18 SECONDARY ELECTRONS 



just in those atoms and molecules which require least energy to be 

 ionized. In other words, just in those substances where an ionization can 

 he produced cheaply, in terms of energy, a large amount of energy has to he 

 spent in excitations. (In substances whose ionization potential has a 

 typical average value, around 10 ev, the relative frequency of excitations 

 and ionizations is of the order of 2 to 1.) 



As a result the ratio of the energy delivered to a material to the 

 number of ionizations produced varies within remarkably narrow limits 

 from one substance to another. This ratio is also, of course, essentially 

 the same for different ionizing radiations, since most excitations and 

 ionizations are produced through glancing collisions affecting the surface 

 layers of atoms. Its numerical value for most gases is in the neighbor- 

 hood of 30-35 ev per ionization. Because of this circumstance, the 

 number of ionizations produced in a gas is very frequently taken as a 

 measure of the energy spent by a radiation within it. 



All these considerations pertain to the action of very fast charged 

 particles. As a particle begins to slow down, the excess of glancing 

 collisions with respect to knock-on collisions is no longer very large. 

 Accordingly, the ionization yield increases a little, because every knock- 

 on collision produces an ionization, whereas glancing collisions produce 

 excitations as well. 



The whole picture changes substantially when a particle slows down 

 to and below the velocity of atomic electrons. (A heavy particle can 

 move more slowly than an atomic electron and still have a substantial 

 kinetic energy. This is the case, for example, for a proton of 20 kev or 

 a nitrogen atom of 300 kev.) Then the particle becomes unable first to 

 ionize and then to excite at all. The residual energy is presumably 

 dissipated through bodily impacts against atoms which can no longer be 

 easily penetrated. The energy dissipated in this manner probably 

 escapes detection by ordinary radiation-measuring devices. Neverthe- 

 less, it may well cause a very substantial dislocation in the structure of 

 matter and thereby acquire a particular biological significance. 



The final products of ionization in a substance like water will include 

 negative ions, like the hydrated 0H~ formed by electron capture. The 

 subsequent chemical action of such molecules may be of high importance 

 in biological material. 



REFERENCES 



1. Bethe, H. A., Handbuch der Physik, vol. 24/1, Berlin, 1933. 



2. Lea, D. E., Actions of Radiations on Living Cells, p. 27, New York, 1947. 



3. See, for example, C. T. R. Wilson, Proc. Roy. Sac, A192: 1923. 



4. Fano, U., Phys. Rev., 70: 44, 1946. 



