The then-conventional use of a single detector rotated about an illuminated sample with intensities collected at specific angles was called " differential light scattering " after the quantum mechanical term " differential cross section ", " ? ( ? ) " expressed in milli-barns / steradian.
22.
Is the magnitude of the relative momenta ( see relative velocity for more on this concept ), and " I " ( " g ", ? ) is the differential cross section of the collision, in which the relative momenta of the colliding particles turns through an angle ? into the element of the solid angle " d " ?, due to the collision.
23.
Where \ frac { d \ sigma } { d \ Omega } is a differential cross section, d \ Omega is an infinitesimal solid angle element, \ alpha is the fine structure constant ( ~ 1 / 137.04 ), \ theta is the scattering angle; r _ c = \ hbar / m _ e c is the " reduced " Compton wave length of the electron ( ~ 0.38616 pm ); m _ e is the mass of an electron ( ~ 511 keV / c ^ 2 ); and P ( E _ \ gamma, \ theta ) is the ratio of photon energy after and before the collision:
