**The Isotopic Composition of Galactic Cosmic Ray Elements from Carbon
to Silicon: the CRRES Satellite Investigation**

M. A. DuVernois^1^, M. Garcia-Munoz^2^, K. R. Pyle^2^, J. A. Simpson^1^
and M. R. Thayer^1^

^1^Enrico Fermi Institute and Department of Physics

^2^Enrico Fermi Institute

The University of Chicago

933 E. 56th Street

Chicago, IL 60637

Submitted August 1995

**Abstract**

** **The separation of the isotopes of carbon, nitrogen, oxygen,
fluorine, neon, magnesium and silicon has been achieved over the energy
range \~30 to \~300 MeV/nucleon with an instrument of new design carried
on the **C**ombined **R**elease and **R**adiation** E**ffects**
S**atellite (CRRES). The galactic source composition has been derived
from the measured isotopic abundance ratios for these elements, taking
account of interstellar propagation and heliospheric modulation. It is
concluded that, except for neon, the isotopic source abundances are
substantially in agreement with those of the solar system. This
conclusion appears to be valid both for single and double exponential
pathlength distributions. The derived source isotopic composition for
these cosmic ray elements is compared with calculated abundance ratios
obtained from earlier cosmic ray investigations as well as radio or
optical investigations of the local interstellar medium.

Solar modulation reached an unprecedented level during the CRRES mission
in 1990 and 1991 and the heliosphere was far from an equilibrium state.
However, the relatively simple steady-state model of heliospheric
modulation we used yielded a remarkable consistency among the recently
published cosmic ray isotopic ratios by other investigators covering a
wide range of energy and modulation levels, when compared with the CRRES
measurements at a common level of modulation.

The instrument and mission are described briefly.

*Subject headings:* Cosmic Rays: Abundances --- Cosmic Rays: General ---
Galaxy: Abundances --- Instruments --- Sun: Activity

**1. Introduction**

The galactic cosmic radiation is, at the present time, the only sample
of matter from the galaxy whose elemental and isotopic composition can
be directly measured in the heliosphere. Thus, the extrapolation of the
measured relative abundances of the cosmic rays to determine their
source composition in the galaxy\--when compared with the composition of
solar system matter (Anders & Grevesse, 1989)\--serves as a critical
guide for the development of self-consistent theories of nucleosynthesis
and stellar evolution, and thereby, for theories for the origin of
cosmic rays (*e.g*. Simpson 1983; Mewaldt 1989).

It is now well established that the elemental abundances of nuclei in
the cosmic rays carry the imprint of a bias thought to be introduced by
the first ionization potential of the atoms during the acceleration
process. This correlation with first ionization potential was also
directly observed in the elemental abundances in matter accelerated by
solar flares (*e.g.*, Meyer 1985).

However, except possibly in the acceleration mechanism that led to the
overabundances in ^3^He-rich flare nuclei (Hsieh & Simpson, 1970), the
isotopic composition of cosmic ray and solar flare nuclei does not
reveal first ionization bias. Therefore, the precise determination of
the similarities and differences between the solar system and the cosmic
ray source isotopic abundances, such as the previously known cosmic ray
^22^Ne excess (Garcia-Munoz, Simpson, & Wefel, 1979), is vital for
understanding the origins of cosmic rays.

The evolution of our ability to determine with confidence the cosmic ray
source abundances has depended on several factors including:

1\) instrumentation to obtain adequate isotopic resolution of the
elements H through Ni;

2\) knowledge of interstellar nuclear spallation cross-sections and
their energy dependences, and;

3\) corrections for the effects of propagation through the heliosphere
to the observer during all phases of a solar cycle.

The principal energy range for these investigations continues to be in
the region where the cosmic rays have their highest fluxes --- namely,
from low energy instrument cut-off to \~1 GeV/nucleon.

In the present paper we show that with a new instrument design resulting
in an improved isotopic resolution we can achieve higher accuracy in the
determination of the relative isotopic abundance of the elements C, N,
O, F, Ne, Mg and Si. We then carry out model propagation calculations
using the available data sets for cross sections to determine the best
values for source isotopic abundances. We take account of the highest
modulation level so far recorded in history, extending from the 1940's
with Department of Terrestrial Magnetism ion chambers and from 1951 with
neutron monitors. We show that the extreme modulation level in 1990-91
(corresponding to \<30% of the solar minimum intensity for \>3GV cosmic
rays at 1 A.U.) requires us to extend modulation theory in order to
compare our new results with other recent investigations in the
carbon-silicon range of isotopes (Gibner *et al*., 1992; Lukasiak *et
al.,* 1993, 1994; Wiedenbeck & Greiner, 1981a,b; Krombel & Wiedenbeck,
1988; Webber 1982; Webber, Kish, & Schreier, 1985). This evidence that
conventional modulation theory can be extended to times when the
heliosphere is clearly not in an equilibrium state is a new result
derived from our investigations.

Our ONR--604^\*^ instrument for the Chemical Release and Radiation
Effects Satellite (CRRES) mission in Earth orbit was intended for
baseline measurements to complement a similar instrument on the Ulysses
spacecraft traveling over the polar regions of the Sun. The charged
particle telescopes for these two missions are virtually identical in
isotopic resolution (Simpson, *et al.,* 1992). Preliminary accounts from
the CRRES instrument have appeared (Gussenhoven, Mullen, & Sagalyn,
1985; DuVernois *et al*., 1993; Garcia-Munoz *et al*., 1993), along with
preliminary reports from our Ulysses High Energy Telescope (HET)
(Connell & Simpson 1993a,b). A further purpose of the present paper is
the comparison of the Ulysses and CRRES results for the isotopes of
carbon through silicon.

**2. The Instrument and the CRRES Mission**

In order to achieve high resolution for the principal elements from
Hydrogen to Nickel we began in 1974 to develop at the University of
Chicago a family of charged particle position-sensing detectors (PSDs)
that made it possible to carry out energy-loss measurements to within
\~0.1% error for an isotropic incidence of cosmic and solar particle
radiation (Lamport *et al*., 1976). These developments were embodied in
the CRRES (FIGURE 1) and Ulysses energy-loss *vs*. residual energy
telescopes. Basic to the design was the introduction of redundant PSDs
and electronics, shown in FIGURE 1(B). In this system one of the top
(D1,2,3) and one of the bottom (D4,5,6) detectors or electronic channels
could fail without significantly degrading particle trajectory
resolution. Indeed, during the CRRES mission one element chain (D1) did
fail 3 months after launch without appreciably degrading the resolution.

The most significant difference between our Ulysses and CRRES*
*instruments is the addition of a 3mm window, a titanium collar and a
heavy magnesium telescope shield for protection from high radiation
levels in the Earth's inner magnetosphere and during intense solar
particle events (see FIGURE 2). A further description of the instrument
and its modes of operation appear in APPENDIX 1.

The CRRES satellite was launched on 25 July 1990 into the geocentric
orbit shown in FIGURE 2 with apogee (33,700 km) near geosynchronous
altitude. This high apogee allowed cosmic ray measurements without
background magnetospheric particle interference over about half of each
orbital period (*i.e.,* for the McIlwain parameter L \>4: altitude
\>26,000 km).

The University of Chicago's telescope was intended to be carried for 3
to 5 years on the NASA-USAF CRRES spacecraft (built by Ball Aerospace,
Inc.) --- one of whose objectives was a 1 A.U. baseline for our High
Energy Telescope (HET) on the *Ulysses* spacecraft (launched in October
1990). Unfortunately, the spacecraft's main power system failed 12
October 1991, probably due to a spacecraft system design problem.

The data gathering period for CRRES occurred at the same time as the
largest amplitude of 11 year heliospheric modulation of cosmic ray
intensity in recorded history. FIGURE 3(A) shows the cosmic ray
intensity for primary nuclei above \~3 GV magnetic rigidity, as measured
by the Climax Neutron Monitor from 1951 through 1994. FIGURE 3(B) shows
the expanded time interval that included the period of our CRRES
measurements. We will discuss the effect of this extreme modulation
level on our measurements compared with the modulation levels of earlier
measurements of the carbon-silicon isotopes. Because of the shortened
duration of the CRRES mission and the low cosmic ray flux levels, the
principal errors in our measurements are statistical.

**3. Data Analysis**

Periods of enhanced solar activity were deleted from the database so as
to make negligible the contamination of galactic cosmic ray components
by charged particles of solar origin.

The trajectory of each charged particle is determined by the six
position-sensitive detectors, shown in FIGURE 1(B). As described in
APPENDIX 1, each of these six detectors requires two 1024 channel pulse
height analyzers (PHA) --- *i.e.,* a set of 12 pulse height measurements
for each particle penetrating the stack of the trajectory-determining
detectors. The energy deposit in each of the thick K detectors was
measured using 4096-channel analyzers.

Several additional criteria have been introduced that are based on the
method of partial energy-loss in one subset of detectors *vs.* residual
energy loss in other detector sets. These criteria are:

1\) The effective thicknesses of the dead layers at the back of each K
detector in FIGURE 1(A) were taken into account. These layers consist of
approximately 3% of the detector thickness and result in a low mass tail
in the mass histograms for events which stop in these layers. The number
of such events was found to be about 3%, what one would expect from the
dead-layer thicknesses. These small tails can be seen in the event
histograms in SECTION IV;

2\) In the analysis procedures we required an overall consistency of
energy deposits in the detector stack; this was a powerful tool for
discriminating against background events.

3\) A chi-square test was invoked requiring consistency among a sequence
of mass determinations obtained by using successive K detectors for the
dE/dx measurement, and the remaining Ks for the residual energy
measurement;

4\) Since the PSD's were intrinsically poorer energy detectors, compared
with the K detectors, the overall energy interval for the analyzed
events in this paper were restricted to particles that triggered K2
(*i.e.*, using the PSDs *only* for the trajectory determination).

**4. Observed Isotopic Abundances**

The mass histograms of the isotopes in elements C, N, O, F, Ne, Mg and
Si are shown in FIGURES 4 and 5. The mass separation of the histograms
is complete except for ^28^Si and ^29^Si. We developed an algorithm for
a maximum likelihood fit to determine the isotopic mass resolution for
each histogram that yields mass resolutions from s^\*^ = 0.13 amu for C
to s = 0.15 amu for Si. FIGURE 5 is a histogram of F and Ne isotopes
used to determine the source abundances of the ratio ^19^F/^20^Ne. Note
that the charge resolution for our CRRES instrument is \~0.04 *charge*
units, so that the actual separation of the two histograms shown is far
better than indicated in the FIGURE.

The relative abundances of these isotopes are calculated from the data
used to derive the histograms. A correction is then applied to take
account of the differences in the energy intervals of the different
isotopes in the detector stack. Since this correction factor depends on
the "shape" of the differential energy spectra, we used a single
spectral shape for all the isotopes. We derived this spectrum from the
helium and oxygen spectra measured by the University of Chicago *IMP-8*
main telescope during the times of our isotope investigation. \[See also
Simpson & Munoz, 1994 and Chenette *et al*., 1994 for spectra measured
by our CRRES telescope\]. TABLE 1 shows in Column 2 the energy intervals
for the isotope ratios we have measured.

**5. Propagation From Source to Observer**

To derive the cosmic ray source abundances from the measurements
reported above, it is necessary to determine the changes in cosmic ray
composition from the source to the observer. This includes both
interstellar propagation in the Galaxy and the effects of solar
modulation during heliospheric propagation.

**5.1. Interstellar Propagation**

The interstellar propagation model used in this work is based on the
standard "leaky box" model, solved using the "weighted-slab" technique
(*e.g.* Fichtel & Reames 1968). It has been shown that for the case of
stable nuclides lighter than iron, the secondary/primary ratios
calculated with this model are practically the same as those calculated
using the more realistic diffusion model of propagation (Ginzburg,
Khazan, & Ptuskin 1980). Furthermore, except for the radioactive primary
and secondary isotopes, the isotopic composition of the stable isotopes
determined from a halo diffusion model and a homogeneous disk model
appears to be essentially indistinguishable (Simpson & Garcia-Munoz,
1988) given the present level of accuracy of nuclear cross-sections,
diffusion coefficients *etc*.. The application of this homogeneous model
also enables us to compare the CRRES experiment source abundances with
other investigators' measurements, listed in TABLE 2.

**5.1.1 The Pathlength Distribution**

The pathlength distribution (PLD) is one of the most critical parameters
in the leaky-box model, and a fuller discussion of it is presented in
Appendix 2. In the paper by Garcia-Munoz *et al.*, 1987 (hereafter
referred to as "GM"), it was found that an energy-dependent truncation
of the PLD in short pathlengths was necessary for agreement between the
calculations and the available B/C and sub-Fe/Fe data. One of the
truncated PLDs investigated in GM was the double exponential (DE) PLD.
This PLD had been suggested previously within the framework of a "nested
leaky box" model (Simon 1977). We use a DE PLD here, and the details of
these calculations and their fits to the observed secondary/primary
elemental ratios are discussed in Appendix 2. It is important to note
that for the isotopes considered in this work the choice of double or
single exponential (SE) PLD has very little effect on the calculated
isotopic source ratios and that the level of truncation of the PLD is
much less important than previously thought.

**5.1.2 Partial Production Cross Sections**

The propagation analysis depends in a sensitive way on the cosmic ray
interaction cross-sections and interstellar medium (ISM) composition
used in these calculations. In recent years, information about both of
these parameters has continually increased. Many new partial
cross-section results have been reported, particularly by two groups,
Webber *et al*. (1990) and the Transport Collaboration (Guzik *et al*.
1990). However, even with the new data, measurements exist only for
about 5% of the reactions used by the propagation code, and often over a
very limited energy range. The partial cross-sections we have used in
our propagation calculations are derived from a combination of reported
measurements and theoretical predictions (Silberberg & Tsao, 1990). We
believe this technique of combining measured data with the time-tested
Silberberg & Tsao formulation is well-founded, and has been used in our
propagation calculations for many years (see GM).

**5.1.3 ISM Parameters and Ionization Energy Loss**

The other major input parameter to the propagation code, and one
important at low energies, is the energy loss rate of the cosmic rays
due to Coulomb interactions with ISM particles. We assume that the
interstellar matter has solar system composition with an ionized
hydrogen (HII) component (Thayer 1995), as in Soutoul, Ferrando, &
Webber (1990)**.** This has the effect of increasing the energy loss
rate by an energy-dependent factor of the order of 1.5 at low energies.
Thayer (1995) has investigated the effects that the addition of this
increased energy loss rate and recent cross section measurements have on
propagation calculations using a single exponential PLD, and find that
they differ from earlier calculations (see also SECTION VI) by the order
of 10% at the lowest energies.

**5.2 Heliospheric Propagation**

The importance of correcting isotope ratios for the level of modulation
has been pointed out by Simpson & Garcia-Munoz (1988) in their analysis
of the ^10^Be/^9^Be ratio. Although the correction decreases with
increasing atomic number, for the ratios of heavier isotopes reported in
our present analysis this dependence on the level of solar modulation is
still important.

We have calculated the effects of heliospheric propagation by assuming a
conventional model of solar modulation in which diffusion, convection
and adiabatic deceleration are in heliospheric equilibrium, as described
by a Fokker-Planck equation (Parker 1965). This equation is solved
numerically using the technique developed by Fisk (1971). In this model
the level of solar modulation is given by the force-field approximation
parameter

F = 1/3 ò V(r)/K(r)dr.

V is the solar wind velocity, the integral is over r, the distance from
the Sun and the diffusion coefficient is of the form K µ bR (where b is
particle velocity in units of velocity of light and R is particle
rigidity). F is independent of rigidity above 0.3 GV (Moraal 1976).

The values of the parameter F as a function of time have been determined
from 1973 through late 1994 using the differential energy spectra of
galactic ^4^He measured by the University of Chicago experiment on board
the satellite IMP-8. It has been shown (Garcia-Munoz, Pyle, & Simpson
1985) that using these values for F the model predicts with good
accuracy the time variations of other elements in the galactic cosmic
rays (*e.g*., protons, carbon and oxygen; see also Simpson 1983).

As noted in SECTION 2 and FIGURES 3(A) and 3(B), the time interval for
the CRRES measurements was the interval with the largest modulation ever
recorded. Furthermore, this period included shorter term, large scale
transients of solar origin (Sanderson *et al.,* 1992; Van Allen &
Fillius, 1992) that kept the heliosphere in a non-equilibrium state.
Nevertheless, we have found that the above steady-state modulation model
satisfies remarkably well all observed spectra for the period in
question. The values of F deduced from this model are shown in FIGURE
3(C) where, for the period of the CRRES measurements the average
modulation parameter was F = 1450 MV (see also Chenette* et al*., 1994).

The measurements reported in TABLES 1 and 2 by other investigators were
carried out at different levels of solar modulation. A meaningful
comparison of these measurements and of model propagation calculations
requires a normalization to a common level of solar modulation. We have
carried out this normalization, using the heliospheric propagation model
described above for the average modulation level F = 1450 MV appropriate
to the CRRES measurements. TABLE 3 shows the reported average modulation
levels of measurements reported by other investigators, their energy
intervals and the factors by which the measured ratios should be
increased in order to compare them with the CRRES isotopic ratios.

The normalized ratio measurements are compared with propagation
calculations as a function of energy in FIGURES 6, 7, and 8. The
importance of the modulation normalization is readily seen by the
example in FIGURE 6 where the ^13^C/^12^C and ^18^O/^16^O ratios, as
reported by investigators without modulation normalization in panel (A)
can be compared after applying this normalization in panel (B). Among
the ratios of isotopes, ^13^C/^12^C and ^18^O/^16^O are easy to measure
because of their relatively large isotopic separation and the favorable
statistics.

**6. Source Abundance**

We have updated the propagation calculation of GM using the new
cross-sections and energy-loss rate described above. We used the
semi-empirical formulation of Silberberg & Tsao (1990) for unmeasured
partial cross-sections. The source spectrum used in the calculations is
described in GM. It is the same form for all sources and is of the form
dJ/dE µ (T + T0)^-g^, where T0 = 400 MeV/n and g = 2.6.

The elemental and isotopic abundances that we have assumed in our
propagation calculations from source to observer are the solar system
abundances except for neon reported by Anders & Ebihara (1982) and
Anders & Grevesse (1989), (listed in TABLE 1 and TABLE 2). We have
investigated two cases concerning the neon isotopes. For the ratio
^22^Ne/^20^Ne = 12% at the source (proposed by Cameron 1982) our
propagated ratios are the long-short-dashed lines in FIGURE 7. The solid
lines are for the enhanced ratio ^22^Ne/^20^Ne = 31%, which is the
source abundance derived from our measured ratio. The results of
calculations performed using a double exponential PLD are shown as solid
lines, while those using the single exponential PLD of GM are
represented as short-dashed lines.

TABLE 1 lists our cosmic ray source isotopic abundance ratios for the
elements carbon to silicon, along with the Ulysses results reported by
Connell & Simpson (1993a,b).

**6.1 Carbon Isotopes**

Our deduced ^13^C/^12^C ratio at the cosmic ray source is 0.25 ± 0.89.
It is below the source ratios reported by Wiedenbeck & Greiner (1981b)
and Webber (1982) and agrees within errors with the source ratios of
Gibner *et al*. (1992) and Lukasiak *et al*. (1994). When corrected for
the effect of solar modulation, all of the isotopic measurements agree
with our propagation calculations using either a double or a single
exponential PLD.

Our measurement is about one standard deviation below the solar system
ratio of 1.11%, suggesting probable depletion or absence of ^13^C** **at
the source. From the point of view of nucleosynthesis ^12^C is a primary
nucleus rapidly formed during CNO processing while ^13^C is formed on a
slower time scale via processing of ^12^C from the CNO stage of former
stellar generations (Wilson & Rood 1994). Crane, Hegyi, & Lambert (1991)
and Stahl & Wilson (1992) report optical measurements of absorption
lines of the interstellar molecules in local interstellar space giving a
ratio ^13^C/^12^C close to 1.4%. Also in the solar neighborhood, radio
measurements of CO transitions in interstellar clouds by Langer &
Penzias (1993) give values from 1.4% to 1.7%.

From their compilation of measurements, Wilson & Rood (1994) conclude
that the interstellar ^13^C/^12^C ratio near the solar system is 1.30 ±
0.01% compared to the solar system value 1.11% Thus, in local
interstellar space there may have been a small increase of about 20%
relative to the solar value since formation of the solar system. This is
in agreement with models of chemical evolution of the galaxy (Audouze
1985). In general radio measurements of the ^13^C/^12^C in the galaxy
show a decrease with increasing distance from the galactic center. In
the solar neighborhood radio and optical measurements of ^13^C/^12^C
give consistent results.

**6.2 Nitrogen Isotopes**

The nitrogen isotopic histograms in FIGURE 4 display approximately equal
abundances --- namely, ^15^N is 55 ± 4% of the measured N. FIGURE 7
shows that the calculated source ratios are in agreement within errors
with all the measurements when normalized at 1450 MV modulation. All the
values of the ratio ^14^N/^16^O at the source in TABLE 1 and TABLE 2 are
in good agreement and converge to a cosmic ray source ratio which is
about one third of the solar system value.

We obtain a negative ^15^N/^16^O source ratio indicating very little or
no ^15^N at the source. All the ^15^N/^16^O ratios at the source
reported in references (a) through (f) in TABLE 2 reinforce this
conclusion.

Radio measurements reveal an increase of the ^15^N/^14^N with decreasing
distance to the galactic center (Wilson & Rood 1994). The solar system
^15^N/^14^N ratio is 0.37%. In the vicinity of the Sun the data are
consistent with a 70% decrease of the ratio since the formation of the
solar system. This decrease with time is consistent with an evolution of
the nitrogen isotopic ratio with stellar CNO processing (Wannier *et
al*. 1991).

**6.3 Oxygen Isotopes**

We find that the experimental ^17^O/^16^O ratio of 1.0 ± 0.4% leads to a
source ratio of --0.85 ± 0.35%, arguing strongly for absence of ^17^O at
the cosmic ray source.

The comparison in FIGURE 7 of our propagation calculations with
published ^17^O/^16^O ratios (normalized to the same level of solar
modulation, 1450 MV) shows that our value lies well below the
propagation calculation; whereas the other published values agree with
our propagation curve. The propagation recalculation of previous
measurements by Lukasiak *et al*. (1994) argues also for absence of
^17^O from the source.

The source ^17^O/^16^O ratio derived from Ulysses is --0.53 ± 0.31%,
also strongly arguing for absence of ^17^O from the cosmic ray source.
For ^18^O/^16^O the Ulysses-determined source abundance is --0.34 ±
0.20% (Connell & Simpson, 1993a), 3s away from the solar system
abundance of ^18^O. The two independent Ulysses and CRRES measurements
of *both* isotopic ratios are in agreement to about the 1s level. The
^18^O/^16^O source ratio found in this work is --0.11 ± 0.32% favoring
absence from the source but differs by only one standard deviation from
the solar system ratio, 0.20%.

Gibner *et al* (1992) report an overabundance of ^18^O in the cosmic ray
source based on a source ^18^O/^16^O ratio of 1.15 ± 0.38% or a value
2.5 standard deviations above the solar system ratio of 0.20%. In TABLE
2 the results for the ^18^O/^16^O ratio of Wiedenbeck & Greiner (1981a)
and Webber (1985) are consistent with solar system ratio and only one
standard deviation from ^18^O absence from the source. On the other hand
Lukasiak *et al*. (1993) and the recalculation of Lukasiak *et al*.
(1994) in column (e), show cosmic ray overabundance over the solar
system ratio by 2.5 and 1.5 standard deviations respectively. It is to
be noted that, as can be seen in FIGURE 6, the CRRES measurements and
that of Wiedenbeck & Greiner (1981a)\--two experiments that have the
highest mass separation\--give the lowest experimental ratios.

All the ^18^O/^16^O measured ratios, normalized for solar modulation, as
in FIGURE 6, show fair agreement with the propagation curves. We
conclude that the difference between results leading to overabundance
and those favoring solar system composition or source absence are due
mainly to differences in the used propagation cross sections. A specific
analysis by Guzik *et al*. (1993) of the calculations of the cosmic ray
source abundances of ^18^O leads to the same conclusion.

Galactic radio surveys reveal that the ^18^O/^16^O ratio increases with
decreasing galactocentric distance. A comparison of the gradients of the
oxygen isotopes suggests that from the point of view of nucleosynthesis
^16^O is a primary and ^18^O a secondary, and it would be expected that
the ^18^O/^16^O ratio would increase with time. Contrary to this, the
local interstellar ratio has been measured as 0.18 ± 0.008%, lower than
the solar system ratio (Wilson & Rood 1994).

**6.4 Fluorine and Neon Isotopes**

* *FIGURE 5 shows our histograms of the isotopes ^19^F and ^20^Ne. The
ratio measured by CRRES is 17.7 ± 3.8%, which translates to a cosmic ray
source ratio of --8.6 ± 2.9%, well below the solar system ratio of
0.026%, arguing for the absence of ^19^F at the source.

In FIGURE 7 published measurements of the ^21^Ne/^20^Ne and the
^22^Ne/^20^Ne ratios, normalized to 1450 MV modulation depth, are
compared to the ratios predicted by our propagation calculations. The
long-short dashed line (double exponential PLD) assumes a solar isotopic
ratio of 12% for ^22^Ne/^20^Ne at the source. The solid lines are for
the standard double exponential PLD and result from the inclusion at the
source of ^22^Ne/^20^Ne = 30.9 ± 7.2%.

TABLE 1 shows that the ^21^Ne/^20^Ne source ratio found in this work,
--6.5 ± 4.1% is well below the solar system ratio of 0.24% and is
consistent with ^21^Ne being absent from the source. Ulysses data on the
^21^Ne/^20^Ne ratio tells quite a different story, however. The source
abundance of ^21^Ne is calculated to be 6.6 ± 2.7% of ^20^Ne,
significantly higher than that derived from the CRRES measurements, as
well as being much higher than the solar system abundance. However, the
Ulysses and CRRES measurements differ from each other by only slightly
more than 2s (Connell & Simpson, 1993b). References (b) through (f) in
TABLE 2 agree with both ^21^Ne/^20^Ne source solar ratios and with
absence of ^21^Ne from the source while (a) gives a higher ^21^Ne at the
source.

For ^22^Ne/^20^Ne all the source ratios agree within 2s errors. The
source abundance of ^22^Ne is derived from Ulysses data to be 32.2 ±
4.7% of ^20^Ne (Connell & Simpson, 1993b), consistent with the CRRES
value and also exhibiting the well-known large enhancement in this
isotope.

The high cosmic ray source ^22^Ne/^20^Ne ratio (Garcia-Munoz, Simpson, &
Wefel 1979), is one of the outstanding differences between the
compositions of the cosmic ray source and the solar system. The solar
system ratio (Cameron 1982) is 12%, derived from the "planetary" Ne
trapped in meteorites. On the other hand Anders & Grevesse (1989) adopt
a value of 7.3% from the solar wind Ne in Al foils. Both values are
clearly below the cosmic ray source ratio (30.9 ± 7.2%) that we report
in this work.

Audouze *et al*. (1981) have given arguments against the hypothesis that
the higher ^22^Ne/^20^Ne ratio could be due to cosmic rays originating
in an interstellar medium which has been enriched in ^22^Ne relative to
the solar system since the formation of the Sun. Woosley & Weaver (1981)
have proposed that ^22^Ne is purely secondary and that cosmic rays may
be produced in remote galactic regions with higher metallicity than the
region where the solar system originated. However, a consequence of this
hypothesis would be an enrichment of neutron-rich isotopes leading to
ratios ^18^O/^16^O, ^25^Mg/^24^Mg, ^26^Mg/^24^Mg, ^29^Si/^28^Si, and
^30^Si/^28^Si enhanced by factors 2 to 2.5. Neither CRRES data nor
Ulysses data (Connell & Simpson, 1993b) support such enhancements.

The difficulties with the assumption of a general ^22^Ne enrichment of
extended areas of the interstellar medium have led (Cassé & Paul 1982)
to the assumption that the cosmic rays may have their origin in regions
near specific sources which can be expected to have an overabundance of
the isotope ^22^Ne, like carbon-rich Wolf-Rayet (W-R) stars and H-poor
stars in general.

On the assumption that W-R stars are stripped cores of short lived
massive stars whose envelopes have been peeled off by strong stellar
winds it is expected that, as a byproduct of He burning, ^22^Ne will be
enhanced in the star surfaces and remain stationary up to the end of He
burning. The stellar winds (and stellar flares) would transport the
^22^Ne enriched material to the cosmic ray acceleration sites.

Prantzos *et al*. (1986) and Prantzos, Arnould, & Arcoragi (1987) have
modeled the composition of the W-R ejecta mixed to the interstellar
medium and have found good agreement of their results with the observed
abundances in W-R stars. One key result is that ^22^Ne is by far the
most overproduced species.

**6.5 Magnesium and Silicon Isotopes**

FIGURE 8 shows a general agreement between measurement and propagation
calculations (both double and single exponential PLD), for the ratios
^25^Mg/^24^Mg, ^26^Mg/^24^Mg, ^29^Si/^28^Si, and ^30^Si/^28^Si, except
for the fairly large discrepancy of Wiedenbeck & Greiner (1991a,b) and
Lukasiak *et al*. (1994) in the case of the ratio ^30^Si/^28^Si.

TABLE 2 shows that the ^25^Mg/^24^Mg, ^26^Mg/^24^Mg source ratios
reported by Wiedenbeck & Greiner (1981b) are enhanced by a factor 1.6
over the solar system and ^29^Si/^28^Si, and ^30^Si/^28^Si by a factor
1.7. The ratios we measured in this work and the results of our
propagation calculations lead to the conclusion that the cosmic ray
source ratios ^25^Mg/^24^Mg, ^26^Mg/^24^Mg, ^29^Si/^28^Si, and
^30^Si/^28^Si are consistent with those of the solar system and do not
confirm the previously-reported overabundances of the neutron rich
isotopes ^25^Mg, ^26^Mg, ^29^Si, and ^30^Si.

**7. Summary**

The new instrument design for the measurement of isotopic composition of
galactic cosmic ray nuclei carried on the CRRES mission reported here,
and on the Ulysses mission (Simpson, *et al*., 1992), has made it
possible to achieve nearly complete separation of the isotopes of the
elements from carbon through silicon. The CRRES measurements at 1 A.U.
lead to the conclusion that, except for neon, the isotopic source
abundances for the elements C, N, O, F, Mg and Si are largely in
agreement with solar system composition. For the low energy spectral
range of the CRRES observations we have shown that these conclusions are
valid for both assumed forms of the PLD, single or double exponential,
but are sensitive (especially for the low abundance isotopes - *e.g.*,
^17^O) to the spallation cross-sections and the rate of energy loss in
the interstellar medium.

Although solar modulation had reached an unprecedented level during the
CRRES Mission, we found that a relatively simple spherically-symmetric
steady-state model of heliospheric modulation allowed an intercomparison
between recently published isotopic ratios and our CRRES measurements.
This intercomparison was performed at the same high level of modulation,
and significant agreement among the data was found. This result is
surprising because the heliosphere was far from an equilibrium state
(Sanderson *et al*., 1992; Van Allen & Fillius, 1992), and the various
measurements covered a wide range of energy and modulation levels.

It is important to note that the data processing and analysis from the
CRRES and Ulysses missions were kept separate from each other as much as
was practical and compared for the first time in the present paper. It
is, therefore, remarkable that these missions, as shown in TABLE 1,
display remarkable agreement, especially for the more abundant isotopes,
where the measurement uncertainties are lower.

In SECTION 6 we also compared our CRRES results with observed or
calculated abundance ratios obtained by radio, optical and other
indirect means, trying to derive a consistent picture of our results and
these other investigations of isotopic composition in the interstellar
medium. Suggestions for the origin of the low energy cosmic radiation,
namely \>95 percent of the radiation, include models requiring
acceleration mechanisms (*e.g*., shocks) in the interstellar medium
instead of supernovae, Wolf-Rayet stars, or super metallicity models.

The present cumulative experimental evidence shows that ^22^Ne is
enriched in cosmic ray source matter relative to the reported solar
system abundances, regardless of whether these abundances are determined
from meteorites (Cameron 1982) or from lunar solar wind deposits (Anders
& Grevesse, 1989).

**8. Acknowledgments**

We are grateful for the sustained sponsorship of our experiment on the
CRRES Mission by the Office of Naval Research (Contract
#N00014-94-1-0868) and NASA (Contract NAS 5-30117 and Grant NAG W-2368).
The development of position-sensing detectors in the period 1974 to
\~1980 was sponsored by NASA (Grant NGL 14-001-006) and carried forward
by A. Tuzzolino, E. LaRue and M. Perkins in LASR. The U.S. Air Force
Space Test Program provided the arrangements for space flight after two
flight cancellations (1979 and 1986) and the Air Force Geophysical
Laboratory (now Phillips Laboratory, Contract #F-19628-90-0025)
supported data reduction and analysis.

We thank E. Murphy, C. Lopate and the LASR staff for development of mass
analysis techniques and data processing. We appreciate the assistance of
J. Wefel, G. Guzik and M. Wiedenbeck. Ball Aerospace, Inc. designed and
built CRRES and integrated our experiment into the spacecraft. The
instrument was designed, built and tested in the Laboratory for
Astrophysics and Space Research by R. Jacquet, M. Perkins, F. DiDonna,
F. Sopron and many others. We thank the Bevalac staff, especially H.
Crawford, for beam calibrations in 1985 and 1989.

The Climax neutron monitor is supported by NSF Grant ATM 94-20790.
M.R.T. wishes to thank NASA for his support under GSRP Fellowship
NGT-51300.

**APPENDIX 1**

**The University of Chicago ONR--604 Experiment Instrument Description**

The high mass resolution needed for isotopic separation of heavy ions
requires knowledge of the trajectory of each charged particle entering
the acceptance cone of the telescope in order to determine the
pathlengths in the detectors. As shown in FIGURE 1(B), each position
sensing detector (PSD) has a series of gold strips on the top surface of
the detector which collect a fraction of the charge liberated by the
passage of the particle. This signal is read out through a resistive
divider network and compared to the total signal from the detector,
measured by the charge collected on the back surface contact. The ratio
of the two 1024 channel pulse height analyzers provide one coordinate of
the point of incidence of the particle (Lamport, *et al*., 1976). Two
sets of PSDs (three per set for redundancy) were used in the telescope
to obtain the trajectory of the particle, or a total of 12 analyzers.
Calibration of this system with accelerator particles has shown that the
position can be measured to better than 0.1 mm: therefore, the angle of
incidence of the particle could be determined to better than 1°.

The charge and mass of each particle was determined by the well-known
technique of multiple dE/dx versus residual energy using the
measurements provided by the stack of 5 mm thick lithium-drifted silicon
detectors, corrected for the trajectory of the individual particle. For
this analysis we required that the particle came to rest within the
detector stack. This determines the energy range over which each type of
particle can be studied. The signal from the scintillator guard counter
was used in anti-coincidence mode to eliminate out-of-geometry events
and to tag particles that exit from the side of the apparatus.

To separate isotopes for elements as heavy as nickel, it is necessary to
measure the dE/dx and residual energy signals to a precision level of
0.1%. This stringent constraint has been met by developing exceptionally
linear, highly stable electronic circuits to analyze the signals from
each detector. Each analysis chain has been individually calibrated over
a wide temperature range, and the entire instrument performance has been
evaluated using high energy heavy ion beams obtained from the LBL
Bevalac accelerator.

The performance of each analysis chain was monitored during flight by an
on-board calibrator which periodically sent a sequence of 2000 pulses
spanning the entire range of signal sizes. Furthermore, in-flight
calibration and normalization was provided by measurement of galactic
cosmic ray events. These high precision measurements required that the
temperature remained stable and constant to within \~3 °C over each
orbit: consequently the instrument power was on during 100% of the
orbital period.

Because the anti-coincidence scintillator guard counter subtended a
large solid angle and was used in the instrument logic, it was shielded
from the high counting rates induced by low energy particles trapped in
the magnetosphere and by intense solar flare fluxes. The shielding
\[shown in FIGURE 1(A)\] corresponded to complete elimination of \<4 MeV
electrons by the AZ31B magnesium shield and the tantalum collar. The
lower part of the scintillator was located well within the instrument
package and was surrounded on five sides by electronic modules offering
additional shielding. Backward moving particles were additionally
shielded by the spacecraft structure.

Since about 50 percent of the time in each orbit was spent in the
intense radiation portion of the magnetosphere (FIGURE 2) a 3mm external
Al window shield was placed over the telescope in order to protect
further against accumulated radiation damage to the detectors Dl, D2 and
D3 (where the average geometrical factor was \~50 cm^2^-ster). This
shield lowered the integrated proton and electron radiation doses for
detectors Dl, D2, D3 while moving upwards only slightly the energy
intervals for measurement of the heavy particles, which were the main
objective of the experiment.

FIGURE 9 shows a functional block diagram of the instrument electronics.
Besides providing pulse height analysis of each particle, the instrument
had internal accumulators that counted and stored both single detector
counting rates and coincidence counting rates, thus allowing the
absolute sampling rate and the absolute flux levels to be deduced.

In addition to heavy particles the instrument measured protons and alpha
particles over the energy range 25-119 MeV with a priority system to
insure that the rare heavy ion events were preferentially analyzed and
stored. In addition, in regions of expected high background radiation
(*i.e.,* the magnetosphere), a commandable heavy particle mode could be
invoked. This raised the thresholds on detectors D1 through D6 to
suppress the high firing rates induced by trapped protons and electrons
while still permitting the rare heavy ions to be analyzed.

The following are examples of particle range/energy in the telescope:

  -- ---------- --------------------------------------------------- ---------------- ------------------------------------------------------- -- ---- ------ ---- ------ -- ---- ------ ---- ------ -- ---- ------ ---- ------ -- ---- ------ ---- ------ -- ---- ------ ---- ------ -- ---- ------ ---- ------ -- ---- ------ ---- ------ -- ---- ------ ---- ------ -- ---- ------ ---- ------ -- ---- ------ ---- ------ -- ---- ------ ---- -------
     Detector   Minimum proton energy to reach detector^\*^ (MeV)   Qmax (degrees)   Maximum proton energy to stop in detector^\*\*^ (MeV)                                                                                                                                                                                                                                                                                                     
     D1         24.8                                                ---              44.5                                                       D2   28.3   87   48.2      D3   31.8   84   49.8      D4   35.2   50   43.9      D5   36.7   48   45.5      D6   38.6   46   46.8      K1   40.0   41   59.4      K2   52.0   39   70.6      K3   62.1   36   79.9      K4   71.3   34   88.5      K5   79.3   32   96.5      K6   87.1   30   103.5
     K7         94.0                                                29               108.4                                                                                                                                                                                                                                                                                                                                                     
     K8         100.8                                               27               116.9                                                                                                                                                                                                                                                                                                                                                     
  -- ---------- --------------------------------------------------- ---------------- ------------------------------------------------------- -- ---- ------ ---- ------ -- ---- ------ ---- ------ -- ---- ------ ---- ------ -- ---- ------ ---- ------ -- ---- ------ ---- ------ -- ---- ------ ---- ------ -- ---- ------ ---- ------ -- ---- ------ ---- ------ -- ---- ------ ---- ------ -- ---- ------ ---- ------ -- ---- ------ ---- -------

\*normal incidence ^\*\*^Qmax angle of incidence

Examples of the geometrical factors for the telescope in FIGURE 1 are:

**[ ]{.underline}[Logic ]{.underline}**^[\*]{.underline}^**[
]{.underline}[Geometrical Factor
(cm]{.underline}^[2]{.underline}^[-ster)]{.underline}**

D1D2ND3D4N(S) 46.1

D13D46N(K1K2S) 10.9

D13K1K2N(K3S) 7.47

D13K1K8AN(S) 6.08

^\*\ ^D, K, A,S are detectors (see FIGURE 1). "N" indicates that all
following detectors within the following parentheses are in
anti-coincidence.

For further details on the instrument and the CRRES spacecraft, see
Gussenhoven, Mullen, & Sagalyn (1985) and Simpson & Garcia-Munoz (1994).

**APPENDIX 2: The Pathlength Distribution**

The leaky-box equations of interstellar propagation are well-known.
Under certain circumstances which are commonly assumed to be met, these
equations are separable (details in Ginzburg & Syrovatskii 1964;
Garcia-Munoz *et al*. 1987). The resulting "slab" equations can then be
solved as a function of pathlength traversed and energy, while the other
equation has a solution which is the Green's function for the slab
equations, namely, the pathlength distribution (PLD). This technique of
solution is known as the "weighted-slab" method (Fichtel & Reames 1968).
In the slab equations, spallation effects, energy loss due to ionization
and radioactive decay are taken into account. The PLD is used to correct
for the distribution of pathlengths traversed by the cosmic rays. The
solutions of the coupled slab equations are number densities as a
function of pathlength and energy Ni(x,E). These are integrated using a
particular PLD(x,E) over the pathlength x to derive a local interstellar
flux of nuclear species i.

The choice of the shape of the PLD used in our calculations is generally
constrained by the requirement that the calculations agree with measured
secondary/primary ratios. Garcia-Munoz *et al. *(1987) found below \~1
GeV/n that a pure exponential PLD with an energy-dependent escape length
"tuned" to agree with the observed B/C data did not agree with the
available experimental data on the (Sc-Mn)/Fe ratio. They showed that a
simultaneous fit of both ratios could be obtained if an energy-dependent
truncation of the PLD was used below \~1 GeV/n, and that the truncation
increased with decreasing energy.

As described in SECTION 5, the PLD is required to fit measured
secondary/primary elemental ratios. It has been found that while a
single exponential PLD may agree with the Ulysses B/C and (Sc-Mn)/Fe
ratios (DuVernois, Simpson, & Thayer, 1995), some slight truncation at
low energies still appears necessary for agreement with all of the
available data, including other recently published data (FIGURE 10). The
truncation was modeled by the use of a double exponential (DE) PLD, as
suggested by Simon (1977). The form of the DE PLD is:

This is properly normalized, and the energy dependence of the individual
means x1(E) and x2(E) is chosen to agree with elemental
secondary/primary ratios. FIGURE 11 shows the PLD means used to generate
the curves in Fig. 10. It is important to note again that the particular
choice of PLD (single exponential *vs*. double exponential or energy
dependence of mean) has very little effect on the isotopic ratios,
however. This can be seen by looking at the curves in FIGURES 6, 7 and
8. The solid lines in all these figures are the results of using a DE
PLD which was used for previous CRRES source calculations (DuVernois *et
al*., 1993; Garcia-Munoz *et al*., 1993), the dot-dashed lines are the
result of using a DE PLD (FIGURE 11) with a much lower amount of
truncation (controlled by mean x2(E)), and the short dashed lines (seen
in FIGURES 6 and 8) are derived from using a single exponential PLD with
the mean as determined in GM. The differences between the curves are in
all cases \<10%, and are certainly much smaller than the uncertainties
in the fragmentation cross sections. It is clear that truncation of the
PLD has a much smaller role in the question of determining isotopic
ratios than may have been previously thought. The minor truncation
required is in qualitative agreement with Webber (1993), who also found
a slight truncation of the PLD at low energies necessary when performing
a similar analysis.

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