Safety_Series_025_1968 - gnssn - International Atomic Energy ...

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This publication is no longer validPlease see http://www.ns-iaea.org/standards/1. 2 . 1. 5. 3. D ose e ffe ct rela tion sh ip fo r late e f f e c t s . One o f thebest-kn ow n late e ffe c ts is ca ta ra ct fo rm a tio n . A fte r irra d ia tio n ,abnorm al ce lls a rise which rem ain inside the lens and form cen treso f op acity. N eutrons are ex trem ely e ffe ctiv e in this sen se and anexposure of 5 - 15 rads o f neutrons was su fficien t to cause radiationcataract among Japanese bomb su rv iv ors. The relationship betweenexposure and degree of lens opacity in the rat is illustrated in F ig. 9.DURATION OF IRRADIATION (h )FIG. 8. Changes in m itotic index (i.e . proportion o f cells in mitosis) o f chick fibroblasts in tissueculture cells following irradiation [ 6 ],A nother late e ffe ct o f irra d ia tion is life -sh o rte n in g . E x p e r im ental anim als exposed to continuous irra d ia tion throughout th eirlife -s p a n g en era lly show a life -s h o rte n in g . A lin ea r rela tion sh ip(F ig . 10) is obtained if the p e r-ce n t reduction of life -sp a n is plottedagainst dose rate (r a d /d ). It is in terestin g that irra d ia tion withneutrons is apparently e ffe ctiv e at v e ry sm a ll d ose ra tes.The data fo r leukaem ia induction am ong the H irosh im a andNagasaki su rvivors indicate that in the range between 100 and900 rads the average rate of in crea se of the incidence with dose was1. 1 ca ses p er m illion p er y e a r p er rad at H iroshim a and 1 .6 ca se s2 4
This publication is no longer validPlease see http://www.ns-iaea.org/standards/p er m illion p er y e a r p er rad at N agasaki. The relation o f averageincidence of con firm ed leukaem ia to dose in rads is plotted in F ig .ll.The relation sh ip is roughly lin ea r fo r both c itie s but the rangeo f e r r o r s is con sid era b le. Between 10 and 100 rads the in ciden cesfo r the two citie s do not d iffer sign ifican tly.NEUTRONS: NEUTRONS: NEUTRONS:DOSE(rads)FIG. 9. Relationship between exposure and degree o f lens opacity [6 ],1. 2. 1. 5. 4. The im p orta n ce o f d ose rate. A given dose, m ay bedelivered o v e r different tim e in tervals, i . e . at different intensities.The exponential d o s e -e ffe c t cu rv e is in m ost ca s e s not affected byapplying the sam e dose with differen t dose rates, becau se separatep rim a ry events act independently. If, on the oth er hand, sfeveralevents are needed in a very short tim e interval to produce an effect,a given dose b ecom es le ss effectiv e when p rotra cted . The reasonfo r the dim inution o f the final effect lies in the re cov e ry p r o c e s s e swhich take place at low radiation intensities. In the case o f theinduction o f certa in ch rom osom e a b erra tion s, fo r exam ple - likech rom osom e exch anges w hich need two sim ultaneous ch rom osom e25
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