Vb=removed thickness/etching time

Thank you Dr. Hiwa Hamad for your answer. 

The answer is density function theory

Thank you Mr. Ali Biganeh 

Dear Dr. Aness

the best method to determine the bulk etch rate f (VB) of the unaffected surface of the detector is given as follows: VB=D/2t

Where the diameter of the track D is computed by taking the average radius of several tracks viewed by camera, and t is the etching time.

my Ms.C. student Layth is apply this method in his research.