Cosmic “rays” are extremely high energy particles that come from outer space. The majority of cosmic rays come from our galaxy, though some of the highest
energy particles come from outside our own galaxy, and some come from the solar wind generated by magnetic storms in the sun (2). About 90% of the cosmic rays are protons, 9% are helium nuclei (a particles), and 1% are electrons (3). The energies of cosmic rays are much higher than anything mankind could create—up to 1020 eV! To put that in perspective, the Large Hadron Collider—the most pow­erful particle accelerator on earth that discovered the Higgs Boson in 2012—can attain proton energies of about 7 TeV (7×1012 eV). As the energetic cosmic rays crash into the upper atmosphere, they create other particles such as pions, muons, neutrons, у rays, and electrons (2, 3). The muons are similar to electrons, but they are about 200 times heavier, and they are the main source of cosmic radiation exposure on the earth’s surface.

Fortunately, we live on a planet that is surrounded by an atmosphere that absorbs much of the cosmic radiation. At lower elevations, the atmosphere is dense and much of the radiation is absorbed, but as you get to higher eleva­tions, there is less atmosphere for the charged particles to interact with, so the dose is higher. That is one of the reasons that the dose is higher in Fort Collins than in Florida, for example. The dose from cosmic rays increases as a second order polynomial with altitude (4) and can easily be modeled. Fort Collins is at an elevation of 5,000 feet, a little lower than Denver, and the cosmic ray dose is 0.38 mSv per year, compared to a US average of 0.33 mSv and a dose in Florida of 0.24 mSv (Figure 8.2). People living in Leadville, a small town high in the mountains in Colorado at 10,152 feet, get an annual dose of 0.85 mSv from cosmic radiation. I have a cabin in the mountains at about 8,200 feet eleva­tion, about the same as Estes Park, and the dose there is 0.6 mSv/yr. The actual

dose rate depends on how much time a person spends outdoors compared to indoors, since the house provides some shielding against the cosmic rays (5). When you fly in an airplane at about 10,000 m (33,000 ft), the dose rate is much higher and depends on the exact flight altitude and path. On average, the dose rate is about 5 to 8 microSv/hr (2), so if you flew from New York City to London, about an 8-hour flight, you would get a dose of 0.04 to 0.06 mSv, about half the dose you would get from a chest X-ray. However, airline pilots and crew average about 500 hours of flight time at high altitude annually (2) and would get doses of about 2.5 to 4.0 mSv/yr.

The earth also has a magnetic field that extends far into space—the magneto­sphere. The magnetic field is important because the charged particles in the cos­mic rays are deflected by the magnetic field, just as an electric current is deflected by a magnet. Since the earth’s magnetic field is roughly aligned with the North and South Poles, the cosmic rays are deflected to the northern and southern regions of the earth. In times of severe magnetic storms on the sun, this leads to the phenomenon of the aurora borealis, or northern lights (and aurora australis, or southern lights, in the Southern Hemisphere). As a result of the magnetic field, there is a variation in cosmic ray exposure with latitude on earth, being greater at higher latitudes, though this is a relatively small factor, adding only about 0.01 mSv/yr to the dose in Colorado as compared to Florida.

There is one other interesting phenomenon associated with cosmic rays. The high energy protons create neutrons, which sometimes interact with nitrogen in the atmosphere. As described in Chapter 5, neutrons can sometimes be captured by nuclei. In this case, 14N captures a neutron but then spits out a proton and the resulting nucleus becomes 14C, a radioactive form of carbon that has a half-life of 5,730 years. Since the cosmic ray flux is relatively constant over time, the pro­duction rate of 14C is constant and becomes a constant fraction of total carbon in the atmosphere. When plants breathe in carbon dioxide to photosynthesize carbohydrates, a certain fraction consists of 14C while the remainder is the normal 12C (and a little 13C). Once plants die, the ratio of 14C to 12C begins changing as 14C p-decays back to 14N. This changing ratio allows scientists to estimate the age at which the plant died, and this forms the basis for radioactive carbon dating of archaeological sites. And, of course, we get a little 14C when we eat plants or ani­mals that feed on plants.