Abstract: The first long-duration balloon flight of TRACER in 2003 provided high-quality measurements of the primary cosmic-ray nuclei over the range oxygen (Z = 8) to iron (Z = 26). The analysis of these measurements is now complete, and we will present the individual energy spectra and absolute intensities of the nuclei O, Ne, Mg, Si, S, Ca, Ar, and Fe. The spectra cover the energy range from 1 GeV/nucleon to more than 10 TeV/nucleon, or in terms of total energy, to several 10 14 eV per particle. We compare our results with those of other recent observations in space and on balloons and notice, in general, good agreement with these data for those regions where overlap exists. We also compare our data with information that has recently been inferred from air shower observations

A Romero-Wolf

C Höppner

D Müller

J Hörandel

M Ave

M Ichimura

P J Boyle

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30TH INTERNATIONAL COSMIC RAY CONFERENCE
Cosmic Ray Energy Spectra of Primary Nuclei from Oxygen to Iron: Results from
the TRACER 2003 LDB Flight
P.J. BOYLE, M. AVE, C. HÖPPNER, J. HÖRANDEL1, M. ICHIMURA2, D. MÜLLER AND A. ROMERO-
WOLF
The University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637, USA
1University of Karlsruhe, Germany
2Hirosaki University, Japan
jojo@donegal.uchicago.edu
Abstract: The first long-duration balloon flight of TRACER in 2003 provided high-quality measure-
ments of the primary cosmic-ray nuclei over the range oxygen (Z = 8) toiron (Z = 26). The analysis
of these measurements is now complete, and we will present the individual energy spectra and absolute
intensities of the nuclei O, Ne, Mg, Si, S, Ca, Ar, and Fe. The spectra cover the energy range from 1
GeV/nucleon to more than 10 TeV/nucleon, or in terms of total energy, to several 1014 eV per particle.
We compare our results with those of other recent observations in spaceand on balloons and notice, in
general, good agreement with these data for those regions where overlap exists. We also compare our
data with information that has recently been inferred from air shower observations.
Introduction
TRACER (Transition Radiation Array for Comic
Energetic Radiation) is a very large instrument
designed to study cosmic ray nuclei above a
TeV/nucleon. In 2003 TRACER had a 10 day bal-
loon flight in Antarctica yielding an exposure of
50 m2 steradian days at a residual atmosphere of
3.9 g/cm2. During the flight TRACER collected 50
million cosmic ray nuclei with charge Z≥ 8. The
analysis of this data-set is now complete and we
present here the energy spectra for eight elements
O, Ne, Mg, Si, S, Ar, Ca and Fe. The results cover
over four decades in energy and are given as abso-
lute intensities, without arbitrary normalizations.
Absolute Intensities
Depending on the method of energy assignment,
each event that passes the data analysis cuts is
classified either as aCerenkov Event, dE/dx Event
or Transition Radiation Event. These events are
sorted into energy bins, using the response curves
described by M̈uller (these proceedings [1]). The
width of each energy bin is commensurate with
the energy resolution and varies with energy E and
charge Z. As the relative intrinsic signal fluctua-
tions decrease proportional to 1/Z, the energy reso-
lution improves with increasing charge. To convert
from the number of events Ni in a particular energy
bin ∆E to an absolute differential fluxdNi/dE at
the top of the atmosphere one must compute the ex-
posure factor, effective aperture, efficiency of the
cuts and unfold the instrument response :
dNi
dE
=
Ni
∆E
·
1
Tl
·
1
εi
·
1
Ai
· Ci (1)
with Tl the live-time,εi the efficiency of analysis
cuts, Ai the effective aperture and Ci the “overlap
correction” due to misidentified events from neigh-
bouring bins. The effective aperture is :
Ai = A · 2π
∫ π/2
θ=0
PI(θ)PD(θ)cosθ d(cosθ) (2)
with A the area of the detector (2.06 m× 2.06m),
PI(θ) the probability of survival in the atmosphere
and the instrument as a function of zenith angle
θ, PD(θ) the probability that a particle passing
through the instrument will be detected. The Over-
lap corrections are determined with Monte Carlo
Proceedings of the 30th International Cosmic Ray Conference
Rogelio Caballero, Juan Carlos D’Olivo, Gustavo Medina-Tanco,
Lukas Nellen, Federico A. Sánchez, José F. Valdés-Galicia (eds.)
Universidad Nacional Autónoma de México,
Mexico City, Mexico, 2008
Vol. 2 (OG part 1), pages 87–90
ID 1192
87
TRACER RESULTS
Kinetic Energy (GeV)
1 10 210 310 410 510 610
)
-
1
 
G
e
V
-
1
 
s
-
1
 
s
r
-
2
F
l
u
x
 
(
m
-2010
-1710
-1410
-1110
-810
-510
-210
10
410
710 HEAO
CRN
TRACER
710⋅O
510⋅Ne
310⋅Mg
Si
-210⋅S
-410⋅Ar
-610⋅Ca
-910⋅Fe
Power Law Fit
Figure 1: Differential energy spectra for the cosmic ray nuclei : O, Ne, Mg, Si, S, Ar, Ca and Fe. Results
from the TRACER 2003 Antarctica flight are indicated by the red squares. Existing data from the HEAO-3
experiment (open diamonds) [2] and the CRN experiment (opencrosses) [3] are shown for comparison.
The dashed line represents an independent power-law fit to each spectrum above 20 GeV/nucleon.
88
30TH INTERNATIONAL COSMIC RAY CONFERENCE
simulations and are typically≤ 20% (i.e. 0.8 ≤
Ci ≤ 1.2). An example of efficiencies etc is given
for oxygen and iron in Table 1 of M̈uller [1]. The
energy assigned for each bin is defined as :
E =
1
E2 − E1
·
1
1− α
·(E1−α
2
−E1−α
1
)−1/α (3)
whereα is the power-law exponent of the differen-
tial energy spectrum. The method is discussed in
detail by Lafferty and Wyatt [4].
Resulting Energy Spectrum
The energy spectra, in terms of absolute intensi-
ties, for the elements O, Ne, Mg, Si, S, Ar, Ca
and Fe are presented in Figure 1. We note the
large range in intensity (ten decades) and particle
energy (four decades) covered by TRACER. This
has been achieved by three complementary mea-
surements in one detector : the Cerenkov counter
(∼ 1011 eV), the relativistic rise of the ionization
signal in gas (∼ 1011−1013 eV) and the Transition
Radiation Detector (> 1013 eV) [1]. Data from
the TRACER 2003 flight are indicated by the red
squares. For clarity the intensity of each element
is multiplied by a factor shown on the left. Exist-
ing data from measurements in space with HEAO-
3 (open diamonds) and CRN (open crosses) are
shown for comparison. As can be seen, the en-
ergy spectra for O, Ne, Mg and Fe extend up to
and beyond 1014 eV. No evidence for any signifi-
cant change in spectral slope is evident at the high-
est energies. The energy spectrum of each element
(from TRACER and CRN) is fit to a power law
above 20 GeV/nucleon. The resulting spectral in-
dices (Figure 2) are remarkably similar, with an av-
erage of 2.65.
Figure 3 compares the TRACER results for iron
with results from a number of other investigations.
Below 1012 eV we show results from HEAO-3 [2]
and at higher energies from CRN [3] on the space
shuttle and from the ATIC-2 [5] and RUNJOB bal-
loon experiments [6]. The dashed line represents
a power-law fit with an exponent of -2.7 and de-
scribes the data well above 1012 eV. Figure 3 also
illustrates the variety of detection techniques used
in measuring the energy of heavy nuclei. Within
Charge Z
5 10 15 20 25 30
S
p
ec
tr
al
 In
d
ex
-2.9
-2.8
-2.7
-2.6
-2.5
-2.4
-2.3
O Ne
Mg Si
S Ar
Ca
Fe Fit-2.66E
Figure 2: Spectral indices of a best power-law fit
to the combined TRACER and CRN data above
20 GeV/nucleon. The line indicates the an average
spectral fit of E−2.65.
the statistical uncertainties (which in some mea-
surements are quite large), the data indicate fairly
consistent results. The Transition Radiation tech-
nique of TRACER can, in principle, provide mea-
surements with energies up to around 1015 eV. The
range of the current results is limited by the expo-
sure available. Also presented in Figure 3 are re-
cent results from the ground based HESS Imaging
Air Cerenkov Telescope using the Direct Cerenkov
Technique (green triangles). These results are the
first examples of a new technique for measure-
ments from the ground [7]. Two flux values are
presented for each energy indicating ambiguities
from different interaction models [8]. Again, these
data are consistent with TRACER.
Comparison with Air-shower Data
Data for oxygen and iron for TRACER are com-
pared in Figure 4 with spectra derived from in-
direct observations of the EAS-TOP collaboration
[9], and of the KASCADE group for two different
nucleus-nucleus interaction models [10]. However,
these groups do not report results for individual el-
ements: the fluxes for the “CNO group” probably
have about twice the intensity than oxygen alone,
89
TRACER RESULTS
Figure 3: Iron energy spectrum above 1010eV
per particle in units of m2 ster−1 s−1, highlight-
ing the complementarity between detection tech-
niques. TRACER (red squares), HEAO-3 [2]
(black diamonds), CRN [3] (black crosses), ATIC-
2 [5] (black open circles), RUNJOB [6] (blue open
circles) and HESS [8] (green stars).
while the “iron group” probably is dominated by
iron. Our results do not yet overlap with the en-
ergy region of the air shower data, but the gap is
becoming smaller, in particular for oxygen. Addi-
tional measurements will indeed lead to significant
constraints on the air shower interpretations.
Conclusion
The TRACER 2003 data represent the most de-
tailed measurements to date for heavy nuclei
above a TeV/nucleon with single charge resolution.
While the results do not reveal any surprising fea-
tures in the cosmic ray energy spectra at high ener-
gies, they begin to provide stringent constraints on
the conventional models on galactic propagation.
The analysis of the TRACER results in the context
of these models is discussed in these proceedings
(Ave et al. [11]).
Figure 4: Energy Spectra from TRACER and from
the interpretation of air shower data of KASCADE
(for two different interaction models) and of EAS-
TOP (two data points for each energy are given,
representing upper and lower limits). The spectra
are for oxygen and for iron for TRACER, but for
the “CNO-group” and the “Fe-group” for the other
observations.
We acknowledge support as summarized in Müller
et al [1].
References
[1] D. Müller et al., Proc. 30th ICRC.
[2] J. J. Engelmann et al., A&A 233 (1990) 96.
[3] D. Müller et al., ApJ 374 (1991) 356–365.
[4] J. D. Lafferty, T. R. Wyatt, NIM A 355 (1995)
541.
[5] A. D. Panov et al., ArXiv e-print.
[6] V. A. Derbina et al., ApJ, L 628 (2005) L41.
[7] D. B. Kieda et al., Proc. 27th ICRC 4 (2001)
1533.
[8] F. Aharonian et al., Phys. Rev. D 75 (4)
(2007) 042004–.
[9] G. Navarra et al., Proc. 28th ICRC 1 (2003)
147.
[10] T. Antoni et al., APh 24 (2005) 1–2.
[11] M. Ave et al., Proc. 30th ICRC.
90


Cosmic Ray Energy Spectra of Primary Nuclei from Oxygen to Iron: Results from the TRACER 2003 LDB Flight

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Cosmic Ray Energy Spectra of Primary Nuclei from Oxygen to Iron: Results from the TRACER 2003 LDB Flight

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