# Optical characterization of lithium fluoride thin-film imaging detectors for monochromatic hard X-rays

M.A. Vincenti, R.M. Montereali, E. Nichelatti, V. Nigro, M. Piccinini, M.

Koenig, P. Mabey, G. Rigon, H.J. Dabrowski, Y. Benkadoum, et al.

► To cite this version:

M.A. Vincenti, R.M. Montereali, E. Nichelatti, V. Nigro, M. Piccinini, et al. Optical characterization of lithium fluoride thin-film imaging detectors for monochromatic hard X-rays. Journal of Instrumentation, 2023, 18 (04), pp.C04012. 10.1088/1748-0221/18/04/C04012. hal-04287326

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# Optical characterization of lithium fluoride thin-film imaging detectors for monochromatic hard X-rays

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M.A. Vincenti<sup>a,1</sup>, R.M. Montereali<sup>a</sup>, E. Nichelatti<sup>b</sup>, V. Nigro<sup>a</sup>, M. Piccinini<sup>a</sup>, M.

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Koenig<sup>c</sup>, P. Mabey<sup>d</sup>, G. Rigon<sup>c</sup>, H.J. Dabrowski<sup>c</sup>, Y. Benkadoum<sup>c</sup>, P. Mercere<sup>e</sup>, P.

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Da Silva<sup>e</sup>, T. Pikuz<sup>f</sup>, N. Ozaki<sup>g</sup>, S. Makarov<sup>h</sup>, S. Pikuz<sup>h</sup>, B. Albertazzi<sup>c</sup>

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<sup>a</sup> ENEA C.R. Frascati, Fusion and Technologies for Nuclear Safety and Security Dept., Via E. Fermi 45, 00044 Frascati (RM), Italy

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<sup>b</sup> ENEA C.R. Casaccia, Fusion and Technologies for Nuclear Safety and Security Dept., Via Anguillarese 301, 00123 S. Maria di Galeria (RM), Italy

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<sup>c</sup> LULI-CNRS, Ecole Polytechnique, CEA, Université Paris-Saclay, F-91128 Palaiseau Cedex, France

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<sup>d</sup> Department of Physics, Freie Universität Berlin, Arnimallee 14, 14195 Berlin, Germany

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<sup>e</sup> SOLEIL synchrotron, L'Orme des Merisiers, Départementale 128, 91190 Saint Aubin

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<sup>f</sup> Institute for Open and Transdisciplinary Research Initiatives, Osaka University, Suita, Osaka 565-0871 Japan

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<sup>g</sup> Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871 Japan

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<sup>h</sup> Joint Institute for High Temperature RAS, Moscow 125412, Russia

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E-mail: aurora.vincenti@enea.it

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**ABSTRACT:** Lithium fluoride (LiF) crystals and thin films have been successfully investigated as X-ray imaging detectors based on optical reading of visible photoluminescence emitted by stable radiation-induced $F_2$ and $F_3^+$ colour centres. In this work, the visible photoluminescence response of optically-transparent LiF film detectors of three different thicknesses, grown by thermal evaporation on Si(100) substrates and irradiated with monochromatic 7 keV X-rays at several doses in the range between 13 and $4.5 \times 10^3$ Gy, was carefully investigated by fluorescence optical microscopy. For all the film thicknesses, the photoluminescence response linearly depends on the irradiation dose in the investigated dose range. The lowest detected dose, delivered to the thinnest LiF film, only 0.5 µm thick, is estimated 13 Gy. Edge-enhancement imaging experiments, conducted by irradiating LiF film detectors at the same energy placing an Au mesh in front of them at a distance of 15 mm, allowed estimating a spatial resolution of (0.38 ± 0.05) µm, which is comparable to the microscope one. This very high spatial resolution in LiF film radiation detectors based on colour centres photoluminescence is combined with the availability of a wide field of view on large areas.

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**KEYWORDS:** lithium fluoride; colour centres; thin films; hard X-rays; imaging detectors.

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¹ Corresponding author.
35 **Contents**

36 **1. Introduction** 1

37 **2. Materials and methods** 1

38 **3. Results and discussion** 2

39 **4. Conclusions** 3

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42 **1. Introduction**

43 High spatial resolution X-ray diagnostic techniques such as micro-radiography, X-ray micros-
44 copy, diffraction and phase-contrast imaging have important applications in various experimental
45 fields ranging from biology to material science [1, 2]. A barrier to the uptake of these techniques
46 is due to the limited characteristics of the employed detectors in terms of spatial resolution, dy-
47 namic range, field of view and non-destructive readout capabilities [3]. In the last decades, lithium
48 fluoride (LiF) crystals and thin films have been successfully investigated as X-ray imaging detec-
49 tors [4-8] based on optical reading of visible photoluminescence (PL) emitted by stable radiation-
50 induced F₂ and F₃⁺ colour centres (CCs) [9]. These aggregate CCs (two electrons bound to two
51 and three close anionic vacancies, respectively) possess almost overlapped absorption bands
52 peaked at about 450 nm (blue spectral region); under optical pumping with blue light, they sim-
53 ultaneously emit broad PL bands peaked at 678 and 541 nm (red and green spectral regions),
54 respectively, which can be read in non-destructive way by using fluorescence microscopy. Passive
55 solid-state radiation detectors based on PL reading of CCs in LiF are characterized by high intrin-
56 sic spatial resolution over a large field of view, wide dynamic range and simplicity of use as they
57 are insensitive to ambient light. They can be considered “sustainable” as they do not need chem-
58 ical development after irradiation and are reusable if subjected to proper thermal annealing pro-
59 cesses. The non-destructive readout capability and the long term stability against fading of the
60 latent images stored in irradiated LiF are other important features of these detectors.

61 In this work we present and discuss some experimental results on LiF thin films of three different
62 thicknesses that were thermally evaporated on Si(100) substrates and irradiated by
63 monochromatic X-rays of energy 7 keV at several doses in the range between 13 and 4.5×10³ Gy
64 (34 mJ/cm³ – 12 J/cm³).

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66 **2. Materials and methods**

67 Radiation imaging detectors based on optically-transparent polycrystalline LiF thin films, of
68 circular shape, with a diameter of 10 mm and nominal thickness 0.5, 1.1 and 1.8 µm, were grown
69 on Si(100) substrates by thermal evaporation at ENEA C.R. Frascati [10]. They were irradiated
70 at several doses with a monochromatic 7 keV X-ray beam at the METROLOGIE beamline of the
71 SOLEIL synchrotron facility (Paris, France), in order to study their PL response. By means of
72 two mutually perpendicular shutters, the X-ray beam area was reduced to a square of size (2×2)
73 mm². Starting from the measurements of incident photon flux on the LiF films, performed before
and after each irradiation by using a photodiode, the values of the irradiation doses were calculated and they resulted to be in a range between 13 and $4.5×10^3$ Gy (34 mJ/cm$^3$ – 12 J/cm$^3$). Edge-enhancement X-ray imaging experiments, aimed to evaluate the spatial resolution of LiF film detectors, were conducted placing an Au mesh (400 lpi, wire thickness of 12 µm) in front of them, at a fixed distance of 15 mm. The X-ray beam energy and the irradiation dose were 7 keV and about $4×10^3$ Gy, respectively. The PL emitted by irradiated areas under blue light illumination was carefully investigated by using a Nikon Eclipse 80i optical microscope operating in fluorescence mode, equipped with an excitation source consisting in a 100 W mercury lamp optically-filtered in the blue spectral range, which simultaneously excited the PL of $F_2$ and $F_3^+$ CCs, and an s-CMOS camera (Andor Neo, 16 bit, cooled at -30°C) as 2D imaging detector. For each irradiated spot, a spectrally integrated PL intensity profile was obtained by acquiring the fluorescence image with the microscope software. Then, the net PL signal was obtained by subtracting the minimum PL intensity (background noise) from the maximum intensity.

## 3. Results and discussion

Figure 1 (a) shows the PL response of LiF film detectors grown on Si(100) substrate together with its linear best fit for each film thickness. In the inset of Fig.1 (a) the fluorescence image of the thickest LiF film (1.8 µm) irradiated with 7 keV X-rays at five doses: $4.5×10^3$, $1.45×10^3$, 652, 157 and 83 Gy (12, 3.8, 1.7, 0.4 and 0.2 J/cm$^3$) – acquired by a conventional fluorescence microscope Nikon Eclipse 80-i (objective magnification 2x, numerical aperture (N.A.) = 0.10, binning 2x2, exposure time 2s) is reported as an example. It clearly shows three of the five areas (spots) on the LiF film detector irradiated at the highest doses, while the dashed white rectangles highlight the lower-dose irradiated spots, which are not visible with the naked eye. For each film thickness, the PL response of LiF film detectors linearly depends on the irradiation dose in the investigated dose range. It can be observed that, at the same irradiation dose, the PL intensity increases with the film thickness. This can be attributed to the corresponding increase in the volume of irradiated LiF, which in turn leads to a higher number of radiation-induced CCs that exhibit PL when optically excited. Even the lowest dose, estimated 13 Gy (34 mJ/cm$^3$), delivered to the thinnest LiF film (0.5 µm) was detected and it is comparable with that obtained in [11] for a 1 µm thick film, despite the smaller thickness of the radiation-sensitive material layer.

Figure 1 (b) shows the PL intensity profile measured along the luminous spots highlighted by the white line shown in the inset, which reports the fluorescence image of the Au mesh stored in the 1.8 µm thick LiF film irradiated with a dose of $3.75×10^3$ Gy (objective magnification = 20x, N.A. = 0.75). Starting from the PL intensity profile, an Au mesh pitch of about 41 µm was estimated. In order to evaluate the spatial resolution of the LiF detectors, the fluorescence image of the same sample was also acquired by using an objective magnification of 100× (N.A. = 0.90) (see the inset of Fig. 1 (c)). The PL intensity profile measured along the luminous spots, highlighted with a white line in the inset, is reported in Fig. 1 (c). Figure 1 (d) shows the PL intensity profile within the region marked with a dashed rectangle in Fig. 1 (c), together with the Gaussian best fit (dashed line) of the highest peak of the diffraction pattern. From the best-fit procedure, a Half Width at Half Maximum of the Gaussian function of (0.38 ± 0.05) µm was obtained. This value can be considered an evaluation of the spatial resolution of the LiF detector, which is comparable to the microscope resolution – this latter is about 367 and 460 nm at the emission wavelengths of $F_3^+$ and $F_2$ CCs, respectively. These imaging tests demonstrate that LiF film detectors grown on Si(100) offer large field of view in combination with high spatial resolution.
<figure><img src="image_2.png" /><figcaption>**Figure 1.** (a) PL response vs. dose of LiF film detectors, irradiated with 7 keV X-rays, together with its linear best fit for each film thickness (0.5, 1.1 and 1.8 µm). Inset of Fig.1 (a): fluorescence image of the thickest LiF film at five doses, image field size (1.67×1.41) cm². (b) PL intensity profile measured along the luminous spots highlighted with a white line in the inset, which reports the fluorescence image of the Au mesh as stored in the 1.8 µm thick LiF film irradiated with a dose of 3.75×10³ Gy, acquired by the optical microscope with a 20× objective, image field size (0.83×0.70) mm². (c) PL intensity profile measured along the luminous spots highlighted with a white line in the inset, which reports the fluorescence image of the same sample acquired with a 100× objective, image field size (0.17×0.14) mm². (d) PL intensity profile within the region marked with a dashed rectangle in Fig. 1 (c), together with the Gaussian best fit (dashed line) of the highest peak of the diffraction pattern.</figcaption></figure>

**4. Conclusions**

Passive X-ray imaging detectors based on polycrystalline LiF thin films of three different thicknesses were deposited by thermal evaporation on Si(100) substrates. After irradiation with 7 keV X-rays at different doses in the range between 13 and 4.5×10³ Gy, their visible PL response, carefully investigated by fluorescence microscopy, has been found to follow a linear behaviour as a function of the irradiation dose. The lowest dose of 13 Gy was successfully detected, a fact that encourages the use of LiF film detectors even at clinical doses. The evaluated high spatial resolution of (0.38 ± 0.05) µm, together with a large field of view, make LiF film promising as imaging detectors in X-ray diagnostic techniques.
157 Acknowledgments

158 Research carried out within the TecHea (Technologies for Health) project, funded by ENEA, Italy
159 and partly supported by KAKENHI (grant no. 21K03499) from the Japan Society for the
160 Promotion of Science (JSPS). JIHT RAS team was supported by MSHE RF (grant No. 075-15-
161 2021-1352).

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163 References

164 [1] M. Endrizzi, X-ray phase-contrast imaging, Nucl. Inst. Methods Phys. Res., A 878 (2018) 88-98.

165 [2] S.C. Mayo, A.W. Stevenson and S.W. Wilkins, In-line Phase-Contrast X-ray Imaging and Tomography for Material Science, Materials 2012, 5, 937-965.

167 [3] A. Faenov, T. Pikuz, P. Mabey, B. Albertazzi, T. Michel, G. Rigon, S. Pikuz, A. Buzmakov, S. Makarov, N. Ozaki et al., Advanced high resolution X-ray diagnostic for HEDP experiments, Sci. Rep. 8, 16407 (2018).

170 [4] G. Baldacchini, F. Bonfigli, A. Faenov, F. Flora, R. M. Montereali, A. Pace, T. Pikuz, L. Reale, Lithium Fluoride as a Novel X-Ray Image Detector for Biological µ-World Capture, J. Nanoscience and Nanotechnology 3,6 (2003) 483-486.

173 [5] F. Bonfigli, A. Faenov, F. Flora, M. Francucci, P. Gaudio, A. Lai, S. Martellucci, R.M. Montereali, T. Pikuz, L. Reale, M. Richetta, M.A. Vincenti, G. Baldacchini, High-Resolution Water-Windows X-Ray Imaging of In Vivo Cells and Their Products Using LiF Crystal Detectors, Microscopy Research and Techniques 71 (2008) 35-41.

177 [6] S.H. Bateni, F. Bonfigli, A. Cecilia, T. Baumbach, D. Pelliccia, F. Somma, M.A. Vincenti and R.M. Montereali, Optical characterisation of lithium fluoride detectors for broadband X-ray imaging, Nucl. Inst. Methods Phys. Res. A 720 (2013) 109-112.

180 [7] P. Mabey, B. Albertazzi, Th. Michel, G. Rigon, S. Makarov, N. Ozaki, T. Matsuoka, S. Pikuz, T. Pikuz and M.Koenig Characterization of high spatial resolution lithium fluoride X-ray detectors, Rev. Sci. Instrum. 90, 063702 (2019).

183 [8] F. Flora, S. Almaviva, G. Baldacchini, D. Batani, S. Bollanti, F. Bonfigli, P. Di Lazzaro, M. Francucci, P. Gaudio, A. Lai, T. Letardi, L. Mezi, R.M. Montereali, D. Murra, E. Nichelatti, A. Poma, A. Scafati, L. Palladino, T. Pikuz, A. Reale, L. Reale, M. Richetta, A. Ritucci, A. Rydzy, M.A. Vincenti and P. Zuppella, X-ray imaging of bio/medical samples using laser-plasma-based X-ray sources and LiF detector, Journal of Instrumentation, JNST 14 (2019) C10006.

188 [9] J. Nahum, Optical Properties and Mechanism of Formation of Some F-aggregate centers in LiF, Phys. Rev. 158 (1967) 814.

190 [10] M.A. Vincenti, M. Leoncini, S. Libera, A. Ampollini, A. Mancini, E. Nichelatti, V. Nigro, L. Picardi, M. Piccinini, C. Ronsivalle, A. Rufoloni, R.M. Montereali, Enhanced photoluminescence of F₂ and F₃⁺ colour centres in lithium fluoride film-based detectors for proton beams, Opt Mat 119 (2021) 111376.

194 [11] T. Kurobori and A. Matoba, Development of accurate two-dimensional dose-imaging detectors using atomic-scale color centers in Ag-activated phosphate glass and LiF thin films, Jpn J Appl Phys 53, 02BD14 (2014).