152 Applied Biophysics 



with this inference is the fact that the vield of neutron-induced 

 interchanges increases in linear proportion to dose.^^- ^^' ^^ 



X-rays ionize by means of electrons, the ionizations in a path 

 being in ckisters spaced apart, except very near the end of the 

 path where the electron has lost most of its energy. Neutrons 

 ionize by means of protons, the ionizations in the path forming 

 a dense column. For a given dose, depending upon the X-ray 

 wave length and the neutron energy respectively, about ten to 

 twenty times as many electrons as protons would traverse a 

 nucleus. It is for this reason that, at the low dosages normally 

 employed, neutron-induced interchanges are predominantly one- 

 hit, while X-ray-induced interchanges are predominantly two-hit. 



Providing that X-ray doses are measured in rontgen units 

 and neutron doses in z'-units, units which represent approxi- 

 mately equal energy dissipations in tissue, the ratio of the yields 

 of chromosome aberrations for equal doses of the two radiations 

 may be taken to be the ratio of the efficiency per ionization of 

 the densely ionizing particles (protons) in neutron experiments 

 to that of the less densely ionizing particles (electrons) in X-ray 

 experiments. This ratio is about 2 to 4 for chromatid breaks 

 and chromosome breaks in Tradescantia pollen grains. 



The X-ray and neutron data taken together may be used to 

 derive an estimate of the distance apart, at the moment of 

 breakage, of breaks which exchange. The order of magnitude 

 is l\i,^^ and this estimate agrees with those based on other 

 data.2. 14 



It has already been seen that a Tradescantia chromosome can 

 be broken by a single ionizing particle. If a single ionization 

 were the causative agent, the efficiency per unit dose should be 

 less for neutrons than for X-rays, since those ionizations in 

 excess of the minimum needed to break the chromosome would 

 be wasted. But neutrons are more efficient and this indicates 

 that several ionizations are usually needed to break a chromo- 

 some. The probabilitv of a chromosome being broken when a 

 proton traverses it is fairly high, most likely between 0.5 and 

 unity. On the other hand, the probability of breakage by an 

 electron is rather low for all of its path except the last densely 



