Discovery of the neutron

The stmcture of the particles inside the nucleus was the next question to be addressed. One step in this direction was the discovery of the neutron in 1932 by Chadwick, and the deterrnination that the nucleus was made up of positively charged protons and uncharged neutrons. The number of protons in the nucleus is known as the atomic number, Z. The number of neutrons is denoted by A/, and the atomic mass is thus A = Z - - N. Another step toward describing the particles inside the nucleus was the introduction of two forces, namely the strong force that holds the protons and neutrons together in spite of the repulsion between the positive charges of the protons, and the weak force that produces the transmutation by P decay. 

In the early years of this century the periodic table ended with element 92 but, with J. Chadwick s discovery of the neutron in 1932 and the realization that neutron-capture by a heavy atom is frequently followed by j6 emission yielding the next higher element, the synthesis of new elements became an exciting possibility. E. Fermi and others were quick to attempt the synthesis of element 93 by neutron bombardment of but it gradually became evident that the main result of the process was not the production of element 93 but nuclear fission, which produces lighter elements. However, in 1940, E. M. McMillan and P. H. Abelson in Berkeley, California, were able to identify, along with the fission products, a short-lived isotope of... 

Fermi had been fascinated by the discovery of the neutron by James Chadwick in 1932. He gradually switched his research interests to the use of neutrons to produce new types of nuclear reactions, in the hope of discovering new chemical elements or new isotopes of known elements. He had seen at once that the uncharged neutron would not be repelled by the positively-charged atomic nucleus. For that reason the uncharged neutron could penetrate much closer to a nucleus without the need for high-energy particle accelerators. lie discovered that slow neutrons could... 

Kennesaw State University. Nuclear Chemistry Discovery of the Neutron (1932). Available online. URL http //www. chemcases.com/nuclear/nc-Ol.htm. 

Discovery of the neutron (Chadwick) and positron (Dirac, Anderson). First nuclear reaction induced in an accelerator (7Li(/ , a) Cockcroft and Walton). Baade and Zwicky suggest a neutron star may be created as residue of a supernova explosion. 

The final piece in this subatomic jigsaw (or, at least, in this simple version) was provided by the discovery of the neutron by James Chadwick (1891 1974) in 1932. Chadwick had been a student of Rutherford s in... 

Soon after the discovery of the neutron (Chadwick, 24 Feb. 1932, letter to N. Bohr), Landau improvised the concept of neutron star in discussion with Bohr (Rosenfeld, 1974). The modern conception of the neutron star origin is... 

Rutherford s atomic model solved problems inherent in Thomson s atomic model, but it also raised others. For example, an atomic nucleus composed entirely of positive charges should fly apart due to electrostatic forces of repulsion. Furthermore, Rutherford s nuclear atom could not adequately explain the total mass of an atom. The discovery of the neutron, in 1932, eventually helped to settle these questions. 

The interpretation of the curious weight of chlorine awaited the discovery of the neutron in 1932. Although all chlorine atoms have 17 protons, different isotopes of the element have different numbers of neutrons. In Table 3-3, the mass numbers of the chlorine isotopes are denoted by superscripts to the upper left of the chemical symbol. 

In 1934 the Japanese physicist Hideki Yukawa postulated the existence of yet another force particle, which he called the meson. In 1932 Yukawa began his academic career with an appointment at Osaka Imperial University, which had been founded the previous year. The discovery of the neutron and the publication of Fermi s theory started him thinking about the nature of the force that bound protons and neutrons together in an atomic nucleus. He realized that, though... 

In 1899 he identified two forms of radioactivity, which he called alpha and beta particles. As we saw earlier, he deduced that alpha particles are helium nuclei. Beta particles are electrons - but, strangely, they come from the atomic nucleus, which is supposed to be composed only of protons and neutrons. Before the discovery of the neutron this led Rutherford and others to believe that the nucleus contained some protons intimately bound to electrons, which neutralized their charge. This idea became redundant when Chadwick first detected the neutron in 1932 but in fact it contains a deeper truth, because beta-particle emission is caused by the transmutation ( decay ) of a neutron into a proton and an electron. 

The neutron is a better hammer than the alpha particle for smashing nuclei. Being electrically neutral, it encounters no electrostatic barrier to penetrating the nucleus. Indeed, slow neutrons often And their way into nuclei more efhciently than fast ones, much as a slow cricket ball is easier to catch. So the discovery of the neutron, in the eyes of the veteran nuclear physicist Hans Bethe, marked a turning point in the development of nuclear physics. 

Several students and associates of Rutherford attempted to make and detect neutrons by these means or via swift protons produeed in nuelear reaetions indueed by a particles protons and electrons were given opportunities to smash into eaeh other. These efforts did not lead to the discovery of the neutron. 

Element abundance data were useful not only in astrophysics and cosmology but also in the attempts to understand the structure of the atomic nucleus. [74] As mentioned, this line of reasoning was adopted by Harkins as early as 1917, of course based on a highly inadequate picture of the nucleus. It was only after 1932, with the discovery of the neutron as a nuclear component, that it was realized that not only is the atomic mass number related to isotopic abundance, but so are the proton and neutron numbers individually. Cosmochemical data played an important part in the development of the shell model, first proposed by Walter Elsasser and Kurt Guggenheimer in 1933-34 but only turned into a precise quantitative theory in the late 1940s. [75] Guggenheimer, a physical chemist, used isotopic abundance data as evidence of closed nuclear shells with nucleon numbers 50 and 82. 

This was initiated by the first description of the atom structure in 1913 by Ernest Rutherford, a British scientist and Niels Bohr, a Danish scientist. Then came the discovery of the neutron in 1932 by James Chadwick (a British student of Rutherford), the discovery of artificial radioactivity by Irene and Frederic Joliot Curie (Nobel Prize in chemistry in 1935) and finally the discovery of fission in 1938 by Lise Meitner, Otto Hahn and Fritz Strassman (German scientists) which brought Hahn the Nobel Prize for physics in 1944. 

Following James Chadwick s discovery of the neutron in 1932, and throughout the decade of the 1930s, the atomic nucleus became the frontier of physical research. And pushing the boundaries of this frontier were many American physicists. 

Several decades ago the number of elementary particles known was limited, and the system of elementary particles seemed to be comprehensible. Electrons had been known since 1858 as cathode rays, although the name electron was not used until 1881. Protons had been known since 1886 in the form of channel rays and since 1914 as constituents of hydrogen atoms. The discovery of the neutron in 1932 by Chadwick initiated intensive development in the field of nuclear science. In the same year positrons were discovered, which have the same mass as electrons, but positive charge. All these particles are stable with the exception of the neutron, which decays in the free state with a half-life of 10.25 min into a proton and an electron. In the following years a series of very unstable particles were discovered the mesons, the muons, and the hyperons. Research in this field was stimulated by theoretical considerations, mainly by the theory of nuclear forces put forward by Yukawa in 1935. The half-lives of mesons and muons are in the range up to 10 s, the half-lives of hyperons in the order of up to 10 s. They are observed in reactions of high-energy particles. 

The most important method of production of the first transuranium elements is neutron irradiation of uranium. After the discovery of the neutron by Chadwick in 1932, this method was applied since 1934 by Fermi in Italy and by Hahn in Berlin. The method is based on the concept that absorption of neutrons by nuclides with atomic number Z leads to formation of neutron-rich nuclides that change by fi decay into nuclides with atomic numbers Z - -1. Unexpectedly, the experiments carried out by Hahn and Strassmann led to the discovery of nuclear fission in 1938. 

With the discovery of the neutron by Chadwick in 1932, the structure of the atomic nucleus was clarified. A nucleus of atomic number Z and mass number A was composed of Z protons and A — Z neutrons. Nuclear diameters arc of the order of several times 10 m. From the iiers ieelive of an atom, which is 10 times larger, a nucleus behaves, for most iiurposes, like a point charge +Ze. 

Chadwick, Sir James. (1891-1974). A British physicist who was awarded the Nobel Prize in 1935 for his discovery of the neutron (1932), the existence of which had been predicted by Rutherford. 

Finally, in this historical introduction, we should recall that neutrons have been the subject of two Nobel Prizes to J. Chadwick for the discovery of the neutron (1935) and to B.N. Brockhouse and C.G. Shull (1994) for their pioneering contributions to the development of neutron scattering techniques (inelastic scattering and diffraction respectively). 

G.E. Bacon (1969). Neutron Physics, Wykeham Publications, London and Winchester, Chapter 2. Discovery of the neutron. 

Fig. 3.9 The evolution of neutron flux since the discovery of the neutron. A charged particle sources, reactor sources, spallation sources.

Since the discovery of the neutron and the positron, therti are clearly other possible, ways in which the nucleus may be thouglit of as built up. Only one of these models, however, lias lieeu found fea,sible, viz. a nucleus composed of protons and n neutrons tlu. nu<. Iear charge number (atomic number) is then Z the atomic mass number is A = p n. The considerations in fa.vour of tliis model will be presented immediately ( 4, p. 57). 

It is remarkable that Fermi introduced this essentially correct interaction only two years after the discovery of the neutron and one year after Pauli s hypothesis of the neutrino. Fermi modeled his interaction after QED, with = 7p, but the actual interaction has to be determined by experiment. After a confusing period in which experiments appeared to indicate tensor-type interactions, the so-called V-A theory was developed, which has the remarkable feature of breaking parity invariance. Specifically, one has Fp = 7 (1 — 75), in which the 7 75 part changes sign under a parity transformation. The V-A interaction creates particles with negative helic-ity, which means that, if they have velocities close to the speed of light, their spins are oriented against the direction of motion. 

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