Earlynpt

S.A. Moszkowski and C.W. Wong, 2-9-99

EARLY NUCLEAR PHYSICS

This is a short history of nuclear physics in its early stages, mainly from the discovery of radioactivity in 1896 to the discovery of the neutron in 1932. There are also some remarks about the weak interactions which go into more modern times. It is meant to put the role of women into the context of the history of the field. The majority of women physicists doing research during the first half of the 20th century were working in the field of 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. This is the phenomenon of spontaneously emitted radiation, which he called Uranic rays, and it marked the beginning of the field of nuclear physics.

While Becquerel went on to do research in atomic physics, Marie Curie ...M. Curie citation... was stimulated by his discovery of the Uranic rays, and she, later joined by Pierre Curie, began to investigate them systematically. Their studies which showed that such rays were not unique to Uranium, led them to the discovery of the new elements Polonium and Radium in 1898, and they coined the term Radioactivity by which the phenomenon has been known ever since.

Four years later Ernest Rutherford and Frederick Soddy found that substances like Uranium and Thorium transmute naturally into other elements.
During the next few years, and continuing for 50 years, many other naturally radioactive substances were found.

Brief discussion of Missing Elements, the last chemical elements discovered after 1920.
I. Noddack, B. Karlik and M. Perey played an important role in some of these.

Citation for Noddack ......Karlik..... .....Perey.....

It was also shown that there are three kinds of radiations, alpha-rays, beta-rays, (both discovered by Rutherford), and gamma-rays. These rays 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. Harriet Brooks..Brooks citation.. 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 decays 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 was shown to be quite independent of chemical and thermal properties of the radioactive substance. This was demonstrated by F. C. Gates. See Gates citation 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 is 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 nucleus of Z positive charges and also A-Z pairs of positive and negative charges surrounded by a sphere of Z uniformly distributed electrons, which 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 orbits 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 into a mathematical formulation called Quantum Mechanics. This theory and Albert Einstein's Theories 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-lives 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 understood in 1928 by Gamow, and independently by Gurney and Condon, as a quantum-mechanical phenomenon. The idea is that 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-rays 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. For the same reason, gamma-rays are much more energetic than atomic X-rays, and therefore must be of nuclear origin. Gamma-rays, like X-rays, are more energetic versions of light radiation.

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. It was shown that the average energy release was, in fact, equal to the mean energy of the beta-rays, (Much crucial work on these and other aspects of beta-decay, was done by Lise Meitner and collaborators...Meitner citation ) The maximum beta energy corresponded to the energy difference between initial and final nucleus, but what happened to the rest of the energy?

Wolfgang Pauli first proposed in 1931 that the deficit between the maximum and the actual energies of the emitted electron is carried away by a new particle 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 is called beta-decay, a process caused by an interaction called the weak interaction. Studies of weak decays in nuclear and subnuclear processes would eventually lead T.D. Lee and C.N. Yang in 1956 to the idea that weak interactions violate parity symmetry, i.e., the idea that the mirror reflections of certain physical phenomena do not exist at all in nature. This picture was experimentally confirmed in 1957 by Chien Shiung Wu and others. C.S. Wu Citation

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 start to understand nuclear structure. That is the subject of the next section.

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