ELECTROMAGNETIC RADIATION 9 



hundred angstroms. Of course, the proximity of any molecular system 

 more complex than the expected water polymer — such as a long nucleo- 

 protein cylinder — will locally much modify this picture. But these 

 problems belong to the chemists. 



Even after the initial particle or its electron secondaries have lost so 

 much energy that they are unable to ionize at all, and their passage is 

 invisible to the cloud chamber or the ionization-sensitive device, they 

 will have a possibly important effect in the stopping material. Heavy 

 particles, however, can ionize appreciably, even after they have become 

 neutral, by capture, though eventually they slow down so far that they 

 cannot transfer to a single electron enough energy to ionize or, finally, 

 even to excite an atom. This limiting energy is not so small, since the 

 maximum transferred energy depends on the velocity of the heavy 

 particle. A carbon recoil from neutron capture in nitrogen might 

 possibly spend a large part of its energy without making a single ion 

 pair. This effect may be of some importance in special cases; it is 

 apparently observed in the effects of bombardment on solid materials. 

 The theory here is far from complete (8) . 



It is plain that the transfer of momentum to the stopping atoms does 

 not always take place wholly along the direction of motion. The 

 primary particles are deflected by these collisions. The many small- 

 energy-transfer collisions impose on a heavy particle a successive series 

 of angular deflections to all sides of the path. For fairly fast, heavy 

 particles the mean square displacement angle will grow proportionally 

 with path traversed; this will give rise to a statistical distribution of 

 energies after a fixed straight-line segment of path. Such struggling 

 can be important at the end of a high-energy track, as Wilson will show. 

 Electrons and very slow, heavier particles will be scattered more drasti- 

 cally; their paths may, in general, differ widely from a straight line. 

 Sufficiently slow electrons will appear to diffuse from collision to collision. 

 In tissue all electrons below some hundreds of kev will be very greatly 

 deviated from a straight-line path. 



Electromagnetic Radiation 



The effect of electromagnetic radiation cannot, of course, be described 

 for the whole spectrum at once. For the usual range of interest it is, 

 however, fairly satisfactory to observe that every quantum absorbed 

 can affect at most one primary absorbing atom. The secondary product 

 of the absorption, an electron ordinarily, will then be set free to repeat 

 the history of an ionizing particle in living matter. For the typical x-ray 

 at, say, 100 kev, the ionizing events due to these electron secondaries 



