Direct Measurement of 3He/4He in the LISM with the
COLLISA experiment
E. Salerno*, F. Biihler*, P. Bochsler*, H. Busemann*, O. Eugster*, G. N. Zastenker^
Yu. N. Agafonov1^ and N. A. Eismont1^
*Physikalisches Institut, University of Bern, Sidlerstrasse 5, CH-3012, Switzerland
^ Space Research Institute (IKI), Russian Academy of Sciences, Profsoyuznaya ul. 84/32, 117997 Moscow, Russia
Abstract.
Results from direct measurements of the helium isotopic ratio in the closest regions of the Local Interstellar
Medium (LISM) are presented. Neutral 3He and 4He atoms coming from the LISM were captured in space
by means of the foil collection technique, a method already successfully used during the Apollo missions to
determine the noble gas isotopic ratios in the solar wind. In the framework of the Swiss-Russian project COLLISA
(COLLection of Interstellar Atoms), beryllium-copper foils were placed on the outer surface of the space station
Mir and directly exposed to the flux of interstellar neutrals. The neutral particles of the LISM cross the heliopause
and reach, almost unaltered, the Mir orbit at 400 km height above the Earth. Here, the kinetic energy of the
interstellar flux ramming against the foils is sufficient to trap the particles into the atomic structure of the metal.
After an exposure of ~60 hours, the foils were recovered by the cosmonauts and brought back to Earth by
the American space shuttle Atlantis. The particles were then extracted with a step wise heating procedure and
their abundances were measured in the mass spectrometric laboratories of the University of Bern. The analysis
performed so far allowed the detection of 3He and 4He atoms of interstellar origin. The measured interstellar
ratio 3He/4He = {l.TO^g'^} x 10~4 is consistent with protosolar values obtained from meteorites and Jupiter's
atmosphere. Such a result seems to confirm the hypothesis that no significant change of the 3He abundance
occurred in the LISM during the last 4.6 Gy.
INTRODUCTION
The measurement of the elemental and isotopic composition in different astrophysical sites (stellar interiors and
winds, planetary rocks and atmospheres, neutral and ionized gas clouds, meteorites, etc.) gives direct information
on the chemical structure of the galactic matter at different galactocentric distances and in different evolutionary
epochs. As discussed by Chiappini (this volume) [1], the
observed chemical abundances are extremely important
since they can be used to test the predictions of Galactic
Chemical Evolution models and to constrain their input
parameters.
For instance, the accurate measurement of the helium isotopic abundances could help to solve one of the
open issues of astrochemistry: the 3He problem. Theoretical models predict that the 3He produced during the
primordial nucleosynthesis undergoes several astration
processes which partially produce it (D is immediately
burnt into 3He in stars of all masses) and partially destroy it (3He is significantly transformed into 4He in
the interiors of massive stars). The resulting 3He net
yield is a steeply decreasing function of the stellar initial mass [2]. This behavior leads to an overestimation
of the 3He solar abundance [3]. The 3He abundance predicted by the models differs, in fact, by almost two orders of magnitude from the abundances observed in both
presolar material [4] and LISM [5] (3He/H ~ 10~5), but
is in agreement with observations of planetary nebulae
(3He/H ~ 10~3) [6, 7, 8]. A solution to the problem was
proposed in 1995 by Charbonnel [9] [see also 10, 11]. It
consists in processing 3He into heavier elements by an
extra-mixing mechanism occurring below the convective
zone of low-mass stars (< 2 M0) on the red giant branch.
The values observed in planetary nebulae, however, indicate that some of these stars have to be net producers of
3
He . Therefore, it has been recently suggested by several authors that extra-mixing occurs only in a fraction
of low mass stars. Galli et al. [12] showed that this fraction should be larger than 80%. Recently Chiappini and
Matteucci [13], adopting a new version of their "twoinfall" model, have predicted the evolution of 3He for
CP598, Solar and Galactic Composition, edited by R. F. Wimmer-Schweingruber
© 2001 American Institute of Physics 0-7354-0042-3/017$ 18.00
275
different percentages of low mass stars in which extramixing should occur. They found that the best fit with
observations is reached when this mechanism occurs in
93% of the stars. A similar result was found by Tosi [2].
According to the "Tosi-1" model, it is in fact possible to
reproduce the abundances of 3He observed in the Sun,
in the LISM and in planetary nebulae if deep mixing
is assumed to operate in ~90% of the low mass stars.
Although the extra-mixing mechanism seems to explain
the apparent inconsistencies between the predicted abundances and those observed in different galactic objects,
further investigations are necessary, both to find out its
possible causes and to check its effects on later stellar
evolution phases.
Since few decades, the observation of astrochemical
"reservoirs" has been a fast growing area of research in
both ground-based astronomy and space science. So far,
many efforts have been made to improve the quality and
the precision of such measurements. Amongst others,
some experiments have been recently performed to determine the physical and chemical properties of the Local Interstellar Medium (LISM): the region of our Galaxy
that extends within few hundreds of pc of the Sun. COLLISA is one of these experiments.
In this work we give a report on the experiment and
present the results of the interstellar 3He/4He measurement. We then compare our ratio with similar values obtained from observations of meteorites, Jupiter's atmosphere and, in general, of all the astrophysical sites that
could be representative of the present-day and the protosolar cloud. The consistency of these values is discussed.
Finally, the agreement between the measured concentration of interstellar helium and the value predicted by the
"hot gas" model is discussed.
missions to collect solar wind ions on the surface of the
Moon [16]. With this method some metal foils are directly exposed in space to a stream of particles. If the
kinetic energy of the particles is sufficiently high, they
get trapped in the foils. After the exposure, the foils are
brought back to Earth and the amount of particles trapped
within them is measured in mass spectrometers.
Studies on the scattering of interstellar helium in the
Earth's atmosphere [15] have indicated that this process
does not affect the collection of interstellar neutral atoms
if the foil exposure is performed during minimum solar
activity at altitudes higher than 200-300 Km. The Russian space station Mir, orbiting the Earth at a distance of
~ 400 km, represents a perfect ground-base for the exposure of trapping foils.
In the framework of the COLLISA project four
beryllium-copper foils (200 cm2 wide and 15 um thick)
were exposed in Spring 1996, for approximately 60
hours, to the flux of the interstellar neutral atoms.
The foils, covered with a beryllium-oxide layer to
further increase their trapping efficiency [17], were
mounted on special cassettes plugged inside two collectors (see Figure 1). The collectors, named KOMZA I
and II, were designed and constructed at IKI with the
participation of the Space Physics Design Bureau. They
were installed on the outside of the Spektr module of the
Mir. The Spektr, already provided with KOMZAs, was
launched and docked to the space station in 1995.
Shutters
THE EXPERIMENT
Cassette 2
The COLLISA project [14, 15] is the result of a cooperation between the Group for Space Research and Planetary Sciences of the University of Bern and the Space
Research Institute (IKI) of the Russian Academy of Sciences. The aim of the experiment is to collect and determine the helium isotopic composition of a sample of
neutral atoms directly coming from the Local Interstellar
Cloud (one of the several gas clouds wich compose the
LISM and wherein the Sun is immersed at the moment).
The experimental procedure is based on the foil collection technique. A detailed description of this method, as
well as the astrophysical conditions necessary for its application to the collection of interstellar particles, is given
by Klecker et al. (this volume). The foil collection technique was developed at the Physics Institute of the University of Bern and successfully used during the Apollo
276
Cassette 1
FIGURE 1. A KOMZA particle collector. Only one of the
two cassettes (dashed lines) is shown.
The collectors were equipped with shutters that remained
open only when the apertures were looking into the
direction of the incoming interstellar atoms. The best
conditions for the capture of the particles were reached
during Spring time. As already mentioned in [18], in this
period of the year the particle collection is particularly
effective since the Earth moves in the upwind direction of
the interstellar flux of neutrals. This condition enhances
the velocity of the neutral atoms relative to Mir from
~25 km/s to -60-80 km/s (~25 eV/AMU) increasing
their trapping probability up to —30% [17].
Care was taken to keep the shutters closed whenever a possible contamination of the foils with solar irradiation or with terrestrial atmospheric particles could
have happened. To avoid foil contaminations, the shutters were also closed during the Mir working activities
(docking, undocking, refuellings, switching on of cruise
or altitude-control engines, etc.). Electrical grids, placed
in the collectors above the foils, rejected < 100 eV electrons and positively charged ions with energies up to 5
keV. This precaution was taken in order to protect the
foils from a possible contamination with terrestrial energetic magnestospheric ions. Heating plates were placed
just below the foils to constantly keep them at 50°C during the exposure. The foils were heated to avoid the formation, on their surface, of condensation layers which
could have reduced the trapping efficiency. After exposure, the foils were recovered by the cosmonauts of Mir
and brought back to Earth by the U.S. space shuttle Atlantis.
THE MASS SPECTROMETRIC
ANALYSIS
Once landed on Earth, the foils were delivered to the
University of Bern for the measurement of the trapped
particles. The first step of the analysis consisted in degassing the foils in a UHV high-temperature furnace.
The extraction was performed in several temperature
steps: 300°, 600°, 1100°, 1400° and 1700°C. Measurements, performed on foils that had previously been artificially bombarded with helium isotopes at different energies, showed in fact that particles implanted with typical interstellar energies (—25 eV/AMU) are released in a
temperature range of 300°-1100°C. At temperatures below 300°C and above 1100°C, particles with lower and
higher implantation energies are released, respectively.
This is due to the fact that according to their velocity
the atoms penetrate the foil to different depths: lower
speeds lead to superficial trappings while high kinetic
energies drive the particles deeper into the foil. As a consequence, the deeper the position of the particles in a
foil, the higher the thermal energy necessary to extract
them. In this way, the stepwise heating provides a further safety measure to separate the interstellar particles
from the low energetic (< 1 eV) atmospheric or high energetic (5000 eV) magnetospheric particles possibly captured by the foils. The measurement of the foils artificially bombarded also provided an estimate of the foil
trapping efficiency "T|" (i.e. the percentage of particles
captured by the foil, compared to the total amount irradiated) [17]. Typical values of the trapping efficiency
277
for the helium isotopes in the beryllium-oxide were:
T|3 = 0.18 ± 0.04 and r|4 = 0.25 ± 0.04, while their mean
ratio was r|3/r|4 = 0.73 ± 0.07.
After the extraction, chemically active gases were
trapped by getters. The helium was then transferred, for
the measurement, into a Mass Analyzer Product 215-50
mass spectrometer with ion counting collector.
RESULTS
Figure 2 summarizes the results of the analysis performed on one (L641-2-1) of the four foils exposed on
the Mir during Spring 1996.
The continuous lines show the accumulated amount
of 3He and 4He (upper and lower panel respectively) released per mg by the foil, at each temperature step. For
comparison, data derived from the analysis of the foils
L461-3-2, L461-3-5, L460-3-1 never flown in space are
also plotted (dashed lines). Such measurements were performed to estimate the background due to noble gases
contained in the foil prior to the flight. Even though measures were taken during the foil preparation to avoid any
possible contamination, small amounts of noble gases
may be present in the atomic structure of the metal foils.
These quantities (namely foil blanks) have to be accurately determined, to correct the yields of the exposed
foils. The values of the foil blanks are obtained from
analysis of foils that were not exposed on Mir but were
treated identically as the exposed ones.
Given errors are mainly due to the background variation. The release profiles indicate that the detected
3
He and 4He have a clear interstellar origin. The highest percentage of gas was in fact released in the
temperature range of 300°-1100°C, which is the expected one for particles with interstellar energies. Above
1100°C the beryllium-copper reaches its melting point,
releasing only terrestrial contaminations. Up to 1100°C,
the 4He extracted from the flown foil is {1.36 ± 0.11} x
109 atoms/cm2. This quantity is more than one order
larger than the amount of gas released by the blank foils
({1.16 ± 0.20} x 108 atoms/cm2), clearly indicating that
the particles detected in the flown foil could be only
trapped in space. While the 4He blank contributes about
8% to the total 4He of the exposed foil, the 3He blank
corresponds to 39% of the total 3He. This is probably
due to the presence of residual tritium inside the metal.
The tritium, maybe present in the atmosphere in higher
levels in the past and already contained in the recycled
copper used for the foil production, decays in 3He with
a half-life of 12.323 years. This time is short enough to
produce tainting amounts of 3He in the beryllium-copper
foils of the COLLISA experiment.
The value of the helium isotopic ratio, determined
35
,—,
n
30
E
25
I
,o
15
*
s
i———I———I———I———I———1———I———I
160
140
120
100
cd
"o
80
60
40
20
0
200
400
600
800
1000 1200 1400 1600 1800 2000
T[°C]
FIGURE 2.
(blank foils)
Helium released from the foils L461-2-1 (exposed on Mir to the interstellar flux), L461-3-2, L461-3-5 and L460-3-1
TABLE 1. 3He/4He derived from observations of different
astrophysical sites.
after correcting the total 3He and 4He release for the foil
blanks and the relative trapping efficiency T|3/T|4, is:
Reservoir
LISM (neutrals)
LISM (pickup ions)
Meteorites (Q-phase)
Meteorites
Jupiter atmosphere
Sun (OCZ)
For the determination of the error bars, a conservative
range of ±25% has been adopted, due to the low number
of foil blanks analyzed. The uncertainty of the upper
limit was further increased to take into account possible
systematic errors introduced in the determination of the
relative trapping efficiency T|3/T|4 which would favor
even more the lighter isotope.
3
He/4He
1.71+0-50 x iQ-4
948+0.68
z
-^o_0.62
1Q-4
X 1U
1.3±0.02xl(T 4
1.5±0.3xl(r 4
1.66±0.05xlO- 4
3.7±0.7xl(T 4 *
Source
[This work]
[5]
[19]
[4]
[20]
[21]
* consistent with Gloeckler et al. [22]
consistent within the error limits, the value of the
3
He/4He ratio in the Local Interstellar Medium observed
with the COLLIS A experiment is lower than that derived
from the measurements of pickup ions [5]. However,
the present-day LISM ratio, inferred from the measurement of neutral atoms is closer to the protosolar values
observed in meteorites and in the Jupiter's atmosphere,
suggesting that no substantial change in the LISM ratio,
and therefore no significant increase of 3He, occurred
DISCUSSION
Comparison with Solar System and LISM
Abundances
Protosolar and present-day LISM values of the
He/4He ratio can be derived, directly and indirectly,
from observations of various astrophysical reservoirs.
Table 1 summarizes some of these values. Although
3
278
during the last 4.6 Gy.
The determination of the 3He/4He ratio in the solar Outer Convective Zone (OCZ), obtained from solar
wind measurements, allows to calculate the protosolar
(3He + D)/H [23]. In the young Sun deuterium was in
fact efficiently converted to 3He. The helium has subsequently remained unprocessed in the material of the
Outer Convective Zone, as is implied by the continuing
presence in this region of the more reactive 9Be. The
3
He/4 He ratio measured today in the outer convective
zone can be therefore considered representative of the
protosolar (3He + D)/H [24]. However, due to the different settling of 3He and 4He out of the OCZ into deeper
layers of the Sun [25], and to possible solar mixing processes [26], the present day 3He/4He in the OCZ could
have increased of a few percent compared to the protosolar value. Geiss and Gloeckler [24], making an estimate of the contribution due to these two effects, found
that this should not exceed (5 ± 3)%. Applying this correction to data derived by Bodmer and Bochsler [21] (in
agreement with those observed by Gloeckler et al. [22])
and using the standard universal ratio He/H~ 0.1, one
obtains:
[(3He + D)/H]
={3.5±0.7}xlO,-5
This value leads to a protosolar 3He/4He ratio which
agrees with those observed in meteorites, Jupiter and
neutral LISM if a deuterium abundance of:
= {1.9±0.7}xl(
5
is assumed. Although consistent with the inferior limit of
the error bars, this value is lower than the one found by
Mahaffy et al. [20] in the Jovian atmosphere:
suggesting a slight overestimation of the primordial deuterium abundance. Such an overestimation seems to be
also confirmed by recent observations of Jupiter's and
Saturn's atmospheres performed with the Short Wavelength Spectrometer onboard the Infrared Space Observatory [27].
Expected and Measured
4
He Concentrations
Model calculations of the expected concentrations of
trapped interstellar 4He have been performed in the
framework of the COLLISA project [15]. The calculations were based on the "hot gas" model that describes
the distribution of interstellar neutral 4He atoms at the
location of the Earth, taking into account the thermal velocity of the particles in the interstellar medium. The parameters for the interstellar helium used in the model are
279
those given by Witte et al. [28] for the period November
1994 - June 1995 (Table 2).
TABLE 2. Interstellar neutral helium properties. Data
from Witte et al. (1996)___________________
Nov. 94 - Jim. 95
Flow Speed
Flow Direction (ecliptic longitude)
Flow Direction (ecliptic latitude)
Temperature
Helium density
Photoionization
24.6± 1.1 km/s
74.7± 1.3°
-4.6±0.7°
5800±700 K
1.4 10~2cm~3
>1.1 10~7sec~1
The velocity distribution was assumed to be a shifted
Maxwellian far upwind from the Sun. At Earth, it was
modified by the solar attraction - differently for each day
of the year, depending on the Earth's orbital velocity and
location. The Mir orientation was known for each exposure, thus the shadowing by the KOMZA walls could reliably be assessed individually for each foil piece.
The model calculations of the expected concentrations
of trapped interstellar 4He on the foil L461-2-1 yield:
4
He = 2.33 x 109atoms/cm2.
Such a value is approximately twice as high as the measured concentration:
4
He = {1.24±0.11} x 109atoms/cm2.
(corresponding to an average accumulation rate
of trapped interstellar atoms of {5.0 ± 0.7} x 103
atoms/cm2s). A similar discrepancy factor has been
found in previous measurements of COLLISA samples
exposed to the interstellar flux [14]. The difference between predicted and measured concentrations could be
due to the uncertainties in the determination of the flux
of neutrals and, marginally, to that of the foil trapping
efficiency.
CONCLUSION
In the framework of the COLLISA project we have determined the helium isotopic ratio in the closest regions
of the Local Interstellar Medium using the foil collection technique. The value of the 3He/4He ratio was obtained from the analysis of one of the foils exposed on
Mir during 1996. The present-day isotopic composition
of neutral helium in the LISM is lower than that derived
from the analysis of pickup ions, but it is consistent with
the presolar cloud value, as derived from meteorites and
Jupiter's atmosphere. The present-day 3He abundance,
derived from the COLLISA ratio, is therefore consistent
with that observed in the presolar cloud, confirming the
hypothesis that no substantial increase of 3He occurred
in the local interstellar medium during the last 4.6 Gy.
Measurements of more foils exposed to the interstellar
flux are planned. They aim at confirming the previously
found helium abundances and isotopic ratio and at detecting, for the first time, the interstellar 20Ne/22Ne ratio.
16.
17.
18.
19.
ACKNOWLEDGMENTS
The authors would like to thank all participants in
the experiment COLLISA. We are specially grateful to
the cosmonauts Sergey Avdeev, Thomas Reiter, Yury
Onufrienko and Yury Usachev for exchanging the cassettes in space and to Armin Schaller for the technical support during the mass spectrometric measurements.
This work was supported by the Swiss National Science
Foundation.
20.
21.
22.
23.
24.
25.
26.
REFERENCES
2.
4.
6.
7.
9.
10.
11.
12.
13.
14.
15.
Chiappini, C., and Matteucci, K, Galactic Chemical
Evolution (2001), this volume.
Tosi, M., "Evolution of D and 3He in the Galaxy", in The
Light Elements and Their Evolution - IAU Symposum,
edited by L. da Silva, M. Spite, and J. de Medeiros, 2000,
vol. 198 of ASP Conference Series.
Rood, R., Steigman, G., and Tinsley, B., Astrophys. J.,
207, L57 (1976).
Geiss, J., "Primordial Abundances of Hydrogen and
Helium Isotopes", in Origin and Evolution of the
Elements, edited by N. Prantzos, E. Vangioni-Flam, and
M. Casse, Cambridge University press, Cambridge, 1993,
p. 89.
Gloeckler, G., and Geiss, J., Space Sci. Rev., 84, 275
(1998).
Rood, R., Bania, T., and Wilson, T., Nature, 355, 618
(1992).
Rood, R., Bania, T., Wilson, T., and Balser, D., "The
Quest for the Cosmic Abundance of 3He ", in The Light
Element Abundances, Proceedings of the ESO/EIPC
Workshop, edited by P. Crane, Springer, Berlin, 1995, p.
201.
Balser, D., Bania, T., Rood, R., and Wilson, T., Astrophys.
J., 483, 320 (1997).
Charbonnel, C., Astrophys. J., 453, L41 (1995).
Hogan, C., Astrophys. J., 441, L17 (1995).
Boothroyd, A., and Malaney, R., astro-phi9512133
(1995).
Galli, D., Stanghellini, L., Tosi, M., and Palla, K,
Astrophys. J., 477, 218 (1997).
Chiappini, C., and Matteucci, R, astro-phi0004030
(2000).
Biihler, K, Bassi, M., Bochsler, P., Eugster, O., Salerno,
E., Zastenker, G., Agafonov, Y., Gevorkov, L., Eismont,
N., Prudkoglyad, A., Khrapchenkov, V., and Shvets, N.,
Astrophys. and Space Sci., 274, 19 (2000).
Bassi, M., COLLISA - An Experiment to Collect
Interstellar Neutral Atoms in Metallic Foils on an
280
27.
28.
Earth-Orbiting Satellite, Ph.D. thesis, Universitat Bern
(1997).
Geiss, J., Biihler, K, Cerutti, H., Eberhardt, P., and
Filleux, C., Apollo 16 preliminary science report, Tech.
Rep. SP-315, Section 14, NASA (1972).
Filleux, C., Morgeli, M., Stettler, W, Eberhardt, P., and
Geiss, J., Radiation Effects, 46, 1 (1980).
Klecker, B., and Bothmer, V. (2001), this volume.
Busemann, H., Baur, H., and Wieler, R., Protosolar and
Circumstellar He Isotopic Ratios Deduced from "phase
Q" in Carbonaceous Chondrites (2001), this volume.
Mahaffy, P., Donahue, T, Atreya, S., Owen, T, and
Niemann, H., Space Sci. Rev., 84, 251 (1998).
Bodmer, R., and Bochsler, P., Astron. & Astrophys., 337,
921 (1998).
Gloeckler, G., Geiss, J., and Fisk, L., Composition of the
Local Interstellar Cloud (2001), this volume.
Geiss, J., and Reeves, H., Astron. & Astrophys., 18, 126
(1972a).
Geiss, J., and Gloeckler, G., Space Sci. Rev., 84, 239
(1998).
Gautier, D., and Morel, P., Astron. & Astrophys., 323, L9
(1997).
Bochsler, P., Geiss, P., and Maeder, A., Solar Phys., 128,
203 (1990).
Lellouch, E., Bezard, B., Fouchet, T, Feuchtgruber, H.,
Encrenaz, T, and de Graauw, T, Astron. & Astrophys.,
370, 610 (2001).
Witte, M., Banaszkiewicz, M., and Rosenbauer, H., Space
Sci. Rev., 78, 289 (1996).