Abstract
1. Introduction
Typically, neutron radiography[
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[
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[
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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
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[
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
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[
As a second approach[
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[
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
The linearity of the LiF detector was checked (see Figure
Additional metrological testing of the LiF crystal neutron imaging sensitivity
provided in Ref. [
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
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
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[
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[
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