Monte-Carlo Methods and Applications in Neutronics, Photonics and Statistical Physics: Proceedings of the Joint Los Alamos National Laboratory - Commissariat à l'Energie Atomique Meeting Held at Cadarache Castle, Provence, France April 22–26, 1985

483 Pages·1985·19.14 MB·English

by  C. A. Philis

80MEASPECTS 0FNU~DATAEVALUATION AT
C.A. PHILI8 and .0 B]~SILIDN
Service de Physique et Techniques Nucl@aires
Centre d'Etudes de Bruy@res-le-Ch~tel
B.P. N" ,21
9~680BR~~Z~ATEL
Abstract
Transport calculations made in different fields of nuclear applications need know-
ledge of microscopic data, both complete and coherent, which for some nuclei exist
as an evaluated nuclear data file. The increasing number of available experimental
results are however, insufficient toa ssurteh e required qualities of a data set. So
we have to use the theory of nuclear reactions. The application varies from the
interpolation between ener~ ranges where there is a large number of experimental
results to data prediction for unmeasurable nuclei. The general method followed and
the application of nuclear models to obtain evaluated data are reported, and some
examples given.
.I INTRODUCTION
The wide field of nuclear applications requires particle transport calculations -
mainly neutron transport calculations. The computer codes describing these processes
demand a large amount of nuclear data to simulate, as well as possible, nuclear
interactions in materials. For these data, known as "evaluated nuclear data", the
definition could be the following: a set of nuclear data, complete, coherent and
recommended, under a computer file format which meets the requirements of the users.
Complete towards their needs, coherent as regards energy and cross-section balances,
and recommended as the best known. set
The purpose of this talk si to give you the outlines of the generalm ethod applied
at BR~-LE-CHATEL (BRC) to obtain these evaluated data [PR83], followed by some
examples. Although the term "evaluated nuclear data" covers the reactions induced by
different incident particles, we will focus onn eutron nuclear data evaluation.
EXISTING N[N/LFAR NE[IT~ON DATA LIBRARIES.
To meet the requirements of the users of an evaluated nuclear data set, the first
step is to look for a convenient evaluation in the existing evaluatedn uclear data
libraries for the application being considered. The main libraries consulted at BRC
are:
- Evaluated Nuclear Data File (ENDF),
_Evaluated - Nuclear Data Library (ENDL),
- esenapa_J _Evaluated Nuclear ataD_ Library (JENDL),
-Kerndatenbibliotek, Karlsruhe (KEDAK),
- Joint Evaluated File (JEF),
in their latest released versions. These data are obtained from the Data Basic
(Nuclear Energy Agency), one of the four interconnected centers, compiling and dea-
ling out through, out the world evaluated ~nd experimental data.
Before recommending such an evaluated data set to a user, we usu~lly check if pos-
sible the quality of the data~ comparing them between different libraries or with a
recent set of experimental results.
As a matter of fact, it happens that some ewaluations are performed for a given type
of application cor- responding to a field (for ex~nple an ener~y r~ige) on which the
evaluators have focused their attention. For other applications, the data will not
be as good. Two examples of inconsistencies which can exist in these files are given
in Figure 1-a. The comparison between the 5U(n,2n) cross-section data given in
FATDF/B-V and JF~DI~2 shows a difference of 50 % at the maximum of the peak. )~igure
1-b shows the differential neutron elastic scattering cross-section on deuterium
calculated in the laboratory fr~ne from both center of mass cross-section and trans-
fer matrix as given in ENDF/B-IV. The negative part of the cross-section between
90 ° and 125" is obviously unphysical.
EVALUATION 0FNUCLF~ DATA.
If the data set requested is not precise enough or does not exist, a new evaluation
has to be performed. Our basic approach to evaluation :si
-to use experimental data directly where possible,
- to systematically analyse experimental results with nuclear models,
- and to use the resulting theoretically-b~sed par~netrizations to clarify experi-
mental discrepancies and to extend evaluations to unmeasured energy regions or
nuclei.
With this philosophy, the outline of our alternative approaches for obtaining
nuclear data is shown in Pig~re ,2 and the nuclear model calculation part in more
detail in Figure .3
In order to handle, process, and display all the necessary data-experimental,
evaluated and calculated-, we have developed the program system "Synopsis". The
basic concept is given in Figure 4.
EXAF[PI~ OF ~VALUATIONS.
This general method is also applied in other laboratories, including some of the
U.S. laboratories with which we collaborate in the nuclear data evaluation field.
The main evaluations performed under this collaboration are sun~narized in Figure .5
To illustrate the complete evaluation of the four major tungsten )W( isotopes (182,
183, 184, 186) [AR81 ,] we have chosen on the one hand the 182W total cross-section
between I and 20 MeV compared with ENDF/B-V and experimental data sets as shown in
Figure ,6 and on the other hand the results of integral data testing for natural W
at 14 MeV incident neutron energy. These data are represented in Figure 7 where we
n a c compare the pulsed sphere experimental results versus Monte-Carlo calculations
from ENDF/B-V )a( and the new evaluation )b( I~T~F[ .] The improvement in the 5-13 MeV
energy range is clear, due mainly to our harder inelastic continuum spectrum which
includes preequilibrium effects.
Among the complete evaluations entirely performed at BRC, the bismuth (Bi) evaluated
data set [BE82] is illustrated in Figure 8. In Figure 8-a are compared the experi-
mental data results with the doppler broadened total cross-section reconstructed
from resonance parameters. The total gamma-spectrum following 14.5 MeV neutron
interaction is shown in Figure 8-b. If the agreement between calculated and experi-
mental [BESO] spectra is good for gamma-energies lower than 4 MeV, the same does not
hold for higher energies, where the cross-section is small and experimental uncer-
tainties are large.
Before concluding, let us come back to the two examples mentioned at the beginning.
The ENDF/B-V and JENDL-2 evaluations of the 23~U(n,2n) cross-section are quite
different, especially around 12 MeV, where ENDF/B-IV gives a cross-section of 0.46 b
whereas J]~NDL-2 gives 0.7 b. - that is a 50 % difference - 0nly new measurements
could remove this ambiguity. The recent measurements [FRSO] in the 6-13 MeV energy
range give near ~ MeV a value of 0.87 b, which is even larger than the two other
values mentioned (figure 9-a). These new results have been taken into consideration
in JEF-I.
In ENDF/B-V the neutron elastic angular scattering by deuterium is described by
Legendre coefficients in the center-of-mass frame. These coefficients are directly
transformed to the laboratory system by using the transformation matrix also given
in the file. Use of this matrix leads to a laboratory angular distribution which can
become negative, as illustrated in Figure 8-b. This undershoot arises for two rea-
sons: the matrix elements are incorrect and the matrix size is not large enough. Use
of a correct transformation matrix [BE83] gives a quite different behaviour for the
angular distribution, with no negative part. The experimental results are reproduced
well.
Among the evaluations in progress, the complete evaluation of deuterium is under
revision for the high energy range, paying special attention to the absolute
double-differential cross-section for the (n,2n) reaction.
~ION.
As far as nuclear data evaluation at BR~ES-LE-CHATEL is concerned, we have to
continue our effort to give the users the best evaluated data set available, to
enable more confident results in transport calculations. Thus, we must continue to
improve and develop:
I - nuclear models to refine data predictions,
2 - data handling systems as regards the increasing amount and varieties of data to
handle,
3 - the checking codes.
We must also be shrewd: to question data which may seen well-established, and to
request new, indispensable experimental results. We are fort~late that a large part
of this effort is made in collaboration with Los Alamos and we hope that it will
continue for the benefit of everyone.
REFER~CES
AR81 : .D.,F ARTHUR, P.G. YOUNG, A.B. SMITH, C.A. PHILIS, "New Tungsten Isotope
Evaluation for Neutron Energies between 1.0 and 20 MEW', Trans. Am. Nucl.
o S c. 39 (1981) 793-794.
B~BO : V.M. BEZOTOSNYI, V.M. GORBACHEV, M.S. ShUYESTSOV, L.M. SUROV, "Group and
Total Cross Sections of Formation of y-Ray Quanta upon The Interaction of
14-MeV Neutron with Various Nuclei", Soy. Atomic Energy 49 (1980) 690-694.
BE82 : .O BERSILLON, B. CAPUT, C.A. PHILIS "A New Evaluation of Neutron Data for
the 209~i between 10-5 eV and 20 MeV", Proceedings of the Internatio~m_l
Conference : Nuclear Data for Science and Technology, 6-10 september 1982,
Antwerp, pp. 665-668.
BE83 : 0.B}~KSILIDN, .A SH~TT, .B CAPUT, "On the transformation of Angular Scat-
tering Probabilities between Reference Systems: Survey and Numerical Ana-
lysis", NEANDC )E( 230 "L",INDC (FR) 59/L, October 1983.
ES81 : G.P. ESTF~, R.C. LITTLE, R.E. SF~ON, ~.D. ARTHUR, P.D. SORAN, "Calculations
Using New Tungsten Isotope Evaluations", Trans. Am. Nucl. Soc. 39 (1981)
794-796.
FR80 : J. A. FRP~IAUT, BF~TIN, .R BOIS, "Measurement of the 235U(n,2n) Cross Section
between threshold and 13 MeV", Nucl. Sci. .gn~] 74 (1980) 29-33.
F071 : D.G. FOSTER Jr. and D. GLASGOW, "Neutron Total Cross Sections, 2.5 -
15 MeV", Phys. Roy. C3 (1971) 576.
GUT9 : K.GUL, .A WAH~D, M. AHMAD, M. SALE~, NAF~AKHAN, "Inelastic Scattering
of 14.8 MeV Neutrons from Deuterons", .J Phys. G 5 (1979) 1107-1116.
GU82 : P.T. GUENTHER, A.B. SMITH, J.F. WHALEN, "Fast Neutron Total and Scattering
Cross Section for 18~, W 184 and 186W", Phys. Rev. C 26 (1982) 2433-2416.
MA67 : R.C MARTIN, P.F. YERGIN, R.II. AUGUSTON, N.N. KAWSHAL, H.A. MEDICUS, E.J.
WI~OLD, "MeV Neutron Total Cross Sections of Ta and W Isotopes", Bull. Am.
Phys. Soc. 12 (1967) 106.
MA72 : D.S. MATHER, P.F. BAMPTON, R.E. COLF~, G. JAMES, P.J. NIND, AWRE Report
° N 072/72 I( 972).
~66 : .S F~SSELT, "Neutron Spectra from Breakup of deute- tons by 14.7 MeV
Neutrons", Nucl. Phys. 48 (1963) 512-516.
PR83 : Compte rendud'activit@ du Service de Physique Neutronique et Nucl@aire pour
l'ann@e 1983", Note CEA N-2396, NEANDC )E( 234 "L", INDC (FR) 62/L, 0ctobre
984.
S176 : U.NSINGH, J. RAINWATER, H. .I LIOU, G. HACKEN, J.B. GARG, "Neutron Resonance
Spectroscopy: 209Bi", Phys. Rev. C 13 (1976) 124-127.
7
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Main evaluations performed in collaboration
with U.S. laboratories.
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