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Nuclear Inst. and Methods in Physics Research B
journal homepage:www.elsevier.com/locate/nimb
Heavy ion PIXE cross sections in Ti, Zn, Nb, Ru and Ta for 4.8–30.0 MeV oxygen and 3.0–12.0 MeV lithium beams
Rainer Siegele
⁎, David D. Cohen, Zeljko Pastuović̀
Australian Nuclear Science and Technology Organisation, Locked Bag 2001, Kirrawee DC, NSW 2232, Australia
A R T I C L E I N F O Keywords:
X-ray production cross section Plane Wave Bohr Approximation (PWBA) ECPSSR
Heavy ion Particle Induced X-ray Emission (PIXE)
Ion beam analysis
A B S T R A C T
Heavy ion Particle Induced X-ray Emission (PIXE) spectroscopy offers a number of advantages over standard proton PIXE, such as higher yields and therefore higher sensitivity. However, in order to be able to use heavy ion PIXE more detailed measurements of the ionisation and X-ray cross sections for heavy ions are required. This issue was recognised by one of the Coordinated Research Projects (CRP) of the IAEA on MeV Secondary Ion Mass Spectrometry (SIMS). ANSTO took part in this CRP and we measured X-ray production cross sections on a range of samples for oxygen and lithium beam in the energy range of 4.8–30 MeV and 3–12 MeV, respectively. Here we report on these X-ray production cross section measurements and compare the results with theoretical models.
Further energy shifts of the characteristic X-ray lines for the different ion-target combinations are presented and discussed.
1. Introduction
Conventional ion beam techniques such as Rutherford Backscattering Spectrometry (RBS) and Particle Induced X-ray Emission spectroscopy (PIXE) using light ions [1], have greatly advanced over the years[2]. Due to improvement of the underlying theoretical models and ongoing improvements of available data analysis tools[3,4]they have also become more reliable and easier to use.
The use of heavy ions for both techniques has been proposed in many papers, assuming that the cross sections will increase by the square of the nuclear charge if the velocity of the ion is the same. This promises much higher yields using heavy ions assuming an accelerator is available to provide the heavy ion beams at the higher energy.
However, the simpleZ2dependence of the ionisation cross sections is based on the plane-wave Born approximation (PWBA)[5,6], which has been replaced by ECPSSR[7,8]theory, which goes beyond the first order Born-approximation and accounts for the energy loss (E) and Coulomb deflection (C) of the projectile and the perturbation of the targets atomic stationary states (PSS) by the projectile as well as the relativistic nature (R) of inner-shell target electron.
On the other hand experimental data on ion ionisation cross section for heavy ions are still very patchy. Therefore part of the IAEA Coordinated Research Project (CRP) F11019 on development of mole- cular concentration mapping techniques using MeV focussed ion beamswas dedicated to measuring Heavy Ion X-ray production cross sections
[9,10]and compile a data base to be used for Heavy Ion PIXE. The work reported in this paper was carried out as part of the Heavy Ions PIXE workpackage of this CRP.
2. Experimental
The experiments were performed on the 10 MV ANTARES accel- erator of the Centre for Accelerator Science (CAS) at ANSTO, using the Heavy Ion Microprobe (HIMP) [11] beamline. This beamline was chosen, because it has X-ray and charged particle detectors for si- multaneous PIXE and backscattering (BS) measurements. Furthermore, it is equipped with automated micro slits enabling us to easily control the beam current, over the wide range of samples and beam energies.
This is necessary in order to keep the count rate for both detectors below 10 kHz in order to avoid pile-up and minimise the dead time.
For all measurements the experimental conditions, such as amplifier and ADC gains as well as detector positions, were kept the same. The amplifier and ADC gains were selected carefully to cover the whole beam energy range for all the samples. This enabled us to quantify peak shifts caused by the heavy ions. All samples were also analysed using 2 and 3 MeV protons, to provide comparison spectra and to give an ac- curate energy calibration (seeFig. 1).
The microprobe is equipped with a 100 mm2high purity Ge detector located at 135° for X-ray detection. The detector has a 10 mm thick Ge crystal, a 25 μm thick Be window and was covered with a 100 μm thick
https://doi.org/10.1016/j.nimb.2018.09.017
Received 17 October 2017; Received in revised form 12 February 2018; Accepted 9 September 2018
⁎Corresponding author.
E-mail address:[email protected](R. Siegele).
Available online 26 September 2018
0168-583X/ Crown Copyright © 2018 Published by Elsevier B.V. All rights reserved.
T
Mylar foil in order to stop backscattered ions hitting the detector. The target to detector distance can be adjusted from 32 mm to 120 mm.
However in this experiment it was kept at 62.5 mm, which is equivalent to a solid angle of 25.6 msr for this detector.
A surface barrier detector with a 100 μm depletion layer was used to measure backscattered projectiles. This detector was located at 165°
relative to the incoming beam. The detector has an active area of 50 mm2and was located 50 mm from the target, resulting in an solid angle of 20 msr. Especially for the heavy ions a significant pulse height defect was observed and fully accounted for.
Experiments were carried out using oxygen and lithium beams of various energies. In the case of oxygen, cross sections were measured for the following beams; 30 and 24.6 MeVO5+, 19.2 and 15.744 MeV O4+as well as 10.8 and 8.856 MeVO3+, while for lithium cross sections were measured with 12 and 7 MeVLi3+as well as 4.5 and 3.0 MeVLi2+. The energy range of 0.5–2.0 MeV/amu was selected to cover both en- ergies comparable to proton PIXE, which is typically at energies of 2–3 MeV/amu, and also typical beam energies used in MeV SIMS.
Thin samples of TiO2, TiN, ZnO, Ta2O5, NbO and RuO2prepared on different substrates were provided by the IAEA as part of the CRP.
These samples cover the full range of elements used in PIXE from Ti to Ru, including a very heavy element (Ta) forLlines. The layer thickness varied from 50 μm for TiO2, TiN, and ZnO samples to 45 μm for the NbO sample. For the heavier elements Nb and Ru even thinner samples of 25 μm thickness were prepared. All the samples were very stable during the measurements, except for the TiN, in which a significant loss of nitrogen was observed. Nevertheless comparison of the results with the TiO measurements shows that no Ti is lost in the sample. At the lowest energies not all the samples could be measured, due to charging effects.
In order to minimise pulse pile-up the target current was varied over a wide range from 0.5–10 nA. The total measurement time and there- fore the total accumulated charge were adjusted to achieve a sufficient number of counts of about 10 k in both the Backscattering and X-ray peaks. Hence, depending on the target material and beam energy, the total accumulated charge varied from 0.3–30µC.
3. Method
In order to correct for uncertainties in the target current measure- ment a Backscattering (BS) spectrum was recorded simultaneously with the PIXE spectrum. The thin target X-ray yield is given by
=
Y EX( )P T E N Na( )x p X ( )Ex X( )EP (1) whereTais the X-ray transmission through the absorber in front of the detector,Npthe number of incoming projectiles,Nthe number of target atoms, the detector efficiency, the solid angle of the xX-ray detector and xX-ray production cross section. For BS the yield is given by:
=
YBS( )EP N Np BS BS( )EP (2)
where BS detector solid angle of the BS detector and BS X-ray pro- duction cross section. When both RBS and PIXE spectra are recorded simultaneously, the X-ray cross sections can be calculated by
Fig. 1.The figure shows the energy calibration using 2 and 3 MeV H spectra.
Fig. 2.The figure shows Zn, Ru and Ta spectra, taken with a 30 MeVO5+beam together with the spectrum fit. For comparison a 3 MeV proton spectrum is also shown.
Fig. 3.The figure shows the energy shift of theK andK lines for Ti, Zn, Nb and Ru as well as theL L, 1andL2lines for Ta, together with the / ratio, as function of the beam energy for oxygen beams. The horizontal line represents the / ratio for light ions.
= =
F E E T E Y E
Y E E ( ) ( ) ( ) ( ) E
( ) ( )
x x a x x ( )
BS x P BS P
BS P
x P (3)
eliminating uncertainties in the target current measurement (NP
=Number of projectiles) as well in the number of target atoms (N). To calculate X-ray production cross sections the detector efficiency as well as X-ray absorption in the Mylar foil covering the detector have to be determined. Both functions were calculated using GeoPIXE [12,13], since GeoPIXE is commonly used in our laboratory to analyse microp- robe data and we have been able to verify the GeoPIXE results with a number of certified standards.
4. Results and discussions
Fig. 2shows the normalised spectra for Zn, Ru and Ta obtained with a 30 MeVO5+beam together with the fit of the spectrum. For com- parison the spectrum taken with a 3 MeV proton beam is also included.
The spectra were fitted using gnuplot[14], with the energy as well as the width of the peaks allowed to vary individually, in order to allow for energy shifts caused by multiple ionisations.
Fig. 2a shows in the case of Zn a clear ( 200 eV) shift forK line, together with a significant broadening in the 30 MeV O spectrum compared to the 3 MeV H spectrum. TheK line shows a smaller shift of about 50 eV.Fig. 2a also shows an increase of theK intensity relative to theK line. This energy shift and line broadening are very similar for the two lightest elements (Ti and Zn) investigated here. The energy shift for the 2 elements as a function of the oxygen beam energy are shown in Fig. 3a. The variation in the / ratio is also included in the figure, with the / ratio scale marked on the right ordinate. The horizontal lines in the figure mark the / for hydrogen PIXE.
Fig. 2b shows the a set of spectra for Ru. For the heavier elements in this investigation (Nb and Ru), the shift of the -line is much smaller, while the broadening is clearly visible. The shift of line is almost zero.
Fig. 3b shows how the energy shift and / ratio vary with the oxygen beam energy. This graph confirms a shift of about 50 eV for theK line, while the variation of the and / ratio is within the error margin.
The difference in the energy shift between the lighter and heavier targets elements can be explained by the effect of multiple ionisations on the energy levels, which is much smaller for heavier elements. For Li beams, compared to the oxygen beam spectra, the energy shift and broadening of the lines is much smaller.
The spectra for Ta are displayed inFig. 2c. The spectra show a very small or no shift in theL andL1lines of the oxygen beam spectrum.
However, for theL2andL lines the shift is clearly visible. In this case, in contrast to theKlines, theL intensity is smaller compared to the 3 MeV H spectrum.
The energy shift as a function of beam energy together with the / ratio for Ta is shown inFig. 3c. The / ratio for protons is indicated by the horizontal line in the figure. This figure clearly shows that for Ti and Zn the K- / ratio increases with the beam energy, the L line ratio decreases with energy.
Using the spectra fitting results together with the BS spectra in- dividual X-ray cross sections were calculated using Eq.3. The BS yield was calculated by adding the counts in the surface peak of the BS spectrum and subtracting the background. The total X-ray production cross sections were calculated by adding the cross sections for the in- dividual lines. This approach was used in order to account for the change in the detector efficiency and transmission through the absorber for the individual lines.
The results of these calculations are summarised inFig. 4andTables 1 and 2. For most beam energies and targets, the statistical errors in the measurements vary between 2–10%, with the exception of low energy Li beams on Ru and Nb targets. In this case the X-ray production cross sections are so low, that sufficient statistics could not be obtained within a reasonable time. All remaining sources of uncertainty, such as
Fig. 4.Comparison of experimental X-ray production cross sections with ECPSSR and PWBA theory.Fig. 4a-d compareK-line cross sections of Li and O beams on Ti, Zn, Nb and Ru, whileFig. 4e compares the totalL-line cross sections of Ta with the theory. The figures show the total X-ray productions cross sections divided by the square of the nuclear charge of the projectile as a function of energy per amu.
the solid angle for both detector, X-ray efficiency of the detector and X- ray absorption in Mylar foil in front of the X-ray detector are estimated to be 7%, leading to a total error of 7–12%.
The figure shows the total X-ray productions cross section divided by Z2as a function of energy per mass unit, together with the uncertainties.
The figure compares the experimental results with calculations using PWBA and ECPSSR theory[8]. According to the (PWBA) at the same velocity of the ion beam, the ionisation will increase with the square of the nuclear charge of the ion, which means the PWBA curves for all ions beams will fall on the same line in a /Z2vs.E M/ graph.
Table 1
Table of the K line cross section results for TiN, TiO, Zn, Nb and Ru using O and Li, together with the line energies from the spectrum fitting.
Energy MeV Charge State Target Beam Energy keV K barn Error % Energy keV K1barn Error % Energy keV K2barn Error % totbarn Error %
30.000 5+ TiN O 4.551 27388.01 13.1 5.063 4660.00 14.5 – – – 32048.0 11.4
30.000 5+ TiO O 4.553 27562.92 12.7 5.063 4704.35 14.2 – – – 32267.3 11.1
24.600 5+ TiN O 4.574 7733.18 10.5 5.090 1293.95 12.7 – – – 9027.1 9.1
24.600 5+ TiO O 4.574 9439.95 10.8 5.090 1581.52 13.0 – – – 11021.5 9.4
19.200 4+ TiN O 4.576 2443.03 8.9 5.093 398.22 11.8 – – – 2841.3 7.8
19.200 4+ TiO O 4.576 2529.05 8.9 5.091 410.81 11.8 – – – 2939.9 7.8
15.744 4+ TiN O 4.577 1000.58 8.9 5.090 154.55 11.8 – – – 1155.1 7.9
15.744 4+ TiO O 4.579 1031.74 8.6 5.091 159.37 12.3 – – – 1191.1 7.7
10.800 3+ TiN O 4.569 172.25 8.7 5.075 22.78 12.0 – – – 195.0 7.8
10.800 3+ TiO O 4.572 170.25 8.6 5.070 23.36 12.7 – – – 193.6 7.7
8.856 3+ TiN O 4.572 81.86 8.6 5.080 9.88 15.8 – – – 91.7 7.9
8.856 3+ TiO O 4.574 78.49 8.8 5.062 8.80 19.0 – – – 87.3 8.1
30.000 5+ Zn O 8.676 801.53 9.1 9.739 135.30 9.7 – – – 936.8 7.9
24.600 5+ Zn O 8.679 359.91 8.8 9.739 59.40 9.7 – – – 419.3 7.7
19.200 4+ Zn O 8.674 119.80 8.4 9.724 18.77 9.8 – – – 138.6 7.4
15.744 4+ Zn O 8.670 54.64 8.3 9.711 7.97 10.3 – – – 62.6 7.4
10.800 3+ Zn O 8.658 12.90 8.2 9.691 1.75 11.4 – – – 14.6 7.3
8.856 3+ Zn O 8.658 6.42 8.2 9.681 0.911 12.8 – – – 7.33 7.3
30.000 5+ Nb O 16.594 39.68 9.2 18.725 6.36 10.3 19.173 1.38 16.2 47.4 7.9
24.600 5+ Nb O 16.585 19.96 8.8 18.707 3.26 10.4 19.173 0.596 19.9 23.8 7.5
19.200 4+ Nb O 16.587 9.52 8.5 18.689 1.55 10.7 19.173 0.240 23.7 11.3 7.3
15.744 4+ Nb O 16.579 4.66 8.3 18.692 0.683 12.2 19.173 0.119 34.2 5.47 7.3
10.800 3+ Nb O 16.577 1.27 8.4 18.651 0.183 18.1 19.173 0.024 67.5 1.48 7.6
8.856 3+ Nb O 16.575 0.532 8.2 18.682 0.096 13.9 – – – 0.629 7.3
30.000 5+ Ru O 19.231 24.00 8.9 21.740 4.32 10.6 22.437 0.574 47.3 28.9 7.7
24.600 5+ Ru O 19.223 12.80 8.7 21.710 2.03 11.7 22.437 0.411 57.9 15.2 7.6
19.200 4+ Ru O 19.220 5.40 8.5 21.717 0.880 13.3 22.437 0.137 86.3 6.42 7.6
15.744 4+ Ru O 19.224 2.75 8.3 21.717 0.471 13.8 22.437 0.082 77.0 3.31 7.5
10.800 3+ Ru O 19.201 0.749 8.9 21.687 0.119 22.8 22.437 0.022 191.9 0.890 9.3
8.856 3+ Ru O 19.223 0.283 8.8 21.697 0.047 25.1 22.437 0.015 159.9 0.345 10.5
12.000 3+ TiN Li 4.548 2161.80 11.5 4.988 274.23 16.4 – – – 2436.0 10.3
12.000 3+ TiO Li 4.546 1938.08 9.6 4.986 248.58 14.8 – – – 2186.7 8.7
7.000 3+ TiN Li 4.546 427.76 9.1 4.991 55.57 13.7 – – – 483.3 8.2
7.000 3+ TiO Li 4.546 387.40 9.3 4.989 50.11 13.9 – – – 437.5 8.4
4.500 2+ TiN Li 4.531 104.38 8.5 4.973 13.36 10.7 – – – 117.7 7.6
4.500 2+ TiO Li 4.531 94.03 8.7 4.966 12.47 11.8 – – – 106.5 7.8
3.000 2+ TiN Li 4.548 25.77 8.3 4.984 2.79 18.0 – – – 28.6 7.7
3.000 2+ TiO Li 4.546 25.72 8.4 4.984 2.99 18.0 – – – 28.7 7.7
12.000 3+ Zn Li 8.650 239.12 9.3 9.607 35.00 11.7 – – – 274.1 8.2
7.000 3+ Zn Li 8.645 36.10 8.5 9.599 5.41 10.8 – – – 41.5 7.5
4.500 2+ Zn Li 8.630 8.30 8.4 9.582 1.17 13.5 – – – 9.47 7.6
3.000 2+ Zn Li 8.643 2.58 8.4 9.529 0.775 14.8 – – – 3.35 7.3
12.000 3+ Nb Li 16.569 12.77 8.8 18.641 2.99 11.1 – – – 15.8 7.4
7.000 3+ Nb Li 16.562 2.51 10.3 18.654 0.276 30.1 – – – 2.79 9.7
4.500 2+ Nb Li 16.519 0.478 10.8 18.620 0.097 34.0 – – – 0.575 10.7
12.000 3+ Ru Li 19.211 9.33 9.7 21.658 1.99 17.6 – – – 11.3 8.6
7.000 3+ Ru Li 19.190 1.01 15.6 20.238 0.000 88.2 – – – 1.01 15.6
4.500 2+ Ru Li 19.142 0.311 19.8 21.686 0.032 144.7 22.437 0.019 329.4 0.362 27.5
Table 2
Table of the L line cross section results for Ta using O and Li, together with the line energies from the spectrum fitting.
Energy Beam Energy Ll Error Energy L Error Energy L1 Error L2 Error Energy L Error tot Error
MeV keV barn % keV barn % keV barn % barn % keV barn % barn %
30.000 O5+ 7.212 80.82 12.5 8.169 1648.31 10.5 9.373 583.39 15.5 288.45 15.6 10.987 71.51 12.6 2672.5 7.5 24.600 O5+ 7.212 45.83 11.1 8.167 960.95 8.7 9.371 352.13 14.4 165.50 14.5 10.981 45.22 10.8 1569.6 6.4 19.200 O4+ 7.200 24.74 11.1 8.164 522.42 8.5 9.366 199.55 14.3 88.85 14.4 10.971 25.30 10.6 860.9 6.3 15.744 O5+ 7.205 13.13 12.0 8.162 282.49 8.4 9.365 116.36 14.2 46.92 14.4 10.968 14.86 10.8 473.7 6.3 10.800 O4+ 7.197 4.06 13.6 8.159 103.85 8.2 9.360 46.52 14.1 16.43 14.3 10.953 6.36 10.3 177.2 6.2
8.856 O3+ 7.210 2.70 12.5 8.159 55.60 8.1 9.363 26.78 14.1 9.25 14.3 10.950 3.82 10.1 98.2 6.2
4.800 O2+ 7.203 0.191 26.4 8.152 4.91 8.1 9.355 3.81 14.1 0.705 15.4 10.925 0.574 11.1 10.2 6.7 12.000 Li3+ 7.187 14.75 11.9 8.152 266.96 8.5 9.355 101.12 14.3 47.19 14.5 10.918 14.36 11.5 444.4 6.3 7.000 Li2+ 7.175 3.51 13.2 8.146 70.71 8.4 9.347 25.67 14.3 12.76 14.4 10.918 3.93 11.7 116.6 6.2 4.500 Li2+ 7.167 0.973 15.9 8.126 24.03 8.2 9.325 8.85 14.2 4.32 14.4 10.902 1.53 11.3 39.7 6.1 3.000 Li2+ 7.167 0.275 17.7 8.152 7.48 8.1 9.352 3.37 14.1 1.53 14.3 10.905 0.526 10.9 13.2 6.1
However, results obtained using ECPSSR theory, show different dependencies for oxygen and lithium ions as indicated in the graph by the dotted lines. The ECPSSR results were calculated using the approach by Cohen and Harrington[8]using fluorescent yield data from Krause [15]. The subfigures (a-e) show the results for the different target ma- terials.Fig. 4a shows the results for both the TiN and TiO2targets, while the figures b-e, show the results for the ZnO, NbO, RuO2and Ta2O5
targets.
The figures show that for theKlines the experimental data agrees closer with the ECPSSR theory than the PWBA calculation. This is especially the case for oxygen beams, while Li measurements are almost in the middle between PWBA and ESPSSR theory. One possible ex- planation is that multiple ionisations are more likely for heavier ions.
Surprisingly for theLlines of Ta the experimental data are closer to PWBA than ECPSSR theory.
5. Conclusions
We measured and tabulated the X-ray cross sections for Ti, Zn, Nb, Ru and Ta for both oxygen and Li beams in the range from 0.5–1.9 MeV/amu. For both Li and O beams agreement between the measurements and ECPSSR theory is better compared to PWBA. This is not surprising, since PWBA theory does not take into account the effects of multiple ionisations, which are very likely for heavier ion beams.
Acknowledgements
The authors would like to acknowledge National Collaborative
Research Infrastructure Strategies (NCRIS) for funding of the Centre for Accelerator Science (CAS) and to CAS staff for access to their ion beam analysis facilities. One of us would also like to acknowledge that this work was made possible by the IAEA CRP F11019 not least for pro- viding the framework for this work together with the samples used.
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