I am new to the fluence concept which is apparently the number of particle passing through an area. However how can this quantity ever be greater than zero....? In my mind there needs to be a timing interval to compute how many particles are passing through the area which I know is referred to as the flux. My professor explains this concept as a snapshot in time but that to me is flux. As the snapshot in time becomes infinitely small this 'fluence' should go to zero... right? Any thoughts???