radioactivity experiment performed by rutherford and willard . Description?
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Rutherford does not stop there, he continued his researches on the properties of radioactive radiations. In the list of experiments to do after his return from Stockholm, he registered the diffusion of alpha particles, that is to say how they are deflected when travelling through matter.
Principle of Rutherford’s experiment
By bombarding a very thin gold foil with alpha particles, Hans Geiger and Ernest Marsden, both students of Rutherford, observed that a small fraction (1 in 8000) of these particles were deflected at large angle as if it bounced off a heavy obstacle. The impacts were observed as scintillations in the dark, under the microscope, on a screen of zinc sulphide. Rutherford concluded that the atom contained a heavy heart, with positive electric charge, able to push away the alpha
DR
In 1903, Philip Lenard, bombarding atoms with cathode rays had noticed that they passed through the atoms as if they could find almost nothing on their trajectory. He summarized his observations by saying that at atomic scale "the solid matter is transparent" and noticed that "the space occupied by one cubic meter of solid platinum is as empty as the space between the stars and the earth" .
Observing that high speed alpha particles were deflected by a thin sheet of mica, Rutherford calculated the electric field inside the mica and deduced that it should have been very powerful. But he thought he should be able to detect and count deflected alpha particles. With his assistant Hans Geiger, Rutherford developed a method to do so. The result of their observations confirmed the existence of strong electric fields. Nevertheless an enigma remains: a few alpha particles were deflected.
Rutherford asked a young assistant Ernest Marsden to see if alpha particles were subjected to a high deflection, and even bounced back, when they went through a thin gold foil. That was the case! It was in darkness and with the naked eye, that Rutherford, Geiger and Marsden counted the scintillations due to the impacts of alpha particles on a screen of zinc sulphide.
Alpha backscattering on nucleus
Rutherford observed the backward bounce of some alpha particles as projectiles sent on the atoms of a thin gold foil. He interpreted this rebound as the "backscatter" of a light nucleus (alpha particle) on the heavy nucleus of a gold atom. Because of the mass ratio (A = 4 versus A = 197), the alpha particle bouncing off (on the left figure) at 150 °, loses only a small portion of its energy - that it gives to the gold nucleus - which allow it to emerge from the gold foil. On the contrary, in the case of a collision (on the right) with a lighter nucleus as oxygen (A = 16), the alpha loses most of its energy in aid of oxygen propelled forward.
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Rutherford pointed out in a phrase made famous, that it was as “It was as if you fired a 15 inch shell at a piece of tissue paper and it came back and hit you" (**) . He studied the phenomenon for a year and found the explanation: the atom positive charge is in a solid and compact nucleus. This nucleus concentrates almost the entire mass of the atom but occupies only a hundred millionth of a millionth of its volume. The atom is almost at one hundred percent empty.
What made by Rutherford and his assistants Geiger and Marsden is perhaps one of the most important experiments of nuclear physics.
The experiments were performed between 1908 and 1913 by Hans Geiger and Ernest Marsden under the direction of Ernest Rutherford at the Physical Laboratories of the University of Manchester.
In the experiment, Rutherford sent a beam of alpha particles (helium nuclei) emitted from a radioactive source against a thin gold foil (the thickness of about 0.0004 mm, corresponding to about 1000 atoms).
Surrounding the gold foil it was placed a zinc sulfide screen that would show a small flash of light when hit by a scattered alpha particle. The idea was to determine the structure of the atom and understand if it were what supposed by Thomson (atom without a nucleus, also known as pudding model) or if there was something different.
In particular, if the atom had an internal nucleus separated from external electrons, then they would have been able to observe events, or particles, with large angle of deviation. Obtained, actually, these results, the New Zealand physicist concluded that the atom was formed by a small and compact nucleus, but with high charge density, surrounded by an electron cloud.
In the image below it is depicted the interaction of the alpha particles beam with the nuclei of the thin gold foil; one can see how the majority of the particles passes undisturbed, or with small angles of deflection, through the “empty” atom, some particles, however, passing close to the nucleus are diverted with a high angle or even bounced backwards.The interaction between an alpha particle and the nucleus (elastic collision) is also known as Coulomb scattering, because the interaction in the collision is due to the Coulomb force. In the diagram below it is shown the detail of the interaction between an alpha particle and the nucleus of an atom.
In the PhysicsOpenLab “laboratory” we tried to replicate the famous Rutherford experiment. With the equipment already used in alpha spectroscopy we built a setup based on an alpha solid-state detector, a 0.9 μCi Am 241 source and a gold foil as a scatterer. In these post we describe the equipment used : Alpha Spectrometer, Gold Leaf Thickness .
The main purpose is not to make precision measurements but to make a qualitative assessment of the scattering as a function of deflection.
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