Rutherford made the first nuclear reactions by bombarding nitrogen-14 nuclei with alpha particles. However, in his famous gold foil experiment on the scattering of alpha particles by gold, nuclear reactions did not occur. What is the difference between these two experiments? What is necessary for the implementation of nuclear transformation of gold when bombarded with alpha particles?
Planetary (or sometimes is called also nuclear) model of atomic structure was proposed by the great English physicist E. Rutherford as a result of a thorough studying the phenomenon of the scattering of alpha particles on the thinnest gold foil. There is no need to give a description of gold foil experiment, as they are well known to anyone. Below we will study a scheme of Rutherford’s experience and see why he was using plates of gold foil to implement the experiment.
Rutherford’s experiments yielded quite an unusual picture of the atom. If it was possible to increase the linear dimensions of the area of gold foil in one billion times, then we would be able to see a huge stack of atoms with a diameter of about 60 cm, each of which would have a volume of more than 30 liters. However, in reality the whole mass of the atom is concentrated in a single particle, in the nucleus with a diameter of only about 0,025 mm, i.e. in the particle, which size is of a small grain of sand. This nucleus of an atom is surrounded by small electrons, moving around at very high speeds. Rutherford’s gold foil experiment is about the perforating these 30-liter atoms with a bundle of small sand grains, each of which will continue to move in a straight line, if you it doesn’t face the same grain of sand, corresponding the nucleus of the atom. It is obvious that the probability of such a collision is extremely small. Alpha particles do not deviate by atom electrons because the alpha particle is much heavier than the electrons.
Gold foil experiment, which was made by Rutherford and his co-workers, was about bombarding the gold foil with alpha particles, which are helium nuclei with a double charge. The a-particles were formed by the radioactive decay of radium. Narrow bundle of the particles was flying out of a deep hole in the lead block. The bundle of alpha particles was directed onto a thin metal foil, which has a thickness of approximately 10000 atoms. Alpha particles were detected by luminescence, which they caused in a collision with scintillation screens – plates coated with zinc sulfide, very similar to the screen of the television tube. The screen was mounted in order for it to be able to move in a circle with its center on a line with the point, where the a-particles collide with foil. Behind the screen was a telescope that was used to record and count even very weak flashes of light that occurred when a single alpha particle was hitting the scintillator screen. The device was placed in a vacuum chamber that did not cause abnormalities related to the collision of alpha particles with gas molecules.
In both cases, mendelevium isotopes were separated from the target material by collecting mendelevium atoms on gold foil, which were formed by the action of a flow of helium ions. In the first experiments for obtaining mendelevium only a few element atoms were found, but they were successfully separated chemically and characterized as having an atomic number 101. Significantly improved version of this method was used in the discovery of elements 102 and Lawrence.
The results of experimental studies on the scattering of helion (alpha particles) in the gold foil experiment, show, according to Rutherford and his co-workers, that the interaction of helion and heavier nuclei occurs without departing from the Coulomb repulsion at distances that are more than 10 Fm. Other experiments have led to fairly accurate values of the size of the nuclei and made it possible to determine the function of the probability distribution of the nucleons inside the nucleus. A study of the scattering of high-energy electrons that were held, in particular, by the American physicist Robert Hofstadter and his colleagues, led to results stating that the nuclear density is constant and equal to about 0.17 nucleon on 1 Fm in the central part of each nucleus (except the lungs), then it falls to zero when changing the radius to the 2 Fm (from the density of 90% of the maximum value to the density of 10%).
In 1906, Ernest Rutherford, who was then professor of physics at McGill University, conducted a series of experiments (including the gold foil experiment) on the scattering of alpha rays with a thin sheet of mica. Similar experiments were carried out on the gold foil at the same time by H. Geiger and E. Marsden in Rutherford’s Manchester laboratory. The results of these studies were published in 1909.
Soon after the discovery of the proton, in 1911, some researchers have attempted to describe the structure of the atom. One of the experiments performed for this purpose, was the bombing of a very thin leaves of gold foil with alpha particles. It was known that alpha particles were emitted by radium, and they carry a positive charge and have a small mass.
When bombarding gold foil with these particles there have been several events.
English physicist Rutherford explained the scattering of alpha particles, suggesting that the atoms of metal foil consist of small positively charged heavy nuclei, surrounded by electrons that are located at relatively large distances from the nucleus. Most of the alpha particles pass through the empty space of the atom, but some of them slightly change their direction. This happens in cases where the alpha particles approach the positively charged nucleus so close that the latter begins to repel them.
Rutherford in his classic gold foil experiment demonstrated that atoms are composed of very small-sized nuclei with positive charge that are surrounded by orbiting electrons. He infiltrated a parallel bundle of alpha particles through the thin foil of gold, platinum, silver, or copper (with the thickness of 0.0004 cm) and marked deviations of alpha particles from the original trajectory according to the light.
After making observations during the gold foil experiment Rutherford made the following conclusions: