Radiation: From A(toms) to S(ieverts)

Radiation is essentially various shrapnel that is released when an atom or atoms undergoes decay, fusion or fission. In this article, I will discuss the basics of radiation, types of danger of radiation, where it comes from, and a framework of risk with radiation. This is meant to inform some of the possible effects of radiation as a result of nuclear accidents or purposeful detonations.

For more on the topic of nuclear non-proliferation you are invited to attend the talk by Ambassador Robert Gallucci, President of MacArthur Foundation, on May 25th at Hughes Auditorium, Northwestern University.

What is radiation?

At the time of formation of the Milky Way and the Solar System, most if not all elements were radioactive. Radioactivity is essentially the potential of an atom to release radiation. If we thought of a radioactive atom as a light-bulb, then radiation is the emanating light. Radiation in general has a number of sources other than atoms, but most nuclear radiation is caused by either atom decay, atomic fusion or fission.

Rays of All Kinds

Radiation comes in a number of flavors and a number of different properties, but the most common from nuclear material are neutron, followed by alpha, beta and gamma rays. Despite conventional wisdom, nuclear material doesn’t normally glow. At specific conditions it can ionize the air which can emit low light. More commonly, it can ionize water which does glow. In room temperature, they can be largely inert. It is said that Luis Alvarez used to keep a flat disk of plutonium as a paperweight on his desk. It produced short-ranged alpha particles, which are stopped by even very thin materials, like paper or skin, so it was practically harmless.

Rays	Composition	Effect
Alpha	2 protons, 2 neutrons	Stopped by paper (skin)
Beta	energetic electron (same energy as alpha)	Numerous collisions, attach themselves to an atom
Gamma	packet of light (1M photons worth)	Usually absorbed by single atom
Neutron	massive no-charge particle	Doesn’t interact w/ atoms, can cause sickness
X-rays	packets of light (1/100 Energy of Gamma)	Absorbed by high-atomic number elements (not low)
Cosmic	Protons, Electrons, Gamma, X-rays, Muons	Very high in penetration (used to x-ray pyramids)
Fission fragments	Decay products	Problems when they stop and re-decay
Cathode rays	Confused for electrons turn of 19th to 20th century	Just the same as electrons (CRT monitors)
Neutrinos	small mass particles, largely inactive	No effect – 10^10 through every cm^2 every second
Cellphone radiation	Microwaves	Cannot break DNA molecules, only heat

Sources: Mueller, “Physics and Technology for Future Presidents”, Chapter 4

Radiation Damage

There are three types of radiation damage: 1) Radiation burns (tissue damage); 2) Radiation sickness (metabolism disruption); 3) Cancer (DNA damage). The first usually accompanies a nuclear explosion, and as such causes immediate damage. If one survives a large dose of radiation, it still can cause radiation sickness as rays such as gamma, which can move through the body and disrupt cellular chemistry. If one survives the radiation sickness, there is a chance that some of the rays were able to shred DNA strands, or interrupt protein sequencing, which can cause cancer cells as a result.

Radiation is measured as a dose, with 3 different considerations – per instance (such as x-ray or CT scan), daily (for visiting a nuclear power plant) or annual (for workers in nuclear power plants). The basic dose is measured in the amount of energy deposited by rays to a certain body mass or volume, and that is measured in SI units called Greys (and older unit still in use is a rad (Radiation Absorption Dose) – 100 rads = 1 Grey). However, different parts of the body absorb different amounts of radiation, e.g. skin absorption rate is less than the lungs absorption rate. That is part of the reason why smoking and second-hand smoking don’t have the same cancer rates. Also, different rays have different effects on the body, and different penetration rates (alpha rays are stopped by skin, while muons pass through almost all matter without interacting).

So, if we include the consideration for the different absorption rates of body and the different penetration rates of the ray, we can calculate the standard units of radiation effect, called Sieverts (and an older unit still in use called a rem (Roentgen Equivalent in Man) – 100 rem = 1 Sievert).

Some common sources of radiation

Living on earth, we are exposed to common types of background radiation, which are usually from cosmic rays and the radon gas, which is released slowly from granite close to the surface. Certain elements, like Potassium, which are indispensable for human existence, also happen to be slightly radioactive, so sleeping next to anyone means receiving a small dose of radioactivity. For the purposes of comparison, the below table switches half way between microSieverts (10^-6) and miniSieverts (10^-3). A miniSievert is roughly equivalent to 200 million Gamma rays passing through every square centimeter of a human body.

Radiation Source	Dose	Units
Eating 1 banana	0.1	μSv
Living within 50mi of a Nuclear Plant	0.1	μSv / year
Living within 50mi of a coal plant	0.3	μSv / year
Dental X-Ray	10	μSv
Sleeping next to someone	30	μSv / year
Chest X-Ray	25	μSv / year
Mammogram	3	mSv
Avg Background	3	mSv / year
NY to Tokyo Flight (airline crew)	9	mSv / year
Smoking 1.5 packs per day (low)	13	mSv / year
Smoking 1.5 packs per day (high)	60	mSv / year
Maximum Dose for Radiation Worker	50	mSv / year
Average dose for radiation sickness	400	mSv
Usually Fatal Radiation poisoning	4000	mSv

Sources: Nuclear Regulatory Commission, Environmental Protection Agency, Mueller “Physics and Technology for Future Presidents”

For a visualization of these numbers and others, please see this radiation chart.

These tables are mostly useful to present the scale of different activities and radiation exhibited by a variety of activities. It shows that nuclear power plants, in normal operation, are far less radioactive than even a human being. Smoking is by far the most dangerous elective activity in terms of radiation (the low to high difference is due to the type of tobacco, kind of smoking, etc.). However, converting this into death risk is very hard to determine. If we assume (like the EPA) that there is a linear relationship between the radiation dose and cancer probability, 250 mSv causes an increase the lifetime risk of cancer by roughly 1%. So, with an average background radiation of 3 mSv, we would expect to see 0.01% cancer deaths from background radiation. However, at smaller values, this relationship is much harder to determine and likely to hard to observe in the total population of cancers.

Overall risk framework

One of the hardest things to frame in radiation is the risk involved, which we usually express as risk of death. The following table summarized average US data for years 2001 to 2007.

Cause of Death	% of Deaths
Perinatal problems	0
Pregnancy/childbirth complications	0
Accidents – Other	0.2
Congenital defects	0.2
Skin diseases	0.2
Undetermined – Homicide, Suicide or Poisoning?	0.2
Blood diseases	0.4
Musculoskeletal diseases	0.6
Non-cancerous growths	0.6
Homicide	0.7
Ill-defined symptoms/causes	1.1
Suicide	1.3
Mental and behavioral disorders	2.2
Urinary tract diseases	2.4
Infectious and parasitic diseases	2.6
Digestive system diseases	3.6
Endocrine/metabolic diseases	4.1
Nervous system diseases	4.1
Accidents	4.2
Respiratory diseases	9.7
Cancers	23.2
Circulatory system diseases	38.6
Total	100

Source: Carnegie Mellon Death Risk Rankings

This table shows that most commonly people in the US die from circulatory system diseases and cancers. What is problematic is that radiation is not the singular cause of cancer, it is usually a minor cause of cancer. And while some cancers are more highly correlated with radiation than others, the small amount of radiation exposure cases means that the relationship is often hard to distinguish from random error. A study done on a very large cohort of similarly exposed subjects (19,000 European pilots) there was not enough data to statistically link cosmic radiation and cancer. This doesn’t mean that the risk should be ignored. Radiation is a real risk, but a risk that can be assessed, evaluated and managed.