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NUCLEAR FISSION
AND
NEUTRON-INDUCED
FISSION
CROSS-SECTIONS
G. D. JAMES {AERE Harwell)
J. E. LYNN {AERE Harwell)
A. MICHAUDON {CEA)
J. ROWLANDS {UKAEA)
G. de SAUSSURE {ORNL)
PERGAMON PRESS
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First edition 1981
British Library Cataloguing in Publication Data
Nuclear fission and neutron-induced fission cross
sections. -(Neutron physics and nucleardata in
scienceand technology; vol. 1)
1. Nuclear fission
I. James, G. D II. Series
539.7'62 QC790 80-41822
ISBN0-08-026125-6
In ordertomakethis volume availableaseconomically
and asrapidly aspossible theauthors'typescripts have
been reproduced in their originalforms. This method
unfortunatelyhasitstypographical limitationsbut itis
hopedthat they inno waydistract thereader.
Printedand boundinGreatBritain by
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PREFACE
This book is the first in a series on Neutron Physics and Nuclear Data in Science
and Technology sponsored by the Nuclear Energy Agency Nuclear Data Committee
(NEANDC). This Committee brings together representatives from the OECD Nuclear
Energy Agency member countries in the interest of international collaboration and
information exchange in the field of the technical applications of nuclear data.
During the past twenty years the principal interest of this committee has been in
the application of neutron physics data in the nuclear energy programmes of the
member countries. While the emphasis in neutron physics during this period has
shifted towards high accuracy measurements to satisfy the ever more stringent
requests from the reactor physics community, neutron physics has continued to
play an important role in the study of the fundamental understanding of nuclear
reactions. For example, in the field of fission studies considered in this book,
the discovery of intermediate structure effects in sub-threshold neutron-induced
fission cross sections has greatly contributed to a better understanding of the
fission process through the double-humped barrier concept. Similar examples
in other fields will be given in further books in this series.
The purpose of this series of books is to provide a thorough description of the
many aspects of neutron physics in various fields of nuclear applications, and at
the same time to attempt to bridge the communication gap between scientists invol-
ved in the experimental and theoretical studies of nuclear properties and those
involved in the technological applications of nuclear data. Many topics of current
interest have been selected by NEANDC and NEACRP , of which Neutron-Induced Fission
is the first. For most of the chosen topics, the publication will typically include
a description of the basic physics involved, a presentation and justification of the
needs for the data relevant to the technological applications, and a thorough des-
cription of the various methods, both experimental and theoretical, which are used
to obtain data of the required accuracy. In such an interdisciplinary approach an
effort is made to treat the various aspects of the subject in a coherent manner,
but some inevitable compromises are necessary between a simplified approach for the
non-expert, and the provision of detailed information for specialist readers.
Although care is taken to make the presentation as homogeneous as possible, some
differences do arise between chapters, reflecting the styles of the authors, and
the different concepts and vocabulary used in the various fields of specialisation.
The scientific level of the series of books is suitable for science graduate stu-
dents, and it is hoped that the subject matter will be of interest over a wide
range of disciplines.
The preparation of such a book requires a considerable voluntary effort from
many people: primarily from the authors who, in addition to presenting their own
fields of specialist activity, have also needed to adapt their presentation in the
interest of homogeneity of the various contributions. The manuscript has been
prepared by the staff of the OECD Nuclear Energy Agency, under the guidance of
Dr. P. Johnston, with secretarial help from Mrs C. Lopez and Miss S. Greenstreet.
The editors' and the authors' thanks are also extended to the many scientists who
have carefully read drafts of the chapters and whose very valuable comments have
contributed to completeness and exactitude of the contents and to Mrs. Godefroy
(CEA) who carefully verified all references.
Nuclear Energy Agency Committee on Reactor Physics (OECD)
v
vi Preface
The future of this series depends first on the competence and the courage of the
scientists who are to write the forthcoming volumes. Writing is rewarding but also
a painful experience. Support and encouragement from the scientific communities are
very helpful and the success of this series strongly depends also on the readers
whose remarks and comments are most welcome to improve the quality of the books to
come.
A. MICHAUDON
September 1980
LIST OF ABBREVIATIONS
AERE Atomic Energy Research Establishment (U.K.)
ANL Argonne National Laboratory (U.S.A.)
BNL Brookhaven National Laboratory (U.S.A.)
CBNM Central Bureau of Nuclear Measurements (JRC-EEC, Belgium)
CEA Commissariat à l'Energie Atomique (France)
CINDA Index to the Literature on Microscopic Neutron Data (I.A.E.A.,
Vienna, Austria)
CJD Centre for Nuclear Data (Obninsk, USSR)
CRNL Chalk River National Laboratory (Canada)
CSEWG Cross-Section Evaluation Working Group (U.S.A.)
ENDF/B Evaluated Nuclear Data File (version B) (U.S.A.)
IAEA International Atomic Energy Agency (Vienna, Austria)
KFK Kernforschungsgentrum Karlsruhe (Federal Republic of Germany)
LASNL (or LASL) Los Alamos Scientific National Laboratory (U.S.A.)
LBNL (or LBL) Lawrence Berkeley National Laboratory (U.S.A.)
LLNL (or LLL) Lawrence Livermore National Laboratory (U.S.A.)
NBS National Bureau of Standards (U.S.A.)
NDS Nuclear Data Section (I.A.E.A., Vienna, Austria)
NEA Nuclear Energy Agency (O.E.CD.)
NEADB Nuclear Energy Agency Data Bank (O.E.CD.)
NEANDC Nuclear Energy Agency Nuclear Data Committee (O.E.CD.)
NEACRP Nuclear Energy Agency Committee on Reactor Physics (O.E. CD.)
NNDC National Nuclear Data Center (U.S.A.)
OECD Organisation for Economic Cooperation and Development (Paris,
France)
ORELA Oak Ridge Electron Linear Accelerator (U.S.A.)
ORNL Oak Ridge National Laboratory (U.S.A.)
WRENDA World Request List for Nuclear Data (I.A.E.A.)
xv
Chapter I
INTRODUCTION
A. Michaudon
Commissariat a l'Energie Atomique, France
Since its discovery in 1938 /HS 20/, the fission phenomenon has played a special
role both in fundamental nuclear physics and in the field of applications. The
break of an actinide nucleus into two heavy 'fragments deexciting "in flight" by
prompt-neutron emission could rapidly be understood, at least qualitatively^ by
a macroscopic description of the process using the liquid drop model /BW 39/.
Yet, many aspects of fission cannot be explained in terms of this crude model.
Even now, despite all the progress made since then, using more refined microscopic
models and very sophisticated experimental methods, a thorough understanding of
the fission process still remains a challenge for nuclear physicists. In contrast
to many other nuclear reactions, fission is of paramount importance for energy
applications. It was realized very early that the large amount of energy (typi-
cally ~ 200 MeV) released in a single fission event combined with the emission of
prompt neutrons capable themselves of inducing other fission events could make
the existence of neutron-chain reactions possible, liberating an enormous quantity
of energy per unit mass of fissionable material (see for example the work con-
tained in /jJC 6_1/) . All this energy could be produced either instantaneously in
the form of a nuclear explosion or continuously and in a controlled manner in a
so-called nuclear reactor. The extreme importance of such energy applications
led to intensive studies of fission and related phenomena, ultimately resulting
in the first man-built nuclear pile in Chicago (1942), under the leadership of
E. Fermi and in the first nuclear explosion at Alamogordo (1945) within the frame-
work of the gigantic and then secret Manhattan project..
To restrict oneself to civilian applications only, one may notice that the subsequent
development of nuclear energy was also very rapid. After the construction of a
moderate-size reactor at Oak Ridge (1943) and of the more powerful reactors at
Hanford (1944), electricity was first obtained from fission at Idaho (1951). The
construction around the world of nuclear reactors which soon followed this early
stage was recently stimulated by the oil crisis which convinced more countries to
adopt nuclear power earlier or more rapidly than they had anticipated. In the
meantime, it was discovered in the soil of Oklo (Gabon) that nature had succeeded
much earlier than man in achieving neutron-chain reactions, over an extended period
of time, about two billion years ago /Neu + 72/.
The case of pulsed reactors is not considered here since they are not used for
energy applications.
1
2 A. Michaudon
The subject of this book concerns fission, but treated in a way that is oriented
towards civilian applications. Among these applications, only nuclear energy is
considered here because it is by far the most important. Therefore, minor appli-
cations such as therapy by irradiation from prompt neutrons emitted by Cf
spontaneous fission sources are ignored. Among energy applications, attention is
paid almost exclusively to nuclear systems that are either in operation or planned
on a large scale. These systems include thermal reactors, in which the fission
neutrons are moderated to thermal energy before they induce fissions, and fast
breeder reactors which contain no moderator but in which nevertheless the fission
neutrons have their energy slightly degraded in the reactor materials before they
induce fissions. Because of their different energy spectra, fissions occur pri-
marily in the fissile nuclei ( U) for thermal reactors, whereas for fast breeders,
fissions are induced not only in the fissile nuclei ( U and Pu) but also in
some non-fissile ones (such as U and Pu) since a fraction of the fast neutrons
have an energy above the fission threshold for these nuclei.
More futuristic designs such as hybrid systems (consisting in a fusion device
surrounded by a blanket of fertile material), accelerator-breeder concepts (in which
an accelerator beam of charged particles produces spallation neutrons by bombard-
ment of a target which is itself surrounded by a blanket of fertile material),
etc... are taken into consideration but not treated in detail here since their
possible development still lies in the remote future.
The above considerations explain the organisation of this book which describes in
broad terms the fission process and also explains in more detail i) why neutron-
induced fission cross-sections are needed for the nuclear energy programme;
which ones and to what accuracy and ii) how these cross-sections can be obtained
both experimentally and theoretically to meet the above requirements.
In Chapter II, a general presentation of our present knowledge of fission is given.
This book does not discuss in detail the fission process which is treated more
thoroughly elsewhere in specialised books, review articles or conference proceed-
ings. The emphasis here is restricted to those aspects of fission relevant to
energy applications. For example, after a broad description of the various phases
of the fission process, attention is focused on the properties of the low-energy
fission of actinide nuclei and on neutron-induced fission. These aspects of fission
play an important role both for nuclear reactors and for the experimental and
theoretical methods used to obtain neutron-induced fission cross-sections of in-
terest for applications.
Chapter III deals with the needs for fission cross-section data in the nuclear
energy programme. The demonstration of the feasibility of a nuclear reactor dates
back to 1942 and many nuclear reactors are now in operation throughout the world.
Therefore, it might seem, at first sight, that the basic nuclear data are already
known. This point of view does not take into account the fact that the calcula-
tional methods for reactors have become very precise and that the basic reactor
parameters need to be known with great accuracy in order to reduce the costs of
constructing and operating these reactors on a large scale. This requires that
many nuclear data, including those for fission cross-sections, which are used in
the reactor calculations are also known with a great accuracy. To be more precise
the uncertainties which can be tolerated in the nuclear data are related to those
of the reactor parameters through sensitivity calculations. The required accuracy
for microscopic nuclear data thus obtained cannot often be met either by experi-
ments or by calculations. It is therefore necessary to supplement the programme
of measuring and calculating nuclear data by integral measurements. All the above
aspects are treated both for thermal reactors and fast breeders, with the main
emphasis on the fast breeders since they represent the most important source of
nuclear data requests for the nuclear energy programme. Requests are usually made
Introduction 3
for a neutron energy range from thermal energy up to 20 MeV. This range is wide
enough to include the energy of fusion neutrons (~14 MeV) produced in the (D + T)
reaction and is therefore quite appropriate to cover also most of the data needs
for anticipated fusion reactors. When more futuristic systems are developed, the
neutron energy range for which fission data are requested is likely to extend
towards even higher energy but this is not considered here. The lists of nuclear
data requests are generally made first at the national level, for example through
nuclear data committees and then compiled at the international level through
relevant agencies (OECD/NEA, IAEA, etc.) where a coordination of the work is made
in order to meet these data requests.
Chapter IV is devoted essentially to the measurements of fission cross-sections.
Some of the requests are so stringent that they can be met only through extremely
precise measurements. In many cases, the required accuracy is even beyond the
capability of presently-available techniques. In order to understand the situa-
tion as thoroughly as possible, this chapter starts by recalling how cross-sections
in general are obtained. It is essential at this stage to make a clear distinction
between absolute and relative measurements. A lot of confusion arises because,
sometimes, measurements are called absolute whereas they are in fact relative, and
no reference is made to what they are relative. When data uncertainties are quoted,
they are usually of statistical origin only, however mention is rarely made of
possible systematic errors in the measurements. This results in underestimated
data uncertainties and in frequent inconsistencies between data obtained by dif-
ferent methods; hence the importance of providing thorough documentation when
giving the results of measurements. In order to understand better the techniques
used in fission cross-sections measurements and, consequently, the possible sources
of systematic errors, great attention is paid here to the detection of fission
events either by the fission fragments or by the particles or radiation emitted
in fission. To illustrate the methods used in the fission cross-sections measure-
ments, selected examples are given. Practically, all these examples concern U,
not only because this nucleus plays an essential role in nuclear energy, but also
because its fission cross-section is the best known of all and is often used as a
standard relative to which many other fission cross-sections are measured. The
vast amount of data obtained with modern neutron generators and their associated
equipment makes data handling, reduction, storage and dissemination difficult.
This aspect is touched upon in this chapter together with data analysis in the
resonance region where the problem is most difficult. The evaluation of nuclear
data is an important aspect of the use of these data for applications. Measured
data, especially if they come from different sources and are inconsistent, cannot
be used directly in big reactor computer codes. These data must firstly be care-
fully compiled and examined, their confidence level must be assessed and, finally,
recommended values with their uncertainties must be obtained and presented in the
required format for reactor calculations. This whole delicate process, called
evaluation, is an essential step in the cycle of nuclear data from their acquisi-
tion to their final use, but is too vast to be treated in detail here. Rather,
evaluation is presented in broad terms in Chapter IV (though theory is also
partially involved) and illustrated by the case of the U fission cross-section,
for consistency with the selected examples of cross-section measurements discussed
above. Finally, a comparison of the measured results with requirements is made
at the end of the chapter and the trend for future measurements is indicated.
In Chapter V, the calculation of fission cross-sections using nuclear theory is
presented. Measurements are already known to be essential for obtaining fission
data especially with the required accuracy. But, measurements cannot be made on
nuclei for all the requests, for example some of them are for very radioactive
materials or for samples available in too small quantities. Therefore, measurements
must be supplemented by calculations. These calculations are not easy for several
reasons. Fission theory is still in childhood despite all the progress achieved
4 A. Michaudon
since the discovery of the process. The basic physics of this process, as given in
Chapter II, is far from being sufficient to make possible accurate calculations of
fission cross-sections. This requires, for example, the precise knowledge of the
fission barrier which cannot be obtained even by using the most sophisticated models
presently available. To obtain reliable fission barrier parameters, they must be
derived from fits to relevant experimental fission data. Also, there is a basic
difference between measurements and calculations of fission cross-sections. In the
first case, the measured fission cross-section can usually be obtained alone, with-
out the knowledge of any of the other cross-sections (this is not quite true for
the determination of the fission width which requires an additional measurement).
In contrast, the calculation of the fission cross-section requires the determina-
tion of all the other cross-sections including the cross-section for compound
nucleus formation and the decay of the compound-nucleus in all channels. It is
therefore necessary to have a complete picture of the interaction of neutrons with
nuclei. As for fission itself, this interaction cannot be described accurately
from pure nuclear theory and requires the adjustment of the relevant parameters to
measured data. These considerations explain why this chapter needs to be fairly
broad in scope and has to treat many aspects of nuclear theory: the compound
nucleus, the transmission coefficients for decay by neutron and gamma-ray emission
as well as for fission, the level density, etc. The specific methods of fission
cross-section calculations are then presented and illustrated by several examples
before the comparison of the calculated results with the requirements is discussed.
A general conclusion is given in Chapter VI.
This book is a joint effort of five physicists living in three different countries.
This explains inevitable changes in style and some overlap from one chapter to
another. The time taken to put the contributions together has meant that work
published after 1978 is not usually considered, except when important up-dating
could be incorporated without major modifications.
The material presented in this book represents also an unusual cut through the
whole subject of fission. Usually, the various aspects discussed here are treated
separately in publications of different types not consulted by the same people.
In this volume, an attempt has been made to mix all the different aspects in a
coherent manner in the hope that the reader will find it a convenient and intel-
ligible overview of the subject.
