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Fluorescent detection of single tracks of alpha particles using lithium fluoride crystals
Nuclear Instruments and Methods in Physics Research B 392 (2017) 41–45
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Fluorescent detection of single tracks of alpha particles using lithium fluoride crystals P. Bilski ⇑, B. Marczewska Institute of Nuclear Physics Polish Academy of Sciences (IFJ PAN), Radzikowskiego 152, PL-31-342 Krakow, Poland
a r t i c l e
i n f o
Article history: Received 3 October 2016 Received in revised form 9 November 2016 Accepted 2 December 2016
Keywords: Radiation measurements Nuclear tracks Fluorescence Lithium fluoride Alpha particles
a b s t r a c t Lithium fluoride single crystals were successfully used for fluorescent imaging of single tracks of alpha particles. This was realized with a standard wide-field fluorescent microscope equipped with a 100 objective. Alpha particles create F2 and F+3 color centers in LiF crystals. The subsequent illumination with the blue light (wavelength around 445 nm), excites these centers and produces fluorescence with a broad band peaked at 670 nm. The observed tracks of alpha particles have diameter of about 500 nm. Focusing of the microscope at different depths in a LiF crystal, enables imaging changes of shape and position of tracks, allowing for visualization of their paths. These encouraging results are the first step towards practical application of LiF as fluorescent nuclear track detectors. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction About ten years ago a new radiation measurements technique using so called Fluorescent Nuclear Track Detectors (FNTD) was developed [1,2]. This technique exploits the fluorescence (photoluminescence) phenomenon in aluminum oxide crystals doped with carbon and magnesium (Al2O3:C,Mg) by analysis performed with a fluorescent confocal laser scanning microscope (CLSM). CLSM is now a well-known microscopic technique, enabling high resolution 3D imaging through scanning of a sample with a laser spot and discriminating the out-of-focus light (principles of CLSM may be found e.g. in Ref. [3]). Ionizing radiation induces F+2 (2 Mg) color centers within Al2O3:C,Mg crystals. When these centers are excited with 620 nm light, they produce fluorescence at 750 nm [4]. It was demonstrated that using a CLSM for analysis of Al2O3:C,Mg crystals it is possible to visualize single ion tracks [5,6] and even delta-rays [7]. The development of the FNTD method opened perspectives of numerous applications for radiation dosimetry including radiotherapy with particle beams, neutron measurements and cosmic radiation dosimetry. Recently, FNTDs were applied in radiobiology studies for colocalization of damage in living cells with track of charged particles [8]. So far FNTD technique has been realized only with Al2O3:C,Mg crystals. Another material showing a stable and intense radiation induced fluorescence is lithium fluoride (LiF). Radiation creates in ⇑ Corresponding author. E-mail address: [email protected] (P. Bilski). http://dx.doi.org/10.1016/j.nimb.2016.12.003 0168-583X/Ó 2016 Elsevier B.V. All rights reserved.
LiF crystals F centers (anion vacancies trapping electrons), which tend to aggregate forming more complex defects [9]. Among them F2 and F+3 color centers are of special interest. F2 center is composed of two anion vacancies with two bounded electrons, while F+3 of three vacancies with two electrons. Both of these centers have overlapped absorption bands peaked around 445 nm. When LiF crystal is illuminated with light of such wavelength, the color centers are excited and emit photoluminescence light of longer wavelength. The photoluminescence emission spectrum (PL) exhibits two peaks at about 670 nm (related to F2) and about 525 nm (related to F+3). The typical emission and excitation spectra of LiF crystals are illustrated in Fig. 1. PL of LiF has been proposed for generating laser action [10], for several applications as dosimeters [11–14], as well as for X-ray imaging applications [15–18]. The described properties indicate LiF as a potential candidate to be also used as fluorescent track detector. Application of LiF crystals offers some potential advantages over Al2O3:C,Mg. Firstly, the natural lithium contains 6Li isotopes, which have a high cross-section for (n,a) reaction, therefore enabling neutron detection. Secondly, the theoretical resolution limit for CLSM is roughly proportional to excitation and emission wavelengths, and both these values are lower in case of LiF. The goal of the present work was therefore to attempt application of LiF single crystals as FNTDs, i.e. for detection of single particle tracks. This goal was achieved using a standard manual wide-field fluorescent microscope, a device much simpler than confocal laser scanning systems (in a wide-field microscope the whole sample is illuminated with light and an image acquired with a CCD camera).
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P. Bilski, B. Marczewska / Nuclear Instruments and Methods in Physics Research B 392 (2017) 41–45
particles to about 3.11 MeV and maximum energy to 4.15 MeV. Range of alpha particles in LiF for these energies is 9.8 lm and 14.2 lm, respectively [20]. Microscopic observations were realized with a Nikon Ni-U fluorescence manual wide-field microscope with a DS-Qi2 CCD camera. For light excitation the Lumen 200 illumination system (Prior Scientific) was used. In order to discriminate between excitation and emission light we used a band-pass filter ET440/40 for excitation, a long-wavelength-pass filter ET515lp for emission and a dichroic mirror FF506-Di03. A Nikon CFI TU Plan Epi ELWD objective (100, NA 0.80) was applied. Image acquisition and analysis were realized with the Nikon NIS-Elements software.
3. Results and discussion
Fig. 1. Typical photoluminescence emission (PL) and excitation (PLE) spectra of LiF crystal (irradiated with Sr-90/Y-90 beta particles). PL measured for excitation with 445 nm light, PLE measured for emission at 670 nm.
2. Materials and methods LiF single crystals with a typical diameter of about 2 cm were grown with the Czochralski method at the IFJ PAN Kraków [19], using undoped LiF powder as the starting material (Fig. 2). The obtained transparent crystals were then cut with diamond saws into about 4 4 0.9 mm samples, polished manually with 0.5 lm diamond abrasive strips and rinsed in acetone. Prior to further experiments, the samples were annealed at 400 °C for 30 min in order to remove pre-existing F2 and F+3 color centers [14]. The LiF samples were irradiated with alpha particles from a radioisotope Am-241 source (produced by Eckert & Ziegler). The source activity was 14 MBq (±30% according to certificate) and dimensions of the active part were 25 12.5 mm. The samples were irradiated through a 100 lm slit at a distance of 4 mm from the source surface. The geometrical configuration during exposures is illustrated in Fig. 3. The reason for such arrangement was to create in LiF crystal a definite fluorescence image, which would be distinctly recognizable from the unirradiated background. Basing on the nominal source activity and on geometrical considerations, the particle fluence rate at the surface of the sample was estimated to be 4.1 105 h 1 mm 2. For some irradiations the slit was widened to 3 mm in order to irradiate a larger area of a crystal. In that case the fluence rate was estimated to be 1.23 107 h 1 mm 2. The time of exposure varied between 1 min and 60 min. The nominal energy of Am-241 alpha particles is 5.486 MeV. However in case of the applied source, energy of particles is strongly degraded, due to crossing 2 lm thick covering gold foil, as well as due to crossing the layer of the active material itself. Spectrometric measurements revealed that the spectrum is very broad and extends from 1 MeV to 4.5 MeV, with a peak at about 3.5 MeV. Crossing 4 mm of air reduces further average energy of
Fig. 2. LiF crystal: a) as grown with the Czochralski method, b) cut into samples.
Fig. 4 presents an example of an image of the LiF crystal irradiated with alpha particles through a slit. The bright band created by alpha particles is well contrasted with the unirradiated part of the crystal. The image is composed of bright spots or short vertical lines sometimes surrounded by a hazy background. The counted number of tracks was 1.33 105 mm 2, which agrees very well with the fluence estimated on the basis of the source activity (1.37 105 mm 2). The used 100 objective had a very short – below 1 lm, depth of focus. This enabled doing a kind of a scan of the samples along the z-axis, i.e. to study changes of the acquired images, when focus was adjusted to different depths into the crystal. Fig. 5 shows some examples of the obtained images. Fig. 5(a) is focused at the very surface of the sample. Fig. 5(b) represents the image acquired 1 lm deeper into the crystal. The appearance of the observed tracks is now different: besides spots there are a lot of lineshaped tracks, which are all directed nearly parallel. A few more micrometers deeper (Fig. 5c and d) the image is changed once more. There are no line-shaped tracks, but again spots, which are now brighter and their number is decreasing with depth. These observations may be interpreted in the following way. The presence of the 0.1 mm slit forced directionality of alpha particles: acceptance angle across the slit is only about 1.4°. The observed lines are created by particles impinging at acute angles to the crystal surface and almost in parallel to the slit. The penetration depth into a crystal at such angular trajectories is obviously smaller, therefore in deeper layers we have nearly not observed line-shaped tracks. Only circular spots created by particles directed nearly perpendicularly to the surface remain. They are brighter than spots at the surface (Fig. 5a), because energy losses are higher at the end of a particle path (Bragg peak), what means higher ionization density (LET in LiF of 3.11 MeV particle is 257 keV/lm and it increases to 407 keV/lm at the Bragg peak; for maximum energy of 4.15 MeV it is 215 keV/lm [20]) and finally more created F2/F+3 color centers. The number of spots decreases with depth, because particles with trajectories deviating from normal incidence are gradually stopped within a crystal. The observed hazy halo around the tracks seems to be due to out-of-focus light coming from layers deeper or shallower than the one under observation and possibly also by some scattered light. These effects, which diminish quality of images, should be absent if instead of a wide-field microscope a CLSM system enabling high spatial discrimination, is used. The tracks were observed in images acquired at depths ranging up to about 12–13 lm. This value is in agreement with 14.2 lm calculated with SRIM code as the maximum possible penetration depth into LiF In order to verify our interpretation of line-shaped tracks as created by particles impinging at acute angles, another experiment was performed. A LiF crystal was positioned for alpha irradiation vertically on its narrow side (Fig. 3b and d), while microscopic
P. Bilski, B. Marczewska / Nuclear Instruments and Methods in Physics Research B 392 (2017) 41–45
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Fig. 3. Schematic of the arrangement for alpha particle irradiation. Dimensions expressed in millimeters. (a) side view, crystal in horizontal position, (b) side view, crystal in vertical position, (b) top view, crystal in horizontal position, (c) top view, crystal in vertical position (c and d not in scale with a and b).
observations were done as previously on the front side. In this way no perpendicularly directed particles can reach surface of the crystal and trajectories at acute angles are more preferred. The result is illustrated in Fig. 6. It is apparent that almost all tracks have a line shape, what confirms the described interpretation. Most lines end with a brighter spot (at the upper side), which most probably corresponds to the Bragg peak at the end of a particle path. The longest track noticed within the image was 12.3 lm long, what agrees with the previous observations and with the SRIM calculations. Finally, one more irradiation was realized in configuration like Fig. 3a, but with the slit width set to 3 mm. By that means, nearly whole crystal surface was uniformly irradiated, without any directionality. Fig. 7 shows that the acquired image agree well with the expectations: it consists of short tracks aiming in various directions. The counted number of tracks was about 1.7 105 mm 2, what roughly agrees with the estimated fluence (2.05 105 mm 2). The diameter of the observed tracks is about 500 nm, what is somewhat more than the diameter of the volume in which an alpha particle deposits dose in LiF. The maximum range of delta
electrons produced by about 4 MeV alphas in LiF is 100–120 nm (calculated with the libamtrack library [21] according to the Tabata model [22]), so the diameter of a track should be below 250 nm. The size of the observed tracks is mainly determined by the resolution of the applied microscope system. For wide-field fluorescent microscopes the resolution is often estimated as k/2NA. For NA = 0.80 and k = 670 nm this amounts to 419 nm. The resolution may be improved if an objective with the higher numerical aperture is used: oil immersion lenses may exceed NA = 1.4.
4. Conclusions This work demonstrated that fluorescence of undoped LiF crystals can be successfully used for imaging nuclear particle tracks. This was achieved using a manual wide-field fluorescent microscope, an instrument which is simpler, much less expensive and much more common than confocal laser scanning systems, which were so far applied for this purpose. Introducing LiF as another type of detector besides Al2O3:C,Mg, as well as application of a
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P. Bilski, B. Marczewska / Nuclear Instruments and Methods in Physics Research B 392 (2017) 41–45
Fig. 4. Fluorescent image of the alpha irradiated LiF crystal. Irradiation time 20 min, fluence 1.37 105 mm 2. Acquisition time 20 s.
Fig. 6. Fluorescent image of the sample irradiated with alpha particles directed nearly parallel to the crystal (see Fig. 3b and d), acquired at depth of about 1 lm. Irradiation time 1 h. Direction of alpha particles is from the bottom side of the image. Acquisition time 40 s.
wide-field microscope, may make FNTD technique more accessible. In spite of the successful use of a wide-field microscope it should be noted that confocal systems would undoubtedly provide images of superior quality concerning S/N ratio and resolution and CLSM will be rather the target technique of measurements. The reported results were obtained using crystal samples which were randomly chosen from the standard LiF crystals grown at the IFJ PAN, without any selection concerning their photoluminescence
properties nor without any special treatment (some preliminary observations indicate that e.g. thermal treatment has significant influence on photoluminescence intensity of LiF). It seems therefore that there are margins for improvement concerning quality of LiF crystals, as well as other factors of the experimental procedure (e.g. application of objective with higher numerical aperture) and such work is under way. The present work may be considered as the first step towards developing of LiF based FNTDs.
Fig. 5. Fluorescent images of the same sample as in Fig. 4 (enlarged) focused at different depths into the crystal: a) ‘surface’, arbitrary assumed as 0 lm, b) 1 lm, c) 6 lm, d) 8.5 lm. Other parameters are the same as in Fig. 4. A video showing images acquired at different focal depth is available.
P. Bilski, B. Marczewska / Nuclear Instruments and Methods in Physics Research B 392 (2017) 41–45
Fig. 7. Fluorescent image of the sample irradiated with a broad uncollimated flux of alpha particles acquired at depth of about 2 lm. Irradiation time 1 min, particle fluence 2.05 105 mm 2. Acquisition time 30 s. A video showing images acquired at different focal depth is available.
Acknowledgments This work was supported by the National Science Centre, Poland (Contract No. UMO-2015/17/B/ST8/02180). The authors are grateful for J.W. Mietelski (IFJ PAN) for spectrometric measurements of the alpha source. We also thank W. Staniszewski (Precoptic/ Nikon) for help in optimum configuring of the microscope system. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.nimb.2016.12. 003. References [1] G.M. Akselrod, M.S. Akselrod, E.R. Benton, N. Yasuda, A novel Al2O3 fluorescent nuclear track detector for heavy charged particles and neutrons, Nucl. Instrum. Methods Phys. Res. B 247 (2006) 295–306. [2] M.S. Akselrod, R.C. Yoder, G.M. Akselrod, Confocal fluorescent imaging of tracks from heavy charged particles utilising new Al2O3:C,Mg crystals, Radiat. Prot. Dosim. 119 (2006) 357–362. [3] R.L. Price, W.G. Jerome, Basic Confocal Microscopy, Springer, 2011.
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[4] M.S. Akselrod, G.J. Sykora, Fluorescent nuclear track detector technology – a new way to do passive solid state dosimetry, Radiat. Meas. 46 (2011) 1671– 1679. [5] S. Greilich, J.M. Osinga, M. Niklas, F.M. Lauer, G. Klimpki, F. Bestvater, J.A. Bartz, M.S. Akselrod, O. Jakel, Fluorescent nuclear track detectors as a tool for ionbeam therapy research, Radiat. Meas. 56 (2013) 267–272. [6] J.A. Bartz, C.J. Zeissler, V.V. Fomenko, M.S. Akselrod, An imaging spectrometer based on high resolution microscopy of fluorescent aluminum oxide crystal detectors, Radiat. Meas. 56 (2013) 273–276. [7] G.O. Sawakuchi, F.A. Ferreira, C.H. McFadden, T.M. Hallacy, D.A. Granville, N. Sahoo, M.S. Akselrod, Nanoscale measurements of proton tracks using fluorescent nuclear track detectors, Med. Phys. 43 (2016) 2485–2490. [8] C.H. McFadden, T.M. Hallacy, D.B. Flint, D.A. Granville, A. Asaithamby, N. Sahoo, M.S. Akselrod, G.O. Sawakuchi, Time-lapse monitoring of DNA damage colocalized with particle tracks in single living cells, Int. J. Radiat. Oncol. Biol. Phys. 96 (2016) 221–227. [9] E. Sonder, W.A. Sibley, Defect creation by radiation in polar crystals, in: J.H. Crawford, L.M. Slifkin (Eds.), Point Defects in Solids, 1972, pp. 201–290, New York. [10] G. Baldacchini, R.M. Montereali, New perspectives of coloured LiF for optoelectronic devices, Opt. Mater. 16 (2001) 53–61. [11] M.K. Murphy, A. Kovács, S.D. Miller, W.L. McLaughlin, Dose response and postirradiation characteristics of the Sunna 535-nm photo fluorescent film dosimeters, Radiat. Phys. Chem. 68 (2003) 981–994. [12] L. Oster, S. Druzhyna, Y.S. Horowitz, Optically stimulated luminescence in LiF: Mg, Ti: Application to solid-state radiation dosimetry, Nucl. Instrum. Methods A 648 (2011) 261–265. [13] B. Marczewska, P. Bilski, E. Mandowska, A. Mandowski, Photoluminescence of gamma-, proton- and alpha-irradiated LiF detectors, Centr. Eur. J. Phys. 10 (2012) 1009–1012. [14] A. Mrozik, P. Bilski, B. Marczewska, B. Obryk, K. Hodyr, W. Gieszczyk, Radiophotoluminescence of highly irradiated LiF:Mg, Ti and LiF:Mg, Cu, P detectors, Radiat. Meas. 71 (2014) 31–35. [15] F. Bonfigli, S. Almaviva, G. Baldacchini, S. Bollanti, F. Flora, A. Lai, R.M. Montereali, E. Nichelatti, G. Tomassetti, A. Ritucci, L. Reale, A.Y. Faenov, T.A. Pikuz, R. Larciprete, L. Gregoratti, M. Kiskinova, Soft X-ray imaging by optically stimulated luminescence from color centers in lithium fluoride, Spectrochim. Acta, Part B 62 (2007) 631–635. [16] T. Pikuz, A. Faenov, T. Matsuoka, S. Matsuyama, K. Yamauchi, N. Ozaki, B. Albertazzi, Y. Inubushi, M. Yabashi, K. Tono, Y. Sato, H. Yumoto, H. Ohashi, S. Pikuz, A.N. Grum-Grzhimailo, M. Nishikino, T. Kawachi, T. Ishikawa, R. Kodama, 3D visualization of XFEL beam focusing properties using LiF crystal X-ray detector, Sci. Rep. 5 (2015) 17713. [17] L. Reale, F. Bonfigli, A. Lai, F. Flora, P. Albertano, M.L. di Giorgio, L. Mezi, R.M. Montereali, A. Faenov, T. Pikuz, S. Almaviva, M. Francucci, P. Gaudio, S. Martellucci, M. Richetta, A. Poma, Contact X-ray microscopy of living cells by using LiF crystal as imaging detector, J. Microsc. 258 (2015) 127–139. [18] R.M. Montereali, F. Bonfigli, M. Piccinini, E. Nichelatti, M.A. Vincenti, Photoluminescence of colour centres in lithium fluoride thin films: from solid-state miniaturised light sources to novel radiation imaging detectors, J. Lumin. 170 (Part 3) (2016) 761–769. [19] B. Marczewska, P. Bilski, W. Gieszczyk, M. Kłosowski, Two-dimensional thermoluminescence method for checking LiF crystals homogeneity, J. Cryst. Growth 457 (2017) 320–324, http://dx.doi.org/10.1016/j.jcrysgro.2016.08.005. [20] J.F. Ziegler, M.D. Ziegler, J.P. Biersack, SRIM - The stopping and range of ions in matter (2010), Nucl. Instrum. Methods Phys. Res. B 268 (2010) 1818–1823. [21] S. Greilich, L. Grzanka, N. Bassler, C.E. Andersen, O. Jäkel, Amorphous track models: a numerical comparison study, Radiat. Meas. 45 (2010) 1406–1409. [22] T. Tabata, R. Ito, S. Okabe, Generalized semiempirical equations for the extrapolated range of electrons, Nucl. Instrum. Methods 103 (1972) 85–91.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Fluorescent detection of single tracks of alpha particles using lithium fluoride crystals P. Bilski ⇑, B. Marczewska Institute of Nuclear Physics Polish Academy of Sciences (IFJ PAN), Radzikowskiego 152, PL-31-342 Krakow, Poland
a r t i c l e
i n f o
Article history: Received 3 October 2016 Received in revised form 9 November 2016 Accepted 2 December 2016
Keywords: Radiation measurements Nuclear tracks Fluorescence Lithium fluoride Alpha particles
a b s t r a c t Lithium fluoride single crystals were successfully used for fluorescent imaging of single tracks of alpha particles. This was realized with a standard wide-field fluorescent microscope equipped with a 100 objective. Alpha particles create F2 and F+3 color centers in LiF crystals. The subsequent illumination with the blue light (wavelength around 445 nm), excites these centers and produces fluorescence with a broad band peaked at 670 nm. The observed tracks of alpha particles have diameter of about 500 nm. Focusing of the microscope at different depths in a LiF crystal, enables imaging changes of shape and position of tracks, allowing for visualization of their paths. These encouraging results are the first step towards practical application of LiF as fluorescent nuclear track detectors. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction About ten years ago a new radiation measurements technique using so called Fluorescent Nuclear Track Detectors (FNTD) was developed [1,2]. This technique exploits the fluorescence (photoluminescence) phenomenon in aluminum oxide crystals doped with carbon and magnesium (Al2O3:C,Mg) by analysis performed with a fluorescent confocal laser scanning microscope (CLSM). CLSM is now a well-known microscopic technique, enabling high resolution 3D imaging through scanning of a sample with a laser spot and discriminating the out-of-focus light (principles of CLSM may be found e.g. in Ref. [3]). Ionizing radiation induces F+2 (2 Mg) color centers within Al2O3:C,Mg crystals. When these centers are excited with 620 nm light, they produce fluorescence at 750 nm [4]. It was demonstrated that using a CLSM for analysis of Al2O3:C,Mg crystals it is possible to visualize single ion tracks [5,6] and even delta-rays [7]. The development of the FNTD method opened perspectives of numerous applications for radiation dosimetry including radiotherapy with particle beams, neutron measurements and cosmic radiation dosimetry. Recently, FNTDs were applied in radiobiology studies for colocalization of damage in living cells with track of charged particles [8]. So far FNTD technique has been realized only with Al2O3:C,Mg crystals. Another material showing a stable and intense radiation induced fluorescence is lithium fluoride (LiF). Radiation creates in ⇑ Corresponding author. E-mail address: [email protected] (P. Bilski). http://dx.doi.org/10.1016/j.nimb.2016.12.003 0168-583X/Ó 2016 Elsevier B.V. All rights reserved.
LiF crystals F centers (anion vacancies trapping electrons), which tend to aggregate forming more complex defects [9]. Among them F2 and F+3 color centers are of special interest. F2 center is composed of two anion vacancies with two bounded electrons, while F+3 of three vacancies with two electrons. Both of these centers have overlapped absorption bands peaked around 445 nm. When LiF crystal is illuminated with light of such wavelength, the color centers are excited and emit photoluminescence light of longer wavelength. The photoluminescence emission spectrum (PL) exhibits two peaks at about 670 nm (related to F2) and about 525 nm (related to F+3). The typical emission and excitation spectra of LiF crystals are illustrated in Fig. 1. PL of LiF has been proposed for generating laser action [10], for several applications as dosimeters [11–14], as well as for X-ray imaging applications [15–18]. The described properties indicate LiF as a potential candidate to be also used as fluorescent track detector. Application of LiF crystals offers some potential advantages over Al2O3:C,Mg. Firstly, the natural lithium contains 6Li isotopes, which have a high cross-section for (n,a) reaction, therefore enabling neutron detection. Secondly, the theoretical resolution limit for CLSM is roughly proportional to excitation and emission wavelengths, and both these values are lower in case of LiF. The goal of the present work was therefore to attempt application of LiF single crystals as FNTDs, i.e. for detection of single particle tracks. This goal was achieved using a standard manual wide-field fluorescent microscope, a device much simpler than confocal laser scanning systems (in a wide-field microscope the whole sample is illuminated with light and an image acquired with a CCD camera).
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P. Bilski, B. Marczewska / Nuclear Instruments and Methods in Physics Research B 392 (2017) 41–45
particles to about 3.11 MeV and maximum energy to 4.15 MeV. Range of alpha particles in LiF for these energies is 9.8 lm and 14.2 lm, respectively [20]. Microscopic observations were realized with a Nikon Ni-U fluorescence manual wide-field microscope with a DS-Qi2 CCD camera. For light excitation the Lumen 200 illumination system (Prior Scientific) was used. In order to discriminate between excitation and emission light we used a band-pass filter ET440/40 for excitation, a long-wavelength-pass filter ET515lp for emission and a dichroic mirror FF506-Di03. A Nikon CFI TU Plan Epi ELWD objective (100, NA 0.80) was applied. Image acquisition and analysis were realized with the Nikon NIS-Elements software.
3. Results and discussion
Fig. 1. Typical photoluminescence emission (PL) and excitation (PLE) spectra of LiF crystal (irradiated with Sr-90/Y-90 beta particles). PL measured for excitation with 445 nm light, PLE measured for emission at 670 nm.
2. Materials and methods LiF single crystals with a typical diameter of about 2 cm were grown with the Czochralski method at the IFJ PAN Kraków [19], using undoped LiF powder as the starting material (Fig. 2). The obtained transparent crystals were then cut with diamond saws into about 4 4 0.9 mm samples, polished manually with 0.5 lm diamond abrasive strips and rinsed in acetone. Prior to further experiments, the samples were annealed at 400 °C for 30 min in order to remove pre-existing F2 and F+3 color centers [14]. The LiF samples were irradiated with alpha particles from a radioisotope Am-241 source (produced by Eckert & Ziegler). The source activity was 14 MBq (±30% according to certificate) and dimensions of the active part were 25 12.5 mm. The samples were irradiated through a 100 lm slit at a distance of 4 mm from the source surface. The geometrical configuration during exposures is illustrated in Fig. 3. The reason for such arrangement was to create in LiF crystal a definite fluorescence image, which would be distinctly recognizable from the unirradiated background. Basing on the nominal source activity and on geometrical considerations, the particle fluence rate at the surface of the sample was estimated to be 4.1 105 h 1 mm 2. For some irradiations the slit was widened to 3 mm in order to irradiate a larger area of a crystal. In that case the fluence rate was estimated to be 1.23 107 h 1 mm 2. The time of exposure varied between 1 min and 60 min. The nominal energy of Am-241 alpha particles is 5.486 MeV. However in case of the applied source, energy of particles is strongly degraded, due to crossing 2 lm thick covering gold foil, as well as due to crossing the layer of the active material itself. Spectrometric measurements revealed that the spectrum is very broad and extends from 1 MeV to 4.5 MeV, with a peak at about 3.5 MeV. Crossing 4 mm of air reduces further average energy of
Fig. 2. LiF crystal: a) as grown with the Czochralski method, b) cut into samples.
Fig. 4 presents an example of an image of the LiF crystal irradiated with alpha particles through a slit. The bright band created by alpha particles is well contrasted with the unirradiated part of the crystal. The image is composed of bright spots or short vertical lines sometimes surrounded by a hazy background. The counted number of tracks was 1.33 105 mm 2, which agrees very well with the fluence estimated on the basis of the source activity (1.37 105 mm 2). The used 100 objective had a very short – below 1 lm, depth of focus. This enabled doing a kind of a scan of the samples along the z-axis, i.e. to study changes of the acquired images, when focus was adjusted to different depths into the crystal. Fig. 5 shows some examples of the obtained images. Fig. 5(a) is focused at the very surface of the sample. Fig. 5(b) represents the image acquired 1 lm deeper into the crystal. The appearance of the observed tracks is now different: besides spots there are a lot of lineshaped tracks, which are all directed nearly parallel. A few more micrometers deeper (Fig. 5c and d) the image is changed once more. There are no line-shaped tracks, but again spots, which are now brighter and their number is decreasing with depth. These observations may be interpreted in the following way. The presence of the 0.1 mm slit forced directionality of alpha particles: acceptance angle across the slit is only about 1.4°. The observed lines are created by particles impinging at acute angles to the crystal surface and almost in parallel to the slit. The penetration depth into a crystal at such angular trajectories is obviously smaller, therefore in deeper layers we have nearly not observed line-shaped tracks. Only circular spots created by particles directed nearly perpendicularly to the surface remain. They are brighter than spots at the surface (Fig. 5a), because energy losses are higher at the end of a particle path (Bragg peak), what means higher ionization density (LET in LiF of 3.11 MeV particle is 257 keV/lm and it increases to 407 keV/lm at the Bragg peak; for maximum energy of 4.15 MeV it is 215 keV/lm [20]) and finally more created F2/F+3 color centers. The number of spots decreases with depth, because particles with trajectories deviating from normal incidence are gradually stopped within a crystal. The observed hazy halo around the tracks seems to be due to out-of-focus light coming from layers deeper or shallower than the one under observation and possibly also by some scattered light. These effects, which diminish quality of images, should be absent if instead of a wide-field microscope a CLSM system enabling high spatial discrimination, is used. The tracks were observed in images acquired at depths ranging up to about 12–13 lm. This value is in agreement with 14.2 lm calculated with SRIM code as the maximum possible penetration depth into LiF In order to verify our interpretation of line-shaped tracks as created by particles impinging at acute angles, another experiment was performed. A LiF crystal was positioned for alpha irradiation vertically on its narrow side (Fig. 3b and d), while microscopic
P. Bilski, B. Marczewska / Nuclear Instruments and Methods in Physics Research B 392 (2017) 41–45
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Fig. 3. Schematic of the arrangement for alpha particle irradiation. Dimensions expressed in millimeters. (a) side view, crystal in horizontal position, (b) side view, crystal in vertical position, (b) top view, crystal in horizontal position, (c) top view, crystal in vertical position (c and d not in scale with a and b).
observations were done as previously on the front side. In this way no perpendicularly directed particles can reach surface of the crystal and trajectories at acute angles are more preferred. The result is illustrated in Fig. 6. It is apparent that almost all tracks have a line shape, what confirms the described interpretation. Most lines end with a brighter spot (at the upper side), which most probably corresponds to the Bragg peak at the end of a particle path. The longest track noticed within the image was 12.3 lm long, what agrees with the previous observations and with the SRIM calculations. Finally, one more irradiation was realized in configuration like Fig. 3a, but with the slit width set to 3 mm. By that means, nearly whole crystal surface was uniformly irradiated, without any directionality. Fig. 7 shows that the acquired image agree well with the expectations: it consists of short tracks aiming in various directions. The counted number of tracks was about 1.7 105 mm 2, what roughly agrees with the estimated fluence (2.05 105 mm 2). The diameter of the observed tracks is about 500 nm, what is somewhat more than the diameter of the volume in which an alpha particle deposits dose in LiF. The maximum range of delta
electrons produced by about 4 MeV alphas in LiF is 100–120 nm (calculated with the libamtrack library [21] according to the Tabata model [22]), so the diameter of a track should be below 250 nm. The size of the observed tracks is mainly determined by the resolution of the applied microscope system. For wide-field fluorescent microscopes the resolution is often estimated as k/2NA. For NA = 0.80 and k = 670 nm this amounts to 419 nm. The resolution may be improved if an objective with the higher numerical aperture is used: oil immersion lenses may exceed NA = 1.4.
4. Conclusions This work demonstrated that fluorescence of undoped LiF crystals can be successfully used for imaging nuclear particle tracks. This was achieved using a manual wide-field fluorescent microscope, an instrument which is simpler, much less expensive and much more common than confocal laser scanning systems, which were so far applied for this purpose. Introducing LiF as another type of detector besides Al2O3:C,Mg, as well as application of a
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P. Bilski, B. Marczewska / Nuclear Instruments and Methods in Physics Research B 392 (2017) 41–45
Fig. 4. Fluorescent image of the alpha irradiated LiF crystal. Irradiation time 20 min, fluence 1.37 105 mm 2. Acquisition time 20 s.
Fig. 6. Fluorescent image of the sample irradiated with alpha particles directed nearly parallel to the crystal (see Fig. 3b and d), acquired at depth of about 1 lm. Irradiation time 1 h. Direction of alpha particles is from the bottom side of the image. Acquisition time 40 s.
wide-field microscope, may make FNTD technique more accessible. In spite of the successful use of a wide-field microscope it should be noted that confocal systems would undoubtedly provide images of superior quality concerning S/N ratio and resolution and CLSM will be rather the target technique of measurements. The reported results were obtained using crystal samples which were randomly chosen from the standard LiF crystals grown at the IFJ PAN, without any selection concerning their photoluminescence
properties nor without any special treatment (some preliminary observations indicate that e.g. thermal treatment has significant influence on photoluminescence intensity of LiF). It seems therefore that there are margins for improvement concerning quality of LiF crystals, as well as other factors of the experimental procedure (e.g. application of objective with higher numerical aperture) and such work is under way. The present work may be considered as the first step towards developing of LiF based FNTDs.
Fig. 5. Fluorescent images of the same sample as in Fig. 4 (enlarged) focused at different depths into the crystal: a) ‘surface’, arbitrary assumed as 0 lm, b) 1 lm, c) 6 lm, d) 8.5 lm. Other parameters are the same as in Fig. 4. A video showing images acquired at different focal depth is available.
P. Bilski, B. Marczewska / Nuclear Instruments and Methods in Physics Research B 392 (2017) 41–45
Fig. 7. Fluorescent image of the sample irradiated with a broad uncollimated flux of alpha particles acquired at depth of about 2 lm. Irradiation time 1 min, particle fluence 2.05 105 mm 2. Acquisition time 30 s. A video showing images acquired at different focal depth is available.
Acknowledgments This work was supported by the National Science Centre, Poland (Contract No. UMO-2015/17/B/ST8/02180). The authors are grateful for J.W. Mietelski (IFJ PAN) for spectrometric measurements of the alpha source. We also thank W. Staniszewski (Precoptic/ Nikon) for help in optimum configuring of the microscope system. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.nimb.2016.12. 003. References [1] G.M. Akselrod, M.S. Akselrod, E.R. Benton, N. Yasuda, A novel Al2O3 fluorescent nuclear track detector for heavy charged particles and neutrons, Nucl. Instrum. Methods Phys. Res. B 247 (2006) 295–306. [2] M.S. Akselrod, R.C. Yoder, G.M. Akselrod, Confocal fluorescent imaging of tracks from heavy charged particles utilising new Al2O3:C,Mg crystals, Radiat. Prot. Dosim. 119 (2006) 357–362. [3] R.L. Price, W.G. Jerome, Basic Confocal Microscopy, Springer, 2011.
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