The LDEF Ultra Heavy Cosmic Ray Experiment (UHCRE) used 16 side viewing LDEF trays giving a total geometry factor for high energy cosmic rays of 30 sq m sr. The total exposure factor was 170 sq m sr y. The experiment is based on a modular array of 192 solid state nuclear track detector stacks, mounted in sets of four in 48 pressure vessels. The extended duration of the LDEF mission has resulted in a greatly enhanced potential scientific yield from the UHCRE. Initial scanning results indicate that at least 1800 cosmic ray nuclei with Z greater than 65 were collected, including the world's first statistically significant sample of actinides. Post flight work to date and the current status of the experiment are reviewed

Bosch, J.

Domingo, C.

Keegan, R.

Osullivan, D.

Smit, A.

Thompson, A.

Wenzel, K.-P.

English

NASA Technical Reports Server

N92-23306
THE LDEF ULTRA HEAVY COSMIC RAY EXPERIMENT
D.O_Sullivan, A. Thompson, J. Bosch and R. Keegan
Dublin Institute for Advanced Studies (DIAS), Ireland
K.-P. Wenzel and A. Smit
Space Science Dept. of ESA, ESTEC, Noordwijk, The Netherlands
C. Domingo
Universitat Autbnoma de Barcelona, Spain
SUMMARY
The LDEF Ultra Heavy Cosmic Ray Experiment (UHCRE) employed sixteen
side viewing LDEF trays giving a total geometry factor for high energy cosmic rays
of 30 m2sr. The total exposure factor was 170 m2sr y. The experiment is based on a
modular array of 192 solid state nuclear track detector stacks, mounted in sets of four
in 48 pressure vessels. The extended duration of the LDEF mission has resulted in a
greatly enhanced potential scientific yield from the UHCRE. Initial scanning results
indicate that at least 1800 cosmic ray nuclei with Z >65 have been collected, including
the world's first statistically significant sample of actinides. Post-flight work to date
and the current status of the experiment are reviewed.
INTRODUCTION
The Long Duration Exposure Facility (LDEF) was deployed into a near circular
orbit of 257 nautical miles altitude and 28.5 ° inclination by the Space Shuttle Chal-
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lenger in April 1984. Almost six years later on January 12 1990 it was retrieved from
a decaying orbit of approximately 180 nautical miles by the Space Shuttle Columbia
and returned safely to Earth. The Ultra Heavy Cosmic Ray Experiment (UHCRE),
which was mounted on the LDEF, was the largest array of cosmic ray particle detec-
tors ever flown in space. It comprised a total of 192 solid state nuclear track detectors
stacks housed in 48 pressure vessels (at 1 arm of dry air) which were mounted in 16
of the LDEF experiment trays. Each stack was 20cm x 26cm in area and contained
mainly lexan polycarbonate interleaved with lead velocity degraders. The average
Lexan equivalent thickness of the detectors was ,-_ 4.7 g/cm 2. (ref.l,2)
Initial inspection and analysis of the UHCI_E hardware took place in the Space-
craft Assembly and Encapsulation Facility II at Kennedy Space Center (KSC) after
which the experiment trays were removed and shipped to the European Space Re-
search and Technology Centre (ESTEC) at Noordwijk.
ACTIVITIES AT KENNEDY SPACE CENTER AND ESTEC
The extended LDEF mission resulted in (i), a greatly increased sample of ultra
heavy cosmic ray nuclei (Z >_ 65) and (ii), a wealth of information on meteoroid and
space debris impacts on the UHCRE hardware. Activities at KSC were related to
the latter investigation.
Impacts of size greater than 0.5 mm on the tray flanges were located by eye
inspection. This was followed by photo documentation of all front and back tray sur-
faces. All sixteen Scheldahl G411500 thermal blankets of the UHCRE were inspected
and the positions of impacts of size greater than 0.3 mm were recorded (ref.3). The
blankets were then trisected and one third of each was archived at KSC while the
remainder were shipped to ESTEC. A detailed report of the preliminary investigation
of meteoroid and orbital debris impacts can be found in these proceedings (ref.4).
Following an eye inspection of the upper surfaces of the cylinders and support frames
all UHCl_E hardware was shipped to ESTEC in the original containers.
At ESTEC a preliminary survey of the thermal covers was carried out by F.
Levadou (ref.5) and further studies were reported at this conference (ref.6). The
aluminium cylinders were removed from their trays in clean room conditions to avoid
any surface contamination. Subsequently, the gas pressure within each cylinder was
measured and it was found that no leakage had occurred. Six of the cylinders con-
taining the detector stacks scheduled for post flight calibration were shipped to the
Bevalac. The detector stacks were removed from the remainder of the cylinders and
were shipped to Dublin for processing and analysis.
368
CALIBRATION OF DETECTORS
Pre-flight and post-flight calibration of the UHCRE detectorswas carried out
at the BerkeleyBevalac. In the preflight calibration which was undertaken in 1983,
a number of stacks were exposed to 960 MeV/N uranium and 300 MeV/N iron
and earlier (1979) to 122 MeV/N iron ions. Post flight calibration in May 1990
consistedof exposuresto high energyuranium (920 MeV/N), gold (1150MeV and
663MeV/N), krypton (1496MeV/N), iron (1690MeV/N and 400 MeV/N). The aim
of these calibrations was twofold; namely, (i) to determine the value of the constants
in the expression used to determine the charge of the individual ultra heavy cosmic
etch rate is of the form Vt=aJ n where j=f(z.__), Zelf is the effectiveray nuclei, (the
charge and a and n are constants determined from calibration) and (ii) to determine
whether there was any 'ageing' of the latent tracks of the particles during the LDEF
Mission.
To date, measurements on the calibration data are at a very preliminary stage.
Several uranium nuclei from the post flight calibration have been followed to their
stopping points and their energy determined to a high degree of accuracy. The
preliminary calibration data shown in fig.lf should be taken as a rough guide until
further data is processed. It is shown here to indicate that the nuclei displayed are
indeed ultra heavy and are in the charge region 70<Z<92. The bar marked uranium
in fig.lf shows the range of values expected for uranium nuclei of _3=0.97 (scaled
up from measurements made at fl--0.77), using the standard model and assuming a
value of n between 2.0 and 2.5, which corresponds to the limits found in previous
experiments. The gold data was calculated using the same data and assumptions.
Comparison of pre flight and post flight calibration data will be available later this
year.
TEMPERATURE HISTORY OF UHCRE DETECTORS
The charge resolution achievable on the ultra heavy cosmic ray experiment is
dependent on the temperature history of the UHCRE modules throughout the LDEF
Mission (ref.7). The thermal design of the experiment was aimed at maintaining the
temperature of the detectors below 30°C(86°F) and ensuring as narrow a band as
possible between maximum and minimum values. Temperatures were measured at
selected locations on the LDEF structure during the first 490 days of flight. These
data were used in post flight analysis to update LDEF thermal models (reL8). The
in-flight parameter data has allowed an accurate assessment of the thermal model
369
used and the resulting temperature uncertainties have been reduced from a preflight
value of +40°F to less than +18°F. Table 1 shows the maximum and minimum tem-
peratures calculated for the locations of all UHCRE trays along with the associated
temperature band (A) for each case.
Table 1 - Maximum and Minimum Temperatures Experienced by UHCRE Trays
Tray Position Min Temp(°F) Max Temp(°F) A
on LDEF
A2 48.7 86.9 38.2
A4 52.2 85.5 33.3
A10 50.6 82.5 31.9
B5 40.7 85.7 45.0
B7 37.9 90.1 52.2
C5 39.5 83.3 43.8
C6 34.8 91.6 56.8
C8 48.7 86.9 38.2
C11 38.9 80.9 42.0
D1 33.2 81.3 48.1
D5 37.5 82.4 44.9
D7 34.3 87.2 52.9
D 11 36.7 77.4 40.7
E2 38.6 74.2 35.6
El0 41.4 71.6 30.2
F4 48.2 77.9 29.7
Further refinement of the LDEF thermal model is continuing but it is unlikely
that the values in Table 1 will alter significantly (private communication, T. Sampair).
Overall the temperature results are very encouraging. The mean temperature of the
individual trays was well below the upper limit chosen as the basis for the thermal
design. Furthermore the mean width of the temperature band was 41.5°F (23°C),
smaller than expected on the basis of preflight analysis.
PRELIMINARY PROCESSING AND ANALYSIS
Three Lexan Polycarbonate detector sheets were removed from a number of
the stacks and were etched for periods varying from 5 to 15 days at the standard
370
1.3
12
11
Z=
1
_go9
E 08
c
N 07
06
0.5
04 _-_---_
1 0
1.3
1.2
1,1
a_ 1
0.9
E 08
0.7
0.6
0.5
0.4
13
12
1.1
_. 1
09
E 0.8
_ 0.'7
0.6
0.5
0.4
-1
STACK NUMBER 61
(a)
1 2 3 4 5
Path Length m Stack (Lexan Equivalent) in cm
STACK NUMBER 146
(b)
--1--
?- o o
0 ! 2 3 4
Path Length in Stack (Lexan Equivalent) in cm
STACK NUMBER 157
(c)
0 1 2 3 4
Path Length in Stack (Lexan Equivalent) in cm
5 6
Figs.|(a),(b),(c) : Etch rate as a function of
depth in stack for ultra heavy nuclei observed
in stacks 61, 146, 157 respectively.
371
't.3
1.1
--_ 1
E 0.8
%
_ 07
0.6
05
04
1.3
1.2
1.1
u=: 1
0.9
E 0.8
c
0.7
>
0.6
0.5
0.4
-1
STACK NUMBER 181
(d)
[2k O
0 1 2 3 4 5 6
Path Length in Stack (Lexan Equivalent) in cm
STACK NUMBER 212
(e)
0 1 2 3 4 5 6
Path Length in Stack (Lexan Equivalent) in cm
1.3
1,2
1.1
.._ 1
0,9
E 0.8
"_ 0.7
0.6
0,5
0.4
I U
Au
COMBINED SAMPLE
(f)
I I I : I ..(
0 1 2 3 4
Palh Length in Stack (Lexan Equivalent) in cm
Figs.](d),(e),(f) : Etch rate as a function of
depth in stack for ultra heavy nuclei observed
in stacks 181,212,and for the combined samples
(a),(b),(c),(d) and (e). See text for explanation
of Fig.l(f).
372
conditions of 40°C temperature and 6.25 N NaOH etchant. The sheets chosen were
from the top, centre and bottom portions of these stacks providing a spread of path
lengths varying from ,-_ 0.01 cm to ,-_ 4 cm in each case. Initial inspection showed
that the surfaces of the sheets were of high quality and that the optical conditions
for locating and measuring the tracks of ultra heavy nuclei were excellent.
Having located and measured the track parameters of a given nucleus in the top
plate, its trajectory through the stack was estimated and the corresponding tracks
were located and measured in the two lower parts of the stack. The number of ultra
heavy nuclei observed ranged from 5 to 13 per stack indicating a total sample of
> 1800 collected during the LDEF mission.
Figure 1 displays the preliminary data obtained for ultra heavy nuclei in five of
the UHCRE stacks. Plots of etch rate versus path length traversed in each stack is
displayed for stacks 61, 146, 157, 181 and 212 (Figs. la, b, c, d, e resp.) These stacks
were mounted in trays which were placed on rows six and eight of the twelve sided
LDEF polygon (the leading edge was number nine).
It can be seen that for each stack the measured etch rates lie between 1.1 ttm/hr
and 0.5 #m/hr indicating a general consistency among the various sets of data. The
different path lengths in the stacks for the centre and lower sections is due to different
angles of incidence of the ultra heavy nuclei.
DISCUSSION
The initial post flight assessment of the UHCRE and preliminary data analysis
indicate that the 69 month exposure in Earth orbit has achieved the major objectives
of the experiment. The data shown in Fig. 1 display the characteristics of high energy
ultra heavy nuclei as they traverse several grams of matter (ref.9). In the majority
of cases there is no appreciable change in etch rate. Where a significant change does
take place, the data is consistent with the occurrence of fragmentation in the stack.
(In this regard it should be noted that, due to the inflight orientation of the LDEF,
particles could enter the detector stacks from both top and bottom). The charge
regime covered by the preliminary data as indicated by the quick look calibration
measurements is consistent with that expected for relativistic cosmic ray nuclei in
Lexan Polycarbonate (charge threshold Z,,_ 70).
The temperature history of the detectors gives rise to optimism with regard to
the registration temperature effect and the long term ageing of latent tracks. On the
basis of the temperature regime experienced by the UHCRE trays, (91.6 °F (33.1°C)
to 33.2 °F (0.7°C) respectively), the uncertainty in charge determination is expected
to be less than two charge units according to estimates made from exposures to UH
373
nuclei at the Bevalac (ref.10). The impact of short term differential latent track
evolution is currently under study.
The total number of ultra heavy nuclei collected during the mission is estimated
to be ,-, 1800. Thus, the UHCRE has provided a sample which is approximately six
times greater than the previous world sample and includes the first significant sample
of cosmic ray actinides.
Initial assessment of the charge resolution achieved indicates that it would be
possible to (i) resolve some of the important charge groups such as platinum and lead
and (ii) determine the abundance of the actinides in the cosmic radiation. The ac-
tinide abundance is determined by the nature of r-process contributions to cosmic ray
source material and early observations suggested the presence of freshly synthesised
r-process material to account for a high value of the ratio of actinides to platinum-
lead nuclides. However, the upper limit of 3% for this ratio found by Binns et al
(ref.11) is consistent with solar system source abundances. (A somewhat larger value
was reported at the same time by Fowler et al (ref.12)). The large UHCRE sample,
combined with a charge resolution which is superior to that achieved in the early
experiments with solid state nuclear track detectors, should clarify the situation.
ACKNOWLEDGEMENTS
The authors wish to acknowledge the assistance of the Bevalac staff at Berkeley
during calibration runs. We thank also, J. Daly, A. Grace-Casey, S. Ledwidge, G.
Broderick, and H. Sullivan who undertook processing and measurements and Wai
Ming Tai whose assistance with data processing was greatly appreciated. Finally, we
wish to express our gratitude to R. Aarts, A.J. Daleman, V. Domingo, M. Froggatt, J.
v.d. Hoek, M.J. Kikkert, F. Levadou, H. Mengs, J.P. van Meygaarden, B.C. Poor, J.
Postema, G. Saenger and M.V. Slogteren who supported the de-integration activities
at ESTEC.
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1. Thompson, A., D. O'Sullivan, K.-P. Wenzel, V. Domingo, C. Domingo, J. Daly,
A. Smit. Proc. 21st ICRC (1990) 4, 441, Adelaide.
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3. See, T., M. Allbrooks, D. Atkinson, C. Simon, M. Zolensky. Meteoroid and Debris
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4. McDonnell J.A.M., M.T. Paley, T.J. Stevenson, A. Thompson, D. O' Sullivan, K.-
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375



The LDEF ultra heavy cosmic ray experiment

The LDEF ultra heavy cosmic ray experiment

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