One the first observations that suggested his nuclear model of the atom to rutherford was that several alpha particles got scattered by metal foils into the backward hemisphere, 7/2 < 0 < r - an observation that was impossible to explain on the basis of rival atomic models, but emerged naturally from the nuclear model. in an early experiment, geiger and marsden measured the fraction of incident alphas scattered into the backward hemisphere off a platinum foil. by integrating the rutherford cross section (14.33) over the backward hemisphere, show that the cross section for scattering with 6 > 90° should be 47 t co (e). using the following numbers, predict the ratio ns,(0 > 90°)/nin,: thickness of platinum foil ti 3 pm, density = 21.4 gram/cm3 , atomic weight = 195, atomic number = 78, energy of incident alphas = 7.8 mev. compare your answer with their estimate that "of the incident a particles about 1 in 8000 was reflected" (that is, scattered into the backward hemisphere). small as this fraction is, it was still far larger than any rival model of the atom could explain.