Cosmic rays are extremely energetic particles, primarily protons , which originate in the sun, other stars and some of the violent cataclisms which occur in the far reaches of space. The cosmic ray particles interact with the upper atmosphere of the earth and produce showers of lower energy particles. Many of these lower energy particles are absorbed by the earths' atmosphere as they travel down to the surface. At sea level the cosmic radiation is composed mainly of muons , with some gamma-rays , neutrons and electrons .
Because the earth's atmosphere acts as a shield the dose rate received from
cosmic rays will be greater at higher elevations than at sea level.
Fig. 4.1 
shows the equivalent dose rate due to different particle
components as a function of height in the atmosphere. The data is for a
latitude of 55 
and for a solar minimum.
Figure 4.1: Equivalent Dose Rate for different particle
types as a function of height in the atmosphere