Contrast mechanisms in positron re-emission microscopy with the Brandeis PRM II

The history of positron microscopy and the design of the second generation Brandeis University transmission geometry Positron Re-emission Microscope (PRM II) are discussed together with a number of investigations into possible sources of contrast in PRM images. The PRM II incorporated three significant changes to enable it to achieve higher magnification and resolution than its predecessor. Firstly the immersion objective lens was redesigned to increase the accelerating field at the surface of the sample. This was accomplished by moving the focusing electrode so that it is no longer between the accelerating electrode and the sample. Secondly, a second projector lens was added to increase the magnification of the microscope. Finally a higher resolution detector was installed to help improve the resolution.; The two main candidates which have been proposed in the past as contrast mechanisms for PRM images, tilt boundaries and variations in dislocation density were studied experimentally in thin Ni (100) foils using the PRM together with Transmission Electron Microscopy, (TEM) and Scanning Transmission Electron Microscopy (S-TEM). The behavior of thermalized positrons in the sample was simulated using the diffusion equation and Monte Carlo methods. The first PRM investigation involved examining the changes in relative re-emission between the dark banded structures which are commonly observed in PRM images of Ni (100) and the surrounding foil as a function of implanted beam energy. This investigation showed a decrease in the relative re-emission of the dark bands with increasing beam energy which is not consistent with either of the proposed contrast mechanisms according to our simulations. A second study of contrast in PRM images was conducted by depositing a binary coded gold fiduciary grid on the re-emitting surface of the foil to allow for comparison of identical regions in PRM images and TEM images. The TEM analysis of the regions on the samples studied with the PRM showed an absence of tilt boundaries and a uniform distribution of dislocations in the sample. In addition our simulations showed that the observed dislocation density of ∼109/cm 2 would require dislocations to have a trapping radius on the order of 1000 Å in order to trap a significant fraction of the positrons in our sample. A number of alternative sources of contrast are discussed together with proposals for future PRM investigations.