Answer to Question #14030 Submitted to "Ask the Experts"

What radiation dosimeter can be used in monitoring cosmic radiation?

I shall assume that your concern is with dose near the surface of the earth. While we often refer to background radiation, which does contribute to dosimeter readings, we seldom make any attempt to discern the cosmic component and, in fact, many common personal or environmental monitoring dosimeters may not be capable of providing a very accurate measure of dose from all significant cosmic components.

Expectedly, the largest ionizing radiation contributors to external dose are high energy muons. The muons come from the decay of pions in the upper atmosphere, the pions originating from primary particle (protons and some helium nuclei) interactions with atmospheric gases. Muons typically account for about 65% to 70% of the total cosmic-generated effective dose at the earth’s surface. The second largest dose contributors are neutrons, originating from various nuclear events in the atmosphere; these typically contribute about 15% to 20% of the effective dose. The third major contributors are x-rays and gamma rays, commonly accounting for about 12% to 15% of the effective dose. Protons and charged pions may add a bit more dose, usually not more than about 3% of the effective dose.

Based on the above, we can conclude that if our dosimeter is capable of measuring muon dose, neutron dose, and photon dose with adequate accuracy, we could obtain acceptably accurate dose estimates. Many of the muons that reach the earth surface are high in energy and exhibit mass collision stopping powers in many materials of about 2 megaelectron volt (MeV) per g per cm² (g^-1 cm²), not much different from many electrons and beta particles often encountered in other aspects of dosimetry. Thus, if the dosimeter is capable of providing adequate response to moderate-to-high-energy beta radiation, such a dosimeter may well yield acceptable muon dose results. Properly designed thermoluminescent dosimeters and optically stimulated luminescence dosimeters are examples of two types that might be suitable.  Similar dosimeter types would also likely be suitable for photon dose measurements, naturally requiring appropriate photon calibration. The neutron dose component assessment would likely require a dosimeter designed to be sensitive to neutrons. The dosimeter would have to provide proper sensitivity over a range of neutron energies from thermal to perhaps 20 MeV to obtain reasonable results. Many available dosimeter types may not be suitable without a calibration consistent with the expected energy distribution. Dosimeters that employ the albedo effect, would require special calibration since these dosimeters exhibit extreme energy dependence in their responses. Possible dosimeters such as those employing CR-39 polycarbonate films may be suitable for some measurements. It would be desirable to check dosimeter measurements against some instrumental measurements possibly using dose-equivalent/ambient dose-equivalent measuring instruments or doses interpreted from Bonner sphere measurements.

It is important to note that the above comments apply to evaluations of doses at the earth's surface. If you are interested in cosmic radiation dose measurements at higher altitudes, much different considerations might apply because of the significant changes in the relative contributions of both primary radiations and secondary radiations. Specialized detectors/dosimeters, not commonly used for dosimetry at the earth's surface, may then be required. These are beyond our present discussion.

I hope this is helpful.

George Chabot, PhD, CHP