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● ● Feature Articles On the Discovery of the Atomic Nucleus Shoji Nagamiya JAEA, 2-4 Shirakata Shirane, Tokai-Mura, 319-1195, Japan KEK, 1-1 Oho, Tsukuba-shi, 305-0801, Japan 1. Two Models for the Atom About 100 years ago the atomic nucleus was discovered. In the early 20th century two different models were proposed to describe an atom. One was the watermelon model (alternatively known as the “plum pudding” model), which was proposed by J. J. Thomson. This model posited that small, negatively charged “plums” (now called electrons) were surrounded by a positively charged “pudding”. The other model, which was proposed later by Thompson’s student, E. Rutherford, is like the solar system: a positively charged nucleus at the center is surrounded by orbiting negatively charged particles (electrons). Rutherford’s model (the right figure of Figure 1) is unstable (and changes with time), because electrons spontaneously emit radiation in the Coulomb type potential, keep loosing energy, and, eventually, are absorbed by the nucleus. In contrast, the motion of electrons in the watermelon model (the left figure of Figure 1) is stable, in particular, when electrons do not move and do not lose energy. Therefore, people had thought that the watermelon model was much more realistic than the other model. Figure 1: The watermelon model (left) and Rutherford’s model (right) for the atom. From the viewpoint of classical electromagnetism, the motion of electrons in the solar system model or Shoji Nagamiya is the President of AAPPS from January of 2011. He is the Director of J-PARC Center, a big center supported by KEK and JAEA in Japan. He also serves as President of Physical Society of Japan. He is a member of Science Council of Japan (SCJ) and serves as a Secretary of Natural Science Section of the SCJ. His original research field is relativistic heavy-ion collisions started at Berkeley and then at Brookhaven. 4 AAPPS BULLETIN Figure 2: J. J. Thomson and E. Rutherford 2. Radiation Radiation was the most attractive subject for many physicists in the early 20th century, and three different types of radiation were discovered. At that time, an α -ray was defined as being a positively charged particle which bends to the left in a magnetic field. On the Discovery of the Atomic Nucleus ● ● Figure 3: Three types of radiation: a positively charged α-ray, a negatively charged β-ray and a neutral γ-ray. A β-ray was defined as being a negatively charged particle which bents to the right in a magnetic field. Finally, a γ -ray was defined as being the radiation that never bends in a magnetic field. Now, in contemporary times, every physicist knows that an α-ray is a 4He nucleus, a β -ray is an electron emitted by the nucleus when a nucleus decays via beta-decay, and γ -rays are radiation from an excited nucleus (Figure 3). Rutherford was a leading figure in the field of radiation. He studied the properties of radiation very carefully and predicted that the strength of radiation, which would be the decays of nuclei, must be doubled when the number of parent nuclei is doubled. Namely, the strength of radiation per unit second is proportional to the number of parent nuclei, as shown in Figure 4 [1]. This leads to the following equation; ) N = N0 exp(-αt) = N0 exp(-t/τ where τis the lifetime of radiation. This work was immediately recognized in the chemistry society and he received the Nobel Prize in chemistry in 1908, before the discovery of the atomic nucleus. In the early 20th century, the Nobel Prize in chemistry was given to microscopic studies such as the study of nuclear decays, whereas the Nobel Prize in physics was given to macroscopic studies such as wireless telegraphy. Episode of Rutherford Some years later, Rutherford received the coat of arms from the Queen. In his case, the design of the coat of arms (Figure 5) was taken from his famous decay curves for radiation (not by the discovery of the nucleus). Note that the design on the shield features the curves as seen in Figure 4, but they are rotated 90 degrees [1]. Rutherford was born in New Zealand, one of the AAPPS countries, and lived there until the age of 25. His mother was the first woman to become an elementary school teacher in New Zealand. He very often wrote letters to his mother. It might imply that his early education was very important for Rutherford (as it is, indeed, for many people). His face appears on the $100 bill in New Zealand. Figure 4: Rutherford’s prediction of radiation strength and his experiment. October 2012 Vol. 22 No. 5 5 ● ● Feature Articles 4. Bohr’s Theory Figure 5: The Rutherford’s coat of arms (left) which is 90 degrees rotated from his famous decay curves, and the New Zealand $100 bill (right). 3. Scatterings of α-rays with Au Nuclei In 1898 Rutherford discovered both α -ray and β -ray. In the same year he moved from Cavendish Laboratory to McGill University in Canada and studied the properties of these forms of radiation. After his return to Manchester in 1907 his research on the nucleus was fully initiated. In 1909 H. Geiger and E. Marsden did the famous Geiger-Marsden experiment in which α -rays were scattered by gold foil. The result, however, was a very puzzling one because α -rays were scattered backward, as shown in Figure 6 (right) [2]. If the atom was like the watermelon model, then, one would not expect the backward scattering as illustrated in Figure 6 (left) [2]. In 1911, two years after the Geiger-Marsden experiment, Rutherford theorized this phenomenon, suggesting that an atom has the features of a high central positive charge concentrated in a central small volume (nucleus), and thereby pioneered the Rutherford model of the atom. This is a famous story on the discovery of the atomic nucleus. Figure 6: Expected results are: αparticles passing through the watermelon model of the atom undisturbed (left); however, a small portion of the particles were deflected, indicating the presence of a small, concentrated, positively charged feature (right). 6 AAPPS BULLETIN Niels Bohr arrived in Cambridge in 1911 at the age of 26. He worked in the group of J. J. Thomson. One weekend, he visited Manchester and met with Rutherford. Rutherford was then 39. Bohr learned Rutherford’s idea regarding the atomic nucleus. His immediate concern was to determine why electrons did not slow down and fall into the nucleus, as described in Section 1. He then spent some time to solve this problem in connection with the Planck’s quantum theory. In 1913, he published a series of three papers, which presented what was then known as the Bohr model for the atom, where an electron orbit was quantized. He sent these three papers to Rutherford, but Rutherford was rather critical to the papers, and recommended the shortening of their length [3]. Evidently, Rutherford was not quite happy with the way that Niels Bohr interpreted the Rutherford discovery of the atomic nucleus, but eventually, he deeply appreciated to it [3]. With Bohr’s quantized” atomic model, the long-standing dilemma regarding an electron’s orbit (to slow down according to the classical electromagnetism) was finally solved. 5. Constituents of the Nucleus In 1919 Rutherford conducted an experiment to bombard a particle from polonium in the air (primarily a nitrogen gas), and observed an ejection of a hydrogen nucleus together with the formation of an oxygen nucleus. This hydrogen nucleus was called a proton. This experiment was the first trial of nuclear transmutation, and awakened Rutherford to the importance of accelerators. J. O. Cockcroft and E. Walton invented an accelerator in 1932 and the first man-made nuclear reaction with this accelerator was accomplished. What does constitute the nucleus? This was the next major question after the discovery of the atomic nucleus. Many researchers believed that the electron must be a constituent of the nucleus, because an electron was ejected from the nucleus via β -decay. Also, the newly discovered “smallest nucleus,” the proton, must be a constituent of the nucleus. Many physicists believed that the nucleus was constituted of protons and electrons for many years. For example, 14N is made of 14 protons and 7 electrons. The above nuclear transmutation can be understood in this analogy. On the Discovery of the Atomic Nucleus ● ● However, we have already seen a serious dilemma here. For example, 14N consists of an even number of protons and an odd number of electrons, so that it must obey the Fermi statistics. On the other hand, the spin of 14 N is 1 and it obeys the Bose statistics. This dilemma was not solved until the discovery of the neutron by J. Chadwick in 1932. By this discovery, it turns out that 14 N is made of 7 protons and 7 neutrons, which allows it to form the spin 1 nucleus. 6. Yukawa Theory Immediately after the discovery of the neutron, physicists focused on a single question of why protons and neutrons were bound to a tiny object of the order of 10 -(12-13) cm, which is the scale of the nucleus. H. Yukawa was one of the physicists who tried to understand this problem successfully. He first tried to calculate many different short-range forces. When the force is the Yukawa type (V = exp(-αr)/r), then it obeys the Klein-Gordon equation with intermediate’s mass of so called the pion The idea came up in his head at the Tamino bridge near the Department of Physics at Osaka University where he worked at that time. Prior to that point H. Yukawa had not published papers and it was a complaint of the department chair, Professor Yagi. The paper written in 1935, which is now known as the famous Yukawa paper, became his first paper. The paper gave a strong impact on the present-day concept of “strong interactions”. Also, it provided a basic new framework regarding interaction, namely, that any interactions were intermediated by particles (which are pions, in this case). The concept became a fundamental guideline to particle physics. Particle physics has gradually been separated from nuclear physics since then. Namely, nuclear physicists have tried to understand properties of the nucleus, whereas particle physicists have tried to understand the particle itself and interactions between particles. References [1] S. Devons, F. R. S., in “Rutherford and the Science of His Day”, Notes and Record of the Roy. Soc. London, 45, 221 (1991). [2] Taken from: http://en.wikipedia.org/wiki/File:Rutherford_ gold_foil_experiment_results.svg [3] R. Peierls, in “Rutherford and Bohr” Resonance, May, 2010. October 2012 Vol. 22 No. 5 7