138 RADIATION BIOLOGY 



dissipation. The equal effectiveness of different ionizing radiations indi- 

 cates only that the aftereffects of successive collisions along the tracks of 

 fast ionizing particles develop independently. Successive collisions may 

 dissipate widely different amounts of energy, and there is no telling 

 whether and how the effectiveness of each collision depends on the energy 

 dissipated. For example, Catcheside and Lea (1943) have been led to 

 think that chromosomal breaks occur only at the tail end of the tracks of 

 fairly energetic secondary electrons (of about 500 ev or more). This 

 possibility is consistent with equal effectiveness of different radiations 

 because the number of energetic secondary electrons is proportional to 

 the energy dissipated. 



5-5b. Higher Effectiveness of Densely Ionizing Radiations. Ionizing 

 particles whose energy is dissipated by particles of comparatively low 

 speed display, in general, a particularly high biological effectiveness. The 

 production of genetic mutations in fruit flies appears to constitute a nota- 

 ble exception to this rule (see Sect. 5-5c). Since successive colhsions 

 follow one another at shorter distances along the tracks of slower 

 particles, the greater effectiveness of these particles indicates some coop- 

 erative action of the aftereffects of different colhsions. There is no 

 definite indication regarding the mechanisms of such cooperative action, 

 but it seems quite plausible a priori that energy dissipated at a high con- 

 centration is particularly effective. 



Experiments with X rays of different energies show an increase of 

 effectiveness^" only when the energy of the photons, and of the photo- 

 electrons which they release, falls below 10 kev. Catcheside and Lea 

 (1943) demonstrated this effect in the production of chromosome breaks 

 in the spiderwort Tradescantia. 



Heavy charged particles with energies of a few million electron volts 

 dissipate energy even more densely than electrons of a few thousand 

 electron volts. Most of the experimental evidence regarding heavy 

 particles was obtained Avith a particles and with neutrons (i.e., with the 

 protons accelerated by the impact of neutrons). Table 1-12 gives 

 data on the comparative effectiveness of heavy particles and of X rays. 



The speed of heavy charged particles varies comparatively little over 

 appreciable lengths of path. Therefore it is possible to study the effec- 

 tiveness of beams of particles that dissipate energy at a fairly uniform 

 and constant rate. (Low-speed electrons have so little energy that they 

 do not travel an appreciable distance.) Zirkle (1935) demonstrated a 

 progressive change of effectiveness along the path of a particles (see 



Table 1-12). 



Heavy-particle beams with energies up to hundreds of million electron 

 volts are now available. The fastest of these particles are as effective as 

 high-energy X rays; the slower ones are generally more effective. It 



«" See Note beginning on page 136. 



