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06 September 2008

Answer to Question #1322 Submitted to "Ask the Experts"

Category: Environmental and Background Radiation — General

The following question was answered by an expert in the appropriate field:

I know that the average person at sea level is exposed to about 20-30 mrem of "cosmic radiation." But what is this "cosmic radiation" that actually reaches the earth's surface made up of? How much of it is composed of photons (that is, gamma rays) as compared to elementary particles (that is, muons)?

The information contained in this answer comes from Report No. 94 published by the National Council on Radiation Protection and Measurements (NCRP). The following fluence rates (flux densities) for cosmic rays at sea level are expressed as particles per square centimeter per second. They are calculated for the northern United States (54 degrees north) at the time of maximum solar activity (Table 2.1 NCRP 94):

Muons—0.019
Electrons—0.00455
Protons—0.000171
Charged pions—0.0000134
Neutrons—0.00646

The contribution from cosmic ray photons is relatively insignificant (ca. 0.1 percent of the total). Relatively little quantitative information is available because, in part, their interactions are difficult to distinguish from those of other particles in the detector systems. Clearly, it is the muon component that dominates at sea level. The median muon energy at sea level is on the order of 2 gigaelectron-volts (2 GeV). At sea level, the dose rates from the various cosmic ray particles are approximately as follows (from Fig. 2.6 NCRP 94):

Muons—2.5 µrad h^-1
Electrons—0.6 µrad h^-1
Protons—0.04 µrad h^-1
Charged pions—0 µrad h^-1
Neutrons—0.04 µrad h^-1

A good source of information about this and other related topics is UNSCEAR 2000, a publication of the United Nations Scientific Committee on the Effects of Atomic Radiation.

Paul Frame, CHP, PhD

Answer posted on 30 October 2001. The information and material posted on this Web site is intended as general reference information only. Specific facts and circumstances may alter the concepts and applications of materials and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice specific to whatever facts and circumstances are presented in any given situation. Answers are correct at the time they are posted on the Web site. Be advised that over time, some requirements could change, new data could be made available, or Internet links could change. For answers that have been posted for several months or longer, please check the current status of the posted information prior to using the responses for specific applications.