The Isotope Magnet Experiment, ISOMAX, a balloon-borne superconducting magnet spectrometer was built with the capability to measure the isotopic composition of the light isotopes (3 ≤ Z ≤ 8) of the cosmic radiation up to 4 GeV/nucleon by using the β vs. rigidity technique with a mass resolution better than 0.25 amu, employing a combination of time-of-flight (TOF) system and silica-aerogel Cherenkov counters for the velocity determination. One of the primary scientific goals of ISOMAX was the accurate measurement of radioactive 10 Be with respect to its stable neighbor isotope 9 Be conveying information on the age of the cosmic rays in the galaxy. ISOMAX had its first flight on August 4-5, 1998, from Lynn Lake, Manitoba, Canada. It provided 13 h of data with a residual atmosphere of less than 5 g/cm^2 . This paper reports the results of the beryllium ratio 10 Be/9 Be = 0.195 ± 0.036 at the top of atmosphere in the energy range from 0.261 - 1.030 GeV/nucleon using the TOF in the 1998 flight. The high energy results of the beryllium ratio up to 2 GeV/nucleon in the Cherenkov regime as well as the lithium results in the TOF energy range are also reported in these proceedings

Barbier, L. M.

Bremerich, M.

Christian, E. R.

deNolfo, G. A.

Geier, S.

Gobel, H.

Gupta, S. K.

Hams, T.

Hof, M.

Menn, W.

Mewaldt, R. A.

Mitchell, J. W.

Schindler, S. M.

Simon, M.

Streitmatter, R. E.

Caltech Authors - Main

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Proceedings ofICRC 200 l: 1655 © Copernicus Gesellschaft 200 I ICRC 2001 
10Be!9Be ratio up to 1.0 GeV/nucleon measured in the ISOMAX 98 
balloon flight 
T. Hams', L. M. Barbier2, M. Bremerich 1, E.R. Christian2, G.A. de Nolfo2, S. Geier3, H. Gobel 1, S.K. Gupta2, M. Hof1, 
W. Menn 1, R. A. Mewaldt3, J. W. MitcheU2, S. M. Schindler3, M. Simon 1, and R. E. Streitmatter2 
1 Fachbereich Physik, Universitat Siegen, 57068 Siegen, Germany 
2Laboratory for High Energy Astrophysics, NASA/GSFC, Greenbelt, MD 20771, USA 
3Space Radiation Laboratory, California Institute of Technology, Pasadena, CA 91125, USA 
Abstract. The Isotope Magnet Experiment, ISOMAX, a 
balloon-borne superconducting magnet spectrometer was 
built with the capability to measure the isotopic composition 
of the light isotopes (3 ::; Z ::; 8) of the cosmic radiation up 
to 4 GeV / nucleon by using the (3 vs. rigidity technique with 
a mass resolution better than 0.25 amu, employing a com-
bination of time-of-flight (TOF) system and silica-aerogel 
Cherenkov counters for the velocity determination. One of 
the primary scientific goals of ISOMAX was the accurate 
measurement of radioactive 10Be with respect to its stable 
neighbor isotope 9Be conveying information on the age of 
the cosmic rays in the galaxy. ISOMAX had its first flight 
on August 4-5, 1998, from Lynn Lake, Manitoba, Canada. It 
provided 13 h of data with a residual atmosphere of less than 
5 g/ cm2. This paper reports the results of the beryllium ra-
tio IOBe/9Be = 0.195 ± 0.036 at the top of atmosphere in 
the energy range from 0.26 I - 1.030 Ge V / nucleon using the 
TOF in the 1998 flight. The high energy results of the beryl-
! ium ratio up to 2 Ge V / nucleon in the Cherenkov regime as 
well as the lithium results in the TOF energy range are also 
reported in these proceedings. 
1 Introduction 
The "secondary" constituents of the cosmic rays, e.g. lithium, 
beryllium, and boron, are produced by spallation in interac-
tions of heavier "primary" cosmic ray nuclei, mainly carbon, 
nitrogen, and oxygen, with the ambient gas in our galaxy. 
Thus, by comparing the secondary to primary ratio in cosmic 
rays for a given propagation model one can derive the amount 
of mean matter cosmic rays encounter during their propa-
gation. However, the measurement of radioactive "clock"-
isotopes, such as 10Be with a half-life of 1.6 x 106 years, 
which is enti rely secondary, are of particular interest allow-
ing the age of cosmic rays to be deduced. The measurement 
Correspondence to: T. Hams 
(hams@ida.physik.uni-siegen.de) 
of this clock-isotope provides a stronger constraint to prop-
agation models if it is carried out al high energies from l -
lOGeV / nucleon where the relativistic time dilation becomes 
significant to the 10Be decay. 
2 Instrument & Flight Performance 
The Isotope Magnet Experiment, ISOMAX, was a balloon-
bome superconducting magnet spectrometer. It was con-
ceived and built to measure the isotopic composition of the 
light elements from lithium to oxygen in the cosmic radiation 
up to 4 GeV / nucleon in multiple flights. A special interest 
of the ISOMAX balloon program was the accurate measure-
ment of radioactive 10Be with respect to its stable neighbor 
isotope 9Be (Streitmatter et al., 1995). 
Incoming cosmic ray particles were identified by measur-
ing their charge, velocity and magnetic 1igidity (momentum/ 
charge) using the velocity-rigidity technique with a mass res-
olution better that 0.25 amu. To achieve this excellent mass 
resolution lSOMAX was comprised of three detector sub-
systems: a magnetic rigidity spectrometer, and the velocity 
measurement of a state-of-the-art time-of-flight (TOF) sys-
tem and two si lica-aerogel Cherenkov counters. The rigid-
ity was obtained from the curvature of a particle in the field 
of the superconducting magnet measured by a stack of three 
high-resolution dri fl chambers (DC). The middle DC was lo-
cated in the high-field region between the two coils of the 
Helmholtz-like magnet. The coils had a separation of 80cm 
and were operated for the 1998 flight at 120 A, yielding a 
field of 0.8 T in the center between the coils and a mean field 
integral of 0.54 Tm. The DC were composed of 480 hexag-
onal drift cells arranged in 24 layers ( 16 in bending and 8 
in non-bending plane). The overall height of the DC stack 
was 150cm. The drift gas was pure C02. The drift cham-
ber demonstrated a spatial resolution of 54 µ,m for relativis-
tic helium and 45 µ,m for beryllium using in-flight data. With 
the given field strength of the magnet and the spatial reso-
lution of the DC we obtained a mean maximum detectable 
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c: 
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Q; 
a. 
700 
(f) 
c 600 Q) 
> 
w 
0 500 Q; 
.D 
E 
;;) 
400 z 
300 
200 
100 
0 
2 3 4 5 6 7 
charge [el 
Fig.1. In-flight charge separation in the top TOF by requiring a 3a 
cut on charge in the other two scintillators and f3 > 0.6. The helium 
peak was scaled by a factor of 70. The values close to the charge 
peaks quote the charge resolution, which is better than 0.2 charge 
units for He - C. 
rigidity (MDR) of 970 MY / c for in-flight helium. Further 
details on the magnetic rigidity spectrometer are reported by 
Hams et al. ( 1999). The TOF system consisted of three layers 
of fast Bicron BC420 scinti llator with a separation from the 
top to middle and top to bottom scinti Ila tor of 206.8 cm and 
260cm, respectively. The time resolution of the TOF sys-
tem was found at 70 ps for heliu m and 60 ps for beryllium. 
In addi tion, the TOF scintillator provided a multiple charge 
measurement using dE/ dx . Figure I shows this charge sep-
aration in the top scintillator with a 3<T cut on charge in the 
other two scintillators and f3 > 0.6. The charge resolution 
for helium through carbon is better than 0.2 charge units. 
Further details of the time-of-flight system can be found in 
Geier et al. (1999). To extend the velocity measurement of 
the TOF system (beyond I GeV / nucleon for beryllium) two 
large-area Cherenkov counters were used. Jn the 1998 flight 
these counters were each equipped with a nominal index-of-
refraction n = 1.14 silica-aerogel radiator resul ting in an en-
ergy threshold of 1.08 GeV / nucleon. For relativistic single-
charged particles (Z = 1, f3 = 1) a total of 22 photoelec-
trons were obtained for both counters (de Nolfo et al., 1999). 
The geometry factor of the instrument was 450 cm2 sr for par-
ticles that had penetrated the sensitive area of the TOF and 
the DC. 
The JSOMAX experiment had its first fli ght on August 4-
5, 1998, from Lynn Lake, Manitoba, Canada. After the flight 
the instrument was recovered in perfect condition near Peace 
River, Alberta, Canada. In this successful flight, the instru-
ment was able to measure the beryllium isotopes in the en-
ergy range from 0.2- 2.0 GeV / nucleon with a mass resolu-
tion better than 0.25 amu. We used ascent data transmitted 
1656 
0.2 < Ekin< l.OGeV/nucleon 
- Pait $ 5 g/cm2 
DC single-track fit: N., :2:: 8 and N 11 :2:: 4 
IZi - 41< 0.5 
Table 1. Applying these cuts to the flight data 424 beryllium events 
were detected. 
via telemetry to opti mize the detector settings for beryllium 
(Z = 4). During this ft ight we obtained 13 hours of data at 
a residual atmosphere of less than 5 g/ cm2 as well as a few 
hours of low-altitude data. A detailed general instrument per-
formance during flight can be found in Mitchell et al. ( 1999) 
and Hof et al. (2000). 
3 Data Selection 
In this section the cuts applied to the fl ight data are discussed 
and the obtained beryllium mass histogram is presented. To 
reduce the effects of atmospheric interaction, only events 
recorded at less than 5 g/ cm2 residual atmosphere were con-
sidered for the analysis. From this subsample of events a 
successful track reconstruction in the diift chamber was re-
quired. A DC fit was successful if at least 8 of the 16 lay-
ers in the bendLng plane, Nx. and 4 of the 8 layers in the 
none-bending plane, Ny. were incorporated into the track ti t. 
In addition, multi- track events where rejected. We chose a 
fa irly loose DC cut to avoid a bias in the isotopic ratio due to 
rigidity-dependent cuts. The DC fi t efficiency for beryllium 
with th is constraint was found at 95% and was independent 
of the rigidity. The beryllium events were then identified 
by requiring a measured charge Z = 4 in all three scintil-
lator layers. Specifically, a cut requiring IZi - 41 $ 0.5 
was imposed, where i denotes the different scintil lator lay-
ers. The velocities of incident particles up to 1 GeY / nucleon 
were determined with the timing of the top and bottom scin-
tillator arrays. Account was taken of energy losses within 
the instrument and the subsequent increase in local dE/ dx . 
By requiring a consistent charge measurement in all three 
TOF paddles, Z = 4, within "-'30" the contamination of the 
beryllium events by misidentified charge is less than 10- 6 
and can be neglected while the efficiency of this cut was de-
termined at 95%. In the discussed energy range from 0.2 
- 1.0 GeV / nucleon, 424 berylli um events were detected by 
applying the cuts discussed above and summarized in Table 
l. Figure 2 shows the mass histogram for these beryllium 
events. From this mass histogram the re lative abundances 
of the beryllium isotopes were obtained by a least-square 
tit using a Monte-Carlo simulation which accounts for in-
night detector performance. This tit of the measured ratio in 
the instrument is also shown in the figure and was found to 
be lOBe/9Be = 0.249 ± 0.046 for the energy range 0.2 -
l.OGeV / nucleon. 
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1657 
Energy 
lnstniment 
Top of Instrument 
Top of Atmosphere 
0.249 ± 0.046 
0.259 ± 0.048 
0.195 ± 0.036 
0.200 - l.OOOGeY / nuc. 
0.233 - l.016GeY / nuc. 
0.261 - l.030GeY / nuc. 
Table 2. Beryllium ratios and energy ranges at different stages of 
the analysis. The quoted uncertainty of the beryllium ratios are sta-
tistical errors only. 
4 Instrumental & Atmospheric Correction 
The charge consistency in all three TOF scintillator layers 
and the single-track requirement in the DC reject interact-
ing beryllium events in the detector stack. The mean instru-
mental correction which accounts for the interaction proba-
bility of beryllium isotopes propagating through the instru-
ment (p = 11.2 g/cm2 ) is found at 1.04 using the method 
of Kox (Kox et al., 1987) for the inelastic cross section. The 
corrected beryllium ratio at top of instrument with the corre-
sponding energy range is shown in Table 2. The beryllium 
80 
70 
60 
50 
40 
30 
20 
10 
0 
5 6 7 8 9 10 11 12 
mass [amu] 
Fig. 2. Mass histogram of 424 beryll ium evel1ls as measured in the 
instrument in an energy range from 0.2 - I .OGeY /nucleon. Solid 
curve is a least-square fit of a Monte-Carlo simttlation. We obtained 
a beryllium ratio of 10Be/9 Be = 0.249 measured i.n the instrument. 
ratio at the top of instrument needs to be corrected for inter-
actions in the atmosphere above the experiment. The average 
residual atmosphere for the high altitude data was 4.4 g/ cm2. 
Taking the geometrical aperture of the instrument into ac-
count., the mean incident zenith angle is 15°, resulting in an 
average grammage of 4.6 g/cm2 which was used for atmo-
sphe1ic correction. The spectra of the elements from boron 
through nickel measured by the ACE-satellite (ACE/CRIS, 
••••••• LBM : Yanasak et al. 2001, n = 0.34 cm"' 
-- LBM : Molnar & Simon 2001, n = 0.23 cm·• ~0.5 
.,, 
........ --
- - - - · O"M • "'"'''""° & 5'ro~ =:.~_.:: ;-~_,: .. , 
0.3 ,' 
--
Q) 
~00 0.4 
, 
, 
, 
0.2 I ~ ~6>- •• /T= 
---
, 
, 
, 
0 1 .. -~ 
. --------- i--- ---
0.0 .__._..._. ................. _..._._....._..........__.__._ ................ ...__.__._,_, 
101 103 104 
kin. Energy (MeV/nucleon] 
Fig. 3. Beryllium ratio at top of atmosphere of ISOMAX (• this 
work and • de Nol fo el al., 200 I) compared with satellite measure-
ments: o ACE (Yanasak el al., 1999), o Ulysses (Connel, 1998), 
<J Voyager l-2(Lukasiak et al., 1997), C> ISEE-3 (Wiedenbeck and 
Greiner, 1980), and<> !MP 7 /8 (Garcia-Mtmoz et al., 1977: Garcia-
Munoz & Wefcl, 198 1 ). The lines show the expected beryll ium ratio 
in different propagation models. The two upper lines are leaky-box 
model (LBM) the lower one is a diffusive-halo model (DHM). 
2000) and by Engelmann et a l. ( I 990) were demodulated and 
thereafter adopted to a solar modulation of 430 MV /c expe1i-
enced during the ISOMAX 1998 flight. In addition, a leaky-
box model (n = 0.3 cm-3) was used to derive an initial 9Be 
spectrum at top of atmosphere (TOA). We assumed a simi-
lar spectral shape for the 10 Be flux. This set of spectra was 
then propagated through 4.6 g/cm2 of atmosphere using the 
partial cross sections of Silberberg et al. (I 998) and Tsao et 
al. ( 1998) and the total inelastic cross sections of Kox et al. 
( 1987). The mean atmospheric correction factor for our en-
ergy interval was found at 0.753 resulting a beryllium ratio 
at TOA of 10Be/9Be = 0.195 ± 0.036, see Table 2. 
5 Results 
Figure 3 shows the 10Be/ 9 Be ratio at top of atmosphere in 
the energy range from 0.26 1 - 1.030 GeV / nucleon measured 
by the TOF together with satellite measurements at several 
JOOMeV / nucleon of ACE, Ulysses, Voyager 1-2, ISEE-3, 
and 1MP7/8. The figure also includes thelSOMAX 10Be/9Be 
ratio above I GeV / nucleon measured by Cherenkov detec-
tors and discussed in detail in these proceedings (de Nolfo 
et al., 200 I ). The error bars represent the statistical errors 
of our measurement. The additional uncertainty from instru-
mental and atmospheric corrections are currently under in-
vestigation. 
Our measurement does not confirm the apparent excess of 
10Be reported by SMILI (Ahlen el al., 2000). SMILI quoted 
the detection of nine 10Be in a total of 26 beryllium events al 
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an average energy of,...., I GeV / nucleon resulting in a ratio of 
10Be/Be = 0.35. Our measured ratio in the instrument cov-
ered by the TOF is 10Be/ Be = 0.09 ± 0.02, more than three 
times smaller. This low 10Be/ Be ratio is also confirmed by 
the ISOMAX Cherenkov data. 
The two upper curves in Fig. 3 are different leaky-box 
models. The top dashed curve by Yanasak et al. (2001) is as-
sum ing an average density of 0.34 atoms/ cm3 of interstellar 
medium (ISM). The solid curve by Molnar & Simon (2001) 
takes a ISM density of 0.23 atoms/ cm3 into account. Fi-
nally, the lower dashed curve is a diffusive-halo model by 
Moskalenko & Strong (2000) with a halo size of 4 kpc and 
that accounts for the local galactic structure. The statistical 
uncertainties of the ISOMAX results, in combination with 
the spread in model predictions, preclude any final conclu-
sions concerning cosmic-ray propagation at present. How-
ever, it is worthwhile mentioning that the steep increase of 
the 10Be/ 9Be ratio as measured by ISOMAX is probably 
difficult to accommodate with the presence of reaccelera-
tion, since reacceleration has the general tendency to produce 
ratios, that are below those calculated in the corresponding 
model without reacceleration. 
6 Conclusion 
The beryllium measurements of TSOMAX (this work; de 
Nolfo et al. , 2001) are the fi rst of its kind extending the mea-
surement up to 2 Ge V /nucleon. Our data indicate a steep in-
crease in the 10Be/ 9Be ratio but final conclusions on model 
predictions are not possible at present, although the steep 
rise of the ratio with energy might be difficult to accommo-
date with reacceleration. Future experiments with compara-
ble features of ISOMAX and improved statistics are needed 
to impose stronger constraints on the comic-ray propagation 
models. 
1658 
Acknowledgements. This experiment was supported by grant NASA 
RTOP 353-87-02 at Goddard, by NAGS-5227 at CalTech, and by 
DFG Si 290/8 at University of Siegen, Germany. We would like 
ro thank the many engineers and technicians that maid ISOMAX a 
success as well as the Nacional Scientific Balloon Facility. 
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^(10)Be/^9Be ratio up to 1.0 GeV/nucleon measured in the ISOMAX 98 balloon flight

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^(10)Be/^9Be ratio up to 1.0 GeV/nucleon measured in the ISOMAX 98 balloon flight

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