Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

is exposed to cosmic radiation near sea level where the dose equivalent rate is ~240f.lSv/year (24 mrem/year). However, smaller populations receive more than five timesthis amount [e.g., Leadville, Colorado, at 3,200 m, ~1.25 mSv/year (125mrem/year)]. Exposures increase with altitude, approximately doubling every 1,500m, as there is less atmosphere to attenuate the cosmic radiation. Cosmic radiation isalso greater at the earth's poles than at the equator, as charged particles encounteredby the earth's magnetic field are forced to travel along the field lines to either theNorth or South Pole. Structures provide some protection from cosmic radiation; theindoor effective dose equivalent rate is ~20% lower than outdoors.Air travel can substantially add to an individual's cosmic ray exposure. For example,airline crews and frequent fliers receive an additional annual equivalent dose onthe order of ~1 mSv (100 mrem); some receive an equivalent dose several timeshigher. A 5-hour transcontinental flight in a commercial jet aircraft will result in anequivalent dose of ~25 f.lSv(2.5 mrem). Supersonic high altitude aircraft, such as theBritish Concord, have on-board radiation monitors to alert the crews to descend tolower altitudes if excessive cosmic radiation dose rates are detected. Spacecraft alsoexperience higher exposures to cosmic rays; the Apollo astronauts received an averageequivalent dose of2.75 mSv (275 mrem) during a lunar mission.A fraction of the secondary cosmic ray particles collide with stable atmosphericnuclei producing "cosmogenic" radionuclides (e.g./iN [n,p] I~C). Although manytypes of cosmogenic radionuclides are produced, they contribute very little (~4f.lSv/year [~0.4 mrem/year] or