[fishtank]

You might want tot look at this:

http://www.radiation-scott.org/deposition/particles.htm

Then, you can do a search on the term "aerodynamic diameter".

A (spherical) heavy particle behaves exactly like a spherical water droplet; the relation between the diameters is based on the square root of the material density.

So, yes, you can easily have a particle of lead that has a 10 micron (equivalent) aerodynamic diameter that can float in the air.

[Herakles]

"Then, you can do a search on the term "aerodynamic diameter". "

Thanks for the info, however much of the research would be oriented to a high energy explosion such as a Nuke where you very well would have a large population of small particles. A dirty bomb however is not a nuke and will not produce this large amount of small particles due to the very nature of the detonation not to mention temperature; low energy in thousands of degrees versus high energy in millions of degrees'

Also, temperature of the explosion would play a significant part in dispersal; low temperature small yields would more or less scatter particles in the immediate area where as high temperature, high yield would result in a chimney effect resulting in a large dispersal radius.

A quick understanding of dispersal could be calculated using standard dispersal formula for distribution from smoke stacks, and as I remember, the higher the stack, the larger the dispersal radius. A low energy ground detonation would result in minimal dispersal of small particles and the majority of the material would be in the immediate area.

Particle size is not everything and when you have done some of these calculations before, you get a feel for what would happen.