The existence of very narrow (FWHM  or approximately = 5 KeV) gamma ray line emission from interstellar grains is pointed out. The prime candidate for detection is the line at 6.129 Mev from O-16, but other very narrow lines could also be detected at 0.847, 1.369, 1.634, 1.779 and 2.313 Mev from Fe-56, Mg-24, Ne-20, Si-28 and N-14. Measurements of this line emission can provide information on the composition, size and spatial distribution of interstellar grains

Lingenfelter, R. E.

Ramaty, R.

English

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GAMMA RAY LINES FROM
INTERSTELLAR GRAINS
(NASA-TM-X-71191) GAMMA RAY LINES FROM	 N76-33117
INTERSTELLAR GRAINS (NASA) 15 p HC $3.50
CSCL 03B
Onclas
G3/93 04573
R. E. LINGENFELTER
R. RAMATY
i
t
AUGUST 1976
GODDARD SPACE FLIGHT CENTER
GREEN3ELT, MARYLAND
T
i E
k
^ I
GAMMA RAY LINES FROM INTERSTELLAR GRAINS
R.E. Lingenfelter*
Department of Astronomy and
Department of Geophysics & Space Physics
University of California
Los Angeles, California 9UO24
and
R. Ramaty
NASA/Goddard Space Flight Center
Laboratory for High Energy Astrophysics
Greenbelt, Maryland 20771
ABSTRACT
We point out the existence of a hitherto unknown compo-
nent of gamma-ray line emission: very narrow (FWHM <5 KeV)
lines from interstellar grains. The prime candidate for detection
is the line at 6.129 Mev from 160, but other very narrow lines
could also be detected at 0.847, 1.369, 1.634, 1.779 and 2.313
Mev from 56 Fe, 24 Mg, 20Ne, 28 Siand 14N. Measurements of this
line emission can provide information on the composition, size
and spacial distribution of interstellar grains.
Subject headings--gamma rays- interstellar grains -cosmic rays.
*This research was supported in part by National Science Foundation
grant AST 67-08178.
-1-
ii
I	 Introduction
Interstellar gamma-ray line emission can result from the
interaction of low-energy (,<,100 Mev/amu) cosmic rays with con-
stituents of the interstellar medium. The expected intensities
of some of these lines and their detectability has been dis-
cussed most recently by Lingenfelter and Ramaty (1976). In
the present letter we wish to point out an important component
of this line emission: the very narrow Da=na-ray lines emitted
from interstellar grains.
Interstellar gamma-ray lines result from the deexcitation
of nuclear levels, mainly in 12 C, 14N, 160, 20Ne, 24 Mg, 28 Si
and 56Fe. Broad lines, with a full width at half maximum (FTWHM)
of about 20% of the line energy, are produced by the deexcita-
tion of cosmic-ray nuclei, while narrower lines, with FWHM of
a few per cent, result from the deexcitation of gas nuclei.
These two components have already been discussed by Rygg and
Fishman (1973), Meneguzzi and Reeves (1975), and Lingenfelter
and Ramaty (1976) for emission in the interstellar medium, and
by Ramaty, Kozlovsky, and Lingenfelter (1975) for emission in
solar flares.
The very narrow line component, which we shall treat here,
is produced by deexcitation of interstellar grain nuclei excited
by low-energy cosmic rays. Unlike the excited nuclei in gases,
which retain most of their recoil energy for times much longer
than the lifetimes of the excited levels, some excited nuclei
in solids can lose all of their recoil energy before they emit
-2
igamma rays. The intrinsic widths of these gamma ray lines are
determined only by the temperature of the solid. For grain
temperatures of 1-15 K, the width is only about 10 ev. Larger
widths, however, result from both the random and systematic
motion of the grains, but these are not expected to exceed about
0.1% of the energy of the lines. These very narrow line widths
not only enhance the detectability of the lines above the back-
ground, but also may permit several new astronomical investi-
gations: namely the determination of the composition of grains
and their size distributions from the relative intensities of
the lines, and the study of the spacial distribution ana systematic
motion of grains in the galaxy from Doppler shifts of the lines.
We defer detailed discussions of these exciting possibilities
for future research. In the present letter we outline the physics
of very narrow line production, present results of calculations
on the formation of the very narrow 6.129-Mev line, and discuss
the relative intensity of other lines with very narrow components.
II	 Discussion
The most important gamma ray lines which are expected to
exhibit the very narrow component are those formed from rela-
tively long-lived nuclear levels in abundant heavy nuclei with
large excitation cross sections. Using detailed compilations
of nuclear cross sections (Ramaty, Kozlovsky and Lingenfelter
1975, 1977) and the elemental abundances of Cameron (1973),
we find that the strongest interstellar gamma ray lines should
-3-
be at 4.438 tlev from 12C and 6.129 !1ev from 160. The mean life-
times of the corresponding excited states are 5.6 x 10 -14 s and
2.4x 10 -11 s, respectively (De Meijer, Plendl and Holub 1974).
The expected ratio of the 4.432 and 6.129-Mev line emissivities
is about 2 for C/O= 0.54.
In proton induced interactions, the excitation cross sections
for the 12C*4.439 and 160*6.131 levels peak at proton energies
of approximately 10 mev. By nuclear kinematics, the heavy nuclei
have recoil energies of a few tens of kev/amu. In solids of
density p ti 1 g cm 3 , such particles stop on time scales of the
order of 10 -12 s (Northcliffe and Schilling 1970; '.•;interbon 1975).
By comparing this time with the lifetimes of the excited levels,
we see that the 5.129- 11ev line of 16 0 will exhibit the very
narrow component, but that the 4.438- 11ev line of 12C will not.
Another strong interstellar line is expected at 0.511 r.ev from
positron annihilation; even though the width of this line can
be as small as a few kev (Crannell et al., 1976), interstellar
grains play no special role in its formation.
In addition to having stopping times shorter than the nuclear
deexcitation times, the emission of the very narrow component
also requires the existence of grains with linear sizes larger
than the vector stopping distance of the recoiling nuclei.
Since the average stopping distances,-.(10- 4 /P) (Northcliffe
and Schilling 1970; Winterbon 1975) are comparable to the linear
dimensions of the grains, we expect that a sicnificant fraction
of the excited nuclei will stop in the grains. Those nuclei
that escape from the grains will emit gamma rays while still
I
-4-
I	 i	 '
in motion and hence will not contribute to the very narrow line
component. However, because they have lost some energy in the
grains these nuclei will produce a narrower line width than
excited gas nuclei.
We have carried out Monte Carlo calculations to determine
both the fraction of 16 0 nuclei that stop in grains, and the
shape of the 6.129-Mev line produced in a mixture of gas and
grains irradiated by low-energy protons. We have assumed that
half of the ambient oxygen is in the gas and the other half
is in the grains (e.g. Morton 1975). We have used the range-
energy relation of oxygen in water (Northcliffe and Schilling
1970), and we took p = 1.5 g cm 3 for the grains. The range
of oxygen in silicates is not significantly different. We have
used detailed nuclear cross sections and kinematics for the
calculation of the energy and angular distribution of the recoiling
nuclei. The method that we have used is similar to that of
Ramaty and Crannell (1976), who have evaluated the shape of
the 6.129-Mev line for the narrow component.
Observations of the reddening and obscuration of starlight
by interstellar grains are consistent with equilibrium grain
size distributions (e.g. Wickramasinghe 1967; Aannestad and
Purcell 1973) of the form N(a) = N0exp(- a/ao ), where the charac-
teristic radius a  is on the order of 0.1p and depends only
weakly on the assumed composition. Such values of ao,althouah
appropriate for dirfuse clouds, may not be representative of
most grains, however, since grains in dense, dark clouds may
contain the bulk of the interstellar grain mass. Observations
-5-
I	 f
of the dark cloud 40ph (Carrasco, Strom and Strom 1973) suggest
that the mean grain size increases with depth in such clouds.
A similar effect is suggested from the study (Jura 1975) of
the apparent dark cloud remnant VY CMa. The high opacity of
dark clouds to visible radiation does not permit study of grains
in the densest region:; of such clouds, where the mean grain
size could be even a couple of orders of magnitude larger (Greenberg
1974). very narrow line gamma ray observations, however, may help
determine the mean sizes of grains in such dark clouds, since
they are transparent at gamma ray energies and the relative
intensity of very narrow line components from different excited
states is fairly strongly dependent on grain size. In the present
calculations therefore we study very narrow line emisoion for
grain size distributions with a range of characteristic radii
ao between 0.03 and 10u.
The resultant gamma ray spectrum is shown in Figure I.
We have averaged the spectrum over energy bins of 5 kev, com-
parable to the expected energy resolution of future gamma ray
line detectors employing solid state devices (Metzger 1973).
The solid line is the sum of the broad, narrow and very narrow
components, the dashed lines represent the sum of the broad
and narrow components only. All spectra are normalized to unit
area under the curves. Half of the integral is under the broad
component, only a portion of which is shown, since it has a
FWHM of til Mev, while the other half is under the sum of the
narrow and very narrow components. As can be seen, for a  >O.lu,
the very narrow component can be resolved. The fraction of
Ili
-6-
photons in a 5-kev bin around 6 . 129 Mev is 0.03, 0 .75, 0.16
and 0 . 23 of the total in the three components for a  equal to
0.1, 0.3, 1.0 and 10u, respectively.
It is difficult to estimate the expected intensity of the
6.129-110v line from interstellar grains because of the large
uncertainties in the possible low-energy cosmic ray and ambient
matter densities (e.g. Lingenfelter and Ramaty 1976). We can,
however, normalize our calculations to the measurements of Haymes
et a".. (1975)t who have reported the detection of several c,amma
ray ^. j,ectra] features from the direction of the galactic center.
In particular, these authors report a broad feature at ti4.4 Mev
with intensity \,10 3photons cm 
2 s
-1 , which could be nuclear
radiation from 12C. Using the results given above on the ratios
of the broad, narrow and very narrow components, and on the
relative 4.438 to 6.129-Mev line fluxes, we find that the flux
of the very narrow 6.129-s•1ev line from the direction of the
galactic center should range from about 4 x 10 -5 to 10-4hhotons
cm-2 s -1 for a  between 0.3 and 10u. These fluxes are well above
the diffuse gamma ray background, which, for a collimated detector
of 0.1 str solid angle at 6 I1ev, is only about 2 > 10 ­ 7 photons
cm 2 s-1 in a 5 kev bin around this energy (Trombka et al. 1976).
Such fluxes should be observable with future satellite-borne
gamma ray detectors (Jacobson 1976 private communication).
The expected emission would of course be lower if some of the
line emission reported by Haymes et al. (1975) were due to nuclear
excitation of material surrounding the detector.
Although the very narrow component of the 6.129-Mev gamma
i
-7-
ray line from oxygen should be the most intense line emitted
by interstellar grains there are several other nuclei in inter-
stellar grains which should also emit very narrow lines of nearly
comparable intensity. These lines are listed in Table 1.
The relative intensities given in this table represent the sum
of the narrow and very narrow components, and are based on our
compilations of nuclear cross sections (Ramaty, KozJovsky and
Lingenfelter 1977) and the abundances of Cameron (1973). The
mean lifetimes are taken from Nuclear Data Group (1973), De Meijer,
Plendl and Holub (1974) and De Meijer, Drentje and Plendl (1975).
Since the stopping times of nuclei with kinetic energies of
a few tens of kev/amu in grains is of the order 10 -12 s, all
the lines listed in Table 1 should exhibit significant very
narrow components. For the iron lines, however, this compo-
nent probably cannot currently be resolved from the gas compo-
nent, because the latter is also quite narrow (FWHM= 5 kev).
It should also be noted that if nearly all of the heavier nuclei
(Mg, Si and Fe) in the interstellar medium are in grains but
only about nalf of the 0 and N and possibly Ne nuclei are in
grains (e.g. Morton 1975), then the relative intensities of
the very narrow line component from the heavier nuclei could
be as much as a factor of 2 greater than the values shown.
Observations of the very narrow line component may help resolve
this question, particularly in such uncertain cases as Ne.
r
-8-
r	 ^
III Summar
We have pointed out the existence of a potentially detect-
able very narrow component of interstellar gamma ray line emission
and have presented results from calculations of the emissivity
of this component in interstellar grains. Gamma ray line emis-
sion from grains result, when low-energy cosmic rays interact
with heavy nuclei embedded in the grains. Very narrow line
emission is produced if both the stopping times and stopping
distances of the :coiling heavy nuclei are shorter than the
lifetimes of the nuclear levels and the sizes of the grains,
respectively. The widths of these lines are determined by the
motions of the grains, and are not expected to exceed about
0.1% of the energy of the lines.
The prime candidate for a line with an observable very
narrow component is the 6.129-Mev line of 160. We have per-
formed detailed calculations on the production of this line
In grains using nuclear cross sections and kinematics, range-
energy relations, and exponential grain size distributions.
For characteristic grain radii of 0.1 to 1 . 0p we find that 10
to 60 % of the nuclei stop in the grains. For these stopped
nuclei, the emitted gamma rays are in an energy bin narrower
than 5 kev.
Very narrow lines should also be emitted at 0.847 and 1.238
Mev from 56 Fe, at 1.369 Mev from 24 Mg, at 1.634 from 20Ne, at
1.779 Mev from 28 Si, at 2.313 Mev from 14 N. The intensities
of these lines range from about 20 to 50't of the very narrow
-9-
^	 I
	
f
6.129-Mev line intensity. Their detection would give unique
information on the composition, size and spacial distribution
of interstellar grains.
Acknowledgements. We wish to acknowledge valuable discussions
with R. Bussard, J. Cohen, M. Jura, B. Ko2loi'sky, J.C. Ling,
B.J. Teegarden, and J. Willet.
REFERENCES
Aannestad, P.A., and Purcell, E.M., 1973, Ann. Rev. Astron.
Astrophys., 11, 309.
Carrasco, L., Strom, S.E., and Strom, K.M., 1973, Ap. J., 182,
95.
Canercn, A . G.W., 1973, Space Sci. Rev., 15, 121.
Crannell, C.J., Joyce, G., Ramaty, R., and Werntz, C., 1976,
Ap. J.	 (in press).
De Meijer, R.J., Drentje, A.G., and Plendl, M.S., 1975, Atomic
Data & Nucl. Data '^ables, 15, 391.
De Meijer, R.J., Plendl, H.S., and Holub, R., 1974, Atomic Data
& Nucl. Data Tables, l3, 1.
Greenberg, J.M., 1974, Ap. J., 189, L81.
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F.T., and Shelton, D . H., 1975, Ap. J., 201, 593.
Jura, M., 1975, Ap. J., 80, 227.
Lingenfelter, R.E., and Ramaty, R., 1976, Proceedings of the
Symposium on the Structure and Content of the Galaxy and
-10-
tGalactic Gamma Rays, C.E. Fichtel and 1. Stecker, eds.
(in press).
Meneguzzi, M., and Reeves, H., 1975, Astron. Astrophys., 40,
91.
Metzger, A., 1973, Gamma Ray Astrophysics, F.W. Stecker and
J.I. Trombka, eds., NASA 3P 339, 97.
Morton, D.C., 1975, Ap. J., 197, 85.
Northcliffe, L.C., and Schilling, R.F., 1970, Nuclear Data Tables,
A7, 233.
Nuclear Data Group, 1973, Nuclear Level Schemes A - 45 Throu(Ili
A = 257 from Nuclear Data Sheets, Academic Press, New York.
Ramaty, R., and Crannell, C.J., 1970', Ate__:,., 203, 766.
Ramaty, d., Kozlovsky, B., and Lingenfelter, R.E., 1975, Space
Sci. RP •, , 18, 341.
Ramaty, R., Kozlovsky, B., and Lingenfelter, R.E., 1977 (in
preparation).
Rygg, T.A., and Fishman, G.J., 1973, Proc. 13th Internat. Cosmic
Ray Conf. (Denver), 1, 472.
Trombka, J.I., Dyer, C.S., Evans, L.G., Bielefeld, M.J., Seltzer,
S.M., and Metzger, A.E., 1976, NASA preprint 3-682-7 -11a.
Wicki-arnasinghe, N.C., 1967, Interstellar
-
Grains, Cha pman & lull,
London, 73.
Winterbon, K.B., 1975, Ion Implantation Range and Energy Deposition
Distributions, IFI/Plenum, New York, 341 pp.
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Figure Caption
Figure 1. The spectrum of 6.129 Mev gamma ray line emission,
q, from 160, excited in low energy cosmic ray inter-
actions with interstellar gas and grains, assuming
equal fractions of oxygen in the gas and grains,
and a grain size distribution such that the number
of grains with a radius between a and da is
N(a)da = No xp(- a/a0)da. The emission from grains
is the difference between the solid and the
dashed curves.
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Gamma ray lines from interstellar grains

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