P1.35 AU9817162 The ANSTO High Energy Heavy Ion Microprobe

R. Siegele, D.D. Cohen, H. Noorman, D. Garton, and A. Croal Australian Nuclear Science and Technology Organisation Private Mail Bag 1, Menai, NSW 2234, Australia

Introduction Ion microprobes have a long tradition at ANSTO as well as in Australia. ANSTO was among the first to use micro-sized beams in the 60s. Since then microbeams have developed considerably and proton and alpha microprobes are a common sight in IBA laboratories around the world. Ion microbeams allow surface imaging of specimens in addition to the depth profiling capabilities of macroscopic beams. Nuclear microprobes have significant advantages in terms of sensitivity for elemental analysis over electron beam microanalysis. The advantages of ions heavier then helium in nuclear micropobes was first pointed out by Martin1 in the late 1970's. However, very few microbeams employing heavy ions at high energies have been realized since then, because of the limited availability of high energy accelerators and the difficulty of the task. The high energy heavy ion microprobe under construction at ANSTO will be a unique, multi-user facility providing new capabilities for ANSTO and the broader Australian re- search community. It will be used in materials research, environmental research, industrial applications and for applications in biology, medicine and geology.

Advantages of High Energy Heavy Ion Microbeams

107 ! The ANSTO heavy ion micro- probe will not only provide H and 106 He ions, but a wide range of ions 9OMeVIJ5RDA 5 10 —• ! at different energies. This results in I a number of advantages. The cross sections of most IBA techniques are L x-ray Z @ 4.5 MeV P, proportional to the square of the nuc- lear charge of the incident ion, Z2. 102 ! /*/7McV NRBS = j/

10° - time, which gives a heavy ion micro- 10 20 30 40 SO 60 70 90 probe a clear advantage over proton Target Atomic Number Z2 microprobes. The higher cross sec- tions are sometimes offset by the fact Figure 1: The figure shows the cross sections for various that the energy also has to be in- techniques. creased in order to achieve the same probing depth or to stay in the Rutherford regime. This, however, is not the case for elastic recoil detection analysis (ERDA) and PIXE. The cross sections for various IBA techniques, such as PIXE, RBS and ERDA for different ions and energies are shown in figure 1. The figure shows the higher PIXE cross- sections for oxygen compared to protons as well as the higher ERDA cross sections for iodine compared to Si. It also shows that the cross sections for ERDA using iodine are much higher compared to the cross sections of 2 MeV He-RBS. Using heavy ions in PIXE analysis also reduces the bremsstrahlung of the primary ion thus lowering the background, which results in an even further increase in sensitivity.

/' / Figure 2: Overview of the ANTARES tandem accelerator and beamlines.

Furthermore the use of techniques such heavy ion RBS (HIRBS) and heavy ion ERDA for microanalysis only becomes possible with a heavy ion microprobe. HIRBS has the advantage of an enhanced depth and mass resolution over conventional RBS using light ions such as H and He. The cross sections in ERDA increase strongly for heavy ions and in fact ERDA becomes a truly universal technique with heavy ions, allowing the detection of all elements from hydrogen up to the mass of the ion used in the analysis. In contrast, PIXE is normally limited to target elements heavier then aluminium and gives no depth information, while RBS is best suited for the depth profiling of medium to heavy mass elements in a light element matrix. The possibility to use ERDA makes a high energy, heavy ion nuclear microprobe a universal tool, that can be used for the depth profiling of heavy and light elements simultaneously.

ANSTO Microprobe Facility Figure 2 shows the ANTARES Accelerator with its various beamlines. The tandem ac- celerator together with its various ion sources is located in the tandem hall, while all the beamlines installed previously are located in the target area. With the ever increasing number of beam lines the space in the target hall has became very limited. Since the micro- probe beamline had to be at least 7 m long a location in the tandem hall was chosen. This location also has the advantage that only one deflection magnet is required, which makes it easier to optimize the beam. A detailed drawing of the microprobe is shown if figure 3. After the analysing magnet the ions travel through energy stabilisation slits situated in front of the object slits. These slits are also used as collimators to prevent damage to the object slits. A set of quadrupoles and deflectors after the accelerator are used to focus and centre the beam on the object slits. Faraday cups before and after the slits are used to set up the beam. Additionally a beam profile monitor and viewer allow monitoring the beam shape. The collimating slits are located 6.10 m from the object slits and and are directly followed by the beam scanning and forming elements. The beam focusing and scanning system consists of the OM-55 quadrupole triplet and the OM-25 scanning system from Oxford Microbeams. The OM-55, a modified version of the OM-50, has a smaller polegap a b c d e

lm

Figure 3: Schematic of the microprobe beamline and target station, a: gate valve, b: Faraday cup, c: beam profile monitor, d: energy stabilisation slits, e: object slits, f: beam viewer, g: pump, h: collimating slits, i: beam scanner, k: quadrupole triplet, 1: target chamber. of 8 mm and thus permits focusing of ions with a mass energy product of up to 120 MeV amu /q2. The stainless steel target chamber has an octagonal design. Each face of the chamber is fitted with two access ports to permit detectors for various applications to be mounted. One of the 135° ports is fitted with a microscope, which will be used for precision placement of the targets and to monitor the focusing of the beam. In the near future on the other 135° port a X-ray detector will be mounted. At the time of writing of this report the construction of the microprobe beamlime at ANSTO as well as target chamber have been completed and we are awaiting the arrival of the quadrupoles for installation. The microprobe quadrupole triplet has been completed and is currently undergoing testing by the manufacturer, Oxford Microbeams. Tests by the manufacturer so far have yielded a 0.9 x 1.2 /um spot size for 3 MeV protons. It is planned to equip the microprobe with various detection systems, that will allow the use of a variety of IBA techniques. One of the advantages of a heavy ion microprobe, is the possibility to use heavy ion ERDA. For this purpose the microprobe will be equipped with a position sensitive large solid angle gas detector.3 Since the target current decreases with the beam spot size detectors with large solid angles are required. A gas detector is currently under design and construction will commence in the near future. In addition it is planned to equip the microprobe with a Scanning Transmission Ion Microscopy (STIM) capability and a secondary electron detection system.

Acknowledgment

We want to acknowledge the valuable assistance of G. Legge and the Melbourne Mi- croprobe group, as well as A. Dymnikov in the design of the microprobe. We also want to thank the technical support team of the ANSTO Ion Beam Analysis group for their assistance.

References 1 F.W. Martin, Nucl. Instr. and Meth. 149, 475 (1978). 2 J.W. Martin, D.D. Cohen, N. Dytlewski, D.B. Garton, H.J. Withlow, G.J. Russel, Nucl. Instr. and Meth. B 94, 277 (1994). 3 W. Assmann, P. Hartung, H. Huber, P. Staat, H. Steffens, Ch. Steinhausen, Nucl. Instr. and Meth. B85, 729 (1994).