- Email: [email protected]
The use of molecular beams in nuclear microprobe imaging
Nuclear Instruments and Methods in Physics Research B 158 (1999) 194±200
www.elsevier.nl/locate/nimb
The use of molecular beams in nuclear microprobe imaging V.M. Prozesky a
a,*
, J. Padayachee a, A. Saint
b,1
, K. Springhorn
a
Van De Graa Group, National Accelerator Centre, P.O. Box 72, 7131 Faure, South Africa b Micro-Analytical Research Centre, Melbourne University, Melbourne, Australia
Abstract Molecular hydrogen ions were ®rst used in nuclear microprobe STIM imaging to enhance contrast. These beams have also been used for PIXE and other techniques of microprobe imaging, although the high-energy electrons associated with the molecules caused problems with charge integration. This paper discusses the use of asymmetric molecular ion beams, to utilise the properties of molecular break-up for enhanced materials analysis. In particular the use of the deuterium-hydrogen (DH ) molecule for STIM imaging to provide increased image contrast and improved density measurement for specimens of widely varying areal densities is discussed. Ó 1999 Published by Elsevier Science B.V. All rights reserved. Keywords: Molecular beams; Nuclear microprobe; STIM; Imaging
1. Introduction The use of molecular beams in STIM imaging is interesting not only in increasing the image contrast, but also as a possible way to look at energy loss and scattering simultaneously to try to learn more about the specimen, as well as possible analytical advantages that can be gained with its use. This study describes the properties of the asymmetric molecule DH and its usefulness in STIM imaging. At the National Accelerator Centre the use of molecular hydrogen ions (H 2 ) [1] was studied to
* Corresponding author. Tel.: +27-21-843-3820; fax: +27-21843-3543; e-mail: [email protected] 1 Present address: GBC Scienti®c, Melbourne, Australia. E-mail: ¯[email protected]
utilise the higher brightness of the ion source for these molecular ions, and therefore the possible higher current for a speci®c beam size in the nuclear microprobe (NMP). These ions were accelerated to 5 MeV (2.5 MeV per proton) and PIXE was used as the analytical technique. The use of these ions turned out to have good application in imaging, but the analytical use was limited due to the fact that current integration was problematic. Similar beams were also used by Lefevre for STIM measurements [2] as well as the use of mixed beams to utilise the advantage of each beam species separately [3]. Saint [4,5] also used mixed beams (H 2 and H 3 ) to study aspects of STIM, IBIC and micro-lithography using magnetic lenses. The DH molecule consists of a deuteron and a proton that are loosely bound by a single electron. The binding energy of the deuteron is 2.23 MeV, which is many orders of magnitude greater than
0168-583X/99/$ - see front matter Ó 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 5 2 2 - 4
V.M. Prozesky et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 194±200
the molecular binding energy. This means that the molecule will almost certainly dissociate in a thin STIM specimen but that the deuteron will have a large probability of remaining intact. The fractional number of deuterons that stay bound through the specimen will depend on the nuclear reaction cross-section for deuterons on the specimen material. Thus if a DH molecule of energy of a few MeV impinges on a thin specimen the binding electron is very easily stripped, leaving a free deuteron and a free proton. Due to the asymmetric mass distribution the deuteron will have an energy of 2/3Ei and the proton will have an energy of 1/3Ei , where Ei is the initial energy of the molecule. The deuteron, due to its greater mass, will thus have twice the energy of its partner proton. The stopping properties of the deuteron are determined by the mass and the nuclear charge, and with the same atomic number as its partner proton the stopping powers of these two particles are intimately linked. This means that upon entering the sample these two particles have the same velocity, and therefore the same stopping power, but at break-up the deuteron has twice the energy of the proton. The result is the same absolute loss of energy by both the proton and the deuteron. With incident energies of the molecule of a few MeV the stopping powers for both particles are far beyond the Bragg peak, and therefore the deuteron will have a slower decrease in velocity compared to the proton, and it turns out that the deuteron has about twice the range of the proton. This allows the imaging of relatively thick target areas using the deuteron signals. Referring to the theoretical STIM specimen shown in Fig. 1, this specimen is of uniform density and has two regions of greatly dierent thickness. If a micro-beam of 3 MeV energy DH ions is scanned across the specimen in the thick region of the specimen, the lower energy protons do not penetrate the thick regions to produce an STIM signal. In the thin region, the deuterons penetrate but they only lose a small amount of energy and thus give poor STIM contrast. The protons, on the other hand, have half the energy of the deuterons and thus give much better STIM contrast in this thin region, even though it is not
195
Fig. 1. Theoretical STIM specimen that shows the interactions in areas of dierent thickness. This specimen is of uniform density and has two regions of greatly dierent thickness. If a micro-beam of MeV energy DH ions is scanned across the thick region of the specimen, the lower energy protons do not penetrate to produce a STIM signal. In the thin region, the deuterons penetrate but they only lose a small fraction of their energy, producing poor contrast. In the thick region the deuterons are the only ions that can be measured and produce contrast.
comparable with the measurement of the DH molecule. The proton, deuteron and DH signals are separated in the spectrum by their energy difference. These dierent areas of application can be explained through the functions of energy deposited by the protons, deuterons and DH in a sample. This is shown in Fig. 2 where the energy deposited by these species are shown as a function of depth within Si. The incident energy was selected to be 3 MeV. Although the stopping of the deuterons and protons are equal at the surface, the energy lost by the proton increases much faster than that of the deuterons, and therefore the protons are stopped at around 8 ´ 1019 at/cm2 , with the deuterons being stopped only after traversing about twice that depth into the sample. Although the DH bond is broken as the molecule enters the surface, the two nuclei can travel essentially along the same path and be measured simultaneously in a transmission measurement. In such a measurement the
196
V.M. Prozesky et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 194±200
Fig. 2. The energy deposited by protons, deuterons and DH as a function of depth within Si with an incident energy of 3 MeV.
combined energy lost by the two particles will be measured, this energy loss is given in the ®gure. The use of molecular ions is of course an eective way to increase the eective stopping of ions, and this is clear when the energy lost by the DH combination is compared with the two nuclei. There are two distinct depth regions, namely from the surface to where the protons are stopped, and the second region to where the deuterons are stopped. In the shallow region, the combined stopping of the proton and the deuteron, in the case of the DH molecule, is always higher than that of the proton or the deuteron alone, indicating that the contrast obtained in STIM measurements should always be highest with DH if the areas probed have thicknesses in this range. From beam±foil spectroscopy studies one can deduce that atomic ion equilibrium charge state distributions are established in the ®rst 10 nm or so, which implies DH dissociation on that distance scale or less. There is, however, an increasing probability with increasing depth that one of the two nuclei might undergo scattering at a large enough angle not to be measured by the detector, and that for increasing depth, the signal for DH can have a lower yield due to this eect. Although not obvious, this may lead to even higher contrast in the measurement of DH , as the setting of gates around the species measured also provides an indication of the continuation of the two individual atoms on a trajectory that will lead to detection in the STIM detector. For this reason, the contrast
between very thin sample regions and that of intermediate thickness (where the proton can still be measured) will be enhanced in the case of the DH signal. In the second region, only the deuterons can penetrate through the sample, and for this reason in relatively thick samples, the signals from the deuterons will be the only STIM signal measured, and therefore contribute toward contrast. It is easy to predict what the STIM spectrum from such an analysis would look like. First all the possible scattering combinations that can occur need to be considered. These possibilities are illustrated in Fig. 3. It is assumed that the original molecular beam energy is 3 MeV. The ®rst scattering mode (no. 1 in Fig. 3) is the most probable in a thin target. In this mode the proton and the deuteron undergo only electronic energy loss and small angle scattering and both impinge on the detector simultaneously to give a peak at an energy E 3 MeV ÿ dEH ÿ dED , where dEH is the total energy lost in the specimen by the 1 MeV H ion and dED is the total energy lost by the 2 MeV D ion. This is schematically indicated in Fig. 4 which shows a theoretical spectrum from such an exper-
Fig. 3. The possible scattering modes for DH using 3 MeV.
V.M. Prozesky et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 194±200
iment. The second scattering mode is when the deuteron undergoes large angle scattering and is not detected. The proton undergoes electronic energy loss only and is detected to give a peak at E 1 MeV ÿ dEH . The third mode shows the scattering of the proton through a large angle and only the deuteron undergoing electronic energy loss producing a peak at E 2 MeV ÿ dED . The fourth mode indicates that the deuteron has undergone a nuclear reaction in the specimen and has produced a proton (or a-particle). In this case the energy of this particle depends on the nuclear reaction that took place. In most low-Z materials deuteron induced nuclear reactions have a large positive Q-value and will thus produce particles that have a very high energy compared to the molecular beam energy and thus can be easily identi®ed in the STIM spectrum. 2. Experimental and results The Van de Graa accelerator linked to the NMP of the National Accelerator Centre was used to produce DH beams with an energy of 3 MeV, and these ions were focussed onto thin targets using the Oxford triplet set of lenses [6]. The NMP setup has been described earlier by Prozesky et al. [7]. For the STIM measurements a surface barrier detector was installed at 0° with respect to the beam direction, using a device where a Faraday cup and the STIM detector can be interchanged by way of rotation.
Fig. 4. A theoretical energy spectrum from an STIM experiment using 3 MeV DH .
197
A synthetic target was prepared by sticking a standard copper grid, such as that used for focussing the beam of the NMP, onto a thin formvar foil. This target was prepared to represent a sample with a thin and thick region as test for the DH STIM measurements. The results are shown in Fig. 5, with the three frames representing the integrated signals of (a) protons, (b) deuterons and (c) DH molecules. The image was constructed by simply putting gates around the signals from the three species, which were well separated in the spectrum. The energy spectrum obtained in this measurement is shown in Fig. 6, with the contribution from each species indicated. The gates set around the peaks are indicated in the ®gure. The energy spread for each species indicates the energy loss distribution as obtained from this target. It is clear from Fig. 5 that the dierent species re¯ect more contrast in dierent areas of the sample. The DH signals show good contrast between areas of only formvar (in the holes of the grid) and the grid itself. On the other hand, the proton signals show good contrast between the thicker and thinner parts of the grid, these thickness variations on the grid are due to the manufacturing method for the grid. It can therefore be deduced that the grid itself is too thick for the DH molecules to be measured simultaneously, but thin enough for the protons to penetrate and yields good contrast for the dierent thicknesses. Although the deuterons easily penetrate through the whole sample, the sample is too thin to allow the deuterons to yield better contrast, i.e., areas where the deuterons are the only species to give a signal. The contrast within the proton window shows contributions from two eects. Firstly there is signi®cant contrast between the thin formvar regions (holes of the grid) and the grid bars, with higher intensity from the grid than from the formvar regions. This is due to the fact that the DH molecules have a high probability of being measured together, as is evident in the DH image, and few signals are measured as protons only. On the grid bars there is signi®cant contrast between the outer rim (intermediate thickness regions) and the centres of the grid bars (thick regions). In this case there are few DH signals, which indicate that
198
V.M. Prozesky et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 194±200
Fig. 5. The results of STIM using a copper grid on thin formvar foil as sample. The three frames represent the integrated signals of (a) protons, (b) deuterons and (c) DH molecules. The image was constructed by simply putting gates around the signals from the three species, as indicated. Lighter shades indicate high intensities.
Fig. 6. The total energy spectrum obtained in the measurement on the copper grid, with the contribution from each species indicated. The energy spread for each species indicates the energy loss distribution as obtained from this target. The gates used to generate the images in Fig. 5 are indicated.
this thickness domain is dominated by scattering eects. Therefore, in the centre of the grid bars fewer proton signals are measured, as more
protons are scattered through angles large enough not to be detected by the detector due to the larger thickness of these regions. A measurement was subsequently performed on a biological sample consisting of a layer of single cells prepared by cryo-techniques and adhering to a thin formvar supporting foil. The results are shown in Fig. 7, again for each species separately displayed: (a) protons, (b) deuterons and (c) DH . The same areas of interest are found in this sample, i.e., the protons have an area of good contrast in the top middle section of the image whereas the thickness here is such that the deuterons give virtually no contrast. This area is too thick for both the deuterons and protons to be measured simultaneously with high enough yield to have good contrast. The bottom section of the image is the thinnest area of the sample, and this is re¯ected in the good contrast shown by the DH results, with poor contrast in the results obtained for the single ions. In this area the single cells are clearly distinguished with areas of diering thickness around these cells.
V.M. Prozesky et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 194±200
199
Fig. 7. Results from a measurement on a biological sample consisting of a layer of single cells prepared by cryo-techniques and adhering to a thin formvar supporting foil. The results are shown for each species separately displayed: (a) protons, (b) deuterons and (c) DH .
3. Other applications of molecular ions The use of molecular ions oers other niche applications, such as the use of HeH ions to perform imaging. For PIXE, there is some advantages to be gained if used at high energy (>5 MeV) to bene®t from the good sensitivities of the He ion for low-Z elements, in contrast to the good sensitivity of protons for high-Z elements. The DH molecular ion can also be useful in the case of performing RBS on targets consisting of both low-Z and high-Z elements, such as that of XN layers on steel, with X representing any nitride forming metal. In this case the protons will yield an increased number of counts due to the enhanced cross-section for scattering from nitrogen, while the D will give good mass resolution for the metals. A simulated spectrum of the use of DH ions on a sample of TiCuN on an Fe substrate is shown in Fig. 8. It is clear from this ®gure that the contributions from the two atomic ions give areas of good information for the dierent elements in the sample,
therefore yielding simultaneous information of analytical importance. This feature can also be used in imaging applications of similar targets. 4. Conclusions The use of molecular ions can yield superior information in applications such as STIM and to a limited extent in analytical applications. These ions are easy to generate in most of the available ion sources attached to nuclear microbeams and the currents that can be generated are more than enough for both analytical applications and STIM measurements. These ions can normally also be focussed using normal NMP lens systems in either the high or low excitation modes. Moreover, the recent advances [8] in distance operation of NMPs have enabled the operation of these facilities remotely from the facility itself, in this case radiation becomes less problematic, especially in the case of using DH ions.
200
V.M. Prozesky et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 194±200
Fig. 8. A simulated spectrum of the use of DH ions on a sample of TiCuN on an Fe substrate. The contributions from the two atomic ions give areas of good information for the dierent elements in the sample, therefore yielding simultaneous information of analytical importance.
References [1] U.A.S. Tapper et al., Nucl. Instr. and Meth. B 77 (1993) 17. [2] H.W. Lefevre, R.M.S. Scho®eld, J.C. Overley, J.D. McDonald, Scanning Microscopy 1 (3) (1987) 879. [3] H.W. Lefevre, R.C. Connelly, G. Sieger, J.C. Overley, Nucl. Instr. and Meth. 218 (1983) 39. [4] A. Saint, M.B.H. Breese, G.J.F. Legge, Rev. Sci. Instr. 67 (1996) 2940.
[5] A. Saint, G.J.F. Legge, Nucl. Instr. and Meth. B 130 (1997) 176. [6] Oxford Microbeams, Quadrupole Microbeam System, Lens system LM150. [7] V.M. Prozesky et al., Nucl. Instr. and Meth. B 104 (1995) 36. [8] C.L. Churms, V.M. Prozesky, K.A. Springhorn, these proceedings (ICNMTA-6), Nucl. Instr. and Meth. B 158 (1999) 124.
www.elsevier.nl/locate/nimb
The use of molecular beams in nuclear microprobe imaging V.M. Prozesky a
a,*
, J. Padayachee a, A. Saint
b,1
, K. Springhorn
a
Van De Graa Group, National Accelerator Centre, P.O. Box 72, 7131 Faure, South Africa b Micro-Analytical Research Centre, Melbourne University, Melbourne, Australia
Abstract Molecular hydrogen ions were ®rst used in nuclear microprobe STIM imaging to enhance contrast. These beams have also been used for PIXE and other techniques of microprobe imaging, although the high-energy electrons associated with the molecules caused problems with charge integration. This paper discusses the use of asymmetric molecular ion beams, to utilise the properties of molecular break-up for enhanced materials analysis. In particular the use of the deuterium-hydrogen (DH ) molecule for STIM imaging to provide increased image contrast and improved density measurement for specimens of widely varying areal densities is discussed. Ó 1999 Published by Elsevier Science B.V. All rights reserved. Keywords: Molecular beams; Nuclear microprobe; STIM; Imaging
1. Introduction The use of molecular beams in STIM imaging is interesting not only in increasing the image contrast, but also as a possible way to look at energy loss and scattering simultaneously to try to learn more about the specimen, as well as possible analytical advantages that can be gained with its use. This study describes the properties of the asymmetric molecule DH and its usefulness in STIM imaging. At the National Accelerator Centre the use of molecular hydrogen ions (H 2 ) [1] was studied to
* Corresponding author. Tel.: +27-21-843-3820; fax: +27-21843-3543; e-mail: [email protected] 1 Present address: GBC Scienti®c, Melbourne, Australia. E-mail: ¯[email protected]
utilise the higher brightness of the ion source for these molecular ions, and therefore the possible higher current for a speci®c beam size in the nuclear microprobe (NMP). These ions were accelerated to 5 MeV (2.5 MeV per proton) and PIXE was used as the analytical technique. The use of these ions turned out to have good application in imaging, but the analytical use was limited due to the fact that current integration was problematic. Similar beams were also used by Lefevre for STIM measurements [2] as well as the use of mixed beams to utilise the advantage of each beam species separately [3]. Saint [4,5] also used mixed beams (H 2 and H 3 ) to study aspects of STIM, IBIC and micro-lithography using magnetic lenses. The DH molecule consists of a deuteron and a proton that are loosely bound by a single electron. The binding energy of the deuteron is 2.23 MeV, which is many orders of magnitude greater than
0168-583X/99/$ - see front matter Ó 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 5 2 2 - 4
V.M. Prozesky et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 194±200
the molecular binding energy. This means that the molecule will almost certainly dissociate in a thin STIM specimen but that the deuteron will have a large probability of remaining intact. The fractional number of deuterons that stay bound through the specimen will depend on the nuclear reaction cross-section for deuterons on the specimen material. Thus if a DH molecule of energy of a few MeV impinges on a thin specimen the binding electron is very easily stripped, leaving a free deuteron and a free proton. Due to the asymmetric mass distribution the deuteron will have an energy of 2/3Ei and the proton will have an energy of 1/3Ei , where Ei is the initial energy of the molecule. The deuteron, due to its greater mass, will thus have twice the energy of its partner proton. The stopping properties of the deuteron are determined by the mass and the nuclear charge, and with the same atomic number as its partner proton the stopping powers of these two particles are intimately linked. This means that upon entering the sample these two particles have the same velocity, and therefore the same stopping power, but at break-up the deuteron has twice the energy of the proton. The result is the same absolute loss of energy by both the proton and the deuteron. With incident energies of the molecule of a few MeV the stopping powers for both particles are far beyond the Bragg peak, and therefore the deuteron will have a slower decrease in velocity compared to the proton, and it turns out that the deuteron has about twice the range of the proton. This allows the imaging of relatively thick target areas using the deuteron signals. Referring to the theoretical STIM specimen shown in Fig. 1, this specimen is of uniform density and has two regions of greatly dierent thickness. If a micro-beam of 3 MeV energy DH ions is scanned across the specimen in the thick region of the specimen, the lower energy protons do not penetrate the thick regions to produce an STIM signal. In the thin region, the deuterons penetrate but they only lose a small amount of energy and thus give poor STIM contrast. The protons, on the other hand, have half the energy of the deuterons and thus give much better STIM contrast in this thin region, even though it is not
195
Fig. 1. Theoretical STIM specimen that shows the interactions in areas of dierent thickness. This specimen is of uniform density and has two regions of greatly dierent thickness. If a micro-beam of MeV energy DH ions is scanned across the thick region of the specimen, the lower energy protons do not penetrate to produce a STIM signal. In the thin region, the deuterons penetrate but they only lose a small fraction of their energy, producing poor contrast. In the thick region the deuterons are the only ions that can be measured and produce contrast.
comparable with the measurement of the DH molecule. The proton, deuteron and DH signals are separated in the spectrum by their energy difference. These dierent areas of application can be explained through the functions of energy deposited by the protons, deuterons and DH in a sample. This is shown in Fig. 2 where the energy deposited by these species are shown as a function of depth within Si. The incident energy was selected to be 3 MeV. Although the stopping of the deuterons and protons are equal at the surface, the energy lost by the proton increases much faster than that of the deuterons, and therefore the protons are stopped at around 8 ´ 1019 at/cm2 , with the deuterons being stopped only after traversing about twice that depth into the sample. Although the DH bond is broken as the molecule enters the surface, the two nuclei can travel essentially along the same path and be measured simultaneously in a transmission measurement. In such a measurement the
196
V.M. Prozesky et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 194±200
Fig. 2. The energy deposited by protons, deuterons and DH as a function of depth within Si with an incident energy of 3 MeV.
combined energy lost by the two particles will be measured, this energy loss is given in the ®gure. The use of molecular ions is of course an eective way to increase the eective stopping of ions, and this is clear when the energy lost by the DH combination is compared with the two nuclei. There are two distinct depth regions, namely from the surface to where the protons are stopped, and the second region to where the deuterons are stopped. In the shallow region, the combined stopping of the proton and the deuteron, in the case of the DH molecule, is always higher than that of the proton or the deuteron alone, indicating that the contrast obtained in STIM measurements should always be highest with DH if the areas probed have thicknesses in this range. From beam±foil spectroscopy studies one can deduce that atomic ion equilibrium charge state distributions are established in the ®rst 10 nm or so, which implies DH dissociation on that distance scale or less. There is, however, an increasing probability with increasing depth that one of the two nuclei might undergo scattering at a large enough angle not to be measured by the detector, and that for increasing depth, the signal for DH can have a lower yield due to this eect. Although not obvious, this may lead to even higher contrast in the measurement of DH , as the setting of gates around the species measured also provides an indication of the continuation of the two individual atoms on a trajectory that will lead to detection in the STIM detector. For this reason, the contrast
between very thin sample regions and that of intermediate thickness (where the proton can still be measured) will be enhanced in the case of the DH signal. In the second region, only the deuterons can penetrate through the sample, and for this reason in relatively thick samples, the signals from the deuterons will be the only STIM signal measured, and therefore contribute toward contrast. It is easy to predict what the STIM spectrum from such an analysis would look like. First all the possible scattering combinations that can occur need to be considered. These possibilities are illustrated in Fig. 3. It is assumed that the original molecular beam energy is 3 MeV. The ®rst scattering mode (no. 1 in Fig. 3) is the most probable in a thin target. In this mode the proton and the deuteron undergo only electronic energy loss and small angle scattering and both impinge on the detector simultaneously to give a peak at an energy E 3 MeV ÿ dEH ÿ dED , where dEH is the total energy lost in the specimen by the 1 MeV H ion and dED is the total energy lost by the 2 MeV D ion. This is schematically indicated in Fig. 4 which shows a theoretical spectrum from such an exper-
Fig. 3. The possible scattering modes for DH using 3 MeV.
V.M. Prozesky et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 194±200
iment. The second scattering mode is when the deuteron undergoes large angle scattering and is not detected. The proton undergoes electronic energy loss only and is detected to give a peak at E 1 MeV ÿ dEH . The third mode shows the scattering of the proton through a large angle and only the deuteron undergoing electronic energy loss producing a peak at E 2 MeV ÿ dED . The fourth mode indicates that the deuteron has undergone a nuclear reaction in the specimen and has produced a proton (or a-particle). In this case the energy of this particle depends on the nuclear reaction that took place. In most low-Z materials deuteron induced nuclear reactions have a large positive Q-value and will thus produce particles that have a very high energy compared to the molecular beam energy and thus can be easily identi®ed in the STIM spectrum. 2. Experimental and results The Van de Graa accelerator linked to the NMP of the National Accelerator Centre was used to produce DH beams with an energy of 3 MeV, and these ions were focussed onto thin targets using the Oxford triplet set of lenses [6]. The NMP setup has been described earlier by Prozesky et al. [7]. For the STIM measurements a surface barrier detector was installed at 0° with respect to the beam direction, using a device where a Faraday cup and the STIM detector can be interchanged by way of rotation.
Fig. 4. A theoretical energy spectrum from an STIM experiment using 3 MeV DH .
197
A synthetic target was prepared by sticking a standard copper grid, such as that used for focussing the beam of the NMP, onto a thin formvar foil. This target was prepared to represent a sample with a thin and thick region as test for the DH STIM measurements. The results are shown in Fig. 5, with the three frames representing the integrated signals of (a) protons, (b) deuterons and (c) DH molecules. The image was constructed by simply putting gates around the signals from the three species, which were well separated in the spectrum. The energy spectrum obtained in this measurement is shown in Fig. 6, with the contribution from each species indicated. The gates set around the peaks are indicated in the ®gure. The energy spread for each species indicates the energy loss distribution as obtained from this target. It is clear from Fig. 5 that the dierent species re¯ect more contrast in dierent areas of the sample. The DH signals show good contrast between areas of only formvar (in the holes of the grid) and the grid itself. On the other hand, the proton signals show good contrast between the thicker and thinner parts of the grid, these thickness variations on the grid are due to the manufacturing method for the grid. It can therefore be deduced that the grid itself is too thick for the DH molecules to be measured simultaneously, but thin enough for the protons to penetrate and yields good contrast for the dierent thicknesses. Although the deuterons easily penetrate through the whole sample, the sample is too thin to allow the deuterons to yield better contrast, i.e., areas where the deuterons are the only species to give a signal. The contrast within the proton window shows contributions from two eects. Firstly there is signi®cant contrast between the thin formvar regions (holes of the grid) and the grid bars, with higher intensity from the grid than from the formvar regions. This is due to the fact that the DH molecules have a high probability of being measured together, as is evident in the DH image, and few signals are measured as protons only. On the grid bars there is signi®cant contrast between the outer rim (intermediate thickness regions) and the centres of the grid bars (thick regions). In this case there are few DH signals, which indicate that
198
V.M. Prozesky et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 194±200
Fig. 5. The results of STIM using a copper grid on thin formvar foil as sample. The three frames represent the integrated signals of (a) protons, (b) deuterons and (c) DH molecules. The image was constructed by simply putting gates around the signals from the three species, as indicated. Lighter shades indicate high intensities.
Fig. 6. The total energy spectrum obtained in the measurement on the copper grid, with the contribution from each species indicated. The energy spread for each species indicates the energy loss distribution as obtained from this target. The gates used to generate the images in Fig. 5 are indicated.
this thickness domain is dominated by scattering eects. Therefore, in the centre of the grid bars fewer proton signals are measured, as more
protons are scattered through angles large enough not to be detected by the detector due to the larger thickness of these regions. A measurement was subsequently performed on a biological sample consisting of a layer of single cells prepared by cryo-techniques and adhering to a thin formvar supporting foil. The results are shown in Fig. 7, again for each species separately displayed: (a) protons, (b) deuterons and (c) DH . The same areas of interest are found in this sample, i.e., the protons have an area of good contrast in the top middle section of the image whereas the thickness here is such that the deuterons give virtually no contrast. This area is too thick for both the deuterons and protons to be measured simultaneously with high enough yield to have good contrast. The bottom section of the image is the thinnest area of the sample, and this is re¯ected in the good contrast shown by the DH results, with poor contrast in the results obtained for the single ions. In this area the single cells are clearly distinguished with areas of diering thickness around these cells.
V.M. Prozesky et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 194±200
199
Fig. 7. Results from a measurement on a biological sample consisting of a layer of single cells prepared by cryo-techniques and adhering to a thin formvar supporting foil. The results are shown for each species separately displayed: (a) protons, (b) deuterons and (c) DH .
3. Other applications of molecular ions The use of molecular ions oers other niche applications, such as the use of HeH ions to perform imaging. For PIXE, there is some advantages to be gained if used at high energy (>5 MeV) to bene®t from the good sensitivities of the He ion for low-Z elements, in contrast to the good sensitivity of protons for high-Z elements. The DH molecular ion can also be useful in the case of performing RBS on targets consisting of both low-Z and high-Z elements, such as that of XN layers on steel, with X representing any nitride forming metal. In this case the protons will yield an increased number of counts due to the enhanced cross-section for scattering from nitrogen, while the D will give good mass resolution for the metals. A simulated spectrum of the use of DH ions on a sample of TiCuN on an Fe substrate is shown in Fig. 8. It is clear from this ®gure that the contributions from the two atomic ions give areas of good information for the dierent elements in the sample,
therefore yielding simultaneous information of analytical importance. This feature can also be used in imaging applications of similar targets. 4. Conclusions The use of molecular ions can yield superior information in applications such as STIM and to a limited extent in analytical applications. These ions are easy to generate in most of the available ion sources attached to nuclear microbeams and the currents that can be generated are more than enough for both analytical applications and STIM measurements. These ions can normally also be focussed using normal NMP lens systems in either the high or low excitation modes. Moreover, the recent advances [8] in distance operation of NMPs have enabled the operation of these facilities remotely from the facility itself, in this case radiation becomes less problematic, especially in the case of using DH ions.
200
V.M. Prozesky et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 194±200
Fig. 8. A simulated spectrum of the use of DH ions on a sample of TiCuN on an Fe substrate. The contributions from the two atomic ions give areas of good information for the dierent elements in the sample, therefore yielding simultaneous information of analytical importance.
References [1] U.A.S. Tapper et al., Nucl. Instr. and Meth. B 77 (1993) 17. [2] H.W. Lefevre, R.M.S. Scho®eld, J.C. Overley, J.D. McDonald, Scanning Microscopy 1 (3) (1987) 879. [3] H.W. Lefevre, R.C. Connelly, G. Sieger, J.C. Overley, Nucl. Instr. and Meth. 218 (1983) 39. [4] A. Saint, M.B.H. Breese, G.J.F. Legge, Rev. Sci. Instr. 67 (1996) 2940.
[5] A. Saint, G.J.F. Legge, Nucl. Instr. and Meth. B 130 (1997) 176. [6] Oxford Microbeams, Quadrupole Microbeam System, Lens system LM150. [7] V.M. Prozesky et al., Nucl. Instr. and Meth. B 104 (1995) 36. [8] C.L. Churms, V.M. Prozesky, K.A. Springhorn, these proceedings (ICNMTA-6), Nucl. Instr. and Meth. B 158 (1999) 124.