Characterization of Velocity Map Imaging for High Energy Electrons

                            Anna Gura                              

Supervisors:  Dr. Itzik Ben-Itzhak, Dr. Matthias Kling and Nora Johnson

Research Project

My research dealt with conducting simulations of electrons in a velocity map imaging spectrometer. Using SIMION, an ion and electron optics simulator, I was able to model the path that electrons would take through an applied electric field. For these simulations I was able make technical modifications to the set-up, adjust the electric field, vary the electron energies, and ultimately characterize the spectrometer. 


The standard velocity map imaging (VMI) model consists of a simple design of three electrodes (a repeller, extractor and a ground electrode) and a detector. An electrostatic lens that projects electrons onto the imaging detector can be formed with the application of appropriate voltages to the first two electrodes (the repeller and the extractor). The detector lies at a relatively long distance away from the three electrodes, a distance that is necessary in order to achieve high resolution. This detector is capable of measuring charged particles. However, because the detector is relatively far away from the electrodes, limits are imposed on the highest possible energies that can be detected. Typically, the standard VMI can measure electrons up to about 100 eV.

In recent years a new VMI has been designed and implemented in the JRM lab. This design consists of many more electrodes and can potentially measure electrons with three-times the energy of highest measurable energies of the standard VMI (around 300 eV), with better resolution.  Already the design has been copied in Florida, Korea, and Australia.

The goal of this project was to characterize, through SIMION simulations, the high-energy VMI, including finding the best possible set of voltages for the electrodes which produces the highest resolution for a range of electron energies. The results from these simulations were then compared to data from the standard VMI and experimental results.

Using these simulations and comparisons to the standard VMI model and experimental results, we see that the high energy VMI allows for the use of higher-energy electrons and produces better resolution.

Results and Interpretation

We investigated several cases of voltage meant to be set on the repeller electrode in the high-energy VMI. For reasons of practicality and availability, the main focus was when the repeller was set to -10kV.

The first challenge was finding the best set of voltages for the rest of the electrodes. The purpose of finding this was to create an electrostatic lens which would send the electrons in such a path that would result in the group of electrons being focused at the detector. Once we found the desirable set of voltages and the electron energies that worked well in that electric field, we had enough data to compare our results to those of the standard VMI design.

Using SIMION we assembled a standard VMI model according to the dimensions of the first VMI spectrometer design, published by Eppink and Parker in 1997. After repeating almost the same process, finding the best set of voltages for the electrodes of the standard VMI design, we were able to produce comparable data.

Using these simulations and comparisons of the high-energy VMI to the standard VMI model and experimental results, we see that the high energy VMI allows for the use of higher-energy electrons and produces better resolution.

For a more detailed description of the project and our findings, please see my final report which can be found on this website.