cross section (classic and quantic)

[sanctus]

Can anyone give an intuitive interpretation of the classic and most importantly the quantum cross section? As well as of the differential one?

[Bo]

Classical:
The cross section is just a measure of the diameter of the target particle.

Quantum
The intuitive view is the same, apart from the fact that in quantum theory the target and the beam particles should be not seen as solid objects, but as waves, described by schroedingers equation. The cross section is an observable, of which an expactation value can be calculated, depending on the 2 wave functions. (i hope this is intuitively enough...)

(note that the the definitions of the cross section is the same in both cases, just the underlying theory changes)

Differential
In most experiments only a fraction of the outgoing particles is seen. The differential corss section is the part of the cross section that bounces particles to this area. (the diff cs integrated over all angles is equal to the normal cs)

Bo

[sanctus]

Thanks very much, it is an interpretation like the one I was looking for.

Because as you might have guessed I'm doing the theory of diffusion (luckily the case of particles without spin...) and have to admit that I never saw the quantum cs like that, I thought it was more kind of a conditional probability.