- Email: [email protected]
The new external beam facility of the Oxford scanning proton microprobe
Nuclear Instruments and Methods in Physics Research B 181 (2001) 66±70
www.elsevier.com/locate/nimb
The new external beam facility of the Oxford scanning proton microprobe G.W. Grime *, M.H. Abraham, M.A. Marsh Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK
Abstract This paper describes the development of a high spatial resolution external beam facility on one of the beamlines of the Oxford scanning proton microprobe tandem accelerator. Using a magnetic quadrupole doublet to focus the beam through the Kapton exit window a beam diameter of <50 lm full width at half maximum (fwhm) can be achieved on a sample located at 4 mm from the exit window. The facility is equipped with two Si±Li X-ray detectors for protoninduced X-ray emission (PIXE) analysis of light and trace elements respectively, a surface barrier detector for Rutherford backscattering spectrometry (RBS) analysis and a HP-Ge detector for c-ray detection. The mechanical and beam-optical design of the system is described. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Ion beam analysis; Microbeam analysis; Non-destructive analysis; PIXE; RBS
1. Introduction Ion beam analysis (IBA) has a well-established role in the characterisation of archaeological and historical objects. The combination of trace element sensitivity and quantitative accuracy oered by the techniques of proton-induced X-ray emission (PIXE) or Rutherford backscattering spectrometry (RBS) presents a solution to many problems involving the study of artifacts and raw materials. Using a suitable micro-focusing system, IBA can be carried out with a spatial resolution of the order of 1 lm, but this is often inappropriate
*
Corresponding author. E-mail address: geo[email protected] (G.W. Grime).
for archaeological objects. This is due to the nature of the materials, which are usually heterogeneous in three-dimensions, containing particulate inclusions with diameters of less than 100 lm. The long penetration depth of MeV light ions averages the signal over the range of the ions (tens of micrometres) and so unless carefully selected thin samples are used, the advantage of using micron or sub-micron beams is lost [1]. The optimum beam diameter for these inhomogeneous samples may be in the range 10±100 lm. Another disadvantage of microbeam facilities for archaeological applications is the requirement to place the sample in an evacuated chamber, which for most archaeological or historical objects requires sampling and may be highly undesirable for precious, delicate objects. Thus an external beam with a diameter of 50 lm may be the optimum solution for the non-
0168-583X/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 1 ) 0 0 6 1 6 - 4
G.W. Grime et al. / Nucl. Instr. and Meth. in Phys. Res. B 181 (2001) 66±70
destructive analysis of these objects. It also has an important role in the analysis of hydrated biological specimens and can provide unique information on the distribution of metals in biological systems. The importance of this technique in the ®eld of archaeometry is underlined by the work of the AGLAE group at the Louvre Museum in Paris, where an external microbeam has been established speci®cally for applications in art and archaeology. The Oxford design draws much from the work of the AGLAE group (e.g. [4,9]). This paper examines the practical considerations which limit the attainment of high spatial resolutions in microbeam systems where the beam is transmitted into air at normal atmospheric pressure and describes the system constructed in Oxford which has a design resolution of 50 lm. 2. Factors limiting the performance of an in-air microbeam facility 2.1. Beam broadening due to scattering The major factor limiting the spatial resolution of an external microbeam system is scattering by the exit window and by the molecules of the ambient gas. Table 1 shows calculations using SRIM2000 [2] showing the magnitude of this eect for 3 MeV protons travelling through 4 mm of air and He at normal temperature and pressure (NTP) assuming three common exit window materials. This shows the beam diameter containing 75% of the total ¯ux d75 , which is approximately 20%
67
larger than the full width at half maximum (fwhm) assuming a gaussian pro®le. Table 1 indicates that for low scattering windows such as silicon nitride the beam diameter is dominated by gas scattering while for thicker windows the most important effect is due to scattering in the foil. In the Oxford design a beam path of 4 mm is proposed, which should give a minimum fwhm of 60 lm using Kapton exit window or 15 lm using silicon nitride with helium. The gas path also introduces an energy loss for the beam particles which can have a signi®cant eect in the analysis, for instance in the use of RBS close to resonant peaks. SRIM-2000 calculations for 3 MeV protons passing through an 8 lm Kapton window into air show that after an initial energy loss of 130 keV in the exit window, the air path introduces a further energy loss of approximately 13 keV per mm. The energy loss in He is signi®cantly less (2 keV per mm). 2.2. X-ray attenuation in the gas The gas surrounding the sample attenuates the emitted X-rays and so imposes a signi®cant limitation on the use of PIXE. Fig. 1 shows calculations of the attenuation of the Ka X-rays of Na, Si and Ca as a function of path length in air and for Na in He (calculations carried out using the GUCSA program in the GUPIX package [3]). This shows that the analysis in air will be restricted to heavy elements unless the path length is
Table 1 Diameter of scattered beam of 3 MeV protons containing 75% of all particles for dierent exit windows and paths of 4 mm in air and helium at normal atmospheric pressurea
a
Window material
4 mm air
4 mm He
None 200 nm Silicon nitride 8 lm Kapton 1 lm Tungsten
23 34 80 280
5 22 84 288
Calculations using SRIM-2000 [2]. The row labelled `None' gives the scattering due to the gas path alone.
Fig. 1. Calculations of the attenuation of the K X-rays of Na, Si and Ca in air at NTP and for Na in helium at NTP as a function of the path length. Calculations carried out using GUCSA [3].
68
G.W. Grime et al. / Nucl. Instr. and Meth. in Phys. Res. B 181 (2001) 66±70
minimised, and that for PIXE analysis of the light elements it is essential not only to maintain a high partial pressure of helium, but also to maintain the He to air ratio constant to a high accuracy, since small changes in the helium concentration will have a large eect on the attenuation of soft X-rays. 2.3. Backscattered particle eects in the X-ray detectors The ¯ux of backscattered high energy protons is much higher in an external beam con®guration than in the corresponding high vacuum situation, since particles recoil from gas molecules as well as from the target. Even with Si±Li detectors which are normally insensitive to small ¯uxes of particles this can create severe problems with resolution and dead time, and may even cause permanent damage to the detector crystal. High energy particles can be excluded from the detectors by using absorber foils with a thickness selected to stop the highest energy particles, but this also results in the attenuation of low energy X-rays. An alternative solution to this problem is to use a magnetic de¯ector as proposed by Calligaro et al. [4] in which a dipole magnet using high ®eld rare-earth ceramic magnets is mounted between the sample and the detector. A magnetic de¯ector based on this design is used for the light element detector.
3. The Oxford external microbeam facility The new external beam system developed for the Oxford scanning proton microprobe facility attempts to minimise the eects of the physical constraints described above. The layout of the system is shown schematically in Fig. 2. Individual features are described below. 3.1. Exit nozzle and beam focusing In conventional external beam facilities the initial beam diameter is de®ned by the diameter of the exit nozzle. This has several disadvantages of which the major ones are the diculty of fabricating very small apertures in material thick enough to stop the beam, the loss of beam ¯ux due to
2.4. Charge measurement In order to quantify the results of the analysis it is often necessary to measure the total charge falling on the sample. Various methods of doing this have been proposed and implemented, but the technique selected for the Oxford system is to carry out simultaneous RBS analysis. The beam charge can then be determined from the area of the RBS spectrum, while at the same time the local sample matrix composition, and the beam energy can be obtained from the shape of the spectrum. This technique [5] is already used routinely for quantifying PIXE and RBS data from the in-vacuum facility at Oxford.
Fig. 2. Schematic plan view of the external beam analysis facility at Oxford. X-rays emitted from the sample are detected by two detectors mounted at 45° on either side of the beam. One detector is ®tted with an X-ray absorber while the other detector has no absorber and is ®tted with a magnetic de¯ector to ensure that high energy protons do not reach the detector. A detector for c-rays is mounted at 90° to the beam. Below the beamline, a detector for recoiling protons allows RBS data to be collected. Not shown in this diagram are a video microscope which uses a mirror to view the front surface of the sample during analysis and a low power alignment laser to assist in positioning the sample for analysis.
G.W. Grime et al. / Nucl. Instr. and Meth. in Phys. Res. B 181 (2001) 66±70
simple collimation, and the generation of spurious signals (X-rays, c-rays) from the interaction of the high incident beam ¯ux with the material of the nozzle. These disadvantages can be minimised by focusing the beam in the vacuum to a diameter much smaller than the exit window. In the Oxford design the exit nozzle is a hollow cylinder of copper tapering in steps from 10 mm diameter where it joins the high vacuum beam line down to 1 mm at the exit face. The exit end has a hole of 300 lm diameter which is covered by an 8 lm Kapton foil to act as a vacuum barrier. Kapton is selected because of its resistance to radiation damage, though a silicon nitride window will be ®tted in future. Copper is selected for the nozzle material in order to reduce the background of c-rays emitted from lighter materials such as Al alloy. No Cu X-rays are observed from the nozzle. The sample is mounted at a distance of 4 mm from the nozzle which is tapered in such a way that a detector of 80 mm2 active area can be positioned at 45° at a distance 20 mm from the sample without obstructing the detector acceptance. The beam is focused to a diameter of approximately 10 lm on the Kapton foil using a doublet of magnetic quadrupoles with a mechanical length of 200 mm and a bore diameter of 25 mm. The quadrupoles are preceded by a four-jaw collimator slit to de®ne the entrance divergence of the beam. The slit box also contains a Faraday cup which may be inserted into the beam to measure the beam current after the slits. For alignment the whole nozzle assembly can be moved onto the position of the beam focus using micrometers. The external beam facility is mounted on the 45° port of the switcher magnet of the Oxford system. A photograph of the nozzle arrangement is shown in Fig. 3. 3.2. Detectors Analytical signals from the sample are detected using the following detectors: 1. A 10 mm2 Si(Li) detector with a thin entrance window for the detection of light elements (above Na). This is ®tted with a magnetic de¯ector as described in [4] which is ¯ooded with He.
69
Fig. 3. Photograph of the nozzle of the Oxford external beam de¯ector. Also shown are the RBS detector with He cap (below the nozzle), the microscope mirror (above the nozzle) and the light element detector (retracted and with the de¯ector removed).
The eect of the magnetic de¯ector reduces the solid angle to 2 msr, but this is not a problem for the high yield X-rays of abundant elements such as Al and Si. 2. An 80 mm2 Si±Li detector for the detection of trace elements. This is ®tted with a an absorber of 125 lm Kapton, which removes all the intense X-rays from the light elements and also stops recoiling protons. This detector has a solid angle of 250 msr and is ®tted with a Delrin cap in the form of a conical nozzle which is ¯ooded with helium. 3. Recoiling protons are detected using a surface barrier detector mounted below the beamline. This is also ®tted with a helium ¯ow nozzle. 4. A high purity germanium c-ray detector can be mounted at 90° to the beam to detect c-rays from (p,c) reactions with light elements in the sample. 3.3. Other features The sample is mounted at present on a manual x±y±z translation stage with a micrometer adjustment range of 200 mm in each axis (although larger movements can of course be made by repositioning the sample) and special arrangements can be employed for large irregular objects. The sample is viewed by a vertically mounted long working distance video microscope with a small mirror mounted just above the beamline to view
70
G.W. Grime et al. / Nucl. Instr. and Meth. in Phys. Res. B 181 (2001) 66±70
Fig. 4. Beam pro®le at 4 mm from the detector nozzle in air measured by mechanically scanning a 4 lm diameter tungsten wire mounted on a PMMA substrate across the ®xed beam. The vertical axis shows the tungsten La X-ray yield normalised to the argon K line (arbitrary units).
the front of the sample. Sample positioning is facilitated by a low power diode laser mounted at 45° to the beam which is aligned using a ¯uorescent screen so that when the sample is correctly positioned the laser spot is coincident with the beam on the video screen. In order to comply with local radiation safety regulations, the ®nal stages of the beamline (from the quadrupoles to the exit nozzle) is enclosed in a separate room with interlocked access so that it is not possible to enter the room while the beam is on. 4. Performance and applications The beam diameter of the external beam was measured by moving a 4 lm diameter tungsten wire across the beam in air and normalising to the argon K X-ray counts. The resulting pro®le is shown in Fig. 4 and yields a fwhm of 50 lm. This was achieved with a beam current of 2 nA (measured immediately after the collimator slits).
The system has been applied to several problems in the ®eld of archaeology and palaeontology. The Ashmolean museum in Oxford holds a tourmaline intaglio of Alexander the Great, which is believed to have been carved during his lifetime. This gem has not been characterised because its high value and the presence of apparent cracks and ®ssures mean that it was not possible to carry out any tests more invasive than optical inspection. The external beam facility was used to characterise this gem, using PIXE, RBS and PIGE, which was employed to obtain the concentrations of Li and F, which are important for the identi®cation of tourmaline end-members. The gem was con®rmed as being a zoned tourmaline of unusual (high manganese) composition [6]. Other recent applications include the combination of external microbeam analysis with a microfocused laser to characterise thick corrosion layers by successive ablation [7] and the analysis of large fossil shells to investigate the factors determining the variation of strontium in aragonitic shells [8]. References [1] G.W. Grime, F. Watt, A.R. Duval, M. Menu, Nucl. Instr. and Meth. B 54 (1991) 353. [2] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon, New York, 1999. [3] J.A. Maxwell, W.J. Teesdale, J.L. Campbell, Nucl. Instr. and Meth. B 95 (1995) 407. [4] T. Calligaro, J.D. Macarthur, J. Salomon, Nucl. Instr. and Meth. B 109 (1996) 125. [5] G.W. Grime, Nucl. Instr. and Meth. B 109 (1996) 170. [6] G.W. Grime, L. Thoresen, Ion Beam Analysis of Gem stones, in: L. Thoresen (Ed.), Archaeogemmology, Getty Museum Publications, Los Angeles, in press. [7] M.H. Abraham, G.W. Grime, M.A. Marsh, J.P. Northover, Nucl. Instr. and Meth. B 181 (2001) 688. [8] L.M.A. Purton-Hildebrand, G.W. Grime, G.A. Shields, M.D. Brasier, Nucl. Instr. and Meth. B 181 (2001) 506. [9] T. Calligaro, J.-C. Dran, E. Ioannidou, B. Moignard, L. Pichon, J. Salomon, Nucl. Instr. and Meth. B 161±163 (2000) 328.
www.elsevier.com/locate/nimb
The new external beam facility of the Oxford scanning proton microprobe G.W. Grime *, M.H. Abraham, M.A. Marsh Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK
Abstract This paper describes the development of a high spatial resolution external beam facility on one of the beamlines of the Oxford scanning proton microprobe tandem accelerator. Using a magnetic quadrupole doublet to focus the beam through the Kapton exit window a beam diameter of <50 lm full width at half maximum (fwhm) can be achieved on a sample located at 4 mm from the exit window. The facility is equipped with two Si±Li X-ray detectors for protoninduced X-ray emission (PIXE) analysis of light and trace elements respectively, a surface barrier detector for Rutherford backscattering spectrometry (RBS) analysis and a HP-Ge detector for c-ray detection. The mechanical and beam-optical design of the system is described. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Ion beam analysis; Microbeam analysis; Non-destructive analysis; PIXE; RBS
1. Introduction Ion beam analysis (IBA) has a well-established role in the characterisation of archaeological and historical objects. The combination of trace element sensitivity and quantitative accuracy oered by the techniques of proton-induced X-ray emission (PIXE) or Rutherford backscattering spectrometry (RBS) presents a solution to many problems involving the study of artifacts and raw materials. Using a suitable micro-focusing system, IBA can be carried out with a spatial resolution of the order of 1 lm, but this is often inappropriate
*
Corresponding author. E-mail address: geo[email protected] (G.W. Grime).
for archaeological objects. This is due to the nature of the materials, which are usually heterogeneous in three-dimensions, containing particulate inclusions with diameters of less than 100 lm. The long penetration depth of MeV light ions averages the signal over the range of the ions (tens of micrometres) and so unless carefully selected thin samples are used, the advantage of using micron or sub-micron beams is lost [1]. The optimum beam diameter for these inhomogeneous samples may be in the range 10±100 lm. Another disadvantage of microbeam facilities for archaeological applications is the requirement to place the sample in an evacuated chamber, which for most archaeological or historical objects requires sampling and may be highly undesirable for precious, delicate objects. Thus an external beam with a diameter of 50 lm may be the optimum solution for the non-
0168-583X/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 1 ) 0 0 6 1 6 - 4
G.W. Grime et al. / Nucl. Instr. and Meth. in Phys. Res. B 181 (2001) 66±70
destructive analysis of these objects. It also has an important role in the analysis of hydrated biological specimens and can provide unique information on the distribution of metals in biological systems. The importance of this technique in the ®eld of archaeometry is underlined by the work of the AGLAE group at the Louvre Museum in Paris, where an external microbeam has been established speci®cally for applications in art and archaeology. The Oxford design draws much from the work of the AGLAE group (e.g. [4,9]). This paper examines the practical considerations which limit the attainment of high spatial resolutions in microbeam systems where the beam is transmitted into air at normal atmospheric pressure and describes the system constructed in Oxford which has a design resolution of 50 lm. 2. Factors limiting the performance of an in-air microbeam facility 2.1. Beam broadening due to scattering The major factor limiting the spatial resolution of an external microbeam system is scattering by the exit window and by the molecules of the ambient gas. Table 1 shows calculations using SRIM2000 [2] showing the magnitude of this eect for 3 MeV protons travelling through 4 mm of air and He at normal temperature and pressure (NTP) assuming three common exit window materials. This shows the beam diameter containing 75% of the total ¯ux d75 , which is approximately 20%
67
larger than the full width at half maximum (fwhm) assuming a gaussian pro®le. Table 1 indicates that for low scattering windows such as silicon nitride the beam diameter is dominated by gas scattering while for thicker windows the most important effect is due to scattering in the foil. In the Oxford design a beam path of 4 mm is proposed, which should give a minimum fwhm of 60 lm using Kapton exit window or 15 lm using silicon nitride with helium. The gas path also introduces an energy loss for the beam particles which can have a signi®cant eect in the analysis, for instance in the use of RBS close to resonant peaks. SRIM-2000 calculations for 3 MeV protons passing through an 8 lm Kapton window into air show that after an initial energy loss of 130 keV in the exit window, the air path introduces a further energy loss of approximately 13 keV per mm. The energy loss in He is signi®cantly less (2 keV per mm). 2.2. X-ray attenuation in the gas The gas surrounding the sample attenuates the emitted X-rays and so imposes a signi®cant limitation on the use of PIXE. Fig. 1 shows calculations of the attenuation of the Ka X-rays of Na, Si and Ca as a function of path length in air and for Na in He (calculations carried out using the GUCSA program in the GUPIX package [3]). This shows that the analysis in air will be restricted to heavy elements unless the path length is
Table 1 Diameter of scattered beam of 3 MeV protons containing 75% of all particles for dierent exit windows and paths of 4 mm in air and helium at normal atmospheric pressurea
a
Window material
4 mm air
4 mm He
None 200 nm Silicon nitride 8 lm Kapton 1 lm Tungsten
23 34 80 280
5 22 84 288
Calculations using SRIM-2000 [2]. The row labelled `None' gives the scattering due to the gas path alone.
Fig. 1. Calculations of the attenuation of the K X-rays of Na, Si and Ca in air at NTP and for Na in helium at NTP as a function of the path length. Calculations carried out using GUCSA [3].
68
G.W. Grime et al. / Nucl. Instr. and Meth. in Phys. Res. B 181 (2001) 66±70
minimised, and that for PIXE analysis of the light elements it is essential not only to maintain a high partial pressure of helium, but also to maintain the He to air ratio constant to a high accuracy, since small changes in the helium concentration will have a large eect on the attenuation of soft X-rays. 2.3. Backscattered particle eects in the X-ray detectors The ¯ux of backscattered high energy protons is much higher in an external beam con®guration than in the corresponding high vacuum situation, since particles recoil from gas molecules as well as from the target. Even with Si±Li detectors which are normally insensitive to small ¯uxes of particles this can create severe problems with resolution and dead time, and may even cause permanent damage to the detector crystal. High energy particles can be excluded from the detectors by using absorber foils with a thickness selected to stop the highest energy particles, but this also results in the attenuation of low energy X-rays. An alternative solution to this problem is to use a magnetic de¯ector as proposed by Calligaro et al. [4] in which a dipole magnet using high ®eld rare-earth ceramic magnets is mounted between the sample and the detector. A magnetic de¯ector based on this design is used for the light element detector.
3. The Oxford external microbeam facility The new external beam system developed for the Oxford scanning proton microprobe facility attempts to minimise the eects of the physical constraints described above. The layout of the system is shown schematically in Fig. 2. Individual features are described below. 3.1. Exit nozzle and beam focusing In conventional external beam facilities the initial beam diameter is de®ned by the diameter of the exit nozzle. This has several disadvantages of which the major ones are the diculty of fabricating very small apertures in material thick enough to stop the beam, the loss of beam ¯ux due to
2.4. Charge measurement In order to quantify the results of the analysis it is often necessary to measure the total charge falling on the sample. Various methods of doing this have been proposed and implemented, but the technique selected for the Oxford system is to carry out simultaneous RBS analysis. The beam charge can then be determined from the area of the RBS spectrum, while at the same time the local sample matrix composition, and the beam energy can be obtained from the shape of the spectrum. This technique [5] is already used routinely for quantifying PIXE and RBS data from the in-vacuum facility at Oxford.
Fig. 2. Schematic plan view of the external beam analysis facility at Oxford. X-rays emitted from the sample are detected by two detectors mounted at 45° on either side of the beam. One detector is ®tted with an X-ray absorber while the other detector has no absorber and is ®tted with a magnetic de¯ector to ensure that high energy protons do not reach the detector. A detector for c-rays is mounted at 90° to the beam. Below the beamline, a detector for recoiling protons allows RBS data to be collected. Not shown in this diagram are a video microscope which uses a mirror to view the front surface of the sample during analysis and a low power alignment laser to assist in positioning the sample for analysis.
G.W. Grime et al. / Nucl. Instr. and Meth. in Phys. Res. B 181 (2001) 66±70
simple collimation, and the generation of spurious signals (X-rays, c-rays) from the interaction of the high incident beam ¯ux with the material of the nozzle. These disadvantages can be minimised by focusing the beam in the vacuum to a diameter much smaller than the exit window. In the Oxford design the exit nozzle is a hollow cylinder of copper tapering in steps from 10 mm diameter where it joins the high vacuum beam line down to 1 mm at the exit face. The exit end has a hole of 300 lm diameter which is covered by an 8 lm Kapton foil to act as a vacuum barrier. Kapton is selected because of its resistance to radiation damage, though a silicon nitride window will be ®tted in future. Copper is selected for the nozzle material in order to reduce the background of c-rays emitted from lighter materials such as Al alloy. No Cu X-rays are observed from the nozzle. The sample is mounted at a distance of 4 mm from the nozzle which is tapered in such a way that a detector of 80 mm2 active area can be positioned at 45° at a distance 20 mm from the sample without obstructing the detector acceptance. The beam is focused to a diameter of approximately 10 lm on the Kapton foil using a doublet of magnetic quadrupoles with a mechanical length of 200 mm and a bore diameter of 25 mm. The quadrupoles are preceded by a four-jaw collimator slit to de®ne the entrance divergence of the beam. The slit box also contains a Faraday cup which may be inserted into the beam to measure the beam current after the slits. For alignment the whole nozzle assembly can be moved onto the position of the beam focus using micrometers. The external beam facility is mounted on the 45° port of the switcher magnet of the Oxford system. A photograph of the nozzle arrangement is shown in Fig. 3. 3.2. Detectors Analytical signals from the sample are detected using the following detectors: 1. A 10 mm2 Si(Li) detector with a thin entrance window for the detection of light elements (above Na). This is ®tted with a magnetic de¯ector as described in [4] which is ¯ooded with He.
69
Fig. 3. Photograph of the nozzle of the Oxford external beam de¯ector. Also shown are the RBS detector with He cap (below the nozzle), the microscope mirror (above the nozzle) and the light element detector (retracted and with the de¯ector removed).
The eect of the magnetic de¯ector reduces the solid angle to 2 msr, but this is not a problem for the high yield X-rays of abundant elements such as Al and Si. 2. An 80 mm2 Si±Li detector for the detection of trace elements. This is ®tted with a an absorber of 125 lm Kapton, which removes all the intense X-rays from the light elements and also stops recoiling protons. This detector has a solid angle of 250 msr and is ®tted with a Delrin cap in the form of a conical nozzle which is ¯ooded with helium. 3. Recoiling protons are detected using a surface barrier detector mounted below the beamline. This is also ®tted with a helium ¯ow nozzle. 4. A high purity germanium c-ray detector can be mounted at 90° to the beam to detect c-rays from (p,c) reactions with light elements in the sample. 3.3. Other features The sample is mounted at present on a manual x±y±z translation stage with a micrometer adjustment range of 200 mm in each axis (although larger movements can of course be made by repositioning the sample) and special arrangements can be employed for large irregular objects. The sample is viewed by a vertically mounted long working distance video microscope with a small mirror mounted just above the beamline to view
70
G.W. Grime et al. / Nucl. Instr. and Meth. in Phys. Res. B 181 (2001) 66±70
Fig. 4. Beam pro®le at 4 mm from the detector nozzle in air measured by mechanically scanning a 4 lm diameter tungsten wire mounted on a PMMA substrate across the ®xed beam. The vertical axis shows the tungsten La X-ray yield normalised to the argon K line (arbitrary units).
the front of the sample. Sample positioning is facilitated by a low power diode laser mounted at 45° to the beam which is aligned using a ¯uorescent screen so that when the sample is correctly positioned the laser spot is coincident with the beam on the video screen. In order to comply with local radiation safety regulations, the ®nal stages of the beamline (from the quadrupoles to the exit nozzle) is enclosed in a separate room with interlocked access so that it is not possible to enter the room while the beam is on. 4. Performance and applications The beam diameter of the external beam was measured by moving a 4 lm diameter tungsten wire across the beam in air and normalising to the argon K X-ray counts. The resulting pro®le is shown in Fig. 4 and yields a fwhm of 50 lm. This was achieved with a beam current of 2 nA (measured immediately after the collimator slits).
The system has been applied to several problems in the ®eld of archaeology and palaeontology. The Ashmolean museum in Oxford holds a tourmaline intaglio of Alexander the Great, which is believed to have been carved during his lifetime. This gem has not been characterised because its high value and the presence of apparent cracks and ®ssures mean that it was not possible to carry out any tests more invasive than optical inspection. The external beam facility was used to characterise this gem, using PIXE, RBS and PIGE, which was employed to obtain the concentrations of Li and F, which are important for the identi®cation of tourmaline end-members. The gem was con®rmed as being a zoned tourmaline of unusual (high manganese) composition [6]. Other recent applications include the combination of external microbeam analysis with a microfocused laser to characterise thick corrosion layers by successive ablation [7] and the analysis of large fossil shells to investigate the factors determining the variation of strontium in aragonitic shells [8]. References [1] G.W. Grime, F. Watt, A.R. Duval, M. Menu, Nucl. Instr. and Meth. B 54 (1991) 353. [2] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon, New York, 1999. [3] J.A. Maxwell, W.J. Teesdale, J.L. Campbell, Nucl. Instr. and Meth. B 95 (1995) 407. [4] T. Calligaro, J.D. Macarthur, J. Salomon, Nucl. Instr. and Meth. B 109 (1996) 125. [5] G.W. Grime, Nucl. Instr. and Meth. B 109 (1996) 170. [6] G.W. Grime, L. Thoresen, Ion Beam Analysis of Gem stones, in: L. Thoresen (Ed.), Archaeogemmology, Getty Museum Publications, Los Angeles, in press. [7] M.H. Abraham, G.W. Grime, M.A. Marsh, J.P. Northover, Nucl. Instr. and Meth. B 181 (2001) 688. [8] L.M.A. Purton-Hildebrand, G.W. Grime, G.A. Shields, M.D. Brasier, Nucl. Instr. and Meth. B 181 (2001) 506. [9] T. Calligaro, J.-C. Dran, E. Ioannidou, B. Moignard, L. Pichon, J. Salomon, Nucl. Instr. and Meth. B 161±163 (2000) 328.