Top of Part 2 Last page Next page 2-3: Scattering of Alpha Particle by an Atom

 Let us summarize the properties of atom that were made clear up to the preceding section.
 (1) The radius of atom is approximately (2) An atom includes electrons, each of which has a negative charge -e, where e is the elementary charge. The total mass of the electrons in an atom is much smaller than that of the whole atom. The number of electrons is about a half of the atomic weight. (3) An atom is electrically neutral. So that it includes "something" whose charge may cancel the charge of all the electrons. This "something" must carry the whole atomic mass.
 What is the internal structure of such an atom? In order to study the structure of an invisible atom, it is better to collide an appropriate projectile against the atom and to look at the reaction. The alpha rays were considered to be suitable for this purpose.

 [The True Nature of the Alpha Rays] Before colliding the alpha rays against an atom, we have to know what the true nature of the alpha rays. The alpha rays are beams emitted at very high speeds from uranium and radium. They were considered to be collections of the alpha particles charged positively. First, the charge-to-mass ratio of the alpha particle, Q/M, was measured by curving its orbit in electric and magnetic fields, where Q is the charge of the alpha particle and M the mass. Thereby it was seen that the charge-to-mass ratio of the alpha particle was 1/2 of that of the hydrogen ion. The measurement method is almost the same as the case of electron shown on the page 1-7-A: Measurement of the Charge-to-Mass Ratio of Cathode Rays . But you should note that, while the charge of electron is negative, that of the alpha particle is positive. Next, the amount of the charge of the alpha particle was measured. On the first step for this purpose, the number of the alpha particles emitted in a unit solid-angle within a unit time was measured by using the system shown in Fig. (A) The scintillation was used for the detector: Namely, the number of the emission of light, i.e. luminescence, which was emitted from the fluorescent material put on the window of the detector when being hit by a charged particle, was counted.

 On the second step, the total charge of the alpha rays radiated in a unit time from the same source is measured by using an apparatus shown in the above figure, Fig. (B). Dividing the resultant total charge by the number of the alpha particles, they obtained the value of electric charge per particle. Thereby Charge of an alpha particle = +2e (e = the elementary charge) was clarified. An alpha particle is therefore speculated to be a helium ion, i.e. the helium atom having lost two electrons. It was later confirmed by E. Rutherford (UK, 1871 - 1937) and T. Royds that this speculation was correct (1908). The schematic drawing of the apparatus used for this purpose is shown in the following figure, Fig. (C).

 In the above apparatus, a radiative material (gas) is enclosed in a very thin glass tube A under a pressure applied by the mercury M2 at the center of a vacuum glass container B. The alpha particles emitted from the radiative gas penetrate the thin glass to be accumulated in the outside glass container B. After leaving this apparatus for several days, the mercury M1 was lifted up so that the gas accumulated in the container B was led up into an upper narrow tube V. They made a discharge by applying a high voltage between the electrodes, and observed the spectrum which was exactly the same as that of the helium atom. Thus they confirmed that the gas accumulated in the container B was helium gas. It was confirmed that an alpha particle came from A to B captured two units of elementary charge from environment to become a helium atom. This means that the alpha particle is a doubly ionized helium atom. For reference, they put helium gas in the thin tube A and leave it for several days, but they could find no helium gas in the outer container B at all; namely, an alpha particle can penetrate the thin glass wall of the tube A, but a helium atom cannot. Let's think why it is so.

 [Scattering of the Alpha Rays by an Atom] The experiment of the scattering of alpha rays by an atom were carried out by H. W. Geiger (Germany, 1882 -1945) and E. Marsden (UK, 1889 - 1970) under Rutherford's leadership (1909). They made alpha rays emitted from radium collide against a thin metal (gold or silver) foil. A schematic graph of the experiment is shown in Fig. (D) and the details of the apparatus are in Fig. (E). In the vacuum scattering chamber in Fig. (D), the alpha particles coming out of the source R hit the metal foil F at the center and they are scattered in various directions. These scattered particles are observed through their scintillations with the microscope M. Thereby it is measured what rate of particle number is scattered in a given solid angle. The results obtained by Geiger and Marsden showed that the almost all alpha particles go straight in the forward angle, but a few number of particles are occasionally scattered in a very large angle like 90 degrees or larger.

 Top Go back to the top page of Part 2. Go back to the last page. Go to the next page.