Editing Rutherford scattering experiments (section)

===Rutherford, Geiger, and Marsden===
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[[Ernest Rutherford]] was Langworthy Professor of Physics at the [[Victoria University of Manchester]]<ref name=PaisInwardBound>{{Cite book |last=Pais |first=Abraham |title=Inward bound: of matter and forces in the physical world |date=2002 |publisher=Clarendon Press [u.a.] |isbn=978-0-19-851997-3 |edition=Reprint |location=Oxford}}</ref>{{rp|188}} (now the [[University of Manchester]]).  He had already received numerous honours for his studies of radiation.  He had discovered the existence of [[alpha rays]], [[beta rays]], and [[gamma rays]], and had proved that these were the consequence of the [[Radioactive decay|disintegration of atoms]]. In 1906, he received a visit from the German physicist [[Hans Geiger]], and was so impressed that he asked Geiger to stay and help him with his research.  [[Ernest Marsden]] was a physics undergraduate student studying under Geiger.<ref name=Heilbron2003pg59>[[#refHeilbron2003|Heilbron (2003)]], p. 59</ref>

In 1908, Rutherford sought to independently determine the charge and mass of alpha particles. To do this, he wanted to count the number of alpha particles and measure their total charge; the ratio would give the charge of a single alpha particle. Alpha particles are too tiny to see, but Rutherford knew about the [[Townsend discharge]], a cascade effect from ionisation leading to a pulse of electric current. On this principle, Rutherford and Geiger designed a simple counting device which consisted of two electrodes in a glass tube containing low pressure gas. (See [[#1908 experiment]].) Every alpha particle that passed through the gas would create a pulse of electrical current that could be detected and counted. It was the forerunner of the [[Geiger-Müller Counter]].<ref name=Heilbron1968>{{Cite journal |last=Heilbron |first=John L. |date=1968 |title=The Scattering of α and β Particles and Rutherford's Atom |url=https://www.jstor.org/stable/41133273 |journal=Archive for History of Exact Sciences |volume=4 |issue=4 |pages=247–307 |doi=10.1007/BF00411591 |jstor=41133273 |issn=0003-9519|url-access=subscription }}</ref>{{rp|261}}

The counter that Geiger and Rutherford built proved unreliable because the alpha particles were being too strongly deflected by their collisions with the molecules of air within the detection chamber. The highly variable trajectories of the alpha particles meant that they did not all generate the same number of ions as they passed through the gas, thus producing erratic readings.  This puzzled Rutherford because he had thought that alpha particles were too heavy to be deflected so strongly. Rutherford asked Geiger to investigate how far matter could scatter alpha rays.<ref name=Heilbron2003>[[#refHeilbron2003|Heilbron (2003)]]</ref>

The experiments they designed involved bombarding metal foil with a beam of alpha particles to observe how the foil scattered them in relation to its thickness and material. They used a phosphorescent screen to measure the trajectories of the particles.  Each impact of an alpha particle on the screen produced a tiny flash of light. Geiger worked in a darkened lab for hours on end, counting these tiny scintillations using a microscope.<ref name=CavendishLaboratory>{{harvnb|Cavendish Laboratory}}.</ref> For the metal foil, they tested a variety of metals, but favoured [[gold]] because they could make the foil very thin, as gold is the most malleable metal.<ref>{{cite book
 |author1=Gary Tibbetts
 |year=2007
 |title=How the Great Scientists Reasoned: The Scientific Method in Action
 |publisher=[[Elsevier]]
 |isbn=978-0-12-398498-2
}}</ref>{{rp|127}}  As a source of alpha particles, Rutherford's substance of choice was [[radium]], which is thousands of times more radioactive than uranium.<ref>Heilbron (2003)</ref>