Category: Atoms And Radioactivity

Rutherford came up with a new model of the atom that accounted for his scattering results: he proposed that the positive charge and its associated mass is concentrated in the nucleus, and that the negatively charged electrons are orbiting around it, like planets. In his 1911 paper discussing the alpha-particle scattering results,20 Rutherford mentioned the planetary…

Meanwhile, Rutherford’s colleague Hans Geiger was exploring the use of thin films of mica—a transparent mineral that can be split into very thin slices—as radiation windows for his detector tubes. Geiger noticed that mica spread out a beam of alpha radiation by slightly deflecting some of its alpha particles. Together with Ernest Marsden—a young undergraduate…

In 1909, Rutherford and his colleague Thomas Royds really wanted to know what made up alpha particles. They carried out many experiments with alpha particles emitted from radon—a gas that occurs naturally as the decay product of radium, and which often accumulates in basements to a point where it may pose health risks. The radon…

The experiment on the penetrating power of radiation (Figure 62) could lead you to believe that alpha radiation is weak because it is so easily absorbed. However, the exact opposite is true. Alpha particles are so massive that they lose their energy by ripping atoms to pieces as they fly through matter. Eventually, alpha particles…

At the time, Rutherford had classified radiation based only on its penetrating power. However, he soon found that an electric or magnetic field could split such emissions into three types of beams. Based on the direction in which the rays were deflected, it was found that alpha rays carried a positive charge, beta rays carried…

We will need a radiation counter to continue our exploration of the subatomic world, so let’s discuss gas-filled radiation detectors, especially the type commonly known as a Geiger counter. As shown in Figure 56, a gas-filled radiation detector is simply a metal cylinder filled with an inert gas. A thin center wire is kept at high…

Thomson concluded that the negatively charged particle of cathode rays must be a fundamental part of matter itself. His model presented the atom containing a large number of smaller bodies, which he still called corpuscles. Since common atoms are electrically neutral, Thomson proposed that the atom comprises separate negative and positive parts. The negative corpuscles (electrons)…

An even simpler, although less accurate, adaptation of Thomson’s setup to measure e/m can be built using a surplus “magic eye” tube. These electron vacuum tubes were commonly used in tube radios as a visual aid for tuning. The purpose of a magic eye tuning tube in these radio receivers was to help tune a station at…

It is possible to use a modern CRT to measure e/m using Thomson’s method. However, it isn’t an easy task, because the deflection plates are relatively small, and their position is not well known due to the graphite coating used to reduce charge accumulation beyond the neck of the CRT. For this reason, specialized CRTs are made…

In his third experiment, Thomson measured the mass-to-charge ratio (m/e) of the cathode rays. His setup allowed him to measure how much energy the rays carried and how much they were deflected by a magnetic field while also being deflected by an electric field. Figure 50 shows a simplified version of the apparatus. A potential difference V accelerates…