Experimental Neutron Resonance Spectroscopy

538 Pages·1970·8.895 MB·English

by  J. Harvey (Eds.)

Contributors to This Volume
ROBERT C. BLOCK
LOWELL M. BOLLINGER
F. W. K. FIRK
W. M. GOOD
E. MELKONIAN
M. S. MOORE
ERNEST R. RAE
EXPERIMENTAL NEUTRON
RESONANCE SPECTROSCOPY
Edited by J. A. HARVEY
Oak Ridge National Laboratory
Oak Ridge, Tennessee
1970
ACADEMIC PRESS New York and London
COPYRIGHT © 1970, BY ACADEMIC PRESS, INC.
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TO
DAD
LIST OF CONTRIBUTORS
Numbers in parentheses indicate the pages on which the authors' contributions begin.
ROBERT C. BLOCK, Department of Nuclear Science, Rensselaer Poly­
technic Institute, Troy, New York (155)
LOWELL M. BOLLINGER, Argonne National Laboratory, Argonne,
Illinois (235)
F. W. K. FIRK, Yale University, New Haven, Connecticut (101)
W. M. GOOD, Oak Ridge National Laboratory, Oak Ridge, Tennessee (1)
E. MELKONIAN, Columbia University, New York, New York (101)
M. S. MOORE,* Idaho Nuclear Corporation, Idaho Falls, Idaho (347)
ERNEST R. RAE, Nuclear Physics Division, Atomic Energy Research
Establishment, Harwell, Great Britain (1, 155)
* Present address: Los Alamos Scientific Laboratory, Los Alamos, New Mexico
ix
PREFACE
The original intention of this book was to cover the experimental tech­
niques of neutron cross-section measurements by the time-of-flight method
up to neutron energies of ^10 keV. This energy limit was selected since a
comprehensive survey for higher energy neutrons was available. [J. L.
Fowler and J. B. Marion, "Fast Neutron Physics," Wiley (Interscience),
1960 and 1963.] However, great improvements of pulsed neutron sources
and experimental equipment have been made in recent years and the time-
of-flight method now excels up to the MeV neutron energy region. Several
conferences have been held during the past decade on various subjects
covered in this book, such as intense neutron sources, neutron time-of-flight
methods, nuclear structure studies with neutrons, neutron cross sections,
and technology, etc.; and there are many publications of detailed results.
However, no unified presentation of the subject is available. As such, all
chapters of this book have comprehensive bibliographies for the reader who
desires supplemental information.
Chapter 1 deals mainly with the characteristics of time-of-flight spectrom­
eters using pulsed electron and positive-ion accelerators, presently the most
versatile of all neutron spectrometers. Other neutron sources continue to be
used since they sometimes have unique characteristics which are valuable
for particular experiments. For example, the "fast-chopper" spectrometer
(a mechanical rotor at a high-flux steady-state reactor) was the major source
of neutron resonance parameters one to two decades ago. This spectrometer
is useful for transmission measurements upon samples available only in
small quantities and is also used for spectral measurements of gamma rays
following the capture of low energy neutrons. The use of a nuclear explosion
as a pulsed neutron source is invaluable for measuring fission and capture
cross sections of highly radioactive nuclides and of nuclides which are
xi
xii PREFACE
available only in minute quantities. For certain experiments on a single
low-energy neutron resonance a crystal spectrometer at a high-flux reactor
may have advantages. However, only the pulsed accelerator spectrometer
is reviewed in detail in this chapter since it can be used for measurements
over the entire neutron resonance energy region.
The other chapters cover the experimental techniques, such as detectors,
data acquisition equipment, methods of analysis, etc., which are used for
neutron cross section measurements, and the interpretation and the signifi­
cance of the results. Chapter 2 on neutron total cross sections contains a
brief treatment of neutron resonance theory which is needed in the analysis
of total cross section data. Parameters of resonances obtained from trans­
mission measurements many years ago were promptly used by theoreticians
for two important discoveries—the Porter-Thomas distribution of neutron
widths and the Wigner distribution of resonance spacings. Total cross
section measurements are still the greatest source of resonance data today.
Scattering measurements, which are used to determine angular momenta
and parities of resonances, and capture measurements, which can be used as
a sensitive technique for detecting very weak resonances, are reviewed in
Chapter 3. Chapter 4 on gamma-ray spectra from the capture of neutrons in
resonances deals with this powerful technique for obtaining information on
both the capturing and final states and also for learning about the neutron
capturing reaction. The detailed and varied experiments which have been
performed on the complicated fission process are discussed in the final
chapter. The recent discovery of intermediate structure in subthreshold
fission can be interpreted in terms of a second minimum in the nuclear
potential well. This final chapter contains a comprehensive summary of the
parameters of the resonances of the fissile nuclides.
The editor would like to thank the authors and publisher for both their
patience and cooperative efforts in trying to bring out a volume as up to date
as the rapidly changing state of the art would allow.
PULSED ACCELERATOR
TIME-OF-FLIGHT SPECTROMETERS
E R N E ST R. R AE
ATOMIC ENERGY RESEARCH ESTABLISHMENT
HARWELL, GREAT BRITAIN
W. M. G O OD
OAK RIDGE NATIONAL LABORATORY
OAK RIDGE, TENNESSEE
I. Introduction 2
A. Scope of Chapter 2
B. Resolution and Intensity 3
II. Pulsed Accelerator Spectrometers with Moderated Continuous
Neutron Spectra 5
A. General Considerations 5
B. Early Work 9
C. Current Technology and Applications 13
D. Comparison of Spectrometer Performances 58
III. The Pulsed Van de Graaff 66
A. Introduction 66
7 7
B. Description of Pulsed Van de Graaff Neutron Spectrometer 68
C. The Li(p,n)Be Neutron Source 73
D. Applications 75
E. Theory and Critique of the Pulsed Van de Graaff . .. 88
References 94
1
2 ERNEST R. RAE AND W. M. GOOD
I. INTRODUCTION
A. Scope of Chapter
It is well known that the resonance level spacing is a nuclear quantity
that decreases in a systematic fashion with increasing mass number except
in the region of magic mass numbers where the spacings become large in
comparison to those of neighboring mass regions. This resonance spacing
may be in the order of 1 MeV for light nuclei, of several kiloelectron volts
for medium mass number nuclei, and less than 1 eV for heavy nuclei. For
the purpose of the present chapter we define the resonance energy region
to cover the energy range where only capture, fission, and elastic scattering
are energetically possible, i.e., inelastic scattering is excluded. Hence, we
consider resonance neutrons as those having energies from ^1 eV to ^100
keV. The only technique that has proved successful for studying this
entire energy region is that of neutron time-of-flight using a continuous
energy spectrum neutron source. From the optical analogy it is appropriate
to speak of such a source as a "white spectrum" source.
It is obvious from the relatively shorter times involved, that the higher
a given neutron's energy, the more precisely its flight time for a given
distance must be determined, or in time-of-flight parlance the shorter must
be the neutron burst. For this reason mechanical means of producing short
bursts of neutrons, namely choppers, for higher energies must eventually
be replaced by electromagnetic devices. Such devices are, of course, pulsed
accelerators, any one of several types, and the neutrons are produced in a
charged-particle nuclear reaction, in which the duration of the neutron
burst is almost the same as the duration of the beam of charged particles
on the neutron-producing target. In accordance with the opening para­
graph, this chapter is concerned with continuous-spectrum neutron sources.
It is convenient in the following discussion to classify continuous-spectrum
neutron sources into three distinguishable types: (a) bounded, continuous-
spectrum sources which are characterized by well-defined upper and lower
energy limits. For these sources the neutron producing target is thin and
the neutron energy spectrum is a consequence of target thickness; and
the instrument most commonly associated with such a source is the pulsed
Van de Graaff. Next (b) boil-off or Maxwellian-type continuous sources.
For these the neutron spectrum is intrinsically boil-off or Maxwellian in
character with a neutron yield which rises to a maximum at some con­
siderable fraction of 1 MeV. Such spectra result from the bombardment
of a high-Z target by any type of particle if the bombarding energy is
above, say, 20 MeV. Examples of instruments of this type are the electron
I. PULSED ACCELERATOR TIME-OF-FLIGHT SPECTROMETERS 3
linac and the positive ion cyclotron. Finally, (c) moderated continuous
spectra in which the original intrinsic spectrum is changed to a l/E spec­
trum by moderator action or the original boil-off spectrum shifted to lower
effective temperature by placing a moderator after the target.
Almost any instrument can be utilized in the form of (c), but to be
effective, the primary beam energy and/or current must be high. Any
instrument producing an intrinsic boil-off spectrum can be used without
moderation as in (b) to study interactions above a few hundred kiloelectron
volts, in such cases, it is advantageous to use deuterons because with
deuterons the direct reaction component serves to enrich the high-energy
part of the neutron spectrum which may frequently be of interest. An
instrument constructed along these lines for neutron research at higher
energies, e.g., above 100 keV, has recently been given special emphasis at
Karlsruhe. It should be noted that for producing the highest neutron ener­
gies, however, charge exchange reactions with protons are most useful since
a cyclotron can accelerate protons to twice the energy of deuterons and the
neutron can take the whole of the proton energy. From our definition of
resonance neutrons, instruments of type (b), although working on the time-
of-flight principle, are outside the scope of this chapter.
The instruments to be considered in this chapter fall either into type (a)
or type (c). Type (a) is limited to the pulsed Van de Graaff; type (c) has
as examples the linacs, the 400-MeV synchrocyclotron (Nevis), and others.
As an instrument for the laboratory devoted exclusively to neutron physics,
the electron linac has emerged as the most versatile of all instruments. It
has emerged in place of the Nevis-type synchrocyclotron because the higher
relative cost of the latter makes it necessary for its activities to be shared
between high-energy physics and neutron physics with some loss of flexi­
bility. It has emerged in preference to the Van de Graaff because the latter
is restricted to neutron energies above 2 keV, and the electron linac is
superior to the Van de Graaff for most cross-section measurements. The
present chapter, therefore, will be devoted in greatest detail to the linac.
Other instruments, especially the 400-MeV synchrocyclotron and pulsed
Van de Graaff, are included essentially as special purpose devices, useful
if available, but not suitable exclusively as instruments for investigation
in resonance neutron physics.
B. Resolution and Intensity
There are two properties, related to some extent, that must be considered
in a discussion of time-of-flight instruments, as indeed they must be con­
sidered in a critical discussion of any measuring instrument. These two
properties are resolution and intensity.