Neutrons: “What are they?”

Neutrons are so important, the massive gravity of this formative and pivotal 5MN article might stretch our five minutes relative to an outside observer! Just a little. Let’s get started.

Neutrons are medium-large particles that do not have charge, but they do have energy. And they can have a lot of it! We know they have a rest energy  because they are “stuff”—they have mass. When neutrons are moving they have a measure of kinetic energy that is vitally crucial for nuclear technologies. Here we discuss what neutrons are, get a basic idea of what they do, and some neutron fun facts. We need to know about fundamental forces to understand these nifty little life-saving buggers.

Neutrons are one of two nucleons that make up every nucleus of any natural chemical element together with the protons. Neutrons and protons themselves are each made of three quarks, which are the elementary particles that form the basis of everything according to the Standard Model of particle physics. Within the nucleus, the neutrons are held very close to the protons by the strong nuclear force. Neutrons can do some very interesting things, some of which are unique strictly because they are neutral in charge. But more on that later here and in related articles.

Neutrons have an incredibly tiny but accurately known mass of 1.674927471 × 10−27 kilograms metric. A hundred million billion neutrons would fit in a fine grain of sand. Incredible! (Scientists, engineers, technologists in many specialties and more need shorthand for very small and large numbers called scientific notation, no less. Here our neutron has a mass of 0.000000000000000000000000001674927471 kg. Did I miss a zero?)

How do we work with so vanishingly small a mass? We need Einstein’s help. The great physicist realized that light speed is strangely constant (represented by the letter c ), and even more bizarre that if we could actually travel that fast (we can’t), time would freeze, the distance in front would shrink to zero, and we would be everywhere simultaneously! He was even more astonished when he also realized a shockingly direct link between mass (m) and energy (E) by the most famous math equation in history:

E = mc2

and so we need only measure its energy to learn its mass. Better. But the immense difficulty in dealing with such an unimaginably small amount of stuff is amazingly wiped away and made doable if we accept the trade-off to use a roundabout method: standardize energy in terms of the third fundamental atomic particle—the even smaller negatively charged electron at about 1,800th the size of the neutron. Eureka!

The energy in joules (J) it takes to move the amount of charge in coulombs (C) on the electron (e or sometimes q, 1.602 × 10−19  C that is the elementary charge and so the same for the proton) through an electric potential difference of 1 volt (V) is 1 electronvolt (eV).

Atomic and subatomic masses are in the millions of electonvolts (MeV) per c squared and this large regime is perfect for measuring extremely small particles. Fantastic! So, the mass of a neutron is 939.56563 MeV/c2 to eight significant figures, where c is 299,792,458 meters per second accurately known to at least nine significant figures, and c2 is a stupendously large number. At last we have the extremely large that helps us with the unbelievably small. Awesome! Thank you, Dr. Einstein. Can we drop your famous c2 term now to make things easier? Sure. It’s famous and well understood.

What’s even more amazing, a neutron is only a tiny fraction bigger (about 0.1%) than a proton’s 938.27231 MeV. This slight mass difference accounts for nuclear stability—i.e., absence of dangerous radioactivity. A band of stability is observed in the elemental isotopes up to lead (with 82 protons) except for technetium (43 protons) and promethium (61 protons). (Greek isos “equal” and topos “place” translates to the location of nucleons, namely the nucleus.) Ultimately, the small extra mass of the neutron keeps atoms of the materials we use every day from perhaps slowly killing us from higher background radiation levels—assuming we knew more about radiation effects on our evolutionary time scale. Thankfully, we’ve had many hundreds of thousands of years to adjust to natural radiation that surrounds us every day. So much for being afraid of radiation. We’re constantly bathed in it. Astounding!

(Here to see the isotopic stability chart from Brookhaven National Laboratory and here for an illuminating description.)

What do they do?

Neutrons have several roles in the universe. But their most important act is to help hold the nuclei of all atoms together by adding to the strong nuclear force—one of the four fundamental forces in nature together with the weak nuclear force that accounts for radioactive decay and is crucial for nuclear fission, the familiar electromagnetic force (cell phones), and gravity (Sandra Bullock).

The idea of how this works is not hard to imagine. The strong nuclear force interacts at two atomic-scale ranges measured in quadrillionths of a meter or femtometers (fm). At short range (0.8 fm), it holds the quarks together that form protons, neutrons, and just about everything else you can think of. At slightly longer range (1 to 3 fm), it turns out that the strong force extends just beyond the effective boundaries of the nucleons. (The idea of an actual nucleon sphere is conceptual to help illustrate.) This residual nuclear force, which I’m sure you’d agree is a very appropriate name, acts slightly outside the nucleon “sphere” to bind neutrons and protons in the nucleus separated by a very slight distance (about 1 fm); very close but not touching. Physicists are actively working at CERN to find out how the strong nuclear force can both bind quarks and nucleons and at the same time keep nucleons from coming too close to each other. Weird! (An amount of nuclear binding energy gets released during nuclear fragmentation and is harnessed in commercial nuclear energy and nuclear fission weapons—much deeper implications for a series of other 5MN articles and content.)

The first bound neutron to the simplest nucleus in nature—normal hydrogen (H) called protium with its single positively charged proton—forms the second natural isotope at the start of the band of stability. So-called deuterium (D) substitutes the two protium atoms in the water molecule (H2O) and forms heavy water (D2O), an excellent neutron moderator. Neutron moderation is crucial in nuclear engineering for power and weapons. And that was oxygen (8 protons) in the water, by the way, that needs no introduction.

Helium is named after the Greek sun-god Helios, symbol He, and is the next chemical element after hydrogen with its two protons. Coulombic force—the whole “opposites attract and likes repel” idea (common with magnetic force) that is the electrostatic force of lightning—would normally never allow helium’s two protons to ever join together into a nucleus if not for its two neutrons. Its first natural isotope (He-4) can lose a neutron and become He-3 that other than H-1 is the only stable isotope of any element with more protons than neutrons! Helium-3 keenly illustrates the threshold case for the residual nuclear force: a sole neutron provides just enough strong force to overwhelm the coulombic repulsion between its two protons that would send them flying apart in nuclear disintegration! One single neutron. Wow.

The first or alpha  mode of natural radioactive decay is when a nucleus heavier than antimony (51 protons) ejects a helium-4 nucleus at high speed. The lightest known alpha emitters are the lighter artificial isotopes of tellurium (52 protons). Polonium-210 (Po, 84 protons), first obtained from uranium ores by Marie and Pierre Curie in 1898, is an infamous alpha emitter. James Chadwick in 1932 used a Po-210 alpha beam aimed at a beryllium (4 protons) target to generate a secondary beam that could not be deflected by the coulombic force of an electric field. He had discovered the neutron! (Here for a gripping nuclear story.)

Because charge interacts with only another charge and neutrons don’t have any, they do not experience coulombic attraction or repulsion. This is really cool, because it allows them to get very close to a nucleus without the electrons or protons having a clue. The neutron is a subatomic ninja! It can sneak in through the electron cloud and get so near to the nucleus that the residual nuclear force takes over like a tractor beam and draws the neutron into the nucleus in a process called neutron capture. Once again, this is all just something that happens in nature. Life as we know it would not exist otherwise!

Well that was nifty. So what can I do with them?

In spite of the fact that we’ve learned the stunning reality neutrons significantly contribute to the basis of life as we know it, the application of our knowledge of the properties of neutrons is also very important in energy production and nuclear medicine. While we now know that the neutral nucleons provide substance to the world, there are still some very interesting applications that usually involve launching them at something else.

Hurtling neutrons toward the heaviest elements such as thorium with its 90 protons or uranium with 92—and by now we can see the sum of nuclear protons is the atomic number for any element—can result in fission of their nuclei under specific conditions. Another possibility is the just-mentioned neutron capture, which induces radioactive decay in a process called neutron activation. Protons can even be ejected from the nucleus in some situations called proton emission that is another fascinating stroll along the path in the particle zoo. There are actually more interactions that will be covered when we discuss neutron cross-sections, which is really awesome because this is how we can predict what the outcome of a nuclear reaction will be!

You see, if we were to shoot at a nucleus a charged particle such as an electron or proton instead of an uncharged neutron, it would have to deal with very large coulombic forces acting on it. A fast proton would never get close enough for the residual nuclear force to grab on below the coulombic barrier. Too bad, Chad! Nature said charged particles cannot be subatomic ninjas. Believe it or not, to accelerate charged particles in an attempt to overcome the coulombic barrier is exactly what nuclear fusion is all about—the Holy Grail of nuclear energy. Look at that! You incidentally learned how nuclear fusion works! You’re so smart! Gold star!

In closing, we’ve learned what neutrons are and some basic atomic physics, that they have really interesting abilities due to their size and lack of charge especially with respect to nuclear stability, what they can do for us, and as a bonus we learned the grand premise behind nuclear fusion. Continued in the 5MN neutron series are discussions of the interaction of neutrons with varying types of matter (what actually happens when we hit stuff with them) and other important properties of neutrons and nuclei.

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Tim Meyer

Former Army Corps of Engineers environmental chemist; B.S. chemistry UWSP 1985, M.S. chemistry ASU 1987. Present media specialist and energy policy advocate.

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About Tim Meyer

Former Army Corps of Engineers environmental chemist; B.S. chemistry UWSP 1985, M.S. chemistry ASU 1987. Present media specialist and energy policy advocate.

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