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|1-3: The Discovery of the Neutron|
[Problem of Isotopes]
J.J. Thomson (UK, 1856-1940) discovered that the natural neon gas is a mixture of two kinds of elements which have different atomic weights (1912). Afterwards, F.W. Aston (UK, 1877-1945) carried out precise mass-spectroscopic analyses and he confirmed that many various elements are mixtures of two or more kinds of isotopes which differ in mass (atomic weight). Namely, there exist some elements (or atoms) which have the same atomic number and, therefore, essentially the same chemical properties but different atomic masses. This means that there exist different nuclei having the same electric charge but different masses. These are also called isotopes.
What is a reasonable explanation of the existence of isotopes ?
[Does the Nucleus Consist of
Protons and Electrons?]
It is known well that some kinds of nucleus decay by emitting electrons. This is called the beta decay. If we consider that the atomic nucleus consists of protons and electrons, then the beta decay could easily be understood. However, the idea that protons and electrons are basic constituents of a nucleus has the following serious difficulties.
(1) Problem on the spin
and the statistics
As for the details of the spin and the statistics, please refer to some other advanced books concerning "Quantum Mechanics". Here, let us give only a rough outline of the discussion about the present problem. (See the page of "Microscopic World -2-: [The Pauli Principle, Spin]" ).
For simplicity of our explanation, let us take as an example the case of nitrogen-14 nucleus () whose atomic number is 7 and atomic weight is 14.
A single proton and a single electron are both fermion. A composit system of even number of fermions is a boson as a whole. And it has the property of Bose-Einstein statistics. Contrarily, a composit system of odd number of fermions behaves as a fermion as a whole, and it has the property of Fermi-Dirac statistics.
If we consider that nitrogen-14 nucleus () consists of 14 protons and 7 electrons, the total particle number is 21 that is an odd number. In contrast to this, the experimental result of the spectroscopy shows that the nitrogen-14 nucleus must be a boson as a whole. This is the first problem.
The second problem is as follows: Each of the proton and the electron has spin . While the total spin of a system of even number of protons (or electrons) is an integral multiple of , that of odd-proton (or odd-electron) system must be an odd-integral multiple of . However, the experimental data show that the spin of the nitrogen-14 nucleus is 0 (= even). This means that total particle number of the nitrogen-14 nucleus must be even. This is also an important problem.
(2) Problem concerning the uncertainty relation
If we consider that the atomic nucleus consists of protons and electrons, an alpha particle (= helium nucleus) should be made of 4 protons and 2 electrons. The size (diameter) of the alpha particle is about . If an electron is confined within this size of space, the uncertainty of the position of the electron should be . Using the uncertainty relation in "Quantum Mechanics", , we can estimate the uncertainty of the momentum of the electron, and then the uncertainty of the speed of the electron will be . This value largely exceeds the light speed. This is quite unreasonable on the basis of the Special Theory of Relativity. Namely, it is completely impossible to confine such a light particle as electron within a narrow space like a nucleus. This is another serious problem.
As discussed above, the problems of the idea that the nucleus consists of protons and electrons could at all once be solved if we assume that there exists an electrically neutral particle with almost the same mass as a proton; so to say a "neutral proton". If it is so, the existence of isotopes could easily be understood.
This was Rutherford's prediction and he sometimes talked this idea to his students in his lecture. One of his top students influenced strongly by him, J. Chadwick (UK, 1891-1974), made an effort to find out that "neutral proton" and finally got the honor of discovering that particle; Chadwick named it "neutron".
[History of the Discovery
of the Neutron]
The discovery of the neutron was based on the results of efforts by some research groups for several years.
(1) Experiment of Bothe and Becker
W. Bothe (Germany, 1891-1975) and his student H. Becker bombarded beryllium with strong alpha-rays from a polonium source and found a high-energy radiation of strong penetrability (1928). They observed similar radiations not only from beryllium, but also from lithium and boron. At first, these radiations of strong penetrability were considered to be high-energy gamma rays.
(2) Experiment of I. Curie and Joliot
Around 1931, I. Curie (France, 1897-1956) and F. Joliot (France, 1900-58) reported that the above high-energy radiation found by Bothe and Becker was able to eject protons from a paraffin layer. I. and F. Joliot-Curie thought that this phenomenon is caused by the Compton effect due to high-energy gamma-rays emitted from the polonium-beryllium source. Namely, they thought that a gamma particle (photon) of Bothe-Becker's high-energy radiation hits a proton in the paraffin layer to be scattered and the observed proton is nothing but the recoil proton from this Compton effect. (As for the Compton effect, refer to the page of "Microscopic World -1-: 3-7:The Compton Effect").
(3) The Discovery of the Neutron by Chadwick
Chadwick was much interested in the report of Joliot-Curie. He repeated the experiments, using the polonium-beryllium (Po-Be) source.
The schematic drawing of his experimental apparatus is shown in a figure below. The high-energy radiation emitted from the (Po-Be) source on the left-hand side is led to the cloud chamber, in front of which a paraffin layer is placed. The radiation is scattered by a proton in the parafin and a photograph of the recoil proton is taken in the cloud chamber on the right-hand side.
Chadwick collided the radiation emerging from the (Po-Be) source not only with proton (paraffin), but also with helium and nitrogen. Comparing the results of these experiments with each other, Chadwick concluded that this mysterious radiation from the (Po-Be) source cannot be interpreted by assuming it to be a gamma ray. He finally concluded that all were able to be understood without any contradiction by assuming that the mysterious radiation is electrically neutral particles with almost the same mass as a proton. This is the confirmation of the existence of the "neutral proton" predicted by Rutherford. Chadwick named this particle "neutron" (1932).
[Experimental Apparatus of Chadwick]
This is a schematic drawing of the experimental apparatus of Chadwick. High-energy alpha rays from polonium on the left-hand side collide the beryllium target and a radiation with strong penetrability (purple arrows = neutrons in fact) comes out to hit the proton in the paraffin layer. The knocked proton is observed in the cloud chamber behind.
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