Remove background, leave foreground intact

[Blue]

Hi everyone,

I'm working on granular flows in a rotating drum for my PhD. In order to aid the particle image velocimetry (PIV) analysis, I'd like to be able to remove the background of my experiments. The drum itself has been painted black and we are using white beach sand; what I'd like to be able to do is say to ImageJ "everything that's darker than X in this greyscale image, set to black. Everything else, leave it alone". The result should be the removal of the equipment, leaving just the sand.

I've used the Threshold tool, but I've found that that removes the background but also sets the foreground to the opposite colour, which is no good for PIV as there are no particles left to track!

Any help greatly appreciated.