Embedded Files
Nuclear physics is the field of physics that studies atomic nuclei. It also studies their constituents and their interactions. It should not be confused with atomic physics. Atomic physics studies the atom as a whole, which includes the electrons.
Henri Becquerel
Nuclear physics, began as distinct from atomic physics, in 1896 with the discovery of radioactivity from Henri Becquerel. Becquerel was investigating phosphorescence in uranium salts.
J. J. Thompson
J. J. Thomson’s discovery a year later of the electron, was indicative of the fact that the atom had an internal structure. In fact, by the beginning of the 20th century, the accepted model of the atom was J. J. Thomson’s “plum pudding model”. In this model, the atom was a positively charged ball with negatively charged electrons lodged inside of it.
Marie Curie
Pierre Curie
Ernest Rutherford
Radioactivity was later investigated with greater scrutiny. This was done in particular by Marie and Pierre Curie and Ernest Rutherford, along with his collaborators. By the turn of the century, three kinds of radiation were discovered that emanated from atoms.
Alpha: This is when an atomic nucleus emits an alpha particle and transforms into another kind of atomic nucleus. An alpha particle consists of two protons and two neutrons.
Beta: This is when a beta particle is emitted from an atomic nucleus. A beta particle is a fast energetic electron or positron. For example, a neutron turns into a proton, by the emission of an electron and its accompanying antineutrino.
Gamma: This normally occurs after alpha or beta decay. The remaining nucleus or “daughter nucleus” as it is known, may be in a high energy state. This high energy state can decay into a lower energy state by emission of a gamma ray photon. This is gamma decay.
An atomic nucleus emitting an alpha particle.
An atomic nucleus emitting a beta particle.
An atomic nucleus emits a gamma ray photon.
Otto Hahn
James Chadwick
Experiments were done in 1911, Otto Hahn and then in 1914, by James Chadwick, showed that the beta decay spectrum was continuous rather than discrete. Electrons were ejected in a continuous range of energies. This is different from the discrete amount of energies observed in the alpha and gamma kinds of radiation. The problem for nuclear physicists was that it appeared that energy was not conserved in those decays.
Becquerel, Marie and Pierre Curie would later win the 1903 Nobel Prize for their work and investigation on radioactivity.
Ernest Rutherford, in 1906, published “Retardation of the alpha Particle from Radium in passing through matter.” Hans Gieger also expanded on this work, communicating to the Royal Society, the experiments that he and Rutherford had done. These experiments involved passing alpha particles through the air, aluminum foil and gold leaf. Gieger and Ernest Marsden, further expanded on this work with it being published by Gieger in 1910. Thus, in 1911 and 1912, Ernest Rutherford went before the Royal Society to explain the experiments, propounding the new theory of the atomic nucleus!
The key experiment: 1910, University of Manchester, Rutherford, Gieger and Marsden
Alpha particles (helium nuclei) were fired at a thin film of gold foil.
According to the plum pudding model, alpha particles should come out of the foil with their trajectories slightly bent at the most.
However, what Rutherford and his team observed, was that a few particles were scattered at large distances. Some were scattered completely backwards.
He compared this to firing a bullet at tissue paper and having it bounce back at you.
This discovery, led to Rutherford’s 1911 model of the atom. In this case, the atom had a very small and dense nucleus, which contained most of its mass. It also consisted of heavy positively charged particles and surrounding electrons. This was to balance out the charge. The neutron was unknown at the time.
Arthur Eddington, in 1920, anticipated the discovery and mechanism of nuclear fusion in stars. His paper was titled, “The Internal Constitution of the Stars”. The source of stellar energy was a complete mystery at the time. Eddington, what he speculated, was that the source of this energy was the fusion of hydrogen into helium. This created enormous energy, according to Einstein’s famous equation: E=mc^2. This is remarkable considering what had yet to be discovered:
Fusion and thermonuclear energy
That stars are largely composed of hydrogen
Google Sites
Report abuse