Editing Beta decay (section)

===Neutrinos===

The study of beta decay provided the first physical evidence for the existence of the [[neutrino]]. In both alpha and gamma decay, the resulting alpha or gamma particle has a narrow energy [[Frequency distribution|distribution]], since the particle carries the energy from the difference between the initial and final nuclear states. However, the kinetic energy distribution, or spectrum, of beta particles measured by [[Lise Meitner]] and [[Otto Hahn]] in 1911 and by [[Jean Danysz]] in 1913 showed multiple lines on a diffuse background. These measurements offered the first hint that beta particles have a continuous spectrum.<ref name="Jensen">{{cite book
 |last1=Jensen |first1=C.
 |year=2000
 |title=Controversy and Consensus: Nuclear Beta Decay 1911-1934
 |url=https://www.springer.com/birkhauser/physics/book/978-3-7643-5313-1
 |publisher=[[Birkhäuser Verlag]]
 |isbn=978-3-7643-5313-1
}}</ref> In 1914, [[James Chadwick]] used a magnetic [[spectrometer]] with one of [[Hans Geiger|Hans Geiger's]] new [[Geiger counter|counters]] to make more accurate measurements which showed that the spectrum was continuous.<ref name="Jensen" /><ref>{{cite journal |last=Chadwick |first=J. |year=1914 |title=Intensitätsverteilung im magnetischen Spektren der β-Strahlen von Radium B + C |journal=[[Verhandlungen der Deutschen Physikalischen Gesellschaft]] |language=de |volume=16 |pages=383–391}}</ref> The results, which appeared to be in contradiction to the [[law of conservation of energy]], were validated by means of calorimetric measurements in 1929 by [[Lise Meitner]] and [[Wilhelm Orthmann]].<ref>{{Cite journal |last1=Meitner |first1=Lise |last2=Orthmann |first2=Wilhelm |date=1930-03-01 |title=Über eine absolute Bestimmung der Energie der primären β-Strahlen von Radium E |url=https://link.springer.com/article/10.1007/BF01339819 |journal=Zeitschrift für Physik |language=de |volume=60 |issue=3 |pages=143–155 |doi=10.1007/BF01339819 |bibcode=1930ZPhy...60..143M |issn=0044-3328}}</ref> If beta decay were simply electron emission as assumed at the time, then the energy of the emitted electron should have a particular, well-defined value.<ref name=Brown>{{cite journal
 |last1=Brown |first1=L. M.
 |year=1978
 |title=The idea of the neutrino
 |journal=[[Physics Today]]
 |volume=31 |issue=9 |pages=23–8
 |bibcode=1978PhT....31i..23B
 |doi=10.1063/1.2995181
}}</ref> For beta decay, however, the observed broad distribution of energies suggested that energy is lost in the beta decay process. This spectrum was puzzling for many years.

A second problem is related to the [[conservation of angular momentum]]. Molecular band spectra showed that the [[nuclear spin]] of [[nitrogen-14]] is 1 (i.e., equal to the [[reduced Planck constant]]) and more generally that the spin is integral for nuclei of even [[mass number]] and half-integral for nuclei of odd mass number. This was later explained by the [[Discovery of the neutron#Proton–neutron model of the nucleus|proton-neutron model of the nucleus]].<ref name=Brown/> Beta decay leaves the mass number unchanged, so the change of nuclear spin must be an integer. However, the electron spin is 1/2, hence angular momentum would not be conserved if beta decay were simply electron emission.

From 1920 to 1927, [[Charles Drummond Ellis]] (along with Chadwick and colleagues) further established that the beta decay spectrum is continuous. In 1933, Ellis and [[Nevill Mott]] obtained strong evidence that the beta spectrum has an effective upper bound in energy. [[Niels Bohr]] had suggested that the beta spectrum could be explained if [[conservation of energy]] was true only in a statistical sense, thus this [[Laws of science|principle]] might be violated in any given decay.<ref name=Brown/>{{rp|27}} However, the upper bound in beta energies determined by Ellis and Mott ruled out that notion. Now, the problem of how to account for the variability of energy in known beta decay products, as well as for conservation of momentum and angular momentum in the process, became acute.

In a [[Electron neutrino#Pauli's letter|famous letter]] written in 1930, [[Wolfgang Pauli]] attempted to resolve the beta-particle energy conundrum by suggesting that, in addition to electrons and protons, atomic nuclei also contained an extremely light neutral particle, which he called the neutron. He suggested that this "neutron" was also emitted during beta decay (thus accounting for the known missing energy, momentum, and angular momentum), but it had simply not yet been observed. In 1931, [[Enrico Fermi]] renamed Pauli's "neutron" the "neutrino" ('little neutral one' in Italian). In 1933, Fermi published his landmark [[Fermi's interaction|theory for beta decay]], where he applied the principles of quantum mechanics to matter particles, supposing that they can be created and annihilated, just as the light quanta in atomic transitions. Thus, according to Fermi, neutrinos are created in the beta-decay process, rather than contained in the nucleus; the same happens to electrons. The neutrino interaction with matter was so weak that detecting it proved a severe experimental challenge. Further indirect evidence of the existence of the neutrino was obtained by observing the recoil of nuclei that emitted such a particle after absorbing an electron. Neutrinos were finally detected directly in 1956 by the American physicists [[Clyde Cowan]] and [[Frederick Reines]] in the [[Cowan–Reines neutrino experiment]].<ref>{{cite journal
 |last1=Cowan |first1=C. L. Jr.
 |last2=Reines |first2=F.
 |last3=Harrison |first3=F. B.
 |last4=Kruse |first4=H. W.
 |last5=McGuire |first5=A. D.
 |year=1956
 |title=Detection of the Free Neutrino: a Confirmation
 |journal=[[Science (journal)|Science]]
 |volume=124 |issue=3212 |pages=103–104
 |bibcode=1956Sci...124..103C
 |doi=10.1126/science.124.3212.103
 |pmid=17796274
}}</ref> The properties of neutrinos were (with a few minor modifications) as predicted by Pauli and Fermi.