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Virtual Laboratory for Ion Scattering to Probe Atomic, Nuclear and Microscopic Structures

David Jamieson and Jon Pearce
School of Physics
The University of Melbourne

1. Introduction

To teach basic concepts of nuclear and atomic physics we have developed a virtual laboratory. In this laboratory students will find an ion accelerator that can be used to explore ion scattering phenomena. Students can also select appropriate detectors for detection of the scattered ionizing radiation. Once the basic concepts have been studied, they can be applied to the investigation of a variety of real world specimens using the technique of ion beam analysis with a nuclear microprobe. These case studies have been selected from actual research data of the Microanalytical Research Centre (MARC) in the School of Physics, University of Melbourne, and elsewhere. In 1995 this virtual workshop will be used in the second year course 'Modern Applied Physics'. Development of the virtual workshop has been supported by a CAUT grant. The virtual workshop has been given the name MARCSHOP.

2. Motivation

Physical processes are THREE DIMENSIONAL and DYNAMIC. Some physical processes are the subject of ACTIVE RESEARCH within the School of Physics and with external collaborators.

Traditional approaches to teaching physics involve learning from lectures and textbooks which provide a STATIC and TWO DIMENSIONAL (at best) view of the topic. Examples are often REMOTE from the student (and lecturer), both in experience and distance.

These two opposing statements provided the motivation for this project. Obviously it is impossible to give undergraduate students real experience of physics research under laboratory conditions, the staff/student ratios are too small and the laboratory equipment is too complicated for an undergraduate to use. Also, because of the time constraints, the students would not have time for the inevitable trouble-shooting, machine calibration, and the like that accompany a real experiment. The student has time to only learn the physical principles behind a real research experiment. The development of a virtual laboratory on a small computer system is a way we propose of overcoming these difficulties. This will allow us to provide the students with contact with actual research data from selected experiments, as well as give them a strong understanding of the important physical principles involved, as well as an insight into the functioning of a real piece of research apparatus via a simplified model simulated by a computer.

Future motivation is provided by our desire to engage students in the actual research of the School. Preliminary surveys conducted in 1994 revealed that most second year students had no knowledge of any of the research activities of any of their lecturers! This implies that although a great deal of Physics was being taught to them, little of it was enhanced by examples taken from, or identified as, the school's work. We believe that introducing current research will have a powerful effect on engaging our students in studying Physics.

3. Method and Content

Simulation of the nuclear microprobe system in the School of Physics offers several advantages to teaching some branches of physics at the undergraduate level. It exploits a wide variety of physical phenomena that are appropriate to introduce into an undergraduate curriculum. These phenomena are applied to measure interesting properties of selected specimens. The specimens can be selected to exploit pre-existing knowledge of the students. The specimens can be selected from suitable interdisciplinary research programs and include bio-medical, geological or technological synthetic specimens.

This analytical technique is essentially a generalisation of the photon or electron scattering used in more conventional light or electron microscopy where particle scattering is also employed to form images:

* Light Specimen(h, h)Specimen

* Electrons Specimen(e, e)Specimen

Using the symbol I to designate a MeV ion, the ion scattering phenomena in a nuclear microprobe include:

* Backscattering spectrometry Specimen(I, I)Specimen

* Particle induced X-ray emission Specimen(I, x-ray)Specimen

* Nuclear reactions Specimen(I, I')Specimen'

* Luminescence Specimen(I, h)Specimen

* Ion induced currents Specimen(I, e-h)Specimen

In a nuclear microprobe, a beam of MeV energy ions is focused to a small spot, called a probe. This probe is then scanned over the region of interest in the specimen. The MeV ions induce various types of characteristic radiation from the specimen. A computer can then construct images of the region of interest by mapping the intensity of the characteristic radiation as a function of the probe position on the specimen.

The techniques has been applied to a wide variety of specimens. Many of these are appropriate for illustrating the basic physics concepts to students. Some examples:

* Biomedical specimens. The spatial distribution of trace elements can be mapped at the cellular and sub-cellular level. Some specific applications include the mapping of Al in brain tissue from Alzheimer's victims, the mapping of Ca and Sr in pancreas tissue from suffers of diabetes.

* Geological specimens. The spatial distribution of economically important metals in mineral ores can be imaged. Further experiments can reveal the actual lattice location of the metal by channelling the probe into the channels of single crystal specimens.

* Synthetic crystals. These typically contain interesting structures, particularly of they have been fabricated with microelectronic devices in them. The three dimensional structure of the device can be probed by measuring the energy of the scattered ions.

In these examples, the students can be motivated to study the specimens because they already know the importance of the underlying research in each specimen, or know of widespread applications of products derived from each specimen.


4.1 Outline

The implementation of these ideas has been funded by a CAUT grant [1]. The basic idea is to provide the students with a sequence of computer screens. These screens are divided into a number of sections that introduce progressively more advanced topics:

* Introductory screens cover basic concepts. These include basic nuclear and atomic structures, the strong nuclear force, Rutherford and non-Rutherford scattering, the kinematics of ion scattering, the energy loss of fast ions in matter, elementary crystal structure, the channelling of ions in crystals, the production of x-rays from vacancies created in inner shells by fast ions, nuclear reactions, the scattering cross section. Here the students explore the nuclear structures.

* Intermediate screens cover elementary spectroscopy. This includes counting statistics, the characteristic X-ray spectrum, the backscattered particle spectrum from thin and thick specimens, the calculation of specimen stoichiometry from the backscattering spectrum, the backscattering spectrum with ion channelling, particle acceleration and detection, the nuclear microprobe. Here the students explore the atomic structures.

* Project screens. Here the students interact with the actual experimental data and interpret the images and energy spectra of the appropriate radiation induced from the specimen using the information from the previous screens. Here the students explore interesting microstructures of the specimens.

A key feature of the screens is the animated energy spectrum. This allows the students to explore, in a natural way, the behaviour of the theoretical models as a function of the parameters of the models. An illustration of this is shown in figure 1. This figure shows the screen of the virtual instrument which models the nuclear microprobe system itself. Clockwise from top right it shows the accelerator which provides the beam of charged particles, the beam lines, the probe forming lens system, the specimen and the particle detector, the control panel which allows the specimen dimensions to be changed and the energy spectrum window. The energy spectrum shown in the window is a simulation for the selected configuration of the system. Sliders on each of the key components allows the important parameters to be changed dynamically. By selecting a specimen that consists of three thin films of metals of different masses, with a beam of He ions, it is possible to investigate graphically the behaviour of the Rutherford scattering cross section by observing the signals from each element as a function of ion energy and detector scattering angle.

Figure 1: The control panel of the virtual instrument used to investigate the effect of scattering angle, beam energy and specimen structure. This appears one of the intermediate screens.

The energy spectrum is read from a database, rather than from an analytical formula. This is done because the program can show simulations in situations where the Rutherford formula beaks down. In this case the simulation is done with actual measured scattering cross sections stored in the data base. This provides the students with graphical insights into the idea of compound nucleus formation since they can observe resonances in signals from the detector. This is where presentation of these ideas using the computer has the advantage over traditional methods, since the transition between the regime where the model applies to the regime where it breaks down may be done seamlessly. The new regime, as in this case, often contains the more interesting physics!

4.2 Implementation

During 1994 a team of software engineers were employed to construct the package. A wide variety of tools were employed for this work including: Multimedia ToolBook by Asymmetrix, Access database by Microsoft, Interactive Physics 2 by Knowledge Revolution and Visual C++ by Microsoft. MARCSHOP is constructed in a way that allows non-specialists to add, or modify, screens as the project develops. In addition, several large computer programs were used to generate simulations, or to pre-process experimental data, for the databases. These included a modified version of the backscattering spectrum simulation program RUMP [2] and the data acquisition and analysis program MPSYS [3].

A feature of the software is that the basic structure is modular and not specific to our project. In principle, the complete structure can be used for the presentation of other topics using alternative screen sequences. This generality also applies to the animations drawn from the data-base. Hence any other physical models, based on analytic formulae or more complicated numerical simulations, could also be displayed.

Figure 2: The data analysis screen which displays images of the microstructures of the selected specimens. The images exploit characteristics of the scattering phenomena explained in the earlier sections. In this example, Rutherford scattering is used to probe the 3D structure of a microelectronic device fabricated at the Telecom Australia Research Laboratories.

It is envisaged that MARCSHOP will be used to replace one lecture and one tutorial each week for two or three weeks of a course that consists of 26 lectures and 13 tutorials spread over 13 weeks. The students will first receive a series of conventional lectures on the theory, with extracts from MARCSHOP used in the lecture theatre to illustrate key points. The students will then work in a computer laboratory, and in their own time, with MARCSHOP, on an individual basis, or in pairs. As they progress through the screens they will keep a log book of their answers to questions posed on the simulations they are studying. In the final sequence of screens, on their individually selected projects to investigate a particular specimen, they will prepare a report on their interpretation of the energy spectra produced from the specimen, with the aim of determining the answer to a specific question posed for each key specimen. For example, in the case of the diabetes cells, it will be necessary for them to determine if trace Sr is absorbed by the cell during a particular treatment.

Evaluation of student knowledge and motivation was conducted in 1994 before introduction of MARCSHOP. This will be repeated in late 1995 following the introduction of the prototype.

5. References

[1] Committee for the Advancement of University Teaching (CAUT) grant 1994.

[2] L.R. Doolittle, Nuclear Instruments and Methods in Physics Research, B9 (1985) 344.

[3] P.M. O'Brien, G. Moloney, A. O'Conner and G.J.F. Legge, Nuclear Instruments and Methods in Physics Research, B77 (1993) 52.

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These pages are maintained by Jon Pearce ( jonmp@unimelb.edu.au), Department of Information Systems. The opinoins on them do not necessarily reflect those of the University of Melbourne. Tel: (613) 8344 1495 Fax: (613) 9349 4596. Last update: September 16, 2003 .