128 Applied Biophysics 



that for all these materials, more structural changes were pro- 

 duced by neutrons than by an equal dose of X-rays, from which 

 we may infer that in all these cases the conditions for break 

 production are of the same general types as those in Tradescantia. 

 There is some evidence, on the other hand, that in Drosophila 

 sperm, a single ion cluster may suffice. 



Since the ion density along an electron track exceeds 200 

 ions per micron only when its energy is less than v3.5 kilovolts, 

 not only is much of the ionization produced by the more energetic 

 electrons generated by, say 200 kilovolt X-rays, wasted as re- 

 gards chromosome-break production in Tradescantia and similar 

 materials, but any one particle is unlikely to break two chromo- 

 somes separated by a distance greater than the range of a 3.5 

 kilovolt electron, i.e., greater than 0.4 micron. 



For this reason, structural changes arising from the inter- 

 change of partners between two broken chromosomes almost 

 always involve the action of two separate electrons. It follows 

 that, when the dose is delivered in a short time, the number of 

 such configurations produced will increase as the square of the 

 dose. Furthermore, as the duration over which the total dose is 

 spread is increased, fewer abnormal configurations will be pro- 

 duced because each individual break may reform the original 

 chromosome, and the chance of this happening in preference to 

 an interchange formation increases with the interval between 

 the production of the two breaks. The same restriction does not 

 apply to the recoil protons generated by neutrons or to alpha 

 particles which maintain the required ion density over distances 

 much greater than the diameter of the whole cell. It thus comes 

 about that in Tradescantia: 



a. Simple breaks produced at any time in the cell cycle, and 

 certain structural changes (the so-called "isochromatid breaks"), 

 arising from the breaking of two sister chromatids lying almost 

 in contact at prophase, increase in proportion to dose, and are 

 independent of the duration of exposure for all radiations. The 

 number produced by a given dose increases with ion density. 



h. Structural changes involving two chromosomes, other than 

 the isQchromatid breaks referred to in a, increase in proportion 



