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In the vast number of discussions, academic lectures, and books/papers regarding nuclear radiation, I have always learned about the "three" basic types of nuclear radiation - alpha, beta, and gamma.

But recently, I watched Dr. Don Lincoln's (Fermilab) video about types of radiation - he discusses the four types of such radiation (the above three plus neutron radiation).

While there's no denying that neutron radiation is equally important as the other three in any research or discussion of nuclear radiation, it seems less mentioned by a large factor. My generalization here is made by personal experience reading and learning about nuclear physics from my atomic-age youth to the present.

Why is this?

Does it have something to do with how the other three are present in naturally-occurring elements (e.g. carbon-14, potassium-40, uranium ores, etc.) whereas neutron radiation is caused by the decay involving one of the other three?

I dare say that neutron radiation is perhaps the nastiest of them all when it comes to biological effects.

Please note that I have already reviewed the below Physics SE questions (and their answers):

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That's not all either: there are also nuclei that decay by emitting protons, including a few near the drip line that emit two protons at once. (I suppose that two-neutron emission is also possible, but harder to observe; I don't know offhand if it's actually been seen.)

Here's the thing: nuclei that decay by nucleon emission are enormously unstable, and decay very quickly. What we dig out of the ground and examine are only isotopes that are long-lived. Basically, the only primary radiation from long-lived isotopes are alpha particles (from heavy nuclei trying to get lighter) and beta particles (from nuclei trying to "fix" a proton-neutron imbalance). Essentially all gamma rays from natural radioactive sources are associated with a previous alpha or beta transition --- only a small handful of excited nuclear states live long enough that you can isolate the gamma emitters from the particle emitters. But since most decay cascades emit one matter particle and several gammas, analyzing the gamma emission is a major part of understanding the radioactivity.

Most neutron emission is a side effect of spontaneous fission (which I guess you could consider another type of radioactivity) or secondary radiation from an energetic particle interacting with another nucleus. These secondary interactions are rare. A passive neutron source like a AmBe alloy might convert a millicurie of alpha radiation (all safely captured in its casing) into less than a microcurie of neutron radiation. It's hard to get the neutrons out.

Now, if you're involved in an experiment where you are producing neutrons --- such as the famous uranium chain reaction that takes place in power plants, or any accelerator experiment where you have particles with energies above about 8 MeV --- then shielding against the neutrons will be your biggest safety concern. Not, mostly, because of any primary damage that the neutrons do, but rather because essentially anything that the neutrons touch becomes a short-lived beta-gamma emitter.

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All neutron emission is essentially instantaneous. This is because there is no Coulomb barrier, so the time scale is essentially just the size of a nucleus (~1 fm) divided by the typical speed of a nucleon (~.05c). This is why we can have proton decay (although it's rare) but not neutron "decay" in the sense of having a bound or metastable state with some significant half-life. A nucleus that's beyond the neutron drip line is not a bound system.

In real-world applications, neutrons are emitted during fission.

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    $\begingroup$ It's very counter-intuitive that adding a purely repulsive Coulomb interaction between an emitted nucleon and its parent nucleus can make the proton emission slower than neutron emission in a mirrored nucleus. But your explanation is (as usual) spot-on. All of the neutron-emitting nuclei with lifetimes long enough to measure are nuclei that emit a neutron after a beta decay. But up around the proton drip line in iridium and gold are some nuclei which decay by proton emission with lifetimes of microseconds to seconds. $\endgroup$
    – rob ♦
    Feb 25, 2019 at 22:02
  • $\begingroup$ It's very counter-intuitive that adding a purely repulsive Coulomb interaction between an emitted nucleon and its parent nucleus can make the proton emission slower than neutron emission in a mirrored nucleus. Yes, it's very counterintuitive. The more classic example is alpha decay. $\endgroup$
    – user4552
    Feb 26, 2019 at 0:15
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Natural sources of large amounts of neutron radiation are quite rare. Neutron radiation in large quantities is really only seen in a nuclear reactor or in a spallation neutron source. So from a practical standpoint, neutron radiation is of much less concern for most people.

Ultimately, even saying there are four types of radiation is oversimplification. You could add on proton radiation, neutrino radiation, muon radiation, and many others. It's just from a safety standpoint they very rarely come up.

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Because natural sources of neutron emission are not abundant. Most of these sources are synthesized in the lab for reactors, like Cf-252. However, neutron detection is a heavily studied subject, perhaps not to the extent as gammas are studied. Neutron detectors are used in accident-tolerant fuel testing, imaging, active interrogation systems, and medical nuclear science.

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