Using LiF crystals for high-performance neutron imaging with micron-scale resolution 
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1. Introduction
Typically, neutron radiography[1] with
a high spatial resolution tries to solve two types of tasks: (i) to perform
self-imaging of large-size (Figure 
and with a high sensitivity (few percent) to variation in material
thickness and structure. For example, in the development of new electrical
devices[2–5] and for neutron source reconstruction
from pinhole imaging developed for inertial confined fusion (ICF) experiments using
laser irradiation[6–9], a spatial resolution on the scale of a
few microns is needed.
Fig. 1. Schematic layouts for neutron radiography by LiF detector. Self-radiography of (a) large-size and (b) tiny neutron sources using a pinhole imaging approach and high-resolution LiF crystal detectors. (c) Neutron radiography of the internal structure of objects. In such a case the object is placed in close contact to the LiF crystal.
Unfortunately, the current best available neutron detectors allow a spatial resolution
of only around 
or worse to be reached. For example, the neutron imaging plate, where
the neutron converter 
is mixed with the photo-stimulated luminescence material (BaFBr:
), has high sensitivity[10], with a measured line spread function of 
[11]. Neutron images
have also been recorded with Gd- or B-doped microchannel plates with cross-delay line
readout, resulting in an estimated spatial resolution of 
[12]. Real-time
neutron imaging has been demonstrated by projecting the fluorescence from a LiZnS
scintillator onto a high-sensitivity CCD camera[13]. Recently, dynamic neutron images have been recorded with a neutron
color image intensifier, enabling real-time observation of dynamic phenomena with
30 frames/s video pictures, at a thermal neutron flux of 
[14]. Recently, great
progress in the development of large neutron imaging systems for ICF has allowed 
spatial resolution to be reached in a wide field of view[8]. In such a case neutrons transmitted by
the aperture are converted to visible light in a scintillator array that is subsequently
recorded by a CCD camera. To achieve a high spatial resolution in such experiments a
large magnification is needed because the spatial resolution of the detector is 
.
At the same time, it is well known that point defects or, as they are also called, color centers (CCs) are produced sufficiently easily under interaction of particles or photons with LiF crystal[15]. Such CCs could be hosted in LiF at room temperature for a very long time and then under excitation by UV radiation the CCs would emit photoluminescence (PL) in the visible spectral range and allow submicron spatial resolution to be reached for soft x-ray imaging when the penetration depth of the photons is on the scale of tens or hundreds of nanometers. Recently, LiF crystal and film detectors have been successfully used for high-performance soft x-ray conventional and phase-contrast imaging[16–22] of different nanofoils or biological objects. They have also started to be widely used for characterization intensity profiles or focusing properties of x-ray laser or high-order harmonic beams[23–30]. Dislocations, point defect clusters and cavities in crystals irradiated by neutrons have been investigated already for many decades[31, 32], but only recently we proposed to use LiF crystals as high-performance neutron imaging detectors[33–36]. In this paper we will give an overview of our main results performed for characterization of such a detector and its use as a high-spatial-resolution thermal neutron radiography and discuss the applications of such a detector in areas where a high spatial resolution with a high image gradation resolution is needed. We also discuss possible applications of LiF imaging detectors for diagnostics of different continuous and pulsed high-intensity neutron sources, including plasma sources such as laser and z-pinch produced plasma.
2. Principles of neutron imaging generation in LiF crystals and experimental procedure
In LiF, the 
and 
CCs are formed by aggregation of the F centers, which are produced by
irradiation of ionizing radiation. In our case of neutron beam irradiation (see Figures
, where the energies of the reaction products are shown in
parentheses[37]. It is necessary
to stress that natural lithium is composed of 7.40% 
and 92.60% 
, and only 
is involved in the processes mentioned above. The ranges of 
and 
in LiF are estimated to be 
and 
, respectively. This means that that the spatial resolution of LiF for
neutron imaging is determined mainly by the range of the particle propagation inside the
LiF crystal. Using such propagation ranges it is possible to estimate that the 
and 
particles will deposit energy of approximately 140 and
33 eV in each lattice cell of the LiF crystal, respectively. In LiF, the F
centers are created by excitation of valence electrons to the conduction band with an
excitation energy[15, 16] of 14 eV. It follows that the energy
deposited by the 
particles is sufficient to create many (
) F centers simultaneously in each lattice, leading to efficient
formation of the 
and 
CCs (Figure
Fig. 3. In image readout, luminescence from the LiF crystal was observed with a laser
scanning confocal luminescence microscope. The 
line of an argon laser was used for excitation and
luminescence from the CCs at 
[33–36].
The thermal neutron radiography facility (TNRF-2) at the research reactor JRR-3M
(20 MW thermal output) at JAEA was used for micron-scale neutron imaging in
our experiments. The beam line for TNRF-2 provides[38] thermal neutrons (peak energy of approximately
30 meV) with a flux of 
, and the 
ratio varies from 100 to 460, where 
is the distance from the reactor to the sample and 
is the aperture size at the reactor. In the TNRF-2 station, the dose
rate of the gamma rays has been measured to be 2.16 Sv/h. The image recording
and acquisition setup for the neutron imaging experiments using LiF crystals is shown in
Figure
After recording, the images of the neutron beam intensity distribution created inside
the LiF crystal by 
and 
CCs were read out using a laser confocal fluorescence microscope
(Olympus model FV-300), as shown in Figure 
was observed. Different samples were used for characterization of the
LiF crystal as a high-performance neutron imaging detector.
3. Features of LiF crystal as a neutron imaging detector
Several important parameters of the LiF neutron imaging detector were characterized during our experiments.
3.1. Spatial resolution
The spatial resolution of the LiF crystal neutron imaging detector was quantitatively
evaluated by two different approaches. First of all, we used specially produced
masks, which consisted of line pair patterns fabricated on a 0.005 mm
thick Gd film evaporated on a glass plate[36]. The images of the small line pairs on the dark field were
observed using a LiF single crystal detector and are presented in
Figure 
wide are clearly seen with good contrast in the images. This result
demonstrates that the LiF crystal neutron detector has potential for use in the
practical evaluation of the spatial resolution of noble gas imaging having an
ultra-high spatial resolution of 
.
Fig. 4. Schematic diagrams and sizes of the line pairs produced on 
thickness Gd patterns coated onto the overall surface of a
glass substrate and their images obtained by using the LiF crystal neutron
imaging detector (top). Line-pair images obtained using the LiF single
crystal detector and line profiles of the pairs with widths of 
[36]
(bottom). The spatial resolution on the scale of 
is clearly seen.
As a second approach[33], we
studied the spatial resolution of the LiF neutron detector by measuring the sharpness
of the edges for low and heavy neutron absorbed materials. In Figure 
thick Cd (low-neutron-absorption material) and a 
thick Gd (high-neutron-absorption material) plate are presented.
The radiography image of the 
thick Cd plate was recorded at only 10 s neutron
exposure. Comparison of this image with a Gaussian error function shows that the best
fit to the experiment corresponds to a spatial resolution of 
, where the spatial resolution is defined as the distance between
10% and 90% of the transmittance of the fitted curve. This spatial resolution agrees
well with the estimated stopping range of 
particles in LiF (
). The neutron images of the 100 
thick Gd plate (see Figure 
takes place. The neutron image of the Gd plate shown in
Figure 
, which is in good consistency with the 
spatial resolution estimated in Figure
Fig. 5. (a) Neutron image of a 
thick Cd plate taken with 10 s exposure time
and a trace of the neutron image across the edge, which is compared with a
calculation at a spatial resolution of 
[33].
(b) Neutron radiography images of a 100 mm thick Gd
plate of triangular shape[33]. Optical microscope and neutron images of the same part
near the edge obviously demonstrate a high-resolution quality of the LiF
neutron imaging detector comparable with optical microscopy imaging. The
magnified image of a small crack in the Gd plate and the line scan of this
part, shown by the blue lines, clearly manifest high contrast and spatial
resolution of such images. We could see that this line scan has a best fit
with a modeled curve with a 
width (dashed curve).
Fig. 6. (a) Comparison of the neutron images of the Au wires of 42, 95
and 
diameter recorded with exposure times of 10 and
30 min[33]. (b) Comparison[33] of the traces of the experimental intensity
transmittance of neutrons through the Au wires (solid curves) with the
theoretical transmittance (dashed curves) for two attenuation beam
coefficients. It is clearly seen that the best coincidence between the
modeling and the experimental curves is obtained for 
(bottom panel). Changes of 
of even 
(
) show a large disagreement between the theoretical and
experimental curves, which testifies to the high quality and sensitivity of
the LiF crystal neutron imaging detector. (c) A plot of the
luminescent intensity from the CCs in LiF versus the neutron fluence on
LiF[33]. The neutron
fluence was varied by the neutron exposure time and the attenuation of the
neutron flux by various filters, such as Au wires, Au foils and Cd plates.
The straight line is a fit to the data, showing a good linear response of
the LiF to the neutron fluence.
3.2. Sensitivity and linearity of the LiF crystal imaging detector
Other very important characteristics of any detector are the sensitivity and
linearity of the detector response to different fluxes of incoming radiation
intensity. We conducted different experiments[33] to measure these important parameters. Figure 
diameter, recorded at 10 and 30 min exposure,
corresponding to neutron fluences of 
and 
, respectively. It is clearly seen from Figure 
diameter, which has an attenuation of only 2.35%. This demonstrates
that LiF has not only high spatial resolution but also high sensitivity, high imaging
contrast and, which is very important for neutron imaging detectors, the image
quality is free from granular noise that sometimes exists in other neutron imaging
detectors. Indeed, for the data presented in Figure 
for the thinnest 
diameter Au wire. As seen in Figure 
(bottom panel). This transmittance corresponds to the energy of the
thermal neutrons applied for the irradiation of the Au wires. We would like to stress
that a very small change of the neutron transmittance coefficient value, for example
to 
, could be clearly distinguished by comparison with experimental
curves (see the upper panel in Figure
Additional proof of the high spatial resolution, high sensitivity and high contrast
of neutron imaging by LiF crystal detectors in the case of imaging of
high-neutron-absorption materials is obviously given by Figure 
could be measured with an accuracy of some microns. Additionally,
all tiny defects on the surface and edge of the foil (for example, such as the burr
in Figure
Fig. 7. Neutron radiography of a 
thick Gd plate. A defect with a size of 
and some micron-scale changes of thickness of the hammered
Gd plate edge (due to cutting the Gd plate with scissors) are clearly seen
in the magnified images of different parts of the sample[35].
The linearity of the LiF detector was checked (see Figure 
to 
. It is demonstrated that the LiF crystal imaging detector has a
very high dynamic range similar to the dynamic range of LiF crystal for x-ray
imaging[16]. We should stress
that luminescence, produced by neutron irradiation, is observed even for the area
covered with highly attenuating filters (see Figure 
. This neutron fluence corresponds to the 
exposure time of the JRR-3M neutron facility used in these
experiments[38]. Indeed, we
could clearly record the neutron images with 10 seconds exposure, as shown
in Figure 
, only 0.26% of the neutrons are absorbed in the 
depth of LiF, which corresponds to the focal depth of the 
microscope objective. To check this point we read out neutron
images not only from the front side but also from the rear side of the
3 mm LiF crystal. In this case clear neutron images were recorded from
both sides of the crystal. The intensity of the image from the rear of the LiF
crystal was sufficiently good, but the spatial resolution and contrast of this image
were worse compared with the image that was read out from the front. This occurred
due to the scattering when neutrons pass through the 3 mm thickness of the
LiF crystal.
Additional metrological testing of the LiF crystal neutron imaging sensitivity
provided in Ref. [34] by using the
standard neutron sensitivity indicator shows that holes with transmittance
differences of less than 2% could be observed. Moreover, all gaps in this indicator
with sizes from 22 to 
were also clearly observed. All of the above mentioned experimental
results show that LiF crystals have excellent characteristics as neutron imaging
detectors with high spatial resolution, high dynamic range and good contrast.
Fig. 8. (a) Neutron images of a ball-point pen obtained by a tiling
sequence of 
magnified images[33]. A metal tube, a roller ball at the top and the ink in
the metal tube with strong neutron attenuation are obviously distinguished.
It is also clearly seen that a small air bubble of 
diameter in the ink has moved to the upper part of the pen
between the first experiment with 30 min neutron exposure and the
second measurement with 10 min exposure. (b) A
schematic drawing of a small fuel cell and the neutron image of
it[35]. Tiny details
with sizes of at least 
of the fuel cell structure and its inhomogeneity along and
perpendicular to the anode–cathode directions are evidently
resolved.
4. Imaging of samples with internal gas or water structures using the LiF neutron imaging detector
One of the main advantages of thermal neutron imaging in comparison to x-ray imaging is
the capability of observing materials comprising high- and low-
elements. In Figure 
diameter at the smallest part of the pen to 
diameter at the largest to be distinguished. The metallic parts such
as the ink tube and the roller ball at the tip are obviously distinguished too.
Furthermore, it is unmistakably seen how an air bubble of 
diameter has accidentally moved from the top of the ink channel with
diameter 
further inside the tube in the direction of the roller bar between the
two measurements (see Figure
A very important application of micron-sized neutron radiography is obtaining detailed
information about the water distribution in the membrane electrode assembly (MEA) and
the gas diffusion layer (GDL) in fuel cells. As is seen from Figure 
can be undoubtedly seen along the full field of view of 
and allows us to distinguish the most minute details of the cell
structure.
The examples presented in this section show that neutron imaging with LiF is suitable
for the observation of detailed structures of low-
materials with high spatial resolution and dynamic range.
5. Conclusion
The experimental results discussed in this paper show that LiF crystals have excellent
characteristics and great advantages compared with traditionally used neutron detectors
in areas where a micron-scale spatial resolution, a high dynamic range and a high
contrast are needed. Indeed, we demonstrated that the neutron images recorded with LiF
are almost free from granular noise, and the spatial resolution reaches 
; the response is highly linear to the neutron fluence with a dynamic
range of at least 
. As drawbacks, we should mention that the sensitivity of LiF to
thermal neutrons is not very high and is approximately 
. At the same time, it is obvious that it could be increased by least
an order of magnitude. Actually, we used natural LiF in our experiments, in which the
concentration of 
is not very high (the abundance is 7.4%). However, it is
expected that this sensitivity will be improved by 
times if we use enriched 
in place of natural LiF. The sensitivity could be further improved by
using poly-crystalline LiF, since poly-crystalline LiF coating has proved to have 
times higher sensitivity than LiF single crystal without degrading the
resolution in soft x-ray imaging[16].
We hope that due to all of the abovementioned advantages of LiF crystal detectors, they
will be useful not only for quantitative evaluation of various object structures in
devices comprising low-
elements (including Li-ion batteries and fuel cells) but also for
diagnostics of different continuous and pulsed high-intensity neutron sources, including
such plasma sources as laser and z-pinch produced plasma. It is necessary to mention
that the application of LiF detectors for fast neutron imaging will require a large
number of neutrons. Indeed, if the cross-sections for thermal neutron interaction with
LiF are 
barn (for neutron energies of tens of meV), the
cross-sections for interaction of DT or DD fusion neutrons with LiF crystals are lower
by practically four orders of magnitude. In such a case, the intensity of the
thermonuclear neutron source should be very high for practical applications of LiF
imaging detectors. Meanwhile, in recent National Ignition Facility (NIF)
experiments[39] the fast neutron
yield has reached the enormous value of 
neutrons per shot, and should be even higher in future experiments.
This means that in future experiments when neutron generation reaches 
neutrons per shot, we could expect that at a distance of
50 cm from the target the neutron flux will be 
. Such a neutron flux should be sufficient for obtaining neutron images
using a LiF crystal detector in single shot at this distance. At the same time, as was
cited in Ref. [40],
deuterium–tritium-loaded capsules at NIF can now regularly produce a plasma
with low-energy ‘ICF-thermal’ neutrons with density 
. This high quantity of neutrons with energy in the range of
1 eV–100 keV gives hope for successful thermal neutron
imaging using LiF detectors due to the higher efficiency of their absorption compared
with MeV neutrons. It is also necessary to mention that, recently, LiF films embedded
with 
have been successfully used as converters of neutrons in tokomak
experiments with thermonuclear neutrons[41,
42]. The subsequent results give
confidence that LiF detectors could be successfully used for high-resolution imaging of
plasma neutron sources. Of course, direct experimental tests at laser fusion, z-pinch or
tokamak installations are needed to confirm the advantages of LiF detectors for
high-performance neutron imaging.
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Article Outline
A. Faenov, M. Matsubayashi, T. Pikuz, Y. Fukuda, M. Kando, R. Yasuda, H. Iikura, T. Nojima, T. Sakai, M. Shiozawa, R. Kodama, Y. Kato. Using LiF crystals for high-performance neutron imaging with micron-scale resolution[J]. High Power Laser Science and Engineering, 2015, 3(4): 04000001.
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