31.5

The Neutrino

When a particle is emitted by a radioactive nucleus, energy is simultaneously released, as Example 6 illustrates. Experimentally, however, it is found that most particles do not have enough kinetic energy to account for all the energy released. If a particle carries away only part of the energy, where does the remainder go? The question puzzled physicists until 1930, when Wolfgang Pauli proposed that part of the energy is carried away by another particle that is emitted along with the particle. This additional particle is called the neutrino, and its existence was verified experimentally in 1956. The Greek letter nu () is used to symbolize the neutrino. For instance, the decay of thorium (see Section 31.4) is more correctly written as

The bar above the is included because the neutrino emitted in this particular decay process is an antimatter neutrino or antineutrino. A normal neutrino ( without the bar) is emitted when decay occurs.
The neutrino has zero electric charge and is extremely difficult to detect because it interacts very weakly with matter. For example, the average neutrino can penetrate one light-year of lead (about ) without interacting with it. Thus, even though trillions of neutrinos pass through our bodies every second, they have no effect. Although difficult, it is possible to detect neutrinos. Figure 31-13 shows the Super-Kamiokande neutrino detector in Japan. It is located 915 m underground and consists of a steel cylindrical tank, ten stories tall, whose inner wall is lined with 11 000 photo-multiplier tubes (see page 995). The tank is filled with 12.5 million gallons of ultrapure water. Neutrinos colliding with the water molecules produce light patterns that the photomultiplier tubes detect.

Figure 31-13 The Super-Kamiokande neutrino detector in Japan consists of a steel cylindrical tank containing 12.5 million gallons of ultrapure water. Its inner wall is lined with 11 000 photomultiplier tubes. (© Kyodo News International)

One of the major scientific questions of our time is whether neutrinos have mass. The question is important because neutrinos are so plentiful in the universe. Even a very small mass could account for a significant portion of the mass in the universe and, possibly, have an effect on the formation of galaxies. In 1998 the Super-Kamiokande detector yielded the first strong, but indirect, evidence that neutrinos do indeed have a small mass. (The mass of the electron neutrino is less than 0.0006% of the mass of an electron.) This finding implies that neutrinos travel at less than the speed of light. If the neutrino’s mass were zero, like that of a photon, it would travel at the speed of light.
The emission of neutrinos and particles involves a force called the weak nuclear force because it is much weaker than the strong nuclear force. It is now known that the weak nuclear force and the electromagnetic force are two different manifestations of a single, more fundamental force, the electroweak force. The theory for the electroweak force was developed by Sheldon Glashow (1932- ), Abdus Salam (1926–1996), and Steven Weinberg (1933- ), who shared a Nobel Prize for their achievement in 1979. The electroweak force, the gravitational force, and the strong nuclear force are the three fundamental forces in nature.

Test your understanding of the material in Sections 31.1, 31.2, 31.3, 31.4 and 31.5 :

· | Nuclear Structure
· | The Strong Nuclear Force
· | The Mass Defect and the Binding Energy of the Nucleus
· | Radioactivity
· | The Neutrino

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