Editing Enrico Fermi (section)

== Professor in Rome ==
[[File:Ragazzi di via Panisperna cropped.jpg|thumb|upright=1.2|Fermi and his research group (the [[Via Panisperna boys]]) in the courtyard of Rome University's Physics Institute in Via Panisperna, {{circa}} 1934. From left to right: [[Oscar D'Agostino]], [[Emilio Segrè]], [[Edoardo Amaldi]], [[Franco Rasetti]] and Fermi]]

Professorships in Italy were granted by competition (''{{lang|it|concorso}}'') for a vacant chair, the applicants being rated on their publications by a committee of professors. Fermi applied for a chair of mathematical physics at the [[University of Cagliari]] on [[Sardinia]] but was narrowly passed over in favour of [[Giovanni Giorgi]].{{sfn|Fermi|1954|pp=37–38}} In 1926, at the age of 24, he applied for a professorship at the Sapienza University of Rome. This was a new chair, one of the first three in theoretical physics in Italy, that had been created by the Minister of Education at the urging of professor [[Orso Mario Corbino]], who was the university's professor of experimental physics, the director of the Institute of Physics, and a member of [[Benito Mussolini]]'s cabinet. Corbino, who also chaired the selection committee, hoped that the new chair would raise the standard and reputation of physics in Italy.{{sfn|Segrè|1970|p=45}} The committee chose Fermi ahead of Enrico Persico and [[Aldo Pontremoli]],{{sfn|Fermi|1954|p=38}} and Corbino helped Fermi recruit his team, which was soon joined by notable students such as [[Edoardo Amaldi]], [[Bruno Pontecorvo]], [[Ettore Majorana]] and [[Emilio Segrè]], and by Franco Rasetti, whom Fermi had appointed as his assistant.{{sfn|Alison|1957|p=127}} They soon were nicknamed the "[[Via Panisperna boys]]" after the street where the Institute of Physics was located.<ref>{{cite web |url=http://www.phys.uniroma1.it/DipWeb/museo/fermi.html |title=Enrico Fermi e i ragazzi di via Panisperna |language=it |publisher=University of Rome |access-date=20 January 2013 |archive-date=20 February 2021 |archive-url=https://web.archive.org/web/20210220125803/https://www.phys.uniroma1.it/DipWeb/museo/fermi.html |url-status=live }}</ref>

Fermi married [[Laura Capon]], a science student at the university, on 19 July 1928.{{sfn|Segrè|1970|p=61}} They had two children: Nella, born in January 1931, and Giulio, born in February 1936.{{sfn|Cooper|1999|pp=38–39}} On 18 March 1929, Fermi was appointed a member of the [[Royal Academy of Italy]] by Mussolini, and on 27 April he joined the [[Fascist Party]]. He later opposed Fascism when the 1938 [[Italian Racial Laws|racial laws]] were promulgated by Mussolini in order to bring Italian Fascism ideologically closer to German [[Nazism]]. These laws threatened Laura, who was Jewish, and put many of Fermi's research assistants out of work.{{sfn|Alison|1957|p=130}}<ref>{{cite web |url=http://fermi.lib.uchicago.edu/fermibiog.htm |publisher=[[University of Chicago]] |title=About Enrico Fermi |access-date=20 January 2013 |archive-date=21 December 2011 |archive-url=https://web.archive.org/web/20111221232508/http://fermi.lib.uchicago.edu/fermibiog.htm |url-status=live }}</ref><ref>{{cite news|url=http://archiviostorico.corriere.it/2001/ottobre/02/Cosi_Fermi_scopri_natura_vessatoria_co_0_0110022882.shtml |author-link=Paolo Mieli |first=Paolo |last=Mieli |title=''Così Fermi scoprì la natura vessatoria del fascismo'' |language=it |newspaper=[[Corriere della Sera]] |date=2 October 2001 |access-date=20 January 2013 |url-status=dead |archive-url=https://web.archive.org/web/20131019204537/http://archiviostorico.corriere.it/2001/ottobre/02/Cosi_Fermi_scopri_natura_vessatoria_co_0_0110022882.shtml |archive-date=19 October 2013 }}</ref><ref>{{cite web|url=http://www.archivi.beniculturali.it/DGA-free/Strumenti/Strumenti_CLXVII.pdf |author=Direzione generale per gli archivi |title=''Reale accademia d'Italia:inventario dell'archivio'' |language=it |year=2005 |publisher=Ministero per i beni culturali e ambientali |location=Rome |page=xxxix |access-date=20 January 2013 |url-status=dead |archive-url=https://web.archive.org/web/20120907065146/http://www.archivi.beniculturali.it/DGA-free/Strumenti/Strumenti_CLXVII.pdf |archive-date=7 September 2012 }}</ref><ref>{{cite web|title=A Legal Examination of Mussolini's Race Laws|url=http://primolevicenter.org/printed-matter/a-legal-examination-of-mussolinis-race-laws/|website=Printed Matter|date=5 June 2014 |publisher=Centro Primo Levi|access-date=7 August 2015|archive-date=17 August 2015|archive-url=https://web.archive.org/web/20150817005628/http://primolevicenter.org/printed-matter/a-legal-examination-of-mussolinis-race-laws/|url-status=live}}</ref>

During their time in Rome, Fermi and his group made important contributions to many practical and theoretical aspects of physics. In 1928, he published his ''Introduction to Atomic Physics'' (''{{lang|it|Introduzione alla fisica atomica}}''), which provided Italian university students with an up-to-date and accessible text. Fermi also conducted public lectures and wrote popular articles for scientists and teachers in order to spread knowledge of the new physics as widely as possible.{{sfn|Bonolis|2001|pp=333–335}} Part of his teaching method was to gather his colleagues and graduate students together at the end of the day and go over a problem, often from his own research.{{sfn|Bonolis|2001|pp=333–335}}{{sfn|Amaldi|2001|p=38}} A sign of success was that foreign students now began to come to Italy. The most notable of these was the German physicist [[Hans Bethe]],{{sfn|Fermi|1954|p=217}} who came to Rome as a Rockefeller Foundation fellow, and collaborated with Fermi on a 1932 paper "On the Interaction between Two Electrons" ({{Langx|de|Über die Wechselwirkung von Zwei Elektronen}}).<ref>{{Cite journal |last1=Bethe |first1=Hans |last2=Fermi |first2=Enrico |date=1932 |title=Über die Wechselwirkung von zwei Elektronen |url=http://link.springer.com/10.1007/BF01348919 |journal=Zeitschrift für Physik |language=de |volume=77 |issue=5–6 |pages=296–306 |doi=10.1007/BF01348919 |bibcode=1932ZPhy...77..296B |issn=1434-6001}}</ref>{{sfn|Bonolis|2001|pp=333–335}}

At this time, physicists were puzzled by [[beta decay]], in which an [[electron]] was emitted from the [[atomic nucleus]]. To satisfy the law of [[conservation of energy]], Pauli postulated the existence of an invisible particle with no charge and little or no mass that was also emitted at the same time. Fermi took up this idea, which he developed in a tentative paper in 1933, and then a longer paper the next year that incorporated the postulated particle, which Fermi called a "[[neutrino]]".{{sfn|Amaldi|2001|pp=50–51}}{{sfn|Bonolis|2001|p=346}}<ref name="Beta decay">{{cite journal| last = Fermi| first = E.| title = Fermi's Theory of Beta Decay (English translation by Fred L. Wilson, 1968)| journal = [[American Journal of Physics]]| year = 1968| url = http://microboone-docdb.fnal.gov/cgi-bin/RetrieveFile?docid=953;filename=FermiBetaDecay1934.pdf;version=1| access-date = 20 January 2013| doi = 10.1119/1.1974382| volume = 36| issue = 12| page = 1150| bibcode = 1968AmJPh..36.1150W| archive-date = 12 May 2013| archive-url = https://web.archive.org/web/20130512011303/http://microboone-docdb.fnal.gov/cgi-bin/RetrieveFile?docid=953;filename=FermiBetaDecay1934.pdf;version=1| url-status = live| url-access = subscription}}</ref> His theory, later referred to as [[Fermi's interaction]], and still later as the theory of the [[weak interaction]], described one of the four [[fundamental forces of nature]]. The neutrino was detected after his death, and his interaction theory showed why it was so difficult to detect. When he submitted his paper to the British journal ''[[Nature (journal)|Nature]]'', that journal's editor turned it down because it contained speculations which were "too remote from physical reality to be of interest to readers".{{sfn|Bonolis|2001|p=346}} According to Fermi's biographer David N. Schwartz, it is at least strange that Fermi seriously requested publication from the journal, since at that time ''Nature'' only published short notes on articles of this kind, and was not suitable for the publication of even a new physical theory. More suitable, if anything, would have been the ''[[Proceedings of the Royal Society of London]]''. He agrees with some scholars' hypothesis, according to which the rejection of the British magazine convinced his young colleagues (some of them Jews and leftists) to give up the boycott of German scientific magazines, after [[Hitler]] came to power in January 1933.{{sfn|Schwartz|2021|p=154}} Thus Fermi saw the theory published in Italian and German before it was published in English.{{sfn|Alison|1957|p=127}}

In the introduction to the 1968 English translation, physicist Fred L. Wilson noted that:{{quote|Fermi's theory, aside from bolstering Pauli's proposal of the neutrino, has a special significance in the history of modern physics. One must remember that only the naturally occurring β emitters were known at the time the theory was proposed. Later when positron decay was discovered, the process was easily incorporated within Fermi's original framework. On the basis of his theory, the capture of an orbital electron by a nucleus was predicted and eventually observed. With time, experimental data accumulated significantly. Although peculiarities have been observed many times in β decay, Fermi's theory always has been equal to the challenge.<br />The consequences of the Fermi theory are vast. For example, β spectroscopy was established as a powerful tool for the study of nuclear structure. But perhaps the most influential aspect of this work of Fermi is that his particular form of the β interaction established a pattern that has been appropriate for the study of other types of interactions. It was the first successful theory of the creation and annihilation of material particles. Previously, only photons had been known to be created and destroyed.<ref name="Beta decay" />}}

In January 1934, [[Irène Joliot-Curie]] and [[Frédéric Joliot]] announced that they had bombarded elements with [[alpha particle]]s and induced [[radioactivity]] in them.<ref name="JoliotCurie1934a">{{cite journal|last2=Joliot|first2=Frédéric|last1=Joliot-Curie|first1=Irène|title=Un nouveau type de radioactivité|trans-title=A new type of radioactivity|journal=Comptes Rendus Hebdomadaires des Séances de l'Académie des Sciences|volume=198|issue=January–June 1934|date=15 January 1934|language=fr|pages=254–256|url=http://gallica.bnf.fr/ark:/12148/bpt6k31506/f254.image|access-date=19 October 2013|archive-date=20 February 2021|archive-url=https://web.archive.org/web/20210220125607/https://gallica.bnf.fr/ark:/12148/bpt6k31506/f254.image|url-status=live}}</ref><ref name="JoliotCurie1934b">{{cite journal|last1=Joliot|first1=Frédéric|last2=Joliot-Curie|first2=Irène|title=Artificial Production of a New Kind of Radio-Element|journal=Nature|volume=133|issue=3354|year=1934|pages=201–202|doi=10.1038/133201a0|url=http://jnm.snmjournals.org/content/5/2/xii.full.pdf|bibcode=1934Natur.133..201J|s2cid=4096977|doi-access=free|access-date=19 October 2013|archive-date=23 November 2020|archive-url=https://web.archive.org/web/20201123021722/http://jnm.snmjournals.org/content/5/2/xii.full.pdf|url-status=live}}</ref> By March, Fermi's assistant [[Gian-Carlo Wick]] had provided a theoretical explanation using Fermi's theory of beta decay. Fermi decided to switch to experimental physics, using the [[neutron]], which [[James Chadwick]] had discovered in 1932.{{sfn|Amaldi|2001a|pp=152–153}} In March 1934, Fermi wanted to see if he could induce radioactivity with Rasetti's [[polonium]]-[[beryllium]] [[neutron source]]. Neutrons had no electric charge, and so would not be deflected by the positively charged nucleus. This meant that they needed much less energy to penetrate the nucleus than charged particles, and so would not require a [[particle accelerator]], which the Via Panisperna boys did not have.{{sfn|Bonolis|2001|pp=347–351}}{{sfn|Amaldi|2001a|pp=153–156}}

[[File:RasettiFermiSegre.JPG|thumb|upright=1.3|Enrico Fermi between [[Franco Rasetti]] (left) and [[Emilio Segrè]] in [[academic dress]]]]
Fermi had the idea to resort to replacing the polonium-beryllium neutron source with a [[radon]]-beryllium one, which he created by filling a glass bulb with beryllium powder, evacuating the air, and then adding 50 m[[Curie (unit)|Ci]] of radon gas, supplied by {{ill|Giulio Cesare Trabacchi|it}}.{{sfn|Segrè|1970|p=73}}<ref name="De Gregorio2005">{{cite journal|last1=De Gregorio|first1=Alberto G.|title=Neutron physics in the early 1930s|journal=Historical Studies in the Physical and Biological Sciences|volume=35 |issue=2|year=2005 |pages=293–340 |doi=10.1525/hsps.2005.35.2.293 |arxiv=physics/0510044|bibcode=2005physics..10044D|s2cid=119489980}}</ref> This created a much stronger neutron source, the effectiveness of which declined with the 3.8-day [[half-life]] of radon. He knew that this source would also emit [[gamma ray]]s, but, on the basis of his theory, he believed that this would not affect the results of the experiment. He started by bombarding [[platinum]], an element with a high [[atomic number]] that was readily available, without success. He turned to [[aluminium]], which emitted an alpha particle and produced [[sodium]], which then decayed into [[magnesium]] by beta particle emission. He tried [[lead]], without success, and then [[fluorine]] in the form of [[calcium fluoride]], which emitted an alpha particle and produced [[nitrogen]], decaying into [[oxygen]] by beta particle emission. In all, he induced radioactivity in 22 different elements.<ref>{{cite journal |journal=Physics in Perspective |title=Enrico Fermi's Discovery of Neutron-Induced Artificial Radioactivity: The Influence of His Theory of Beta Decay|last1=Guerra |first1=Francesco |last2=Robotti |first2=Nadia|author2-link=Nadia Robotti |date=December 2009 |volume=11 |issue=4 |doi=10.1007/s00016-008-0415-1 |pages=379–404|bibcode = 2009PhP....11..379G |s2cid=120707438}}</ref> Fermi rapidly reported the discovery of neutron-induced radioactivity in the Italian journal ''La Ricerca Scientifica'' on 25 March 1934.<ref name="De Gregorio2005" /><ref>{{cite journal|first=Enrico|last=Fermi|title=Radioattività indotta da bombardamento di neutroni|journal=La Ricerca Scientifica|volume=1|issue=5|date=25 March 1934|page=283|url=http://www.phys.uniroma1.it/DipWeb/museo/collezione%20Fermi/documento2.htm|language=it|access-date=20 October 2013|archive-date=24 February 2021|archive-url=https://web.archive.org/web/20210224071249/https://www.phys.uniroma1.it/DipWeb/museo/collezione%20Fermi/documento2.htm|url-status=dead}}</ref><ref name="FermiAmaldi1934">{{Cite journal | last1 = Fermi | first1 = E. | last2 = Amaldi | first2 = E. | last3 = d'Agostino | first3 = O. | last4 = Rasetti | first4 = F. | last5 = Segre | first5 = E. | title = Artificial Radioactivity Produced by Neutron Bombardment | doi = 10.1098/rspa.1934.0168 | journal = Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences | volume = 146 | issue = 857 | page = 483 | year = 1934 |bibcode = 1934RSPSA.146..483F | doi-access = free }}</ref>

The natural radioactivity of [[thorium]] and [[uranium]] made it hard to determine what was happening when these elements were bombarded with neutrons but, after correctly eliminating the presence of elements lighter than uranium but heavier than lead, Fermi concluded that they had created new elements, which he called [[ausenium and hesperium]].{{sfn|Bonolis|2001|pp=347–349}}{{sfn|Amaldi|2001a|pp=153–156}} The chemist [[Ida Noddack]] suggested that some of the experiments could have produced lighter elements than lead rather than new, heavier elements. Her suggestion was not taken seriously at the time because her team had not carried out any experiments with uranium or built the theoretical basis for this possibility. At that time, fission was thought to be improbable if not impossible on theoretical grounds. While physicists expected elements with higher atomic numbers to form from neutron bombardment of lighter elements, nobody expected neutrons to have enough energy to split a heavier atom into two light element fragments in the manner that Noddack suggested.{{sfn|Amaldi|2001a|pp=161–162}}{{sfn|Bonolis|2001|pp=347–349}}

[[File:Beta-minus Decay.svg|thumb|left|[[Beta decay]]. A [[neutron]] decays into a [[proton]], and an [[electron]] is emitted. In order for the total energy in the system to remain the same, Pauli and Fermi postulated that a [[neutrino]] (<math>\bar{\nu}_e</math>) was also emitted.]]
The Via Panisperna boys also noticed some unexplained effects. The experiment seemed to work better on a wooden table than on a marble tabletop. Fermi remembered that Joliot-Curie and Chadwick had noted that [[paraffin wax]] was effective at slowing neutrons, so he decided to try that. When neutrons were passed through paraffin wax, they induced a hundred times as much radioactivity in [[silver]] compared with when it was bombarded without the paraffin. Fermi guessed that this was due to the hydrogen atoms in the paraffin. Those in wood similarly explained the difference between the wooden and the marble tabletops. This was confirmed by repeating the effect with water. He concluded that collisions with hydrogen atoms slowed the neutrons.{{sfn|Bonolis|2001|pp=347–352}}{{sfn|Amaldi|2001a|pp=153–156}} The lower the atomic number of the nucleus it collides with, the more energy a neutron loses per collision, and therefore the fewer collisions that are required to slow a neutron down by a given amount.<ref>{{cite web |url=http://energyfromthorium.com/2007/02/13/a-few-good-moderators-the-numbers/ |title=A Few Good Moderators: The Numbers |publisher=The Energy From Thorium Foundation |access-date=24 September 2013 |date=13 February 2007 |archive-date=24 February 2021 |archive-url=https://web.archive.org/web/20210224071432/https://energyfromthorium.com/2007/02/13/a-few-good-moderators-the-numbers/ |url-status=live }}</ref> Fermi realised that this induced more radioactivity because [[slow neutron]]s were more easily [[neutron capture|captured]] than fast ones. He developed a [[diffusion equation]] to describe this, which became known as the [[Fermi age equation]].{{sfn|Bonolis|2001|pp=347–352}}{{sfn|Amaldi|2001a|pp=153–156}}

In 1938, Fermi received the [[Nobel Prize in Physics]] at the age of 37 for his "demonstrations of the existence of new radioactive elements produced by neutron irradiation, and for his related discovery of [[nuclear reaction]]s brought about by slow neutrons".{{sfn|Cooper|1999|p=51}} After Fermi received the prize in [[Stockholm]], he did not return home to Italy but rather continued to New York City with his family in December 1938, where they applied for permanent residency. The decision to move to America and become US citizens was due primarily to the racial laws in Italy.{{sfn|Alison|1957|p=130}}{{sfn|Sullivan|2016|p=19}}