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Dose response and Bragg curve reconstruction by radiophotoluminescence of color centers in lithium fluoride crystals irradiated with 35 MeV proton beams from 0.5 to 50 Gy
Journal Pre-proof Dose response and Bragg curve reconstruction by radiophotoluminescence of color centers in lithium fluoride crystals irradiated with 35 MeV proton beams from 0.5 to 50 Gy M. Piccinini, E. Nichelatti, A. Ampollini, G. Bazzano, C. De Angelis, S. Della Monaca, P. Nenzi, L. Picardi, C. Ronsivalle, V. Surrenti, E. Trinca, M. Vadrucci, M.A. Vincenti, R.M. Montereali PII:
S1350-4487(20)30039-1
DOI:
https://doi.org/10.1016/j.radmeas.2020.106275
Reference:
RM 106275
To appear in:
Radiation Measurements
Received Date: 29 October 2019 Revised Date:
14 February 2020
Accepted Date: 17 February 2020
Please cite this article as: Piccinini, M., Nichelatti, E., Ampollini, A., Bazzano, G., De Angelis, C., Della Monaca, S., Nenzi, P., Picardi, L., Ronsivalle, C., Surrenti, V., Trinca, E., Vadrucci, M., Vincenti, M.A., Montereali, R.M., Dose response and Bragg curve reconstruction by radiophotoluminescence of color centers in lithium fluoride crystals irradiated with 35 MeV proton beams from 0.5 to 50 Gy, Radiation Measurements, https://doi.org/10.1016/j.radmeas.2020.106275. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
P3-088 Dose response and Bragg curve reconstruction by radiophotoluminescence of color centers in lithium fluoride crystals irradiated with 35 MeV proton beams from 0.5 to 50 Gy M. Piccinini1*, E. Nichelatti2, A. Ampollini1, G. Bazzano1, C. De Angelis3, S. Della Monaca3, P. Nenzi1, L. Picardi1, C. Ronsivalle1, V. Surrenti1, E. Trinca1, M. Vadrucci1, M.A. Vincenti1 and R.M. Montereali1 1
ENEA C.R. Frascati, Fusion and Technologies for Nuclear Safety and Security Department, Via E. Fermi 45, 00044 Frascati (Rome), Italy.
2
ENEA C.R. Casaccia, Fusion and Technologies for Nuclear Safety and Security Department, Via Anguillarese 301, 00123 S. Maria di Galeria (Rome), Italy. 3
Istituto Superiore di Sanità, Core Facilities, Viale Regina Elena 299, 00161 Rome, Italy.
* Corresponding author: [email protected]
Abstract. Passive solid-state radiation detectors, based on the radiophotoluminescence of stable color centers in nominally pure lithium fluoride (LiF) crystals, have been used for advanced diagnostics of the segment up to 35 MeV of the TOP-IMPLART proton linear accelerator for protontherapy applications, under development at ENEA Frascati. In LiF, proton beams generate a spatial distribution of F2 and F3+ aggregate color centers, which efficiently emit radiophotoluminescence in the red-green spectral range under optical pumping around 450 nm. LiF crystals were irradiated with 35 MeV nominal energy protons at several doses of interest for protontherapy (from 0.5 to 50 Gy). For the first time is shown here the possibility of measuring the radiophotoluminescence spectra of proton irradiated LiF even at such low doses and how the signal showed a linear behavior with dose with a good signal-to-noise ratio. A Bragg-curve image, stored as color-center distribution in a LiF crystal after a 50 Gy-dose irradiation in a particular geometry, was acquired in a conventional fluorescence microscope. After appropriate image processing, the signal intensity resulted to be proportional to the linear energy transfer along the whole Bragg curve. By best-fitting it with an ad-hoc analytical formula, the depth-dose distribution was reconstructed and the proton beam energy and energy spread were estimated.
Keywords. LiF; color centers; proton beams; radiophotoluminescence; Bragg curve
1. Introduction Lithium fluoride (LiF) is an alkali halide which is widely used in dosimetry in doped form [1]. It can host stable radiation-induced electronic defects, known as color centers (CCs), which can be created by several kinds of ionizing radiation (X-rays, γ-rays, electrons, neutrons, protons, α-particles and heavier ions). The aggregate F2 and F3+ CCs (two electrons bound to two and three anion vacancies, respectively), when optically pumped in their almost overlapping absorption band centered at 450 nm [2], simultaneously emit broad radiophotoluminescence (RPL) bands peaked at 678 and 541 nm, respectively [3]. In the last decade, the use of hadrons in oncological radiotherapy has remarkably grown due to the release of most of their energy at the end of their path in tissue with modest lateral diffusion, thus preserving the surrounding healthy organs [4]. In the framework of the TOP-IMPLART project [5], a 150 MeV proton linear accelerator, made of a sequence of accelerating modules, is under construction at ENEA Frascati, with the aim of protontherapy applications. We have started studying the optical properties of CCs induced in LiF crystals and thin films by low-energy protons in the (103-106) Gy dose range [6,7] and recently we have demonstrated the suitability of these LiF-based radiation detectors for advanced diagnostics of the TOP-IMPLART accelerator [8,9,10].
In this paper, we report on the RPL response of nominally pure LiF crystals irradiated with a 35 MeV nominal energy proton beam in a dose range of interest for protontherapy: (0.5-50) Gy. Although at such low doses the RPL signal in LiF is several orders of magnitude lower than in the previously investigated (103-106) Gy dose range [6,7], here for the first time we show the possibility of measuring the RPL spectra of proton irradiated LiF even at the minimum dose of 0.5 Gy, with a signal-to-noise ratio of 6.7 at the F2 center peak emission wavelength of 670 nm. Moreover, we report on how to acquire in a conventional fluorescence microscope an RPL Bragg curve image, stored in a LiF crystal after a 50 Gy-dose irradiation in a suitable geometry and how to process the acquired image to reconstruct the depth-dose distribution along the Bragg curve after best fitting the RPL profile with an ad-hoc analytical formula, allowing also the estimation of the proton beam energy and energy spread.
2. Materials and Methods Commercially available 5x5 mm2 and 1 mm-thick nominally pure LiF crystals were irradiated in air with a proton beam at a nominal energy of 35 MeV in the (0.5-50) Gy dose range. Before irradiation, the crystals were annealed for 2 hours at 500°C [11]. The beam spot diameter at the 50 µm-thick Ti exit window of the accelerator was ~2 mm; a lateral spread of the beam was obtained placing a 210 µm-thick Pb plate at 209 mm from the Ti exit window and performing
all the irradiations with the LiF crystals placed at 1656 mm from the Pb plate in free air, attached behind a 1.4 mm-thick plate of polystyrene. A 2D ionizing chamber [12], with a water-equivalent thickness of 170 µm, was placed 451 mm before the LiF crystal to monitor the proton beam in real time. This setup assured both the possibility of delivering low doses with good control and a beam homogeneity better than 3% within a 17x17 mm2 square area, where LiF crystals to be irradiated and a 60Co calibrated microDiamond detector mod. 60019 (PTW Freiburg, Germany) [13] for dose measurement were placed. In each irradiation only one LiF crystal could be positioned and all the samples were irradiated with a dose rate of 3.5 Gy/min. The concentrations of F2 and F3+ CCs, created by protons in LiF, are proportional to the locally deposited energy [7,14] when this latter is below saturation values [9]. After irradiation, the F2 and F3+ CCs were simultaneously pumped in their overlapping absorption bands [3] with a 10 mW and 1.7 mm beam diameter continuous wave 445 nm laser. This wavelength corresponds to the peak of the F2 CCs absorption band [3], thus allowing to maximize the F2 RPL intensity respect to using the 458 nm line of an Argon laser [6,7]. RPL spectra were acquired by an Andor iDus 401 CCD cooled to -55°C and attached to an Acton Research Spectra Pro 300i monochromator equipped with a 150 lines/mm grating (λblaze= 500 nm). Before the monochromator entrance slits a Semrock BLP01-458R long-pass filter (λcutoff= 458 nm) was placed to prevent the pump laser from entering the monochromator. The width of the
monochromator slits was set at 1 mm and the spectra acquisition time was 1 s. This setup allowed us to reach a RPL sensitivity high enough to detect the signal of the LiF unirradiated crystal (see Figure 1), which was used as a background to be subtracted from the signals of the irradiated samples, as specified later on. By mounting a LiF crystal with its 1 mm-thick side exposed perpendicularly to the impinging protons, the CC concentration with depth results proportional point by point to the linear energy transfer (LET) curve, provided CC saturation is not reached [9]. The RPL image, due to both the F2 and F3+ CCs, of the irradiated crystal was acquired by a Nikon 80-i fluorescence microscope equipped with a Hg-lamp, a 2x objective and an Andor NEO s-CMOS camera. A region of interest (ROI) was selected in the image, so that an experimental RPL intensity profile, corresponding to the underlying Bragg curve, could be obtained by integrating the RPL pixel intensity transversally to the proton beam propagation axis in ImageJ software [15]. In our irradiation conditions, at the dose of 50 Gy (known from the experiment) released by the impinging protons in the LiF crystal at the air-LiF interface, the RPL image intensity was quite low (see Figure 3), so it was necessary not only to set a long camera exposure time (10 s, at 11 bit), but also to correct the RPL profile by subtracting the background signal profile, corresponding to the same ROI in the frame, obtained by acquiring an image with the same exposure time after removing the sample from the microscope stage.
3. Results and Discussion Figure 1 shows the laser-excited RPL spectra of LiF crystals irradiated at several doses in the range (0.5-50) Gy by exposing the 5×5 mm2 polished crystal faces perpendicularly to the proton beam propagation axis; within the 1 mm thickness of the crystals, the absorbed dose is practically constant for the considered proton energy, being the Bragg peak placed well deeper than 1 mm in LiF, and it is equal to that found at the air-LiF interface. The spectra consist of the superposition of two broad RPL emission bands, peaked in the red and green spectral ranges, due to F2 and F3+ CCs, respectively [3]. The logarithmic RPL intensity scale allows for an enhanced visibility of the spectra at the lowest doses respect to the unirradiated sample spectrum (dashed curve), consisting of a band peaked at 780 nm that could be ascribed to impurities in the blank LiF crystal. Where the emission intensity of the F2 centers band is maximum, the "background" intrinsic signal of the unirradiated sample is minimum, thus the signal-to-noise-ratio is maximum at 670 nm and it results equal to 6.7 at the lowest dose of 0.51 Gy.
Figure 1. RPL spectra, as recorded under laser-excitation at 445 nm, of both an unirradiated nominally pure LiF crystal and of the irradiated ones in the dose range (0.5-50) Gy with protons of 35 MeV nominal energy.
Figure 2 shows the RPL peak value of the F2 CCs emission band at the wavelength of 670 nm as a function of dose. Such RPL peak intensity is linear with dose, as shown by the linear best-fit in Figure 2. Such a linear behavior has already been observed in LiF crystals irradiated with a 6 MV X-ray clinical beam up to 100 Gy [16] and also with low-energy protons from 103 to ~105 Gy [6,7].
Figure 2. RPL dose-response behavior of the F2 emission at the wavelength of 670 nm, as derived from the RPL spectra in Figure 1, and its linear best-fit (red line).
By mounting a LiF crystal with the 1 mm thick side exposed perpendicularly to the impinging protons, the 5×5 mm2 faces were parallel to the beam propagation axis (see inset of Figure 3). In this geometrical configuration, the 35 MeV nominal energy protons are completely absorbed within the LiF material; they create CCs whose local concentrations with depth are proportional to the linear energy transfer (LET) curve. Figure 3 shows the RPL image, acquired at the fluorescence microscope, of the F2 and F3+ CC spatial distribution created in the LiF crystal by proton irradiation in this mounting geometry, and the yellow rectangle indicates the selected ROI. Integration within this latter of the image RPL signal transversally to the proton beam
propagation axis, after background profile subtraction, allows to obtain the experimental Bragg curve plotted in Figure 4.
Figure 3. RPL image, acquired with a conventional fluorescence microscope, of a LiF crystal irradiated with the 1 mm-thick side exposed perpendicularly to the impinging protons (see inset); a ROI is also shown to extract the Bragg curve profile.
Recently, we have successfully extended to LiF [17] the analytical approach introduced by Bortfeld to approximate Bragg curves due to protons in water [18]. Suitable coefficients have been determined for the mathematical laws ruling range and range-straggling width by best fitting Monte Carlo (SRIM v. 2013.00) [19] simulation results obtained for protons in LiF in the energy range of (5-65) MeV and from tabulated data of the electronic density of LiF [20]. The
reliability of the analytical model was confirmed by calculating several LET curves which were quite well reproduced by SRIM simulated ones [17]. This analytical approach has been here applied to perform the best-fit of the experimental RPL Bragg curve of Figure 4, with the purpose of evaluating the proton beam energy (E) and energy-straggling width (σe), which were used as fit parameters.
Figure 4. RPL Bragg curve profile (dots) obtained from the ROI in the image of Figure 3 after background profile subtraction and its best fit/dose profile with depth (solid line) obtained by the analytical approach described in the text.
The best-fitting LET curve is reported in Figure 4 (solid line), corresponding to a proton beam with E= (26.10 ± 0.17) MeV and σe= (382 ± 15) keV. The beam energy at the air-LiF interface
resulted to be lower than the nominal 35 MeV value, due to several beam interactions with air and the other layers listed in the previous section. As the dose of 50 Gy delivered at the beam entrance in LiF was known from experiment, it has been possible to calculate the dose-depth distribution along the whole Bragg curve (see Figure 4). Moreover, the LINAC multi-particle beam-dynamics code [21], utilized to design the accelerator, was used to calculate the longitudinal and transverse motion of the beam up to the accelerator output; then the SRIM code took the output coordinates in the 6-dimension phase space of the particles computed by LINAC and transported them through air and the sequence of layers as in our experimental conditions up to the LiF crystal. According to the LINAC code, the calculated parameters of the beam impinging on the LiF crystal were E= 26.04 MeV and σe= 330 keV, in quite good agreement with the values obtained from the RPL Bragg curve best-fit. The successful best fit of the RPL Bragg curve by an analytical model for LET in LiF [17] demonstrates that the RPL response in nominally pure LiF is linear with dose in the investigated dose and energy range not only as far as the F2 peak is concerned (Figure 2), but also for the spectrally integrated RPL of Figure 4, and that the linearity extends at least up to 250 Gy, corresponding to the absorbed dose at the Bragg peak.
4. Conclusions We have shown the possibility of measuring the RPL spectra of LiF crystals irradiated by a 35 MeV nominal energy proton beam in the (0.5-50) Gy dose range, with a signal-to-noise ratio of 6.7 at the F2 center peak emission wavelength for the minimum considered dose of 0.5 Gy under laser excitation at 445 nm. Moreover, a Bragg-curve image, stored as color-center distribution in a LiF crystal after a 50 Gy (value at the air-LiF border) dose irradiation could be acquired in a conventional fluorescence microscope and, after appropriate image processing, the whole RPL Bragg curve profile could be best-fitted with an ad-hoc analytical formula, allowing to determine the dose distribution with depth, the beam energy and energy spread. The linear RPL response in the dose range from 0.5 to 50 Gy, and also along the whole Bragg curve up to 250 Gy, demonstrates that nominally pure LiF crystals are a good candidates to be used as dosimeters for low-energy proton beams by exploiting the visible RPL signal of stable radiation-induced F2 and F3+ CCs in a dose range of interest for protontherapy. Moreover, as the CCs are stable even at room temperature, the RPL signal acquisition can be repeated as many times as needed. On the other hand, CCs can be erased by means of a 2 hour-long annealing at 500°C [11], that makes LiF crystals reusable for further irradiations. Work is in progress to extend the LiF RPL response characterization at energies higher than 35 MeV for dosimetry applications in protontherapy.
Acknowledgements Research carried out within the TOP-IMPLART (Oncological Therapy with Protons – Intensity Modulated Proton Linear Accelerator for RadioTherapy) Project, funded by Regione Lazio, Italy.
References [1] Lakshmanan, A.R., Madhusoodanan, U., Natarajan, A. and Panigrahi, B.S., 1996. Photoluminescence of F‐aggregate centers in thermal neutron irradiated LiF TLD‐100 single crystals. Phys. Status Solidi (a) 153, 265. [2] Nahum, J. and Wiegand, D.A., 1967. Optical properties of some F-aggregate centers in LiF. Phys. Rev. 154, 817. [3] Baldacchini, G., De Nicola, E., Montereali, R.M., Scacco, A., Kalinov V., 2000. Optical bands of F2 and F3+ centers in LiF. J. Phys. Chem. Solids 61, 21. [4] Tamborini, A., Raffaele, L., Mirandola, A., Molinelli, S., Viviani, C., Spampinato, S., Ciocca, M., 2016. Development and characterization of a 2D scintillation detector for quality assurance in scanned ion beams. Nucl. Instrum. Methods A 815, 23. [5] C. Ronsivalle at al., 2011. The TOP-IMPLART Project. Eur. Phys. J. Plus 126, 68.
[6] Piccinini, M., Ambrosini, F., Ampollini, A., Carpanese, M., Picardi, L., Ronsivalle, C., Bonfigli, F., Libera, S., Vincenti, M.A., Montereali, R.M., 2014. Solid state detectors based on point defects in lithium fluoride for advanced proton beam diagnostics. J. Lumin. 156, 170. [7] Piccinini, M., Ambrosini, F., Ampollini, A., Picardi, L., Ronsivalle, C., Bonfigli, F., Libera, S., Nichelatti, E., Vincenti, M.A., Montereali, R.M., 2015. Photoluminescence of radiation-induced color centers in lithium fluoride thin films for advanced diagnostics of proton beams. Appl. Phys. Lett. 106, 261108. [8] Piccinini, M., Nichelatti, E., Ampollini, A., Picardi, L., Ronsivalle, C., Bonfigli, F., Libera, S., Vincenti, M.A., Montereali, R.M., 2017. Color centers in LiF thin film detectors for proton beam dose-mapping by fluorescence microscopy. Europhys. Lett. 117, 37004. [9] Nichelatti, E., Piccinini, M., Ampollini, A., Picardi, L., Ronsivalle, C., Bonfigli, F., Vincenti, M.A., Montereali, R.M., 2017. Bragg-curve imaging of 7 MeV protons in a lithium fluoride crystal by fluorescence microscopy of colour centres. Europhys. Lett. 120, 56003. [10] Piccinini, M., Ronsivalle, C., Ampollini, A., Bazzano, G., Picardi, L., Nenzi, P., Trinca, E., Vadrucci, M., Bonfigli, F., Nichelatti, E., Vincenti, M.A., Montereali, R.M., 2017. Proton beam spatial distribution and Bragg peak imaging by photoluminescence of color centers in lithium fluoride crystals at the TOP-IMPLART linear accelerator. Nucl. Instrum. Methods A 872, 41. [11] Baldacchini, G., Montereali, R.M., Nichelatti, E., Kalinov, V.S., Voitovich, A.P.,
Davidson, A.T., Kosakiewicz, A.G., 2008. Thermoluminescence in pure LiF crystals: Glow peaks and their connection with color centers. J. Appl. Phys. 104, 063712. [12] Cisbani, E., et al., 2016. Micro Pattern Ionization Chamber with Adaptive Amplifiers as Dose Delivery Monitor for Therapeutic Proton LINAC. Proceedings of IBIC2016. DOI:10.18429/JACoW-IBIC2016-TUPG51. [13] Mandapaka, A.K., Ghebremedhin, A., Patyal, B., Marinelli, M., Prestopino, G., Verona, C., Verona-Rinati, G., 2013. Evaluation of the dosimetric properties of a synthetic single crystal diamond detector in high energy clinical proton beams. Med. Phys. 40, 121702. [14] Nichelatti, E., Piccinini, M., Ampollini, A., Picardi, L., Ronsivalle, C., Bonfigli, F., Vincenti, M.A., Montereali, R.M., 2019. Modelling of photoluminescence from F2 and F3+ colour centres in lithium fluoride irradiated at high doses by low-energy proton beams. Opt. Mat. 89, 414. [15] ImageJ v.1.47, National Institutes of Health, USA, http://imagej.nih.gov/ij. [16] Villarreal-Barajas, J.E., Piccinini, M., Vincenti, M.A., Bonfigli, F., Khan, R.F., Montereali, R.M., 2015. Visible photoluminescence of color centers in LiF crystals for absorbed dose evaluation in clinical dosimetry. IOP Conference Series: Materials Science and Engineering 80, 12020.
[17] Nichelatti, E., Ronsivalle, C., Piccinini, M., Picardi, L., Montereali, R.M., 2019. An analytical approximation of proton Bragg curves in lithium fluoride for beam energy distribution analysis. Nucl. Instrum. Methods B 446, 29. [18] Bortfeld, T., 1997. An analytical approximation of the Bragg curve for therapeutic proton beams. Med. Phys 24, 2024. [19] Ziegler, J.F., Ziegler, M.D., Biersack, J.P., 2010. SRIM - The stopping and range of ions in matter. Nucl. Instrum. Methods B 268, 1818. [20] Janni, J.F., 1982. Proton range-energy tables, 1 keV-10 GeV. At. Data Nucl. Data Tables 27, 147. [21] Crandall, K.R., Weiss, M., 1994. TERA 94/34 ACC 20, internal note.
Dose response and Bragg curve reconstruction by radiophotoluminescence of color centers in lithium fluoride crystals irradiated with 35 MeV proton beams from 0.5 to 50 Gy
Highlights •
RPL spectra of 35 MeV proton-irradiated LiF crystals measured from 0.5 to 50 Gy
•
RPL response linear vs dose with good signal-to-noise ratio under laser excitation
•
RPL Bragg curve image obtained by a fluorescence microscope at low doses
•
RPL response along the whole Bragg curve reproduces well the LET curve in LiF
S1350-4487(20)30039-1
DOI:
https://doi.org/10.1016/j.radmeas.2020.106275
Reference:
RM 106275
To appear in:
Radiation Measurements
Received Date: 29 October 2019 Revised Date:
14 February 2020
Accepted Date: 17 February 2020
Please cite this article as: Piccinini, M., Nichelatti, E., Ampollini, A., Bazzano, G., De Angelis, C., Della Monaca, S., Nenzi, P., Picardi, L., Ronsivalle, C., Surrenti, V., Trinca, E., Vadrucci, M., Vincenti, M.A., Montereali, R.M., Dose response and Bragg curve reconstruction by radiophotoluminescence of color centers in lithium fluoride crystals irradiated with 35 MeV proton beams from 0.5 to 50 Gy, Radiation Measurements, https://doi.org/10.1016/j.radmeas.2020.106275. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
P3-088 Dose response and Bragg curve reconstruction by radiophotoluminescence of color centers in lithium fluoride crystals irradiated with 35 MeV proton beams from 0.5 to 50 Gy M. Piccinini1*, E. Nichelatti2, A. Ampollini1, G. Bazzano1, C. De Angelis3, S. Della Monaca3, P. Nenzi1, L. Picardi1, C. Ronsivalle1, V. Surrenti1, E. Trinca1, M. Vadrucci1, M.A. Vincenti1 and R.M. Montereali1 1
ENEA C.R. Frascati, Fusion and Technologies for Nuclear Safety and Security Department, Via E. Fermi 45, 00044 Frascati (Rome), Italy.
2
ENEA C.R. Casaccia, Fusion and Technologies for Nuclear Safety and Security Department, Via Anguillarese 301, 00123 S. Maria di Galeria (Rome), Italy. 3
Istituto Superiore di Sanità, Core Facilities, Viale Regina Elena 299, 00161 Rome, Italy.
* Corresponding author: [email protected]
Abstract. Passive solid-state radiation detectors, based on the radiophotoluminescence of stable color centers in nominally pure lithium fluoride (LiF) crystals, have been used for advanced diagnostics of the segment up to 35 MeV of the TOP-IMPLART proton linear accelerator for protontherapy applications, under development at ENEA Frascati. In LiF, proton beams generate a spatial distribution of F2 and F3+ aggregate color centers, which efficiently emit radiophotoluminescence in the red-green spectral range under optical pumping around 450 nm. LiF crystals were irradiated with 35 MeV nominal energy protons at several doses of interest for protontherapy (from 0.5 to 50 Gy). For the first time is shown here the possibility of measuring the radiophotoluminescence spectra of proton irradiated LiF even at such low doses and how the signal showed a linear behavior with dose with a good signal-to-noise ratio. A Bragg-curve image, stored as color-center distribution in a LiF crystal after a 50 Gy-dose irradiation in a particular geometry, was acquired in a conventional fluorescence microscope. After appropriate image processing, the signal intensity resulted to be proportional to the linear energy transfer along the whole Bragg curve. By best-fitting it with an ad-hoc analytical formula, the depth-dose distribution was reconstructed and the proton beam energy and energy spread were estimated.
Keywords. LiF; color centers; proton beams; radiophotoluminescence; Bragg curve
1. Introduction Lithium fluoride (LiF) is an alkali halide which is widely used in dosimetry in doped form [1]. It can host stable radiation-induced electronic defects, known as color centers (CCs), which can be created by several kinds of ionizing radiation (X-rays, γ-rays, electrons, neutrons, protons, α-particles and heavier ions). The aggregate F2 and F3+ CCs (two electrons bound to two and three anion vacancies, respectively), when optically pumped in their almost overlapping absorption band centered at 450 nm [2], simultaneously emit broad radiophotoluminescence (RPL) bands peaked at 678 and 541 nm, respectively [3]. In the last decade, the use of hadrons in oncological radiotherapy has remarkably grown due to the release of most of their energy at the end of their path in tissue with modest lateral diffusion, thus preserving the surrounding healthy organs [4]. In the framework of the TOP-IMPLART project [5], a 150 MeV proton linear accelerator, made of a sequence of accelerating modules, is under construction at ENEA Frascati, with the aim of protontherapy applications. We have started studying the optical properties of CCs induced in LiF crystals and thin films by low-energy protons in the (103-106) Gy dose range [6,7] and recently we have demonstrated the suitability of these LiF-based radiation detectors for advanced diagnostics of the TOP-IMPLART accelerator [8,9,10].
In this paper, we report on the RPL response of nominally pure LiF crystals irradiated with a 35 MeV nominal energy proton beam in a dose range of interest for protontherapy: (0.5-50) Gy. Although at such low doses the RPL signal in LiF is several orders of magnitude lower than in the previously investigated (103-106) Gy dose range [6,7], here for the first time we show the possibility of measuring the RPL spectra of proton irradiated LiF even at the minimum dose of 0.5 Gy, with a signal-to-noise ratio of 6.7 at the F2 center peak emission wavelength of 670 nm. Moreover, we report on how to acquire in a conventional fluorescence microscope an RPL Bragg curve image, stored in a LiF crystal after a 50 Gy-dose irradiation in a suitable geometry and how to process the acquired image to reconstruct the depth-dose distribution along the Bragg curve after best fitting the RPL profile with an ad-hoc analytical formula, allowing also the estimation of the proton beam energy and energy spread.
2. Materials and Methods Commercially available 5x5 mm2 and 1 mm-thick nominally pure LiF crystals were irradiated in air with a proton beam at a nominal energy of 35 MeV in the (0.5-50) Gy dose range. Before irradiation, the crystals were annealed for 2 hours at 500°C [11]. The beam spot diameter at the 50 µm-thick Ti exit window of the accelerator was ~2 mm; a lateral spread of the beam was obtained placing a 210 µm-thick Pb plate at 209 mm from the Ti exit window and performing
all the irradiations with the LiF crystals placed at 1656 mm from the Pb plate in free air, attached behind a 1.4 mm-thick plate of polystyrene. A 2D ionizing chamber [12], with a water-equivalent thickness of 170 µm, was placed 451 mm before the LiF crystal to monitor the proton beam in real time. This setup assured both the possibility of delivering low doses with good control and a beam homogeneity better than 3% within a 17x17 mm2 square area, where LiF crystals to be irradiated and a 60Co calibrated microDiamond detector mod. 60019 (PTW Freiburg, Germany) [13] for dose measurement were placed. In each irradiation only one LiF crystal could be positioned and all the samples were irradiated with a dose rate of 3.5 Gy/min. The concentrations of F2 and F3+ CCs, created by protons in LiF, are proportional to the locally deposited energy [7,14] when this latter is below saturation values [9]. After irradiation, the F2 and F3+ CCs were simultaneously pumped in their overlapping absorption bands [3] with a 10 mW and 1.7 mm beam diameter continuous wave 445 nm laser. This wavelength corresponds to the peak of the F2 CCs absorption band [3], thus allowing to maximize the F2 RPL intensity respect to using the 458 nm line of an Argon laser [6,7]. RPL spectra were acquired by an Andor iDus 401 CCD cooled to -55°C and attached to an Acton Research Spectra Pro 300i monochromator equipped with a 150 lines/mm grating (λblaze= 500 nm). Before the monochromator entrance slits a Semrock BLP01-458R long-pass filter (λcutoff= 458 nm) was placed to prevent the pump laser from entering the monochromator. The width of the
monochromator slits was set at 1 mm and the spectra acquisition time was 1 s. This setup allowed us to reach a RPL sensitivity high enough to detect the signal of the LiF unirradiated crystal (see Figure 1), which was used as a background to be subtracted from the signals of the irradiated samples, as specified later on. By mounting a LiF crystal with its 1 mm-thick side exposed perpendicularly to the impinging protons, the CC concentration with depth results proportional point by point to the linear energy transfer (LET) curve, provided CC saturation is not reached [9]. The RPL image, due to both the F2 and F3+ CCs, of the irradiated crystal was acquired by a Nikon 80-i fluorescence microscope equipped with a Hg-lamp, a 2x objective and an Andor NEO s-CMOS camera. A region of interest (ROI) was selected in the image, so that an experimental RPL intensity profile, corresponding to the underlying Bragg curve, could be obtained by integrating the RPL pixel intensity transversally to the proton beam propagation axis in ImageJ software [15]. In our irradiation conditions, at the dose of 50 Gy (known from the experiment) released by the impinging protons in the LiF crystal at the air-LiF interface, the RPL image intensity was quite low (see Figure 3), so it was necessary not only to set a long camera exposure time (10 s, at 11 bit), but also to correct the RPL profile by subtracting the background signal profile, corresponding to the same ROI in the frame, obtained by acquiring an image with the same exposure time after removing the sample from the microscope stage.
3. Results and Discussion Figure 1 shows the laser-excited RPL spectra of LiF crystals irradiated at several doses in the range (0.5-50) Gy by exposing the 5×5 mm2 polished crystal faces perpendicularly to the proton beam propagation axis; within the 1 mm thickness of the crystals, the absorbed dose is practically constant for the considered proton energy, being the Bragg peak placed well deeper than 1 mm in LiF, and it is equal to that found at the air-LiF interface. The spectra consist of the superposition of two broad RPL emission bands, peaked in the red and green spectral ranges, due to F2 and F3+ CCs, respectively [3]. The logarithmic RPL intensity scale allows for an enhanced visibility of the spectra at the lowest doses respect to the unirradiated sample spectrum (dashed curve), consisting of a band peaked at 780 nm that could be ascribed to impurities in the blank LiF crystal. Where the emission intensity of the F2 centers band is maximum, the "background" intrinsic signal of the unirradiated sample is minimum, thus the signal-to-noise-ratio is maximum at 670 nm and it results equal to 6.7 at the lowest dose of 0.51 Gy.
Figure 1. RPL spectra, as recorded under laser-excitation at 445 nm, of both an unirradiated nominally pure LiF crystal and of the irradiated ones in the dose range (0.5-50) Gy with protons of 35 MeV nominal energy.
Figure 2 shows the RPL peak value of the F2 CCs emission band at the wavelength of 670 nm as a function of dose. Such RPL peak intensity is linear with dose, as shown by the linear best-fit in Figure 2. Such a linear behavior has already been observed in LiF crystals irradiated with a 6 MV X-ray clinical beam up to 100 Gy [16] and also with low-energy protons from 103 to ~105 Gy [6,7].
Figure 2. RPL dose-response behavior of the F2 emission at the wavelength of 670 nm, as derived from the RPL spectra in Figure 1, and its linear best-fit (red line).
By mounting a LiF crystal with the 1 mm thick side exposed perpendicularly to the impinging protons, the 5×5 mm2 faces were parallel to the beam propagation axis (see inset of Figure 3). In this geometrical configuration, the 35 MeV nominal energy protons are completely absorbed within the LiF material; they create CCs whose local concentrations with depth are proportional to the linear energy transfer (LET) curve. Figure 3 shows the RPL image, acquired at the fluorescence microscope, of the F2 and F3+ CC spatial distribution created in the LiF crystal by proton irradiation in this mounting geometry, and the yellow rectangle indicates the selected ROI. Integration within this latter of the image RPL signal transversally to the proton beam
propagation axis, after background profile subtraction, allows to obtain the experimental Bragg curve plotted in Figure 4.
Figure 3. RPL image, acquired with a conventional fluorescence microscope, of a LiF crystal irradiated with the 1 mm-thick side exposed perpendicularly to the impinging protons (see inset); a ROI is also shown to extract the Bragg curve profile.
Recently, we have successfully extended to LiF [17] the analytical approach introduced by Bortfeld to approximate Bragg curves due to protons in water [18]. Suitable coefficients have been determined for the mathematical laws ruling range and range-straggling width by best fitting Monte Carlo (SRIM v. 2013.00) [19] simulation results obtained for protons in LiF in the energy range of (5-65) MeV and from tabulated data of the electronic density of LiF [20]. The
reliability of the analytical model was confirmed by calculating several LET curves which were quite well reproduced by SRIM simulated ones [17]. This analytical approach has been here applied to perform the best-fit of the experimental RPL Bragg curve of Figure 4, with the purpose of evaluating the proton beam energy (E) and energy-straggling width (σe), which were used as fit parameters.
Figure 4. RPL Bragg curve profile (dots) obtained from the ROI in the image of Figure 3 after background profile subtraction and its best fit/dose profile with depth (solid line) obtained by the analytical approach described in the text.
The best-fitting LET curve is reported in Figure 4 (solid line), corresponding to a proton beam with E= (26.10 ± 0.17) MeV and σe= (382 ± 15) keV. The beam energy at the air-LiF interface
resulted to be lower than the nominal 35 MeV value, due to several beam interactions with air and the other layers listed in the previous section. As the dose of 50 Gy delivered at the beam entrance in LiF was known from experiment, it has been possible to calculate the dose-depth distribution along the whole Bragg curve (see Figure 4). Moreover, the LINAC multi-particle beam-dynamics code [21], utilized to design the accelerator, was used to calculate the longitudinal and transverse motion of the beam up to the accelerator output; then the SRIM code took the output coordinates in the 6-dimension phase space of the particles computed by LINAC and transported them through air and the sequence of layers as in our experimental conditions up to the LiF crystal. According to the LINAC code, the calculated parameters of the beam impinging on the LiF crystal were E= 26.04 MeV and σe= 330 keV, in quite good agreement with the values obtained from the RPL Bragg curve best-fit. The successful best fit of the RPL Bragg curve by an analytical model for LET in LiF [17] demonstrates that the RPL response in nominally pure LiF is linear with dose in the investigated dose and energy range not only as far as the F2 peak is concerned (Figure 2), but also for the spectrally integrated RPL of Figure 4, and that the linearity extends at least up to 250 Gy, corresponding to the absorbed dose at the Bragg peak.
4. Conclusions We have shown the possibility of measuring the RPL spectra of LiF crystals irradiated by a 35 MeV nominal energy proton beam in the (0.5-50) Gy dose range, with a signal-to-noise ratio of 6.7 at the F2 center peak emission wavelength for the minimum considered dose of 0.5 Gy under laser excitation at 445 nm. Moreover, a Bragg-curve image, stored as color-center distribution in a LiF crystal after a 50 Gy (value at the air-LiF border) dose irradiation could be acquired in a conventional fluorescence microscope and, after appropriate image processing, the whole RPL Bragg curve profile could be best-fitted with an ad-hoc analytical formula, allowing to determine the dose distribution with depth, the beam energy and energy spread. The linear RPL response in the dose range from 0.5 to 50 Gy, and also along the whole Bragg curve up to 250 Gy, demonstrates that nominally pure LiF crystals are a good candidates to be used as dosimeters for low-energy proton beams by exploiting the visible RPL signal of stable radiation-induced F2 and F3+ CCs in a dose range of interest for protontherapy. Moreover, as the CCs are stable even at room temperature, the RPL signal acquisition can be repeated as many times as needed. On the other hand, CCs can be erased by means of a 2 hour-long annealing at 500°C [11], that makes LiF crystals reusable for further irradiations. Work is in progress to extend the LiF RPL response characterization at energies higher than 35 MeV for dosimetry applications in protontherapy.
Acknowledgements Research carried out within the TOP-IMPLART (Oncological Therapy with Protons – Intensity Modulated Proton Linear Accelerator for RadioTherapy) Project, funded by Regione Lazio, Italy.
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Dose response and Bragg curve reconstruction by radiophotoluminescence of color centers in lithium fluoride crystals irradiated with 35 MeV proton beams from 0.5 to 50 Gy
Highlights •
RPL spectra of 35 MeV proton-irradiated LiF crystals measured from 0.5 to 50 Gy
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RPL response linear vs dose with good signal-to-noise ratio under laser excitation
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RPL Bragg curve image obtained by a fluorescence microscope at low doses
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RPL response along the whole Bragg curve reproduces well the LET curve in LiF