- Email: [email protected]
Detection of neutrons with LiF fluorescent nuclear track detectors
Accepted Manuscript Detection of neutrons with LiF fluorescent nuclear track detectors P. Bilski, B. Marczewska, M. Kłosowski, W. Gieszczyk, M. Naruszewicz PII:
S1350-4487(18)30283-X
DOI:
10.1016/j.radmeas.2018.06.022
Reference:
RM 5944
To appear in:
Radiation Measurements
Received Date: 20 April 2018 Revised Date:
3 June 2018
Accepted Date: 25 June 2018
Please cite this article as: Bilski, P., Marczewska, B., Kłosowski, M., Gieszczyk, W., Naruszewicz, M., Detection of neutrons with LiF fluorescent nuclear track detectors, Radiation Measurements (2018), doi: 10.1016/j.radmeas.2018.06.022. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
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Detection of neutrons with LiF fluorescent nuclear track detectors P. Bilski*, B. Marczewska, M. Kłosowski, W. Gieszczyk, M. Naruszewicz
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Institute of Nuclear Physics, Polish Academy of Sciences (IFJ PAN), PL-31-342 Krakow, Poland
Measurements of neutron Hp(10) with LiF FNTDs Single tracks well visible even for 1 Gy gamma-ray background Detection of fast neutrons through recoil of Li nuclei
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*Corresponding author: [email protected], phone: +48 12 662 8414
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Fluorescent nuclear track detectors (FNTD) based on natLiF crystals were successfully applied for measurements of neutron doses. FNTDs were exposed on a phantom to Hp(10) doses ranging from 1 mSv to 20 mSv. The number of observed tracks was found to be linear with the dose. The actual limit of detection is certainly much lower than 1 mSv and it depends on the scanned volume of a crystal. The very advantageous feature of LiF FNTDs is that the unirradiated detectors show no tracks similar to that produced by thermal neutrons, which means that background (zero-dose signal) is basically equal to zero. These results were obtained with detectors based on natural lithium and might be substantially improved if 6Li enriched lithium is used for crystal growth. It was also found that LiF FNTDs can be used for direct detection (without any converter) not only of thermal, but also of fast neutrons by using recoil of lithium nuclei.
1. Introduction
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The technique of fluorescent nuclear track detectors (FNTD) was developed by Akselrod and coworkers, basing on Al2O3:C,Mg single crystals (Akselrod et al., 2006; Akselrod and Akselrod, 2006; Akselrod and Sykora, 2011; Greilich et al., 2013). Besides detection of directly ionizing particles, it was also effectively applied for measurements of neutron doses. For this purpose special neutron/alpha or neutron/proton converters were used (Sykora et al., 2008). Recently another material apart from Al2O3 has been also successfully exploited for fluorescent detection of nuclear tracks – lithium fluoride crystals (Bilski and Marczewska, 2017; Bilski et al., 2018). The irradiated LiF emits, under blue light excitation, photoluminescence light within a broad spectrum peaked at 670 nm (Bilski et al., 2017). LiF is a particularly interesting material for detection of neutrons, as one of lithium isotopes, 6Li, possesses a very high cross-section for (n,α) reaction with thermal neutrons. The natural lithium contains about 7.5% of 6Li isotope. The possibility of direct visualization of tracks of (n,α) reaction products was already demonstrated (Bilski et al., 2018). The goal of the present work is to investigate more quantitatively and in more detail some aspects of neutron measurements with LiF FNTDs.
2. Materials and methods Lithium fluoride single crystals with typical diameter of about 2 cm were grown with the Czochralski method at the IFJ PAN Kraków, using undoped natLiF powder as the starting material. The obtained transparent crystals were then cut with diamond saws into pieces of typical size about 4x4x1 mm. The samples were next polished manually with 0.5 µm diamond abrasive strips and rinsed in acetone. The reference neutron irradiations were performed at the accredited calibration laboratory of the National Centre for Nuclear Research in Otwock with a Pu-Be source (emission rate ~107 n.s-1) (Domański et al., 2018). The exposures were conducted in terms of Hp(10), with 2
ACCEPTED MANUSCRIPT samples placed on the front surface of a water phantom or in terms of H*(10), free in air. The neutron fluence rate was 128.6 n.cm-2s-1.
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Additional neutron test exposures were carried out at the IFJ PAN, using also a Pu-Be source (emission rate 5×105 n.s-1). In order to obtain thermal neutron field, the source was placed behind a 10 cm layer of polyethylene. The thermal neutron fluence rate was estimated using 6 LiF:Mg,Ti detectors, previously calibrated in a reference thermal neutron field (Burgkhardt et al., 2006) and found to be about 4 n.cm-2s-1. Some irradiations were performed with the bare Pu-Be source. In that case neutron fluence rate was calculated basing on the emission rate and a distance from the source.
3. Results and discussion
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Microscopic observations were conducted using Nikon Ni-U wide-field fluorescent microscope with DS-Qi2 CCD camera and Lumen 200 halogen lamp. Excitation light was transmitted through a band-pass filter ET440/40X (transmission window 420-460 nm), while emission light through a long-pass filter ET570lp. An objective with magnification 100× and NA=0.80 was applied. The field of view was limited by a diaphragm and had a quasi-circular shape with the diameter of about 90 µm (area about 6900 µm2). The exposure time for each image was 30s. The images were analyzed using the Fiji software (Schindelin et al., 2012).
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Figure 1 illustrates typical shapes of tracks registered after irradiation with thermal neutrons. Figure 1a represents a single image taken at the depth of 12 µm below the crystal surface. A well visible long track has length of about 39 µm and consists of two distinguishable parts: shorter and brighter (6 µm) and longer (33 µm). These lengths agree very well with the predicted ranges in LiF of products of 6Li(n,α)3H nuclear reaction: an alpha particle and a tritium nucleus. The whole length of the discussed track was quite well visible, because incidentally it was perfectly aligned with the observation plane. In case of particles crossing the plane under an angle, we observe a sharp image of only a part of the track. This effect is illustrated in Figure 2 for a single alpha/triton track. The reason is a short focal depth of the used optical system, which may be estimated to be below 1 µm (what actually is an advantageous effect, as it greatly reduces a disturbing influence of light emitted from other layers of the crystal). The whole track may be registered, if instead of a single image, a stack of images is acquired with a small step in depth into a crystal. Such stack may be basically used for a 3D reconstruction of a track, but within this work a more simple method was used: the maximum intensity projection. The result of such approach is presented in Figure 1b, which is the maximum intensity projection of 23 images registered with the step of 1 µm from 10 µm to 32 µm. The same approach was adopted in all measurements: typically stacks of about 20 images were registered with the step of 1 µm. In this way, a volume of about 1.4×105 µm3 was analyzed per one location in a crystal. Figure 3 presents dependence of the number of the registered tracks on the thermal neutron fluence. For an unirradiated crystal essentially no tracks was observed. The number of the tracks increases proportionally to the fluence, up to the last data point for about 1.4 n.cm-2, which shows underresponse by around 10%. This is an obvious result of tracks overlapping, which may cause that some less visible tracks are unnoticeable. The linearity range might certainly be extended by developing a more optimized procedure of analysis. 3
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Fig. 1. Examples of fluorescent tracks registered after irradiation with thermal neutrons (fluence ca. 106 n.cm-2). a) – image acquired 12 µm below the crystal surface, b) – maximum intensity projections of 23 images for depths from 10 µm to 32 µm.
Fig. 2. A series of images showing a single alpha/triton track, taken with 1 µm steps from 16 µm (left upper) to 24 µm (right lower) below the crystal surface. 4
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Fig. 3. The number of registered tracks per field of view versus thermal neutron fluence. One track per field of view corresponds to about 7000 tracks per mm3. For the zero fluence no track was observed. The line represents a linear fit to the first four data points.
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In order to check performance of LiF FNTDs for applications in individual dosimetry, the detectors were exposed in the calibration laboratory to reference doses of individual dose equivalent, Hp(10). Such irradiations are performed on a water phantom, which becomes a source of thermalized albedo neutrons. Figure 4 illustrates typical images registered for doses of 1 mSv, 10 mSv and 20 mSv. The quantitative results, expressed as the number of registered tracks per field of view versus dose equivalent, are presented in Figure 5. It is apparent that the dose of 1 mSv, which produced by average somewhat less than one track per a scanned volume, may be easily measured and taking into account that for the zero dose no tracks is observed, the actual minimum measurable dose is certainly much lower. Accuracy of such measurement and hence the limit of detection, will depend on the number of registered stacks, i.e. on the scanned volume. A good statistic is needed, as relatively high variation between number of tracks observed in single stacks was found. For instance, for 10 mSv dose, the number of registered tracks was ranging from 4 to 12 tracks per field of view. The obtained results may be compared with those of the aluminum oxide FNTD automatic system, for which a capability of measuring doses in the range of 0.2-0.3 mSv was demonstrated (Akselrod et al., 2014; Yukihara et al., 2018) and lower limit of detection was estimated basing on standard deviation of unirradiated detectors to be around 0.05 mSv. This was however achieved by scanning quite large areas, exceeding 1 mm2.
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Fig. 4. Examples of images registered after irradiation with different neutron Hp(10) doses: a) 1 mSv, b) 10 mSv, c) 20 mSv.
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Fig. 5. Hp(10) dose response of LiF FNTDs: number of registered tracks per field of view versus neutron individual dose equivalent. One track per field of view corresponds to about 7000 tracks per mm3. For the zero dose no track was observed. The line represents a linear fit. The number of the registered tracks might be substantially increased by using lithium enriched with 6Li isotope for crystal growth. Natural lithium contains 7.5% of 6Li, while material with enrichment factor up to 95% is also available. Simple calculation of the neutron absorption ratio indicates that over 10 times more tracks may be expected when enriched lithium is used. This should shift the minimum measurable dose well below 0.1 mSv. Detection of thermal neutrons is obviously the most promising application of LiF FNTDs, However, it was found that this material also offers possibility to detect fast neutrons. This is illustrated in Figure 6. This image was registered for a crystal exposed to a bare Pu-Be source, neutron spectrum of which extends up to 11 MeV, with average energy of about 5 MeV. In this image, besides a few long tracks attributable to thermal neutrons (presence of 6
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which may be always expected due to scattering of neutrons), many much shorter tracks are also well visible. Their length is various: from several micrometers down to even less than 1 µm. Some of these short tracks are quite bright. It seems that the most probable interpretation of these tracks is that they are caused by recoil lithium nuclei, which are sufficiently light to acquire a significant amount of kinetic energy in a collision with a neutron. The energy of a recoil nucleus is described by the equation: 4ܣ ܧ = ݏܿ ܧଶ ߠ ሺ ܣ+ 1ሻଶ where A – mass number of an isotope, En – neutron energy and Θ – scattering angle. For 7Li the maximum recoil energy is therefore equal to: Emax( 7Li) = 0.44 En
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This means, that for example 5 MeV neutron may impart energy of 2.2 MeV to 7Li nucleus. The range in LiF crystal of a 7Li ion with such energy is 4.5 µm. For the maximum neutron energy of 11 MeV, this range may reach 8.6 µm. Obviously any shorter ranges may be also present, depending on angle Θ and neutron energy.
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Fig. 6. Examples of fluorescent tracks registered after irradiation with fast neutrons from a bare Pu-Be source (fluence ca. 109 n.cm-2). The discussed results were obtained after irradiation with relatively high neutron fluence, estimated to be ~109 n.cm-2. In order to check capability of direct, i.e. without any moderator nor converter, application of LiF FNTDs in measurement of fast neutron doses, the detectors were exposed in the calibration laboratory free in air to 20 mSv of ambient dose equivalent, H*(10), which corresponds to the fluence of 2×107 n.cm-2. In this case however, practically no tracks were observed. It seems therefore that such direct detection of fast neutrons is not suitable for dose range which is mainly of importance for radiation protection. This approach might be useful when higher neutron fluences are encountered. In practical dosimetric measurements neutrons are always accompanied by some doses of gamma radiation. Separating both components of the radiation field is often a difficult task. In thermoluminescent dosimetry it is usually accomplished by using pairs of detectors enriched in 6Li and 7Li isotopes and calculating neutron dose from the difference of their 7
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signals. This approach becomes problematic if the neutron induced signal is much lower than the gamma background. Additionally, even highly enriched 7LiF TLDs show some sensitivity to neutrons (Burgkhardt et al., 2006). On the other hand, the chemically etched track detectors are basically insensitive to gamma radiation. In order to check this aspect of the performance of LiF FNTDs, the detectors previously irradiated with neutrons, were then exposed to various doses of 137Cs gamma-rays. The results are illustrated in Figure 7, showing examples of the neutron induced tracks for different gamma background ranging from zero dose up to 1 Gy. The dose of 50 mGy has basically no visible effect on the image. For 100 mGy a brightening of the background is apparent, but tracks are still very well contrasted and the background signal should not have any deteriorating effect on the counting efficiency. For 1 Gy the image is very bright and hence less contrasted, so one may expect that some weaker tracks could be missed. This concerns mainly triton tracks, especially if only a part of a track is present in the image. However, alpha particle tracks are still very clearly visible, as well as several triton tracks.
Fig. 7. Examples of images acquired after irradiation with thermal neutrons (fluence ca. 106 n.cm-2) and additional doses of 137Cs gamma radiation.
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Conclusions
The performed investigations confirmed the promising properties of LiF FNTDs for application in neutron dosimetry. Hp(10) dose of 1 mSv was easily measured. The actual limit of detection is certainly much lower and will depend on the scanned volume of a crystal. The very advantageous feature is that the unirradiated detectors show no tracks similar to that
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produced by thermal neutrons, which means that background (zero-dose signal) is basically equal to zero. Further improvement may be achieved by growing LiF crystals from 6Li enriched lithium, which would significantly decrease the lower detection limit, as well as by optimizing image analysis procedures. An interesting finding is that LiF FNTDs can be used for direct detection (without any converter) not only of thermal, but also of fast neutrons This can be done by using recoil of lithium nuclei. However, efficiency of this process is not very high and it may be too low for application in radiation protection. A photoluminescence induced by an increased gamma radiation background does not hamper detection of neutrons. Single tracks are still well visible for 1 Gy of gamma background.
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Acknowledgments
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This work was supported by the National Science Centre, Poland (Contract No. UMO2015/17/B/ST8/02180
References
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Akselrod, G.M., Akselrod, M.S., Benton, E.R., Yasuda, N., 2006. A novel Al2O3 fluorescent nuclear track detector for heavy charged particles and neutrons. Nucl. Instr. and Meth. B 247, 295-306. Akselrod, M.S., Akselrod, A.E., 2006. New Al2O3:C,Mg crystals for radiophotoluminescent dosimetry and optical imaging. Radiat. Prot. Dosim. 119, 218-221. Akselrod, M.S., Fomenko, V.V., Bartz, J.A., Haslett, T.L., 2014. Automatic neutron dosimetry system based on fluorescent nuclear track detector technology. Radiat. Prot. Dosim. 161, 86-91. Akselrod, M.S., Sykora, G.J., 2011. Fluorescent nuclear track detector technology - A new way to do passive solid state dosimetry. Radiat. Measur. 46, 1671-1679. Bilski, P., Marczewska, B., 2017. Fluorescent detection of single tracks of alpha particles using lithium fluoride crystals. Nucl. Instr. and Meth. B 392, 41-45. Bilski, P., Marczewska, B., Gieszczyk, W., Kłosowski, M., Nowak, T., Naruszewicz, M., 2018. Lithium fluoride crystals as fluorescent nuclear track detectors. Radiat. Prot. Dosim. 178, 337-340. Bilski, P., Marczewska, B., Zhydachevskii, Y., 2017. Radiophotoluminescence spectra of lithium fluoride TLDs after exposures to different radiation modalities. Radiat. Measur. 97, 14-19. Burgkhardt, B., Bilski, P., Budzanowski, M., Bottger, R., Eberhardt, K., Hampel, G., Olko, P., Straubing, A., 2006. Application of different TL detectors for the photon dosimetry in mixed radiation fields used for BNCT. Radiat. Prot. Dosim. 120, 83-86. Domański, S., Tulik, P., Boimski, B., 2018. Reference neutron fields in calibration laboratory; simple dosimetric parameters and their changes in time. Radiat. Prot. Dosim., doi: 10.1093/rpd/ncy052 Greilich, S., Osinga, J.M., Niklas, M., Lauer, F.M., Klimpki, G., Bestvater, F., Bartz, J.A., Akselrod, M.S., Jakel, O., 2013. Fluorescent nuclear track detectors as a tool for ion-beam therapy research. Radiat.Measur. 56, 267-272. Schindelin, J., Arganda-Carreras, I., Frise, E., Kaynig, V., Longair, M., Pietzsch, T., Preibisch, S., Rueden, C., Saalfeld, S., Schmid, B., Tinevez, J.-Y., White, D.J., Hartenstein, V., Eliceiri, K., Tomancak, P., Cardona, A., 2012. Fiji: an open-source platform for biological-image analysis. Nature Methods 9, 676. Sykora, G.J., Salasky, M., Akselrod, M.S., 2008. Properties of novel fluorescent nuclear track detectors for use in passive neutron dosimetry. Radiat. Meas. 43, 1017-1023. Yukihara, E.G., Akselrod, M.S., Fomenko, V., Harrison, J., Million, M., Assenmacher, F., Stabilini, A., Meier, K., 2018. Comparison between PADC and FNTD neutron detector systems in blind tests. Radiat. Prot. Dosim., doi: 10.1093/rpd/ncx171 9
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S1350-4487(18)30283-X
DOI:
10.1016/j.radmeas.2018.06.022
Reference:
RM 5944
To appear in:
Radiation Measurements
Received Date: 20 April 2018 Revised Date:
3 June 2018
Accepted Date: 25 June 2018
Please cite this article as: Bilski, P., Marczewska, B., Kłosowski, M., Gieszczyk, W., Naruszewicz, M., Detection of neutrons with LiF fluorescent nuclear track detectors, Radiation Measurements (2018), doi: 10.1016/j.radmeas.2018.06.022. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
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Detection of neutrons with LiF fluorescent nuclear track detectors P. Bilski*, B. Marczewska, M. Kłosowski, W. Gieszczyk, M. Naruszewicz
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Institute of Nuclear Physics, Polish Academy of Sciences (IFJ PAN), PL-31-342 Krakow, Poland
Measurements of neutron Hp(10) with LiF FNTDs Single tracks well visible even for 1 Gy gamma-ray background Detection of fast neutrons through recoil of Li nuclei
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Highlights:
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*Corresponding author: [email protected], phone: +48 12 662 8414
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ACCEPTED MANUSCRIPT Abstract
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Fluorescent nuclear track detectors (FNTD) based on natLiF crystals were successfully applied for measurements of neutron doses. FNTDs were exposed on a phantom to Hp(10) doses ranging from 1 mSv to 20 mSv. The number of observed tracks was found to be linear with the dose. The actual limit of detection is certainly much lower than 1 mSv and it depends on the scanned volume of a crystal. The very advantageous feature of LiF FNTDs is that the unirradiated detectors show no tracks similar to that produced by thermal neutrons, which means that background (zero-dose signal) is basically equal to zero. These results were obtained with detectors based on natural lithium and might be substantially improved if 6Li enriched lithium is used for crystal growth. It was also found that LiF FNTDs can be used for direct detection (without any converter) not only of thermal, but also of fast neutrons by using recoil of lithium nuclei.
1. Introduction
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The technique of fluorescent nuclear track detectors (FNTD) was developed by Akselrod and coworkers, basing on Al2O3:C,Mg single crystals (Akselrod et al., 2006; Akselrod and Akselrod, 2006; Akselrod and Sykora, 2011; Greilich et al., 2013). Besides detection of directly ionizing particles, it was also effectively applied for measurements of neutron doses. For this purpose special neutron/alpha or neutron/proton converters were used (Sykora et al., 2008). Recently another material apart from Al2O3 has been also successfully exploited for fluorescent detection of nuclear tracks – lithium fluoride crystals (Bilski and Marczewska, 2017; Bilski et al., 2018). The irradiated LiF emits, under blue light excitation, photoluminescence light within a broad spectrum peaked at 670 nm (Bilski et al., 2017). LiF is a particularly interesting material for detection of neutrons, as one of lithium isotopes, 6Li, possesses a very high cross-section for (n,α) reaction with thermal neutrons. The natural lithium contains about 7.5% of 6Li isotope. The possibility of direct visualization of tracks of (n,α) reaction products was already demonstrated (Bilski et al., 2018). The goal of the present work is to investigate more quantitatively and in more detail some aspects of neutron measurements with LiF FNTDs.
2. Materials and methods Lithium fluoride single crystals with typical diameter of about 2 cm were grown with the Czochralski method at the IFJ PAN Kraków, using undoped natLiF powder as the starting material. The obtained transparent crystals were then cut with diamond saws into pieces of typical size about 4x4x1 mm. The samples were next polished manually with 0.5 µm diamond abrasive strips and rinsed in acetone. The reference neutron irradiations were performed at the accredited calibration laboratory of the National Centre for Nuclear Research in Otwock with a Pu-Be source (emission rate ~107 n.s-1) (Domański et al., 2018). The exposures were conducted in terms of Hp(10), with 2
ACCEPTED MANUSCRIPT samples placed on the front surface of a water phantom or in terms of H*(10), free in air. The neutron fluence rate was 128.6 n.cm-2s-1.
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Additional neutron test exposures were carried out at the IFJ PAN, using also a Pu-Be source (emission rate 5×105 n.s-1). In order to obtain thermal neutron field, the source was placed behind a 10 cm layer of polyethylene. The thermal neutron fluence rate was estimated using 6 LiF:Mg,Ti detectors, previously calibrated in a reference thermal neutron field (Burgkhardt et al., 2006) and found to be about 4 n.cm-2s-1. Some irradiations were performed with the bare Pu-Be source. In that case neutron fluence rate was calculated basing on the emission rate and a distance from the source.
3. Results and discussion
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Microscopic observations were conducted using Nikon Ni-U wide-field fluorescent microscope with DS-Qi2 CCD camera and Lumen 200 halogen lamp. Excitation light was transmitted through a band-pass filter ET440/40X (transmission window 420-460 nm), while emission light through a long-pass filter ET570lp. An objective with magnification 100× and NA=0.80 was applied. The field of view was limited by a diaphragm and had a quasi-circular shape with the diameter of about 90 µm (area about 6900 µm2). The exposure time for each image was 30s. The images were analyzed using the Fiji software (Schindelin et al., 2012).
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Figure 1 illustrates typical shapes of tracks registered after irradiation with thermal neutrons. Figure 1a represents a single image taken at the depth of 12 µm below the crystal surface. A well visible long track has length of about 39 µm and consists of two distinguishable parts: shorter and brighter (6 µm) and longer (33 µm). These lengths agree very well with the predicted ranges in LiF of products of 6Li(n,α)3H nuclear reaction: an alpha particle and a tritium nucleus. The whole length of the discussed track was quite well visible, because incidentally it was perfectly aligned with the observation plane. In case of particles crossing the plane under an angle, we observe a sharp image of only a part of the track. This effect is illustrated in Figure 2 for a single alpha/triton track. The reason is a short focal depth of the used optical system, which may be estimated to be below 1 µm (what actually is an advantageous effect, as it greatly reduces a disturbing influence of light emitted from other layers of the crystal). The whole track may be registered, if instead of a single image, a stack of images is acquired with a small step in depth into a crystal. Such stack may be basically used for a 3D reconstruction of a track, but within this work a more simple method was used: the maximum intensity projection. The result of such approach is presented in Figure 1b, which is the maximum intensity projection of 23 images registered with the step of 1 µm from 10 µm to 32 µm. The same approach was adopted in all measurements: typically stacks of about 20 images were registered with the step of 1 µm. In this way, a volume of about 1.4×105 µm3 was analyzed per one location in a crystal. Figure 3 presents dependence of the number of the registered tracks on the thermal neutron fluence. For an unirradiated crystal essentially no tracks was observed. The number of the tracks increases proportionally to the fluence, up to the last data point for about 1.4 n.cm-2, which shows underresponse by around 10%. This is an obvious result of tracks overlapping, which may cause that some less visible tracks are unnoticeable. The linearity range might certainly be extended by developing a more optimized procedure of analysis. 3
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Fig. 1. Examples of fluorescent tracks registered after irradiation with thermal neutrons (fluence ca. 106 n.cm-2). a) – image acquired 12 µm below the crystal surface, b) – maximum intensity projections of 23 images for depths from 10 µm to 32 µm.
Fig. 2. A series of images showing a single alpha/triton track, taken with 1 µm steps from 16 µm (left upper) to 24 µm (right lower) below the crystal surface. 4
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Fig. 3. The number of registered tracks per field of view versus thermal neutron fluence. One track per field of view corresponds to about 7000 tracks per mm3. For the zero fluence no track was observed. The line represents a linear fit to the first four data points.
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In order to check performance of LiF FNTDs for applications in individual dosimetry, the detectors were exposed in the calibration laboratory to reference doses of individual dose equivalent, Hp(10). Such irradiations are performed on a water phantom, which becomes a source of thermalized albedo neutrons. Figure 4 illustrates typical images registered for doses of 1 mSv, 10 mSv and 20 mSv. The quantitative results, expressed as the number of registered tracks per field of view versus dose equivalent, are presented in Figure 5. It is apparent that the dose of 1 mSv, which produced by average somewhat less than one track per a scanned volume, may be easily measured and taking into account that for the zero dose no tracks is observed, the actual minimum measurable dose is certainly much lower. Accuracy of such measurement and hence the limit of detection, will depend on the number of registered stacks, i.e. on the scanned volume. A good statistic is needed, as relatively high variation between number of tracks observed in single stacks was found. For instance, for 10 mSv dose, the number of registered tracks was ranging from 4 to 12 tracks per field of view. The obtained results may be compared with those of the aluminum oxide FNTD automatic system, for which a capability of measuring doses in the range of 0.2-0.3 mSv was demonstrated (Akselrod et al., 2014; Yukihara et al., 2018) and lower limit of detection was estimated basing on standard deviation of unirradiated detectors to be around 0.05 mSv. This was however achieved by scanning quite large areas, exceeding 1 mm2.
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Fig. 4. Examples of images registered after irradiation with different neutron Hp(10) doses: a) 1 mSv, b) 10 mSv, c) 20 mSv.
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Fig. 5. Hp(10) dose response of LiF FNTDs: number of registered tracks per field of view versus neutron individual dose equivalent. One track per field of view corresponds to about 7000 tracks per mm3. For the zero dose no track was observed. The line represents a linear fit. The number of the registered tracks might be substantially increased by using lithium enriched with 6Li isotope for crystal growth. Natural lithium contains 7.5% of 6Li, while material with enrichment factor up to 95% is also available. Simple calculation of the neutron absorption ratio indicates that over 10 times more tracks may be expected when enriched lithium is used. This should shift the minimum measurable dose well below 0.1 mSv. Detection of thermal neutrons is obviously the most promising application of LiF FNTDs, However, it was found that this material also offers possibility to detect fast neutrons. This is illustrated in Figure 6. This image was registered for a crystal exposed to a bare Pu-Be source, neutron spectrum of which extends up to 11 MeV, with average energy of about 5 MeV. In this image, besides a few long tracks attributable to thermal neutrons (presence of 6
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which may be always expected due to scattering of neutrons), many much shorter tracks are also well visible. Their length is various: from several micrometers down to even less than 1 µm. Some of these short tracks are quite bright. It seems that the most probable interpretation of these tracks is that they are caused by recoil lithium nuclei, which are sufficiently light to acquire a significant amount of kinetic energy in a collision with a neutron. The energy of a recoil nucleus is described by the equation: 4ܣ ܧ = ݏܿ ܧଶ ߠ ሺ ܣ+ 1ሻଶ where A – mass number of an isotope, En – neutron energy and Θ – scattering angle. For 7Li the maximum recoil energy is therefore equal to: Emax( 7Li) = 0.44 En
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This means, that for example 5 MeV neutron may impart energy of 2.2 MeV to 7Li nucleus. The range in LiF crystal of a 7Li ion with such energy is 4.5 µm. For the maximum neutron energy of 11 MeV, this range may reach 8.6 µm. Obviously any shorter ranges may be also present, depending on angle Θ and neutron energy.
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Fig. 6. Examples of fluorescent tracks registered after irradiation with fast neutrons from a bare Pu-Be source (fluence ca. 109 n.cm-2). The discussed results were obtained after irradiation with relatively high neutron fluence, estimated to be ~109 n.cm-2. In order to check capability of direct, i.e. without any moderator nor converter, application of LiF FNTDs in measurement of fast neutron doses, the detectors were exposed in the calibration laboratory free in air to 20 mSv of ambient dose equivalent, H*(10), which corresponds to the fluence of 2×107 n.cm-2. In this case however, practically no tracks were observed. It seems therefore that such direct detection of fast neutrons is not suitable for dose range which is mainly of importance for radiation protection. This approach might be useful when higher neutron fluences are encountered. In practical dosimetric measurements neutrons are always accompanied by some doses of gamma radiation. Separating both components of the radiation field is often a difficult task. In thermoluminescent dosimetry it is usually accomplished by using pairs of detectors enriched in 6Li and 7Li isotopes and calculating neutron dose from the difference of their 7
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signals. This approach becomes problematic if the neutron induced signal is much lower than the gamma background. Additionally, even highly enriched 7LiF TLDs show some sensitivity to neutrons (Burgkhardt et al., 2006). On the other hand, the chemically etched track detectors are basically insensitive to gamma radiation. In order to check this aspect of the performance of LiF FNTDs, the detectors previously irradiated with neutrons, were then exposed to various doses of 137Cs gamma-rays. The results are illustrated in Figure 7, showing examples of the neutron induced tracks for different gamma background ranging from zero dose up to 1 Gy. The dose of 50 mGy has basically no visible effect on the image. For 100 mGy a brightening of the background is apparent, but tracks are still very well contrasted and the background signal should not have any deteriorating effect on the counting efficiency. For 1 Gy the image is very bright and hence less contrasted, so one may expect that some weaker tracks could be missed. This concerns mainly triton tracks, especially if only a part of a track is present in the image. However, alpha particle tracks are still very clearly visible, as well as several triton tracks.
Fig. 7. Examples of images acquired after irradiation with thermal neutrons (fluence ca. 106 n.cm-2) and additional doses of 137Cs gamma radiation.
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Conclusions
The performed investigations confirmed the promising properties of LiF FNTDs for application in neutron dosimetry. Hp(10) dose of 1 mSv was easily measured. The actual limit of detection is certainly much lower and will depend on the scanned volume of a crystal. The very advantageous feature is that the unirradiated detectors show no tracks similar to that
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produced by thermal neutrons, which means that background (zero-dose signal) is basically equal to zero. Further improvement may be achieved by growing LiF crystals from 6Li enriched lithium, which would significantly decrease the lower detection limit, as well as by optimizing image analysis procedures. An interesting finding is that LiF FNTDs can be used for direct detection (without any converter) not only of thermal, but also of fast neutrons This can be done by using recoil of lithium nuclei. However, efficiency of this process is not very high and it may be too low for application in radiation protection. A photoluminescence induced by an increased gamma radiation background does not hamper detection of neutrons. Single tracks are still well visible for 1 Gy of gamma background.
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This work was supported by the National Science Centre, Poland (Contract No. UMO2015/17/B/ST8/02180
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