Alpha-particle scattering

In practice one studies the excitation of the GDR by alpha particle scattering (isoscalar probe). By comparing the experimental cross section with the theoretical one (calculated as a function of the ratio AR/ R) the value of AR can be deduced [32],... 

A series of episodes in the historical development of our view of chemical atoms are presented. Emphasis is placed on the key observations that drove chemists and physicists to conclude that atoms were real objects and to envision their stracture and properties. The kinetic theory of gases and measmements of gas transport yielded good estimates for atomic size. The discovery of the electrorr, proton and neutron strongly irtfluenced discttssion of the constitution of atoms. The observation of a massive, dertse nucleus by alpha particle scattering and the measrrrement of the nuclear charge resrrlted in an enduring model of the nuclear atom. The role of optical spectroscopy in the development of a theory of electronic stracture is presented. The actors in this story were often well rewarded for their efforts to see the atoms. 

Figure 3. Energy spectra of 18-MeV alpha particles scattered from mylar-based target at 55 ...

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

The differential cross section for 30 MeV alpha particles scattered by hehum (Graves2) shows minima at about 40° and 70° in the centre of mass system and requires s, d and g waves for a theoretical fit. 

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. 

Alpha-particle scattering from metal foils... 

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. 

Describe the nuclear model of the atom. How does this model explain the results of alpha-particle scattering from metal foils ... 

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. 

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