514 L. H. GRAY 



proportion of ionization in a more effective form. In this instance 

 the 8 rays are more effective by virtue of being more densely ionizing 

 than the primary radiation. Conversely, in radiochemical reactions 

 the 8 rays along an a-particle track may be more effective by being 

 less densely ionizing than the primary radiation. An interesting 

 example of this is provided by the recent work of Dale, Meredith 

 and Gray (33). The ionic yield for the inactivation of the enzyme 

 carboxypeptidase is ten to twenty times smaller for a than for X 

 rays, a result that may be ascribed to the fact that the highly concen- 

 trated radicals formed by the ions along the a-particle track are in- 

 accessible to the enzyme, particularly at low enzyme concentrations. 

 However, contrary to expectation, the a-ray yield did not tend to 

 zero at low enzyme concentrations but to a constant value of about 

 5% of the X-ray value. This result becomes intelligible when ac- 

 count is taken of the fact that 5% of the total a-ray ionization is pro- 

 duced not along the track of the a particles but along 5-ray tracks, 

 which are not sensibly different in ion density from the secondary 

 electron tracks resulting from exposure to X rays. 



For a fuller discussion of the significance of 5-ray ionization in the 

 interpretation of radiobiological experiments Lea's book should be 

 consulted (1). 



Lea has computed the energy distribution along 8 rays generated 

 per micron of path of the primary particle for electrons, protons, and 

 a particles, using the theoretical formulas of Bethe (34), which de- 

 scribe the loss of energy by charged particles. The experiments of 

 Hornbeck and Howell (36) and those of Shearin and Pardue (36) indi- 

 cate that the theory is correct for the production of fast 8 rays by fast 

 primary electrons. The computed emission of 8 rays by fast a par- 

 ticles agrees fairly well with the experiments of Alper (57) but in the 

 case of slow a particles the agreement is poor, the computed emission 

 being greater than that observed. In view of the obvious radiobio- 

 logical importance of 5-ray ionization it is unfortunate that there 

 should be so little detailed experimental evidence available. 



5. Relative Positions Occupied by Positive and Negative Ions 

 at Moment of Formation and Their Subsequent Movement 



Ionizing particles very rarely indeed eject an electron from an atom 

 through which they do not actually pass. The primary positive ions, 

 therefore, lie strictly along the geometrical path of the particle. The 



