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Safety_Series_025_1968 - gnssn - International Atomic Energy ...

Safety_Series_025_1968 - gnssn - International Atomic Energy ...

This publication is no

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

This publication is no longer validPlease see http://www.ns-iaea.org/standards/E lectron a ccelera tion m ay be obtained either by e le c tric or bym agn etic fo r c e s . E le c tr ic a c c e le r a tio n o f e le c tr o n s is u sed inc la s s ic a l X -r a y equipm ent, fo r ra d io d ia g n o stic and con v en tion a lradiotherapy pu rp oses. The e le c tr ic potential d ifferen ces used arein the range of 50 000 to 400 000 V.E lectron s can a lso be a cce le ra te d by m eans o f p ow erfu l m agnetic fo r c e s . Equipm ent of the b etatron type p rod u ces a c ir c u la ra cceleration of the electrons situated in a magnetic field the lines offo rce o f which are perpendicular to the plane on which the electron sm ove.The a cce le ra te d electron s are p ro je cte d on a target made o f ap articularly heat-resistant m etal. The interaction between the e le c ­tron radiation and the ta rget atom s g iv e s r is e to X -r a d ia tio n andit is this X -ra d ia tio n which is used fo r the p u rp ose o f m ed ica lra d iolog y .The X -r a y beam thus obtained is ch a ra cte riz e d p r im a r ily byits quantity, quality and d ire ctio n .(2) E lectron radiation gen erators are based on the sam e prin ciplesas the foregoin g; they com p rise two parts only, an ele ctro n gen e­ra to r and an electron a c c e le r a to r . The electron beam , instead ofim pinging on a target, is used as it co m e s from the apparatus.A m ong the g en era tors studied above, on ly th ose capable o f givingelectron s su fficien t a ccelera tion a re used, electron radiation beingo f no in terest fo r th erapeu tic p u rp o se s excep t when the e le ctro nenergy is high enough to enter fa irly deeply into human tissu e. G e­n era tors used fo r this purpose a re m ainly lin ear a c c e le r a to r s andbetatron s. H ere again, the e le ctron beam is ch a ra cte riz e d by itsquantity, quality and d irection .2. 2. 1.1. 2. Sealed radionuclide s o u r c e s . Sealed radionuclide sou r­ces are used in m edical radiology for external irradiation when theyem it g am m a o r b eta ra d ia tion o f su ita b le in ten sity and q u a lity .(1) G a m m a -ra y s o u rce s a re m ainly p rovid ed by th ree ra d io ­nuclides, radium -226, cobalt-60 and c a e s iu m -137. The ch a ra cteristicso f such s o u rce s a re dependent on the h a lf-life of the ra d ionu clid e, tne radiation output p e r unit o f a ctiv ity and the radiationen ergy. The h a lf-liv e s o f 226Ra, 60Co and 137Cs a re 1600, 5, and30 y ea rs re sp e ctiv e ly . The output of a o n e -cu rie sou rce at onem etre per hour is 0.84 R for radium -226; 1.35 R for cobalt-60; and55

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