Answer to Question #609 Submitted to "Ask the Experts"

I have been asked to answer your question about neutron radiation. First of all, neutrons are well known and have been extensively studied since they were first discovered. But, they just are not that common for human exposure. Radiation is a term we use to talk about energy being released from atoms that have too much energy for one reason or another. Radiation is in the form of energetic particles or electromagnetic waves.

While there are many different radiations, the four we tend to deal with the most are alpha particles, beta particles, gamma, and x rays. The fifth one is the neutron. First let me give you a little history. While radiation has always been a part of our world, we learned about it starting in 1895, when Wilhelm Roentgen's attention was drawn to a glowing fluorescent screen on a nearby table. Roentgen immediately determined that invisible rays were originating from the partially evacuated glass Hittorf-Crookes tube and he named the mysterious rays x rays.

Upon learning how Wilhelm Roentgen discovered x rays by observing the fluorescence they produced, Henri Becquerel chose to work with a double sulfate of uranium and potassium which he exposed to sunlight and placed on photographic plates wrapped in black paper. He demonstrated that these salts also produced energy or radiation. At about the same time, Madame Marie Curie and her husband Pierre began investigating the phenomenon of radioactivity recently discovered in uranium ore, which led to the discoveries of the elements polonium and radium. For their work, Roentgen was awarded the first Nobel Prize in physics in 1901, Becquerel was awarded the 1903 Nobel Prize in physics, and the Curies were awarded the 1903 Nobel Prize in physics. Madame Curie won a second Nobel Prize in 1911.

In the research that followed these discoveries, the beta and alpha particles were shown to have mass and charge, being natural products of the decay of uranium, radium, polonium, and other elements. Gamma and x rays were also found to cause effects in air and materials, being the product of radioactive elements and x-ray tubes. At that time, many scientists were investigating the properties of matter and how atoms were made up.

Now back to the neutron. James Chadwick, at the end of World War I, went to England and joined forces with the eminent scientist Ernest Rutherford. Intrigued by Rutherford's speculation about a subatomic particle with no charge, Chadwick began a series of experiments to demonstrate the existence of such a particle. Initially, none of the experiments succeeded. Then, in 1930, Walther Bothe and Herbert Becker described an unusual type of gamma ray produced by bombarding the metal beryllium with alpha particles. Chadwick recognized that the properties of this radiation were more consistent with what would be expected from Rutherford's neutral particle. The subsequent experiments by which Chadwick proved the existence of the neutron earned him the 1935 Nobel Prize in physics.

Neutrons then did arrive years following the discovery of radioactivity, x rays, alpha, and beta particles. But, starting in 1930, scientists worked, researched, and found new uses for neutrons. Just before World War II, scientists theorized that by bombarding isotopes of uranium, it would be possible to split the uranium atom, releasing energy and more neutrons, the process which could go on in a chain reaction. This fissioning of uranium led to atomic weapons and nuclear reactors, which by the way are good sources of neutrons.

A neutron is a particle that is found in the nucleus, or center, of atoms. It has a mass very close to protons, which also reside in the nucleus of atoms. Together, they make up almost all of the mass of individual atoms. Each has a mass of about 1 amu, which is roughly 1.6×10^-27kg. Protons have a positive charge and neutrons have no charge, which is why they were more difficult to discover. Protons, alphas, betas, and gamma and x rays cause direct ionization, meaning that they transfer their energy upon interaction with matter by giving energy to electrons of atoms. These ions formed are easy to measure, simply using devices like Geiger counters.

Neutrons, being noncharged particles, interact in totally different ways. Without getting into too much physics or math, I can explain that neutrons interact by collisions with atoms. They transfer energy during these collisions. Consider an experiment, while not real, that may help explain how neutrons interact with matter. In this, you will shoot a neutron at an atom. I like to think of it like a pool or billiard game. As the moving neutron (cue ball) strikes an atom (eight ball), it will ricochet off but cause the other atom to move, hence imparting energy to it. As seen while playing pool, you have a variety of outcomes: the cue ball may transfer all of its energy to the eight ball, so that the eight ball moves off with the same speed as the cue ball. They could share energy, so that both continue to move. Or consider that instead of the eight ball, someone left a bowling ball on the table. The cue ball would ricochet off the bowling ball, not moving it at all.

This illustrates many of the basic concepts: the collisions can be 0-100 percent energy transfers, depending on the speed, angle, and size of the components, but all following the laws of physics. With this in mind, it should be noted that the best energy transfer is between a neutron and a target of the same size. As noted above, based on size, one of the best targets for energy transfer is a proton. Protons, like the nucleus of a hydrogen atom, if struck by a neutron will absorb energy and move. Now, instead of having a noncharged particle moving through the material, you have a charged particle, which will give up its energy through ionization. As the neutrons slow, they may end up being absorbed by atoms. This is one way that material is made radioactive, although the absorption of neutrons does not always lead to a radioactive atom.

The sources of neutrons could be nuclear reactors, making neutrons by fission and decay of fission products. Neutrons will also be produced in spent fuel. Also, several neutron sources are made by combining alpha-emitting isotopes like polonium or radiation with beryllium. An interesting neutron source is also made of the transuranium element ²⁵²Cf, which undergoes spontaneous fission. Some accelerators also produce neutrons, either by photon neutron production or by smashing a deuterium atom into tritium, producing fusion and neutrons. So, to actually answer your questions:

First, is this because neutron radiation is not as well understood? Neutron radiation itself is understood very well. The main reason you do not see much on neutrons is that they are only encountered in very set circumstances. Other radiations are used a lot more frequently in industry, research, education, and medicine. Neutrons are used for each of the above too, but just not on the same level as the other radiations.

Secondly, can you provide more information, especially in terms of health effects? The health effects of neutron radiation will be the same as any other ionizing radiation. It will cause, indirectly, ionization by interacting with the molecules of the body. Humans are made up mostly of water and water is made up of two hydrogen atoms and the neutrons have a probability of colliding with the protons. Upon collision, the neutrons will transfer energy to the proton, which then may go on to cause ionizations of surrounding tissue. The main area of cells that we are concerned with is ionization near the DNA/chromosome. DNA damage is either repaired or leads to the cell dying. In the rare instance that the damage is not repaired or is repaired wrong and the cell survives, the main risk is an increased risk of cancer.

For more on risk of radiation, see the Radiation Information Network under the General Section. There are some added difficulties with estimating the amount of damage caused by neutrons though. Again, it goes back to the way neutrons interact with matter. Neutrons of different energies will have different probabilities of causing collisions. Very high-energy neutrons have a low probability of interacting with protons, and very low-energy neutrons do not have enough energy to knock the proton around much.

So, there is a direct relationship between the amount of biological damage and the energy of the neutron. Using experiments and computer simulations, we believe we understand the relationship well. In assigning doses to humans from neutrons, we split neutron energies up into ranges and give each range a value of biological damage. To see what this looks like, see the 10 CFR §20.1004 Units of Radiation Dose Web page. Under c, you will see the assigned biological damage (Q) to different ranges of neutrons, again, based on energy. There are some good texts, reports, and studies on neutrons I can also recommend if you would like more information. I hope this helps with your questions. If you have something specific in mind that I did not touch on, feel free to email me back.

Bruce Busby RSO Genentech