All 100989644

iSe «1M^1S74 C 1 NeS-PUB-C I! A

NBS MONOGRAPH 138

MeV Total Neutron Cross Sections
NATIONAL BUREAU OF STANDARDS

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Pan Ccnler Im Radljllon Roeuch, Lwjitd ai Bouldtr, Cf.|or,do Som.

Fin ..f Tdt CerlCf (or BuUdin
JUL 1 6 1974

u 55fc

1-10

0.2

MeV Total Neutron Cross Sections

Robert B. Schwartz, Roald A. Sohrack. and H. Thompson Heaton, H

Nuclear Sciences Dhision

Institute for Basic Standards

1/ . S . National Bureau of Standards Washinglon, D.C. 20234

U.S. DEPARTMENT OF COMMERCE, Frederick B. Denf, Secre/( NATIONAL BUREAU OF STANDARDS. Richard W. Roberls, D/recfor

Issued January 1974
Library of Congress Catalog Numben 73-600276

National Bureau of Standards Monograph 138

Not- Bur, Siflod. (U^.), MonofT. 138, 160 pages (Jan. 1974) CODEN: NBSMA6

UJ. GOVERNMENT PRINTING OFFICE WASHINGTON: 1971

For sale by the SupcrinLcndenI cf Documeni,, U.S. Govemmenl Printine Offic*. Wttahin(n<.n. D.C. 20102

(Order by SD Caialog No. 013,44:136). Price 13.60

Slocl: Number 0303-01 189
Contents

CROSS SECTION CURVES

1. Introduction.. Curves

APPENDIX-EXPERIMENTAL TECHNIQUES

Al. Experimental Arrangement

A2. Electron Linear Accelerator ^ ^ A3. Neutron Producing Target

A4. Instrumentation ^ ^ A5. Monitoring ^ ^ A6. Sample Thickness Considerations

A7. Corrections to the Data. . „ A-6

A8. Summary

- A-7

A9. References A-7

iii
I
MeV Total Neutron Cross Sections*

Robert B. Schwartz, Roald A. Schrack, and H. Thompson Heaton. II

Thia report ia a compilation of the MeV neutron total cross section data measured at the National Bureau of Standard, over the past several years. The measurements eenerally span the enersy in- terval from 0.5 to 15 or 20 MeV; data are presented in graphical form for twelve normally occurring

elements, plus the separated isotopes ="U. ="U, and ""Pu. An appendix is included which gives complete details of the experimental technique. Key words; MeV neutrons; neutron time-of-flight; neutron total cross sections

1. Introduction

This report is a compilation of the MeV Total Neutron Cross Section data measured fLTf fv. B^eau ot Standards. The neutron tirae-of-fiight method was used for the measurements, tvith the NBS electron linear accelerator as the pulsed neutron source. The measurements were made over the energy range 0.5 to 20 MeV (in some cases, only to 15 MeV). Except for the uranium and plutonium isotopes, the measurements were made on the naturally occurring isotopic mixtures of the elements

in question In each case, the data are presented in the form of graphs, both linear and logarithmic. Descriptive notes are also included for each element, giving details concerning the samples. Most of these data have not as yet been formally published. In most cases however the data have been presented orally at various meetings of the American Phystatks Literature Reference" is given which refers to the abstracts for these

Details of the experimental technique are given in the appendix.

I.l. Quality of Data

A. Energy Resolution At low energies the resolution is largely determined by the neutron detector thickness (12.7 cm); at high energies the electronic response function plays the dominant role. For our 40 m (light path, the resolution varies from 0.2 ns/m at 500 keV

to 0.08 ns/m at 15 MeV. The resolution as a function of e in the appendix. nergy is shown in figure 5

B. Energy Scale Uncertainty The energy scale uncertainty is 0.04 ns/m. There is excellent agreement between

our energy assignments and, for example, the precision neutron energy determina- tions of Davis and Noda (NucL Phys. AI34, 361 (1969)).

C. .Absolute .Accuracy The absolute accuracy of the data is estimated to be within ±1 percent This estimate is based largely on the excellent « 1%) internal consistency of data taken mth ditferent sample thicknesses, and under different experimental conditions, as

well as the excellent l< 1%) agreement between our hydrogen data (Schwartz et al Phys. Letters SOB, 36 (19691) and previously measured values, as represented by the shape-independent effective range theory. Detailed comparisons between our data and those from other laboratories will be given in later publications; we simply state here that such comparisons have generally shoivn good agreement consistent with our estimated 1 percent accuracy.

D. Statistical Precision The statistical errors are generally 1 percent to 2 percent per point, but somewhat

poorer at the extreme high and low energy regions of our data. In any case, the statistical errors are indicated by the usual vertical hnes at every tenth point on our

curves, except in cases where the error bars are smaller than the points.

1.2. Experimental Technique A brief account of our experimental setup has been given previously (R. B. Schwartz, H. T. Heaton II, and R. A. Schrack, Proc. Symposium Neutron Standards and Flux Normalization, AEC Conf-701002, Argonne, Illinois, p. 377 (October 21,

1970)). This description is brought up to date, and further details given, in the

appendix.

lupported in part by the U.S. Defense Nucl r Agency. Washington, D.C. i

1

HYDROGEN

Sample materia!: polyethylene (CHOx

open: high-purity carbon

Sample diameter: 12.7 cm

Sample thickne! 18.3 cm

)! = 1.438 atoms/barn of hydrogei

Analysis: polyethylene: stochastic ratio of hydrogen to carbon within 0.04 percent of theoretical value

carbon: Volatile and nonvolatile contamination less than 0.01 percent

Literature Reference: R. B. Schwartz, R. A. Schrack, and H. T. Heaton II. Physics Letters SOB, 36 (1969); also, Proc. Symp. Neutron Standards and Flux Normalization. AEC CONF-701002. Argonne, Illinois, p. 57

(Oct. 21, 1970).

Comments: Equality of carbon atoms/barn in the carbon and polyethylene samples can be seen from the absence of structure in the observed hydrogen cross section. The statistical precision is degraded where there are large peaks in the carbon cross section (e.g., 2.1 MeV), but there are no net fluctuations.

3
4
4.0 5.0 6.0 7.0 8.0 9.0 10.0 20. 30- 40. NEUTRON ENERGY ,MEV

NEUTRON ENERGY .MEV

7

COZ 3

CCm

CO

to

CD

LJ

(X

"1 1 r "1 I r

I I I I I I I I L_

-1 1 1 1

1

1

I 1 r n 1 r

_i I

I I L- 15 . 20 . NEUTRON ENERGY .MEV

9

25 .

H

CO

3

cnz

CD

1—

(_) 2 LU

CO

CO

CO

Cd

CJ

RL

1

1

Q

1—

BERYLLIUM

Sample Material: metallic beryllium

Sample Diameter; 12.7 cm

Sample Thicknesses: 3.3 cm; ti=0.4069 atoms/bam

7.65 cm; n=0.9447

Analysis: s= 99.39 percent beryllium

principal impurity: < 0.6 percent oxygen

Literature Reference: R. A. Schrack, R. B. Schwartz, and H. T. Heaton II, Bull. Am. Phys. Soc. 16,495 (1971).

11

10

0 . ll 4 .0 5.0 6-0 7 .0 .0 9.0 10.0

NEUTRQN

20 . 30 . ENERGY , MEV

0 .

1

40 .




CE

CD

UJ 2 LO

COCO

CD

LJ

6.0

CECD

LJUJ

to

OJ

CD

cc

(_>

(X

CJ

J 1 1 L_ 8.0 10 . I 0 12. NEUTRON ENERGY .MEV
Be

4

15 . 20 . NEUTRON ENERGY ,nEV

25 .

crCD

CO

COCD

01

CJ

CE

CARBON

Sample Material: pressed graphite

Sample Diameter: 12.7 cm; 5.08 cm.

Sample Thickness: 5.08 cm ?? = 0.4803 atoms/bam

10.16 cm 7i=0.9557 atoms/barn

17.78 cm » = 1.6750 atoms/barn

Analysis: volatile components < .01 percent

nonvolatile components < .01 percent

Literature Reference: R. B. Schwartz, H. T. Heaton II, and R. A. Schrack, Bull. Am. Phys. Soc. 15, 567 (1970).

Comments: Density fluctuations within a sample are less than 1 percent. There was no evidence of absorption

impurities on exposure to air.

21

T

0 . iL 5.0 6 .0 7.0 8 . 0 9.0 10.0

NEUTRON ENERGY ,MEV

10 .

cr

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cr

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c

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cr

I—CD

NEUTR0N ENERGY ,hEV

24
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i i

1 r

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4 .

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C

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1

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NEUTRON ENERGY ,(1EV
LJUJ 2 . CO

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4

-

- -

i

' T

-

1

1

1 1 1 1 1

1 1 1 1 1 1 1 _l 1 1

-

1 1 1

CC

CO

CJ

UJto

CO

CO

Ql

LJ

_JCC

CD

0 .

NEUTRQN ENERGY .MEV

29

NITROGEN

Sample Material: liquid nitrogen

Sample Diameter: 12.7 cm

Sample Thickness; 27.9 cm, n - 0.9684 atoms/barn

13.5 cm. n = 0.4683

Literature Reference: H. T. Heaton 11, R. B. Schwartz, and R- A. Schrack, Bull. Am. Phys. Soc. 15, 568 (1970).

CommentB- Special dewars were constructed witl, end windows of known thickness of pyrex. Open runs were madeComments, bpe ^^^^^^^ t^am. The nitrogen cross section was also obtamed by run_

ning melamine (CH,N.) against polyethylene ICH-.). A powdered sample of ""'^ ''^o C

atoms/barn of nitrogen was run aga.nst a sample of polyethene havmg the same number of atoms of C

and H Figure A shows the ratio of cross sections obtained using melamme and liquid ratrogen.

Figure A,

31

10

T

\

0 . iL

4 .0 5.0 5.0 7.0 8.0 9.0 10.0

NEUTRON ENERGY .MEV

,0. N

cr

OD

1 . U

en

CD

LJ

ex

CD

Jo.

40 .




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to

tou

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1

1 1 1

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i 1

1 1

1 T i

1 1

1

'

4

-

-

1 —1—1—1

•

1 1 ^ \ \ \ \ L 1

1

1 12

cc: crQD

LJ

LUto

eoCO

CD

q;

ej

_icc

I—

NEUTRON ENERGY ,riEV

38
-I 1 1 r

-1 1

1

1 r- N

ccCD

cr

ca

QJ 2 CO

CO

COo

LJ

cr

E3

' I I I 1_ I I I L_

I I I I

1 L_ 20 . 25 . NEUTRON ENERGY ,MEV

39

2 bJ

CO

CO

cn

CE

I—CD

OXYGEN

Sample Material:

open:

Sample diameter:

Sample thickness:

Literature Referen

quartz (Si02) single crystals

semiconductor grade silicon

5.08 cm

7.54 cm; n= 0.396 atoms/barn

29.98 cm; n = 1.576 atomsAiam

R. A. Schrack, R. B. Schwartz, and H. T. Heaton II, Bull. Am. Phys. Soc. 17. 555 (1972).

whe« there are large peaks in the silicon cross section (e.g., 570 keV), but there are no net fluctuations.

41
10.

1

1

A

\

O.ll t

I 1 1 1 4; o 3 4. 0.4 0.5 0.6 0.7 0.8 0.9 1.0 • NEUTRQN ENERGY .MEV
0 . IL 4 .0 5.0 6.0 7.0 8.0 9.0 10.0

NEUTRON ENERGY ,MEV

20 . 30 0 . 1 40 .



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5.0

1 \ 1 1 1 1 1

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1 1 1 1 1 1—1

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cr

CO

<_) UJ

LO

CO

LO

LJ

_JH

NEUTRON ENERGY ,nEV
NEUTRON ENERGY .tlEV

ALUMINUM

Sample Material: metallic aluminum plate

Sample Diameter: 12.7 cm

Sample Thickness: 11.3 cm, u= 0.6858 atoms/barn

A„a„,,s: The samples were fabricated fron, 1100 .rade a.u„,i„u,„. The principa, impurities were 0.62 percent iron

and 0.16 percent copper. AU other impurities were less than 0.1 percent.

51

10

cr

I—CD

"jTo sTo tli tTo 8 .0 9 .Q 10 -0 NEUTRON ENERGY ,riEV

10 .

0 .

20 . 30 . 40 .



4 Al

cc

CD

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LO

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»

cc

1 I I 1 L 3 .0 4.0 5.0

NEUTRON ENERGY .MEV

cc

cn

2 LU

CO

COa

CJ

6.0
6.0

NEUTRQN ENERGY ,MEV
Al

4

crCD

cr

CD

CO

COu

CE

15 . J 1 1 L, 20 .

I I I I

1 L. 25 . NEUTRON ENERGY .MEV

CO

CO

CO(3CC

CX

SILICON

Sample Material: polycrystalline silicon bar

Sample Diameter: 5.08 cm

Sample Tliickness: 4.6 cm. i. - 0.2294 atoms/barn

17.9 cm. « = 0.8957 atoms/barn

Literature Reference: E. A. SchraC. R. B. Schwart. and H. T. Heaton 11. Bull. Am. Phys. See. 16, 495 ,1971).

Comments: The samples were semiconductor grade silicon having impurity levels <

10-.

61
10.

crCD

1 • u

UJ

enQ

crI—

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0.4 Q . S Q • 7 0.8 0 . 9 1-0 MEUTRQM ENERGY , MEV

0 .

1

4 .
10

CECO

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LO

LOCO

CD

CC

C-J

CL

CD

0 .

4 .0 5.0 6.0 7.0 .0 9.0 10.0

NEUTRON ENERGY ,MEV

20 . 30 .

10 . Si

CLCO

LJLU

CO

CO

CO

CKLJ

cr

I—CD

0 .

40 .



NEUTRQN ENERGY , liEV
Si

CD

LU 2 LO

LO

LOCD

cr

LJ

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cr

6.0 1.0

ex

CD

2 UJ

LO

LO

LO

crLJ

cr

I—CD

10 . 1 2 . NEUTRQN ENERGY ,MEV
NEUTRON ENERGY ,MEV

m

CALCIUM

Sample Material: metallic calcium

Sample Diameter: 12.7 cm

Sample Thickness: U.f) cm, 0.3331 atoms/barn

44.7 cm, )!= 1.027 atoms/barn

Analysis- The samples were fabricated from 99 percent purity metal and placed m me al contamers to prevent exces- Analysis.

J^^e -mp^^._^^ ^ .^^ spectrograph.c analysis of tl,e samples d.sclosed '^'''-^"'f^lfj^^^^^^^

concentration were magnesium and strontium, both present .n concentrations greater than 0.1 percent but

less than 1 percent.

Literature Reference: R. A. Schrack, R. E. Schwartz, and H. T. Heaton II. Bull. Am. Phys. Soc. 17, 555(1972).

71
Co 10..

cc

crCD

(_> 1

CO

CO

LJ

CC

I—

0 . IL 0 .

1- * •

0.5 0.6 0.7 0.8 0.9 1.0 NEUTRQN ENERGY ,MEV

3 .
ccCD 0 . il 4 .0

__u 20
0.5 0-6

MEUTRQM ENERGY .MEV

az ' U

coo 6

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LOO

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IMEUTRQN ENERGY .MEV
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NEUTRON ENERGY .MEV
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6.0

ct; CE03

2 uj

(JO CD

a:

CL

8.0

12 . NEUTRON ENERGY .MEV
4 Ca

cr. az

CD

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CO

COCD

J I I L

I I I 1

1 L

1 5 . 20 . 25 . NEUTRON ENERGY .MEV

LJ

2 LU

CO

COCD

LJ

TITANIUM

Sample Material: metallic titanium bar

Sample Diameter: 12.7 cm

Sample Thickness: 10.7 cm, n - 0.6368 atoms/barn

Literature Reference: R. B. Schwartz, R. A. Schrack, and H. T. Heaton 11, Ball. An,. Phys. Soc. 14, 494 (1969).

Analysis: A qualitative spectro^aphic analysis ctthe san«,le showed no impurit.es at levels greater than 0.1 percent.

81
Ti 10 .

crCD

l-J 1

CO

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LJ

cr

I—

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^ NEUTRON ENERGY ,hEV

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10 .

cr

CD

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CC

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ccCD

cc

CD

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CO

LO

CD

Q£

LJ

CE

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LO

LO

CD

CHLJ

CC

1

CD

0 . 1

0 . u4.0 5.0 6-0 7 .0 .0 9.0 10-0

NEUTRQN

20 30 . 40 . ENERGY , MEV

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~i I

i r

1

1 r I I

1 r "1 I

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1 .2

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1 . 4
Ti

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4 14

Ai

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NEUTRQN ENERGY ,nEV

6.0

IRON

Sample Material: metallic iron

Sample Diameter: 12.7 cm

Sample Thickness: 2.94 cm. n = 0.2490 atoms/barn

11.8 cm, « = 0.9997 atoms/barn

Analysis: The samples were fabricated of SAE 1017 steel containing 0.9 percent manganese. The presence of the manganese may, at worst, introduce spurious fluctuations of up to 3 percent in amplitude in the energy region where there are sharp resonances (< 2 MeV) and should have negligible effect at higher energiel
10 .

t y • •

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NEUTRQN ENERGY, MEV

20 . 30 .

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»

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1

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6 CD

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NEUTRQN ENERGY ,MEV

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cr

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NEUTRON ENERGY, MEV

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Fe

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20 . NEUTRON ENERGY .MEV

97

25 .

NICKEL

Sample Material: nickel 200 cold drawn rod

Sample Diameter: 5.08 cm

Sample Thickness: 4.46 cm, 71=0.4047

11.37 cm, n= 1.032

Analysis: Qualitative spectro^raphic analysis of the sample showed no impurities greater than 0.1 percent.

99
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cr

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101

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1 .2 NEUTRQN ENERGY. MEV

103

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104
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NEUTRON ENERGY. MEV

107

LEAD

Sample Material: metallic lead sheet

Sample Diameter: 12.7 em

Sample Thickness: 7.6 cm, 71 = 0.2614 atoms/barn

Analysis: A qualitative spectrographic analysis was made on the sample; the principal impurity was copper present to less than 0.1 percent. All other impurities were less than 0,01 percent PP", present

109
D U i) 1 • U NEUTRQN ENERGY ,MEV
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1

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NEUTR0N ENERGY, MEV

URANIUM

Sample Material:

Sample Diameter:

Sample Thickness:

Analysis:

enriched metallic uranium

1.91 cm

4.45 cm, w = 0.2136 atoms/barn

1.00 percent

93.22

0.38

5.39

C: 200 ppm by weight

Si: 200

Pe: 200

0: 80

P: 50

Ni: 30

Al: 20

Literature Reference:

Comments: The

R. B. Schv/artz, H. T. Healon II. J. Menke. and R. A. Schrack, Bull. Am. Phvs. Soc. 18 (1973)

539.

asured cross section has been corrected for the ""U and carbon content, using our previously nieasured values of these cross sections. Since the maximum correction was 0.4 percent any errors in the corrections would introduce a totally negligible error in the final cross section. Systematic errors

in the ftnal corrected cross sections due to the remainingimpurities are < 0.1 percent.

Acknowledgment: We should like to thank the Los Alamos Scientific Laboratory for providing us with the sample, and for doing the chemical and isotopic analysis.

119
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25 .

""'URANIUM

Sample Material: depicted uranium metal

Sample Sizes: 5.08 cm diam; 1.91 cm thick

5.08 cm diam; 4.45 cm thick,

1.91 cm diam; 4.45 cm thick;

= 0.0914 - 0.2128 = 0.2132

Analysis:

"U: 0.19 percent "U: 99.80 350 ppm by weight

400

10085

30

15

Literature Reference: R^B. Schwartz, H. T. Heaton I,, Menke, and R. A. Schraok, Bull. Ann. Phys. Soc. 18 a9,3,

Comments: l'—^^^^^^^^^^^^

inthe

the

final,

corrections would introduce a Slfn Jl lreT the' "/nil' '""""'T V' corrected, cross section due to tL gLtriti"e: a^Vrptr^nt'''^'TM'""

Acknowledgment: We should like to thank the Los Alamos Scientific r =h , , andforperformingthechemicalandTs^topTc anafysi ' "

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^"'PLUTONIUM

Sample Material: plutonium metal

Sample Diameter: L91 cm

Sample Thickness: 4.45

Analysis:

n = 0.2207 otoms/gran0.01 percent

0.37

0.02

260 ppm by weight

15

1510

Literature Reference: R, B, Schwartz. H. T. Heaton II, J. MenUe, and R. A. Schrack, Bull. Am. Phys. S„c. 18,1973, 539

139
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AI. Experimental Arrangement

Figure 1 is a bird's eye view of our tarect area w» n, i j t electron linear accelerator as a source of Su^rons The h

' f

of tlic picture, and passes throuT.

"'"'"^o"" Tlie beam enters from tile bottom

monitor' Since'tl,is se s ZJned f^rboZw"'"".'"""''"' "

neutron cross section measuremeS^ ireautmenrr"^^^^^^^

For cross section measurementTthe mamet i^T^ 1 ? f'i " """"""^

tion. The electron beam ererges' from thfrnaTM t"^

f"""^" our neutron producing target

Appendix-Experimental Techniques

PZ IONIZATION CHAM6E

NEUTRON SOURCE

NEUTRON TIME-OF-FLIGHT EXPERIMENTAL AREA

iRT PHOTOMULTIPLIER

ELECTRON BEAM LINE

FIGUBE 1 . Neutron time-of-flight target area.

' Figureain brackets indicate the lit«ratur« references at the end of the appendix.

^^etr'anTM?,!""""""? " "> evacuated flight path. The transmission samples are placed three meters from the neutron source. A2. Electron Linear Accelerator

„nl "^r^ accelerator has been described previously [2J, and hence TMl'rbe''Ltt"°n:dh:Jr " tlme.of.fllgi;t'ixpeHmrt:

BO nfeV The'T 'f' ">'"'»''«'"'="t» the linac is generally run at about hirer ihanthh T'f" °' ""ifary, as long as it is considerably higher than the highest energy neutrons to be measured. On the other hand, once the energy ,s chosen, ,t must remain stable over the course of a measuremen (several ?hetrar, [h 1

'^'^" ""^ P"' "< *e neutron spectrum remaTns the same

'he^'eutroli'p^o^^ctj L"rge^ '"""^^ ' '^'"'"^

and since the' vilTdT'''/"' "^'^fTM ""'^ Pe-- "eam burst, and since the yield of neutrons is fairly high, only very low beam currents are reouired

Ts IvT tr : "V °' APP-imately ten times more currentIS available in a 2 ns wide pulse.

whi I''!l'l'"'"' '"'"I""' " TM"'entional triode gun,= driven by a special pulser which allows a wide range of pulse widths: from several microseconds down to less

puls"es p"er "cordTM TM" " '° ^ " --ePe'i'i"" "te of 720 The average dark current is generally held to 0.01 percent of the average beam current, and hence causes no problem. ^..igcueam The beam spot size is - 3 mm in diameter. Once tuned up. the linac is generally quite stable in operation. Specifically the beam spot is stable in size and position, the beam current fluctuates by less than 10 percent, the energy remains within the 2 percent limits and the pulse shape remains

takinj"' ^'^ '''^^ ""''''''J' "ery important for accurate data

A3. Neutron Producing Target

To some extent, it is possible to "tailor" the neutron spectrum by the choice of neutron producing target. Very generally, in the McV energy range, higher Z targets produce more neutrons, but of low energy and lower Z materials will give fewer neutrons, but of higher energy. The observed spectrum will, of course, be modified by the variation in the detector efficiency ns a function of energy. In our early measurements (which only went to - 5 MeV neutron energy), cadmium targets 1 to 2 cm thick were used. The yield from cadmium tails off sharply above about 6 MeV, however so a com bination of - 0.1 cm tungsten, backed by - 3 cm beryllium is now used. The tungsten

provides the low energy neutrons and also acts as an efficient bremsstrahlung radiator

' Applied Radiation Corporation Model 10. (Thi. particular piece of equipment, and certain other com- merciai e,uipment. instrumenU and materials are identified in thi. Monosraph in order to .dequately specify the experimental procedure. In no case does such identification imply recommendation or endorse- ment by the National Bureau of Standards, nor does it imply that the material or equipment identified is necessarily the best svaiiabie (or the purpose.) equipment loentined is
for the beryllium which, in turn, supplies the higher energy neutrons. The difference

in thickness between the W and the Be is typical of the difference in yields between

high Z and low Z materials. The use of a target which is mainly low-Z is also important

in solving some of the problems associated with the gamma flash, as will be discussed

in section A4.3.

A4. Instrumentation

A4.1. Detector and Fast Timing Electronics The main function of the fast electronics is simply to provide a "start" signal, related in time to the production of the neutrons, and a "stop" signal, similarly related

in time to the detection of a neutron at the end of the flight path. These signals are then analyzed and encoded by a Time Interval Counter, (sec. A4.3) A block diagram of the electronics is shown in figure 2. Not included are such ancillary' units as fanoiits, pulse shapers, and delay cables, nor do we show the slow

electronics necessary to produce the logic signals for the on-line data handling system.

Figure 2. Btot^k diagram of electronics.

The shower developed when the electron beam strikes the neutron producing tar- get impinges upon a bare RCA 931A phototube (i.e., no scintillator is used). The photo- tube anode signal is fed to a zero crossing discriminator [3] whose output furnishes the

"start" signal The start signal performs several functions. It is fed to the Time

Interval Counter, initiates delayed gates and a marker generator, and is scaled by the computer. The neutron ("stop") detector is a 13 in. diam. by 5 in thick NE 21P liquid scintil- lator, viewed by 3 Amperex type 58AVP photomulti pliers. The anode signals are fed

to zero cross-over discriminators [3]. The outputs of the zero crossing discriminators go to a Two-Out-of-Three [4] coincidence gate and an OR gate. The Two-Out-of-Three coincidence requirement reduces the noise counts. The actual "stop" timing is derived from the output of the OR circuit. Before each run, the delays among the three phototubes and zero crossing discriminators are

carefully matched so that the centroids of the timing distributions for the three tubes

are exactly coincident in time (i.e.. total spread less than 200 ps) at the input to the OR circuit. For each individual event, however, there will generally be some spread

in the time of arrival of the three pulses. Since the Time Interval Counter is triggered by the leading edge of its input pulse, timing from the output of the OR circuit means

that the timing is actually determined by whichever of the (nominally coincident) pulses arrives first. This procedure makes the best use of the timing information from

the scintillator. Signals from dynode 14 of each phototube are summed in a fast adder-discriminator. Since the gains of the phototubes are carefully matched, this provides a con- venient method for setting the overall bias level, which is usually set at about 160 keV energy neutrons. The outputs of the Two-Out-of-Three coincidence, the "OR" and the fast adderdiscriminator are suitably shaped and delayed, and applied to the inputs of an AND

circuit, along mth the output of a delayed gate generator which is used to define the timing range and hence the energy range. If no neutron is detected during the time the delayed gate is open, the marker pulse generator provides an artificial stop pulse which

resets the Time Interval Counter.

A4.2. System Response Function We have used two approaches in determining the response function of our system:

synthesis and analysis. In the synthesis approach, we fold all the known contributions

to the response function to obtain the final overall response function. We have used

contributions from the following sources:

1. Electronic response function

2. Neutron detector thickness

3. Neutron source thickness

4. Electron beam pulse width

5. Timing channel bin width The electronic response function was determined by a coincidence technique using

^^Na y rays. For y rays whose pulse height was approximately equal to that of 0.5 MeV

neutrons, the response function was 3.5 ns FWHM. The detector and source response

functions are truncated exponentials determined by the mean free path of the neutrons and their flight time in the medium. The electron beam pulse shape was assumed

to be a 2 ns wide Gaussian, as determined from measurements of the gamma flash shape through thick absorbers.

* Manufactured by Nuclear Enterprises, Inc.
RESPONSE FUNCTION COMPONENTS AT .5

NANOSECONDS

FIGURE 3. Ehme„U of ,„po,„e /,,„ei,'„„, „alM<:i al-Sm k,V.

All of these contributions, evaluated at approximately 600 keV neutron ener«,

The t--^^:^^-t^:^ response function was also determined " by analysis of the 634 keV line in Si svstem soThaTth "i '^T. " -"-derably less than the response function of the ^unctl sh t! T i

transmission line is a good indication of the response £ a ea anaLr^TT' " '^-''-^^e"" -hape (determined by a, ea analysis of the transmission) was unfolded from the transmission. The re- sponse function thus obtained has a width of 2.4 kilovolts or 9 ns. The response func- tion used at 530 kilovolts is the compromise shown by the dashed line in figure 4 rnn i,' •f,'"'"''' '=''P<>"''e function improves rapidly above about mil keV neutron energy, since the electronic response function as well as effects due to source and detector thicknesses all improve at higher energies. The measured ana yzed) response function is about 6.5 ns (11 keV) at 2.1 IMeV and about 3 4 ns

(120 keV) at 14 MeV. Figure 5 shows the width of the response function as a function

Va m"i'TM" """•'Sy- The points are from analyses of narrow resonances. (The point at

14 MeV is the measured direct response to 14 MeV neutrons generated by the NBS Van de Graaff.)

A4.3. Gamma Flash When the linac electron beam strikes the neutron-producing target an inten.. th^ :^csrbrr:t isrctr'' -^--^ ./.i^Xot-::

pro'blemsr^' """"

2. An after-pulse, occurring approximately (iOO ns after the main pulse- and gamrrshVuTse"" approximately 750 ^s after the

ng The after-pulse is caused by the photoelectrons from the gamma-flash pulse ioniz the residual gas in the region between the photocathode and the first d mode t :"str^obird"afrpX'^-"°"°-'^ - th^pttti^^:

elector :^e?rcrtbrfirst"dytd::7:'sir:"t?e%ttr^^^^^^

i e., pulsing off the first few dynodes, w^ll not h?lp It was potted m^ "sua way

Malvano [6,, however, that a phototube can ^.Vi:rS.trZ'oilTSL:t
focus electrode. In our case, we are not really concerned with turning off the photo- tube per se: our object is specifically to reverse the electrostatic field near the photo- cathode so that the enormous number of photoelectrons generated by the gamma

flash are not accelerated down the photocathode-first dynode space. We do this by applying a 600 volt negative pulse to the focus electrode of the 58AVP during the gamma flash. Figure 6 shows the electrostatic field lines in the front end of a 58AVP as mapped

out on an analog electrostatic field plotter. (For clarity, we show the field lines which would exist with the photocathode at ground and -1-2600 volts on the anode; in actual practice the anode is at ground and -2600 volts is applied to the cathode.) Figure 6a shows the field lines during normal operation; figure 6b shows the field lines with -600 volts applied to the focus. Note the reversal of the field at the photocathode.

so that photoelectrons are not accelerated down to the first dynode.

After the applied pulse, the photomultiplier gain recovers as rapidly as the pulse decays, which in our case is about 100 ns (caused mainly by the stray capacity of the leads and the phototubes.) This method of ehminating the after-pulse also, of course, essentially eliminates most of the primary gamma flash pulse itself. Further details of the phototube pulsing as well as a circuit diagram of the pulser, have already been published [7]. While the phototube becomes rather noisy for several microseconds after the applied pulse, the Two-out-of-Three coincidence requirement eliminates this as a

58 AVP FIELD DISTRIBUTIONS

-600 Volt Pulse

Figure 6. Electrostatic field linen in the type 5SAVP photcnnuitiplier. The upper part of the figure shows the field lines when the -GOO volt pulse is applied

to the focus electrode, G,. The lower part of the fi^re shows the applied voltages and

resultant field lines during normal operation. For clarity, the voltages shown are those which would exist with the photocathode at ground and +2600 volts on the anode- In

actual use, the anode is run at dc RTound and -2600 volts is applied to the cathode. The

field lines are, of course, the same in either case.

problem. A much more serious noise problem is caused by the gamma flash itself. While the noise from the gamma flash pulse decays after about 750 ns, it is so intense during this period that a significant number of accidental coincidences are recorded. Requiring a three-fold (rather than 2/3) coincidence helps, but results in an appreciable

loss of smaller pulses. In addition to pulsing off the phototube, it was also found neces- sary to reduce the intensity of the gamma flash by using a 1 inch thick tungsten

filter in the beam and by using a neutron-producing target consisting largely of low-Z material (see sec. A3). In this way, we are able to make accurate measurements out to about 20 MeV, or 500 ns after the gamma flash pulse.

A4.4. Time Interval Counter As indicated earlier, the "start" and "stop" signals are analyzed by the Time

Interval Counter. This is a commercial digital timing device^ with one nanosecond

' Manufactured by Eldorado Electrodata Corporation.
and is mthin one m. " " ">"ker generator based on such a standard,

is stirtef put' ge^t^r td'foi" H

' ^o-ter

source. When properlfad^Tsted the ZZH^l'''v'""" r

" which cancels out in a L„Lisstonmett::e°^^ "

A4.5. Energy Calibration Our system allows convenient, absolute enerev calihr.tinn <R„ i, . »

f^:^rj^^-^j;:-Lr--drt^^^

2^rp-^JCtir:'rz^:e^rri-^

Since

lute

the TMdths of the timing channels on the counter are known, th a srtves an abso then measurement of the neutron time-of-flight. A measurement of th? path "enih gives a direct measurement of the neutron energy

tainSes i„"'th"","" """"^ Pnniarily from uncer- tainties n the timing measurements. These include nonlinearities of the Time Counter, long.term (i.e., a few days) drifts in the timing, and systematic timing differ cnces between neutrons and gamma rays. The non-linearities and long term drifts have both been carefully checked, and are less than 1 ns and V. ns, respectively While lor„h ^7 l'°

in the timing response of the scintilla- tor-phototubes-discrimmators between neutrons and gammas, it is known that NE does not show any difference in response between neutrons and gammas. The slewing between small pulses (due to low energy neutrons) and large pulses (from the gamma flash) is approximately 1 ns. The path length has been measured with a geodimeter» to an accuracy of 0 02 percent: this contributes a neghgible error to the energy scale uncertainty. A minor problem arises due to the detector thickness. The detector is approximately 4 mean

tree paths thick at 0.6 MeV, and 0.3 mean free paths at 20 MeV. Therefore (he low energy neutrons effectively stop near the front face of the scintillator, whereas, on the

average, the high energy neutrons stop nearer the center. Thus, the effective path

length IS greater for high-energy than for low energy neutrons. This difference in path length (amounting to almost 0.1%) can, however, easily be calculated and is cor- rected for. The final energies are all calculated relativistically. Taking all the above factors into consideration, the over-all uncertainty in the energy scale determination is 0.04 ns/m (e.g.. ± 3 keV at a neutron energy of 2 MeV). Below about 2 MeV this compares very favorably with the best work reported from Van de Graaff accelerators. At higher energies, some high-accuracy Van de Graaff experiments [8] have smaller errors, but, in any case, the time-of-flight method has

' We are grateful to Mr. George Lesley of the U.S. Coast and Geodetic Survey for making this measurement.

the great virtue of allowing a direct measurement. It is worth noting that there is renorted hvThlw " our energy scale and that reported by the Wisconsin group [8] (i.e., agreement within - 8 keV at 6 MeV neutron

energy).

A4.6. Data Handling The data from the Time Interval Counter, as well as the counts from the moni- tors various scalers clocks, etc. are all handled by the NBS on-line data handling system The general features of this system have been described previously [9], and

a detailed report of our use of the system has been published [10); hence, only the major features will be reviewed here. The on-line data handling system has been constructed around an XDS 920 computer. (16K core memory, 24 bit word, and 8ms cycle time). The modular concept has been applied to both the software and hardware design of the system. Associated with the 80 levels of priority interrupts are programs used to record the experimental data. The priority hierachy insures that the most important data will be recorded flrst after which lime the computer can do more routine functions, such as present a CRT display, output the data etc.

A5. Monitoring Two monitors are used: a secondary emission monitor (SEM) to monitor the elec- tron beam, and an NBS P2 ionization chamber (11) which monitors the shower created

in the neutron producing target. The ratios among the monitor counts and the total neutron counts are printed out periodically during an experiment. The constancy of these ratios during the course of the experiment is a measure of the overall stability

It IS important to note, however, that the spread in value of these ratios only repre- sents an upper limit to the error caused by any errors in monitoring. This is because in

ovir transmission measurements the samples are cycled in and out every twenty

minutes. Hence drifts, with time constants long compared to twenty minutes vrill cancel out.

'

A5.I. Secondary Emission Monitor The secondary emission monitor (SEM) consists of three aluminum foils each approximately 1 mg/cm= thick. A thin layer of platinum (- 25 Mg/cm') is evaporated on the aluminum surfaces to reduce ageing effects. Although work in this laboratory

indicates that an SEM may not give very reproducible results for high (> 20 /lA) elec- tron beam currents, the SEM is satisfactory for the low currents (- 100 nanoamps)

used in this experiment. The ratio of neutron counts to SEM readings show RMS

fluctuations of ~ Vh percent during the course of an experiment (i e two to three

days).

A5.2. Ionization Chamber

The more stable of our two monitors is a conventional NBS P2 ionization chamber

[111. The P2 is placed immediately behind the neutron producing target, and hence

effectively monitors the shower produced in this target. Although the P2 may over- load (due to recombination effects) at high currents, again, as in the case of the SEM,

at the low currents used in these experiments there is no sign of overloading. The

neutron-to-P2 ratios fluctuate by ± Vi percent during the course of an experiment.
A6. Sample Thickness Considerations

It had been customary to use fairJy thin samples-SO percent to 70 percent aver- age transniission-in total cross section measurements. In a low background situation, however, there are two factors which argue for the use of thicker samples-say, < 15 percent transmission.

First, Rose and Shapiro [13] long ago showed that where the backgi'ound is low, the optimum sample for minimum statistical error is approximately two mean free paths thick; i.e., transmission ~ 15 percent. Secondly, the over-all cross-section accuracy (as opposed to the statistical preci- sion) may be improved by the use of a thick sample. This is easily seen: since T= e^"", where T is the measured transmission, n is the areal density of the sample (in units, of, say, atoms/barn) and cr is the total cross section,

fl<T 1 dT

(We call the dimensionless quantity iicr tlie "sample thickness.") Thus, for example, an error in normalization will give a constant fractional error

in the trans »nission. Equation (1) shows that for this case, the resultant error in the

cross section may be reduced by simply running a thick sample. Equation (1) also ex- plains why a particular cross section measurement may be accurate where the cross

section cr is high, but have large fractional errors where a is low. On the other hand, eq (1) does not, of course, mean that one can achieve arbitrarily small errors in the final measured cross section by using arbitrarily thick samples.

In addition to losing statistical precision for samples whose thickness is far from optimum, for very thick samples (»a- >5; T < 1%) the in-scattering correction (see

sec. A7.3) becomes large and difficult to calculate, even in good geometry. More important, for thick samples, the background correction becomes large, and the ever- present uncertainties in the background can become the dominant error. In addition, if the instrumental resolution width is wider than the natural width of any structure to be measured, the observed peak cross sections will be higher for thinner samples. In other words, a thick sample worsens the apparent resolution. This is sometimes referred to as a "beam hardening" effect. A final complication derives from the simple fact that cross sections vary mth

energy, hence a particular sample which is "thick" at one energy may be very "thin"

at another energy. Thus, while it is clear that the sample thickness has an important function in de- termining the quality of a total cross section measurement, the procedure for choosing the "n" value of the sample is much less clear. Careful consideration must be given to

all the sources of error in a particular experiment, as well as the use to which the

final data will be put, before the optimum sample thickness can be sensibly chosen. In our case, the background is very low and well-behaved (see sec. A7.1). In addition, an important use of our data was to be as input in neutron transport calculations; for deep-penetration calculations the minima in the cross section are most important. Thus, relatively large "«" values seem to be called for. Such samples might, however,

be too thick to give any useful information in regions where the cross section is high" hence we generally ran with two different samples. The value for the thicker sample was commonly > 1; "n" for the thinner sample was a factor of 2 to 4 less.

We thus had samples appropriate to cover a wide range of cross section values and,

in addition, the data from the two samples provided an important check on internal

consistency. (The exact "i>" values used are listed for each element in the main body

of this report. It will be seen that even the "thin" samples are rather thicker than has been customary in this type of measurement.)

A7. Corrections to the Data

A7.1. Background The background is measured by simply inserting a "shadow bar" (32 cm of copper

plus 15 cm of polyethylene) in place of the transmission sample. The backgi-ound count- ing rate is generally less than 0.3 percent of the open beam counting rate. About half

of the background is due to cosmic rays and natural room background, and the other

half is associated with the linac beam. The background is flat and structureless, and

quite constant during the course of a run. We have investigated the question of whether the true background is adequately determined by this simple shadow-bar measurement. We have, for example, measured

counting rates with the detector off-axis, at "long" times-of-flight between machine

bursts, -with the beam purposely missteered, with various combinations of shielding and beam stops, etc. We find no significant background beyond that measured with the shadow bar technique. We note that our cross section measurements tend to be rather immune to background caused by neutrons which scatter off the walls of the measure- ment room and return to the detector, since such an event must occur within 4 fis of the beam burst (at which time our gate closes) and must give a pulse larger than a 160 keV recoil proton. The "black resonance" technique commonly used to measure backgrounds in the eV and keV regions can not be used in the same way in the MeV region, simply because

the peak heights are so much lower. Nevertheless, a measurement with black (or almost black) resonances does provide a valuable check on backgrounds determined from shadow bar measurements. This was generally the case in our "thick" sample measurements and the results (albeit often with rather poor statistics) were consistent with our assumed background. The 2.95 MeV resonance in carbon, for example, was

studied with a quite thick sample {n = 1,2) for which the transmission T was 2.4 percent, and with a thinner sample {n = 0.48) with 7'= 23 percent. The final value for the peak

cross section was the same for the two samples, despite the factor often difference in transmission, indicating that the background was accurately known.

A7.2. Dead Time

With a low duty cycle machine (maximum rep. rate = 720 pulses per second) and

an electronic system which will only record one count per pulse, it is difficult to obtain good counting statistics in each of several thousand channels in a reasonable amount

of running time. In addition to optimizing the sample thickness (sec. A6). it is also necessary to run at high counting rates, which unfortunately, means that large dead

time corrections are required. In practice we select a beam current such that during

the "open" runs, an average of one neutron per burst is detected bv the "stop" detector For this case, Poisson statistics show that the average number of neutrons actually recorded by the electronics is (1-1/e) per burst, and in the last timing channel (lowest energy neutrons) the ratio of true counts to recorded counts is equal to e. For a -one-shot" electronic system which completely recovers between pulses.

It has been shown, [14] however, that to first order the dead-time correction is a sim-
SpecSy;"'""'"" --'-"^ .-ntities.

recorded counts tro^ cZnIZ :LtTH-l7 " ^"^ °'

the c^r::;r°:^r:;^?i o;:r-r- o^d^^r^c^Lr:;:.^^-^-"-^-^-^

=i^'Sic5f~^

iiffj.lt different ""^'^ "P^'^ions experimentally, primarily' by running at neutron counfn,; rates and by recording "fiat" timing spectra from radio active sources at counting rates equivalent to those obtained under actuaT^unnin,

troduce negligibly small errors ,n tlie final result. This is further verified by the good agreement between thick and thin sample measurements. By us.ng thick samples and counting at relatively high rates, in ~ 60 hours of running time the statistical errors are approximately 2 percent over most of the 3500 channels.

A7.3. Inscattering The good geometry which is intrinsic in a time-of-flight experiment almost auto- matically insures that the inscattering correction will be small. The fractional change in the total cross section, due to inscattering is given by (16);

icr, ri-)!-...-) ro-(0)l

where

n is the sample-to-source solid angle,

r-. is the detector-to-sample solid angle,

is the detector-to-source solid angle,

cr(0) is the scattering cross section at 0°, in b/sr.

and ^rZL'^n'^?^ ^TM "inscattering index" by Foster LneSriy 0 or Ls' ' ?

* °" ' ^ '^<^ -cond term general y

O.o or less. The last term, which is essentially a multiple scattering correc tion to the in-scattenng varies betwppn i q e . ... . ^^'"'"s these measurement, H.„ 1 ^ sample thicknesses used in ir,yToTari::t'her"re: g:f„ir:7yT^:er^ —

AS. Summary

In the appendix we have described the NBS system for making accurate total cross section measurements in the MeV energy region, the main body of th?s Mono

future TMbl,"^ °' "^'^ — TM* this system. In future publications we will discuss some of these measurements in detail, and make comparisons with results from other laboratories.

We should like to thank Julian Whittaker for his inestimable contributions to the electronics and instrumentation. It is very doubtful that this program would have succeeded without his active assistance.

A9. References

't:;r.u„"„ maiizaiion, aecc'Sp ^lz' ""'°"n,"- »" ^ AtL CUNI--701002, Argonne, Illinois, p. 377 {Oct 21 1970) m wk'.';

' r^'/^t^

Accelerator Conf., Loa Alamos, New Mexico, p. 20 (Sept. 1666).

[3] Whitlaker.J.IC.IEEETranB.onNudearScienceNS-n.No 1 399(1966)

(4) Whittaker, J. K., Nucl. In8tr.andMeth.45. 138(1966)

[51 Mor^„.G^A Smith. H. M .„d W...eTM.„, R., ,EEE Tr.n.. Nucl. Sd. NS-I4. N,. 1. 443,1967,; Shin.

' ^' Glavint.. C, and Rawhns, J, A.. Nuel. Instr. and Meth. 58. 363 (1968)

I6j t anneUi. U., and Maivano, R., Kev. Sci. Instr. 29. 699 (1958, S

"''"°»' ""' K- B.. Nucl. In.tr. and Meth. 77. 176(1970,

[8] Davia. J. C, and Noda. F. T,. Nuel. Phya. A134. 361 (1969,

191 Broh.rj. J. B, IEEE Tr.n,. Nuc). Sd. NS-13, Nu. 1. 192 (1966,; Wyakuff, J. M., I.e. cil. 199.

[101 Heaton, H. T, 11. Nat. Bur. Stand. (U.S., Tach. Not. 516. 31 paee. (Jan 1970) M i! i

""•<•" 18 P«e«> (J^ne 1952).

[121 M. l.r. D. W.. Past Neutron Phy.ic. Part II, Chapter V. A., p. 986. J. B. Maiion and J. L Fo.l.r Ed. (Intersdence Publishers, New York, 1963,. u o

. i.. r owier. tda.

[13] Rose, M. E., and Shapiro, M. M., Phya. Rev. 74, 1863 (1948,.

(14) Boilmeer L^M^, arid Thomas, G E., Rev. Sci. Iratr. B2. '1044 (1961); Kirkbride, J.. Yate.. E, C. and Crandall, D. G., Nucl. Instr. and Meth. 52, 293 (1967).

[15] Bratenahl, A., Peterson, J. M., and Stoering. J. P., Phya, Rev. 110, 927 (1957)

[16] Foster, D. G., Jr., and Glasgow, D. W.. Phya. Rev. C3, 576 (1971).
roBM HBS-INA ll-iD

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4. TITLE AND SUBTITLE

NeV Total Neutron Cross Sections

January 1974

6. Perfonning Otgnoization Code

7- AUTHOB(S) R.B. Schwartz, R.A. Schrack, and H.T. Heaton II

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^ repoxt is a compilation of the McV neutron total cross section data measured at the National Bureau of Standards oirer the past several years. The measurements generally span the energy Interval from 0.5 to 15 or 20 MeV- data are Cotoo^f 2&,S1S^'"' '"S^ti"'

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