Energy distributions of the neutron dose equivalents in the 0.1 to 300 Mev interval for the Ames and Hess spectra are compared. The Ames spectrum shows no evaporation peak, moves the bulk of the flux away from the region of elastic collision and spreads it more evenly over higher energies. The neutron spectrum in space does not seem to hear out the Ames model. Emulsion findings on all manned missions of the past consistently indicate that evaporation events are a prolific source of neutrons in space

Schaefer, H. J.

English

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A NOTE ON THE REVISED GALACTIC NEUTRON SPECTRUM
OF THE "AMES COLLABORATIVE STUDY"
by
Hermann J. Schaefer,
University of West Florida,
Pensacola, Florida.
Final Report
prepared under
NASA Contract NAS 9-15417
March 1980.
N80-10002
OA %01	 N 
IRE REVISED
AM ES
 SPECTRUM CF THE
	
ES	 Unolas
C TIC NEOT&ON	 Final ReFoL^ ola.)	 G3/93 1yg02
GALA	 V STUDY	 Fen..ac CSCL 03ECOLLABORATI Eof West FloLiaa,
(University
9 P RC A02/hF A01
# Since the contract includes several subtasks,
a major part of the final results is con-
tained in three preceding reports distributed
in April 1978, November 1978 9 and September
1979. For a concise summary, the Sept 1979 re-
port should be con`ulted.
CNIJ
MkR	 ^yC.
0INTRODUCTION
preceding report (1), hereafter referred to as the 1978
t, the available information on the energy spectrum of
cosmic-ray produced neutrons in the Earth's atmosphere has
been reviewed and evaluated in terms of the dose equivalent
(DE). In the time since, new comprehensive measurements of
the galactic neutron spectrum have been reported in the "Ames
Collaborative Study" (2). The Ames spectrum modifies the Hess
spectrum (3) used in the 1978 Report inasmuch as it questions
the existence of the so-called evaporation peak. If corrobo-
rated, this finding would have important implications for
neutron dosimetry in space. To be sure, galactic radiation ac-
counts for only one part of the total neutron DE on a near-
Earth orbital mission. Neutrons from interactions of trapped
protons contribute a substantial additional share. Separate
measurements of the two fractions are extremely difficult, to
say the least. In fact, even the undifferentiated grand total
neutron DE in space has never been accurately determined. Re-
corded entirely outside the influence sphere of trapped pro-
tons, the Ames spectrum reflects the neutron build-up of the
unadulterated galactic radiation and therefore constitutes a
first step toward the eventual disentanglement of the com-
bined neutron spectrum from trapped and galactic protons in
space.
The cross sections for neutron production in coll v isions of
high-energy protons are weak and smooth functions of proton
energy and Atomic Number Z of the target material. Therefore,
the galactic neutron build-up in the atmosphere can serve,
within limits, as a model for the corresponding build
 up in
the compact matter of a vehicle in orbit. The Ames group re-
ports (l.c., 2) a neutron DE rate of 220.microrems/hour mea-
sured at 48 0 N geomagnetic latitude and 12.5 km altitude
(41,000 feet or 180 g/cm2
 residual atmosphere). Also repor-
ted is an additional dose rate of 410 microrads/hour from all
Ar
ionizing components. These values impressively demonstrate
the significance of galactic radiation in general and the
role of neutrons in it in particular.
THE GALACTIC NEUTRON SPECTRUM: THE AMTS VS. THE HESS MODEL.
Reflecting the energy spectrum of the primary radiation from
which they originate, galactic neutrons cover an extremely
wide spectrum extending from thermal to ultra-relativistic
energies. However, only fast neutrons contribute significantly
to the DE because of their high Quality Factors. accordingly,
only the upper part of the total spectrum beginning at about
0.1 Mev is of interest in the present context. Since the Ames
group has reported data only up to 300 Mev, the comparative
evaluation of the two spectral models is . -further reduced to
the interval from 0.1 to 300 Mev.
Figure 1 shows the two energy spectra over a common logarith-
mic energy scale as abscissa. We recognize at once the diffe-
rent configurations in the region about 1 Mev where the Hess
spectrum exhibits the evaporation peak whereas the Ames spec-
trum shows a smooth monotonic decline. The term evaporation
designates the most common type of nuclear interaction at
high energies in which the constituent nucleons of the target
nucleus "boil off" in a quasi-thermodynamic fashion carrying
off, in the case of neutrons, individual energies from zero
to a few Mev.
It must be mentioned that the Hess group has presented two
versions of the neutron spectrum with somewhat different
evaporation peaks. The 1978 Report evaluates the mode] with
the more pronounced peak (l.c., 3) whereas the preFjnt re-
port follows the Ames Collaborative Study and presents, in
Figures 1 and 2 9 the model with the less pronounced peak (4).
If the neutron flux per unit energy interval dN/dE is plotted
over a log scale of energy, equal areas under the curve do
not represent equal fluxes. For full equivalence of area and
2
r1
flux over the entire energy scale, we have to plot neutron
flux per unit log E rathsr+'than per unit E, i.e., dN/d(log E)
instead of dN/dE. Since d(log E) = (dE/E)(log e) where a is
the base of natural logs, the two different flux units are
connected by the relation dN/d(logE) = (dN/dE)(E/log e). Fi-
gure 2 shows the spectra of Figure 1 converted accordingly.
Equal areas under the curves now represent equal neutron flu-
xes for any energy interval and the graphs present the two
spectra in their true dimensions.
Assessment of the dosimetric implications requires conversion
of flux and energy to kerma and DE. For this step, we apply,
to the two spectra, the flux/DE relationship for neutrons re-
commended by the National Council on Radiation Protection and
Measurements (5). Numerical integration 'over the energy in-
terval from 0.1 to 300 Mev furnishes grand total DE rates of
380 microrems/hour for the times spectrum and 510 microrems/
hour for the Hess spectrum. In comparing the two radiation
levels, one should be aware of the above mentioned fact that
the Ames study defines the spectrum only up to 300 Mev. Exa-
mining the trend of the spectra, we recognize that the Ames
spectrum, quite differently from the Hess spectrum, strongly
suggests a substantial additional contribution to the trial DE
from the undefined region above 300 Mev. In other words, the
DE rate of 380 microrems/hour appears to fall short of the
true value by a sizeable margin whereas the DE rate of 510
microrems/hour for the Hess spectrum should approach the true
value closely. The two models, then, actually yield more
closely matching DE rates than evaluation of the limited in-
terval from 0.1 to 300 Mev indicates. To boure, behind this
equality of the DE's as such, a basic difference in the
energy regions furnishing the DE's remains hidden. For the
Hess spectrum, the bulk of the DE originates in a comparati-
vely narrow energy interval centering on about 1 Mev. For
the Ames spectrum, a much broader interval extending from a
few to several hundred Mev furnishes the DE. Taking the total
3
DE rates of 380 and 510 microrems/hour for the two models
at face value and normaliAng to 100 per cent, we show in
Table I their distributions in four sections of the total in-
terval from 0.1 to 300 Mev. The basic difference of the two
distributions shows up quite clearly even in this coarse
breakdown.
TABLE I
Energy Distributions of the Neutron Dose Equivalents
in the 0.1 to 300 Mev Interval for the
Ames and Hess Spectrum
Energy Interval,
Mev
Per Cent of Total
Dose Equivalent,
Ames	 Lass
0.1	 - 1 10 30
1	 - 10 25 48
10 - 100 45 17
100 - 300 20 5
CONCLUSIONS
The particular problem of neutron dosimetry in space centers
on the limited section of the spectrum where recoil protons
of short range and very high LET produced in elastic colli-
sions account for the energy transfer. The critical energy
interval extends from a fraction of 1 to some 10 Me^ eWe re-
cognize in Figure 2 that the interval coincides with evapora-
tion peak of the Hess spectrum. Beginning at about 15 Mev,
nuclear interaction overtakes elastic collision as the most
probable event hence short-ranged recoils are no longer the
predominant agents of energy dissipation. Examining Figure 2
once again we see that the Ames spectrum, showing no evapora-
tion peak, moves the bulk of the flux away from the region of
4
elastic collision and spreads it more evenly over higher ener-
gies. The crucial point is';that the neutron spectrum in space
does not seem to bear out the Ames model. Emulsion findings
on all manned missions of the past (6) consistently indicate
that evaporation events are a prolific source of neutrons in
space.
The assumption is near at hand that the high frequency of eva-
poration events in space is due to trapped protons producing a
Hess-type spectrum superimposed on an Ames-type spectrum with-
out an evaporation peak. Since the spectrum of trapped protons
centers on mach lower energies than the galactic spectrum, it
favors the production of evaporation over knock-on stars and
the corresponding spectra of secondary neutrons should indeed
differ in the indicated way. Disentangling the two spectra in
the integral response of a passive dosimeter such as nuclear
emulsion poses insurmountable difficulties. Recording separate-
ly, with active instrumentation, the combined spectra in the
Anomaly and the pure galactic spectrum outside seems a better
approach. Although such separate measurements would not be re-
quired for the exposure records of personnel, they appear in-
dispensable for defining the sensitivity and response charac-
teristics of a passive dosimeter. The task is formidable: In a
heavy background with an LET spectrum extending far beyond the
LET of recoil protons, a neutron DE is to be recorded which ac-
crues from two sources with instantaneous DE rates strongly and
independently varying over the orbital cycle.
REFERENCES
1. Schaefer, H. J. Personnel Neutron Monitoring in Space.
Faculty of Physics, University of West Florida. Pensacola,
Florida, 1978.
2. Hewitt, J.E. and Hughes, L., Baum, J.W. and Kuehner, A.V.,
McCaslin, J.B., Rindi, A., Smith, A.R., Stephens, L.D.,
and Thomas, R.H., and Griffith, R. V. Ames Collaborative
Study of Cosmic-Ray Neutrons: Mid-Latitude Fligi-ts.
Health Phys. 349 375 - 384, 1978.
5
3. Patterson, H.W., .Hess,	 W.N., Moyer,	 B.J., and Wallace,
R.W.	 The Flux and Spectrum of Cosmic-Ray Produced Neut-
trons as a Function of Altitude. 	 Health Phys, 2, 69 -
72,
	
1959.
4. Hess, W.N., Patterson, H.W., and Wallace, R.W.	 Cosmic-
Ray Neutron Energy Spectrum. 	 Phys. Rev. 116, 445 - 457,
1959•
5. Protection against Neutron Radiation.	 National Council
on Radiation Protection and Measurements. Report 38.
Washington, D.C., 	 1971.
6. Schaefer, H.J.	 A Combined TLD/Emulsion Method of Sampl-
ing Dosimetxy Applied to Apollo Missions. Faculty of Phy-
sics, University of West Florida. Pensacola, Florida,
1979.
6
IN
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	 300
LOG LOG PLOTS OF GALACTIC NEUTRON
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7
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ADJUSTED
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LIN LOG PLOTS OF GALACTIC
SPECTRA OF AMES AND HESS
H ESS
AMES
FIGURE: 2.
8


A note on the revised galactic neutron spectrum of the Ames collaborative study

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