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Nuclear microprobe performance in high-current proton beam mode for micro-PIXE
Nuclear Instruments and Methods in Physics Research B xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Nuclear microprobe performance in high-current proton beam mode for micro-PIXE P. Vavpeticˇ a,⇑, M. Kelemen a,b, B. Jencˇicˇ a, P. Pelicon a a b
Jozˇef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia Jozˇef Stefan International Postgraduate School, Jamova 39, SI-1000 Ljubljana, Slovenia
a r t i c l e
i n f o
Article history: Received 10 August 2016 Received in revised form 9 January 2017 Accepted 13 January 2017 Available online xxxx Keywords: PIXE Focused ion beam Elemental mapping Element quantification Lateral resolution Proton beam focusing
a b s t r a c t The performance of a nuclear microprobe is dominantly determined by the brightness of the injected ion beam. At Jozˇef Stefan Institute (JSI), negative hydrogen ion beams are created in a multicusp ion source and injected into a 2 MV tandetron accelerator. The output characteristics of the multicusp ion source were tuned in order to obtain matching proton beam intensities for the ion accelerator and for the object slits as well. For the optimal focusing of the proton beam in a high-current mode (I > 100 pA) to the submicrometer dimensions, dedicated thin nanostructures with sharp edges have been manufactured. Set of nanostructures was micromachined by focused ion beam (FIB) at film reference material, produced by Institute for Reference Materials and Measurements (IRMM) and constituted of 57 lg/cm2 of titanium on vitreous carbon substrate. The proton beam profiles were measured by beam scans across the nanostructures over long measuring times, indicating eventual slow drifts of the sample from a reference beam direction. Overall, proton beam dimensions of 600 nm were obtained, demonstrating appropriate stability for micro-PIXE (micro-Particle Induced X-ray Emission) at sub-micrometer resolution for elemental analysis of biological tissue samples prepared in a freeze-dried state or in a frozen-hydrated state. The resulting performance required for micro-PIXE analysis in a high current mode with a 3 MeV proton beam is presented. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction The brightness of the injected ion beam dominantly determines the end performance of a nuclear microprobe. The negative hydrogen ion beam is created in a multicusp ion source [1] and injected to a 2 MV tandem accelerator at Jozˇef Stefan Institute (JSI). Both the multicusp ion source and the tandetron accelerator were manufactured by High Voltage Engineering Europa B.V. [2]. The multicusp ion source was tuned to achieve the maximum possible brightness at the entrance to the accelerator and later at the object slit on the nuclear microprobe beam-line. The entire optics was then further tuned to achieve the maximum possible beam current on the target within the nuclear microprobe measuring chamber with all of the slits and magnet triplet lenses configured for achieving the best possible resolution for use with samples from biomedical field [3] that occupy about 80% of the dedicated beam-time on the nuclear microprobe at JSI [4–6]. The overall performance of the entire system needed to be evaluated since there is a significant
⇑ Corresponding author. E-mail address: [email protected] (P. Vavpeticˇ).
tendency in the biomedical field to progress the micro-PIXE (micro-Particle Induced X-ray Emission) elemental resolving capability with its absolute elemental quantification [7] towards the sub-cellular research. For one to achieve the resolution needed for elemental mapping inside the biological cells there are some conditions and demands which cannot be avoided. The beam diameter must be in the sub-micrometer range and the beam current must be in the order between 100 and 300 pA depending on the sample analyzed. To achieve these conditions the magnetic lenses for the beam diameter demagnification [8] must be aligned to the highest precision possible, the object slits and collimating slits must be configured in a way to achieve the smallest beam diameter possible and to minimize aberrations from the lenses. With such configuration of the slits one must produce the highest beam brightness possible by tuning the ion source coupled to the accelerator to meet the beam current demands at the desired beam energy. The entire system also needs to be free of any vibrations that can significantly decrease the desired resolution and stability needed for achieving the best possible results. When the proton beam is focused to the sub-micrometer dimensions it is also very important to have an accurate focusing standard structure or material to check the beam diameter and to maintain the high-
http://dx.doi.org/10.1016/j.nimb.2017.01.023 0168-583X/Ó 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: P. Vavpeticˇ et al., Nuclear microprobe performance in high-current proton beam mode for micro-PIXE, Nucl. Instr. Meth. B (2017), http://dx.doi.org/10.1016/j.nimb.2017.01.023
2
P. Vavpeticˇ et al. / Nuclear Instruments and Methods in Physics Research B xxx (2017) xxx–xxx
current mode (I > 100 pA) operation. This approach is sometimes the only option to maintain the exact position of the entire nuclear microprobe measuring system used for micro-PIXE analysis to avoid possible later relaxation in an arbitrary direction that ruins the dimensional resolution of the acquired listmode data over longer periods of acquisition time typical for biological tissue analysis.
2. Experimental methods The usual approach using standard copper grid (Gilder, G2000HS-C3) [9] to check the beam profile (Fig. 1) produces a to high X-ray yield with a fixed detector position and object slit configuration typical for biomedical applications at JSI [10,11]. The problem was observed when the PIXE detectors (HPGe – High Purity Germanium and SDD – Silicon Drift Detector) were moved to a position further away from the sample to decrease the countrate. Because in this way the detectors solid acceptance angles are decreased and thus the countrate is decreased. This disrupted the self-relaxation of the detectors to some stable equilibrium position that is crucial to maintain throughout the entire acquisition of the listmode file when performing micro-PIXE measurements if good lateral resolution is desired [3]. The HPGe detector is mounted on the microprobe target chamber (Fig. 2) with a flexible vacuum bellow and its holder is fixed to a concrete block positioned on a rubber cushion to minimize vibration from the surroundings. The detector that is also heavy because of its cooling tank for liquid nitrogen does introduce some amount of torque to the chamber that can influence the position of the chamber in reference to the rest of the microbeam setup. The SDD detector is mounted on the chamber directly through a custom design vacuum bellow that enables small alteration of detector-to-target angle [12]. By moving both of the detectors back to the measuring position (Fig. 2), the entire detection system must come to some stable equilibrium once more, therefore its exact position in reference to the beamline with object slit and the accelerator remains stable for operation. The problem is that this relaxation can take quite some time to end. Both PIXE detectors are retracted from their measuring positions whenever new samples are placed into the target chamber. The sample holder inside the target chamber can be moved via a goniometer, that is mounted ontop of the chamber itself. It has a fixed position in reference to the target chamber. Movement of the sample holder inside the chamber does not influence the stable and relaxed position of the target chamber to a measurable effect.
The standard copper grid [9] also lacks the accuracy needed for beam tuning (Fig. 1) when one gets below one micron in beam diameter. For a long time, when we were focusing the beam on the 2000 mesh copper grid, we were noticing the round edges in the K production yield elemental maps from the HPGe detector in the corners (Fig. 1) of the intersecting grid lines and contributed that to overlapping of the scan pattern of the beam. We believed that this was our best possible beam profile and thus the lateral resolution. When examining the elemental map of the L production yield of the copper grid from the SDD detector, acquired simultaneously with elemental map from HPGe detector, we observed some noticeably round structures in the grid (Fig. 1) from the perspective of the SDD detector, which led us to believe that the copper grid is not exact and precise enough, to use it as a resolution standard for the microprobe. The SDD detector is positioned at 45 degrees in both azimuthal and polar angle in reference to the target normal (Fig. 2), while HPGe detector is positioned in the polar angle of 45 degrees in reference to the target normal only and opposite to the SDD (Fig. 2). This difference between both detectors enabled us to observe the previously unnoticed round shapes of the copper grid, since we removed and replaced the Si(Li) – Silicon Lithium doped detector, that was also positioned at 55 degrees in the polar angle only [13] in reference to the target normal and also opposite to the HPGe, for a MeV-SIMS detection setup [14] shown in Fig. 2.
3. Discussion The new and more exact tool for focusing the beam and to tune the entire nuclear microprobe to best performance possible was required. A material from Institute for Reference Materials and Measurements (IRMM) with already micro-machined structures on a chip [15] was acquired. The obstacle was that this microchip is constituted of permalloy (19% iron and 81% nickel) on a silicon substrate and because of this it also yielded too high count rate on the PIXE detectors. Because of the previous discussion regarding the detector movement and then the time to wait until they relax again to some more stable position, we required a different material that would be more appropriate to use as a reference standard for the micro-beam. We required a material that would be satisfactory for both of the PIXE detectors and to produce a moderate count-rate on both detectors when exposed to a proton beam with current of 200 pA. We selected a thin reference material from IRMM [16] constituted of 57 lg/cm2 of vacuum deposited titanium on a vitreous carbon substrate. This material was then micromachined by Focused Ion Beam (FIB) and we selected some struc-
Fig. 1. Elemental maps of L and K production yield from Cu grid with 2000 mesh per inch with indicated round structures and edges that limit precision when focusing the beam. Scan size: 20 20 lm. The L production yield elemental map is acquired with SDD PIXE detector and K production yield elemental map is acquired with HPGe PIXE detector. Measuring time was 1 h.
Please cite this article in press as: P. Vavpeticˇ et al., Nuclear microprobe performance in high-current proton beam mode for micro-PIXE, Nucl. Instr. Meth. B (2017), http://dx.doi.org/10.1016/j.nimb.2017.01.023
P. Vavpeticˇ et al. / Nuclear Instruments and Methods in Physics Research B xxx (2017) xxx–xxx
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Fig. 2. Schematic view of the nuclear microprobe measuring setup from the top with all detectors and additional equipment indicated.
tures that would best suit our need to use as a focusing tool or standard. We developed some cross-like structures (Fig. 3) on the thin titanium standard and decided for thin stripes of 1, 3 and 5 lm thicknesses to use with OM-DAQ internal beam monitor utility [17]. The standard material performed as expected and enabled us to focus the beam in a high current mode without movement of any of our detectors attached to the microprobe measuring chamber. This proved to be the right decision because we are now able to focus the beam to a sub-micrometer diameter and we start to notice some other problems that we were not aware of before. We noticed a very slow drift of the sample from a reference beam direction when performing very long measuring time over night. Such long measuring times are sometimes needed, because when one needs to perform for instance an elemental mapping of cadmium inside some biological samples [18–20] and the cadmium is in trace level concentrations, just above the detection limit, this
longer measuring times are a necessity and unavoidable at selected energy of 3 MeV. 4. Results When performing the overnight run of the 1 lm cross-like structure on the thin titanium film, slow drifts from a reference beam direction were recognised and the hourly excerpts from the listmode (Fig. 4) were made, where one can observe the sample drifting to the right direction from the initial reference position for the first four hours shown. After a jump of ten hours, when the sample did not move noticeably, we observe, that it slowly drifted back toward the left direction in the meantime and then again to the right for the last four hours. One can also observe the beam resolution becoming significantly degraded to the end of the listmode run. This is because the sample also moved along the beam direc-
Fig. 3. Upper part: SEM pictures acquired with backscattered electrons from FIB structures manufactured on Ti film reference material that provide the necessary precision and that best address the demands needed when focusing the proton beam in a high-current mode. The surface film of Ti is sufficiently thin and thus enables high-current operation and tuning the entire micro-beam system to its best performance. Lower part: Titanium elemental maps of FIB structures from HPGe PIXE detector. Scan sizes are 160 160 lm for the left most map and 20 20 lm for the three remaining maps on the right. Measuring time for first, second (1 lm cross) and fourth (5 lm cross) elemental map is 1 h and for third (3 lm cross) is 20 min. In the fourth elemental map the L-shaped scan pattern for the beam profile determination (X, Y) is indicated in purple.
Please cite this article in press as: P. Vavpeticˇ et al., Nuclear microprobe performance in high-current proton beam mode for micro-PIXE, Nucl. Instr. Meth. B (2017), http://dx.doi.org/10.1016/j.nimb.2017.01.023
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Fig. 4. Consecutive hourly titanium maps i.e. excerpts from a single 18 h listmode file of a 1 lm cross-shape FIB for the first 4 h (upper row) and the last 4 h (lower row) of the entire acquisition time. Scan size is 6 6 lm. The analysis of the listmode file shows a slow drift of the sample which destroys the end resolution of the elemental map if the entire 18 h run was used.
tion, meaning the sample moved out of the focal plane of the target reference position. All of this was never observed to such extent before, because one has to select very small scan areas and must also have at hand the very exact focusing tool, to observe this kind of behaviour. When scanning the samples with a beam diameter anything above one micrometer, observing this kind of behaviour can go by unnoticed. The performance of the nuclear microprobe at JSI delivering the 3 MeV focused proton beam in a high-current mode is best shown on Fig. 5, where we show the picture of beam monitor utility from OM-DAQ. Also on Fig. 6, where one can observe the elemental map of titanium from the 3 lm cross-like FIB structure, where we have examined the stability of the highest meaningful magnification with scan size of 5 5 lm. The beam profile shown in Fig. 5 is acquired from internal OM-DAQ beam monitor utility [17] that
uses FWHM estimate of error function fitting of the L-shaped line scan over the 5 lm cross-like FIB structure (Fig. 3) in X (horizontal) and Y (vertical) direction. The beam spot with dimensions of 600 700 nm was measured with this tool after scanning the 5 lm cross-like FIB structure for 20 min. There also exists the possibility that we can observe the very small curvature indicated in the corner of the cross-like FIB structure on Fig. 6. This small curvature can also be seen in the SEM (Scanning Electron Microscope) pictures acquired with backscattered electrons from the cross-like structure in Fig. 3. This will be further examined and we will need to produce even more exact structures with FIB micromachining to address whether this is the consequence of the beam scan pattern overlapping as mentioned before in the previous discussion and possibly combined with some other unobserved vibrations or is
Fig. 5. Beam profile of a 3 MeV proton beam with dimensions of 600 700 nm in high-current mode acquired with OM-DAQ beam monitor utility on 5 lm cross-like FIB structure. The left peak represents horizontal (X) and the right peak represents vertical (Y) scan of the cross-like FIB structure also indicated in Fig. 3.
Please cite this article in press as: P. Vavpeticˇ et al., Nuclear microprobe performance in high-current proton beam mode for micro-PIXE, Nucl. Instr. Meth. B (2017), http://dx.doi.org/10.1016/j.nimb.2017.01.023
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Fig. 6. Zoom details on the 3 lm cross-like FIB structure with indicated curvature. Scan sizes are 16 16 lm, 10 10 lm and 5 5 lm. Measuring time was 1 h for each detail individually.
this the actual curved structure we can see in the SEM image (Fig. 3). We still need to address the issue of what causes the slow drifts we observed during the overnight scan. This can be because of the goniometer rod that holds the sample is slowly relaxing to some equilibrium position or this drift can possibly be contributed to some arbitrary relaxation of the microprobe setup during the overnight measurement due to minimal thermal changes of the microprobe components. Or the most possible reason that this is the consequence of the liquid nitrogen for cooling the HPGe detector, that is slowly evaporating from the cooling tank and changing the torque i.e. strain on the detector holder and that this is what we observe in the acquired elemental map (Fig. 4). 5. Conclusion For the optimal focusing of the proton beam in a high-current mode to the sub-micrometer dimensions set of detailed microstructures with good precision on the nano-scale was micromachined by focused ion beam (FIB) on film reference material, produced by IRMM and constituted of 57 lg/cm2 of titanium on vitreous carbon substrate. The proton beam profiles were measured by beam scans across the nanostructures over long measuring times, indicating eventual slow drifts of the sample in reference to a beam direction. Overall, proton beam dimensions of 700 nm or better were obtained by measuring the newly designed FIB structures on thin film titanium demonstrating appropriate stability for micro-PIXE elemental analysis at sub-micrometer resolution for future detailed examination of biological tissue samples [21– 26]. Future work is considered regarding prevention of slow drifts of the sample from the reference beam direction. References [1] P. Pelicon, N.C. Podaru, P. Vavpeticˇ, L. Jeromel, N. Ogrinc Potocˇnik S. Ondracˇka, A. Gottdang, D.J.M. Mous, A high brightness proton injector for the tandetron accelerator at Jozˇef Stefan Institute, Nucl. Instrum. Methods B (2014). [2] http://www.highvolteng.com. [3] P. Vavpeticˇ, K. Vogel-Mikuš, L. Jeromel, N. Ogrinc Potocˇnik, P. Pongrac, D. Drobne, Zˇ. Pipan Tkalec, S. Novak, M. Kos, Š. Koren, M. Regvar, P. Pelicon, Elemental distribution and sample integrity comparison of freeze-dried and frozen-hydrated biological tissue samples with nuclear microprobe, Nucl. Instrum. Methods B348 (2015) 147–151. [4] K. Vogel-Mikuš, P. Pelicon, P. Vavpeticˇ, I. Kreft, M. Regvar, Elemental analysis of edible grains by micro-PIXE: common buckwheat case study, Nucl. Instrum. Methods B 267 (2009) 2884. [5] Zˇ. Pipan Tkalec, D. Drobne, K. Vogel-Mikuš, P. Pongrac, M. Regvar, J. Štrus, P. Pelicon, P. Vavpeticˇ, N. Grlj, M. Remškar, Micro-PIXE study of Ag in digestive glands of a nano-Ag fed arthropod (Porcellio scaber, Isopoda, Crustacea), Nucl. Instrum. Methods B 269 (2011) 2286–2291. [6] A. Detterbeck, P. Pongrac, S. Rensch, S. Reuscher, M. Pecˇovnik, P. Vavpeticˇ, P. Pelicon, S. Holzheu, U. Kraemer, S. Clemens, Spatially resolved analysis of variation in barley (Hordeum vulgare L.) grain micronutrient accumulation, New Phytol. 211 (4) (2016) 1241–1254.
[7] S.A.E. Johanson, J.L. Campbell, PIXE: A Novel Technique for Elemental Analysis, Willey, Chichester, 1988. [8] J. Simcˇicˇ, P. Pelicon, M. Budnar, Zˇ. Šmit, The performance of the ljubljana ion microprobe, Nucl. Instrum. Methods B 190 (2002) 283. [9] http://www.gildergrids.co.uk. [10] B. Cestone, K. Vogel-Mikuš, M.F. Quartacci, N. Rascio, P. Pongrac, P. Pelicon, P. Vavpeticˇ, N. Grlj, L. Jeromel, P. Kump, M. Necˇemer, M. Regvar, F. Navari-Izzo, Use of micro-PIXE to determine spatial distributions of copper in Brassica carinata plants exposed to CuSO4 or CuEDDS, Sci. Total Environ. 427–428 (2012) 339–346. [11] L. Lyubenova, P. Pongrac, K. Vogel-Mikuš, G. Kukec Mezek, P. Vavpeticˇ, N. Grlj, P. Kump, M. Necˇemer, M. Regvar, P. Pelicon, P. Schroeder, Localization and quantification of Pb and nutrients in Typha latifolia by micro-PIXE, Metallomics 4 (2012) 333–341. [12] N. Grlj, P. Pelicon, M. Zˇitnik, P. Vavpeticˇ, D. Sokaras, A.G. Karydas, B. Kanngießer, Construction of a confocal PIXE set-up at the Jozˇef Stefan Institute and first results, Nucl. Instrum. Methods B 269 (2011) 2237–2243. [13] Hatan E. Gholami, M. Lamehi-Rachti, P. Vavpeticˇ, N. Grlj, P. Pelicon, Surface topography of 1 EURO coin measured by stereo-PIXE, Nucl. Instrum. Methods B 306 (2013) 90–93. [14] L. Jeromel, Z. Siketic´, N. Ogrinc Potocˇnik, P. Vavpeticˇ, Z. Rupnik, K. Bucˇar, P. Pelicon, Development of mass spectrometry by high energy focused heavy ion beam: MeV SIMS with 8 MeV Cl(7+) beam, Nucl. Instrum. Methods B 332 (2014) 22–27. [15] U. Wätjen, C. Dücsö, A. Tajani, F. Munnik, F. Lechtenberg, A novel microstructured reference material for microbeam analysis, Nucl. Instrum. Methods B 161–163 (2000) 359–365. [16] http://irmm.jrc.ec.europa.eu. [17] http://www.microbeams.co.uk. [18] P. Pongrac, K. Vogel-Mikuš, P. Vavpeticˇ, J. Tratnik, M. Regvar, J. Simcˇicˇ, N. Grlj, P. Pelicon, Cd induced redistribution of elements within leaves of the Cd/Zn hyperaccumulator Thlaspi praecox as revealed by micro-PIXE, Nucl. Instrum. Methods B 268 (2010) 2205. [19] I. Lefevre, K. Vogel-Mikuš, L. Jeromel, P. Vavpeticˇ, S. Planchom, I. Arcˇon, J.T. Elteren van, G. Lepoint, S. Gobert, J. Renaut, P. Pelicon, S. Lutts, Differential cadmium and zinc distribution in relation to their physiological impact in the leaves of the accumulating Zygophyllum fabago L., Plant, Cell Environ. 37 (2014) 1299–1320. [20] L. Lyubenova, P. Pongrac, K. Vogel-Mikuš, Mezek G. Kukec, P. Vavpeticˇ, N. Grlj, M. Regvar, P. Pelicon, P. Schröder, The fate of arsenic, cadmium and lead in Typha latifolia: a case study on the applicability of micro-PIXE in plant ionomics, J. Hazard. Mater. 248 (249) (2013) 371–378. [21] P. Pongrac, I. Kreft, K. Vogel-Mikuš, M. Regvar, M. Germ, P. Vavpeticˇ, N. Grlj, L. Jeromel, D. Eichert, B. Budicˇ, P. Pelicon, Relevance for food sciences of quantitative spatially resolved element profile investigations in wheat (Triticum aestivum) grain, J. R. Soc. Interface 10 (84) (2013) 1742. [22] P. Vavpeticˇ, P. Pelicon, K. Vogel-Mikuš, N. Grlj, P. Pongrac, L. Jeromel, N. Ogrinc, M. Regvar, Micro-PIXE on thin plant tissue samples in frozen hydrated state: a novel addition to JSI nuclear microprobe, Nucl. Instrum. Methods B 306 (2013) 140–143. [23] P. Pongrac, K. Vogel-Mikuš, L. Jeromel, P. Vavpeticˇ, P. Pelicon, B. Kaulich, A. Gianoncelli, D. Eichert, M. Regvar, I. Kreft, Spatially resolved distributions of the mineral elements in the grain of tartary buckwheat (Fagopyrum tataricum), Food Res. Int. 54 (1) (2013) 125–131. [24] S. Novak, D. Drobne, J. Valant, Zˇ. Pipan Tkalec, P. Pelicon, P. Vavpeticˇ, N. Grlj, I. Falnoga, D. Mazej, M. Remškar, Cell membrane integrity and internalization of ingested TiO2 nanoparticles by digestive gland cells of a terrestrial isopod, Environ. Toxicol. Chem. 31 (5) (2012) 1083–1090. [25] N. Ogrinc Potocˇnik, P. Pelicon, P. Vavpeticˇ, M. Kelemen, N. Grlj, L. Jeromel, S. ˇ olic´, A. Beran, Quantitative assay of element mass inventories in Tomic´, M. C single cell biological systems with micro-PIXE, Nucl. Instrum. Methods B 306 (2013) 121–124. [26] H. Bothe, K. Vogel-Mikuš, P. Pongrac, M. Likar, N. Stepic, P. Pelicon, P. Vavpeticˇ, L. Jeromel, M. Regvar, Metallophyte status of violets of the section Mellanium, Chemosphere 93 (9) (2013) 1844–1855.
Please cite this article in press as: P. Vavpeticˇ et al., Nuclear microprobe performance in high-current proton beam mode for micro-PIXE, Nucl. Instr. Meth. B (2017), http://dx.doi.org/10.1016/j.nimb.2017.01.023
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Nuclear microprobe performance in high-current proton beam mode for micro-PIXE P. Vavpeticˇ a,⇑, M. Kelemen a,b, B. Jencˇicˇ a, P. Pelicon a a b
Jozˇef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia Jozˇef Stefan International Postgraduate School, Jamova 39, SI-1000 Ljubljana, Slovenia
a r t i c l e
i n f o
Article history: Received 10 August 2016 Received in revised form 9 January 2017 Accepted 13 January 2017 Available online xxxx Keywords: PIXE Focused ion beam Elemental mapping Element quantification Lateral resolution Proton beam focusing
a b s t r a c t The performance of a nuclear microprobe is dominantly determined by the brightness of the injected ion beam. At Jozˇef Stefan Institute (JSI), negative hydrogen ion beams are created in a multicusp ion source and injected into a 2 MV tandetron accelerator. The output characteristics of the multicusp ion source were tuned in order to obtain matching proton beam intensities for the ion accelerator and for the object slits as well. For the optimal focusing of the proton beam in a high-current mode (I > 100 pA) to the submicrometer dimensions, dedicated thin nanostructures with sharp edges have been manufactured. Set of nanostructures was micromachined by focused ion beam (FIB) at film reference material, produced by Institute for Reference Materials and Measurements (IRMM) and constituted of 57 lg/cm2 of titanium on vitreous carbon substrate. The proton beam profiles were measured by beam scans across the nanostructures over long measuring times, indicating eventual slow drifts of the sample from a reference beam direction. Overall, proton beam dimensions of 600 nm were obtained, demonstrating appropriate stability for micro-PIXE (micro-Particle Induced X-ray Emission) at sub-micrometer resolution for elemental analysis of biological tissue samples prepared in a freeze-dried state or in a frozen-hydrated state. The resulting performance required for micro-PIXE analysis in a high current mode with a 3 MeV proton beam is presented. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction The brightness of the injected ion beam dominantly determines the end performance of a nuclear microprobe. The negative hydrogen ion beam is created in a multicusp ion source [1] and injected to a 2 MV tandem accelerator at Jozˇef Stefan Institute (JSI). Both the multicusp ion source and the tandetron accelerator were manufactured by High Voltage Engineering Europa B.V. [2]. The multicusp ion source was tuned to achieve the maximum possible brightness at the entrance to the accelerator and later at the object slit on the nuclear microprobe beam-line. The entire optics was then further tuned to achieve the maximum possible beam current on the target within the nuclear microprobe measuring chamber with all of the slits and magnet triplet lenses configured for achieving the best possible resolution for use with samples from biomedical field [3] that occupy about 80% of the dedicated beam-time on the nuclear microprobe at JSI [4–6]. The overall performance of the entire system needed to be evaluated since there is a significant
⇑ Corresponding author. E-mail address: [email protected] (P. Vavpeticˇ).
tendency in the biomedical field to progress the micro-PIXE (micro-Particle Induced X-ray Emission) elemental resolving capability with its absolute elemental quantification [7] towards the sub-cellular research. For one to achieve the resolution needed for elemental mapping inside the biological cells there are some conditions and demands which cannot be avoided. The beam diameter must be in the sub-micrometer range and the beam current must be in the order between 100 and 300 pA depending on the sample analyzed. To achieve these conditions the magnetic lenses for the beam diameter demagnification [8] must be aligned to the highest precision possible, the object slits and collimating slits must be configured in a way to achieve the smallest beam diameter possible and to minimize aberrations from the lenses. With such configuration of the slits one must produce the highest beam brightness possible by tuning the ion source coupled to the accelerator to meet the beam current demands at the desired beam energy. The entire system also needs to be free of any vibrations that can significantly decrease the desired resolution and stability needed for achieving the best possible results. When the proton beam is focused to the sub-micrometer dimensions it is also very important to have an accurate focusing standard structure or material to check the beam diameter and to maintain the high-
http://dx.doi.org/10.1016/j.nimb.2017.01.023 0168-583X/Ó 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: P. Vavpeticˇ et al., Nuclear microprobe performance in high-current proton beam mode for micro-PIXE, Nucl. Instr. Meth. B (2017), http://dx.doi.org/10.1016/j.nimb.2017.01.023
2
P. Vavpeticˇ et al. / Nuclear Instruments and Methods in Physics Research B xxx (2017) xxx–xxx
current mode (I > 100 pA) operation. This approach is sometimes the only option to maintain the exact position of the entire nuclear microprobe measuring system used for micro-PIXE analysis to avoid possible later relaxation in an arbitrary direction that ruins the dimensional resolution of the acquired listmode data over longer periods of acquisition time typical for biological tissue analysis.
2. Experimental methods The usual approach using standard copper grid (Gilder, G2000HS-C3) [9] to check the beam profile (Fig. 1) produces a to high X-ray yield with a fixed detector position and object slit configuration typical for biomedical applications at JSI [10,11]. The problem was observed when the PIXE detectors (HPGe – High Purity Germanium and SDD – Silicon Drift Detector) were moved to a position further away from the sample to decrease the countrate. Because in this way the detectors solid acceptance angles are decreased and thus the countrate is decreased. This disrupted the self-relaxation of the detectors to some stable equilibrium position that is crucial to maintain throughout the entire acquisition of the listmode file when performing micro-PIXE measurements if good lateral resolution is desired [3]. The HPGe detector is mounted on the microprobe target chamber (Fig. 2) with a flexible vacuum bellow and its holder is fixed to a concrete block positioned on a rubber cushion to minimize vibration from the surroundings. The detector that is also heavy because of its cooling tank for liquid nitrogen does introduce some amount of torque to the chamber that can influence the position of the chamber in reference to the rest of the microbeam setup. The SDD detector is mounted on the chamber directly through a custom design vacuum bellow that enables small alteration of detector-to-target angle [12]. By moving both of the detectors back to the measuring position (Fig. 2), the entire detection system must come to some stable equilibrium once more, therefore its exact position in reference to the beamline with object slit and the accelerator remains stable for operation. The problem is that this relaxation can take quite some time to end. Both PIXE detectors are retracted from their measuring positions whenever new samples are placed into the target chamber. The sample holder inside the target chamber can be moved via a goniometer, that is mounted ontop of the chamber itself. It has a fixed position in reference to the target chamber. Movement of the sample holder inside the chamber does not influence the stable and relaxed position of the target chamber to a measurable effect.
The standard copper grid [9] also lacks the accuracy needed for beam tuning (Fig. 1) when one gets below one micron in beam diameter. For a long time, when we were focusing the beam on the 2000 mesh copper grid, we were noticing the round edges in the K production yield elemental maps from the HPGe detector in the corners (Fig. 1) of the intersecting grid lines and contributed that to overlapping of the scan pattern of the beam. We believed that this was our best possible beam profile and thus the lateral resolution. When examining the elemental map of the L production yield of the copper grid from the SDD detector, acquired simultaneously with elemental map from HPGe detector, we observed some noticeably round structures in the grid (Fig. 1) from the perspective of the SDD detector, which led us to believe that the copper grid is not exact and precise enough, to use it as a resolution standard for the microprobe. The SDD detector is positioned at 45 degrees in both azimuthal and polar angle in reference to the target normal (Fig. 2), while HPGe detector is positioned in the polar angle of 45 degrees in reference to the target normal only and opposite to the SDD (Fig. 2). This difference between both detectors enabled us to observe the previously unnoticed round shapes of the copper grid, since we removed and replaced the Si(Li) – Silicon Lithium doped detector, that was also positioned at 55 degrees in the polar angle only [13] in reference to the target normal and also opposite to the HPGe, for a MeV-SIMS detection setup [14] shown in Fig. 2.
3. Discussion The new and more exact tool for focusing the beam and to tune the entire nuclear microprobe to best performance possible was required. A material from Institute for Reference Materials and Measurements (IRMM) with already micro-machined structures on a chip [15] was acquired. The obstacle was that this microchip is constituted of permalloy (19% iron and 81% nickel) on a silicon substrate and because of this it also yielded too high count rate on the PIXE detectors. Because of the previous discussion regarding the detector movement and then the time to wait until they relax again to some more stable position, we required a different material that would be more appropriate to use as a reference standard for the micro-beam. We required a material that would be satisfactory for both of the PIXE detectors and to produce a moderate count-rate on both detectors when exposed to a proton beam with current of 200 pA. We selected a thin reference material from IRMM [16] constituted of 57 lg/cm2 of vacuum deposited titanium on a vitreous carbon substrate. This material was then micromachined by Focused Ion Beam (FIB) and we selected some struc-
Fig. 1. Elemental maps of L and K production yield from Cu grid with 2000 mesh per inch with indicated round structures and edges that limit precision when focusing the beam. Scan size: 20 20 lm. The L production yield elemental map is acquired with SDD PIXE detector and K production yield elemental map is acquired with HPGe PIXE detector. Measuring time was 1 h.
Please cite this article in press as: P. Vavpeticˇ et al., Nuclear microprobe performance in high-current proton beam mode for micro-PIXE, Nucl. Instr. Meth. B (2017), http://dx.doi.org/10.1016/j.nimb.2017.01.023
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Fig. 2. Schematic view of the nuclear microprobe measuring setup from the top with all detectors and additional equipment indicated.
tures that would best suit our need to use as a focusing tool or standard. We developed some cross-like structures (Fig. 3) on the thin titanium standard and decided for thin stripes of 1, 3 and 5 lm thicknesses to use with OM-DAQ internal beam monitor utility [17]. The standard material performed as expected and enabled us to focus the beam in a high current mode without movement of any of our detectors attached to the microprobe measuring chamber. This proved to be the right decision because we are now able to focus the beam to a sub-micrometer diameter and we start to notice some other problems that we were not aware of before. We noticed a very slow drift of the sample from a reference beam direction when performing very long measuring time over night. Such long measuring times are sometimes needed, because when one needs to perform for instance an elemental mapping of cadmium inside some biological samples [18–20] and the cadmium is in trace level concentrations, just above the detection limit, this
longer measuring times are a necessity and unavoidable at selected energy of 3 MeV. 4. Results When performing the overnight run of the 1 lm cross-like structure on the thin titanium film, slow drifts from a reference beam direction were recognised and the hourly excerpts from the listmode (Fig. 4) were made, where one can observe the sample drifting to the right direction from the initial reference position for the first four hours shown. After a jump of ten hours, when the sample did not move noticeably, we observe, that it slowly drifted back toward the left direction in the meantime and then again to the right for the last four hours. One can also observe the beam resolution becoming significantly degraded to the end of the listmode run. This is because the sample also moved along the beam direc-
Fig. 3. Upper part: SEM pictures acquired with backscattered electrons from FIB structures manufactured on Ti film reference material that provide the necessary precision and that best address the demands needed when focusing the proton beam in a high-current mode. The surface film of Ti is sufficiently thin and thus enables high-current operation and tuning the entire micro-beam system to its best performance. Lower part: Titanium elemental maps of FIB structures from HPGe PIXE detector. Scan sizes are 160 160 lm for the left most map and 20 20 lm for the three remaining maps on the right. Measuring time for first, second (1 lm cross) and fourth (5 lm cross) elemental map is 1 h and for third (3 lm cross) is 20 min. In the fourth elemental map the L-shaped scan pattern for the beam profile determination (X, Y) is indicated in purple.
Please cite this article in press as: P. Vavpeticˇ et al., Nuclear microprobe performance in high-current proton beam mode for micro-PIXE, Nucl. Instr. Meth. B (2017), http://dx.doi.org/10.1016/j.nimb.2017.01.023
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Fig. 4. Consecutive hourly titanium maps i.e. excerpts from a single 18 h listmode file of a 1 lm cross-shape FIB for the first 4 h (upper row) and the last 4 h (lower row) of the entire acquisition time. Scan size is 6 6 lm. The analysis of the listmode file shows a slow drift of the sample which destroys the end resolution of the elemental map if the entire 18 h run was used.
tion, meaning the sample moved out of the focal plane of the target reference position. All of this was never observed to such extent before, because one has to select very small scan areas and must also have at hand the very exact focusing tool, to observe this kind of behaviour. When scanning the samples with a beam diameter anything above one micrometer, observing this kind of behaviour can go by unnoticed. The performance of the nuclear microprobe at JSI delivering the 3 MeV focused proton beam in a high-current mode is best shown on Fig. 5, where we show the picture of beam monitor utility from OM-DAQ. Also on Fig. 6, where one can observe the elemental map of titanium from the 3 lm cross-like FIB structure, where we have examined the stability of the highest meaningful magnification with scan size of 5 5 lm. The beam profile shown in Fig. 5 is acquired from internal OM-DAQ beam monitor utility [17] that
uses FWHM estimate of error function fitting of the L-shaped line scan over the 5 lm cross-like FIB structure (Fig. 3) in X (horizontal) and Y (vertical) direction. The beam spot with dimensions of 600 700 nm was measured with this tool after scanning the 5 lm cross-like FIB structure for 20 min. There also exists the possibility that we can observe the very small curvature indicated in the corner of the cross-like FIB structure on Fig. 6. This small curvature can also be seen in the SEM (Scanning Electron Microscope) pictures acquired with backscattered electrons from the cross-like structure in Fig. 3. This will be further examined and we will need to produce even more exact structures with FIB micromachining to address whether this is the consequence of the beam scan pattern overlapping as mentioned before in the previous discussion and possibly combined with some other unobserved vibrations or is
Fig. 5. Beam profile of a 3 MeV proton beam with dimensions of 600 700 nm in high-current mode acquired with OM-DAQ beam monitor utility on 5 lm cross-like FIB structure. The left peak represents horizontal (X) and the right peak represents vertical (Y) scan of the cross-like FIB structure also indicated in Fig. 3.
Please cite this article in press as: P. Vavpeticˇ et al., Nuclear microprobe performance in high-current proton beam mode for micro-PIXE, Nucl. Instr. Meth. B (2017), http://dx.doi.org/10.1016/j.nimb.2017.01.023
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Fig. 6. Zoom details on the 3 lm cross-like FIB structure with indicated curvature. Scan sizes are 16 16 lm, 10 10 lm and 5 5 lm. Measuring time was 1 h for each detail individually.
this the actual curved structure we can see in the SEM image (Fig. 3). We still need to address the issue of what causes the slow drifts we observed during the overnight scan. This can be because of the goniometer rod that holds the sample is slowly relaxing to some equilibrium position or this drift can possibly be contributed to some arbitrary relaxation of the microprobe setup during the overnight measurement due to minimal thermal changes of the microprobe components. Or the most possible reason that this is the consequence of the liquid nitrogen for cooling the HPGe detector, that is slowly evaporating from the cooling tank and changing the torque i.e. strain on the detector holder and that this is what we observe in the acquired elemental map (Fig. 4). 5. Conclusion For the optimal focusing of the proton beam in a high-current mode to the sub-micrometer dimensions set of detailed microstructures with good precision on the nano-scale was micromachined by focused ion beam (FIB) on film reference material, produced by IRMM and constituted of 57 lg/cm2 of titanium on vitreous carbon substrate. The proton beam profiles were measured by beam scans across the nanostructures over long measuring times, indicating eventual slow drifts of the sample in reference to a beam direction. Overall, proton beam dimensions of 700 nm or better were obtained by measuring the newly designed FIB structures on thin film titanium demonstrating appropriate stability for micro-PIXE elemental analysis at sub-micrometer resolution for future detailed examination of biological tissue samples [21– 26]. Future work is considered regarding prevention of slow drifts of the sample from the reference beam direction. References [1] P. Pelicon, N.C. Podaru, P. Vavpeticˇ, L. Jeromel, N. Ogrinc Potocˇnik S. Ondracˇka, A. Gottdang, D.J.M. Mous, A high brightness proton injector for the tandetron accelerator at Jozˇef Stefan Institute, Nucl. Instrum. Methods B (2014). [2] http://www.highvolteng.com. [3] P. Vavpeticˇ, K. Vogel-Mikuš, L. Jeromel, N. Ogrinc Potocˇnik, P. Pongrac, D. Drobne, Zˇ. Pipan Tkalec, S. Novak, M. Kos, Š. Koren, M. Regvar, P. Pelicon, Elemental distribution and sample integrity comparison of freeze-dried and frozen-hydrated biological tissue samples with nuclear microprobe, Nucl. Instrum. Methods B348 (2015) 147–151. [4] K. Vogel-Mikuš, P. Pelicon, P. Vavpeticˇ, I. Kreft, M. Regvar, Elemental analysis of edible grains by micro-PIXE: common buckwheat case study, Nucl. Instrum. Methods B 267 (2009) 2884. [5] Zˇ. Pipan Tkalec, D. Drobne, K. Vogel-Mikuš, P. Pongrac, M. Regvar, J. Štrus, P. Pelicon, P. Vavpeticˇ, N. Grlj, M. Remškar, Micro-PIXE study of Ag in digestive glands of a nano-Ag fed arthropod (Porcellio scaber, Isopoda, Crustacea), Nucl. Instrum. Methods B 269 (2011) 2286–2291. [6] A. Detterbeck, P. Pongrac, S. Rensch, S. Reuscher, M. Pecˇovnik, P. Vavpeticˇ, P. Pelicon, S. Holzheu, U. Kraemer, S. Clemens, Spatially resolved analysis of variation in barley (Hordeum vulgare L.) grain micronutrient accumulation, New Phytol. 211 (4) (2016) 1241–1254.
[7] S.A.E. Johanson, J.L. Campbell, PIXE: A Novel Technique for Elemental Analysis, Willey, Chichester, 1988. [8] J. Simcˇicˇ, P. Pelicon, M. Budnar, Zˇ. Šmit, The performance of the ljubljana ion microprobe, Nucl. Instrum. Methods B 190 (2002) 283. [9] http://www.gildergrids.co.uk. [10] B. Cestone, K. Vogel-Mikuš, M.F. Quartacci, N. Rascio, P. Pongrac, P. Pelicon, P. Vavpeticˇ, N. Grlj, L. Jeromel, P. Kump, M. Necˇemer, M. Regvar, F. Navari-Izzo, Use of micro-PIXE to determine spatial distributions of copper in Brassica carinata plants exposed to CuSO4 or CuEDDS, Sci. Total Environ. 427–428 (2012) 339–346. [11] L. Lyubenova, P. Pongrac, K. Vogel-Mikuš, G. Kukec Mezek, P. Vavpeticˇ, N. Grlj, P. Kump, M. Necˇemer, M. Regvar, P. Pelicon, P. Schroeder, Localization and quantification of Pb and nutrients in Typha latifolia by micro-PIXE, Metallomics 4 (2012) 333–341. [12] N. Grlj, P. Pelicon, M. Zˇitnik, P. Vavpeticˇ, D. Sokaras, A.G. Karydas, B. Kanngießer, Construction of a confocal PIXE set-up at the Jozˇef Stefan Institute and first results, Nucl. Instrum. Methods B 269 (2011) 2237–2243. [13] Hatan E. Gholami, M. Lamehi-Rachti, P. Vavpeticˇ, N. Grlj, P. Pelicon, Surface topography of 1 EURO coin measured by stereo-PIXE, Nucl. Instrum. Methods B 306 (2013) 90–93. [14] L. Jeromel, Z. Siketic´, N. Ogrinc Potocˇnik, P. Vavpeticˇ, Z. Rupnik, K. Bucˇar, P. Pelicon, Development of mass spectrometry by high energy focused heavy ion beam: MeV SIMS with 8 MeV Cl(7+) beam, Nucl. Instrum. Methods B 332 (2014) 22–27. [15] U. Wätjen, C. Dücsö, A. Tajani, F. Munnik, F. Lechtenberg, A novel microstructured reference material for microbeam analysis, Nucl. Instrum. Methods B 161–163 (2000) 359–365. [16] http://irmm.jrc.ec.europa.eu. [17] http://www.microbeams.co.uk. [18] P. Pongrac, K. Vogel-Mikuš, P. Vavpeticˇ, J. Tratnik, M. Regvar, J. Simcˇicˇ, N. Grlj, P. Pelicon, Cd induced redistribution of elements within leaves of the Cd/Zn hyperaccumulator Thlaspi praecox as revealed by micro-PIXE, Nucl. Instrum. Methods B 268 (2010) 2205. [19] I. Lefevre, K. Vogel-Mikuš, L. Jeromel, P. Vavpeticˇ, S. Planchom, I. Arcˇon, J.T. Elteren van, G. Lepoint, S. Gobert, J. Renaut, P. Pelicon, S. Lutts, Differential cadmium and zinc distribution in relation to their physiological impact in the leaves of the accumulating Zygophyllum fabago L., Plant, Cell Environ. 37 (2014) 1299–1320. [20] L. Lyubenova, P. Pongrac, K. Vogel-Mikuš, Mezek G. Kukec, P. Vavpeticˇ, N. Grlj, M. Regvar, P. Pelicon, P. Schröder, The fate of arsenic, cadmium and lead in Typha latifolia: a case study on the applicability of micro-PIXE in plant ionomics, J. Hazard. Mater. 248 (249) (2013) 371–378. [21] P. Pongrac, I. Kreft, K. Vogel-Mikuš, M. Regvar, M. Germ, P. Vavpeticˇ, N. Grlj, L. Jeromel, D. Eichert, B. Budicˇ, P. Pelicon, Relevance for food sciences of quantitative spatially resolved element profile investigations in wheat (Triticum aestivum) grain, J. R. Soc. Interface 10 (84) (2013) 1742. [22] P. Vavpeticˇ, P. Pelicon, K. Vogel-Mikuš, N. Grlj, P. Pongrac, L. Jeromel, N. Ogrinc, M. Regvar, Micro-PIXE on thin plant tissue samples in frozen hydrated state: a novel addition to JSI nuclear microprobe, Nucl. Instrum. Methods B 306 (2013) 140–143. [23] P. Pongrac, K. Vogel-Mikuš, L. Jeromel, P. Vavpeticˇ, P. Pelicon, B. Kaulich, A. Gianoncelli, D. Eichert, M. Regvar, I. Kreft, Spatially resolved distributions of the mineral elements in the grain of tartary buckwheat (Fagopyrum tataricum), Food Res. Int. 54 (1) (2013) 125–131. [24] S. Novak, D. Drobne, J. Valant, Zˇ. Pipan Tkalec, P. Pelicon, P. Vavpeticˇ, N. Grlj, I. Falnoga, D. Mazej, M. Remškar, Cell membrane integrity and internalization of ingested TiO2 nanoparticles by digestive gland cells of a terrestrial isopod, Environ. Toxicol. Chem. 31 (5) (2012) 1083–1090. [25] N. Ogrinc Potocˇnik, P. Pelicon, P. Vavpeticˇ, M. Kelemen, N. Grlj, L. Jeromel, S. ˇ olic´, A. Beran, Quantitative assay of element mass inventories in Tomic´, M. C single cell biological systems with micro-PIXE, Nucl. Instrum. Methods B 306 (2013) 121–124. [26] H. Bothe, K. Vogel-Mikuš, P. Pongrac, M. Likar, N. Stepic, P. Pelicon, P. Vavpeticˇ, L. Jeromel, M. Regvar, Metallophyte status of violets of the section Mellanium, Chemosphere 93 (9) (2013) 1844–1855.
Please cite this article in press as: P. Vavpeticˇ et al., Nuclear microprobe performance in high-current proton beam mode for micro-PIXE, Nucl. Instr. Meth. B (2017), http://dx.doi.org/10.1016/j.nimb.2017.01.023