History of Science -- Part Nineteen: Neutrons

James Chadwick

The hydrogen atom is very unique. Since it is the simplest atom with only one proton and one electron (and typically no neutrons) it didn’t present the problem that the atoms of the other elements did. Namely, what holds together a group of protons whose like positive charges should repel them and force the nucleus to fly apart.

We now know that the next larger atom, helium, with its two protons and two electrons also has two neutral particles in its nucleus called "neutrons." These neutrons are essential in establishing a new force of nature, now called the “strong nuclear force,” and it is this new force that overcomes the electric force’s repulsion to hold the nucleus tightly together.

While it was easier to discover electrons since they are on the “outside” of the atom and can be freed relatively easy to be studied with devices like the Crookes tube, getting inside the nucleus was another matter. A hydrogen atom stripped of its electron gave access to the proton, and the helium atom stripped of its two electrons exposes a nucleus consisting of two each of protons and neutrons. Still, the lack of an electric charge made the neutrons difficult to detect.

The early study of these stripped atoms were thought of as “rays.” Some of the first discoveries involved radioactivity, the breakdown of the nucleus of very large atoms and how these “rays” affected photographic film.

There were three types of radiation identified by these researchers:

1. Alpha rays are are fast moving helium atoms. They have high energy, typically in the MeV range, but due to their large mass, they are stopped by just a few inches of air, or a piece of paper.

   An electron-volt or eV is a measure of energy, a very small measure. It is appropriate for use at the atomic scale because it is defined as the kinetic energy (energy of motion) gained (or lost) by a single electron when accelerated across an electric potential of one volt.

   This is analogous to the kinetic energy gained by a rock falling in a gravitational field. A single electron volt is 1.6 x 10^-19 joule where a joule is the amount of energy (or power) required to produce one watt of power for one second — not a very large lightbulb at all. So, if joule’s are small, eV are teeny-tiny. But this is the atomic scale, after all, and there are billions and billions of atoms, so it can add up.

   An MeV is a mega-electron-volt or one million electron volts … still a tiny amount of energy on a human scale.

2. Beta rays are fast moving electrons. They typically have energies in the range of a few hundred keV to several MeV. Since electrons are much lighter than helium atoms, they are able to penetrate further, through several feet of air, or several millimeters of plastic or less of very light metals.

3. Finally, Gamma rays are photons, just like light, except of much higher energy, typically from several keV to several MeV. X-Rays and gamma rays are really the same thing, the difference is how they were produced. Depending on their energy, they can be stopped by a thin piece of aluminum foil, or they can penetrate several inches of lead.

These rays were known prior to the “quantum age.”

In addition to a lack of understanding of what force held the nucleus of larger atoms together, there was another concern that bothered scientists about the nucleus. That was the difference between the mass of the simple hydrogen nucleus, which they figured was the mass of a single proton, and the mass of higher elements. For example, helium, which should contain two protons, had a nucleus that was about four times as massive as the hydrogen nucleus. This disparity in mass was apparent in all the elements.

In chemistry and physics, the atomic number (also known as the proton number) is the number of protons found in the nucleus of an atom. It is conventionally represented by the symbol Z. The atomic number uniquely identifies a chemical element. The periodic chart of the elements, an important tool for understanding atoms as well as chemical and physical properties, is based on the atomic number.

In 1920, Ernest Rutherford conceived the possible existence of the neutron. In particular, Rutherford considered that the difference found between the atomic number of an atom and its atomic mass could be explained by the existence of a neutrally charged particle within the atomic nucleus. He considered the neutron to be a neutral double consisting of an electron orbiting a proton.

This seemed to make sense because we now know the neutron is just slightly heavier than the proton and the electron is a very small particle with less than one-thousandth the mass of the either nucleons — the term for protons and neutrons. Through the 1920s, physicists had generally accepted an (incorrect) model of the atomic nucleus as composed of protons and electrons. It was known that atomic nuclei usually had about half as many positive charges than if they were composed completely of protons, and in existing models this was often explained by proposing that nuclei also contained some "nuclear electrons" to neutralize the excess charge. Thus, the nitrogen-14 nucleus would be composed of 14 protons and 7 electrons to give it a charge of +7 but a mass of 14 atomic mass units.

However, the new quantum mechanics implied that a particle as light as the electron could not be contained in a region as small as the nucleus with any reasonable energy. In 1930, Viktor Ambartsumian and Dmitri Ivanenko in the USSR found that, contrary to the prevailing opinion of the time, the nucleus cannot consist of protons and electrons. They proved that some neutral particles must be present besides the protons.

In 1931, Walter Bothe and Herbert Becker in Germany found that if the very energetic alpha particles emitted from polonium fell on certain light elements, specifically beryllium, boron, or lithium, an unusually penetrating radiation was produced. At first this radiation was thought to be gamma radiation, although it was more penetrating than any gamma rays known, and the details of experimental results were very difficult to interpret on this basis.

The next important contribution was reported in 1932 by Irène Joliot-Curie (Madame Curie) and Frédéric Joliot in Paris. They showed that if this unknown radiation fell on paraffin, or any other hydrogen-containing compound, it ejected protons of very high energy. This was not in itself inconsistent with the assumed gamma ray nature of the new radiation, but detailed quantitative analysis of the data became increasingly difficult to reconcile with such a conclusion.

Finally, in 1932, James Chadwick performed a series of experiments at the University of Cambridge, showing that the gamma ray hypothesis was untenable. He suggested that the new radiation consisted of uncharged particles of approximately the mass of the proton, and he performed a series of experiments verifying his suggestion. These uncharged particles were called neutrons, from the Latin root for "neutral" and the Greek ending "-on" (such as the names of the electron and proton).

James Chadwick was a Physicist by profession. He was born in 1891 in Manchester, England. Chadwick got admitted in Victoria University, Manchester. He was more interested in studying mathematics but instead he was admitted in the field of physics mistakenly. Chadwick was pretty bashful, so he did not make any attempt to amend the error. In 1911, he graduated from the Honors School of Physics. He continued his studies at the same school in the laboratory of Ernest Rutherford.

Now with the discovery and confirmation of the existence of neutrons, the model of the atom was complete. Atoms in their normal state consist of a nucleus containing protons providing a positive charge and neutrons with no electric charge. This nucleus is surrounded by a cloud of electrons. Normally the number of electrons with their negative charge and the number of protons are equal. If an atom gains or loses an electron or two, it becomes an “ion,” a charged atom. Positive ions have lost electrons and negative ions have gained electrons.

That seemed a good and simple model and worked well for chemistry and electrical devices until science looked closer. Again radiation from large atoms provided some clues as did the powerful “rays” from space called cosmic rays provide others. Soon scientists realized that these three basic particles were not all there is. The new, sub-atomic particles being discovered soon became a zoo of new and strange objects. But,before we start splitting atoms, we should discuss this strong force that could hold the nucleus together and the role that neutrons play in that force, for there is yet another nuclear force to be found … next time.

The History of Science -- Part Twenty