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Journal ArticleDOI

Neutron interactions and atomic recoil spectra

01 Oct 1994-Journal of Nuclear Materials (North-Holland)-Vol. 216, pp 29-44

Abstract: This paper presents a discussion of neutron interactions with materials that lead to activation, transmutation, and atomic displacements. The emphasis will be on current applications including neutron irradiation facilities, neutron dosimetry techniques, and computer codes for spectral adjustment and radiation damage calculations.
Topics: Neutron cross section (67%), Neutron scattering (66%), Neutron (65%), Neutron source (61%), Neutron flux (58%)

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L. R. Greenwood
August 1993
Presentedat the
InternationalSummer Schoolon
Fundamentalsof RadiationDamage
August I-12, 1993
Urbana, Illinois
Work supportedby MASTER
the U.S. Departmentof Energy
' under ContractDE-ACO6-76RLO1830
Richland,Washington 9935Z
This report was prepared as an account of work sponsoredby an agency of the United States
Government. Neither the United States Government nor any agency thereof, nor any of their
employees, makes any warranty, express or implied, or assumes any legal liability or responsi-
bility for the accuracy, completeness, or usefulness of any information, apparatus, product, or
process disclosed, or represents that its use would not infringe privately owned rights. Refer-
ence herein to any specific commercial product, process, or service by trade name, trademark,
manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recom-
mendation, or favoring by the United States Government or any agency thereof. The views
and opinions of authors expressed herein do not necessarily state or reflect those of the
United States Government or any agency thereof.

Neutron Interactions and Atomic Recoil Spectra
L. R. Greenwood
Battelle, Pacific Northwest Laboratories
This chapter presents a discussion of neutron interactions with materials that lead to
activation, transmutation, and atomic displacements. The emphasis will be on current
applications includingneutron irradiationfacilities, neutron dosimetry techniques, and
computer codes for spectral adjustment and radiation damage calculations.
The effects of neutron irradiations on materials depend on the neutron spectra and the
length of irradiation. Materials effects have been measured in many different types
of facilities, including fission reactors and accelerator-based neutron sources. If we
want to compare materials effects produced by different types of facilities, then we
must consider an exposure parameter that takes into account the differences in the
neutron spectrum. For example, we cannot compare an irradiation at a 14 MeV
neutron source with one at a fission reactor on the basis of neutron exposure alone
since 14 MeV neutrons produce significantly more damage per neutron than lower
energy neutrons; furthermore, the nuclear transmutation may be quite different for
these two facilities. A widely used exposure parameter that accounts for some of
these spectral differences is displacements per atom (dpa), which is a calculated
representation of the number of primary and secondary atoms that are displaced from
their lattice sites as a result of neutron bombardment. Dpa incorporates, to a first
approximation at least, the neutron energy dependent response of the material under
irradiation. This is illustrated in Figure 1, which shows that some materials effects
measured for 316 stainless steel correlate quite well on the basis of dpa for three
radically different spectra including 14 MeV, a fission reactor, and a Be(d,n) neutron
source.[1] The rest of this chapter will describe the techniques and measurements
that are used to perform neutron spectral measurements and subsequent radiation
damage calculations to determine neutron irradiation exposure parameters that can be

used to correlate matc, ;als effects between widely different types of facilities and to
predict materials effects in other facilities, such as fusion power reactors.
Neutron Sources and Spectra _ _'_._;'_
Several terms are used to define neutron exposure. Flux is defined as the number of
neutrons passing through a unit area regardless of their direction of travel; flux units
are thus neutrons/cm=-s. Fluence is simply time integrated flux with units of
neutrons/cm 2. The neutron energy spectrum describes the energy dependence of the
neutron flux. Neutron energies of interest typically run from below room temperature
at 0.0253 eV (referred to as thermal neutrons) up to about 20 MeV. Neutrons have
about the same mass as a proton; however, neutrons differ in that they have no
electrical charge. Hence, unlike protons, neutrons can strongly interact with atoms
at very low energies. The most common types of reactions that we are concerned
with include scattering (like billiard balls), inelastic scattering (where the target
nucleus is left in an excited state), and neutron capture (where the atomic weight of
the target is increased by one mass unit). Following neutron capture, the nucleus may
undergo gamma emission, fission, charged particle reactions (where a proton or alpha
particle is emitted) or multiple neutron emission. These latter reactions can produce
transmutation that is defined as the change from one element to another. It is easy
to predict such effects using the Table of the Isotopes as a guide to determine the
reaction products for each reaction as well as the subsequent radioactive decay
products. Frequently, the transmuted element may be produced as a radioactive
isotope; this is referred to as neutron activation.
Neutrons are produced either from nuclear fission of heavy elements, such as uranium,
or by charged particle reactions induced by accelerators. In the fission process, two
or more neutrons are produced for every fission. The fission neutron energy spectrum
can be described as a maxwellian with a nuclear temperature of about 1.5 MeV, as
shown in Figure 2. Two basic types of fission reactors have been developed: - fast

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