An experiment to measure neutrons in the upper atmosphere was performed on a balloon flight from Palestine, Texas, at an altitude of about 32 km. The experimental arrangement is discussed briefly, and results of a preliminary analysis of the data for neutrons in the energy range 3 to 30 MeV are given

Delorenzo, J. T.

Love, T. A.

Mcnew, C. O.

Zobel, W.

English

NASA Technical Reports Server

NEUTRON SPECTRAL MEASUREMENTS IN THE UPPER ATMOSPHERE I
W. Zobel
T. A. Love
J. T. Delorenzo
C. O. McNew
Oak Ridge National Laboratory
Oak Ridge, Tennessee 38930
An experiment to measure neutrons in the upper atmosphere has been
performed on a balloon flight from Palestine, Texas, at an altitude of
about 32 kin. The experimental arrangement is discussed briefly, and re-
sults of a preliminary analysis of the data for neutrons in %he energy
range 3 to 30 MeV are given.
The decay of neutrons, produced by the inter-
action of cosmic rays in the atmosphere and subse-
quently leaving it (so-called "cosmic-ray albedo
neutrons" ), was suggested initially by Singer
(ref. l) as a mechanism to explain the observed
protons in the Van Allen belt. Hess et al. (ref. 2)
had measured the spectrum of neutrons in the atmos-
phere, and these measurements were used as the basis
for subsequent calculations designed to test the
"Cosmlc-Ray Albedo Neutron Decay" (C._AND) theory
(refs. 3-5). Additional measurements of neutrons
(refs. 6-9) at high altitudes appeared to confirm
the spectrum to about 20 MeV since calculated ex-
trapolations from these measurements agreed with
the calculated leakage flux of Newkirk (ref. 4)
and Lingenfelter (ref. 5) which use the measure-
ments of Hess et al. (ref. 2). Note that the ex-
periments referenced in 6-9 arrive at the "spectrum"
by assuming a spectral shape and converting it into
the measured quantity, either pulse-height spectra
(refs. 6-8) or count rate (ref. 9).
Calculations of the proton injection into the
belt from CRAND neutrons by Dragt et al. (ref. i0)
using Lingenfelter's estimate for the neutron
source, indicated, however, that the CRAND source
is too small by a factor of 50 for particles with
energies greater than about 20 MeV. Later measure-
ments of the proton spectrum in the belt for pro-
tons above' i0 MeV appear to confirm this discrep-
ancy (refs. 11-12). While a recent paper by Farley
et al. (ref. 13) goes far toward resolving the
A Research funded by the U. S. Atomic Energy Com-
mission under contract with Union Carbide Corpora-
tion; flight costs and telemetry support provided
by the NASA Office of Space Science and Applica-
tions and the George C. Marshall Space Flight
Center.
discrepancy, it too uses Lingenfelter's spectrum
with the observation that experimental verification
of this spectrum for energies above i0 MeV is
lacking.
It is unfortunately true that the measurements
by Hess et el. were performed with counters which
had minimal sensiti-v-ity in the energy region from
roughly lO to 100 MeV. It is in this very region
that the greatest discrepancies are observed. A
program was therefore started in 1966 at the Oak
Ridge National Laboratory to reduce the uncertainty
in the neutron spectrum at high altitude in the
energy region from 2 to 60 MeV, the upper limit
being chosen because of the availability of detector
calibration facilities to this energy, and the lower
limit following from the experimental scheme em-
ployed. The data-gathering program culminated in
a balloon flight on 17 June 1969 from Palestine,
Texas. The device floated at an altitude of
31,950 • 200 m (corresponding to about 9 g/am s
residual atmosphere.) for about 9-1/2 hours. This
paper will describe the instrument used and give
the results of a preliminary analysis of the data
obtained from this flight.
The heart of the spectrometer is a liquid
scintillator (Nuclear Enterprise s Inc. NE-_I3) in
which neutron interactions are detected by measuring
the light pulses produced by recoil protons. Gamma-
ray interactions in the scintillator result in
pulses which can be separated with pulse-shape
discrimination (PSD) techniques. Since incident
charged particles can give rise to pulses similar
to those from neutron interactions in the detector,
the latter is enclosed in a plastic scintillator
"mantle"_ of NE-110 (Nuclear Enterprises, Inc.,
San Carlos, California). The output from this
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“mantle“ is usually put in anticoincidence with the 
output from the liquid scintillator, thus preventing 
any counts due to events which trigger both detec- 
tors. The arrangement used is shown in figure 1. 
The NE-213 is contained in an aluminum can which is 
mounted on an RCA Type-8575 photomultiplier tube. 
The mantle is also viewed by an 8575 tube. 
bottom of the assembly can be seen the preampli- 
fier used with the NE-2l3 detector and the high- 
voltage parer supplies for both tubes. 
shown on the left is 1/16-in.-thick aluminum, used 
as a pressure vessel to maintain the assembly at 
ground pressure to eliminate high-voltage sparking. 
The aluminum can was then surrounded by about 1 4 2  
in. of foam insulation to maintain proper operating 
temperature in the assembly. 
At the 
The can 
F’IGUR3 1.- Detector Assembly. 
In order to reduce locally produced background, 
we decided to laver the detector assembly after 
launch about 90 ft below the gondola on which were 
mounted the electronics and batteries as well as 
miscellaneous support equipment. Since it was im- 
practical to retract the detector assembly to the 
gondola prior to landing, the foam insulation was 
in turn contained in a 16-gauge sheet-steel can 
designed to take the landing impact. 
effectiveness of this arrangement was obtained when 
-,recovered the equipment and it showed no de- 
terioration in performance. 
Proof of the 
A block diagram of the electronics used in 
this experiment is shown in figure 2. 
tions must be distinguished: 
crimination, and pulse-height analysis. 
As can be seen from figure 2, the pulse-height 
Two f’unc- 
pulse-shape dis- 
analysis was conventional. The major item of note 
here is that the discriminators had their thresholds 
adjusted so that the pulse height to be antdyzed 
was determined by the threshold of the zero-crossing 
discriminator. 
U 
FIGURE 2.7 qock Diagram of the Electronics. 
The pulse-shape discrimination was based on 
the difference in the base-line crossing time of 
the amplifier output pulses due to the different 
relative amounts of slow light output from proton 
and electron recoils. The crucial feature of such 
a scheme lies in the proper adjustment and stability 
of the timing involved. 
determined by the input signal to the fast dis-  
criminator connected to the NE-213 photomultiplier, 
and this signal eventually was used as the start 
signal to the time-to-amplitude converter (TAC). 
The TAC stop signal was provided by the zero cross- 
ing discriminator after an arbitrary delay chosen 
so as to put the desired signals into the middle 
of the TAC range. 
The time of an event was 
923 
TheTACoutput was also digitized in a separate
analog-to-digital converter (ADC); in addition, a
single channel analyzer used the TAC output to set
a bit in the telemetry whenever the TAC signal ex-
ceeded a predetermined level corresponding to pulses
due to incident neutrons. For each event, then,
the output to the telemetry consisted of the output
from the two ADC's and two flags, one to identify
the PSD ADC and the other to distinguish neutrons
from gamma rays. A separate telemetry channel
carried the information from the 8-channel multi-
scaler shown; yet another telemetry channel, not
shown, carried housekeeping information such as
pressures, voltages, and temperatures.
A preliminary analysis of the data has been
performed, based on the hardware-set bias for the
pulse-shape discrimination. A pulse-height spectrum
of the events so ascribed to incident neutrons is
shown in figure 3. While a few pulses can be seen
extending to about channel 230 (corresponding to
50 MeV incident neutrons), most of the spectrum
lies below channel 130, or about 30 MeV.
The analysis performed on these data makes
use of the calibration of a similar NE-213 detector
by Verbinski et al. (ref. 14) and a spectrum un-
folding scheme due to Burrus (ref. 15 ). The cali-
bration of the detector was extended to about kO
MeV (ref. 16), and a slightly different version of
the unfolding code (i.e., FEED rather than FERDoR)
was employed. The results of this analysis are
shown in figure 4. Also shown on the figure are
the results of the group at New York University
(ref. 8) in the 2 to lO MeV energy range.
Several points must be made in connection
with this figure. The first and most important is
that it is still preliminary; for example, no
attempt has been made to estimate in any detail the
required correction for locally produced background
neutrons. A very crude, zeroth-order calculation
indicates that such a correction should not amount
to more than 25% of the total number of counts ob-
served, but no detailed calculation to determine
the spectral shape of this correction has yet been
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FIGURE 3.-- Pulse-Height Spectrum from Neutron
Events.
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ORNL- DWG 7t-t9t0
I I I IIIII I I I
NYU DATA
I ORNL DATA
I I I till [ il[ lllII I I
2 5 t0 20 50 t00 200
NEUTRON ENERGY(MeV)
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m
m
1
500
FIGURE 4.--Neutron Energy Spectrum at _9 g/cms
Between 95.5 ° - 100.7 ° W, and 31.3 ° 31.8 ° N.
92/_
performed. A second point to note is that the
analysis procedure results only in the determination
of a 6_ confidence interval within which the result
can be expected to lie. It is for this reason that
no data points are shown but only the limits of
this confident interval. The width of the interval
is due not only to the statistical deviations ex-
pected from the input data but also includes the un-
certainties associated with the unfolding process
except those associated with the response matrix.
For the comparison with the NYU data it must be
remembered that their data were obtained during a
period of solar minimum, whereas our data were
taken during solar maximum. One would, therefore,
expect their results to lie above ours, by perhaps
25¢.
A comparison with the calculations of Linger-
felter is best accomplished by comparing our re-
sults with his in the form of an integral over a
common energy region. Part of his calculations
yield 0.106 neutrons-cm-e-sec-l-MeV -l for the
integrated flux from 5.16 to lO MeV at 8 g/cm 2
residual atmosphere for 40 ° geomagnetic latitude
(ref. 17). Our preliminary data, integrated from
3 to i0 MeV for 9 g/cm 2 residual atmosphere at
about 41° latitude give an upper limit of 0.155
and a lower limit of 0.142 neutrons/cm-S-sec -1-
MeV- i.
Further analysis of the data, as well as an
attempt to calculate the neutron spectrum to be
expected, is in progress.
We wish to gratefully acknowledge the vital
contribution to the project made by Mr. H. W. Par-
ker and his associates from George C. Marshall
Space Flight Center, who furnished and operated the
telemetry equipment, and, of course, the invaluable
cooperation of the personnel at the NCAR Scientific
Balloon Facility, Palestine, Texas, who conducted
the launch, flight, and recovery operation. Mr.
R. H. Baldry, formerly with the Laboratory, designed
and supervised construction of the necessary inter-
facing to the telemetry equipment. Thanks are also
due Dr. R. B. Mendell of New York University for
making available their data prior to publication,
and to Dr. R. E. Lingenfelter for use of his un-
published results.
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Rev., vol. ll6, 1959, P. h45.
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Neutron spectral measurements in the upper atmosphere

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