Locate circular objects and integrate their intensity in noisy background

Hi everyone,

I have a noisy image of a SnO2 nanoparticle in projection. Sn atoms have a higher intensity and O atoms a weaker one.

Firstly, I would like to locate the O atoms on the surface (edge) of the nanoparticle adjacent the noisy background (vacuum) (in red box).

Secondly, I would like to create a circular mask of known width in pixels around each O atom and finally calculate their average intensity.

The best output would be the coordinates of the atoms and their corresponding average intensity and an image showing the atom indices.

Your help in any way is appreciated,

Arthur

(A version of the image without the red box is also attached.)