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Performance of BaFCl:Eu2+ scintillating composites for X-ray imaging screens

12 Sep 2019-
About: The article was published on 2019-09-12 and is currently open access. It has received 1 citations till now.

Summary (1 min read)

1.1 Engineering Advancements to Radiographic Detection

  • X-ray imaging techniques applicable to the medical industry, such as computed tomography (CT), have undergone significant reformation and development over the past few decades.
  • Here, the authors discuss a two-fold design improvement to modern X-ray detectors; the first of which is a materials development, and the second an engineering refinement.
  • A reflective surface bordering the pillar can further improve the number of visible photons reaching the detector by redirecting the light towards the detector pixel.
  • Each design exhibits unique features which make it innovative; however, careful considerations on the cost must also be made.

3.2 Composite Fabrication and Performance

  • Here, it was determined that the IPA study performed previously was more complicated than originally thought.
  • As time progressed, the viscosity of the composite increased as more IPA evaporated.
  • Composites fabricated during the mixed resin study showed significant improvement in both uniformity and translucency.
  • For each composite, the translucency under UV exposure appears very uniform and can be attributed to the increase in IPA during the mixing process.
  • The white regions located in the 75/25 mix and 50/50 mix are due to slight damage incurred while removing from the application surface.

4. Conclusions

  • It has been shown that a large-scale flux-growth synthesis is capable of producing increased quantities of high quality BaFCl:Eu2+ powder.
  • The resulting scintillator displays very bright photoluminescence and has a high quantum yield near resonance excitation.
  • Eu2+ were fabricated with improved uniformity by increasing the amount of IPA included prior to mixing, but allowing ample time for it to evaporate after mixing, also known as Composites containing the BaFCl.
  • The translucency of the composites were also improved by incorporating NOA 170 optical adhesive into the mixture, thereby reducing the difference in index of refraction of the combined resins and the scintillator.
  • Funding Funding for this project was provided by Los Alamos National Laboratory.

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LA-UR-19-29163
Approved for public release; distribution is unlimited.
Title: Performance of BaFCl:Eu2+ scintillating composites for X-ray imaging
screens
Author(s): Richards, Cameron Gregory
Intended for: Report
Issued: 2019-09-12

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Los Alamos National Laboratory, an affirmative action/equal opportunity employer, is operated by Triad National Security, LLC for the National
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technical correctness.

1
Performance of BaFCl:Eu
2+
scintillating
composites for X-ray imaging screens
CAMERON G. RICHARDS,
1,2,*
1
Graduate Research Intern, MST-7 Engineered Materials, Los Alamos National Laboratory, Bikini Atoll
Rd., SM 30, Los Alamos, NM 87545, USA
2
Optical Materials and Devices, Masters Industrial Internship Program, 1252 University of Oregon
Eugene, OR 97403, USA
*crichar7@uoregon.edu
Abstract:
A cost-effective solution for improving X-ray imaging detectors is presented through the use
of a translucent scintillating composite and pixelated screen structure. A large quantity of
BaFCl:Eu
2+
polycrystalline scintillator powder was synthesized via flux growth and its optical
properties characterized through photoluminescence (PL) and absolute quantum yield (QY).
UV-curable composites containing the BaFCl:Eu
2+
material were fabricated with differing
amounts of NOA1665 and NOA170 optical adhesives. It was determined that, when the two
resins were combined, the translucency of the resulting composite could be enhanced. The
BaFCl:Eu
2+
composite was also cast into a pixelated glass screen containing an array of 160 μm
x 160 μm holes by two separate methods: composite paste application and powder-resin
application. Based on visual inspection, using a composite paste to fill the apertures proved to
be more effective in creating a uniform packing.
1. Introduction
1.1 Engineering Advancements to Radiographic Detection
X-ray imaging techniques applicable to the medical industry, such as computed tomography
(CT), have undergone significant reformation and development over the past few decades. With
global concerns over the detrimental effects of radiation exposure and cancer related illnesses,
new materials and designs are being developed to help reduce the risks associated with
radiometric practices. Parallel to this effort, modern research is driven by the need for faster,
more efficient, and more cost effective solutions pertaining to detector elements within the X-
ray imaging process. Inherently, these demands are highly complex and require a multi-faceted
approach in order to meet them.
Here, we discuss a two-fold design improvement to modern X-ray detectors; the first of which
is a materials development, and the second an engineering refinement. In 1983, Cusano et al.
[3] proposed an idea for a composite scintillating material containing an optical binding agent
and scintillator of equal refractive indices. This index-matching condition creates a more
translucent medium for visible photon propagation by reducing scattering events due to
interfacial reflections at surface boundaries. The increase in the optical light transport can lead
to a substantially greater detector response, improve image quality, and reduce the necessary
measurement time. Generally, activator-type inorganic scintillators, such as those commonly
used for X-ray detection, typically have high refractive indices (1.8-2+) [9] whereas the indices
of most commercial binders lie below 1.7. For this reason, among others discussed later, we
explore the use of BaFCl:Eu
2+
since its refractive index lies between 1.65 and 1.71 over the
visible spectrum. Fig. 1 shows the dispersion curve for BaFCl:Eu
2+
with the indices of two
commercially available optical adhesives for reference.

2
Fig. 1. Dispersion curve of BaFCl:Eu
2+
over the visible spectrum. Norland Optical Adhesives 1665 and 170 are
included for reference. Work in this paper explores mixing the two resins to match the index of BaFCl:Eu
2+
at 385 nm
(the primary emission wavelength of the Eu
2+
ion).
In addition to the potential improvements resulting from using a translucent composite, the use
of a pixelated screen in conjunction with the composite can further enhance the detection
capability. Pixelated structures are advantageous due to the fact that they can enhance spatial
resolution by constraining the visible photons within pillar-like structures; this geometry can
be used to directly align an array of photo-detectors. A reflective surface bordering the pillar
can further improve the number of visible photons reaching the detector by redirecting the light
towards the detector pixel. Hence, by channeling the visible light in the effective waveguide,
cross-talk between pixels can be reduced and signal-to-noise can be improved.
A variety of designs utilizing this concept have been explored, ranging from modular systems
to rigid structures [19]. Each design exhibits unique features which make it innovative;
however, careful considerations on the cost must also be made. Many of the “modern” detector
designs incorporate very expensive materials (i.e. tungsten, single-crystal scintillators) and
could easily take on the order of months to fully construct. Therefore, a relatively quick and
cost-effective solution is to use a lithographically formed pixelated screen and fill the apertures
with a composite scintillator. The benefits of this route are that the scintillator can be formed
as a polycrystalline material, the composite can be easily applied into the screen, and the whole
process can be done on the order of weeks.
1.2 Optical Properties of BaFCl:Eu
2+
The europium-doped barium fluoro-halides (BaFBr:Eu
2+
, BaFCl:Eu
2+
) have been known to
actively produce scintillation photons for over 30 years [20]. Since the time of their discovery,
an extensive amount of research has rendered a desirable level of maturity for their properties.
It is well known [1, 2, 5, 7, 13, 16-18, 20] that under UV- and X-ray irradiation, BaFCl:Eu
2+
typically exhibits luminescence emissions near 365 nm, 385 nm, and occasionally 430 nm due
to electronic transitions of the Eu
2+
activator ion. These emission wavelengths are subject to
broadening and shifts resulting from solid-state system effects. The lifetimes of the 385 nm and
430 nm emissions have been reported to be in the μs and sub-μs regime, respectively [17].

3
These values were confirmed in the last paper using Time-Correlated Single Photon Counting
(TCSPC), yielding primary lifetime decay constants of 2.947 μs and 0.4719 μs for the 385 nm
and 430 nm emissions. The speed, emission wavelengths, and stability in atmosphere make
BaFCl:Eu
2+
a suitable choice for the radiographic applications.
Previous studies on the effects of synthesis parameters on the optical properties of BaFCl:Eu
2+
indicated a correlation between the luminescence and structural properties of the crystal;
extended dwelling time provided sufficient conditions for the nucleation of an alternate phase
(Ba
12
F
19
Cl
5
). The presence of this Ba
12
F
19
Cl
5
phase can be related to the reported 430 nm
emission. With reduced dwell time, the 430 nm emission was no longer present and the intensity
of the 385 nm emission increased three-fold. The discovery of the secondary phase was found
through XRD analysis and supported by recent literature discussing the secondary phase [11].
Other research [4, 8] has suggested oxygen impurities can migrate into the crystal, leading to
similar shifts in the transition energies. Future work may be performed to investigate the
properties of the secondary phase and to elucidate the true origin of the 430 nm transition.
1.3 BaFCl:Eu
2+
Scintillating Composites
A composite material utilizing the synthesized BaFCl:Eu
2+
, Norland Optical Adhesive 1665
(NOA 1665), and IPA was previously fabricated and optimized for balanced translucency and
density. In the previous paper, it was determined that a 50% by weight contribution of IPA and
50% volume fraction of NOA 1665 to the total volume yields the desired traits. These values
were based on UV-VIS absorption spectroscopy and radio-luminescence spectroscopy.
For the intended application, it is necessary to maintain a particle size less than 40 μm to ensure
adequate filling and packing of the screen apertures. Due to the large distribution of sizes
following the synthesis, post processing of the powder is necessary. Currently, the powder is
crushed by hand using a mortar and pestle. Though effective for reducing size, it was also
shown in the previous work that this technique may quench the luminescence intensity by up
to 30% after multiple iterations. Therefore post synthesis crushing is kept to a minimum to
maintain optical quality.
2. Experimental
2.1 BaFCl:Eu
2+
Composite Fabrication
A large scale flux growth synthesis of BaFCl:Eu
2+
was performed with BaF
2
(s), BaCl
2
(s), and
EuF
2
(s) reagents. The constituents were mixed in a large glassy carbon crucible and allowed to
dwell at 1100˚C for 2 hours. Polycrystalline BaFCl:Eu
2+
is formed during cooling. Once fully
cooled, the yield was crushed and washed with DI water to filter out excess BaCl
2
. Subsequent
annealing of the scintillator yield at 120˚C dehydrated the material. PL emission spectra were
obtained for multiple samples of the synthesis yield on a Photon Technology Inc. fluorescence
spectrometer. The yield was then crushed and sieved to isolate particle size distributions. Four
particle size subsets were created using a multi-layered sieve with mesh sizes 75 µm, 53 µm,
and 38 µm. Only powder from the smallest subset was used for further experimentation.
The quantum yield, or ratio of photons emitted to photons absorbed, of the BaFCl:Eu
2+
powder
was measured on a Hammamatsu Quantarus-QY Absolute PL Quantum Yield Spectrometer
C11347 system. The excitation wavelength was set to 335 nm for all measurements,
corresponding to the wavelength for maximum excitation. To account for variation within the
system, the sample was rotated by 45˚ after every 10 measurements, subtending a full 360˚. A
total of 90 measurements were taken.

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A cost-effective solution for improving X-ray imaging detectors is presented through the use of a translucent scintillating composite and pixelated screen structure.