Weak Force and Interactions
There are four fundamental forces in nature related to physical phenomena. Although gravity is, by far, the weakest force, the force mediating particle transformations is termed the weak force. The weak force is actually a submicroscopic force working only within the atomic nucleus, whereas gravity has an infinite range. When the range of the weak force is considered, it is truly weak compared to the strong nuclear force. Accordingly, the weak force is considered as a weak nuclear force.
The beginning of the discovery of the weak force can be traced back to German physicist Wilhelm R¨ontgen who discovered x rays. R¨ontgen inadvertently left wrapped photographic plates near a Crookes tube, and returned later to find them exposed. R¨ontgen then wrapped a Crookes tube in black cloth and noted a glow on a piece of paper covered with barium platinocyanide. R¨ontgen realized invisible rays, which he called x rays, were being given off by the Crookes tube.
French physicist Antoine-Henri Becquerel postulated that some substances radiated or gave off after being exposed to sunlight. Becquerel worked with uranium salt. Becquerel put photographic plates in a drawer covered with black cloth and metal plate, on top of which he deposited a golf ball-sized piece of uranium salt. Then he closed the drawer. The day was not sunny and Becquerel wanted to be sure the plates were not exposed. The material lay in the darkened drawer. Three days later when Becquerel opened the drawer, he expected the plates to remain unexposed because the uranium salt had not been exposed to the Sun. Instead, Becquerel found a clear image on the plates. The uranium salt had given off invisible rays, even in total darkness. Becquerel continued to experiment with uranium and discovered that it had some very interesting properties. The uranium rays were different from x rays. They did not penetrate materials. The emitted their rays spontaneously, day or night, for seemingly endless periods of time. While x rays required energy input into a Crookes tub; uranium gave off its rays spontaneously, and no outside energy source was required. This form of radiation is now understood as nuclear or particle radioactivity.
At the same time in Paris, French physicists Marie and Pierre Curie were able to extract two new radioactive elements from uranium ore, the elements polonium and radium. Radium was discovered to emit radiation a million times more intensely than uranium. Every sample of radium pours out energy in torrents. But an unknowing public used it to light watch dials, make flashy costumes, and gamble with luminous chips. It was quickly discovered that radium destroyed cancer cells, but also killed healthy tissue, as well as causing cancers and radiation poisoning.
Another powerful discovery was made by New Zealand-born English physicist Ernest Rutherford. Using cloud chambers, Geiger counters, and scintillation screens, Rutherford showed that three different types of radiation (alpha, beta, and gamma rays) were given off. Rutherford showed alpha particles were actually helium atoms, beta particles were electrons, and gamma rays were similar to x rays. Rutherford also found, when he carefully reviewed all his data, that the atom was somewhat like a miniature solar system with negative electrons orbiting a positively charged nucleus.
Danish physicist Niels Bohr pushed the idea of a planetary model into the realm of quantum physics. Because nucleus was much too massive for the amount of positive charge it was carrying, it could not be made entirely of protons. English physicist James Chadwick discovered the neutron, a particle with slightly more mass than the proton, but without an electric charge. It became clear that the nucleus was composed of positively charged protons and neutral neutrons around which orbited the negatively charged electrons.
It was beta (electron) decay that inspired the notion of the weak nuclear force. Bohr and Austrian physicist Wolfgang Pauli studied beta decay. It was found that the electron emitted in beta decay had a wide range of kinetic energy ranging anywhere from zero up to a maximum value of 156 keV. It was as if the law of the conservation of energy was being violated. Pauli suggested that perhaps an additional new particle was being emitted which was difficult to detect. Such a particle could be carrying off the energy, momentum, and angular momentum required to maintain the conservation laws. Italian physicist Enrico Fermi named such a particle the neutrino, meaning little neutral one.
Fermi studied beta decay intensively. The mass of any nucleus was approximately the sum of the masses of the protons and neutrons it contained. The simplest manifestation of beta decay is the decay of a free neutron into an electron, a proton, and an antineutrino. Fermi thought only of a neutrino, until French physicist Louis de Broglie introduced the term antiparticle in 1934, after which Fermi developed a theory of beta decay that included the antineutrino. Fermi considered the above reaction to be the prototype of weak interactions. Fermi described the wide energy variations of the beta particle (electron) and developed a theory to compensate for such variations. Fermi stated that at a single point in space-time, the quantum-mechanical wave function of the neutron is transformed into that of the proton, and that the wave function of the incoming neutrino (which is equivalent to the outgoing antineutrino we actually see) is transformed into that of the electron. Fermi described the reaction as four fermions reacting at a single point. In beta decay, the electron emitted is not an orbital electron, but is created within the nucleus itself.
Fermi also postulated the existence of a fourth force in nature which regulates this decay, the weak nuclear force. It was not until 1956 that the existence of the neutrino was confirmed, and Fermi's theory verified. The weak force theory included the possible energy levels of the expelled electron and also explained the range of weak force (less than 10-18 meters) and the fact that the fastest beta decays take on the average of about a hundredth of a second, slow compared with the typical time scale of processes caused by the strong nuclear force, which is about a millionth of a second.
There were problems with Fermi's theory. Chief among these was how the weak force depends on the relative orientation of the spins of the participating particles, and why the weak force reaction seemed to violate the conservation of charge conjugative (C) symmetry and parity (P) symmetry. These problems have been solved with models using the properties of three observed boson particles that transmit the weak nuclear force, namely, the W+ , W- , and Z°. The weak force is responsible for beta decay and for interaction of the neutrino with matter.
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