Editing Rutherford scattering experiments (section)

==Summary==

===Thomson's model of the atom===
[[File:Thomson atom seven electrons.svg|thumb|The "[[plum pudding model]]" of an atom with seven electrons, as imagined by [[J. J. Thomson]] in 1905]]
{{main | Plum pudding model}}
The prevailing model of atomic structure before Rutherford's experiments was devised by [[J.&nbsp;J.&nbsp;Thomson]].<ref name=GilibertiLovisetti/>{{rp|123}} Thomson had discovered the [[electron]] through his work on cathode rays<ref>{{cite journal |author=J. J. Thomson |url=http://web.lemoyne.edu/~GIUNTA/thomson1897.html |title=Cathode rays |journal=Philosophical Magazine |volume=44 |issue=269 |pages=293–316 |year=1897}}</ref> and proposed that they existed within atoms, and an electric current is electrons hopping from one atom to an adjacent one in a series. There logically had to be a commensurate amount of positive charge to balance the negative charge of the electrons and hold those electrons together. Having no idea what the source of this positive charge was, he tentatively proposed that the positive charge was everywhere in the atom, adopting a spherical shape for simplicity.<ref name=GilibertiLovisetti/>{{rp|123}}<ref>J. J. Thomson (1907). ''The Corpuscular Theory of Matter'', p. 103: "In default of exact knowledge of the nature of the way in which positive electricity occurs in the atom, we shall consider a case in which the positive electricity is distributed in the way most amenable to mathematical calculation, i.e., when it occurs as a sphere of uniform density, throughout which the corpuscles are distributed."</ref> Thomson imagined that the balance of electrostatic forces would distribute the electrons throughout this sphere in a more or less even manner. Thomson also believed the electrons could move around in this sphere, and in that regard he likened the substance of the sphere to a liquid.<ref>J. J. Thomson, in a letter to [[Oliver Lodge]] dated 11 April 1904, quoted in Davis & Falconer (1997):<br/>
"With regard to positive electrification I have been in the habit of using the crude analogy of a liquid with a certain amount of cohesion, enough to keep it from flying to bits under its own repulsion. I have however always tried to keep the physical conception of the positive electricity in the background because I have always had hopes (not yet realised) of being able to do without positive electrification as a separate entity and to replace it by some property of the corpuscles.<br/>
When one considers that, all the positive electricity does, on the corpuscular theory, is to provide an attractive force to keep the corpuscles together, while all the observable properties of the atom are determined by the corpuscles one feels, I think, that the positive electrification will ultimately prove superfluous and it will be possible to get the effects we now attribute to it from some property of the corpuscle.<br/>
At present I am not able to do this and I use the analogy of the liquid as a way of picturing the missing forces which is easily conceived and lends itself readily to analysis."</ref> The positive sphere was more of an abstraction than anything material. He did not propose a positively-charged subatomic particle; a counterpart to the electron.

Thomson was never able to develop a complete and stable model that could predict any of the other known properties of the atom, such as emission spectra and valencies.<ref>Thomson (1907). ''The Corpuscular Theory of Matter'', p. 106: "The general problem of finding how ''n'' corpuscles will distribute themselves inside the sphere is very complicated, and I have not succeeded in solving it"</ref> The Japanese scientist [[Hantaro Nagaoka]] rejected Thomson's model on the grounds that opposing charges cannot penetrate each other.<ref>[[#refDaintithGjertsen1999|Daintith & Gjertsen (1999)]], p. 395</ref> He proposed instead that electrons orbit the positive charge like the rings around [[Saturn]].<ref>{{cite journal
 |author1=Hantaro Nagaoka
 |year=1904
 |title=Kinetics of a System of Particles illustrating the Line and the Band Spectrum and the Phenomena of Radioactivity
 |url=http://www.chemteam.info/Chem-History/Nagaoka-1904.html
 |journal=[[Philosophical Magazine]] |series=Series 6
 |volume=7 |issue= 41|pages=445–455
 |doi=10.1080/14786440409463141
 |ref=refNagaoka1904
|url-access=subscription
 }}</ref> However this model was also known to be unstable.<ref name=Heilbron1968/>{{rp|303}}

===Alpha particles and the Thomson atom===
An [[alpha particle]] is a positively charged particle of matter that is spontaneously emitted from certain radioactive elements. Alpha particles are so tiny as to be invisible, but they can be detected with the use of phosphorescent screens, photographic plates, or electrodes. Rutherford discovered them in 1899.<ref>{{cite journal |author=Ernest Rutherford |year=1899 |title=Uranium Radiation and the Electrical conduction Produced by it |journal=Philosophical Magazine |volume=47 |issue=284 |pages=109–163|url=https://archive.org/details/londonedinburgh5471899lon/page/108/mode/2up}}</ref> In 1906, by studying how alpha particle beams are deflected by magnetic and electric fields, he deduced that they were essentially [[helium]] atoms stripped of two electrons.<ref>{{cite journal
 |author1=Ernest Rutherford
 |year=1906
 |title=The Mass and Velocity of the α particles expelled from Radium and Actinium
 |journal=Philosophical Magazine |series=Series 6
 |volume=12
 |issue=70
 |pages=348–371
 |doi=10.1080/14786440609463549 
 |url=https://zenodo.org/record/1430814
 |ref=refRutherford1906
 }}</ref> Thomson and Rutherford knew nothing about the internal structure of alpha particles. At the time, scientists did not know exactly how many electrons a helium atom had (nor atoms of other elements for that matter), so a helium atom stripped of two electrons might still have ten or so left for all they could tell.<ref name=Heilbron1968/>{{rp|280|q=The α particle, because of its mass, was considered a formidable projectile of ''atomic dimensions''}}

Thomson's model was consistent with the experimental evidence available at the time. Thomson studied [[beta particle]] scattering which showed small angle deflections modelled as interactions of the particle with many atoms in succession. Each interaction of the particle with the electrons of the atom and the positive background sphere would lead to a tiny deflection, but many such collisions could add up.<ref name=Heilbron1968/>{{rp|274}} The scattering of alpha particles was expected to be similar.<ref name=Heilbron1968/>{{rp|281|q=...Rutherford and his colleagues followed the multiple scattering approach in the case of α particles as well}} Rutherford's team would show that the multiple scattering model was not needed: single scattering from a compact charge at the centre of the atom would account for all of the scattering data.<ref name=Heilbron1968/>{{rp|289}}

===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>

===Scattering theory and the new atomic model===
[[File:Geiger-Marsden experiment expectation and result.svg|upright=1.5|thumb|Left: Had Thomson's model been correct, all the alpha particles should have passed through the foil with minimal scattering.<br/>Right: What Geiger and Marsden observed was that a small fraction of the alpha particles experienced strong deflection.]]
In a 1909 experiment, Geiger and Marsden discovered that the metal foils could scatter some alpha particles in all directions, sometimes more than 90°.<ref name="BelyaevRoss2021">{{Cite book |last1=Belyaev |first1=Alexander |url=https://link.springer.com/10.1007/978-3-030-80116-8 |title=The Basics of Nuclear and Particle Physics |last2=Ross |first2=Douglas |date=2021 |publisher=Springer International Publishing |isbn=978-3-030-80115-1 |series=Undergraduate Texts in Physics |location=Cham |language=en |doi=10.1007/978-3-030-80116-8|bibcode=2021bnpp.book.....B }}</ref>{{rp|4}} This should have been impossible according to Thomson's model.<ref name="BelyaevRoss2021"/>{{rp|4}} According to Thomson's model, all the alpha particles should have gone straight through.

In Thomson's model of the atom, the sphere of positive charge that fills the atom and encapsulates the electrons is permeable; the electrons could move around in it, after all. Therefore, an alpha particle should be able to pass through this sphere if the electrostatic forces within permit it. Thomson himself did not study how an alpha particle might be scattered in such a collision with an atom, but he did study [[beta particle]] scattering.<ref name=Heilbron1968/>{{rp|277}} He calculated that a beta particle would only experience very small deflection when passing through an atom,<ref name="ThomsonScattering1910"/> and even after passing through many atoms in a row, the total deflection should still be less than 1°.<ref>Beiser (1968). ''Perspectives of Modern Physics'', p. 109</ref> Alpha particles typically have much more momentum than beta particles and therefore should likewise experience only the slightest deflection.<ref>Rutherford (1911): "This scattering is far more marked for the β than for the α particle on account of the much smaller momentum and energy of the former particle."</ref>

The extreme scattering observed forced Rutherford to revise the model of the atom.<ref name=Baily2013/>{{rp|25}}  The issue in Thomson's model was that the charges were too diffuse to produce a sufficiently strong electrostatic force to cause such repulsion. Therefore they had to be more concentrated. In Rutherford's new model, the positive charge does not fill the entire volume of the atom but instead constitutes a tiny nucleus at least 10,000 times smaller than the atom as a whole. All that positive charge concentrated in a much smaller volume produces a much stronger electric field near its surface. The nucleus also carried most of the atom's mass. This meant that it could deflect alpha particles by up to 180° depending on how close they pass. The electrons surround this nucleus, spread throughout the atom's volume. Because their negative charge is diffuse and their combined mass is low, they have a negligible effect on the alpha particle.<ref name=LeoneRobotti2018/>

To verify his model, Rutherford developed a scientific model to predict the intensity of alpha particles at the different angles they scattered coming out of the gold foil, assuming all of the positive charge was concentrated at the centre of the atom. This model was validated in an experiment performed in 1913. His model explained both the beta scattering results of Thomson and the alpha scattering results of Geiger and Marsden.<ref name=Heilbron1968/>{{rp|285}}

===Legacy===
There was little reaction to Rutherford's now-famous 1911 paper in the first years.<ref name="PaisInwardBound"/>{{rp|192}} The paper was primarily about alpha particle scattering in an era before particle scattering was a primary tool for physics. The probability techniques he used and confusing collection of observations involved were not immediately compelling.<ref name=Heilbron1968/>{{rp|304}}

==== Nuclear physics ====
{{main | Nuclear physics| Scattering}}
[[File:AlphaTrackRutherfordScattering3.jpg|thumb|upright=1.0| In a [[cloud chamber]], a 5.3&nbsp;MeV alpha particle track from a [[lead-210|<sup>210</sup>Pb]] source (1) undergoes Rutherford scattering (2), deflecting by an angle of about 30°. It scatters once again (3), and finally comes to rest in the gas. The target nucleus recoils, leaving a short track (2). (cm scale)]]
The first impacts were to encourage new focus on scattering experiments. For example the first results from a [[cloud chamber]], by [[Charles Thomson Rees Wilson|C.T.R. Wilson]] shows alpha particle scattering and also appeared in 1911.<ref>{{Cite journal |title=On an expansion apparatus for making visible the tracks of ionising particles in gases and some results obtained by its use |journal=Proceedings of the Royal Society of London. Series A, Containing Papers of a Mathematical and Physical Character |date=1912-09-19 |volume=87 |issue=595 |pages=277–292 |language=en |doi=10.1098/rspa.1912.0081 |bibcode=1912RSPSA..87..277W |issn=0950-1207 |last1=Wilson |first1=C. T. R. |doi-access=free }}</ref><ref name=Heilbron1968/>{{rp|302}} Over time, particle scattering became a major aspect of theoretical and experimental physics;<ref name=Barrette2021/>{{rp|443}}  Rutherford's concept of a "cross-section" now dominates the descriptions of experimental particle physics.<ref name=GilibertiLovisetti>{{Cite book |last1=Giliberti |first1=Marco |chapter-url=https://link.springer.com/10.1007/978-3-031-57934-9_6 |title=Old Quantum Theory and Early Quantum Mechanics. Challenges in Physics Education. |last2=Lovisetti |first2=Luisa |date=2024 |publisher=Springer Nature Switzerland |isbn=978-3-031-57933-2 |location=Cham |pages=229–268 |language=en |chapter=Rutherford's Hypothesis on the Atomic Structure |doi=10.1007/978-3-031-57934-9_6 }}</ref>{{rp|247|quote=The idea of using the scattering of particles against a target to determine the internal structure of matter, as Rutherford did, turned out to be one of the most prolific ideas of experimental physics of the twentieth century and continues today in particle colliders to be one of the basic methods we have for determining the nature of things.}} The historian [[Silvan S. Schweber]] suggests that Rutherford's approach marked the shift to viewing all interactions and measurements in physics as scattering processes.<ref>{{Cite book |last=Schweber |first=S. S. |title=QED and the men who made it: Dyson, Feynman, Schwinger, and Tomonaga |date=1994 |publisher=Princeton University Press |isbn=978-0-691-03685-4 |series=Princeton series in physics |location=Princeton, N.J}}</ref>{{rp|xiv}} After the nucleus - a term Rutherford introduced in 1912<ref name=PaisInwardBound/>{{rp|192}} - became the accepted model for the core of atoms, Rutherford's analysis of the scattering of alpha particles created a new branch of physics, nuclear physics.<ref name=PaisInwardBound/>{{rp|223}}

==== Atomic model ====
{{main|Rutherford model|Rutherford–Bohr model}}
Rutherford's new atom model caused no stir.<ref name=Baily2013/>{{rp|28}} Rutherford explicitly ignores the electrons, only mentioning [[Hantaro Nagaoka]]'s [[Saturnian model]] of electrons orbiting a tiny "sun", a model that had been previously rejected as mechanically unstable. By ignoring the electrons Rutherford also ignores any potential implications for atomic spectroscopy for chemistry.<ref name="PaisInwardBound"/>{{rp|302}} Rutherford himself did not press the case for his atomic model: his own 1913 book on "Radioactive substances and their radiations" only mentions the atom twice; other books by other authors around this time focus on Thomson's model.<ref>Andrade, Edward Neville Da Costa. "The Rutherford Memorial Lecture, 1957." Proceedings of the Royal Society of London. Series A. Mathematical and Physical Sciences 244.1239 (1958): 437-455.</ref>{{rp|446}}  
	
The impact of Rutherford's nuclear model came after [[Niels Bohr]] arrived as a post-doctoral student in Manchester at Rutherford's invitation. Bohr dropped his work on the Thomson model in favour of Rutherford's nuclear model, developing the [[Rutherford–Bohr model]] over the next several years.  Eventually Bohr incorporated early ideas of [[quantum mechanics]] into the model of the atom, allowing prediction of electronic spectra and concepts of chemistry.<ref name=Heilbron1968/>{{rp|304}}

[[Hantaro Nagaoka]], who had proposed a Saturnian model of the atom, wrote to Rutherford from Tokyo in 1911: "I have been struck with the simpleness of the apparatus you employ and the brilliant results you obtain."<ref>Letter from Hantaro Nagaoka to Ernest Rutherford, 22 February 1911. Quoted in Eve (1939), p. 200</ref> The astronomer [[Arthur Eddington]] called Rutherford's discovery the most important scientific achievement since [[Democritus]] proposed the atom ages earlier.<ref name=Reeves2008/> Rutherford has since been hailed as "the father of nuclear physics".<ref name=Father>{{cite web |title=Ernest Rutherford |url=https://ehs.msu.edu/lab-clinic/rad/hist-figures/rutherford.html |website=Environmental Health and Safety Office of Research Regulatory Support |publisher=Michigan State University |access-date=23 June 2023 |archive-date=22 June 2023 |archive-url=https://web.archive.org/web/20230622163634/https://ehs.msu.edu/lab-clinic/rad/hist-figures/rutherford.html |url-status=live }}</ref><ref>{{cite web |title=Ernest Rutherford: father of nuclear science |url=https://media.newzealand.com/en/story-ideas/ernest-rutherford-father-of-nuclear-science/ |website=New Zealand Media Resources |archive-url=https://web.archive.org/web/20210612184534/https://media.newzealand.com/en/story-ideas/ernest-rutherford-father-of-nuclear-science/ |archive-date=12 June 2021 |language=en |url-status=dead}}</ref>

In a lecture delivered on 15 October 1936 at Cambridge University,<ref>''Report on the Activities of the History of Science Lectures Committee 1936–1947'', Whipple Museum Papers, Whipple Museum for the History of Science, Cambridge, C62 i.<br />The report lists two lectures, on October 8 and 15. The lecture on atomic structure was likely the one delivered on the 15th.</ref><ref>''Cambridge University Reporter'', 7 October 1936, p. 141<br/>The lecture took place in the lecture room of the Physiological Laboratory at 5 pm.</ref> Rutherford described his shock at the results of the [[#1909 experiment|1909 experiment]]:

{{blockquote|Then I remember two or three days later Geiger coming to me in great excitement and saying, "We have been able to get some of the α-particles coming backwards...". It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you. On consideration, I realised that this scattering backward must be the result of a single collision, and when I made calculations I saw that it was impossible to get anything of that order of magnitude unless you took a system in which  the greater part of the mass of the atom was concentrated in a minute nucleus. It was then that I had the idea of an atom with a minute massive centre, carrying a charge.<ref>''The Development of the Theory of Atomic Structure'' (Rutherford 1936). Reprinted in [https://archive.org/details/backgroundtomode032734mbp/page/n85/mode/2up ''Background to Modern Science: Ten Lectures at Cambridge arranged by the History of Science Committee 1936'']</ref>}}

Rutherford's claim of surprise makes for a good story but by the time of the Geiger-Marsden experiment, the result confirmed suspicions Rutherford developed from previous experiments.<ref name=Heilbron1968/>{{rp|265}}
{{Clear}}