The 1909 experiment Geiger–Marsden experiment

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in 1909 paper, on diffuse reflection of α-particles, geiger , marsden described experiment proved alpha particles can indeed scattered more 90°. in experiment, prepared small conical glass tube (ab) containing radium emanation (radon), radium (actual radium), , radium c (bismuth-214); open end sealed mica. alpha particle emitter. set lead plate (p), behind placed fluorescent screen (s). tube held on opposite side of plate, such alpha particles emitted not directly strike screen. noticed few scintillations on screen, because alpha particles got around plate bouncing off air molecules. placed metal foil (r) side of lead plate. pointed tube @ foil see if alpha particles bounce off , strike screen on other side of plate, , observed increase in number of scintillations on screen. counting scintillations, observed metals higher atomic mass, such gold, reflected more alpha particles lighter ones such aluminium.


geiger , marsden wanted estimate total number of alpha particles being reflected. previous setup unsuitable doing because tube contained several radioactive substances (radium plus decay products) , alpha particles emitted had varying ranges, , because difficult them ascertain @ rate tube emitting alpha particles. time, placed small quantity of radium c (bismuth-214) on lead plate, bounced off platinum reflector (r) , onto screen. found tiny fraction of alpha particles struck reflector bounced onto screen (in case, 1 in 8000).









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