22 BIOLOGICAL EFFECTS OF RADIATION 



These are to be multiplied by h; then the negative quantities —hB and 

 — hA are the energy values of two stationary states of the atom, reckoned 

 from the energy of the ionized atoms as zero. To obtain the energy 

 values of these states reckoned from the normal state of the neutral atom, 

 one must add the ionizing energy of the atom to —hA and —hB. Quanti- 

 ties such as A and B are technically known as "terms"; they are some- 

 times tabulated instead of lines, and are usually expressed in wave 

 numbers. 



ABSORPTION IN THE X-RAY REGION 



In the X-ray absorption spectra there is a striking difference from 

 those which we have hitherto been considering; instead of absorption 

 lines being prominent and continua being inconspicuous, the continua 

 of an X-ray absorption spectrum are its principal features. 



Like the others, these X-ray continua signify absorption of photons 

 attended by expulsion of electrons. Generally, any one of them presents a 

 relatively sharp edge toward the low-frequency side, and a gradual 

 fading out toward the high-frequency side: as the frequency is increased 

 from an initial low value, the absorption coefficient suddenly rises from a 

 very low to a high value, thereafter to decrease gradually and smoothly 

 as the frequency is further raised. (At frequencies slightly below that 

 of the absorption edge, discrete absorption lines may be observed, 

 recalling those of the line series adjoining a continuum in an optical 

 spectrum.) Denote by z^o the frequency of an absorption edge: then 

 hvo represents the energy just sufficient to detach an electron from the 

 atom. Let the atoms (we will imagine a stratum of some chemical 

 element, lead or argon, for example) be irradiated by a beam of X-rays 

 of frequency v greater than i^o. The expelled electrons will then possess 

 kinetic energy K given by the formula 



K = hv - hvo (i7) 



The similarity with equation (8) is manifest: we have a linear relation 

 between kinetic energy of expelled electrons and frequency of incident 

 light — its slope is h — there is an additive constant of negative value. 

 There are, however, striking differences : The electrons all have about the 

 same kinetic energy, instead of a distribution of energies from zero to a 

 maximiun value to which alone the equation is appUed; and the additive 

 constant is not equal to {eWi - eWa), but as a rule it is very much greater. 

 All this signifies that the electrons in question were not originally free 

 electrons circulating in the metal (if the irradiated element was a metal) 

 but "bound" electrons definitely attached to atoms in a fixed and constant 

 way, the energy hvo being that required to overcome the binding and make 



