S.A. Moszkowski  and  C.W.Wong


 This is a short history of nuclear physics in its early stages. It is meant to provide a historical context for the role of some of the women in the CWP archive.  Several of the most important discoveries, starting with radioactivity, were made by women physicists, and these are highlighted in this history.  A relatively large number of women physicist doing research in the early part of the 20th century worked in nuclear physics.

 In 1896, Henri Becquerel studied the radiation emitted by phosphorescent materials. He was intrigued by Roentgen's recent discovery of X-rays and looked for X-rays in Uranium salts. But the unexpected happened, as it has on numerous other occasions in physics: Becquerel discovered that these salts emit a new form of radiation, different from both phosphorescent light and X-rays, which he called Uranic rays. This marked the beginning of the field of nuclear physics.

 While Becquerel went on to do research in atomic physics,  Marie Sklodowska Curie was interested in his discovery of the Uranic ray and  began to investigate them systematically. Soon afterwards her husband, Pierre Curie, joined her in this research. Their studies led them to propose that the radiation was emitted from single atoms. These ideas, based on the not yet fully confirmed theory of atomic structure of the elements, led them to the discovery of new elements Polonium and Radium.They showed that other elements besides Uranium emitted such rays, andcoined the term Radioactivity by which the phenomenon of this sort of spontaneously emitted radiation has been known ever since.

 A few years later Ernest Rutherford and Frederick Soddy found that substances like Uranium and Thorium radioactively transmute naturally into other elements. At the time of Becquerel's discovery, there were still 15 gaps in the periodic table. Most of these were filled in, mainly by the discovery of natural radioactive substances, and also rare gases. By 1920, there were only six elements missing from the  periodic table, which at that time ended with Uranium(Z=92). See table of missing elements. All of these were found during the next 30 years. Three women who played a leading role in these discoveries were  Berta KarlikIda Noddack, and  Marguerite Perey.

 It was also shown that there are three kinds of radiations, alpha-rays, beta-rays, (both discovered by Rutherford), and gamma-rays. These ray of radioactivity have very different penetrating powers: Alpha-rays can be stopped by a thick sheet of paper, beta-rays can go through a sheet of metal, while gamma-rays are even more penetrating. It would take several decades before the detailed nature of radioactivity would be fully understood  by physicists. Rutherford's graduate student in the early days , Harriet Brooks ,  discovered an effect which was later shown to be nuclear recoil after emission of radiation.

 Rutherford found that a certain fraction of a radioactive substance decays in a given time interval. This means that the original amount decay exponentially with time; the time it takes for half the material to decay is known as the half-life. For each radioactive decay, there is a characteristic half-life, which  Fanny Gates along with others showed was  quite independent of chemical and thermal properties of the radioactive substance. It was eventually learned that alpha-rays are just helium atoms without electrons, carrying two units of positive charges each. This means that when an alpha ray i emitted, the atomic number Z of the atom decreases by two units, and the atom is transmuted into another atom two steps below it in the Periodic Table.

 An observation of unexpectedly large angle backscatterings when alpha particles hit a gold foil led Rutherford in 1911 to the theoretical picture of an atom. Rutherford's atom was made up of a small heavy nucleu of Z positive charges and A-Z  neutral bound pairs of positive and negative charges surrounded by a sphere of Z uniformly distributed electrons.  The electron as a carrier of negative charge had been known since 1898. This discovery of the atomic nucleus would have far-reaching impact not only in physics, but also in war and politics.

 Rutherford's nuclear model pointed the way to the new world of modern physics, but it was Niels Bohr who opened its door. In 1913, he constructed a dynamical model of the hydrogen atom with an electron circulating a hydrogen nucleus, (which later acquired the name proton) in stable orbit called stationary states. By allowing the electron to emit light only when it jumps between these stationary states, Bohr was able to explain the known energies of light emitted by excited hydrogen atoms. Bohr's model was soon developed by others in a mathematical formulation called Quantum Mechanics. Quantum Mechanics and Albert Einstein's Theory of Relativity provide the conceptual basis for the theoretical description of all physical phenomena known to us today.

 One of the early successes of quantum mechanics was its explanation of alpha decay. It had been known for some time that alpha decay half-live depend very sensitively on the decay energy.  Doubling the decay energy from 4 to 8 MeV causes the typical half-life to decrease from 1010  years to 10-2 seconds, a change by a factor of 10-19!  This extreme energy dependence was finally explained in 1928 by Gamow,and independently by Gurney and Condon, as a Quantum Mechanical phenomenon. The alpha-particle, held inside the nucleus by a potential barrier caused by the positive nuclear charges, cannot escape from it, according to classical physics. However, Quantum Mechanics does allow the alpha-particle to escape by "tunneling" through the barrier, with an energy-dependent half-life consistent with experiment.

 A year after the discovery of the electron in 1898, beta-ray were found to be electrons too, but of very high velocity, not much smaller than the velocity of light. When a beta-ray is emitted, the atomic number increases by one unit. It was not until Bohr had constructed his atomic model in 1913 that it became obvious that the energies of beta-rays are too high to be of atomic origin, and that these electrons must have come from the nucleus.  Gamma-rays are much more energetic than atomic X-rays and for the same reason must be of nuclear origin,  but, like X-rays, they are more energetic versions of light radiation.

 We have seen that the history of early nuclear physics had several surprises whose resolution stimulated rapid advances. On the other hand, until 1932 little progress could be made in understanding the internal structure of the atomic nuclei. It was taken for granted that these nuclei are composed of protons and electrons, the only particles known at the time. Only when the neutron was found (in 1932) could physicists begin to understand nuclear structure.

The discovery of the neutron ushered in the field of modern nuclear physics in which nuclei  are  regarded as composed of  neutron and protons.  Today, we view nuclear structure physics along line of the nuclear shell model, which was  discovered in 1949 by  Maria Goeppert Mayer and Hans Jensen.

Another great discovery  in nuclear physics occurred a decade earlier, in 1938, when Otto Hahn, Lise Meitner, and Fritz Strassman discovered  nuclear fission.

 In the study of beta-rays, a major paradox developed. It was found that the emitted electrons, unlike alpha-rays or gamma-rays, do not have a definite energy, but a continous spread, or spectrum, of energies. The paradox was that the average energy release, as measured in a calorimeter, was, definitely less than the maximum electron energy, i.e. the energy difference between initial and final nucleus. What happened to the rest of the energy? This paradox had first been pointed out by Chadwick and Ellis, and was confirmed by  Lise Meitner and  W. Orthmann.

Wolfgang Pauli then proposed that the deficit between the maximum and the actual energies of the emitted electron is carried away by a new particle which he called a neutrino. This postulate was readily accepted when Fermi succeeded in explaining the continuous beta-spectrum with its help. However, experimental evidence for neutrinos was not obtained until 1956, by Frederick Reines and Clyde Cowan.

 The emission of beta-rays from nuclei, i.e.beta-decay, is, a proces caused by an interaction called the weak interaction. Studies of weak decay in nuclear and subnuclear processes eventually led T.D. Lee and C.N. Yang to suggest that weak interactions violate parity symmetry; i.e., the idea that the mirror reflections of certain physical phenomena do not exist in nature.This radical proposal was experimentally confirmed in 1957 by  Chien Shiung Wu and others.

 The following books are good references on the history of early nuclear physics:

M. Mladjenovic,  The History of Early Nuclear Physics (1896-1931),World Scientific, Singapore, 1991
E. Segre, From X-Rays to Quarks,  W.A. Freeman &Co, San Francisco, 1980
A. Pais, Inward Bound,  Clarendon Press, Oxford, 1986.
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