Scanning transmission ion microscopy (STIM), a new technique for density mapping of microstructures

The density of a cosmic dust particle is an important parameter in calculating the orbital evolution timescale and temperature profile on atmospheric deceleration. Sutton and Flynn inferred the densities of 12 stony micrometeorites from the stratospheric dust collection using synchrotron x-ray fluorescence. This method used irons mass determinations to infer the particle mass and optical microscope measurements to determine volumes. These results coupled with density measurements by others suggest that stony micrometeorite densities fall into two distinct density groups, with mean values of 0.6 and 1.9 g/cm{sup 3}. Such a bimodal distribution has dramatic implications for natural segregation of dust by gravitational resonances and interpretation of degree of heating in terms of proportions of asteroidal and cometary populations. The synchrotron-inferred densities have estimated uncertainties of {plus minus} 25% due largely to the difficulty in determining the volumes of these irregular objects. We are presently exploring the value of Scanning Transmission Ion Microscope (STIM) for directly mapping the density distribution in individual stratospheric particles and other micrometer-sized objects. In this approach, the energy loss experienced by protons as they traverse the specimen is used to produce 3-dimensional microtomographic images of the internal structure of objects. One of the key advantages ofmore » this technique is the ability to focus this charged particle beam to below 100 nm and thereby attain extremely high resolution tomographic images.« less