4.10: Neutrons

Figure \(\PageIndex{1}\) (Credit: Sidney Paget; Source: http://commons.wikimedia.org/wiki/File:Sherlock_rose_Naval_Treaty.JPG(opens in new window); License: Public Domain)

Was Sherlock Holmes real or the product of someone's imagination?

The most famous detective in history and literature never existed.  Sherlock Holmes was the creation of the British author Sir Arthur Conan Doyle. This mythical person had capabilities far beyond those of mere mortals.  Holmes was capable of spotting the tiniest clue, the smallest piece of evidence to solve the crime.  He could link all sorts of seemingly irrelevant data into a coherent whole to clear up whatever mystery he was dealing with.

The Quest for the Neutron

Clues are generally considered to involve the presence of something – a footprint, a piece of fabric, a bloodstain, something tangible that we can measure directly. The discoveries of the electron and the proton were accomplished with the help of those kinds of clues. Cathode ray tube experiments showed both the negatively charged electrons emitted by the cathode and the positively charged proton (also emitted by the cathode). The neutron was initially found not by a direct observation, but by noting what was not found.

Research had shown the properties of the electron and the proton. Scientists learned that approximately 1,837 electrons weighed the same as one proton. There was evidence to suggest that electrons went around the heavy nucleus composed of protons. Charge was balanced with equal numbers of electrons and protons which made up an electrically neutral atom. But there was a problem with this model – the atomic number (number of protons) did not match the atomic weight. In fact, the atomic number was usually about half the atomic weight. This indicated that something else must be present. That something must weigh about the same as a proton, but could not have a charge – this new particle had to be electrically neutral.

In 1920, Ernest Rutherford tried to explain this phenomenon through the presence of another particle in the nucleus. He proposed that the "extra" particles were protons and electrons that had combined into a new particle in the nucleus (this did not turn out to be the case). These new particles would have a mass very similar to a proton, but would be electrically neutral since the positive charge of the proton and the negative charge of the electron would cancel each other out.

In 1930, German researchers bombarded the element beryllium with alpha particles (helium nuclei containing two protons and two neutrons with a charge of +2). The particles produced in this process had strong penetrating power, which suggested they were fairly large. In addition, they were not affected by a magnetic field, so they were electrically neutral. The French husband-wife research team of Frederic and Irene Joliot-Curie used these new "rays" to bombard paraffin, which was rich in protons. The unknown particles produced a large emission of protons from the paraffin.

The English physicist James Chadwick (1891-1974) repeated these experiments and studied the energy of these particles. By measuring velocities, he was able to show that the new particle has essentially the same mass as a proton. So we now have a third subatomic particle with a mass equal to that of a proton, but with no charge. This particle is called the neutron. Chadwick won the Nobel Prize in Physics in 1935 for his research.

Neutron Applications

Neutrons can be used in a variety of ways. One important use is in nuclear fission to produce new isotopes. A neutron will collide with a large atom (such as uranium) and cause it to split into smaller atoms, such as in figure below.

Figure \(\PageIndex{2}\): A neutron collides with a large atom, splitting it into smaller atoms. (Credit: User:Fastfission/Wikimedia Commons; Source: http://commons.wikimedia.org/wiki/File:Nuclear_fission.svg(opens in new window); License: Public Domain)

Nuclear reactors utilize chain reactions involving neutrons to heat water which drive turbines for the generation of electricity. When a neutron collides with a large atom, the atom splits with the release of more neutrons and also a large amount of energy. The energy converts water to steam for the turbine, while the neutrons serve to continue the chain reaction (see figure below).

Figure \(\PageIndex{3}\): How nuclear fission produces new isotopes. (Credit: User: Fastfission/Wikimedia Commons; Source: http://commons.wikimedia.org/wiki/File:Fission_chain_reaction.svg(opens in new window); License: Public Domain)

Summary

• Rutherford proposed that "extra" particles in nucleus were combinations of protons and electrons.
• Bombardment of beryllium with alpha particles produced large, neutral particles.
• Chadwick determined the mass of the neutron.
• Nuclear fission produces new elements.
• Nuclear reactors use chain reactions to produce heat.

Review

1. How did Rutherford try to explain the differences between the number of protons in the nucleus and the atomic weight?
2. What did German researchers find when they bombarded beryllium with alpha particles?
3. What did Chadwick determine about these new particle (observed by the German scientist and the Curies)?