Safety_Series_025_1968 - gnssn - International Atomic Energy ...

This publication is no longer validPlease see http://www.ns-iaea.org/standards/A s a w hole then natural ra d ioa ctiv ity re su lts in an ex p osu rew hich a v era ges about 100 m illir e m s a y e a r in sedim entary a re a s.C osm ic radiation in crea ses with altitude, being three tim es asm uch at 3000 m etres as at sea level. T here is little variation withlatitude.The dose d eliv ered to the body by the natural radion u clid es itcontains does not vary as the body content o f and 14C is stable.On the other hand the abundance of radioactivity in the soil may resultin wide variation s in extern al d ose. In som e parts of the w orld, inB ra zil o r India fo r exam ple, the corresp on d in g dose is as m uch as1500 m illire m s a y e a r. T hese a re excep tion al c a s e s , but in v e rym any c a s e s the d ose is betw een 150 and 400 m illir e m s a y e a r,e sp e cia lly fo r populations livin g in g ra n itic d is tricts w h ere h ou sesa re built with m a teria ls drawn fro m the s o il.2.2. M EDICAL EXPOSU RE2. 2. 1. S ources o f m edical exposureThe radiation sou rces used for m edical purposes may be cla s s i­fied as extern al s o u rce s o r internal s o u rce s a cco rd in g to w hetherthey a re applied outside o r in side the human body.2. 2. 1. 1. E xtern al s o u rcesE xtern al s o u r c e s in clude ra d ia tio n -p ro d u cin g equipm ent andseale d ra d ion u clid e s o u r c e s .2. 2. 1. 1. 1. Radiation-producing equipm ent. The radiation-producingequipment used in m edical radiology is, in practice, all based on thesam e p rin cip le . A cce le ra te d e le ctro n s a re p ro je cte d on a ta rgeto r em itted fro m the apparatus. In the fo r m e r ca s e a bea m o f X -ra y s is g en erated , in the la tter a bea m o f e le c tr o n s .(1) X -ra y equipment com p rises three separate parts: (a) an e le c ­tron generator; (b) an acceleratin g d evice; and (c) a target.E lectron g en era tors a re b a sed on the th e r m o -io n ic e m issio n .A m etal heated to a high tem perature em its electron s which can besca tte re d fro m the m etal if they a re su b jected to e le c t r ic fo r c e s .54