What is Radiation?

The first requirement is1

Radiation is a generic term in science and physics which is part of what leads to so much confusion in popular literature and the press. If you live in an old house, you may have steam radiators. A physicist has no problem referring to the heat that emanates from those as radiation. Or maybe you have a radiant heat system other than steam. Same thing, the heat coming from that is radiation (notice the radiant in the name, same root word).

In physics, we generally break radiation into two types: ionizing and non-ionizing. The difference is important in the context of nuclear physics. Both types are found naturally, and you are exposed to both on a daily basis. Your ancestors were too, even hundreds and thousands of years ago.

Non-ionizing radiation includes the heat radiating from your steam radiator, baseboard radiant heat system, or that portable space heater. It also includes the light coming from the fixture in your room and from the computer/tablet/phone screen you are using the read this. It include radio waves, microwaves, and similar. Those are examples, but you may still be wondering, what is it?

It's easier to start with ionizing radiation. Non-ionizing radiation is everything else. So...what is ionizing radiation.

Ionizing Radiation

The short answer is that ionizing radiation is radiation that has the ability to create ions, that is, it has the ability break chemical bonds. This is primarily a matter of the energy of the radiation. This ionization comes in two forms: particles which are emitted, and electromagnetic "radiation."

Alphas, Betas, and Gammas, Oh My!

Historically, when radiation was first discovered and measured, the types of radiation were grouped into three types named after the first three letters of the Greek alphabet. Why? Basically, no one yet knew what they were, so the types were categorized based on their measured effects in early detectors. Alpha radiation had the lowest penetration of ordinary objects and the strongest deflection by magnetic fields. Alpha radiation is stopped by a sheet of paper. Beta radiation was considerably more penetrating and could pass through several millimeters of aluminum. Gamma radiation is even more penetrating.

Initially, all three types were referred to as "rays" as in alpha rays, beta rays, and gamma rays.

Early on, Henri Becquerel made measurements of the charge-to-mass ratio of beta radiation particle and found it was the same as that for the (then newly discovered) electron. Ernst Rutherford and Frederick Soddy made measurements that showed that nuclei that decayed by emitting an alpha or beta particle changed types and that alpha emission resulted in changing the nuclear charge by 2 units and nuclear mass by 4 units. Those observations, and others since, have lead us to identify the alpha "ray" as an alpha particle which is the same as an ordinary helium 4 nucleus. The beta "ray" is the beta particle and is, in fact an electron.

But what about those gamma rays? They still hold that name and are identified as high energy photos, i.e., a high energy light, more high energy than ultraviolet light.

While alpha particle are non-penetrating, they do have enough energy to break chemical bonds and ingesting an alpha source (apart from any dangers of being poisoned by the chemistry of the alpha source) is very serious because now the alpha emission is right up next to the fast dividing cells of your intestines.

But Wait, There's More!

This neat division into three types of radiation is, however, incomplete. While it is describes all the forms of radiation emitted by naturally radioactive elements such as the classic U²³⁵, this is not all of the forms of radiation to which humans are naturally exposed. Any high energy particle is a potential radiation hazard. The earth is constantly bombarded by high energy "cosmic rays" (which are mostly relativistic protons possibly generated light years away in supernova explosions) which, when they collide with gas molecules in the upper atmosphere, produce showers of other high energy particles which make it all the way down to sea level, even passing through several feet of concrete.2

The Danger

The danger from ionizing radiation comes from it's ability to break chemical bonds. This include the chemical bonds that form our DNA, the proteins in our bodies, and simpler molecules that, when their bonds are broken, can form free radicals that do further damage. While the human body is an amazing machine and has numerous repair mechanisms, this damage cannot always be repaired and can lead to long term disease including (the most notorious) cancer. If enough ionizing radiation is received in a small period of time, so much damage can be done that the body begins to fail before repairs can progress. We'll come back to this later. For now, there are two take-away points: (1) ionizing radiation is the type that is particularly dangerous to human health, and (2) some sources of ionizing radiation are quite literally unavoidable.

Non-ionizing Radiation

Non-ionizing radiation is all the rest. Really. It's the stuff that makes up sunshine, radio waves, those nice cell phone 4G signals, WiFi signals and all the rest. So it's the safe stuff, right?

Mostly, but not quite. While non-ionizing radiation cannot break chemical bonds and thus can't do that genetic damage that results in cancer, remember that microwaves can cook you food. Non-ionizing radiation can still do damage, though that is mostly due to localized heating. While you want you microwave to cook or heat up your food, you don't want them to cook you. You want your steam radiator to irradiate your room with infrared radiation to make it nice and cozy on that snowy night, but you don't want it to cook you or leave you with heat prostration. Fortunately, our bodies are equipped to sense heat and infrared sources (like your steam radiator or space heater) are easy to detect and correct.

Microwaves can act quickly which is why microwave transmitters are guarded by fences and warning signs and why your microwave has a wire mesh on the window; this prevents the microwaves from passing through the window and into your face while you peak in to check on your food. But I would not recommended spending hours with your nose pressed against the glass.

References

1. Ionizing Radiation, on Wikipedia
2. Ionizing Radiation and Humans - The Basics, from American Physical Society
3. What is Ionizing Radiation? from the World Health Organization
4. IDTIMWYTIM - Radiation, YouTube SciShow
5. What is Ionizing Radiation, YouTube CDC. Note that the speaker quickly moves from using the more specific term "ionizing radiation" to the generic "radiation." This is common among scientists where the context is understood to mean ionizing radiation but also leads to most people not realizing there is a difference.

• 1. https://meritbadge.org/wiki/index.php/Nuclear_Science, November 2017
• 2. A standard experiment when I was an undergraduate at MIT was to measure muon velocity. The muon source was naturally created muons from cosmic rays and the experiment was conducted indoors in a room that had multiple concrete floors above us. That lead to the quick realization that muons are very penetrating.