RADIATION DOSE 



contributed solely by particles of either extreme of LET. Even monochro- 

 matic sources of X, y, and neutron radiation give rise to spectra of secondary 

 particles, and each of these particles loses energy at a continuously varying 

 rate before coming to rest. The particles, therefore, contribute to the dose at 

 a continuously varying LET. Typical LET spectra resulting from the 

 exposure of biological material to ""Co y-rays, 200 kV X-rays, 2 MeV 

 neutrons, and polonium a-particles, are shown in Figure 6. Even these spectra 

 fail to give the full picture of energy loss in one important respect, since they 

 take no account of the secondary ionizing particles of low energy (8-rays) 

 which are set in motion along the tracks of the primary particles. These §-rays 

 are seen very clearly as little spurs in all the cloud chamber photographs of 

 Figure 2, except the lowest one, which depicts energy loss along the last 

 micron of an a-particle track where the S-rays have so little energy that their 

 ionization merges with that of the primary particle at the level of resolution 



12 



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200kV 

 X-rays 

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5MeV Neutrons 

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5-5MeV 

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500 1000 

 ekV/fJL 



Figure 6. LET spectra 



(Reproduced by kind permission of P. Howard-Flanders) 



of the photograph. In the case of a-rays, the accompanying S-rays represent 

 a small contribution of the dose from particles of much lower LET than that 

 of the main column of ionization. In the case of X and y radiation the reverse 

 is the case. The S-rays represent a contribution of some 10 per cent to the 

 total dose from particles of very much higher LET — -a contribution which, 

 though small, is in many cases of very great importance since it falls in a 

 range of LET which has an extremely high R.B.E. Lea^^ in fact concluded 

 that chromosome structural changes induced in Tradescantia microspores by 

 X and y radiation can be ascribed almost entirely by such very slow elec- 

 trons. The range of LET values covered by S-rays is roughly that of the band 

 between the broken vertical lines in Figure 6. 



Attempts have been made^** to compute LET spectra which take account 

 of the S-rays, i.e. to derive distributions of dose in LET for material exposed 

 to some of the more common sources of X and y radiation. These show the 



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