of total track density is explained by the three-dimensional shielding model of the P0006
stack. These results can be used to validate 3D mass model and transport code calculations and also for
predictions of the outer radiation environment for the space station Freedom.
INTRODUCTION
Cosmic ray charged particles contribute to the health risk of crew members of manned flights and
produce single event upsets (SEUs) in microelectronics in space. Risk estimations are usually based on
measurements of the outer charged particle radiation environment and use three_limensional mass
"Work partially supported by NASA grant No. NAG8-168 (NASA-Marshall Space Flight Center,
Huntsville).
- 187
https://ntrs.nasa.gov/search.jsp?R=19930020450 2020-03-17T04:28:18+00:00Z
models and computer transport codes to calculate inner radiation fields. Measurements of the inner
radiation environment are also essential to validate transport codes and in some cases to provide direct data
for risk estimation. Plastic nuclear track detectors (PNTDs) have been widely used to measure both outer
(charge and energy spectra of galactic cosmic rays (GCRs) and trapped particles) and inner (LET-spectra,
charge and energy spectra of secondary particles) charged particle radiation fields [1,2,3,4,5,6,7].
A series of PNTD stacks were exposed on the LDEF (Long Duration Exposure Facility) satellite and
the ongoing analysis of these stacks makes the cosmic ray charged particle experiments performed on
LDEF probably the most comprehensive and, as of today, the most important ever conducted with PNTDs.
Unique features of the LDEF satellite make this even more reasonable. The long duration, almost six years
in space, makes it possible to study low abundant cosmic rays and measure the high LET-tail with good
statistical accuracy. The gravity-stabilized orientation gives the possibility of studying the directionality
distribution of cosmic ray charged particles. Also, a detailed mass model of LDEF is available, which
makes it possible to compare the inner radiation field and activation measurements with model calculations.
"Outer radiation field measurements" usually actually means "inner radiation field measurements",
taking into account the shadowing effects of shielding. This shadowing effect and the contribution of
secondary particles to the primary particle fluxes makes a strong difference in the inner and outer radiation
fluxes. Hence the understanding of three-dimensional shielding effects on charged particle fluences is
essential when interpreting experimental data measured at different locations on LDEF under different
shielding distributions. In the present paper we present data of the measurement of galactic cosmic ray Fe
particles and stopping trapped proton particles in the main and four side stacks of the P0006 experiment
flown on LDEE The contribution of the three-dimensional shielding effects to the observed anisotropy of
Fe particles and to the 9osition dependence of total track density is discussed.
i
I
: _:: 77
7
!
EXPERIMENTS AND RESULTS
The P0006 Experiment
The P0006 experiment on LDEF is a complex experiment to measure the inner and to a certain extent
the outer radiation field of cosmic rays. It consisted of activation materials, neutron detectors,
thermoluminescent detectors and plastic nuclear track detectors to obtain different kinds of information
about the inner radiation field.
The P0006 experiment was located in the F2 tray of LDEF on the west end and close to the space end
of the satellite. Its experimental unit consisted ofa mainand four side stacks as shown in Figure 1. The
main stack consisted of 9 sub-stacks of PNTDs, the planes of which were perpendicular to the east-west
direction. The side stacks wereattached to the four sides of the main stack with the normal vectors pointing
out from the main stack into space with the approximate directions as follows: north-space (side stack A),
north-Earth (side stack B), Earth-south (side stack C) and south-space (side stack D). (The orientation of
the side stacks is assumptive in the sense that experimental results obtained are in agreement with and only
188
East
Spacecraft
<
North
:ral
X South
Space t
Side
_..ack D
Side
Stack B _Dde
ck C
West
Figure 1: The P0006 experimental block on the LDEF satellite.
with this assumption. Unfortunately it was not possible to confirm the orientation of the side stacks from
the data available about the disassembly of the LDEF satellite.)
The P0006 experimental block was placed in a sealed aluminum canister with 1 atmosphere normal
air inside. In the surrounding of the P0006 experiment there were 4 other canisters of the P0004 corn seed
experiment representing a significant amount of shielding from those directions.
Measurement of Relativistic Fe Particles
Two layers of CR-39 PNTDs were selected from the center of the main stack (6.5 g/cm 2 shielding
from the top of the canister) and from the side stacks (1.3 g/cm 2 shielding) for scanning and measuring. The
central area of each layer was scanned for tracks in coincidence at the adjacent surfaces. Track pairs were
further investigated as to whether they have corresponding tracks on the top surface of the top layer and on
the bottom surface of the bottom layer (4-surface tracks). Comparing the size of the tracks at all four
surfaces to each other we found that the size of the tracks did not change within experimental errors which
means that these tracks were produced by high energy particles. Comparing the differential LET-spectra of
4--surface tracks found in LDEF samples to a differential LET-spectra of 4-surface tracks found in a
sample flown on a shuttle flight with similar altitude and inclination to LDEF, we found that practically all
4--surface tracks measured in LDEF samples must be produced by relativistic energy Fe particles. In the
case of the sample from the shuttle flight other galactic cosmic ray particles are also included in the
differential LET-spectra and the peak of the relativistic Fe particles can clearly be seen because of the
189
Sout, h
Space
................ 40 o ...............
..... .... .....
"" " ......... 30" ......"-.,
../ j.-'* • • "'--... "%.,..
.... ..... " p . _ %" ..... g .....
.... ,:'." .....v- 'i;, " ':
:: • . 2 • • '. • ;:;
/ e..' 06-- • • • @@ "., _ \/ • / e_• f e-'" "-.t,J • • _ • • :.
: , • • • ",o |e _ • "
_, e%;o.'_?o " 0_. ._ • "i i
"0 i'o% i •_ ,'t ; Q • # :" -:
"',,- '', .... • • .... S • .........
"_ .,. • • • ..." ../
-\ ..-
.. .Y
........................................................:;iiii_
Earth
Z
Figure 2: Arriving directions of GCR Fe particles with relativistic energies looking to the west direction
from the center of the P0006 experiment on LDEF. Particles with dip angle greater than 40 ° could not be
measured because of the critical angle cut-off of the detector.
relatively high abundance of these particles to the neighboring elements. Lighter than iron relativistic
particles could not be seen in the case of the LDEF samples because of the short etching time needed.
300 particles in the mainstack and 100 particles in each side stack were identified as relativistic
galactic cosmic ray Fe particles in the present experiment. The azimuthal and dip angle distributions of the
particles were measured. Since the moving direction along theparticle trajectory could not be measured we
assumed that all the particles measured arrived from the west direction and moved toward the east
direction. Experimental data in the side stacks are consistent with this assumption.
Figure2 shows_from which directions 300 GCR Fe pal?ti'cles .aTri'ved to the center of the main stack of
the P0006 experiment. No particles c9uld be detected with dip angle (measured from the normal of the
detector layer) greater than about 40 ° because of the dip angle cut-off of the detector. Scanning efficiency
close to the cut-off dipangle was also significantly less ih-an-100% due to the difficulty of finding
corresponding trac__k_s0nthe first and fou_rth surfaces of the CR-39_ doublet. Figure 2 shows that arriving
directions of particles from the west are very is•tropic with some extra particles coming from the
south-west, parallel with the Earth's surface.
In Figure3, empty circles showthe arriving directions;filled circles show the leaving directions of
100 particles in each side stack. The shielding of the LDEF satellite is the east side and the shielding of the
P0004 canisters are at the center of the graphs. It seems that these large (about 40-150 g/cm 2) shieldings
190
A side B side
West i
Space
.............. 400 ..............
.. o ",,,
•" ....._.." 300" ......... ,, "".....
o. "'" '",. "%
.- .,.' O "_, "..
..... o ,oo 2 ;..... ".
.... ....... o ......200 ..... "..... •
:" 0 "a o ".. \
o . o o ..-100 .. •
i ' '." !,,i oi !
! _ _\o , :,.
- 00° _ - o
O0 ', O_ '.. .J'• •
. % '"-_......................., _-" ...
....,, "-o_.. #.,'_ ....
........ ..-"
"% o ..,-''"/"
Earth
/
i
/
East West°
Space =
..... •............. 40 ° ...............
,.." • "--.,,,
o ./" ",-
......o o .........300" -.
...-" o •'"'" "- , '"
/..' -
.. 0 .,..,.' o o
_'"_'o ._'_ o 20"
f o @'o o o° ...... " "'"o
i ,4 ..-"o
....-{ o!O "
0 / 0 "-
/ "k
i i ° )i° i ' ,
i \ ° ..... "--........................
'_0 _ ",. o •
_. oO..o .............. ._ ..
\ o "o .................... • 'e
"., ",,. O ,/
......i,', %
• •• .
'\,
d I
.i
P !•
i
x.
e ¸
!" EasL
?
/
Earth
_o
C side D side
J
East l
k
:/
?"
Space
..................... 40 ° ......................
. y.,--
,,,..,,,
• ........300 o o .....
../" ..... .... -.................
".. o
'e • 6 • o -.
: •, .'i, " 100 o_
[ p
•/," "" "- \ o_
.{ i _ io i
"; • '\ / o i
,, _,. ............* ......i_ .'o
': _. _ o o.../ -
_'\ "" • o ."0 /
6 '.. _,o _;.,, • o..."o /"
., .... • • ...._.....i ......._ 0/6.-" /:"
., ,-"
., /'
'-..'.. -. .........• ......................0-"" 0 ...."'/'
"- • o ......'J
"'""...................................................
Earth
"'_.,,.,.
'\
/
//
West
Space
- 400
'.,.
/y,*-'"
........... 30 _
• ,,.'" ............. -,,
,' o '".,
• . "-..
2if, -
,,.._ ...... ..,
.., • .. -._,.
o o
• .. .... ,.. 10° ..
/ .:-' -, _, o \
. _ ............ ,,/
East i ,i " _ • !
'- _-" o o 0 c_
_r _ ":. e I 0 0 " '=
•,, :,, • o g .- .,
'.%. /.""
'%. _,'"
%'"'-.. ................ -.-"
•. o
",.,
Ear I,h
West
Figure 3: Arriving directions of GCR Fe particles with relativistic energies in the side stacks of the P0006
experiment.
191
canabsorbrelativisticFeparticlesandonly a veryfewpercentcanpenetratethem.However,some
anisotropyof theGCRFe particlescannotberelatedto shieldingeffects,hencewebelievethereis an
anisotropyin theflux of theprimaryGCRFeparticles.
!
Total Track Density Measurements
We have measured two--dimensional total track density profiles in each side stack using an image
analyzer. The image analyzer was able to pick up tracks darker than a certain brightness threshold and
having contrast high enough to detect edges. Track size criteria were also applied in the counting,
Overlapping tracks were separated by the software with very high efficiency. Total track density
measurements were also performed by manual scanning and counting and the results were used to calibrate
the automatic system. A reasonably good agreement between manual and automatic scanning has been
established.
Total track density in CR-39 consists of tracks of different particles. At low shielding, such as the
case for the side stacks, a large contribution is due to stopping primary trapped protons. A significant
contribution is due to secondary stopping particles and recoils and also from GCR Fe particles. At higher
shielding depths, these particles dominate the total track density as the high flux of low energy trapped
protons is absorbed in the shielding.
On the other hand, not all of the particles can be detected by CR-39. A stopping proton can be
detected with very high efficiency if the particle stops in the upper layer of the detector, which has a
thickness comparable with the layer removed by the chemical etching of the detector. This thickness was
about 8-10 t*m in our samples. The critical angle for the detection of a proton in this case is about 700
measured from the normal of the detector surface. This critical angle decreases rapidly with the residual
range of the proton if the particle stopped below the post-etched surface which, we found, is usually the
case for proton tracks. A detailed detection efficiency calculation is in progress taking into account a
variable track etch rate ratio model for stopping particles. Internal calibration will also be performed using
tracks of stopping proton particles.
Detection efficiency for other than proton particles is almost 100% if the particle stops in the removed
layer, which means that CR-39 can detect practically all heavy recoils that have a part of their trajectories
in the removed layer thickness. Relativistic Fe particles, on the other hand, are detected only from a small
field angle around the normal of the detector with a critical angle of about 40 °.
Trapped protons are expected to have a strong directionality. In the case of the P0006 stack they are
expected to arrive from the west direction, mostly parallel with the Earth's surface and with the surfaces of
the CR-39 layers in the side stacks. It means that the track density of stopping primary trapped protons is
expected to decrease with the depth in the main stack as the low energy trapped protons and their
secondaries are absorbed in it. The secondaries of the high energy trapped protons, however, are not
expected to show strong shielding effects because they can penetrate the whole stack with approximately
constant interaction cross sections. Hence, a strong location dependence of total track density is expected at
low shieldings and some relatively smooth track density profiles at higher protected areas.
192
90
80
70
60
50
40
30
20
10
0
A side B side C side D side
__'_ . ,:> ..._.
I
1
0 20 40 60 10 30 50 70 10 30 50 70 10 30 50 70
I 500,000
540,000
580,000
620,000
660,000
700,000
740,000
780,000
820,000
860,000
900,000
940,000
Figure 4: 2D total track density profiles in CR-39 from the side stacks of P0006. Bottom edges of the layers
are the space side edges.
Figure 4 shows some preliminary 2D total track density profiles measured in the side stack CR-39
samples. The total track density varied between 0.6 and 1.3 million tracks/cm _. Side stacks A and D had
significantly higher track densities than side stacks B and C facing toward the Earth. On the side stacks
facing toward space the side stack D has higher track density than side stack A, indicating more particles
from the south-west than from the north-west direction. This observation is in good agreement with the AI
activation measurements around the LDEF satellite which also show a maximum in the south-west
direction.
On each sample the track density is highest at the space edge of the sample and decreases toward the
LDEF satellite. This is especially demonstrated on side stacks B and C. The sudden drop of track density
moving from the space edge toward the satellite probably reflects the absorption of low energy trapped
protons arriving from the west-space direction and slowing down as they penetrate the main stack. The
dominance of the trapped protons from the west-south-space direction is also suggested by the relatively
uniform track density on the side stack D which has practically no shielding from this direction.
CONCLUSIONS
It is demonstrated that the three-dimensional shielding effect has to be taken into account when
cosmic ray charged particle fluences are measured under different shielding conditions. The shadowing
effect of the LDEF satellite on the relativistic GCR Fe particles was found. Additional observed anisotropy
193
in the fluence of these particles is assumed due to the anisotropic directional distribution of the particles
which has to be explained by propagation codes.
The variation of total track density as a result of self shielding of the P0006 stack was also observed.
The strong location dependence of the track density was found to be in good agreement with the anisotropic
trapped proton environment and 3D geometrical considerations of the P0006 experimental block.
These experiments clearly indicate the need for detailed mass model calculations together with which
they may provide an excellent opportunity to validate model calculations and help update flux data of the
outer radiation environment.
REFERENCES
1. Armstrong, T. W.; and Colborn, B. L.: Radiation model predictions and validation using LDEF data.
Second LDEF Post-Retrieval Symposium, NASA CP-3194, 1993.
2. Benton, E. V.; Heinrich, W., Parnell, T. A.; Armstrong, T. W.; Derrickson, J. H.; Fishman, G. J.;
Frank, A. L.; Watts, J. W.; and Wiegel, B.: Ionizing radiation exposure of LDEFi Pre-recovery
estimates. Nuclear Tracks and Radiation Measurements, Vol. 20, No. 1 1992, pp. 75-100.
3. Benton, E. V.; Frank, A. L.; Benton, E. R.; Csige,I.; Parnell, T. A.; and Watts, J. W. Jr.: Radiation
exposure of LDEF: Initial Results. LDEF-69 Months in Space." First Post-Retrieval Symposium,
NASA-CP 3134, Part 1, Washington DC, 1991, pp. 325-338.
4_ Csige,'I.; Benton, E. V.; Frankl A. L'; Frigo, L. A.; Benton, E. R.; Parnell, T. A.; and Watts, J. W. Jr.:
Charged particle LET-spectra measurements aboard LDEE LDEF-69 Months in Space: First
Post-Retrieval Symposium, NASA-CP 3134, Part 1, Washington DC, 1991, pp. 339-346.
5. Harmon, B. A.; Fishman, G. J.; Parnell, T. A.; Benton, E. V.; and Frank, A. L.: LDEF radiation
measurements: preliminary results. Nuclear Tracks and Radiation Measurements, Vol. 10, No. 1,
1992, pp. 131-136.
6. Watts, J. W. Jr.; Parnell, T. A.; Derrickson, H. J.; Armstrong, T. W.; and Benton, E. V.: Predictions of
_E--F ionizing radlaiiqnr env'tronment. LDEF-69 Months in Space." First Post-Retrieval Symposium,
NASA-CP 3134, Part 1, Washington DC, 1991, pp. 213-224.
, Reitz, G.; Bficker, H.; Facius, R.; Horneck, G.; Schaefer, M.; Schott, J. U.; Bayonove, J.; Beaujean,
R.; Benton, E. V.; Delpoux, M.; Heilmann, C.; Heinrich, W.; Kranz, A. R.; Planel, H.; Gasset, Y.;
GaUbin, G.; Portal, G.; Graul, E. H.; Riither, W.; Schopper, E.; Tobias, C. A.; and Yang, T. C. First
biological and dosimetric results of the free flyer biostack experiment A0015 on LDEF. LDEF--69
Months in Space." First Post-Retrieval Symposium, NASA-CP 3134, Part 1, Washington DC, 1991,
pp. 1639-1642.
194


Three-dimensional shielding effects on charged particle fluences measured in the P0006 experiment of LDEF

Three-dimensional shielding effects on charged particle fluences measured in the P0006 experiment of LDEF

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server