The gamma ray line intensities due to cosmic ray spallation reactions in clouds, the galactic disk and accreting binary pulsars are calculated. With the most favorable plausible assumptions, only a few lines may be detectable to the level of 0.0000001 per sq. cm per sec. The intensities are compared with those generated in nuclear excitation reactions

Letaw, J. R.

Silberberg, R.

Tsao, C. H.

English

NASA Technical Reports Server

369 OG: 3.2-9
GAMMA RAY LINE PRODUCTION FROM COSMIC RAY SPALLATION REACTIONS
Silberberg, Rein and Tsao, C.H., NRL, Washington, D.C. 20375-5000 USA
Letaw, John R., Severn Communications Corp., Severna Park, MD 21146 USA
The gamma ray line intensities due to cosmic-ray spallation
r'eactions in clouds, the galactic disk and accreting binary
pulsars are calculated. With the most favorable plausible
assumptions, only a few lines may be detectable at the level of
10-6 per cm2-sec. The intensities are compared with those
generated in nuclear excitation reactions.
I. Introduction. Measurements of gamma ray line intensities and
energies from astrophysical sites permit the study of two types of
nuclear processes: (I) Nuclear collisions by cosmic rays and other high
energy particles. Possible sites for these reactions are the galactic
disk (Ramaty et al. 1979), interstellar clouds, supernovae in clouds,
accreting sources such as beaming pulsars in •close binary systems, and
active galactic nuclei, with high-energy beams interacting in the
accretion disk. (2) Recent nucleosynthesis (i.e. build-up of nuclei) in
supernova and nova precursors, as well as during their explosive phase
_layton, 1973, Arnett, 1978, Mahoney et al. 1982, Clayton, 19_4)._ The
A1 line has already been observed at a flux level of 5x10--/cm_-sec.
tad (Nahoney et al. 1984 and Share et al. 1985). The latter line is
probably due to nucleosynthesis in novae (Clayton, 1984).
In nuclear collisions, there are three processes that give rise to
gamma ray lines: (I) Quasi-elastic collisions at low energies that
generate excited nuclei. The relevant cross sections for these
reactions have been reviewed by Ramaty et al. (1979). The main
uncertainty here is the abundance of the low energy nuclei (near 10
MeV). (2) Nuclear spallation reactions, with the spallation products in
various excited states, that de-excite promptly by gamma-ray line
emission. Here the uncertainty is due to lack of information of the
population of the various excited states. (3) Gamma rays produced upon
the decay of various radioactive nuclides generated in nuclear
spallation reactions. The associated gamma-ray energies and emission
probabilities for the latter case are sufficiently well known. In this
paper we shall concentrate on processes (2) and (3).
In Section 2, we calculate the spallation yields and the relative
gamma ray line intensities. In Section 3, we apply these calculations
to the interstellar clouds, the galactic disk, and to pulsars in an
accreting system. In Section 4, the conclusions are presented, and the
detectability with the GRO/OSSE detector is discussed.
2. The Spallation Yields and Relative Gamma Ray Intensities. The rate
at which a nuclide of type j is created per unit volume in the
interstellar medium from proton interactions (for protons in a given
energy interval) is:
Rj (nuclides/cm 3 sec)= 10-27 J nH Fj (I)
369 OG: 3.2-9 
GAMMA RAY LINE PRODUCTION FROM COSMIC RAY SPALLATION REACTIONS 
Silberberg, Rein and Tsao, C.H., NRL, Washington, D.C. 20375-5000 USA Letaw, John R., Severn Communications Corp., Severna Park, MD 21146 USA 
The gamma ray line intensities due to cosmic-ray spallation ~e~ctions in clouds, the galactic disk and accreting binary pulsars are calculated. With th  most favorable plausible assumptions, only a few lines may be detectable at the level of 10- 6 per cm 2-sec. The intensi i s are compared with those generated in nuclear excitation reactions. 
1. Introduction. Measurements of gamma ray line intensities and energies from astrophysical sites permit the study of two types of nuclear processes: (1) Nuclear collisions by cosmic rays and other high energy particles. Pos ible sites for these r actions are the galactic disk (Ramaty et ale 1979), interstellar clouds, supernov e in clouds, accreting sources such as beaming pulsars in .close binary systems, and active galactic nuclei, with high-energy beams interacting in the accretion disk. (2) Recent nucleosynthesis (i.e. build-up of nuclei) in supernova and nova pr cursors, as well as during their explosive phase ~~layton, 1973, Arnett, 1978, Mah ney et a1. 1982, Clayton, 19§4).2 The Al line has already been observed at a flux level of 5 10- Icm -sec. rad (Mahoney et ale 1984 and Share et ale 1985). The latter line is probably due to nucleosynthesi  in novae (Clayton, 1984). 
In nuclear collisions, there are three processes that give rise to gamma ray lines: (1) Quasi-elastic collisions at low energies that generate excited nuclei. Th  relevant c ss sec ions for these reactions have been reviewed by Ramaty et ale (1979). The main uncertainty her  is the abundance of the low energy nuclei (near 10 MeV). (2) Nuclear spallation reactions, with he spallation products in various excited states, that de-excite promptly by gamma-ray line emission. Here the uncertainty is due to lack of information of the population of the various exci ed states. (3) Gamma rays produced upon the decay of various rad oactive nuclides generated in nuclear spallation reactions. The associated gamma-ray energies and emission probabili ties for the latter case ar  sufficiently w ll known. In this paper we shall oncentrate on pro ess s (2) and (3). 
In Section 2, we calculate the pallation yields and the relative J gamma t'ay line intensities. In Section 3, we apply these calculations to the. interstellar clouds, the galactic d sk, and to pulsars in an accreting system. In Section 4, the co clusions are presented, and the detectability with the GROIOSSE d tector s discussed • 
• 2. The Spallation Yields and Relative Gamma Ray Intensities. The rate at which a nuclide of type j is created per unit volume in the interstellar medium from proton in eractions (for protons in a given energy interval) is: 
Rj (nuclides/cm3 sec) = 10-27 J nH Fj (1) 
https://ntrs.nasa.gov/search.jsp?R=19850025692 2020-03-20T18:04:18+00:00Z
370 00: 3.2-9
I0-272is the conversion factor from mb to cm2, J = cosmicHere ray
protons/cm -sec, in a given energy interval. (We assume that the
contribution of nuclei heavier than protons results in a second-order
correction, though at energies of a couple of MeV/u, the contribution of
alpha-induced reactions is important.) nH is the number of hydrogen
atoms in the medium, per cm3, and
: Zo..f., (2)
FU_ i 31 1
where o.. is the cross section for protons with nuclide i, in units of3 1
mb yiel_ing nuclide j, and fi is the abundance of i per B atom in the
collision medium.
50 l l = = I I I = I I I = I _ I
E _ mO
:----- Hc0
"-------14 N
Cl _ 6Li
I __ 7Li
"1" _lo B
_ ";'Be
_L I0
5 I I i I i i
0. I 1.0 I0
ENERGY (GeV)
Fig. I. The energy dependence of F_, the production rate function for
J
the nuclides whose secondary yields are largest.
Fig. I shows the values of F. as a function of cosmic ray proton
energy E, using the elemental and _sotopic abundances of Cameron (1982).
Assuming that most of the prompt de-excitations pass to the ground state
via the first excited state, the most prominent _ines induced by _ghenergy cosmic r_ protons would be 4_4 MeV from I-C, 2.12 MeV from
5.27 MeV _r°m-JN_I, 5.18 MeV from "_0. In addition, the radioactive
decay of JO and C yields positrons. Fig. 2 shows the F. values of
other abundant products. Some of these F. values are _ow at low
energies, providing a test for the presenceJor absence of low-energy
cosmic rays.
The rate of gamma ray line emission from a volume V is R.Vk, where k
is the gamma ray multiplicity from the formation of nucl_de j. The
gamma ray line flux from nuclide j at earth is:
RjVk (per cm2-sec), (3)
4_d2
where d is the distance from the gamma ray source to earth.
370 OG: .2-9 
er  10-27 2is the conversion factor from mb to cm2 , J = cosmic r  
protons/cm -sec, in a given energy interval. ( e assu e that the 
ntri uti n f cl i eavier t  r t s lts i   d-order 
rr cti n, t  t er i s f  pl  f eV/u, t  ntri uti n f 
alpha-induced reactions is 3 important.) n  is the number of hydrogen to s i   edi , r  ,  
F
J
. = La .. f. , 
. Jl 1 
1 
here 0 ..    t   t s it  cli    nits f 
mb yiel~lng nuclide j, and fi is the abundance of i per H atom in the 
ll  edi . 
 
o 
+-
o 
I 
::c 
'-
.0 
::t. 1  
~ __ ~ __ ~-L~LL~L-__ ~ __ -L-L-L~~ 
1 .  
Y ) 
1  
i . 1. he e er  dependence f ., t e r ucti  r t  f cti  f r 
t  cli es hose s dary i l  J r  l r est. 
i . 1 s s t  al es f . s  f cti  f s i  r  r t  
er y , si   ental  l i  ndances f er  82). 
ss i  t t ost f t  r pt -excitati ns  t  t  r  t t  
via the first excited state, the most prominent 1~ines induced by ~~gh 
energy cosmic r~~ protons would be 41~4 MeV from C, 2.12 MeV from B, 
5.27 MeV f5'0m N1'1 5.18 MeV from O. In addition, the radioactive 
eca  f 0 a   i l s sitr s. i .  s s t  . l s f 
t er undant r ducts. e f t se . alues r  10  t l  
ergies, r vi i  a t t f r t  r s  J or s ce f l -energy 
cos ic ra s. 
The rate of ga a ray line e ission from a volu e  is . k, here k 
is t e ga a ra  ultiplicity from t e for ation f clibe j. he 
ga a r  lin  flu  from cli  j t rt  i : 
(per cm2_sec), 
here  i  t  ist e from t  a a r  s r  t  rt . 
• 
371
OG: 3.2-9
__ OBe
_- ,oc
V"--_ __.-- oB
°I _ / // --14C
IL I- / /.I--_"- -._--leF
I / _ / / _',--"N
_L I_ %-')°Ne)If I I _ i ll)
0.I 1.0 I0
ENERGY (GeV)
Fig. 2. The energy dependence of F. for less abundant product nuclei.
J
3. The Cosmic-Ray Induced Gamma Ray Flux from Clouds, the Galactic Disk
and from Accreting Pulsars. The flux from a large cloud is calculated,
using the properties discussed by Issa et al. (1981) i.e. we assume a
cloud mass of about 105 solar masses, or 10 _2 hydrogen atoms. A
distance of 400 pc is assumed. If one adopts Cameron's abundances for
the calculations, F=O.03 for the *_C 4.44 MeV llne. J, for the local
interstellar cosmic ray intensity is about 10/cm2-sec. With the most
conservativ_ assumptions, the flux of the 4.4 MeV carbon line is about
2xi0 -9 y/cm_-sec. However, Issa et al. (1981) noted that the cosmic ray
flux is higher by a factor of 5, especially in clouds associated with
O-B stars, where frequent supernovae occur. Furthermore, such C-B
regions should have nucleosynthetlc enhancement of elements heavier than
helium. Adopting a factor of 5 for this enhancement the flux is 5xi0 -8.
Reeves (1978), based on the general abundances of isotopes of B, has
concluded that there is a very large flux of low-energy particles. A
large low-energy cosmic ray flux coupled to the large low-energy cross
sections would further increase the 4.4 MeV gamma ray_lu_ from a large
cloud by an order of magnitude, to values near 5 x 10- /cm -sec. With a
large proton flux between 5 and 30 MeV, such as considered by Ramaty et
al. (1979), the contribution of nuclear excitations to the gamma ray
line flux becomes dominant; the 4.4 MeV gamma ray line flux could then
be boosted further by a factor of about 10.
With the "optimistic" latter assumptions, the flux from the galactic
disk (near 4 MeV) due to spallation is about 3 x 10-S/cm2-sec, and with
nuclear excitation included, about 3x10-s/cm2-sec (Ramaty et al. 1979).
The gamma ray background from the galactic disk (due largely to
bremsstrahlung) is about 10-_ photons/cm2-sec MeV at 4 MeV and about
10-2 at I MeV (O'Neill et al. 1983). This background renders the lines
from the galactic disk (near 4 MeV) difficult to observe if the fluxes
are less than 10-5/cm2-sec, unless the line is narrow (i.e. if the
recoil nuclei are produced in grains and come to rest therein) and the
instrument has an excellent resolution (about 5 keV).
Consider a pulsar beam incident on an accretion disk that is
i . . 
E 
o 
..... 
o 
I 
:::r:: 
........ 
.0 
:i. 
0.1 
371 
1.0 
ENERGY (GeV) 
OG: 3.2-9 
10 
he energy dependence of F. for less abundant product nuclei. J 
1.'.  s ic-Ray Induced Gamma Ray Flux from Clouds, the Galactic Disk  from ccreting Pulsars. The flux from a large cloud is calculated,   perties discussed by Issa et al :-(198'1")T.e. we assume a l d ass of about 10  solar masses, or 10 62 hydrogen atoms. A ce f 00 pc is assumed. If one adopts Cameron's abundances for th  l ulations, F=0.03 for the 12C 4.44 MeV line. J, for the local in .stellar cos ic ray intensity is about 10/cm2 -sec. With the most c ~rvativ2 ass ptions, the flux of the 4.4 MeV carbon line is about 2 1   y/cm -sec. o ever, Issa et ale (1981) noted that the cosmic ray flux is higher by a factor of 5, especially in clouds associated with O-B sta , w ere frequent supernovae occur. Furthermore, such C-B reg s sho  ha  n l synthetic enhancement of elements heavier than heliu . Adopting a factor of 5 for this enhancement the flux is 5x10- a • R s (19 8), based on the general abundances of isotopes of B, has co ed that there is a very large flux of low-energy particles. A larg  low-energy cosmic ray flux coupled to the large low-energy cross sections would furth  in se the 4.4 MeV gam a r ~~lu~ from a large clou  by an order of magnitude, to values near 5 x 10 /cm -sec. With a larg  proto  flux between 5 and 30 MeV, such as considered by Ramaty et ale (197 ), the contribution of nuclear excitations to the gamma ray line flux becomes do inant; the 4.4 MeV gamma ray line flux could then be boo  further by a factor of about 10. 
W th the "opti istic" latter assumptions, the flux from the galactic disk (near 4 MeV) due to spallation is about 3 x 10-6 /cm 2-sec, and with nucle  excitatio  included, about 3x10-s /cin2-sec (Ramaty et a!, 1979). The gam  ray back und from the galactic disk (due largely to brem l ng) is about 1O-~ photons/cm2-sec MeV at 4 MeV and about 10- 2 at 1 MeV (O'Neill et ale 1983). This background renders the lines from the galactic disk (near 4 MeV) difficult to observe if the fluxes are les  than 1O-s /cm 2 -sec, unless the line is narrow (Le. if the 
recoil nuclei are produced in grains and come to rest therein) and the instrument has an excellent resolution (about 5 keV). 
Con i r a pulsar beam incident on an accretion disk that is 
3 7 2 OG: 3.2-9
sufficiently thick for most particles to collide. Adopting (a)
Cameron's (1982) abundances for the accretion disk material, (b) power
10 ergs/sec per decade of energy interval, and (c) a distanceinput of sT
of I kpc, we estimate a flux of about 10-_f/cm2-sec for the 4.4 MeV _2C
spallation-induced gamma rays. (The flux is about 10 times larger if
low-energy nuclear excitation reactions are considered.) Here f is the
gamma ray suppression factor due to opacity of the accretion disk; f<1.
Table I summarizes the 4.4 MeV :_C fluxes from the sources discussed
above. The 5.1-5.3 MeV fluxes of IsO, _SN, _N are nearly as abundant.
Table I. The 4.4 MeV '2C Flux (cm-_ sec -*) from Various Sources
Dominant Inner Galactic
Reaction Large clouda Disk Accreting Pulsar a
Spallation 5 x 10-7 3 x 10-6 10-6 f
Excitation b 5 x 10-6 3 x 10-5 10-5 f
aThe assumed proton fluxes and properties of these sources are given in
_he text.Excitation dominates over spallation if low-energy cosmic rays
dominate.
4. Conclusions. Gamma ray lines due to nuclear spallation reactions
should 2be detectable with a detector that has a threshold of10-S/cm -sec from sources like the galactic disk (if the low-energy
20-50 MeV cosmic ray flux is high) and from pulsars and active galactic
nuclei that beam into accretion disks. The above detection threshold is
about 10 times lower than that of GRO/OSSE. However, if the flux
between 5-30 MeV is high, as suggested by Reeves et al. (1978) and
Ramaty et al. (1979), nuclear excitation lines will be observable from
the above sites (with the GRO/OSSE detector) as well as from supernova
remnants in clouds (Morfill et al. 1981).
References
Arnett, W.D. 1978, Gamma Ray Spectroscopy in Astrophysics, ed. T.L.
Cline and R. Ramaty, NASA Tech. Memo. 79619, P. 310.
Cameron, A.G.W. 1982, Ap. Space Sci., 82, 123. &
Clayton, D.D. 1973, Explosive Nucleosynthesis, ed. D. Schramm, and
W.D.Arnett, Univ. Texas Press, Austin, p. 263.
Clayton, D.D. 1984, Ap. J. 280, 144. C
Issa, M.R. et al. 1981, 17th ICRC (Paris), _, 150.
Mahoney, W.A. et al. 1982, Ap. J., 262, 742.
Mahoney, W.A. et al. 1984, Ap. J., 286, 578.
Morfill, G.E. and Meyer, P. 1981, 17th ICRC (Paris), 2, 56.
O'Neill, T. et al. 1983, 18th ICRC (Bangalore), 9, 45.
Ramaty, R. et al. 1979, Ap. J. Suppl., 40, 487.
Reeves, H. et al. 1978, Gamma Ray Spectroscopy in Astrophysics, ed.
T.L. Cline and R. Ramaty, NASA Tech. Memo 79619, p. 283.
Share, G.H. et al. 1985, Ap. J. (Lett.), 292, L61.
372 G: 3. -  
s fficiently thick for ost particles to collide. dopting (a) 
ameron's ( ) ndances f r t  accreti n is  aterial, ( ) po er 
i ut f 10 37 ergs/sec per decade of ener~y interval, and (c) a distance 
of 1 kpc, we esti ate a flux of about 10- f/c 2-sec for the 4.4 eV 12C 
spallation-induced ga a rays. ( he flux is about 10 ti es larger if 
l -energy uclear citati n r cti ns r  onsidered.) er  f i  t  
gan a ray suppression factor due to opacity of the accretion disk; f<1. 
able 1 marizes  .  e  12   om  urces i ssed 
a ove. he . -5.3 e  fl s f IS , 15 , I~N r  early as a undant. 
able 1. he .  e  12  l  ( -Z  I  fro  ari us ources 
ominant 
eaction 
pallati n 
xci t ti b 
a arge cloud 
er alactic 
is  ccreting pulsara 
------------------------------------------------------------------------
a he assu ed proton fluxes and properties of these sources are given in 
l5he text. 
xcitati n inates er all t   l -energy ic s 
inate. 
4. onclusions. a a ray li es due t  nuclear s allati  reacti ns 
should be detectable with a detector that has a threshold of 
O- 6 /c _  from s r es li  t  alactic is  i  t  l -energy 
20-50 e  cos ic ra  flux is high) and from ulsars and acti e alactic 
clei t t bea  i t  accreti  is s. he above etecti  t res l  is 
out  es er  at f RO/O SE. owever,    
bet een 5-30 eV is high, as suggested by eeves et a1. (1978) and 
Ra aty et al. (1979), nuclear excitation lines ill be observable from 
t  e  it  t  O/ SS  t ct r)  ell  rom r va 
nants  s orfi l t l  1). 
efer ces 
rnett, .O. 1978, Gamma Ray Spectroscopy in strophysics, ed. T.L. 
line and R. a aty, NASA ech. e o. 79619, p. 310. 
a eron, . . . 1982, ~p. Space Sc1., 82, 123. 
Clayton, D.O. 1973, Explosive Nucleosynthesis, ed. D. Schramm, and 
.O. rnett, niv. Texas Press, ustin, p. 263. 
Clayton, 0.0.1984, Ap. J. 280,144. 
Issa, .R. et a1. 1981, 17thrCRC (Paris), 1, 150. 
ahoney, W.A. et al. 1982, Ap. J., 262, 742: 
ahoney, .A. et al. 1984, Ap. J., 286, 578. 
orfill, G.E. and eyer, P. 1981, 17th ICRC (Paris), 9, 56. 
' eill, T. et a1. 1983, 18th ICRC ( angalore), .2., 45: 
Ra aty, R. et a1. 1979, Ap. J. Supp1., 40, 487. 
Reeves, H. et al. 1978, Gamma Ray Spectroscopy in strophysics, ed. 
. . li   . at ,  .  , . . 
Share, G.H. et a1. 1985, Ap. J. (Lett.), 292, L61. 


Gamma ray line production from cosmic ray spallation reactions

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