Problems are listed in approximate order of difficulty. A single dot (•) indicates straightforward problems involving just one main concept and sometimes requiring no more than substitution of numbers in the appropriate formula. Two dots (••) identify problems that are slightly more challenging and usually involve more than one concept. Three dots (•••) indicate problems that are distinctly more challenging, either because they are intrinsically difficult or involve lengthy calculations. Needless to say, these distinctions are hard to draw and are only approximate.
•• When the Joliot-Curies observed the reaction α + 9Be → n + 12C, they suggested (erroneously) that the neutral particle was a photon. Chadwick’s main argument against this concerned the large momentum (up to 100 MeV/c) of the protons that these supposed photons ejected from wax. Assuming that it was photons that ejected the protons and given that a proton gets the largest momentum in a head-on collision, do the following: (a) To a fair approximation, the photon would bounce back with its energy unchanged; in this approximation, what is the incident photon energy required to eject a proton with momentum 100 MeV/c? (b) Using conservation of energy and momentum, redo this calculation without the approximation of part (a).
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