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In December of 1930 Wolfgang Pauli postulated the existence of the ...
Pauli calls this particle a neutron, today we know it as a neutrino...
Neutrinos have a very low cross-section, they are very penetrating ...
This is a copy of the original letter (in German) by W. Pauli in 1930.
[This is a translation of a machine-typed copy of a letter that Wolfgang Pauli sent to a group of physicists
meeting in Tübingen in December 1930. Pauli asked a colleague to take the letter to the meeting, and the
bearer was to provide more information as needed.]
Copy/Dec. 15, 1956 PM
Open letter to the group of radioactive people at the
Gauverein meeting in Tübingen.
Copy
Physics Institute Zürich, Dec. 4, 1930
of the ETH Gloriastrasse
Zürich
Dear Radioactive Ladies and Gentlemen,
As the bearer of these lines, to whom I graciously ask you to listen, will explain to you in more
detail, because of the "wrong" statistics of the N- and Li-6 nuclei and the continuous beta spectrum, I
have hit upon a desperate remedy to save the "exchange theorem" (1) of statistics and the law of
conservation of energy. Namely, the possibility that in the nuclei there could exist electrically neutral
particles, which I will call neutrons, that have spin 1/2 and obey the exclusion principle and that further
differ from light quanta in that they do not travel with the velocity of light. The mass of the neutrons
should be of the same order of magnitude as the electron mass and in any event not larger than 0.01
proton mass. - The continuous beta spectrum would then make sense with the assumption that in beta
decay, in addition to the electron, a neutron is emitted such that the sum of the energies of neutron and
electron is constant.
Now it is also a question of which forces act upon neutrons. For me, the most likely model for the
neutron seems to be, for wave-mechanical reasons (the bearer of these lines knows more), that the neutron
at rest is a magnetic dipole with a certain moment μ. The experiments seem to require that the ionizing
effect of such a neutron can not be bigger than the one of a gamma-ray, and then μ is probably not
allowed to be larger than e • (10
-13
cm).
But so far I do not dare to publish anything about this idea, and trustfully turn first to you, dear
radioactive people, with the question of how likely it is to find experimental evidence for such a neutron
if it would have the same or perhaps a 10 times larger ability to get through [material] than a gamma-ray.
I admit that my remedy may seem almost improbable because one probably would have seen
those neutrons, if they exist, for a long time. But nothing ventured, nothing gained, and the seriousness of
the situation, due to the continuous structure of the beta spectrum, is illuminated by a remark of my
honored predecessor, Mr Debye, who told me recently in Bruxelles: "Oh, It's better not to think about this
at all, like new taxes." Therefore one should seriously discuss every way of rescue. Thus, dear radioactive
people, scrutinize and judge. - Unfortunately, I cannot personally appear in Tübingen since I am
indispensable here in Zürich because of a ball on the night from December 6 to 7. With my best regards to
you, and also to Mr. Back, your humble servant
signed W. Pauli
[Translation: Kurt Riesselmann]

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