We present a new measurement of the isotopic composition of cosmic ray iron in the energy

interval ~1550-2200 MeV /nucleon. The data were collected during the May 1984 flight of a balloon-borne spectrometer and show an average mass resolution of ~0.7 amu. The instrument employed the Cerenkov-Energy technique for mass determination. The observed ^(54)Fe/^(56)Fe ratio, 0.14 +0.18/-0.11 at the top of the atmosphere, is consistent with a solar system composition at the cosmic ray source. We also place an upper limit on the ^(58)Fe/^(56)Fe ratio at the top of the atmosphere of ≤.07. Both are consistent with previous

measurements at lower energies

Buffington, A.

Christian, E. R.

Grove, J. E.

Mewaldt, R. A.

Rasmussen, I. L.

Schindler, S. M.

Stone, E. C.

Caltech Authors - Main

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A tv!EASUllEMENT OF' THE ISOTOPIC COMPOSITION OF' COSMIC RAY llWN 
J.E. Grove*, E. fl. Christian, IL A. Mcwaldt, S. M. Schindler, E. C. Stone 
California Inst,itutc of Technology, Pasadena, CA ~)11 :!5 USA 
A. Buffingt.on 
University of California at San Diego, La Jolla, CA U20D~1 USA 
I. L. Rasm usscn 
Danish Spa.cc Research Institute, Lyngby, Denmark 
*current address: E.O. Hulburt, Center for Space Research, NllL, 
Washington DC, 20375 USA 
Abstract: 
V·le present a new measurement of the isotopic composition of cosmic ray iron in the energy 
interval .......,1550-2200 MeV /nucleon. 'fhe data were collected during the May 1984 flight of 
a balloon-borne spectrometer and show an average mass resolution of r---0. 7 am u. The 
instrument emf,loyed the Cerenkov-Energy technique for mass determination. The 
observed 54 Fe/5 Fe ratio, 0.14!gN at the top of the atmosphere, is consistent with a solar-
svstem composition at the cosmic ray source. VVe also place an upper limit on the 
5SFe/56Fe ratio at the top of the atmosphere of ::;o.07. Both are consistent with previous 
mea.su remen ts at lower energies. 
Instrument: The Caltech/DSRI High Energy Isotope Spectrometer Telescope (HEIST) is 
a balloon-borne mass spectrometer designed to measure the isotopic abundance of the 
cosmic rays from Ne to Ni (10::; z::;28) at energies from -..I.5 GcV/nucleon to "'2.2 
GcV /nucleon at the top of the atmosphere. Masses of particles which stop in the instru-
ment are measured by the Cerenkov x Energy technique, while at higher energies masses of 
penetrating particles can in principle be measured by the Cerenkov x 6£ x Cerenkov tech-
nique (Buffington et al., 1983). Mass is given by the ratio of energy loss to velocity change, 
Lill M = -. 
l:l.1 
The instrument consists of a stack of twelve Nal(Tl) scintillators bounded on each end 
by a Cerenkov counter and plastic scintillator (Buffington et al., 1983). The effective 
geometry factor varies with particle depth in the stack, ranging from 0.18 to 0.11 m 2sr. 
The layers of the Nal(Tl) stack provide a direct measurement of energy loss 1:l.E and also 
measure particle positions to an accuracy of r-..().5 cm for Fe {Grove, 1989; or Schindler et 
al., 1985). 
The top Cerenkov counter contains a silica aerogel radiator with index of refraction 
n ~ 1.1, which corresponds to a threshold energy of ""'1300 MeV /nucleon (Rasmussen el al., 
1983). A calibration of the detector at the Bevalac in November l 982 showed that for the 
aerogel counter, the number of photoelectrons from a Z = l, /3=1 particle was 18±2. How-
ever, the light output was subsequently observed to degrade at an average rate of "'2% per 
month over a period of rvl8 months, thus dropping by a factor of rv2 by the time of the 
flight (Grove, 1989). Note that this rate is ""2-4 times greater than that reported by other 
investigators {Poelz, l 986, and references therein). Although the precise source of the 
degradation remains unknown, it is presumed to have resulted from organic contamination 
of the aerogels, perhaps by the ou tgassing products of the BaS04 high-reflectivity paint 
used in the counters. Identical blocks of acrogel stored in a dessicator for a period of .......,5 
years do not show significant degradation. 
The instrument was launched from Palestine, Texas, on 1-1 May 19811, and spent >35 
hours at a mean float altitude of 6.0 g/ cm 2 of residual atmo.5phere. The flight path ranged 
in vertical geomagnetic cutoff rigidity from .......,4_1 to .......,5.4 CV. 
Analysis: The current analysis is restricted to Fe nuclei which stopped in the Nal{Tl) 
stack; thus only the Cerenkov x Energy t.cchniquc was employed. Stopping events which 
53 
© Organizing Committee of the 21st Int. Cosmic Ray Conf • Provided by the NASA Astrophysics Data System 
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did not suffer charge-changing interactions were selected through a multi-step process. An 
initial rough selection of stopping Fe~group events (24 ~ Z ~ 28) was achieved by the 
dE/dx x Energy method and by rejecting those events showing evidence of interaction 
daughters in the layers following the stopping point. A subsequent detailed comparison of 
response in the Nal(Tl) stack with the expected Bragg curve gave a charge measurement at 
several p9ints in the instrument. A total of 32 stopping, non-charge-changing Fe nuclei 
were identified, while a Monte Carlo simulation predicted that 54 such events should have 
been observed (Grove, 1989). A number of uncertainties exist in the simulation, including 
the assumed spectrum at the top of the atmosphere and the filtering by the geomagnetic 
field, so that the disagreement may not be considered significant. Figure 1 shows a mass 
histogram of these events, along with the best fit discussed below. 
10 
8 
en 6 
...,_) 
c:: 
Q) 
> 
t=J 4 
2 
total 
----- s7Fe 
·-·-·-·-·-
58Fe 
·-·-·-·· 
55Fe 
- - - - - - - - s.&F e 
45 50 55 
Mass (amu) 
60 
Figure 1: Mass histogram of the 32 Fe 
events, renormalized by the maximum-
likelihood mass scale factor. The smooth 
curves are the calculated mass distributions 
for the isotopes of Fe. 
The isotopic composition of Fe measured in the instrument is modified from that at 
the top of the atmosphere by nuclear interactions in the atmosphere and in the detectors. 
This includes a non-negligible number of neutron-stripping interactions in the Nal(Tl) 
stack, leading to a low-mass tail for a beam of a single isotopic species. We therefore used 
a maximum-likelihood technique to fit the measured masses with calculated mass distribu-
tions that accounted for these neutron-stripping interactions in the at.mosphere and instru-
ment. The calculated mass distributions were derived by convolving an assumed Gaussian 
resolution function with the result of a Monte Carlo calculation of the expected amount of 
neutron stripping. Cross sections on high-Z materials were derived from those of Webber 
(1987) on hydrogen. We have demonstrated that this procedure gives good agreement 
between the observed and the calculated mass distribution for 55Mn using Bevalac calibra-
tion data (Grove, 1989). 
Free parameters of the fit in Figure 1 were the 58Fe/56Fe and 54Fe/56Fe abundance 
ratios at the top of the atmosphere, along with the mass scale and mass resolution. We did 
not attempt to fit the abundances of 57Fe and 55Fe -instead we assumed a solar-system 
abundance of these isotopes at the cosmic ray source (GCRS) and propagated them to the 
top of the atmosphere as described below. Note that the solar-system abundance of the 
electron-capture nuclide 55Fe is negligible, and thus its abundance at the top of the atmo-
sphere is equal to the amount produced by the galactic propagation .. We assumed that 55Fe 
is stable once accelerated to cosmic ray energies because the probability of electron attach-
ment from the ISM and subsequent decay is negligibly small at these energies (Grove, 
1989). 
54 
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The average ma.ss resolution obtained wa.s O.G7±0.17 amu, consist.ent wit.h t.hcoret.iea.1-
cstimates. The principal contrihut.ions to the mass resolution were from phot.o<dect.ron sta-
tist.ical fluctuations, aerogel mapping uncert.ainties, and aerogel response 11orm~tlizat.ion 
errors, all of which were increased by the degradation of the light yield of t.he rn.dia.t.or. 
'T'hc observed resolution from the 13evalac calibration done in Novcm bcr I U82 was 0.52±0.01 
arnu for 55Mn. . 
To interpret the measured composition, we propagated trial Pc isotopic a.bu nda.nces at 
the GCRS to the top of the atmosphere using a Leaky Box model that accou n t.cd for spalla-
tion interactions and deceleration in the ISM (assumed to be hydrogen), and also allowed 
unstable species to decay. Interaction cross sections were from Webber (1987) .. We used 
the source spectrum and rigidity-dependent mean escape length from Engelman.n el al. 
{1985), and a mean mass density of 0.3 amu/cm 3 in the propagation volume. We used the_ 
force-field approximation of Gleeson and Axford ( 1968) to account for solar mod u la.tion, 
although its effect on composition at these energies is small. · 
Note that 54Mn may decay by 13- emission to 54Fe during propagation wit.h a ha.lf~life 
which is rather uncertain. This decay can have a significant effect on the in tcrpr.ctation ·of 
the 54Fe/56Fe ratio observed at Earth (Grove et al., 1990). Assuming T '?. = 2 x 106 years 
~Koch et al. 1981) and a solar-srstem 3:bundance ratio at the GC~S (i.e., "1Mn absent a.nd 
·tFe/56Fe = 0.0632), the 54Fe/5 Fe ratio at 1750 MeV /nucleon mcreases by '""'15% with 
respect to the ratio derived assuming that the weak decay does not occur. 
Discussion: The table below summarizes our observations in the energy int.crval ""1550-
2200 Me V /nucleon at the top of the atmosphere, with 68% confidence intervals. and 84% 
confidence upper limits. Note the large lower error bar for the observed 51Fe/56Fe rat.io. 
Uncertainties in the propagation correction are large enough that our estimate of the 
&4Fe/56Fe ratio at the GCRS may also be considered to be an upper limit. Also listed for 
comparison are the solar system ratios (Anders and Ebihara, 1982). 
Ratio 
Fe isotopic abundance ratios 
,....,,, . -?? 
o.14~g·i1r 
::0.044 
::0.024 
~0.07 
0.12~8.·l~ 
=:0.0 
=:0.023 
~0.07 
*MMn decay r" = 2xl06 years. 
p =:0.3 amu/cm3 
Solar S stem 
0.063 
0.0 
0.023 
0.0032 
To improve the upper limit placed on the 58Fe/56Fe ratio given above, we have combined 
our data with those of Mewaldt et al. (1980). We assume (conservatively) that one 58Fe 
event was observed-the event with h~est mass from the mass histogram in Figure 1, out 
of 26.5 56Fe from this data set and 27 Fe from the Mewaldt et al. data set. The Poisson. 
84% confidence upper limit for the expected number of events when fewer th.an two arc 
observed is 3.3 events, from which we derive a ratio of 58Fe/56Fe =0.062. Our abundance 
ratios are consistent with previous measurements at lower energies (Tarlc el al. 1979, 
Mewaldt et al. 1980, Webber 1981, and Young et al. 198ll. 
Figure 2 summarizes five measurements of the 54Fe/ 6Fe abundance ratio at the CCRS 
as a function of energy. The data are plotted approximately at the average of their energy 
intervals at the top of the atmosphere or the orbit of Earth. All observed ratios_ have been 
propagated from Earth back to the GCRS by the same algorithm described above. The 
uncertainties of the current measurement are large due to the moderate mass resolution 
(comparable to that of Tarte et al. and Young et al.) and low counting stat.istics. The 
dashed line indicates the solar-system ratio. In all cases, the observations are consistcn t 
with a solar-system source composition, although they do permit significant <>nhanccments 
in 54 Fe and 58Fe. 
55 
© Organizing Committee of the 21st Int. Cosmic Ray Conf • Provided by the NASA Astrophysics Data System 
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Q) g .3 
ro 
"O 
c: 
~ 
..0 < .2 
OG5.2-J2 
(/) 
0:: 
u 
CJ 
---------- --1I-------l-----
0 ~~--'-1~_.____._I ~l_._.1---.....1_._._~~---~ 
100 200 500 1000 2000 
Energy (MeV /nuc) 
Figure 2: Calculated MFe 56Fe abundance 
ratios at the cosmic ray source. The data are 
plotted approximately at the average of their 
energy intervals at the top of the atmosphere 
or the orbit of the Earth. The dashed line 
marks the solar-system ratio. Open circle, 
Mewaldt el al., (1980); filled square, Tarle el 
al., (1979); cross, Webber (19Sl); open square, 
Young et al., (H>Sl); filled circle, this work. 
Acknowledgements: This work was supported in part by NASA under grant NGR 05-
002-160. We are grateful to P. S. Gibner for his contributions to this work and to the per-
sonnel of the National Scientific Balloon Facility and the Wallops Flight Facility for their 
excellent support. 
References: 
Anders, E., and ~·I. Ebihara, Geochim. at'Cosmochim. Acta, 46, 2363 (19821. 
Buffington, A., et al, Proc. 18th ICRC {Bangalore), 2, 49 {1983). 
Engelmann, J. J., et al., Astron. and Astroph., 148, 12 {1985). 
Gleeson, L. J., and W. I. Axford, Ap. J., 154, 1011 (HJ68). 
Grove, J. E., et al., Proc. £1st ICRC {Adelaz"de}, OG 8.3-5 {1990). 
Grove, J. E., Ph.D. Thesis, California Institute of Technology (1989). 
Koch, L., et al., Astron. and Astroph. {Letters), 102, L9 (1981). 
Mewaldt, R. A., et al., Ap. J. {Letters), 236, Ll21 (1980). 
Poelz, G., in Aerogels, ed. J. Fricke (Springer-Verlag: Berlin, 1986). 
Rasmussen, I. L., et al., Proc. 18th ICRC (Bangalore), 8, i7 (1983). 
Schindler, S. M., et al., Proc. 19th ICRC (La Jolla), 2, 103 {1985). 
Tarle, G., et al., Ap. J., 230, 607 (1979). 
Webber, W. R., Proc. 17th ICRC (Parz"s}, 2, 80 (1981). 
Webber, W. R., Proc. £0th ICRC {Moscow), 2, 463 (1987). 
Young, J. S., et al., Ap. J., 246, 1014 (1981). 
56 
© Organizing Committee of the 21st Int. Cosmic Ray Conf • Provided by the NASA Astrophysics Data System 


A Measurement of the Isotopic Composition of Cosmic Ray Iron

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A tv!EASUllEMENT OF' THE ISOTOPIC COMPOSITION OF' COSMIC RAY llWN 
J.E. Grove*, E. fl. Christian, IL A. Mcwaldt, S. M. Schindler, E. C. Stone 
California Inst,itutc of Technology, Pasadena, CA ~)11 :!5 USA 
A. Buffingt.on 
University of California at San Diego, La Jolla, CA U20D~1 USA 
I. L. Rasm usscn 
Danish Spa.cc Research Institute, Lyngby, Denmark 
*current address: E.O. Hulburt, Center for Space Research, NllL, 
Washington DC, 20375 USA 
Abstract: 
V·le present a new measurement of the isotopic composition of cosmic ray iron in the energy 
interval .......,1550-2200 MeV /nucleon. 'fhe data were collected during the May 1984 flight of 
a balloon-borne spectrometer and show an average mass resolution of r---0. 7 am u. The 
instrument emf,loyed the Cerenkov-Energy technique for mass determination. The 
observed 54 Fe/5 Fe ratio, 0.14!gN at the top of the atmosphere, is consistent with a solar-
svstem composition at the cosmic ray source. VVe also place an upper limit on the 
5SFe/56Fe ratio at the top of the atmosphere of ::;o.07. Both are consistent with previous 
mea.su remen ts at lower energies. 
Instrument: The Caltech/DSRI High Energy Isotope Spectrometer Telescope (HEIST) is 
a balloon-borne mass spectrometer designed to measure the isotopic abundance of the 
cosmic rays from Ne to Ni (10::; z::;28) at energies from -..I.5 GcV/nucleon to "'2.2 
GcV /nucleon at the top of the atmosphere. Masses of particles which stop in the instru-
ment are measured by the Cerenkov x Energy technique, while at higher energies masses of 
penetrating particles can in principle be measured by the Cerenkov x 6£ x Cerenkov tech-
nique (Buffington et al., 1983). Mass is given by the ratio of energy loss to velocity change, 
Lill M = -. 
l:l.1 
The instrument consists of a stack of twelve Nal(Tl) scintillators bounded on each end 
by a Cerenkov counter and plastic scintillator (Buffington et al., 1983). The effective 
geometry factor varies with particle depth in the stack, ranging from 0.18 to 0.11 m 2sr. 
The layers of the Nal(Tl) stack provide a direct measurement of energy loss 1:l.E and also 
measure particle positions to an accuracy of r-..().5 cm for Fe {Grove, 1989; or Schindler et 
al., 1985). 
The top Cerenkov counter contains a silica aerogel radiator with index of refraction 
n ~ 1.1, which corresponds to a threshold energy of ""'1300 MeV /nucleon (Rasmussen el al., 
1983). A calibration of the detector at the Bevalac in November l 982 showed that for the 
aerogel counter, the number of photoelectrons from a Z = l, /3=1 particle was 18±2. How-
ever, the light output was subsequently observed to degrade at an average rate of "'2% per 
month over a period of rvl8 months, thus dropping by a factor of rv2 by the time of the 
flight (Grove, 1989). Note that this rate is ""2-4 times greater than that reported by other 
investigators {Poelz, l 986, and references therein). Although the precise source of the 
degradation remains unknown, it is presumed to have resulted from organic contamination 
of the aerogels, perhaps by the ou tgassing products of the BaS04 high-reflectivity paint 
used in the counters. Identical blocks of acrogel stored in a dessicator for a period of .......,5 
years do not show significant degradation. 
The instrument was launched from Palestine, Texas, on 1-1 May 19811, and spent >35 
hours at a mean float altitude of 6.0 g/ cm 2 of residual atmo.5phere. The flight path ranged 
in vertical geomagnetic cutoff rigidity from .......,4_1 to .......,5.4 CV. 
Analysis: The current analysis is restricted to Fe nuclei which stopped in the Nal{Tl) 
stack; thus only the Cerenkov x Energy t.cchniquc was employed. Stopping events which 
53 
© Organizing Committee of the 21st Int. Cosmic Ray Conf • Provided by the NASA Astrophysics Data System 
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did not suffer charge-changing interactions were selected through a multi-step process. An 
initial rough selection of stopping Fe~group events (24 ~ Z ~ 28) was achieved by the 
dE/dx x Energy method and by rejecting those events showing evidence of interaction 
daughters in the layers following the stopping point. A subsequent detailed comparison of 
response in the Nal(Tl) stack with the expected Bragg curve gave a charge measurement at 
several p9ints in the instrument. A total of 32 stopping, non-charge-changing Fe nuclei 
were identified, while a Monte Carlo simulation predicted that 54 such events should have 
been observed (Grove, 1989). A number of uncertainties exist in the simulation, including 
the assumed spectrum at the top of the atmosphere and the filtering by the geomagnetic 
field, so that the disagreement may not be considered significant. Figure 1 shows a mass 
histogram of these events, along with the best fit discussed below. 
10 
8 
en 6 
...,_) 
c:: 
Q) 
> 
t=J 4 
2 
total 
----- s7Fe 
·-·-·-·-·- 58Fe 
·-·-·-·· 
55
Fe 
- - - - - - - - s.&F e 
45 50 55 
Mass (amu) 
60 
Figure 1: Mass histogram of the 32 Fe 
events, renormalized by the maximum-
likelihood mass scale factor. The smooth 
curves are the calculated mass distributions 
for the isotopes of Fe. 
The isotopic composition of Fe measured in the instrument is modified from that at 
the top of the atmosphere by nuclear interactions in the atmosphere and in the detectors. 
This includes a non-negligible number of neutron-stripping interactions in the Nal(Tl) 
stack, leading to a low-mass tail for a beam of a single isotopic species. We therefore used 
a maximum-likelihood technique to fit the measured masses with calculated mass distribu-
tions that accounted for these neutron-stripping interactions in the at.mosphere and instru-
ment. The calculated mass distributions were derived by convolving an assumed Gaussian 
resolution function with the result of a Monte Carlo calculation of the expected amount of 
neutron stripping. Cross sections on high-Z materials were derived from those of Webber 
(1987) on hydrogen. We have demonstrated that this procedure gives good agreement 
between the observed and the calculated mass distribution for 55Mn using Bevalac calibra-
tion data (Grove, 1989). 
Free parameters of the fit in Figure 1 were the 58Fe/56Fe and 54Fe/56Fe abundance 
ratios at the top of the atmosphere, along with the mass scale and mass resolution. We did 
not attempt to fit the abundances of 57Fe and 55Fe -instead we assumed a solar-system 
abundance of these isotopes at the cosmic ray source (GCRS) and propagated them to the 
top of the atmosphere as described below. Note that the solar-system abundance of the 
electron-capture nuclide 55Fe is negligible, and thus its abundance at the top of the atmo-
sphere is equal to the amount produced by the galactic propagation .. We assumed that 55Fe 
is stable once accelerated to cosmic ray energies because the probability of electron attach-
ment from the ISM and subsequent decay is negligibly small at these energies (Grove, 
1989). 
54 
© Organizing Committee of the 21st Int. Cosmic Ray Conf • Provided by the NASA Astrophysics Data System 
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The average ma.ss resolution obtained wa.s O.G7±0.17 amu, consist.ent wit.h t.hcoret.iea.1-
cstimates. The principal contrihut.ions to the mass resolution were from phot.o<dect.ron sta-
tist.ical fluctuations, aerogel mapping uncert.ainties, and aerogel response 11orm~tlizat.ion 
errors, all of which were increased by the degradation of the light yield of t.he rn.dia.t.or. 
'T'hc observed resolution from the 13evalac calibration done in Novcm bcr I U82 was 0.52±0.01 
arnu for 55Mn. . 
To interpret the measured composition, we propagated trial Pc isotopic a.bu nda.nces at 
the GCRS to the top of the atmosphere using a Leaky Box model that accou n t.cd for spalla-
tion interactions and deceleration in the ISM (assumed to be hydrogen), and also allowed 
unstable species to decay. Interaction cross sections were from Webber (1987) .. We used 
the source spectrum and rigidity-dependent mean escape length from Engelman.n el al. 
{1985), and a mean mass density of 0.3 amu/cm 3 in the propagation volume. We used the_ 
force-field approximation of Gleeson and Axford ( 1968) to account for solar mod u la.tion, 
although its effect on composition at these energies is small. · 
Note that 54Mn may decay by 13- emission to 54Fe during propagation wit.h a ha.lf~life 
which is rather uncertain. This decay can have a significant effect on the in tcrpr.ctation ·of 
the 54Fe/56Fe ratio observed at Earth (Grove et al., 1990). Assuming T '?. = 2 x 106 years 
~Koch et al. 1981) and a solar-srstem 3:bundance ratio at the GC~S (i.e., "1Mn absent a.nd 
·tFe/56Fe = 0.0632), the 54Fe/5 Fe ratio at 1750 MeV /nucleon mcreases by '""'15% with 
respect to the ratio derived assuming that the weak decay does not occur. 
Discussion: The table below summarizes our observations in the energy int.crval ""1550-
2200 Me V /nucleon at the top of the atmosphere, with 68% confidence intervals. and 84% 
confidence upper limits. Note the large lower error bar for the observed 51Fe/56Fe rat.io. 
Uncertainties in the propagation correction are large enough that our estimate of the 
&4Fe/56Fe ratio at the GCRS may also be considered to be an upper limit. Also listed for 
comparison are the solar system ratios (Anders and Ebihara, 1982). 
Ratio 
Fe isotopic abundance ratios 
,....,,, . -?? 
o.14~g·i1r 
::0.044 
::0.024 
~0.07 
0.12~8.·l~ 
=:0.0 
=:0.023 
~0.07 
*MMn decay r" = 2xl06 years. 
p =:0.3 amu/cm3 
Solar S stem 
0.063 
0.0 
0.023 
0.0032 
To improve the upper limit placed on the 58Fe/56Fe ratio given above, we have combined 
our data with those of Mewaldt et al. (1980). We assume (conservatively) that one 58Fe 
event was observed-the event with h~est mass from the mass histogram in Figure 1, out 
of 26.5 56Fe from this data set and 27 Fe from the Mewaldt et al. data set. The Poisson. 
84% confidence upper limit for the expected number of events when fewer th.an two arc 
observed is 3.3 events, from which we derive a ratio of 58Fe/56Fe =0.062. Our abundance 
ratios are consistent with previous measurements at lower energies (Tarlc el al. 1979, 
Mewaldt et al. 1980, Webber 1981, and Young et al. 198ll. 
Figure 2 summarizes five measurements of the 54Fe/ 6Fe abundance ratio at the CCRS 
as a function of energy. The data are plotted approximately at the average of their energy 
intervals at the top of the atmosphere or the orbit of Earth. All observed ratios_ have been 
propagated from Earth back to the GCRS by the same algorithm described above. The 
uncertainties of the current measurement are large due to the moderate mass resolution 
(comparable to that of Tarte et al. and Young et al.) and low counting stat.istics. The 
dashed line indicates the solar-system ratio. In all cases, the observations are consistcn t 
with a solar-system source composition, although they do permit significant <>nhanccments 
in 54 Fe and 58Fe. 
55 
© Organizing Committee of the 21st Int. Cosmic Ray Conf • Provided by the NASA Astrophysics Data System 
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Q) g .3 
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"O 
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OG5.2-J2 
(/) 
0:: 
u 
CJ 
---------- --1I-------l-----
0 ~~--'-1~_.____._I ~l_._.1---.....1_._._~~---~ 
100 200 500 1000 2000 
Energy (MeV /nuc) 
Figure 2: Calculated MFe 56Fe abundance 
ratios at the cosmic ray source. The data are 
plotted approximately at the average of their 
energy intervals at the top of the atmosphere 
or the orbit of the Earth. The dashed line 
marks the solar-system ratio. Open circle, 
Mewaldt el al., (1980); filled square, Tarle el 
al., (1979); cross, Webber (19Sl); open square, 
Young et al., (H>Sl); filled circle, this work. 
Acknowledgements: This work was supported in part by NASA under grant NGR 05-
002-160. We are grateful to P. S. Gibner for his contributions to this work and to the per-
sonnel of the National Scientific Balloon Facility and the Wallops Flight Facility for their 
excellent support. 
References: 
Anders, E., and ~·I. Ebihara, Geochim. at'Cosmochim. Acta, 46, 2363 (19821. 
Buffington, A., et al, Proc. 18th ICRC {Bangalore), 2, 49 {1983). 
Engelmann, J. J., et al., Astron. and Astroph., 148, 12 {1985). 
Gleeson, L. J., and W. I. Axford, Ap. J., 154, 1011 (HJ68). 
Grove, J. E., et al., Proc. £1st ICRC {Adelaz"de}, OG 8.3-5 {1990). 
Grove, J. E., Ph.D. Thesis, California Institute of Technology (1989). 
Koch, L., et al., Astron. and Astroph. {Letters), 102, L9 (1981). 
Mewaldt, R. A., et al., Ap. J. {Letters), 236, Ll21 (1980). 
Poelz, G., in Aerogels, ed. J. Fricke (Springer-Verlag: Berlin, 1986). 
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A Measurement of the Isotopic Composition of Cosmic Ray Iron

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