EPJ Web of Conferences 231, 05006 (2020)
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Scalable Neutron Imaging Systems at Compact Sources
Knud Thomsen*, Eberhard Lehmann, and Markus Strobl
Laboratory for Neutron Scattering & Imaging, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland
Abstract. Neutron imaging offers unique capabilities not limited to the structural characterisation of
materials, components and processes at a large variety of neutron sources and thus is highly qualified to be a
prime candidate for instrumentation at compact neutron sources. The peculiar characteristics of neutrons,
and in particular the cross sections for their interaction with matter enable imaging results that are directly
comparable but, most notably, are effectively complementary to those of wide spread X-ray imaging
characterisation. Here, it is highlighted how neutron imaging can significantly add to the value and the
versatility of basically all and especially future compact neutron sources. Diverse advantages are identified
at all possible sizes and levels of sophistication of compact sources. These areas range from source
characterisation and students training to state of the art neutron imaging in 3D to applications and method
development, which are possible at compact sources and some of which can take particular advantage of
advanced source characteristics such as e.g. several target stations at a single accelerator.
1 Introduction
Neutron imaging (NI) offers a number of unique features
qualifying it as a prime candidate for instrumentation at
compact neutron sources. The peculiar characteristics of
neutrons, in particular their interaction cross sections,
allow for image contrast that is in principle comparable
but, in particular, complementary to that of easily
available X-ray imaging. Neutron imaging, initially on
film, was first established as a non-destructive testing
tool for industrial use, but through digitalisation and ever
increasing resolution capabilities quickly developed into
a powerful research tool in many fields of applications.
The most advanced NI facilities, defining the state-ofthe-art of the technique, are located at large scale neutron
sources where the access routes to beam time are strictly
regulated and time consuming.
The establishment of neutron imaging techniques at new
compact sources has several key aspects:
Sustainable in a landscape of a decreasing number
of large sources, in particular reactor based
Potential impact of individual methods to the design
and tuning of source properties, which can be
optimized according to the different requirements
Flexibility in the instrument layout (e.g. multiple
beams, time structure, variable collimation, sample
environment, …) in order to optimize for method
and application
Easy access to beam time for researchers and
industry in particular in the vicinity of the source
and in-house, and also for students
*
1.1 Aspects Relevant to Compact Neutron
Sources
Technically, the range for possible installations is vast;
this likewise holds for price, sophistication and potential
use. Neutron imaging is in principle possible at rather
small sources yielding only a relatively low neutron flux.
However, e.g. a demanding state-of-the-art detection
systems (very high sensitivity, long exposure enabled) is
a pre-requisite for impactful application. A very
instructive example is the 2W research reactor of the TU
Dresden where imaging is pursued (compare image data
in Fig. 1).
At the other end of the scale it is possible to take full
advantage of a most versatile compact source concept
with several independent target stations, like it is
intended e.g. in the High Brilliance Source (HBS)
project. In such case one can conceive even unparalleled
opportunities e.g. for dynamic neutron imaging, like in
the idea of crossed neutron beams for stereoscopy with
neutrons. Such option would hardly be accessible at any
other type of neutron source. However, such options
require detailed consideration concerning applications
and corresponding instrumentation by the source
designers from the very beginning. This should entail a
high priority for neutron imaging as a technique with
outstanding potential and capabilities and well suited to
be established at such neutron sources.
1.2 Current situation for Neutron Imaging at
existing Compact Source facilities and Projects
In the overview given in [1] 23 neutron sources are listed
and described in some details. From these sources, 11
declare to intend (or perform) some neutron imaging.
However, regarding scientific literature or even the
Corresponding author: knud.thomsen@psi.ch
© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0
(http://creativecommons.org/licenses/by/4.0/).
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homepages of the facilities only about 6 display relevant
and impactful results.
A very positive example in this regard are the Japanese
sources which are additionally networked in the JCANS
collaboration [2] and provide a valuable platform to even
progress imaging methods relevant for the large scale
source of JPARC during its construction and
commissioning. And such valuable work continues due
to beam availability limitations during large scale user
operation.
Some of the new projects like HBS Jülich (D), SONATE
Saclay (F), projects in Hungary, and LENS (USA) have
intentions to establish neutron imaging capabilities. It is
of outmost importance, however, that these projects
early on gain access to expertise and practical knowledge
in the respective technology of neutron imaging and its
applications in order to enable optimum design
decisions.
Fig. 1. Example image obtained at 8x103 n/cm2s in 30 min
(AKR-2, TU Dresden); image courtesy C. Lange, TU Dresden,
Germany
Access at a small source is inherently easier than at big
facilities, and hands-on experiments are possible, which
cannot be done at more powerful installations. This, in
turn allows for a “pathfinder” function where
experiments are tried and optimized before expensive
beam time at a big source is applied for. As a proof of
principle, simple neutron radiography can yield valuable
images at fluxes as low as104 n/cm2s (see Fig. 1). In
parallel to increasing possibilities at larger and more
complex, still compact, neutron sources, preparatory
experiments can be performed before going to flagship
neutron facilities with optimized set-ups.
2 Imaging as a Key Application at
Sources of Different Power and Various
Levels of Sophistication
Neutron imaging holds the potential to provide several
valuable functions for a small scale source project which
include self-supporting applications for the project,
industrial use cases and science cases of various nature
as well as method development. In the following, some
key fields of neutron imaging are briefly introduced.
2.1 Self-supporting Project Applications
2.1.1. Source/Moderator Characterisation
Sources and moderators can be optimized for specific
instrumentation and in particular for neutron imaging
and even different imaging modalities. Specifically,
wavelengths/spectra, sizes, flux, time structure and
homogeneity have to be considered and carefully
designed. Taking the perspective the other way around,
neutron imaging set-ups allow for a detailed
characterization, in particular concerning flux, spectrum,
divergence and respective homogeneity of emanating
neutron beams. Thus pinhole imaging can support
efficient facility development and validation of
numerical simulation tools. In particular small sources
can provide significant flexibility for ongoing
optimization of the source layout, and will hence
especially provide from such assessment capability.
2.1.3. Public Relations Capabilities
The results of neutron imaging are particularly well
suited to provide insights to and generate understanding
in the general public for neutrons and neutron science,
thus supporting the project and facility. Likewise results
and valuable science or industrial applications directly
accessible through the observation of high quality
images are also exceptionally well suited to generate
impact at funding bodies and governance and thus ensure
sustainability.
2.2 Industrial Uses
Neutron imaging can provide information of interest to
diverse industry and cover a vast range of questions.
Quality Assurance topics, in particular improving
processing, quality and reliability of products can be
claimed to lie at a centre of gravity of relevant
investigations in industry. Neutrons deliver especially
valuable results e.g. for pyrotechnic components, joining
parts, comprising welding, soldering/brazing and, in
particular, adhesive bonding (see Fig. 2) but also e.g.
processing such as sintering.
2.1.2. Training and Preparation Capabilities
Advantageous educational aspects are a direct
consequence of these manifold possibilities: students can
easily become acquainted with neutron-based
instrumentation, make their first steps and can almost
immediately obtain useful and illustrative results.
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supporting process optimisation and simulation of
metallic materials and components. The main focus is on
bulk metallic materials, which cannot be penetrated by
other radiation for imaging purposes, and the achieved
porosity and porosity distribution, dependent on process
parameters. Novel imaging methods are also used to map
strains and phases in the built material, which bear key
aspects of mechanical behaviour of the material, material
strength and durability under service conditions.
Fig. 2. Example image of assessment of soldering quality,
semi-transparent 3D representation highlighting the Boron
containing brazing solder distribution.
Further examples include humidity distributions in
building materials, but also investigations on the effect
of storage on syringes containing medicine, rocket [3]
and automotive components, to name but a few.
A specific example of an application at a small scale
source is provided in Fig. 3. It concerns a relatively new
field, which is to assess relevant characteristics of
specimen produced by metal additive manufacturing.
Fig. 4. Example image of porosity in additively manufactured
gold (courtesy R. Loge, EPFL) at the sub-10µm scale obtained
with the PSI neutron microscope detector (illustrating a far aim
for compact sources) [4]
Novel manufacturing technologies like additive ones are
often used for sophisticated and expensive parts; typical
areas include aerospace and medical industries. For
these, special requirements apply also with respect to
quality control. Neutron imaging is a tool of choice for
such applications.
Figure 4 displays a specific example targeting the
porosity in additive manufacturing of precious metals,
which is of interest for jewellery and watch industries.
Other key topics concern welding, phase transformations
and strain in steels or magnetic domain structures in
transformer steels as well as hydrogen and hydrogen
embrittlement in metals, corrosion and many others
more.
2.3.2. Energy Research
The specific high sensitivity of neutrons for some light
elements, namely hydrogen and lithium, make neutrons
and especially neutron imaging an outstanding research
and development tool in the key field energy conversion
techniques especially with respect to fuel cells, hydrogen
storage and batteries, but also nuclear safety. These also
spur industrial interest and applications and examples are
numerous in recent literature [5-13].
Many of these investigations are also suited for small
sources, in particular in observations of steady states,
which can be done in their own right but also establish
ideal preparation work for extensions to in operando
characterisations.
Fig. 3. Example of image of hip replacement, obtained with a
moderated 14 MeV D-T source, image courtesy M. Taylor,
Phoenix Lab, Madison, USA.
Figure 3 shows the result obtained using a small
commercial neutron source, which employs the D-T
reaction. Neutrons are moderated and 3 hours of
exposure at a thermal flux of approximately 1x104
n/cm2s were required to obtain the presented result.
2.3 Science Cases
2.3.1. Engineering materials research
Additive manufacturing is also a dominating topic in
engineering materials research and neutron imaging is
utilized as post processing characterisation tool
2.3.3. Soil, Plants and Environment
Recent literature also underlines the distinguished
capabilities of neutron imaging in the field of geology,
soil and plants and particular the interplay of the latter in
the context of water balance and uptake.
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2.4 Development of new Methods
The development of novel methods at dedicated small
sources, potentially even featuring multiple targets and
moderators offers an open-ended avenue. Advantages
with respect to method development are (i) better
availability of beamtime for tests and experiments, (ii)
more flexible instrumentation, potentially (iii) down to
the neutron source itself.
Due to the fact that neutron imaging is an excellent tool
for the observation of water and liquid flow in porous
media a large variety of geological and agriculturally
relevant processes and phenomena can be observed with
high spatial and time resolution, and especially
outstanding means for quantification of related transport
phenomena.
2.3.4. Cultural Heritage
Unexpected images of familiar as well as of unknown
objects like deciphering the internal make-up of, e.g.,
ancient sacred statues are considered interesting by most
people and important to those researching ancient culture
and technology (Fig 5).
Fig. 6. Example image of recent application aimed at
improving imaging- as well as simulation-approaches: moving
inclusions in electromagnetically agitated liquid metal,
comparison image obtained with neutrons versus fluid dynamic
numerical simulation [5]
Outstanding examples are the developments for time-offlight imaging at JPARC performed at HUNS in
Hokkaido, including sophisticated techniques like
polarized neutron imaging [15]. Another example might
be the test beamline for ESS in Berlin supporting the
development of a sophisticated wavelength frame
multiplication chopper system not only for neutron
imaging [16]. Despite the neutron source BERII being a
medium flux reactor source, with regards to time-offlight through mimicking the ESS time structure, it can
be considered a small source in this context.
Some small sources might also provide so far
unparalleled opportunities for new concepts and ideas to
be tested and realized. A small source with several target
stations might for example enable to cross neutron
beams from different targets at a sample position. This
might provide the combination of different spectra, time
structures or techniques. An example in neutron imaging
might be found related to recent studies at PSI
investigating the movement of inclusions in liquid metal
as relevant for industrial manufacturing processes (Fig.
6) and required for the development of reliable
simulation tools [5]. The challenge to observe the
process with 3D spatial resolution, despite the speed of
the process which does not enable tomographic scans,
could be solved through neutron stereoscopy utilizing
crossed neutron beams.
Fig. 5. Example for cultural heritage studies: neutron image of
a Tibetan Buddha from the 15th century, showing the filling
with scrolls
Neutron imaging is an invaluable tool for the
investigation of invaluable cultural heritage as it allows
to gain insights in a non-destructive manner, which is
often the only possibility to investigate unique and
valuable artefacts in details and thus provide unique
insights into our past and origin.
3
General Considerations & Conclusion
Neutron Imaging is a non-invasive inspection technique
with high relevance in several areas of research and
industry.
Based on modern digital detection techniques and
numerous methodical developments in the past years it
has become attractive for research fields like e.g. electrochemistry, engineering materials, energy applications,
construction materials, geology and also archaeometry.
It is very often such applications that offer a unique way
to demonstrate to the common taxpayer as well as to
funding agencies the high potential and manifold
usefulness of neutron-based techniques starting at flux
levels as low as 1000 n/cm2s.
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On the other hand, industry can take profit directly in the
investigations of advanced manufacturing processes and
products, as well as in R&D activities like in the past for
Diesel particular filters but also representative
inspections like e.g. of adhesive joints, to highlight some
prominent examples only.
Nevertheless, state-of-the-art neutron imaging is
currently established only at distinct places, where strong
neutron sources and dedicated beam lines are available.
A list of the neutron imaging installations operated in a
user lab are provided in [17], but about 100 facilities
used around the globe are attempting to apply neutron
imaging [18].
suitable and technologically advanced detection system
is set to enable valuable and impactful NI also beyond
the traditional settings of large-scale facilities.
Aspects that require special (re-)consideration
concerning imaging at small sources include:
•
all nuclear reactions initially provide high energy
neutrons first (on the order of some MeV) and
moderation reduces the available flux significantly; can
and how can fast neutrons be used efficiently for highly
relevant neutron imaging applications?
•
what is the optimum source and moderator for
specific relevant neutron imaging techniques and can
their parameters be tuned even supplying an individual
beamline?
Reasons for the currently low exploitation of this
rewarding technique might include:
•
Strong neutron sources have been built in the past
dedicated to different use cases: nuclear fuel
development
(irradiation
experiments),
isotope
production, neutron scattering, nuclear engineering
education and neutron activation analysis.
•
what are the requirements to the time structure and
pulsed beam flux to be relevant for advanced ToF
imaging?
•
are there relevant advanced techniques and in
particular imaging methods and applications that can
profit from novel concepts such as crossed beams from
multiple target stations realized at a single accelerator
(compare HBS project [19])?
•
most neutron sources are not mobile or even
portable (like X-ray sources) and the inspection of
specimen has to be performed “on-site” with often
restricted access for important stakeholders of neutron
imaging (limiting also awareness)
•
while current neutron imaging detector technique
serves well the current state of the art in neutron
imaging, novel sources and instruments still provide
challenges for efficient utilisation. What specific
demands are expected in particular from future small
neutron sources?
•
neutron scattering communities with coherent
applications and purely academic intentions have been
dominating the design and applications of research
neutron sources for the past decade
•
the neutron imaging user community is highly
diverse and thus less suited to effectively lobby for their
needs
For the moment there are only very few suppliers of
compact sources on the market. Some test facilities have
been built and upgraded and a few new installations
appear to be planned and to advance. None of them is
optimized for the exclusive and dominant use for neutron
imaging. However, informed design work and applied
expertise in neutron imaging are needed to build the best
options and to identify challenging applications and use
cases matching the particular conditions.
•
modern neutron imaging with digital detectors and
capable of quantitative also scientific studies emerged
delayed to diffraction and spectroscopy techniques
The approach of building compact and tuneable neutron
sources offers new challenges in efficient exploitation
also for the NI community. It is an outstanding
opportunity to build dedicated imaging stations,
optimized from the cradle to the grave from the initial
source to the detector. At most sources so far, in
contrast, a compromise has to be found between the
different instruments and stakeholders in the design and
particularly the neutronic design of the source.
Today thermal and cold neutron spectra are mainly used
for state-of-the-art NI, while fast neutrons are utilized
only in few cases and for special purposes. While highflux pulsed neutron beams are only available at very few
powerful spallation sources (JPARC, ISIS, IBR-2, …)
time-of-flight (TOF) methods at continuous sources
entail significant flux penalties.
However, very sensitive neutron imaging detectors are
meanwhile available and in operation which enable
working also with low beam intensities and provide
neutron images with reasonable image quality. The
combination of tuneable neutron sources with the most
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