Alpha particle scattering experiments

Knowledge Required The general features of Ernest Rutherford s alpha-particle-scattering experiment. 

The alpha-particle scattering experiments of Rutherford established that the atom contains a dense, positively charged nucleus. The later work of Chadwick demonstrated that the atom contains neutrons, which are particles with mass, but no charge. Rutherford also noted that light, negatively charged electrons are present and offset the positive charges in the nucleus. Based on this experimental evidence, a model of... 

On the basis of these results, Rutherford in 1911 postulated that the atom consists of a tiny central positively charged region, which he subsequently termed the nucleus. The nuclear positive charge was balanced by electrons revolving round the nucleus at a considerable distance. The results of the alpha particle scattering experiments were thereby explained the positive alpha particle would experience little or no deviation unless it happened to approach very close to the positively charged nucleus. 

The nucleus of an atom also has a structure the nucleus is composed of two different kinds of particles, protons and neutrons. The type of alpha-particle scattering experiment that led to the nuclear model of the atom was also instrumental in clarifying this structure of the nucleus. 

The neutron was also discovered by alpha-particle scattering experiments. When beryllium metal is irradiated with alpha rays, a strongly penetrating radiation is obtained from the metal. In 1932 the British physicist James Chadwick (1891-1974) showed that this penetrating radiation consists of neutral particles, or neutrons. The neutron is a nuclear particle having a mass almost identical to that of the proton but no electric charge. When beryllium nuclei are struck by alpha particles, neutrons are knocked out. Table 2.2 compares the masses and charges of the electron and the two nuclear particles, the proton and the neutron. 

In 1911, Rutherford s alpha-particle scattering experiments were controversial. In the Rutherford model of the atom, all of the positive charge was crammed into the dense, tiny nucleus. Like charges repel, so the nucleus of the atoms should not be stable, yet it was. The relationships of classical physics that worked so well in explaining large-scale systems did not work on atom-sized systems. Thus, someone had to develop a new approach to understanding the atom. The breakthrough that was needed was the development of the field of study now known as quantum mechanics. 

Alpha-particle scattering experiments on a number of elements showed that the charge on the nucleus was always a multiple of the charge of the proton. It was, therefore, reasonable to conclude that any particular element is characterized by having a certain fixed number of protons in the nucleus, this number (the atomic number, Z) being the same as the number of electrons in the atom (the latter determining the chemical behavior of the element). It was clear, however, that additional information was necessary to complete the picture, since, for the typical atom, the combined mass of the number of protons required to supply the known nuclear charge was somewhat less than one half of the observed mass of the nucleus. 

Alpha particle A helium nucleus He2+ ion, 30 emission, 513 scattering experiment, 26... 

During 1910-1911, Sir Ernest Rutherford suggested ail experiment, carried out by Geiger and Maisden, in which alpha panicles from a radioactive source were scattered from thin foils, The angles at which the alpha particles were scattered were found to be such as could best be... 

FIGURE 2.5 The Rutherford scattering experiment, (a) When a beam of alpha particles is directed at a thin gold foil, most particles pass through the foil undeflected, but a small number are deflected at large angles and a few bounce back toward the particle source. 

A/Q = 3 (tritons), those species with mass-to-charge ratios of 2 or less were reduced by more than 2 orders of magnitude. (Scattered background events limited the measurement of this reduction factor.) This mass-to-charge filtering is particularly important in our future experiments because the protons, deuterons, and alpha particles are produced with yields that are, respectively, 105, 104, and 103 times larger than those of the ions for which measurements are planned. 

Scattering experiments, often using electrons as "bullets," yield a scattering cross section for hadrons, and from this cross section one can obtain an estimated size for the hadrons. The original Rutherford88 experiment, which arrived at the small value for the nuclear size, relative to the size of atom, used a beam of (2He4)++ nuclei (alpha particles) from a Ra source. 

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