Lithium fluoride fluorescent nuclear track detectors (FNTD) irradiated with alpha-particles were subjected to annealing at temperatures ranging up to 400 ◦C. It was found that for the annealing temperatures between 200 ◦C and 300 ◦C the intensity of fluorescent tracks increases.
Radiation Measurements 157 (2022) 106845 Contents lists available at ScienceDirect Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas Thermal enhancement of the intensity of fluorescent nuclear tracks in lithium fluoride crystals M Sankowska *, P Bilski, B Marczewska Institute of Nuclear Physics Polish Academy of Sciences, Krak´ ow, Poland A R T I C L E I N F O A B S T R A C T Keywords: FNTD Photoluminescence LiF Track detectors Lithium fluoride fluorescent nuclear track detectors (FNTD) irradiated with alpha-particles were subjected to annealing at temperatures ranging up to 400 ◦ C It was found that for the annealing temperatures between 200 ◦ C and 300 ◦ C the intensity of fluorescent tracks increases The maximum enhancement factor of 2.5 is reached for a temperature of 290 ◦ C For temperatures exceeding 300 ◦ C the intensity decreases and above 340 ◦ C the tracks are not visible The effect of track enhancement is permanent - no differences were observed for several weeks after the thermal treatment An interesting phenomenon is the influence of the particle fluence: with increasing fluence the enhancement factor decreases and for the fluence around 2.9*107 cm− 2, the intensity of light registered after heat treatment is lower than the initial one It is still unclear what is the reason behind the observed effect It however seems probable that the mechanism involves transforming part of F centers into F2 centers Nevertheless, the effect is promising, as it may lead to signal-to-noise ratio improvement and improved measuring capabilities of LiF FNTDs Introduction Photoluminescence of color centers in lithium fluoride (LiF) has been studied for decades (Baldacchini, 2002), but the recent years brought a new interest to this topic The reasons are in new emerging applications of LiF crystals for imaging radiation dose distributions at the micro scopic level One direction of these applications are measurements of various parameters of proton beams (Marczewska et al., 2017; Mon tereali et al., 2019; Nichelatti et al., 2020; Piccinini et al., 2017, 2020; Vincenti et al., 2021) Another newly developed technique is fluorescent imaging of single nuclear particle tracks (Bilski et al., 2018a, 2018b, 2019a, 2019b, 2020) This method was originally developed for Al2O3 crystals (Akselrod et al., 2006; Akselrod and Sykora, 2011; Greilich et al., 2013), but recently successfully worked out also for LiF Fluo rescent nuclear track detectors (FNTD) based on LiF crystals were shown to be capable of imaging tracks of energetic ions, alpha particles, pro tons, and through secondary particles, even photons and neutrons The idea of this technique is based on the creation of color centers by ionizing particles when passing through a crystal Lattice defects pro duced by radiation in LiF consist mainly of F centers (electrons trapped by an anion vacancy) and their aggregates: F2 and F+ While F centers not produce any photoluminescence, the excitation of F2/F+ centers (which possess a common absorption band around 445 nm) with the blue light leads to strong photoluminescent emission within two bands: green (weaker, peaked near 525 nm, corresponding to F+ ) and red (stronger, peaked near 670 nm, corresponding to F2) (Baldacchini, 2002) The red photoluminescence of LiF was exploited for obtaining images of nuclear particle tracks (Bilski et al., 2018a) LiF FNTDs are basically reusable The heating of crystals at suffi ciently high temperatures removes color centers and resets the intensity of photoluminescence to the background level Within the series of ex periments aimed at optimizing conditions of such thermal treatment, we found that the heating of previously irradiated crystals sometimes does not remove the color centers, but quite oppositely increases the intensity of the fluorescence, making the nuclear tracks much more clear and better separated from the background This unexpected finding, which may lead to a substantial improvement of LiF FNTD measuring capa bilities, motivated a more systematic investigation of this phenomenon, and the present paper reports the obtained results Materials and methods Lithium fluoride single crystals were grown with the Czochralski method at IFJ PAN in Krak´ ow (Bilski et al., 2018a) As a starting material * Corresponding author Institute of Nuclear Physics PAN, ul Radzikowskiego 152, 31-342, Krak´ ow, Poland E-mail address: malgorzata.sankowska@ifj.edu.pl (M Sankowska) https://doi.org/10.1016/j.radmeas.2022.106845 Received 22 June 2022; Received in revised form 27 July 2022; Accepted 16 August 2022 Available online 20 August 2022 1350-4487/© 2022 The Authors Published by Elsevier Ltd This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) M Sankowska et al Radiation Measurements 157 (2022) 106845 Fig Microscopic fluorescent images registered with the same LiF crystal (acquisition time s) after annealing at gradually increasing temperatures Samples irradiated with Am-241 source (particle fluence around 1.60*104 cm− 2) an undoped, pure LiF powder was used The grown bulk crystals were then cut with diamond saws into square plate samples of a standard size of × × mm Samples were later polished with abrasive straps and rinsed in acetone in an ultrasonic washer for After that, samples were pre-heated for 10 at a temperature close to the melting point of LiF (820–830 ◦ C) in argon atmosphere before the first use This pro cedure improves the surface quality of samples by removing small scratches created during polishing and at the moment is routinely applied in our laboratory to all LiF crystal samples The main experiments were carried out with the crystal samples irradiated with alpha particles from two Am-241 sources (Eck ert&Ziegler: AMRB5718 with activity 10.7 MBq and AMR14 with ac tivity 40 kBq) Irradiations were carried out using special metal collimators to ensure the same conditions for each sample and provide a high number of tracks perpendicular to the sample’s surface The used collimators had, respectively, a thickness of mm and a round hole with a diameter of mm for a source with an activity of 10.7 MBq and a thickness of 2.5 mm and a round hole with a diameter of mm for a source with an activity of 40 kBq The time of irradiation depended on the desired fluence and the source used and varied from a few seconds to several hours Some additional exposures were performed with gamma radiation from a Cs-137 source in the calibration laboratory of the IFJ PAN After irradiation, microscopic images were taken using Nikon Eclipse Ni–U fluorescent wide-field microscope with a CCD camera DS Qi-2 As the excitation light source, a pE-100 illumination system with 440 nm LEDs (CoolLED) was used together with band-pass filter ET445/30 (Chroma) For emission, a long-pass filter ET570lp (Chroma) was applied Most images were taken using a 100x TU Plan ELWD (NA 0.80) objective lens The field of view was limited by a diaphragm and had a quasi-circular shape with a diameter of about 90 μm Images of samples irradiated with alpha particles were taken at depth of μm beneath the sample’s surface For samples irradiated with gamma radiation, images were taken at μm beneath the sample’s surface Computer analysis of the obtained microscopic images was performed using ImageJ software (with Fiji interface) (Schindelin et al., 2012) For observed tracks the background signal was subtracted from the maximum intensity in a track The background signal was calculated as a modal value of in tensity in the circle of a radius of 50 pixels (c.a 3.5 μm) around a track As the final result a mean value of intensity of tracks after background subtraction was used To calculate the change in intensity, the results after heat treatment were divided by the results obtained before applying any post-irradiation annealing This ratio will be later called the enhancement factor The particles’ angle of incidence has an impact on tracks intensity For that reason from all the observed tracks we used only the ones with a circular shape as they originate from particles almost perpendicular to the sample’s surface The photoluminescence emission spectra were measured using Ocean Optics QE pro 00689 spectrometer mounted into a setup of a Nikon Eclipse Ni–U fluorescent wide-field microscope instead of the CCD camera The spectral measurements were performed using a 5x TU Plan Epi objective lens and a long-pass 510lp filter After the irradiations we waited at least 24 h before applying heat treatment The reason for this delay is the observed small increase in tracks intensity during the first few hours after irradiation (even without annealing) Post-irradiation heating of the samples was carried out using the Linkam THMS600 heating stage The heating of the crystals was performed, depending on the experiment, in one of two ways: Method 1: multi-step heating - a sample was submitted to a series of subsequent heat treatments at increasing temperatures ranging from room temperature up to 400 ◦ C with the step of 20 ◦ C Method 2: one-step heating to the set temperature For each tem perature another crystal sample was used The temperature was varied with the step of 10 ◦ C Fig Light intensity increase vs temperature of annealing (heating the same sample at increasing temperatures) Samples were irradiated with Am-241 source (particle fluence around 1.60*104 cm− 2) Error bars represent one standard deviation M Sankowska et al Radiation Measurements 157 (2022) 106845 Fig Tracks intensity increase - comparison between one-step heating and successive heating Samples irradiated with Am-241 source (particle fluence around 1.60*104 cm− 2) Fig Tracks intensity increase - heating time optimization Samples irradi ated with Am-241 source (particle fluence around 1.60*104 cm− 2) enhancement for a given temperature, combined with the standard de viation of the enhancement for all crystal samples after heat treatment at 290 ◦ C (measured separately to account for possible differences between various samples) Results, presented in Fig 3, are very similar to those observed for Method The biggest enhancement factor was observed for 290 ◦ C, and this temperature was used as the standard in the further measurements Another step that was undertaken to optimize the post-irradiation heat treatment was testing the influence of the time of annealing on the change in tracks intensity We used a standard one-step heating procedure to 290 ◦ C (Method 2) with annealing time varying between 10 s and 10 Fig shows the relationship between the duration of the heat treatment and the observed increase in tracks intensity Values of in tensity increase and uncertainties were determined analogously to the heat treatment temperature optimization measurements The effect gets stronger with increasing time up to about 60 s Above that time we noticed only small changes, which did not show any strong trend and were within the limits of uncertainty As the measurements were con ducted for different samples, it is safe to say that extending the time of heat treatment above 120 s does not significantly change the strength of the observed increase For these reasons, we decided to continue using as our standard annealing time and be consistent with our previous results We also checked the effect of performing additional heating on a sample which already underwent such treatment and found that the increase of the track intensity remains unaffected What is important, the applied temperature treatment does not cause any deterioration of track images due to migration of defects Fig presents enlarged images of tracks acquired before and after 290 ◦ C annealing There is no blurring nor broadening of the track image Fig 5c compares profiles measured along the lines perpendicular to the tracks The width of both tracks is similar Even when crystals were heated at higher temperatures and the tracks began to fade out, their shape remained unchanged This indicates low mobility of F2 centers, even at elevated temperatures The results presented in the previous figures were obtained for microscopic images that were taken directly after heating and cooling of the samples In everyday usage, however, we may need to perform multiple measurements on the same sample and take pictures even after a long time FNTD is basically a non-destructive technique, except for slight bleaching during readout We checked that samples that were submitted to the heat treatment also not show any changes in the In both methods crystal plates were placed on the heating stage at room temperature and then heated to the set temperature with a heating rate of 150 ◦ C/min After reaching the desired temperature samples were annealed for After that time, the heating was turned-off and samples were cooled, remaining on the heating stage for another Then they were removed from the heating stage and placed on the aluminum plate to cool down quickly to room temperature All spectral and microscopic measurements were performed at room temperature Results and discussion To determine the influence of the temperature treatment on the in tensity of single-particle tracks, in the first step we subjected a LiF crystal sample previously irradiated with alpha-particles (the fluence around 1.60*104 cm− 2) to the heat treatment according to the Method Microscopic images illustrating changes occurring after heat treatment are presented in Fig The increase of the brightness of both, tracks and the whole image is evident A shortcoming of the heat treatment is the enhancement of the background signal It was observed for every tested sample and depended strongly on the quality of the sample’s surface Fig presents the dependence of the observed increase of tracks intensity on the temperature of the applied heat treatment For each temperature point, approximately 45 tracks from three different parts of a crystal sample were measured It can be seen that up to around 200 ◦ C the intensity of the tracks remains the same Above that temperature, we noticed a significant increase in track intensity, as well as in background light, with the maximum increase observed for temperatures around 280–300 ◦ C For these temperatures, the intensity measured after annealing was almost 2.5 times higher than the initial value The in crease in tracks intensity after heat treatment starts to diminish quickly when we apply temperatures higher than 300 ◦ C After annealing at temperatures above 340 ◦ C, the tracks are no longer visible The same effect of the increase in tracks intensity was observed also when samples were submitted to one-step heating (Method 2), instead of a series of heatings to subsequent temperatures (Method 1) To choose the most optimal temperature of the applied treatment, we performed tests in a narrowed temperature range from 240 to 320 ◦ C, with a step of 10 ◦ C For each temperature value, five images were taken at different parts of a sample and the mean value of the track intensity was calcu lated for approximately 60–80 tracks The uncertainties were calculated as standard deviations of the mean value of the track intensity M Sankowska et al Radiation Measurements 157 (2022) 106845 Fig Exemplary comparison of the fluorescent tracks before and after heating at 290 ◦ C Panel c) presents profiles of the tracks (normalized intensity) measured along the yellow lines perpendicular to the tracks’ axes, marked on panels a) and b) different fluences of alpha radiation ranging from 1.6*104 cm− to 5.8*107 cm− and subjected them to the standard thermal treatment The results are presented in Fig and Fig For each fluence value, five images were taken at different parts of a sample for three different crystal samples The mean value of the track intensity was calculated for fluences up to 4.8*105 cm− For higher fluences, when single tracks were no longer recognizable, the light intensity increase was determined as the ratio of the average brightness in the field of view of a diameter of 90 μm measured after and before heating to 290 ◦ C Acquisition time varied from s for lowest fluences to 200 ms for highest value of fluence It was, however, always the same for each compared pair of images (before and after the heat treatment) It appears that the enhancement in the intensity of tracks reaches its highest value of about 2.5 only for very low fluences When the density of tracks grows, the increase is less prominent and finally, for the value of fluence around 2.9*107 cm− 2, the intensity of light registered after a heat treatment is lower than the initial one The results confirmed that the fluence indeed was the cause of the discrepancy between the microscopic and the spectral data It seems that signal enhancement is high only if an area of a highconcentration of color-centers (i.e a track) is surrounded by an area with a low number of centers This unexpected finding makes in vestigations aimed at understanding the nature of the observed photoluminescence signal over time After months of storage the track intensity enhancement stayed within 5% from the initial enhancement observed right after heat treatment This proves that the increase in tracks intensity is a permanent effect To better understand the mechanism of photoluminescence signal increase after post-irradiation heat treatment we performed emission spectra measurements for the sample irradiated with alpha particles from Am-241 source with a fluence of 5.5*1012 cm− The irradiated sample was gradually heated from room temperature up to 400 ◦ C with a standard multi-step heating procedure Fig presents the comparison between emission spectra obtained for a sample that was not subjected to any heating and for the same sample after multi-step heating to 290 ◦ C It is clear that we not observe the effect of signal enhance ment, described above, but oppositely, a big decrease of the intensity of the 670 nm emission band related to the F2 color centers is present We also observe a complete disappearance of a peak related to F+ color centers (located near 525 nm) The main difference between the conditions of the spectral mea surements and microscopic observations of tracks was in the used alphaparticle fluence (~1012 cm− and ~104 cm− 2, respectively) In order to verify whether the higher fluence might be the reason for the lack of the luminescence intensity increase, we irradiated LiF samples with M Sankowska et al Radiation Measurements 157 (2022) 106845 Also, the background is much brighter This might be caused not only by surface imperfections (as in the case of alpha irradiations), but maybe even more by the enhancement of the fluorescent signal originating from the color centers which not form visible tracks Such centers are certainly present, as the gamma-ray dose is deposited pretty uniformly in the whole volume of a crystal, but due to the limited sensitivity, we can only see as tracks the spots of locally higher ionization density, i.e higher density of color-centers The number of visible tracks is also increased after the annealing The smaller increase of the light intensity than in the case of alphaparticles (1.5 instead of 2.5) may be interpreted as consistent with the observed dependence of the enhancement factor on the density of tracks Looking for a possible reason for the observed enhancement of the fluorescent tracks after thermal treatment, two kinds of processes might be considered: the creation of additional F2 color centers in LiF crystals or mitigation of some processes competitive to the F2-related photo luminescence Concerning the latter, the existence of such competitive processes in LiF has never been reported, so the formation of new F2 centers seems more probable Interactions between defects and trans formation of one type into another obviously occur in LiF (Schwartz et al., 2010; Voitovich et al., 2011, 2013) These processes may follow various paths and the presently available data not allow us to draw any firm conclusions However, taking into account that F2 centers are the most abundant ones in LiF besides F centers, it seems to be rather unlikely that the transformation of any other defects may increase the population of F2 to a degree sufficient to explain a 2.5-times increase in the fluorescent intensity On the other hand, as the number of F centers in LiF many times exceeds that of F2 centers, even a small fraction of F centers turned into F2 may account for such enhancement of the track intensity, therefore the involvement of F centers in the process seems probable Fig Photoluminescence emission spectra for a sample irradiated with alpha particles from Am-241 source (fluence 5.5*1012 cm− 2) for excitation at 440 nm The spectrum is clipped from the low wavelength side by the applied filter Conclusions Fig Change in tracks intensity after 290 particle fluence ◦ The performed investigations fully confirmed that thermal treatment may increase the intensity of fluorescent tracks in LiF crystals This in crease occurs for heating a crystal at temperatures ranging between 200 ◦ C and 300 ◦ C, with a maximum increase of 2.5 at 290 ◦ C Enhancement of tracks intensity is a permanent effect that does not change in time An interesting phenomenon that we discovered during our research is a strong dependence of the photoluminescent signal enhancement on particle fluence The enhancement becomes less and less prominent with increasing fluence and completely disappears for fluences over 2.9*107 cm− Above this value, the photoluminescence signal after postirradiation annealing at 290 ◦ C is lower than the initial one It is unclear what is the reason behind the observed effect of pho toluminescence increase As different color centers disintegrate or interact with each other and create new species, their concentrations change while LiF crystal is subjected to heat treatment Unfortunately, as the effect of photoluminescence enhancement only occurs for very low fluences of ionizing particles, emission and absorption spectra measurements are not helpful in solving that problem The effect of tracks intensity increase is very desirable for LiF-based FNTDs Although more research is still needed on this topic, it may lead to signal-to-noise ratio improvement and improved measuring capabil ities As the proposed heat treatment is fast and very easy, it could be routinely used in measurements C annealing vs alpha- enhancement of the tracks intensity, more difficult It seems that the measurement of luminescence spectra, and all the more, the measure ments of the absorption spectra, which require even higher doses, will not be able to provide data useful for this purpose In our study, we focused mostly on the crystals irradiated with alpha particles, as their tracks are easy to observe with the FNTD method, and the irradiations are quick and uncomplicated The effect of enhancement is however not limited only to these particles Fig presents the images acquired from the crystal irradiated with 50 mGy of Cs-137 gamma radiation The tracks produced by photons not have a shape resem bling the real path of secondary electrons, but have just a form of small dots, a little brighter than the background, and of the size equal to the spatial resolution of the applied microscope system (c.a 400 nm) They may be interpreted as a result of locally enhanced energy deposition e.g at the end of an electron track or due to the overlapping of several electron tracks The effect of annealing on the photon-induced image is clearly visible also in this case The enhancement factor is about 1.5 Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper M Sankowska et al Radiation Measurements 157 (2022) 106845 Fig Illustration of track intensity changes due to 290 ◦ C heating for various alpha-particle fluence Fig Effect of 290 ◦ C annealing on gamma-rays irradiated crystal (Cs-137, 50 mGy) Acquisition time 20 s Data availability Acknowledgments Data will be made available on request This work was supported by the National Science Centre, Poland (Contract No UMO-2020/37/N/ST5/01975) M Sankowska et al Radiation Measurements 157 (2022) 106845 References Nichelatti, E., Piccinini, M., Ronsivalle, C., Picardi, L., Vincenti, M.A., Montereali, R.M., 2020 Evaluation of saturation dose in spatial distributions of color centers generated by 18 mev proton beams in lithium fluoride Nucl Instrum Methods B 464, 100–105 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., 2020 Dose response and bragg curve reconstruction by radiophotoluminescence of color centers in lithium 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was carried out using the Linkam THMS600 heating stage The heating of the crystals was performed, depending on the experiment, in one of two ways: Method 1: multi-step heating... on the density of tracks Looking for a possible reason for the observed enhancement of the fluorescent tracks after thermal treatment, two kinds of processes might be considered: the creation of