Progress of the study of cosmic ray neutrons is described. Data obtained aboard flights from Hawaii at altitudes of 41,000 and 45,000 feet, and in the range of geomagnetic latitude 17 N less than or equal to lambda less than or equal to 21 N are reported. Preliminary estimates of neutron spectra are made

Hewitt, J. E.

Hughes, L.

Mccaslin, J. B.

Smith, A. R.

Stephens, L. D.

Thomas, R. H.

English

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LIST.-6738
11C- 34b
TID-4S00-R66
M A S l" ERl
MfI:S COLLA )RATM: S'11MY Ol :
 C0SMI C- RAY
IrTafl't2pNS. H. LOW- AND MID-LATI'l MI : ITMII'S
L. D. Stephens, J. 13. McCaslin, A. R. Smith,
R. 11. Thomas, J. F. Hewitt, and L. Ihighes
A1arch 1, 1978
Prepared for the U. S. Department of L.nergy
imder Contract W-740S-tNG-48
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-2-
"The untenzed kodmoe my abode
Ipa,aa, a w. t6ut 4tkmlgen;
My mi6tte6.6 atilt the open hoad
And the bn i.gh t eyea o6 dangeA. "
— from Youth and Love
Robert Louis Stevenson
1850-1894
I .	 1 N'fROD11C I WN
'Pic collaborative stLxl y to more precisely define the pronertics
of the cosmic r,i y produced neutron spectrum 1mve been nreviouzly
described in detail [He761 and preliminary results obtained from three
flights at northern mid-geon-ognetic latitudes (30°N - a , 48°N) puhl ishcd
[He78j .
In this second report of experimental data, the data ohtained from
six flights ,mde in the period August - October 1975 are descrihed and
discussed. All data were taken aboard the NASA Airborne Observatory,
a C-141 aircraft, haled at tii^ Anxs Research Center.
2.	 SUMMARY OF FLIGHT PATIlS
Four f  ights (flights ntun)erod 4 through 7) originated from IIickam
1 : icld, Ilawai i ,	 and	 the fl i tihv paths are shown on Vi g. 1 .	 lliese	 fl ights,
whicli were made primarily to	 facilitate astronomi,-al observations, happened
to	 folloi: fli	 ht paths of ,rl ►nost constant geoma g netic latitude,	 in	 the
ran , :e 17°N 5 a < 21'N, which was very convenient for the measurement of
cosmic ray radiation. 1ltinide during the flights varied from 12.5 km
to	 15.7 len (41,000 ft to 45,000	 ft).
Following the four flights from Hawaii, which took place in August
197, tv.o fIij;hts originating from Moffett Field verc utilized to cal ihratc
the instruments flomi from Ilickham Field. 1111 flight paths of these
latter- flights (#8 and 4'9) are shorn in Fig. 2. With the exce ption of
one leg on flight #J, there was a large variation in geomagnetic latitude
on these two flights.
3 I
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3.	 INSTRUMENTATION
Ilewitt et al [He76, He78] have described the radiation detectors
tha, may be routinely flown aboard the C-141 aircraft.
A Router-Stokes ionization chani)cr and a Bonner sphere neutron
spectrometer obtained data during; the Hawaii based flignts. In addition,
the operation of a recoil proton scintillation counter spectrometer [He761
and a large area bismuth fission chandler [Mc651 were tested during flight
conditions.
The dimensions of the moderators of the Bonner sphere neutron
spectrometer are suiianari zed in Table 1.
TABLE I. Bonner sphere spectrometer moderators
(flights 04 - ON
Moderator	 Moderator
S i Ze	 material
5.1 cm (? in.)	 (:IfI
7.6 cm (3 in.)	 CI I)
12.7 cm (5 in.)	 CHI
10.3 c ►n (8 in.)	 11?0
25.4 cm (10 in.l
	
H2O
30.S cm (12 in.)	 1120
45.7 cm (18 in.)	 H2O
Iiie smaller ,moderators (S inch diaumeter and less) were of polyethy-
leric, while the larger moderators consisted of water-filled aluminum shells.
Interp.etation of the data taken in Ilawaii is made difficult because these
mixed moderators were used and also because a LiI crystal 8 mum dia. X 4 mm
hi0> was used to detect thermal neutrons, rather than the standard
12.7 nlm di a. x 1'.7 mm high (0.5 in. k X1.5 in.) crystal for barb response
functions have heen calculated.
The experimental data taken with the smaller crystal were later
normalized to that which would have heen observed had a standard-sire
crystal detector been uscd . Piese normal izat ion factors were obtained
(hiring }lights 08 and # 9 at higher gconclgnetic latitlkles (44°N to SOW.
lhlrirlg tllL fl it;hts 01-it;11VJt i1 ,, g from Allies Resc arch tenter (l l ig}lts ''R :Ind
#J) ahnuin ► un activation detectors wore flown in addition to the Bonner
sphere spectrouleter.
4.	 L'XPERIhi}•,NTAL DATA AND ANALYSIS
4.1
	
Ioniz a tion Wasureme.°nts
The hi,,h p ressurc, argon -1 i l Icd ionization chad)er was aboard all
f l i ohts. Prior to the llawai ian series of' measurements i t ►.as cal iterated
uv:ing all instr ► nnent ►%hich had been calibrated at the standard range of
the Health Physics Group, Stanford University. The experimvntal data
obtained are presented in Table 2 is the range of geomagnetic latitude
17'N -< X v 2l'X and at :1! t i tudes between •11 ,000 ft and 15,000 ft.	 1110
exposure rate varied from 200 ;Y/hr to 230 pR/hr. During flights ey and
f?'.), at a geoiwignet is latitude of' 45°N the exposure rates measured were
375 A/hr :Ind 383 pR/hr respectively
4.2	 Neutron McaStn_ement s wrd SPectr ►nrl 1 )etelT1111at 1On
ne average of the cowit ing rates obtained for each sphere :raring
the flights originatin,. from Hawaii are surtiL{rized ill fahle 3. Data were
5 200
227
G 202
230
200
210
230
7
17	 - 19
15.5 - 21
18.5 - 21
41 ,000
45,000
41,000
45,000
41,000
43,000
45,000
-8-
TABU. 2. Ionization chanbf- measurements.
c;eorrmignetic:
1 : 1 ight
	 latitude, A.	 A]titude
munher	 (del;. North)	 (ft)
4	 10.5 - 21	 4] , 000
4j,000
45,000
I :XPOSUre
rate
(uk/hr)
202
215
225
8	 45	 41,000	 383
9	 45	 41,000	 375
TABLE 3. Average counting rates mcasured during the ]laiNai i
flights	 (normalized
u.5	 in. high, and a
to a i.i 1	 detector 0.5
gcomignetic latitucle
in	 Ilia. X
o1*	 ? O 'N i
"Werator count ing Standard
diameter altitude rate deviation
(inches) (ft) (crm) (±ol
0 11,()O0	 ft 1	 1. 22
2 A = 20 0 N 27.5 19
3 38.9 12.8
5 5	 .9 S. -I
8 5/1.0 4.4
10 50.7 5.5
12 39.4 5.3
18 14.8 13
0 45,Onn	 ft	 -- --
a	 2.,.5
)S. 2 8.c^
S Oo.; S.1
12 15.2 n, l
lg 1'.O II.S
Tr11iT 4. Northern latitude fliphts; Lit d, Lector mounting
rates 0.5 in -, 0.5 in.
Inner sphere	 Al t i tuck•	 Cotuit i nf, rate	 Standard
diam (in.)	 (ft)	 (cPm)	 deviation
0	 4 ► , On g ► 54 2 6
n	 2 136.8 1.r
r,	 3 223.4 S.4
S 302. S 1 .5
8 283.17 1.8
o	 10 224.S 1.2
12 165.4 1.6
is -- --
8-in. alminim disc 26.6 3.8%
0 41,01))	 51.3 S.4
L? 2 1118. 2.5
=^ 3 103. 1.4
5 257, 1.4
a 8 236.5 1.4
i; 10 193. 1.3
12 143.9 1.2
-	 -
18
-	 -	 -	 -	
-	 -
69.5
-	 -	 -	 -	 -	 ••	 -	 -	 -	 -	 -	 -	 -	 -	 -	 -	 -	 -
2.4
-	 -	 -	 -	
-	 -	 -
8-in. almintm disc	 24.1	 5.80
obta ined at two aI It It tiles	 - •11 ,000 I • t cold . 1 ;,iIi10 ft .	 \ moderated BF.
cott►lter wa y used to Illp ►litOl the . ,rrriat ions in colon ing rate LIHC to cllani;es
in gcoituignetic latitude and aIitutude and to correct the cot trltiIIg rates
observed III 	 varim,; Ixxicrators of the Bonner sphere • spectrometer Ille"HI.
The data of Table .3 arc nonnal i:ed to a geomagnetic latitude of' 20'N.
III addition to norrta l i a t i on w ith respect to g('ollslgnct i s I :I t l t i Ide ,
the data of latele .) have been novii^al ized to the coltllt Ind; ,'.ate that would
have beell observed, had a titandard (). i In diam. , 0. i IIt. high 1,1 I cry'titaI
been used. AS discussed in Section 3 ) a detector of non- standard size was
used. Because the moderator re::ponse ftlll 1,- tiO114 haVC beell calcul.Itcd 1-01-
a tl.S in diam. , 0.5 in. high Lil crystal, It was necessarY to determine
nonual i--ation !;icto^r,. 'lllese factors were Qhtained during. I • l ights #8 and
1, 9. lle factors obtained in this way are uncertain to the extent that
the y Mary i^itlt neutron spcctrtan Alld the cosmic rah- ne utron spe rm varies
with gcONNI --act is lat ItudC. At the present t I me, errors i11trodlI( A by this
fio nciI i ^at ion 1)rocedtIre are juJiled to he much smaI ler than errors dite to
other sources.
'cable .1 sumTlari es the experimental coLtnting rates observed ill
c,II iteration fl ights 18 and 09.
Measurements made with the recoil proton spectrometer were perturb.d
by aircraft vibrati^r during flight. (711is problem has subsequently hecn
mi t i gated by ar i ►m p roved motnit i ng system.) Because of this vibration rile
data show a larger statistical Spread than had been obtained in previous
fli-lets Ille76, IIe781. 'lllc spectrometer was used to determine the neutron
flux density in the en:r^, range 3 `IeV ^ En < 1; ^IeV and the data obtained
COTIpIred with measurements of the moderated RP_ counter (which measures
12-
neutrons from thermal energies to about 20 MeV) are suroi4ari zed in 'fable S.
During operation the proton spectrometer was calibrated with a standa,'d
NBS PuB(a,n) neutron source and checked with a 252 C neutron source.
TABLE S. Neutron density measured at t-wo latitudes
(altitude 41,000 ft).
Flux density	 Flux density,
measured by the
	
measured by the
Geomagnetic	 rF.coil proton
	
modcratcd BF	 1^
latitudes, a	 spectrometer
	
counter
(deg. N)	 (n/cm 2 - sec)
	
(11/cm2 - sec)
50.7	 0.35 ± S%
	 2.28 ± 1.S%
20	 0.084 ± 14%	 0.43 7 ; 1..9 a
Ratio of the flux
density at 50.;°N	 4.2 ± 15%
	 S.2 ± 2.40
to that at 20°N:
4.3 Neutron differential Energy ^pectrum
The experimental data obtained witl, the Bonner sphere spectrometer
have been used to estimate the neutron differential energy spectrum with
the compW cr program LOIIII I 1 Kou69 I. `Thc results are shown -Ln Fig. 3.
The LOUH I unfolding routine penni is varying constraints to be
placed on the rapidity and magnitude of fluctuations in the solutions it
generates. This is achieved by the use of a smoothing parameter, y, which
nonnally is given values between 0 and 1.0. The higher value of y, the
more constrained are fluctuations in the solution generated and the loser
the ability to resolve sharp structure in the spectrum. Figure 3 shows	 -
-13-
G
0	 C 11  - 71 --1 1 11 T-T--I -r- -T-F -TT--T-r-7 ---j
105
104
103
aI i 0
U
`r'	 10i
NE 10 0U
c 10 
I
0
a^ 102
E 103
10
a^aN 10- 5
-6
c
10
-7
w 10
a
10-6
10-9
10 10
-11
10
10 8	 106	 104	 102	 1100	 102	 104
Neutron energy (Mev)
XBL 7711-10885A
Fig. S. Neuti-cn differential ener,,,Y spectra tuifolded from the
Hawaii .lata [41,000 ft; -eoma:metic latitnk 20°1' for
different values of the smoothing 	parruneter Y].
_14_
SO! ut ions ;or values of -r =I .0 (heavi I  damped) , -y = 0.1 (moderate I y d imped)
and y = 0.01 (slightly damped) . Sil;ni F icant differences in the calculated
spectra are evident, but it is not entirely clear whether , this inconsistency
is due to fluctuations of statistical origin or system,tic errors introduce(]
by the use of two detectors of different size and a mixed set of moderator,
Table 6 surrurarizes data taken during Fli ght #9 on .7,0 October 1975
and Fig. 4 shows the differential neutron spectrinn calculated by the LOIIII l
routine. 11his spectrum is ing)ortanr in that it is obtained from data
taken with a cogd ete sit of Boliner moderators and from an aluminum
activation detector. It represents a significant step forward in our
goal to obtain an Improved determination of the cosmic ray neutron spectrum.
JAW :
 0. Moffett l-ield I 1 fight #9, 30 October 1975.
0.5 in. ( i.un. x 0.5 in. high Li I zinc]
aluminum -ictivation detector counting rates.
Moderator	 Count irlg
diameter
	
rate (clan)
0 111. S 	 .3
2 in. M.
3 i ► l. 193.
S In. 257.
8 in. 236.
10 rrl. 193.
12 in. 144.
18 i11. 69.S
Al `
^,
-^ Na` 24.1
-1s-
107
6
10
105
4
10
103
102
10
10 0
10 -1
10-2
10-3
10-4
10-5
10-6
10-7
10 8
10-9
10
-e
	10 0
y = 0.1
10- 4
	10 2	 100	 102	 104
Neutron energy (Mev)
U
N
E
U
W
C
O
73
O
C
U
0
OLN
v
c
a^
0
XBL 7711-10884A
Fi;,. 3. Neutron differential energy spectra unfo lded from
clata obtained during Flight #9 ori^-inatiug from
`foffett Field.	 [:Altitude 41 ono ft; geomagnetic
latitude 4S *N; smoothing; par.-►neter y = 0.11.
In
Analysis of the data obtained during Flights 04 through 09 have
shown that:
a) Data obtained with the Bonner spheres must have a statistical
accuracy of about ±1% if the neutron spectrum in the region
0.1 to 10 NleV is to he well defined. (At the present time
our data contain statistical errors ranging from +lo to ±20%.)
b) It will he desirable to obtain a complete set of experimental
data with a moderator nude of one material.
c) Data Should be taken with Lil detectors of the standard
dimensions (O.S in. diam. x O.S in. high).
S.	 CONCLUSIONS
Ile data taken aboard the six flights described here have enabled
significant progress to be made.
Fi rst, and n:.,st irn)ortantly, we now know that hor icr sphere data
of accuu'acy adCgirate to determine the cosmic ray neutron spectrum may be
obtained in flight of duration 4-6 hours. The taking of data simultaneously,
on all moderators will enable the required statistical precision of ±lo
to be obtained and will remove any systematic uncertainties introduced by
our present normalization technique.
Second, we know that the moderator response functions calculated by
Samna are internally consistent.
'Ibird, we have been able to combine Bonner sphere data with
aluminum activation detector data.
Fourth, we have flow the hismrrth fission c:,urter and found it to
be stable and reliable in operation during flights at high altitudes.
The ultimate addition of data from this detector is most important to the
determination of the high-energy neutron flux density.
Fifth, we have obtained a great deal of insight into the operation
of LOU I .
ACKNOWLEDGMENTS
The authors express their gratitude to Dr. Bans 'lark, former Director,
Ames Research Center, for his interest and assistance in this work. They
also  wish to thank '.-Is.	 . Henson and Dr. A. Ri nd i , Lai" rence Berkeley
Laboratory, University of California, who played an important role in
using the LOUlll program to determine the neutron spectra.
Thanks are also due to Dr. J. ;MkIaughl in and 1 ► 1 • . F. llainal of the
Department of Lncrgy, Environmental Nbasurements La;ooratore, Nei .; York,
for providinti;	 information  rand encouragement. Pi-. R. `lendell ,
New York Universit y , provided ►Lti with our initial liquid scintillation
detector.
Special thanks are also clue to R. Kirby , a student a San Jose Mate
University, and Prof. A. 'fucker, who participated in data acquisition
and analysis. R. M. Cameron and C. M.Gi1lespie, Jr., Ames Research Center,
acre extremel y helpful in arranging for flight time and installation of
the experimental equipment on the C-141. G. 1;. Kojima rind A. L. Canmbell,
Ames Research Center, aided by modify ing the electronic circuitry of this
system and interfacing all the experimental packages with the aircraft
environment.
This work was supported ')y the U.S. Departr ►ent of Energy, the National
Aeronautics and Space Administration, and Ames Research Center, Moffett
Field, California.
-18-
ITITRL'NCFS
[11e761
	
Hewitt, J.E., 1—Hughes, J.B.McCaslin, L.II.Stephens, A.Rindi,
A.R.Smith, R.H.Thomas, R.V.Griffith, C.G.Welles, .J.W.Bauni,
and A.V.Kculiner, ones Collaborative Study of Cosmic Ray
Neutrons, NASA 174 X-3329 (1976).
IHe781	 -----, L.11ulhe.s, .J.W.BaUnn, A.V.Kuehner, .J.B.McCaslin, A.Rindi,
A.R.Sm.ith, L.D.Stephens, R.H.Thomas, R.V.Gril - fith, and C.G.Welles,
Ames Collaborative Study of' Cosmic Ray Neutrons: Mid-Latitude
blights (1978); Health  Physics (in press) .
McCaslin, J.B., II.W.Patterson, A.R.Smith, and L.D.Stephens,
Sore Recent Developments ii. 'feclinique!-- for 'Monitoring High-Energy
Radiation, in Proc. of first Inte rnat'l. Com I ress o f Radiation
Protection, Rome, September 1960 Worgamon Press, Oxford, 1968),
pp. 1131-1137.
(1009]	 Routti, J.'I'., Iligh - Fnerly Neutron Spectroscol,y with Activation
Detectors, Lat"ence iierkcley Laboratory Report llla 1.-18514 (1969).
DATE F LNIED
^0^o3^7s


Ames collaborative study of cosmic-ray neutrons. 2: Low- and mid-latitude flights

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