LA-4780 (ENDF-174)

1

:

(

G:10s

CIC-14REPORT”CULLECTl~~REPRODUCTION

cow

A Preliminary Evaluation of the Neutron

and photon-production Cross Sections

of Oxygen

)alamosscientific tiaboratory

of the University of California

LOS ALA MOS, NEW MEXICO 87544

UNITED STATES

ATOMIC ENERGY COMMISSION

CONTRACT W-7405 -ENG. 36

*This report was prepared as an account of work sponsored by the United

States Government. Neither the United States nor the United States Atomic

Energy Commission, nor any of their employees, nor any of their contract-

ors, subcontractors, or their employees, makes any warranty, express or im-

plied, or assumes any legal liability or responsibility for the accuracy, com-

pleteness or usefulness of any information, apparatus, product or process dis-

closed, or represents that its use would not infringe privately owned rights.

Printed in the United States of America. Available from

National Technical Information Service

U. S. Department of Commerce

5285 Port Royal Road

Springfield, Virginia 22151

Price: Printed Copy $3.00; Microfiche $0.95

.

.

,

,

10

LA-4780 (EIUDF- 174)

UC-34

ISSUED: August 1972

Iamosscientific laboratory

of the University of California

LOS ALAMOS, NEW MEXICO 87544

A Preliminary Evaluation of the Neutron

and photon-production Cross Sections

of Oxygen

.

by

D. G. Foster, Jr.

P. G. Young

This work was supportedby the Defense Nuclear Agency under Subtask PC-102

TASLE OF CONTENTS

Page

ABSTRACT

1. moDucTIoN

2. NEUI’RCNAND PHUI’ON-PRODUCI!ION CROSS Sl?CTIONS

2.1 Total Cross Section

2.2 Radiative Capture Cross Section

2.3 Inelastic ScatteringCross Sections

2.3.1 The 160(n,n’) and 160(n,n’7)Cross Sections for

~(160) <~M~V

2.3.2 The 160(n,n’) Cross Section for &(160) > 13 MeV

2.4 The 160(n,p)@ and %(n,p7)1~ cross Sections

2.5 me %(n,d)UN Cross Section

2.6 The %(n,a)% and %(n,w)% Cross Sections

2.7 Elastic Scattering Cross Section

3. ANGUT.ARDISI’EC03Ul’IONS

3.1 Elastic Neutron Angular Distributions

3.2 InelasticNeutron Angular Distributions

3.3 Secondary-photonAngular Distributions

4. DISCUSSIC$i

5. ACKN~~S

REFERENCES

1

1

2

2

6

6

6

13

13

15

15

21

22

22

24

25

23

27

27

.

.

.

#

ii

.

A PRELIMINARYEVALUATION OF THE NEUTRON AND

PHOTON-PRODUCTIONCRCSS SI12TIONSOF OXYGEN

by

D. G. Foster, Jr.,and P. G. Young

ABWRACT

~ preliminaryevaluation of’the neutron-inducedcross sect,ionsof 1 0 has been completed for the energy region 10-5 eV to 20 MeV.

Energy and angular distributionsof secondary neutrons and photonsare included. The recommendeddata are based mainly on experiment,although model calculationswere used to augment the measurementsin cetiain areas. In addition, results from an earlier evaluationperformed at the Knolls Atomic Power Laboratory are also includedto a limited extent. The evaluated data are available on magnetictape in ENDF/B(III) format.

1. INTRODUCTION

A preliminary evaluationof the neutron and

photon-productioncross sections of oqgen has been

c-leted for the energy range 10-5 eV to 20 MeV.

Except in the region of the “window”at 2.35 MeV,16natural oxygen is treated entirely as O, end the

0.23?$of 170 and180 is ignored. The data are in

ENDF/B (III) format as MAT 1134 and have been pro-

vided to the Radiation Shieldlng InformationCenter

at Oak Ridge and to the National Neutron Cross Sec-

tion Center at Brookhaven.

The primary purpose of this work is to provide

the Defense Nuclear Agency with an interim set of

cross-sectiondata which contains a consistent set

of evaluated inelastic-neutronand photon-production

cross sections and which includes considerationof

several importantnew measurements. A complete

literature survey was not performed for this study,

although many new measurementsare included. Parts

of the )-965 evaluationby Slaggie and Reynolds

(s16>) at the Knolls Atomic Power Laboratory (KAPL)

have been incorporatedin the evaluation. The pre-

sent study is incomplete in

be regarded as preliminary.

several areas and should

A survey of the reactions of interest is given

schematicallyin Fig. 1 (Aj>9, AJ70, Aj71). The

(n, Y) reaction has been omitted from Fig. 1 be-

cause it is very smell above the electron-volt

region and can be neglected for most purposes. The16

Q-values and thresholds for various n+ O reactions

are given in Table I.

The only significant channel bela~ the (n,ao)

threshold at 2.334 MeV is elastic scattering,and

in practice the Coulomb barrier inhibits the (n,ao)

reaction up to almost 4 MeV. Similarly, the 3.09-

MeV photon from the (n,al) reaction to the first13

excited state of C does not become detectable un-

til above 7 MeV, although its threshold occurs at

5.64 MeV. Above 7MeV, inelastic scattering is the

dominant reaction channel. Although the (n,n’a)

channel opens at 7.61 MeV, parity conservation16

inhibits the levels in O below 9 MeV excitation

from’decayingby alpha emission, and the (n,n’a)

reaction does not become significantuntil above

11 MeV. At energies above 12.89 MeV, proton emis-16

sion from excited states in O com?eteswith alpha

emission; however, at all energies the (n,n’P)

reaction remains less important than the (n,n’cY)

reaction. In this study the (n,n’p) and (n,n’a)

1

20 -

18-

16-

14 -

12-

Io-

8-

6-

4-

2-

0-

10.84

. 9

73 7.65.a

4.44 160(n,np)15 n

Q=-12,13

Q=-9,64o

160(n,nd12CQ=-7.16

IQ--l

L-Jo

’60 (n,a)l 3CQ=--2.21

!0

8

6

4

2

0

8

6

4

2

0

ELAB‘60(n,n’!60

Q=0.0‘LAB

Fig. 1. Composite energy-leveldiagram for the residual nuclei of major interest formed in n +160 reactions.

Except for the “E,ah” scales, the energies are in the center-of-masasystem, and all energies aregiven in MeV.

-—-

reactions are included in the discrete (n,n’) ENDF/B

files, with appropriate flags to indicateparticle16emfss%on from the residual O excited atcvtes.

The threshold for the (n,p) reaction occurs at

10.25 MeV, followed closely by the (n,d) threshold

at 10.53 MeV. The latter reaction is relatively

unimportant, as its cross section is less than

16 mb at all energies. The cross section for the

(n,2n) reaction is less than 3mbbelow20MeV

(Br61), and it iS not included in the evaluation.

Similarly, the (n,t), (n,3He),and (n,2a) reactions

listed in Table I are expected to have very small

cross sections and have been ignored.

The neutron total cross section was obtained

at all energies from experimentaldata. Below

11 MeV, the elastic cross section was determinedby

subtractingthe sum of all reaction cross sections,

which were determinedmainly from measurements, from

the evaluated total cross section. Between 11 and

14 MeV, increasingemphasiswas placed on the mea-

sured elastic cross section, and above 14 MeV the

nonelastic cross section was determinedby sub-

traction of the elastic from the total. The non-

elastic cross section was e&ended to 20 MeV by

optical model calculations. Above 14 MeV the (nip),

(n,d), and (%a) cross SectionsWere takenfrom

measurements,and the (n,n’) cross section to highly

excited states of 16Owas adjusted to give the cor-

rect nonelastic cross section.

2. NEUTRON AND PHU1’ON-PRODIE’TIONCROSS SECTIONS

2.1 Total Cross Section

The total cross section has been completely

re-enluated. Because our main purpose was to in-

corporate recent measurements,we gave less atten-

tion to the older work, especially in the million-

electron-voltregion.

evaluated total cross

Belox 700 keV, however, the

section is based entirely on

.

.

,

2

.

Reaction

16O(n,n)160

16O(n,n’)

160*

16O(n,y)170

160(n,p)~6N

I-60(n,d)l*N

1.6O(n, t)14N

16O(n, 3He)14C

1.60( n,a) 13C

160(n,n ‘a)‘C

16O(n, n ‘p)15N

16O(n,2n)150

16O(n,~) ‘Be

(M%)

o

-6.050

+4.143

-9.639

-9.901

-14.479

-14.616

-2.214

-7.161

-12.126

-15.668

-12.865

Threshold(MeV)

o

6.432

0

10.247

10.526

15.392

15.538

2.354

7.613

12.891

16.656

13.676

aAll Q-values used in this evaluationare takenfrom the 1964 mass tables of Mattach et al. (Ma65).

very old measurements. From a preliminary approxi-

mate fit to the data of Okazaki (Ok55), the first

resonancewas found to have Er = 442 keV,

a= 13.03 b, and r = 48 keV, all in the laboratoryosystem. By subtractingthe tail of the resonance

from three measurementsbelow 250 keV (Adb9, Jo@,

Meb9), we obtained a weighted average of 3.666 ~

0.015 b for the low-energy potential-scattering

cross section. If the (n,y) cross section is assum-

ed to vary as I/v from Jurney’s value (Ju71) of

178 pb at 22OO m/see, it contributesan additional

9 mb to the total cross section at 10-5 eV, but is

negligible above the electron-voltregion. The

potential-scatteringbackgroundwas assumed to vary

linearly In the neighborhoodof the bb2-keV reson-

ance in such a manner that the total cross section

joins snoothly at 700 keV onto the curve deduced at

higher energies. The result is shwm in Figs. 2 and

3, together with the data. Near 300 keV the total

cross section 1s a few percent higher than the KAPL

evaluation.

The remainder of the evaluated total cross sec-

tion is based primarily on three major time-of-

flight measurementswhich have become available

since the 1965 evaluation. The measurement of

Foster and Glasgow (F071) had the poorest resolu-

tion and the least reliable energy scale (especially

belw 3.5 MeV), but was part of an extensive series

of systematicmeasurementswhich allows additional

comparisonsto be made with other work on other

elements. It agrees with the results of Schwartz

(SC71) within about 0.5$, on the average, in the

overlap fnterval of 2.25 to 15 MeV. Since the two

measurements used different samplea (B203-2Band

Si02-Si, respectively),and similarly good agree-

ment has been observed previously in nitrogen (Y072)

and aluminum (Yo72a),theFoster and Glasg~Z and the

Schwartz measurementswere adopted as standards for

the million-electron-voltregion.

The measurement by Cierjacks et al. (Ci68),

who used Al O with an aluminum blank, is the only23

one of the original 1968 group of Karlsruhe measure-

ments that has not subsequentlybeen assigned a

recorrectionfor dead-time errors, and it is be-

lieved to be correct within about 20 mb at all

energies above 700 keV (Ci71). However, the

Cierjacks data disagree with other time-of-flight

measurements in much the same way as Young and

Foster (Yo72a) obsened for aluminum (after recor-

rection), namely, there is a smooth variation

ranging between 1$ lower and 4$ higher than the

Foster and Glasgow (F071) and Schwartz (SC71) re-

sults. Agreement is within 1$ at almost all

energieswhen a correction similar to that used for

aluminum (yo72a) is applied.

The evaluated cross section from 0.7 to 12 MeV

was taken directly from Schwartz’swork (SC71).

However, since the Karlsruhe results had the best

resolution of the three time-of-flightmeasurements,

we used inserts of those result.s,normalized by the

correctiondiscussed above, wherever necessary to

preserve the resolution. Between E and 20 MeV we

used a composite of the work of Schwartz and Cier-

jacks. Below 4 MeV, the extremely narrow resonances

found in the work of Fowler and Johnson (F070, J067)

were corrected for resolution and superimposedon the

base provided by the time-of-flightwork. Unfor-

tunately,we overlooked the fact that the earlier*

work of Fossan et al. (F061),which was the pri-

~IY basis fOr the 1965 evaluation above 4 MeV, had

6

u)zam.zo

;3Inmmocc(J

I 1 I 1. r 1 1 I t “

c1

v“-v @)aw JONES. 1946

A

~ HELKONIPN. 19490 HOFIIR. 1949 Q

La..—., 1 I 1 Lu.AJ t t 1 I UI..Ii

10-’0 10-7 IO-4,,()-I

NEUTRON ENERGY, MEV

Fig. 2. Measured and evaluated total cross section for160 below 0.1 MeV.

, I 1 1 1 1 1 I 1 # I 1 1 1

x

A IIELKONtfTN. 19490 FIDRIR. 1949X OKFIZRKI. 1955X CIERJRCKS, 1968+ SCH14RfiTZ. 197t

A

a

La o -—— .c1

0N

0 , , 1 , ! I 1 1 , , tOO.O 0.1 0.’2 0.3 0.4 0.5 0.8 0.9. 1.0 1.1 1.2 1.3 1.4

I&!ITROl”LNERGy. m.s

Fig. 3. Measured and evaluated total cross section for160 from 0.01 to 1.5 MeV. The experimentalpoints

shown are averages of 35 points for the Cier,jacksdata (Ci68) and 25 points for the Schwartz

.

data (SC71). -

4

.

(JYsum.zCJ1-(Jw(n

(nIn

Z

.

x CIERJRCXS. 19S0+ SCHRRRTZ. 1s71O FO14LER. 1970● FOSTER. 1971

IKFIPL 1965

!s

i#-

11LR5L 1971

F&. 4. Measured and evaluated total cross section for O from 1.4 to 2.8 ~4eV. me enertiental pointsshown are averages of 11 points for the Cierjacks data (Ci68) and 8 points for the Schwartz

.

data (SC71).

better resolution*at some energtes than the work

of Cier~acks,although it differs from the recent

time-of-flightmeasurements in absolute cross sec-

tion, especiallyat higher energies. Therefore,

although the present evaluation should be more accu-

rate in energy and absolute cross section, in iso-

lated areas above b MeV its resolution is inferior

to the 1965 evaluation.

Figure b shows the measured and evalusted

cross section between 1.4 and 2.8 MeV, including

the “window” in the total cross section near 2.35

MeV. An energy shift between the older and more

recent work is clearly visible at the 1.83-MeV res-

onance. For the window itself, two additional

measurementswere considered. After we applied

correctionsfor resolution ranging from 80 mb (Fo71)

*Note that in Figs. 3-7 the data of Cier,jaclcsandSchwartz are plotted as averages of many adjacentpoints. This expedient introducesa further degra-dation of resolution In the figures but not in theevaluation.

to negligible (Ci68),we found that a value of

130 ~ 10 mb for the minimum cross section of natural

oxygen agreed with the three time-of-flightmeasure-

ments and Fowler’s (Fo71a)point-by-pointwork.*

Four different samples had been used for the four

measurements (Fowler et al. used BeO-Be). We re-

jected, for the present, a preliminary result of

65 + 15 mb made by Kalyna et al. (Ka71),who used

a 5-ft-long sample of liquid oxygen. Later values

reported by this group are higher and should be in-

corporated in the next evaluation. The present

evaluation gives a window which is broader and shal-

lower than that of the 1965 evaluation and which

aPPe?rs at a sllshtl-ylower energy (Fig. 4).

*Final correctionswere not made until after wecompleted the evaluation;the total cross sectionat the minimum in the ENDF/B (III) data set is139 mb.

Figures 5-7 show the evaluated total cross sec-

tion in the energy range from 2.6-20 MeV, together

with the data used for the present evaluationand

the measurement of Fossan et al. (F061),which was

not used. The loss of resolution can be seen at

several energies. At about 8 MeV, the newer measure-

ments and the present evaluation show a steady rise

over the Fossan data and the 1965 eval~tion. Near

11 MeV there is a 12$ differencewhich has an appre-

ciable effect on air-transportcalculationsfor

14-MeV neutrons. In addition, some of the propor-

tionately larger changes in the elastic cross sec-

tion discussed tn Sec. 2.7 are due to subtraction

from this higher total cross section.

2.2. Radiative Capture Cross Section

For the 1bO(n,y)170 cross section for thermal

neutrons, we used the Jurney and Motz (Ju63) value

of 232 pb. The cross section at all other neutron

energieswas assumed to vary as I/v from the thermal

value. This assumption results in an (n,y) cross

section that is too small in the region of the in-

verse photonucleargiant resonance. However, the

actual cross section in the million-electron-volt

region is undoubtedlytoo low to be of interest In

most practical problems.

The capture gamma-ray spectrum for thermal

neutrons was obtained from the measurementsof

Jurney (Ju71) and is gfven.schematical.lyin Fig. 8.

The total multiplicity for thermal neutrons is 2.854

photons per capture.

2.3. InelasticScattering Cross Sections

2.3.1. The 160(n,n’) and 160(n,n’y)Cross

Sections for EX(160) < 13 MeV. The

(n,n’) cross sections to the most importantphoton-

producing levels in 16Owere determinedmainly from

(n,n’y)measurements. The level decay scheme for160

which relates the (n,nf) and (n,n’y) cross sections

was obtained from the compilationsof Ajzenberg-

Selove (AJ59, Afi’0)and is given in Fig. 9. The pho-

ton transitions included in the evaluationare given

in Table II.

d

+

*

0ma

q t I I I 1 I 1 # 1 1 1 1

‘%.6 2.8 9.01

S.2 S.4 S.6 4.4 4.6 4.8 S.0 S.2NEU%N E&GY,4;EV

.4

Fig. 5. Measured and evaluated total crossshown are averages of 9 points fordata (SC71).

6

section for 160 from 2.6to 5.4 MeV. The experimental pointsthe Cierjacks data (Ci68) and > points for the Schwartz

.

.

.

.

.

F055FIN. 1961~OSTER. 1S71CIERJFICK5. 1968scHkifiRTz. 1971

! 1 f

%.o S.S 6.0 5.5 ;.o 7’,s1 1 ! ! 1 ! 1 I

9.5 10.0 10.5 11.0 11.5 1~.oNEIJ!R\N E;: KG Y,9;EV

Fig. 6. Measured and evaluated total cross section for 160 from 5.o to 12.o MeV. The experimentalpointsshown are averages of 6 points for the Cierjacks data (Ci68) and k points for the Schwartzdata (sc71).

0y

.m!:

.

s FOSSS44. 1961~ ● FOSTER. 1971

X CIER.JKHS. 1S68+ scwmrz.1971

g * , t , , , t , , I-12.0 12.5 13.0 13.s 14.0 14.5 15.0 15.5 16.0 16.5 17.0 17.5 18.0 18.5 19.0NEUTRON ENERGY. t’lEV

19.5 20.0

Fig. 7. Measured and evaluated total cross section for IGO from 12.o to 20.o MeV. The expertientalpointsshown are averages of 4 points for the Cierjacks data (Ci68) and 2 points for the Schwartzdata (SC71).

7

TABLE II

160(: )WRMEIONS

E‘initial ‘fl.nal

& (MeV) (MeV)

8.872 8.872 0

7.119 7.119 0

6.917 6.917 0

6.131 6.131. 0

4.949 11.080 6.131.

4.163 11.090 6.917

3.833 10.952 7.119

2.741 8.872 6.13I.

2.208 11.080 8.872

1.955 ‘ 8.872 6.917

1.753 8.872 7.119

o.510a 6.050 0

aEach 6.05w O transition i8 a8sumed to produce two0.51-MeV annihilationphotons.

The 160 nucleus has two peculiar properties thet

should be noted. First, the 6.@O-NeV first excited

state has ? = O+ and can therefore decay to the 0+

ground state only by internalpair-production. Since

provlslon has not been made in the ENDF/B system to

indicateprompt electron decay following (n,nt) re-

actions, we were not able to Include in the ENDF/B

files the energy deposited by the electron-positron

pair. The annihilation photons from the positron,

however, do appear in the photon-production files

with the appropriate threshold and cross section.

The second property of ’60 that should be noted

deals with the relatively low threshold for the

(n,n’a) reaction. States above 7.161-MeV excitation

are unstable to alpha emission (see Fig. 9); hmfever,

because both’2 C and the alpha particle have 0+16ground states, only states in Owith parity equal

to (-l)J can decay to the ground atate of 12C. The

8.872-, 1O.952-, and 11.&O-MeV states do not satis-

fy this crtterion and therefore de-exciteby photon

emission. Thus, the (n,n’cx)reaction is unimportant

below 10 MeV although ita threshold occurs at 7.61

MeV. Above 11.6@-Me~ excitation, states in 160 can

alpha-decay to the 2+ first-excitedlevel of 12C

witiloutthe above restriction,and we have assumed

that states in this region decay 10@ by particle

emission.

4.143

“+ ’60

(14.6) (85.4),——-- ——___ —___ - ----

1 10.8708 3.055

0-“

3.055 1/2+

ou

0.87081/2+ ●

Tou

o 5/2+ 1170

.“

Fig. 8. Decay scheme and branching ratios for “04The transitionsthat occur following thermalneutron captures are given in parenthesesatthe top of the diagram in photons per 100captures.

I 11.6~ 3-x\

11.5212+11.605

\ 0+’2C(4.44)11.4403-

\ ‘a

11.260 0+\

a

11.096 4+\ ‘

a

160

Fig. 9. Energy level decay scheme for 160.

8

r 1 1 T ! 1 I 1 1 1 1 I [

‘--.,KRPL\\ \ . \ \ . \.

. \ \ \ \ \ \ \ \ . d

* flc06NflL0 . 1966y O[cKENS. 1370a GRPHFIN. 1s70X Cl]CKENS. 137~a CLRYEW . 1963.> ENG5SSER, 13s70 NY13ERG, 1970+ tiRLL. 19s9x CRLOtiELL, 1960x 6UCHRNRIV,1971v ORflKE. 1970M KOZLOHSK [ , 1965

P0. (7000

.5736-.5736(7. 00000.0000

0 .rlooo.1736

0.00000.0000

.5736

.5700

f 1 1 ! I I t t 1 I 8 1 1 !3 7.0 6.0 9.(J loco 11.0 12.0 13.0 14.il 15.0 16.0 17.0 lt3.o 19.0

NEUTRON ENERGY, MEV.-

Fig. 10. Measured and evaluated (n,n’y) cross section for the 6.131-MeV photon.

The evaluatedphoton-productioncross section

for the ground-statetransition from the 6.131-MeV

level is compared in Fig. 10 to the ICAPLevaluation*

(s165) and to the available experimentaldata. All

the measurements in Fig. 10 are single-angle

(k = COS o) differentialresults that have been mul-

tiplied by kIT. The LASL curve 1s based mainly on

55° measurementsmade by Dickens and PereY (Di70)

below 11 MeV and on a composite of several measure-

ments near 15 MeV. The single datum at 7.5 MeV by

Drake et al. (Dr70) is part of a complete angul.ar-

distributionmeasurement. The Drake data indicate

that a significanterror (--&@) is introducedat 7.5

MeV by assuming the integratedcross section to be

4@(55”). This assumption is made throughout the

*Because photon-productiondata are not included inthe KAPL evaluation,the dashed curve of Fig. 10was computed from the KApL (n,nt) cross sectionsand the decay scheme of Fig. 9.

1.0

present evaluationbecause very few photon sngular.

distributionmeasurements exist for oxygen, and we

did not perform a detailed theoretical analysis. The

differencesbetween the 55° and 90° measurements of

Dickens and Perey (Di70) indicate that the anisotropy

of the 6.131-MeV photon persists at most neutron

energies below 11 MeV. The KAPL curve in Fig. 10 Is

apparently based on the 1959 measurements of Hall

et al. (Hasg) and is roughly a factor of two higher

than the IASL curve below 11 MeV.

Similar comparisonsare given in Figs. 11 and

12 for the ground-state transitions from the 6.917-

and 7.119-MeV levels. Figure 13 includes the photon-

production cross sections for the 1.753- and 2.7Jl-

MeV photons, both of which result from de-excitation

of the 8.872-MeV level in 160, and the cross section

for the 2.2&-MeV photon from the 11.&O-MeV level

is given in Fig. 14. The KAPL evaluation (sI.65) is

generally higher than our results in Figs. 11-14.

9

N

or I 1 i I 1 I 1 1 I I I 1 I

E=Y

tWPL,z --’-s/ -. .)’ . \

1’ . \ PI

6.917 WJ “O NYBEf7G. 1970 .1736

1’ Cl CWPHPN, 1970 -.~736\\ Y OICKENS. 19WI .s736

1’ \I \\ x DICKENS. 13~fl 0. moo

/I?J

A CLRYEW. 1963 0.0000/ I o ENGEssCf?. 1967 C.cooo

I

1’I ! 1 1 1 9 I 1 1 ! ) ) ! 1

06.0 7.0 8.0 ~.~ 10.0 11.0 12.0 i3.O 14.0 15.0 i6. O 17.0 ia.o i9. o

NEUTRON ENERGY. MEV

Fig. 11. Measured and evaluated (n,n’y) cross section for the 6.917-MeV photon.

m1-

0

m0

%(i

[ I 1 1 I 1 1 #

KRPL ,’’--”,/

, \

I

\I \

/ \ ,

-L/ “\

1’ \\\\\\\\\

L-1-—.-

—

—

1 I 1 T 1

Y DICKENS. 1970x DICKENS, 1970•l ORPWW. 1970

<)

+

?I NYBEf?Ge 1970A CLIIYEUX. 1363~ ENGESSER. 1967

P.5736

O.mo-. s736

-17360. moo0.0000

+

1 \% \

1 \

1\

+

\\

.

.N

0

w

o -

w EY= 7.119 MEVD0 -

A

0/lj

%0 7.0 0.0 9.0! I I 1 ! 1 1 I 1 1 ! i

10.0 11.0 12.0 13.0 14.0 15.0 16.0 17.0 1$.0 19.0

NEIJTRGN ENERoY, MEV

‘o.9

0.0

Fig. 12. Measured and evaluated (n,n’y) cross section for the 7.119-MeV Photon.

10

The 55° and 90” measurementsmade by Dickens and

Perey (Di70) Indicatethat at most neutron energies

the anisotropy in the angular distributionsfor these

photons is significantlyless than for the 6.131-MeV

photon.

The (n,n’) excitation cross sections for the

6.131-, 6.917-, 7.119-, and 8.872-MeV levels that

resulted from our analysis of (n,n’y)measurements

are given in Fig. 15. Beyond 15 MeV, the shapes of

the curves are taken from compound-nucleusreactfon-

theory calculationsmade with the code C@lMUC (Du71).

The results shown for the 6.@2-MeV level also result

from CC1.fJUCcalculations, normalized with the same

factor required to bring the calculated excitation

cross seckions for the photon-producing levels into

approximateagreement with the (n,n’y) measurements.

Similarly, the curves shown in Figs. 15-17 for levels

with excitationenergy between 8.9 and 13 MeV are

based on CCMNUC calculations. The normalizationfor

these calculationswas chosen so that the total cross

section minus the reaction cross section near 14 MeV

resulted in an elastic cross section consistentwith

measurements (see Fig. 35 in Sec. 2.7).

The sums of the excitation cross sections for

the 6.05- and 6.13-MeV levels and the 6.92- and

7.12-MeV levels are in excellent agreement with di-

rect measurements near 14 MeV made by Bauer et al.

(w63) and McDonald et al. (Mc66a). The excitation

cross section for the 8.87-MeV level deduced from

the (n,n’y)measurements is, however, higher by

roughly a factor of 2 than the direct measurement of

McDonald et al. (Mc66a).

The total inelastic cross section that results

from the above analysis is compared in Fig. 19 to

the evaluation of SI.aggieand Reynolds (s165). The

present results are substantiallylower than the

KAPL curve at energies below 13 MeV, mainly reflec-

ting the differencebetween the older (n,n’y)meas-

urements of Hall et al. (Ha59) and the newer measure-

ments of Dickens and Perey (Di70) and Orphan et al.

R (or70)..F. GWtilRW PRWrlLN L7!OS6 sCCTIW

ey= 2.741 HEv

GWWUlRIW PRrXP.KTlffl 0706S SECT1ffl

+ z 1 .7s9 IiEv

g% 7.6 ,., ,.0‘-’2 ;,.,i., ;,.0;’.,;,* ;,,0;,,e ;,.#;,.,

— tEuTRoNC!ERGV,WV

Fig. 13. Measured and evaluated (n,n’y) cross sec-tions for the 1.753- and 2.7hl-MeV photonsresulting from the 8.872 +7.119-M V and

%8.872 +6.131-MeV tmnsitions fn 1 ~.

,’,’II

1’//I//Ii////1

1

1 /

Ey= 2.208 llEVKf?PL

,,-. . .-\\\\

i

1

1’8;~1.O !2.0 13.0 14.0 15.0 16.0 i7.O lB.O 18.0 20.0

NEUTRONENERGY. t!EV

Fig. 14. Measured and evaluated (n,n’y) cross sec-tion for the 2.208-MeV photon resultingf om the 11.@O *8.872-MeV transition in15.,

11

I,-

1 1 , , 1 1 1 1 1

I

/

Ex= 60131 M~v

II ~

6.052

z

‘6.0 7.0 8.0 9.0 10.0 11.0 iz. o 13.0 14.0 15.0 16.0 17.0 18.0 19.0NEIJTRON ENERGY. t’lEV

Fig. 15. Evaluated croes sections for inelastic scattering to levels in 160 with Ex < 10 MeV.

.

‘i

F~. 16. Evaluated cross sections for inelasticscatteringto levels in 160 with ~ be-tween 10.354 and 11.>21 MeV.

20.9

$~’1”’’”9=”9’\.-1- .,,e.. . . . . .

UK./12.628

12.967

0 13.0 14.0 1s.0 16.0 17.0 180 19.0 ‘.0

Fig. 17. Evaluated cross sections &r inelasticscattering to levels in 1 0 with ~ be-tween 11.63 and 12.967 MeV.

.

12

.

2.3.2. The 160(n,n’)Cross Section for EX(160)

> 13 MeV. At excitation energies above

13 MeV, the level structure in160 becomes increas-

ingly less certain, and a simple evaporationmodel

was used to estimate the relative (n,n’) cross sec-

tion to levels in this region. We used an expression

of the form

un,n’

a (E. - Ex) exp (Ex/T)

where E. is the total energy available in the center-16

of-mass system, Ex is the excitationenergy in O,

and T is the nuclear temperature. A value of 2 MeV

was used for the temperature. The relative cross

sections generated in this manner were normalized so

that the nonelastic cross section agreed with the

total and elastic cross sections. These results are

representedas discrete (n,n’) reactionsto 0.3-MeV-16wide bands of excitation energy in O and are given

inFi.s. 18. This representation was used rather than

the more usual form of continuous energy spectra to

permit more accurate calculation of the effects of

center-of-mass motion on the neutron angular distri-

butions (Yol’2).

The 160(n,p)1% and 160(n,py)1% Cross Sections2.4.

The evaluated (n,p) cross section of Slaggie

and Reynolds (s165)below 15 MeV was adopted in

the present study. From 15 to 17 MeV, we used

experimentaldata to determine the (n,p) cross sec-

tion, and above 17 MeV we joined a shape calculated

with the code CO!! to the the lower-energyresults.

The evaluated curve is compared to the available

measurements in Fig. 20. Most of the high energy

Fig. 18. Evaluated (n,gj) cross sections to groupsof19isat

~evel~ ~ LOO with & between 15 and

MeV. Each 0.3 MeV in excitation energyrepresentedas a single ffctftio~ levelthe midpoint energy.

measurements (e.g., B067, De60, De62, Ka62, Ma54,

Mi66, pa53, pr66, se62) restit from activation

studies; the evaluated curve therefore does not in-

clude (njp) reactions to particle-unstablestates*

of ‘%.

Several low-energyphotons are produced by the160(n,py)16N reaction, as indicated In the

~6N level

decay scheme of Fig. 21. No experimentaldata are

available on these transitions,and the individual

(n,p) level excitation cross sections are not inc-

luded explicitly in the evaluation. We estimated

the (n,py) cross sections, however, by distributing

the evalmted (n,p) cross section of Fig. 20 among

the particle-stablestates of 16N, assuming the pop-

ulation of each to be proportional to (2J+1).

Table III gives a summary of the (n,py) transitions

included in the evaluation. TWO of the transitions

have been combined, as indicated.

E

&

TABLE III

Einltial(MeV)

‘final(MeV)

0.397 0.397 0

0.292

{

0.297 0

0.397 }0.121

0.120

{

0.121

0.397

0,

}0.297

‘In this context we exclude beta emission in defin-ing “particle-stable”levek.

Fig. 19. Evaluated total inelastic cross sectionfrom 6 to 20 MeV. The LASL curve includes(n,n’~) and (n,n’p) contributions.

13

0

I

m

Eg.

EE(n

mcclC3(E(-l

NEUTRON ENERGY, MEV

Fig. 20. Measured and evalusted total (n~P) cross section for 160 from 10 to 20 ‘ev”

3.355 1+\

2.487

0.3971-~+ ‘5N

w m mq w 0.

0.2973-0 0 0

*

0_.

0.121 0- &

0~

0 2- ● T ●

16N

Fig. 21. Energy-leveldecay scheme for 1$.

+ IWNTELE. 1962X IIRRTIN. 1954A OEJUREN. 1962D SEEMRNN. 1962* BORWINN.19674 LILLIE. 19S2x PFIUL. W53v OEJIJREN. 1260Y PRRSRO. 19S6x nITRf3. 1966

).0

1!. 1

valuated total (n,d) cross‘ig” 22” ~~~ndf~rdl%O from 10 to 20 MeV.

.

.

14

TABLE IV2.5. The160(n,d)15NCross Section

The (n,d) cross section is given in Fig. 22.

The evaluated curve is based on a compound-nucleus

reaction-theorycalculationwith the code CC21NUC,

normalized to the lone measurementby Lillie (L152)

near 14 MeV. Because the threshold for the (n,dy)

reaction does not occur until 16.I.MeV, photon pro-

duction from (n,d) reactionswas ignored.

2.6. The 160(n,a)13Cand 16O(n,ay)13CCross Sections.

At excitationenergies higher than 4.947 MeV,13C levels decay predominantlyby neutron emission,

as indicated in the decay scheme of Fig. 23. The

excitation cross sections to the ground and first

three excited states are included in the present

evaluation,and the sum of these is given as the

total (n)a) cross section. The (n,on) cross section

through the higher levels 1s lumped with the (n,n’a)

cross section discussed in Sec. 2.3.2. The photon

transitions included in this study are given in

Table IV.

Figure 24 shows the (n,ao) cross section from

threshold to 6 MeV. The Slaggie and Reynolds (s165)

evaluationwas adopted in this region, and the

160(%l&mm&&TS

E

(M:V)~

3.8>4

3.684

3.096

Einitial(MeV)

3.854

Efinal(MeV)

.

0

3.684

3.096

{

3.854

3.684

3.854

0

0

0.683

3.6840.170

6.5345/2+\

3.S545/2+ 4.947“+12C

83.S843/2- 0

3.0S61/2+ 6

0u

o l/2-

13C

Fig. 23. Energy-leveldecay scheme for 13c.

N

& 1 I 1 1 1 I T I 1 1 t I f

I

.0

illmzCEam I

b-l

0‘1e I

k ?e

rt%d+

Yd~<-{1

+e

+Q

Qe

e

++

4.0 4.2 S.0 S.2 S.4 S.6 5.6NEU:R:N Ek\GYc4;EV

NL

% ot 0 NPLTON. 1957 ? P 01

+ OIw[s.o SE[TZ.

196319s5

ot

A OIVFITIP.1S67

m0

0

a0L‘3.’2

.

-/3.0

I /

3.6I

3.4

Fig. 24. Measured and evaluated (n,ao) cross section between 5.2 and 6.o MeV.

15

agreement with the availablemeasurements is reason.

able. Above 6 MeV, we reevaluated the (n,ao) reac-

tion, and the results are compared to experiment in

Fig. 25. At higher energies the curve is based

mainly on the measurement of Sick et al. (Si68).

The (n,al) cress section below 15 MeV was de-

termined from measurementsof the direct (n,~) cross

section to the 3.086-MeV level and from measurements

of the ground-state photon transition from that level,

as given in Fig. 26. The excitation cross sections

to the 3.684- and 3.85k-Me’Jlevels are based on mea-

surements of the ground-statephotons below 15 MeV,

using the decay scheme shown in Fig. 23, and the re-

sulting photon-productioncross sections are given

in Figs. “27and 28. At energies above 15 MeV, we

obtained the (n)a) cross sections to the first three

excited levels by dividing the total (nja) cross

section available to these levels, as measure& by

Sick et al. (Si~) and sh~ in Fig. 29, among the

three states aasuming each is populated in the same

ratio as occurs near 15 MeV.

The total (n,a) cross section 1s given by the

solid curve in Fig. 30. The expertientalresults in

the figure are actually measurementsof the alpha-

emission cross section, and the rapid increase in

the measurementsabove 12 MeV is caused by the onset

of the (n,n’a) reaction. The differencebetween the

KAPL and LASL (n,a) curves occurs because part of

the (n,n’a) cross section was apparently included in

the KAPL curve, whereas we chose to include that re-

action in our (n,n’) files, designating alpha emis-

sion Bs the mode of decay of the residual nucleus.

The total alpha-emissioncross section con-

structed from our (n,n’cy)and (nja) cross sections

Is also gfven in Fig. 30. At all energies above

13 MeV, our resu.ltaare higher than the experimental

data. It should be noted, hcwever, that the law-

energy portion of the alpha spectrum i6 difficult to

measure accurately, ao the agreement is probably

reasonablebelow 17 MeV. Above 17 MeV, however, the

evaluated curve is too high, probably because proton

decay following (n,n’) reactions [i.e., the (n,n’p)

reaction] is competingmore effectivelywith alpha

decay than we have assumed. ~is problem was not

discovered until the evalwtion had been completed.

The cross section for proton emission, corresponding

to the sum of the total (n$p) and (n,n’p) reactions,

is given in Fig. 31, together with the alpha-emission

cross section* and the total (n,n’) cross section to

gamma-decayinglevele. A slight increase in the

(n,n’p) cross section with a correspondingdecrease

in (n,n’a) would correct the problem above 17 Mi?Vin

Fig. 30.*The structure in the alpha-emiaaioncross sectionabove 15 MeV In Fig. jl fs artificial and resultsfrom the articular representationthat we used in

7the ENDF B files.

N

7 1 1 I 1 1 1 1 1 1 1 1 t I 1

JN

0t

~;

1I

m“ ,00

0

0

~

06.0 7.0 8.0 9.0 10.0 11.0 12.0 13.0 14.0 15.0 16.0 17.0 18.0 19.0 20.0NEUTRON ENERGY. MEV

Fig. 25. Measured and evaluated (n,ao) cross section between 6 and 20 MeV.

16

,

D.0 , 1 ,

(n, al ) CROSS SECTION

EX = 3.086 flEV

+ ORV16, 1963X HSU, 1967

\

, * ! , , , t

GRMflFl RRY PRODUCTION CROSS SECTION

II

I

EY= 3.086 PIEV

l--g~ 1“u)= -

mg.

:50

.7a

s0 -

~, , , , , , t 1

‘6.0 7.0 a.u 9.0 10.0 11.0 12.0 13.0 14.0 15.0 i6 .0 17.0 18.0 19.0

NEUTRON ENERGY, tlEV

Icl.n

Measured and evaluated (n,al) and (n,ay) cross sections for the 3.@36-MeV level and gro~d-statetransition in 13C.

17

.R0 , I 1 , , , , ,

+

‘Y= 3.684 MEV

P

1.

;clL%PMW. 1970 -.5736x Olciums.1970 0.0000Y OICNENS.1970 .5736

.4 ~ CMYEUXO 1s63 o .Woo~ ENGESS.W,1267 0.-0 N?BERG.1970 .1736● l)ocm~ . 1363 q.-

%“

—

8 t , , ‘\o.o 11.0 12.0 13.0 14.0 15.0 16.0 17.0 10.0 19.0

NEUTRON ENEROY. flEV “

Fig. 27. Measured and evaluated (n,ay) cross section for the 3.684-MeV photon.

‘Y= 3.054 ?lEV

.P-.s7360.00000.Kul.1736.5736

0.CXmo

E

06.0 7.0 8.0 S.0 \o.0 11.0 12.0 13.0 14.0 1s.0 16.0 17.0 18.0 19.0

NEUTRON ENERGY. MEV

Fig. 28. Measured and evaluated (n,ay) cross section for the 3.854-MeV photon.

D.0

).0

,

;0 r f

Fig. 29. lleasuredand evalu ed composite (n)a) cross eection to the firat three

excited levels of !&.

, ,

1LASL TOTAL ALPHA EMISSION

X ORNOV. 1960+ ORV16.1963~ 01VIITI17,13670 u0Rlt!lr&4, 1%3Q I.lLLIE. 1952

/ --l---

I--

-./’h●/1 ‘1/// ’8’ “

/;

t

g \,

06.0 7.0 8.0 9.o, , I , , , 1

10.0 11.0’” 12.0 13,0 14.0 15.0 16.0 17.0 10.0 19.0NEUTRON ENERGY, tlEV

o

Fig. 30. Measured and evaluated total alpha emission cross eection compared to theLASL and KAFL (n,ct)evaluations.

/

ALPHA

\

NEUTRON(y)j\ /’

\\

\

g/ , , , 1 , , , ,

06.0 7.0 8.0 9.0 10.0 11.0 12.0 13.0 14.0 1s.0 16.0 17.0 18.0 19.0

NEIJTRON ENERGYD flEV

Fig. 31. The total alpha antiproton emission cross sections compared to the portionof the inelasticneutron cross section that results in photon emission.

12ofu

Ey= 4.439 Ho

PP

● CLfwLlx.1s69 O.OCKOe cNo256m . lsm omcu&x 61JcmNl=!N.1971Q ORIWW. 1970 -.5736● fMICW.fWXV,1963 0.0000

/; L‘1

~0

t ,> ?: ‘,,,,

+

g

O11.O 12.0 13.0 14.0 15.0 16.0 17.0 1s.0 19.0

NEUTRON ENERGY. flEV

0.0

Fig. 32. Measured and evaluated (n,n’ay) cross sectionfor the k.439-MeV photon.

20

A significantportion of the (n, n’a) cross

section results in excitationof the first excited

level at 4.439 MeV in12C, with subsequent emission

of a photon of that energy. We made a crude estimate

of the (n,nlry) cross section from the (n,n’a) data

using a technique described earlier (Yo’72),and the

results are compared to the availablephoton-produc-

tion measurements in Fig. 32. The threshold for

photon production following (n,n’p) reactions does

not occur until 18.49 MeV, and we have ignored pho-

tons from this reaction in this study.

2.7. Elastic ScatteringCross Section

Although the threshold for the (nja) reaction

is 2.35 14eV,this channel does not become important

until nearly 4 MeV, and at lower energies the elastic

cross section is essentiallyeq~l to the total cross

section. The elastic cross section obtained from

our analysis of the total is compared in Fig. 33 to

tbe available elastic measurementsbetween O and

2.8 Mev. similarly, Fig. 3k includes the elastic re-

sults between 2.8 and 6.OMeV. Above 4 MeV, the (n,a)

cross section has been subtractedfrom the total to

give the elastic curve. Clearly, the elastic mea-

surements shown in Figs. 33 and 34 are in reasonable

agreement with the evaluated curvej althowzh the

measurementsdo not show much of the structure dis-

played by the total cross section. Below 6 MeV, the

Slaggie and Reynolds (s165) elastic data are onlY

slightly different from the present results, and

the only importantdifference occurs in the window

near 2.35 MeV, as noted in Sec. 2.1.

The elastic cross section from 6 to 20MeV is

given in Fig. 35. The KAPL elastic (s165) is in-

cluded for comparison. Most of the experimental

points in Fig. 55 were obtained byfitting the mea-

sured angular distributionswith Legendre expansions.

The fitted curves above 7 MeV were anchored at front

and back angles by inserting into the fit fictitious

‘0N 1 1 I 1 1 1 1 1 1 I T 1 1

1

U-Jz

D

w

0

m

0

0

&~m

0J ! M LOVCHIKOVR.1962

. 0q 6KflZRK[.1955

a“X FOULER.19s8v tlfiRTIN. 1962

I~ FIIJNZINCER. 1962

,> + LRNE. l%flZ? 4-(J3M -

wIn

g: -

0 +:

++ +J

g0&

0L 1 I 1 1 I ! I 1 1 I #

“0.0 0.2 0.4 0.6 O.B 1.0 1.8 2.0 2.2 2.4 2.6NEU;F&N E~:RGYolfEV

Fig. 33. Measured and evaluated elastic scattering cross section for O from o to 2.8 NeV.

-’4

21

points at 0° based on Wick’s limit (Wik3) and points

at back angles based on optical model calculations.

The evaluated curve below 11 MeV was obtal.ned

by subtractingthe sum of all reaction cross sec-

tions from the total cross section. Between 11 and

lk MeV the emphasiswas shffted to the elastic mea-

surements,and near 14 MeV the elastic was obtained

directly from experimental data. To extend the

elastic cross section to higher energies, the para-

meters of a spherical optical model were fitted to

the angular distributionmeasurementsnear lb MeV.

The depth of the real potentialwas then varied to

reproduce the total cross section to 20 MeV, and the

elastic cross section was obtained by subtracting

from the fluctuatingtotal cross section a smooth

nonelastic cross section based on the calculations.

The evaluated elastic cross section is in

good agreementwith most of the measurements in

Fig. 35, although the curve is somewhat higher than

the measurementsnear 11 and 11.6 MeV. Above 9 MeV,

the evaluated curve is substantiallyhigher than the

KAPL results (s165). The 1953 cloud chamber measure-

ment by Conner (c053),which roughly agrees with the

KAPL data, was not emphasized in the present study.

3. ANGULAR DISTRIBUTIONS

3.1* Elastic Neutron Angular Distributions

The evaluated angular distributionsfor elastic

scatter@ by Sleggie and Reynolds (s165)were

adopted for neutron energies belcw 5 MeV. From 5 to

14 MeV smooth curves were drawn through the coeffi-

cients obtained from Legendre fits to the available

experimentaldata. Above 7 MeV the fits were anchor-

ed at front and back angles using Wick’s limit (Wi43)

and optical model calculations,as noted earlier.

Above 14 MeV the behavior of the Legendre coeffi-

cients was estimated from the optical model calcula-

tions that were used to extrapolatethe elastic

cross section to 20 MeV (Sec. 2.7).

In Fig. 36 the evaluated Legendre coefficients*

for f = 1 through f = 4 are compared to the fitted

values for neutron energies above 5 MeV. Similarly,

the coefficientsfor f = 5 through 1 = 10 are given

in Fig. 37. Near 11 MeV there are significantdif-

ferences in the f = 7 and t = 8 coefficientsfrom

*These coefficientsare in the ENDF form defined

9T,.—

.7byu(e) =“~ .0 (21+l)f1P1(cos~), with

fo= and Iffl< 1.

I 1 , , 1 ! , I 1 , r ,I

Y LISIFR, t966o HljPz INKR, 1962.$ ftIILLM’S$”l S6L

0 JOHW2N> 1267M FOULER, 1364

~ CJiRSE, 1361

0 , , 1 , , 1 I , I # $ 1 I ,02.8 3.0 3.2 3.4 3.li 3.0 4.0 4.0 S.u 5.2 5.4 S.6 S.6

NEU;R;N E;:RGY.4:EV.0

Fig. 34. Measured and evalwted elastic scattering cross section for16

0 from 2.8 to 6.o MeV.

22

.

.

I 1 1 1 1 1 1 1 1 I 1 ! I

I x*

1,1 b

CONNER. 19S3f’HILLIPS. 1961CHRSE. 19S1

1111 X NELL IS, 19716flUERo 1963BERCH. 1967tICOONf7L0. 1966

1 I 1 1 1 1 , 1 1 I 1 1 10 7.0 8.0 9.0 10.0 11.0 12.0 13.0 14.0 15.0 16.0 17.0 18.0 19.0 20.0

NEUTRON ENERGY. MEV

Fig. 35. Measured and evaluated elastic scattering cross section for 160 from 6 to 20 MeV.

ELASTIC SCATTERINGLEGENORE COEFFICIENTS

0.6 - 4.1● CHASE, 1961

. NELLIS, 1971

0.6 - ● BAUER, 1963● BEACH. 1967

0.4 -

02

,L .

0.2Ird .3

0 –

02

1- WC-4 I

1 I I I I I I I I I I I I I I5 8 II 14 17 20

NEUTRON ENERGY. MeV

Fig. 36. Measured and evaluated Legendre coeffi-cients for 1 = 1 through t = h at neutronenergies above 5 MeV. The ENDF/B conven-tion is used for the coefficients (foal).

I I I I I I I I I I I I I I 10.24 ELASTIC SW ,TTERING J

10’

0.08

0.04

0[

0.04

0.02[

~s 8 II [4 20

NEUTRON ENERGY. MeV

Fig. 37. Measured and evaluated Legendre coeffl-ciente for 1 = 5 through f = 10 at neutronenergies above 5 MeV.

23

the Nel.liset al. (Ne71) and the Chase et al. ((!h61)

measurements. While these differencesmay be real

because there is structure in the total cross sec-

tion near 11 Me’?,we used average values for the

coefficientsin this analysis. Significantdiffer-

ences also occur in the coefficientsfrom the Beach

(B~7) data as compared to those from the Bauer

et al. (BS63) and McDonald et al. (MC6&) measure-

ments near 14 MeV. The results from the latter ex-

periments were used In this analysis.

The elastic angular distributionsare compared

in Fig. 38 to the measurementsbetween 4.99 and

6.53 MeV. Similar comparisonsare given in Figs. 39

and 40 for the elastic measurementsbetween 7 and

14.1 MeV; The values used in the figures for Wick’s

limit were averaged roughly over the energy resolu-

tion of the experimentalmeasurements. The differ-

ences noted earlier between the Nellis (Ne71) and

Chase (Ch61) measurementsnear l.1MeV are apparent

in Fig. 39. Similarly, in Fig. 40 the evaluated

1011,~1 .

Cosi?em

curve at 14.1 MeV differs significantlyfrom the

Beach (Be67) measurement.

Angular distributionsobtained from the optical

model calculations are given in Fig. 41 for neutron

energies of 16, 18, end 20 MeV. The distributions

become progressively more ’fomard-peaked as the

neutron energy increases..

3.2. InelasticNeutron Angular Distributions

Measurements of inelasticneutron angular dis-

tributions for some of the low-lying levels of160

have been made at 9 and 11 MeV by Nellis et al.

(Ne71) and near 14 MeVby Baueret al. (ss63) and

McDonald et al. (Mc66a). The Nellis data indicate

that the sum of the angular distributionsto the

6.05- and 6.13-MeV levels 1s somewhat forward-peaked

at 9 and 11 MeV, and similar effects are observed in

the measurementsnear 14 MeV (Ba63, Mc66a). The sum

of the angular distributionsto the 6.92- and 7.12-

MeV levels is nearly isotropic at 11 MeV (Ne71),

L -1

1 PHILLIPS, 1961100 6.0 M9V ~.. d

I

I--_d1.0 -1.O

C&’L?=m.

Fig. 38. Measured and evaluated angular distributions for elastic scattering between 4.99 and 6.53 MeV.The solid curves represent the evaluated shapes normalized to the measurements;the dashed curvesgive the same shapes normalized to the evaluated elastic cross sections. Wick’s limit is indicatedby the arrows at the left margin.

.

24

.

.

10 . j

I I I I1.0 -1.0

Cos”em

Fig. 39. Measured and evaluated angular distributionsSee caption to Fig. 38 for details.

although slight forward-peakingis seen in the

lh-MeV measurements (Ba63, Mc66a).

A complete analysis of the (n,n’) a%tiar dis-

tributions should include the results described16

above, together with all available O(p,p’) angular

distributiondata. We did not make such an analysis

for this eval-tion, and the inelasticneutron

angular distributionsare assumed to be isotropic

throughout. These results are regarded as prel~-

inary, and improvements are planned for the future.

3.3. Secondary-PhotonAngular Distributions

The angular distributionsof all photonswere

assumed isotropic in the evaluation. This asscssp-

tion is known to be invalid for the 6.13-MeV photon

at 7.5 MeV, where the measurement of Drake et al.

(Dr70) indicatespronounced forward-peaking,with

U(OO)/CT(900)being roughly equal to 2.2. Similarly,

near 14 MeV the results of Kozlowski et al. (K065)

and Buchanan et al. (Bu71) establishU(OO)/CJ(900)to

be approximately1.5. Since the 6.13-MeV photon IS

very important in the million-electron-vo>tneutron

1000 I I I 44 w\-\

}\\ NELLIS, 1971

1

i~-

,\ 11.0McV

’000 w \ —+-..- \

\ ‘v ~’ *\

\ //

\ v=~ * LL

\

ii-

-1000 w ‘\ CHASE, 1961~

/ -- -_ .11.6 MeV%

/\ /

:s , ‘“

/--

--

Iw\

CONNER, 1953

‘ “;-- “::Mev\ ●

10\

\\

.

t i

l~,oLo

Co:”oc.m.

for elastic scatteringbetween 7.0 and 14.1 MeV.

energy region and since knowledge of the angular dis-

tribution is needed to evaluate the integratedcross

section properly, the results given in this evalua-

tion are regarded as tentative, and future work in-

cluding theoretical considerationsis planned.

The results of Morgan et al. (M06h)* indicate

that the assumption of isotropy is reasonably valid

at 14.8 MeV for the 2.74-, 3.09-, 3.68-, 3.85-,

4.44-, 6.92-, and 7.12-MeV photons.

4. DISCUSSION

We have mentioned several areas where the pre-

sent evaluation is deficient and improvementsare

needed. The most important of these are the 6.1.3-

MeV photon angular distrlbutlons,which also affect

the (n,n’) and (n,n’y) cross sections inferred from

*The data of Morgan et al. (M06J) are supersededbythe results of Buchanan et al. (Bu71); the latterreference, however, contains explicit data only onthe angular distribution of the 6.13-MeV photon.

25

BAUER, 1963

y

a

-vb-100n

10F

t 1

Ffg. 40. Measured and evaluated angular distributionsfor elastic scatteringat 14.1 MeV. Seecaption to Fig. 38 for detaila.

TAME v

F&z. 41. Evaluated angular distributionsfor elas-tic scatteringat 16, 18 and 20 MeV.

ENDF/2CrOme Section Desfgaation

Totala MF=3, m-l

Else.tica MF=3,ur=2

Nonelastic MF-5, 1.?r=3

Total(n,n’) MF=3,Mr=4

Discrete(n,n’)MF-3EX(160)<13MeV !’U!=51-70Discete(n,n’) MF=3%( l&o)>13~iev !.U!=71-89

(n)y) KF=3?fl?=lo2

1000

1000

~ IOQO

E.-.

E&b [oo .

10r

(n)p) KF=3,Mr=lo3

(n,d) KF=3, MP104

(n,a) nF=3,Mr=lo7

Total (n,xy) MF=13,sumof1.?r=4,22,1ce,

103,107

Individual MF=13(n,~) li.nea Mr=4;22,102,

103,lal

%’he error in the total and elaticIn larger.

Therunl 0.1MeV.—* 4$ i 4$

4$ 4$

14% order

of msg-

nitude

I I I I J1.0 0.0 -m

CQsec.n

=IMATED13WRS IUTHSEVALIJATED IXW’2RCRCSS SW1’ICXl

Neutron Ihera1 MeV—2.!sL_w2L9ELwEL A!ML?!2EL

● 1$ * 1% & l’j% * 1+ ● 1$ * 1$ & 1$

1$ 1$ 3$ 6% 15$ 10I 20%

order 20,4 2d 25% 3c$ 20jJ 3cljof lllag-nitxle

30% 3d 301 30X

- . 30% 30% 3ol 3ox

. 5dlwer

14% llmltonly

. .

.

crme nectiona near

. 20$ 2aL 201

. . . 5L% 30Z 50X- “20% 20% 2Ct% 2(X4 2d’ 20%

. . . 3ol 3C% 303 3C%

- . 3d 30X 30$ “30$

the minfmum at 2.35 hfeVis * 1$ or * O.O15b, whichever

.

.

26

the photon measurements. Areas involvingthe in-

elastic neutron angular distributionsand the elastic

scattering angular distributionsbelow ~ MeV also

need improving. The KAPL evaluation (s165)was

used for the latter below 6 MeV, and a detailed

resonance-theoryanalysis incorporatingmore recent

measurementswould improve the data set. Finally,

more careful reaction-theorycalculationsshould

lead to improvementsin the inelasticneutron cross

sections at higher energies.

A summary of the estimateduncertaintiesIn the

various cross sections is given in Table V at several

representativeneutron energies. The estimatea are

rough and,are not expected to hold true in detail.

In addition, the errors are averaged over the struc-

ture in the cross sections,and allowance for unknown

structure is not included. Time-of-flightspectra of

neutrons emerging from liquid oxygen spheres pulsed

by 14-MeV neutrons have been calculatedby Harris

et al. (Ha71) using the present data set and compared

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29

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HK/cb:675/400

30