A brief survey is made of our present knowledge of the composition and energy spectra of the primary cosmic radiation. The total energy carried by all forms of cosmic radiation that have appreciable penetrability into matter has been evaluated

Waddington, C. J.

English

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The Chemical Composition and Energy Content of the Energetic Cosmic Radiation*
C. J. Waddington
School of Physics and Astronomy
University of Minnesota
A brief survey is made of our present knowledge of the composition
and energy spectra of the primary cosmic radiation. The total energy
carried by all forms of cosmic radiation that have appreciable pene-
trability into matter has been evaluated. This information, when
combined with a knowledge of the rate at which the different components
deposit energy in traversing matter, would permit calculations of the
radiation dose that would result from exposure to the primary cosmic
radiation. It is concluded that overall these radiation effects are
rather small but it is emphasized that local damage can be of much
greater significance.
Introduction.
Any detector or object placed outside
the protection of the earth's atmosphere
i_ irradiated by cosmic electromagnetic
and corpuscular radiation that can pene-
trate deeply into the object. The
radiation effects produced by this
exposure depend critically on the
physical and biological conditions in-
volved and are one of the main topics of
this conference. In this paper I present
some of the data concerning the nature of
this cosmic radiation that is relevant to
any calculations of the magnitudes of
these effects. Specifically I have
summarized our current knowledge of the
primary cosmic radiation and have dis-
cussed some of the factors that should be
of importance. In what follows I have
neglected radiation of solar origin and
that present in the radiation belts,
since these will be discussed by other
authors.
The majority of the cosmic radiation
is corpuscular in nature and is con-
sequently subject to the effects of
solar and geomagnetic modulation. Geo-
magnetic modulation is relatively well
understood in that at any point in space
within the magnetosphere, the geomagnetic
field simply imposes a cut-off rigidity
b_low which particles coming from a
particular direction cannot penetrate.
Solar modulation has been extensively
studied as a temporal phenomena and
examples of the resulting variations
in particle intensities are given later.
However, spatial studies have been less
successful and at present we do not
really know either how the intensities
vary throughout the solar system, nor
what they are in interstellar space.
Measurements of the cosmic ray gradient
have been conflicting and we have to rely
on theory to calculate the demodulated
spectra. Fortunately these solar
*Work supported by the Office of Naval
Research under Contract
N00014-67-A-0113-0021.
209
modulation processes mainly affect the
lower energy particles and therefore it
is probable that the total energy content
of the cosmic radiation is not seriously
influenced by these uncertainties.
In addition to the corpuscular
radiation, there are also X and y-ray
components and these will not be
appreciably temporally or spatially
dependent unless some cosmic cataclysm
occurs, such as a nearby super-novae
explosion. In what follows I will first
outline our current knowledge of the
Composition and energy spectra and then
evaluate the amount of energy carried by
each component. A knowledge of the
incident energy, combined with a know-
ledge of the absorption characteristics,
permit an evaluation of the gross, or
average radiation effectiveness of any
particular cosmic ray component. Such
an averaging approach does, of course,
neglect the fact that when we consider
the cosmic radiation we are concerned
with particles that have a spectrum of
energies and a small fraction in any
component will be of extremely high
energy. Indeed, in fact, a single
particle may have up to 10 _ ergs.
These particles, or quanta, have the
potential of delivering all or most of
their energy into a very small volume,
producing localized radiation effects of
much more serious consequence than those
suggested from the overall level. A
similar phenomena can be produced by
the highly charged particles in the
cosmic radiation, which because of the
Z 2 dependence of the ionization energy
loss, can also deliver a large amount
of energy into a very localized volume.
The effects produced by these large local
radiation doses on biological or solid
state systems can be much more serious
than would be inferred from the values
for the average doses.
X and y-Rays.
The emission of cosmic X-rays has been
observed from various point sources and
as a diffuse background of galactic or
metagalactic origin. High energy,
https://ntrs.nasa.gov/search.jsp?R=19720009977 2020-03-17T03:48:05+00:00Z
E > 50MeV,y-ray observations have gen-
er_lly shownthat the energy spectra
observedat X-ray energies either
extrapolate sensibly or steepenat higher
energies. It is thus not unreasonableto
integrate the observeddifferential X-ray
spectra to infinity to obtain an upper
limit to the energy input. In all cases
the energy intensities are rather small.Table I showsthe values for the in-
tensities above10 KeVfor the repre-
sentative point sources ScoX-I and
TauX-I as well as for the isotropic
background. Clearly, irrespective of
howthe energyis deposited the average
radiation effects must be small although,
once again, individual energetic gamma
ray photonsmayproducelarge local
effects.
Table I
(x-Ray Incident Energies)
Tau X-I (Peterson (1970)).
dN 20 E-2"3_E 2= photons/cm .sec.keV
I(>E) = 67_ -0"3 keV/om2.sec.
I(>10 keV) = 4.6 x 10 -3 ergs/cm2,day.
Sco X-I (Peterson (1970)).
I _ II0 e -E/4'3 keV/cm2.sec.keV.
I (>E) = 470 e -E/4"3 keV/cm2.sec.
I(>10 keV) = 6.3 x 10 -3 ergs/cm2.day.
has collected various estimates which
suggest that probably the energ[ density
does not greatly exceed 3 eV/cm=,i.e.
about an order of magnitude greater than
that of the corpuscular cosmic radiation.
(Note that for ultra relativistic
particles i00 ergs/cm2.sr, day =
0.3 eV/cmS.)
Charged Particles.
i) Electrons.
The true spectrum of the electrons
in the cosmic radiation is somewhat
controversial at the present time.
Figure 1 shows the spectra reported in
a recent paper, Marar et al. (1971), from
which it can be seen that there are at
least two plausible representations of
the true spectrum at energies above
5 GeV, which differ in intensity by at
least a factor of five. At lower
energies the situation is equally
complicated, although in this case the
principal cause appears to be the effects
of solar modulation rather than experi-
mental inconsistencies. Figure 2 shows
a schematic representation of the data
available in this energy region. These
results permit us to evaluate the in-
cident energy carried by the electron
component. The resulting energy intensity
spectra for the various cases are shown
in Figure 3.
Diffuse Background (Schwartz (1969)).
dN l = 10 E-1"5 dE photons/cm2.sec.sr.keV
for E < 25 keY.
dN 2 = 225 E -2"5 dE photons/cm2.sec.sr.keV
for E > 25 keY.
I(>E) = 20 (5-EI0"5) + 450 E2-0"5 keV/cm2.sr.sec.
I(>10 keV) = 1.75 x 10 -2 ergs/cm2.sr.day.
(A possible bump in the spectrum at 1-5 meV, Vette et al. (1970),
would only raise I (>i0 keY) by a few percent).
Here, as in the remainder of this
paper, the energy intensities are
expressed in units of ergs/cm2.day or
ergs/cm2.sr.day. Remembering that a Fad
corresponds to the absorption of 100 ergs
per gram of irradiated material, one
could calculate the radiation effects
from these energy intensities, if one
knew the absorption rates as a function
of energy. Of course this is a big if
and represents one of the principal
problems in this field.
Neutrinos.
For completeness it is appropriate to
discuss neutrinos here, since they are
certainly a penetrating form of cosmic
radiation. Obviously their extreme
penetrability implies negligible energy
deposition and hence, unimportant
radiation effects. This is fortunate
since we know very little regarding the
flux of cosmic neutrinos. Burbidue {1970)
210
DANIEL et. al.
d L ANAND et. aL 1968
t
oJ ANAND et. ol. 1969
• MARSOEN et. al. •
* ORAN et. al. •
a RUBSTOV et. al. •
• AGRINIER et. al..
o ROGKSTROH et. aL
• BLEEKER et, ol.
• NISHIMURA et. oi.
SPARK CHAMBER
® LINE SCAN
• AREA SCAN
IOO
ELECTRON ENERGY IN GeV
IOOO
Figure i. The integral energy spectrum of primary cosmic ray
electrons above about 2 GeV, as measured by various
authors. For references see the original paper by
Marar et al. (1971).
_ 1.0
w
_z
>-
5
z 0.2
0
z
I--
_z
SOLAR
MAX.
L OW
E-2.8
I() 2 IO -I I.O
ENERGY IN GeV
Figure 2. A schematic representation o£ the integral energy
spectrum of low energy cosmic ray electrons as
measured at times typical of minimum and maximum
solar modulation.
I0.0
' ' ' ' ''1 1
ELECTRONS_
(HIGH)
ELECTRONS_
( LOW ) _,,._ _ -
...... J , , _ , , , ,=J
o.I I.O
ENERGY IN GeV
Figure 3. The integral energy intensitles for cosmic ray
electrons at _xim_ and minim_ solar _dulation
end ass_ing either the high or l_ spectr_ given
in Figure i.
IO.0
2) Hydrogen and Helium Nuclei.
The energy spectra of these nuclei
have been extensively studied by many
workers. Figure 4 shows the differential
energy spectra at times typical of solar
minimum and solar maximum as recently
compiled by Lezniak and Webber (1970) from
selected data. At higher energies,
> 2 GeV per nucleon, the spectra of both
_omponents can be well represented as a
power law in total energy with
(T+moC2)-2"5dE. If the kineticdJ=K
energy per nucleon T, is expressed in GeV
per nucleon then K=4500 for protons and
400 for s-particles. At very high energies,
> i0 *s eV, deviations do occur from these
Epectra, with apparently a steepening to
an exponent of about _.0, followed at
around i0 le eV by a flattening to the
original exponent. However, the total
energy carried by these energetic
particles is a negligible fraction of
the total.
Both components contain small frac-
tions of isotopes other than the main one.
Deuterons make up 1 or 2% of the total
hydrogen component while about 10% of the
helium component is probably He 3. In
both cases We only know these proportions
at low energies, < 500 MeV per nucleon,
but there seems little reason to expect
that they would be much larger at higher
energies.
3) Heavy Nuclei.
The energy spectra of the cosmic ray
nuclei in the range between lithium and
nickel, 3 < Z < 28, have been studied by
a large number--of workers and detailed
comparisons exist between these spectra
and that of the helium nuclei. For
example, Figure 5 is from a recent review,
Waddington (1970) and shows the ratios of
the helium abundance to those of various
groups of heavy nuclei expressed as a
function of energy. It can be seen that
while there is some suggestion that at
least some of these ratios are energy
dependent below about 1 GeV per nucleon,
it is not unreasonable to use as a working
assumption th_ concept that all nuclei
have similar energy spectra. The apparent
deviations from this that are seen do not
seriously affect our estimates of the
total energy carried by each component.
At very high energies, > 1016 eV/nucleon,
composition changes do _ccur that probably
result in all the particles above =1018 eV
being protons. The value assumed here
for these ratios are given in Table II,
together with an estimate of the-mean mass
number in each group. From these values
we can calculate the incident energies
carried by each component. As an example,
the table also gives the total incident
energy carried by nuclei with T _ i00 MeV
per nucleon.
211
>.
=
%
_o
_z
....... I ' ''I
HYDROGEN
$ MIN
i(_ 2 ........ I ........ I
162 16j I
ENERGY IN GeV PER NUCLEON
Figure 4. The differential energy spectra of hydrogen _d
h,li_ nuclel below about 2 GeV per nucleon a=
=ea,urea by ,sve=al groups. After Lezni_ _
Webber (1970).
that those nuclei that we believe are
predominately the consequence of secondary
production during interstellar propa-
gation from the 'source' of cosmic rays
to show energy dependent abundances, since
we know that the nuclear parameters des-
cribing their production are energy
dependent.
Energy Intensities.
The data summarized in the previous
sections has been used to calculate the
energy intensities, both integral and
differential, carried by each major
component of the cosmic radiation. The
enerqy spectra of these intensities are
shown in Figures 5 and 6. For comparison
it is possibly relevant to note that the
total energy intensity of cosmic ray
particles at sea level and high latitudes
is about 5 ergs/cm_.sr.day, Hayakawa
(1969), or 3-4% of that above the
atmosphere. Although the radiation dose
due to sea level cosmic rays is about
30 mrem/year, this cannot be taken as
implying that the space cosmic ray dose
is just 25 to 30 times greater. First of
all there is the factor due to the iso-
tropy of space cosmic rays compared to
the non-isotropy of those at sea level.
Secondly, and more important, is the
greatly different nature of the particle
TABLE II
Charge Group Abundance Ratio _ I (>0.1 GeV/n)
Relative to Ergs/cm2.sr.day
Helium
L-nuclei 3 _ Z _ 5 1/48 9 1.53
M-nuclei 6 _ Z _ 9 1/16 14 7.07
LH-nuclei I0 < Z < 14 1/75 22 2.38
MH-nuclei 15 _ Z < 19 1/600 35 0.475
VH-nuclei 20 < Z < 30 1/200 52 2.12
SVH-nuclei Z _ 30 1/8 x 105 _i00 1 x 10 -3
Helium Nuclei 32.5
The abundances of individual elements
are still imperfectly known, with the
uncertainties generally increasing at
higher charges. A recent survey by
Shapiro and Silberberg (1970) probably
represents the best values currently
available, although several of the
abundances quoted are still controversial
and the values are somewhat inconsistent
with the ratios quoted above. Table III
shows the abundances reported by the
above authors, normalized to carbon=100.
These values should be regarded as being
typical of those for energies between
2-5 GeV per nucleon and are uncorrected
for the generally small effect of geo-
magnetic cut-offs on nuclei of different
Z/A ratios. Physically we would expect
212
TABLE III
(After Shapiro and Silberberg (1970))
Abundances of HeaVy Primary Nuclei at the Top of the Atmosphere
(Normalized to Carbon = I00)
Element Z Relative Element Z Relative
Abundance Abundance
Lithium 3 16 + 2 Sulphur 16 3.5 + 1
Beryllium 4 ii + 3 Chlorine 17 0.5 + 0.3
Boron 5 27 + 3 Argon 18 2 + 0.5
Carbon 6 100 Potassium 19 0.6 + 0.3
Nitrogen 7 27 +_ 2 Calcium 20 2 + 0.3
Oxygen 8 86 + 4 Scandium 21 0.3 + 0.2
Fluorine 9 2 + 1 Titanium 22 2.0 + 0.5
Neon 10 20 + 2 Vanadium 23 1.0 + 0.3
Sodi_u ii 3 + 1.5 Chromittm 24 3.5 + 1.0
Magnesium 12 21 + 2 Managanese 25 0.9 + 0.3
Aluminum 13 2 + 1 Iron 26 11.3 + 1.4
Silicon 14 15 + 2 Cobalt 27 <0.2
+ 1.4
Phosphorous 15 0.6 - 0.5 Nickel 28 ~0.2
300 I I
_J
20(]
z
>-
J
o
w I00
-r
o
E
VH-NUGLEI
VON ROSENVINGE: et. al.
( 1969 )
FREER & WADOINGTON(1969)
LH -NUCLEI
M -NUGLEI
i I
102 103 104
KINETIG ENERGY MeV/NUGLEON
Figure 5. Measured values of the ratios of the abundance of
helium to those of various groups of heavier nuclei,
Waddington (1970).
I00
%
_" I0
z
w
I
<
t63
3 x IO4
...... i
S. MIN.
TOTAL
S. MIN.
P
S. MAX.
S. MIN.
" S. MAX.
...... I
VH
,o'
VVH
, ..... I ,I
O.I LO
ENERGY IN GeV OR GeV PER NUCLEON
Figure 6. The integral energy intensities for cosmic ray
nuclei.
I0.0
213
%I0
z
w
-- O_
I J
TC
Y_. ROTONS
HEAVY NUGLEI GIVEN BYHELIUM _ HIGH
flZ) = 4.7 x 10 -2 L-NUCLEI
• 236 x IO -I M- NUCLEI
• 7,5 x 10 -2 LH-NUCL£1
= 1.46 x 10 -2 MH-NUCLEI
=3.I x _o*SvvH-NUCLEI
ELECTRONS
l , , , ,,,,,P , , , , ,,,
0,1 1.0
EN£RGY IN GeV OR GeV PER NUCLEON
I0.0
radiation at the two locals. At sea
level the particles are predominately
muons, and the rate of energy deposition
is small, while in space the deposition
rate is considerably higher. This rate,
which of course determines the dosage
received by a sample immersed in the
radiation, depends in a complex way on
the charge, energy and nucleon interaction
parameters of the incident particle. How-
ever, in no case can one envisage it being
so high that the total energy of a compo-
nent would be dissipated in traversing as
little as one gram of material. Hence
the average cosmic ray doses must always
be very small, never exceeding a few
rad/day. The localized dose, on the other
hand, as pointed out before, may well be
very considerable, since a single particle
may release a great deal of energy in a
small volume. As an extreme example, a
nucleus of U 23e having a total kinetic
energy of 55 GeV has a range of 1 g/cm 2
in carbon of density 2.0 gm/cm 3. This
energy is mostly dissipated by producing
electrons of less than i0 keY that are
absorbed within 10Um of the primary
trajectory and hence in a volume of
1.5 x 10- cm . The energy deposited is
thus equivalent to about l0 s ergs/gm and
gives a localized dose of 103 rads, which
is sufficient to produce serious damage
in many systems.
While this is admittedly an extreme
example it is clear that these highly
charged nuclei may well produce a line
of damage. Similarly very energetic
particles can produce either small
volumes of damage, by nuclear inter-
actions, or lines of damage, by pro-
duction of cascades, nuclear or electro-
magnetic. The effects of producing
these localized regions of damage in
radiation sensitive systems do not appear
to be well known and it seems important
that further research should be under-
taken to clarify our understanding of
these problems.
REFERENCES
i. Burbidge, G.: Intergalactic Matter and Radiation, Invited paper
presented at I.A.U. Symposium No. 44, 1970.
2. Freier, P. S.; and C. J. Waddington:Phys. Rev.,vol. 75, 1968, p. 1641.
3. Hayakawa, S.: "Cosmic Ray Physics" pub. Wiley-Interscience,1969, p.437.
4. Lezniak, J.A.; and W.R.Webber: The Solar Modulation of Cosmic Ray
Protons, Helium Nuclei and Electrons, Univ. of New Hampshire preprint
UNH-70-06, 1970.
5. Marar, T.M.K.,; P.S.Freier; and C.J.Waddington: J. Geophys. Res.
vol. 76, March 1971.
6. Peterson: "Properties of Individual X-Ray Sources Non-Solar X and
Gamma-Ray Astronomy" Ed. L. Gratton, Reidel Publ Co., 1970, p. 59-80.
7. Schwartz, D. A.: "The Spatial Distribution of the Diffuse Component
of Cosmic X-Rays" Ph.D. Thesis, U of Calif.,San Diego, 1969.
8. Shapiro, M.M.; and R. Silberberg: Ann. Rev. of Nuclear Phys.
vol. 20, 1970, p. 323.
9. Vette, J.I.; D. Graber; J. L. Matteson;and L. E. Peterson: Ap. J.
Letters, vol. 160, 1970, p. LI61.
10. von Rosenvinge, T. T.; W. R. Webber; and J. F. Ormes: Astrophys.
and Space Sci., vol. 5, 1969, p. 342.
ii. Waddington, C. J.: "Some Remakrs on the Composition of the Cosmic
Radiation" Proc. of the Sixth Interamerican Seminar on Cosmic Rays
and U of Minn. Tech. Rep. CR-150. 1970.
214


The chemical composition and energy content of the energetic cosmic radiation

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