CONTEMPORARY ADVANCES IN PHYSICS 151 



More formidable is the third kind, the gamma-rays or short electro- 

 magnetic waves. They also vary from one element to another, and 

 different wave-lengths may proceed from a single element; some are 

 innocuous, but some are much more penetrating than even beta-rays. 

 One must be careful about the language here; one must not say that 

 they have a great range. To speak of "range" implies that when a 

 number of particles is started off side by side with equal speed along 

 parallel paths into a stratum of matter, they continue to follow parallel 

 paths — albeit with diminishing speed — until they are brought to a stop, 

 this termination coming for each of them at the same distance from 

 their source. Of alpha-particles this is nearly a true statement, of 

 beta-particles it is a fair approximation. Of gamma-corpuscles or 

 photons (for gamma-rays are high-frequency light, and may therefore 

 be regarded alternatively as corpuscles and as waves) it is not true at 

 all. If a number of these, all of identical wave-length, is started off 

 side by side along parallel paths into a stratum of matter, one after 

 another impinges on an atom, or (it is probably better to say) upon an 

 electron belonging to some atom. Until such an impact, the photon 

 does not ionize at all, but passes unperceived. The number which at a 

 distance x from the source have not yet suffered an impact varies as an 

 exponential function e~''^ of that distance. 



At such an impact the gamma-ray photon loses energy and suffers a 

 deflection, which may or may not be considerable. It also liberates the 

 electron, which is an act of ionization; but usually the electron itself, 

 being started off w^ith considerable speed, liberates a great number of 

 additional electrons from other atoms of the gas, so that the direct 

 ionization by the gamma-rays falls far behind the indirect; most of the 

 wnization by gamma-rays is done by the intermediacy of fast electrons. 

 One may say that the lower the value of tx, the more penetrating, or 

 "harder," are the gamma-rays; one may compare values of p. for 

 gamma-rays of different w^ave-lengths ; but one may not compare a 

 value of IX for these with a value of range for beta-particles or alpha- 

 particles (unless one should arbitrarily define the range of a beam of 

 gamma-rays as the distance over which its intensity is reduced by say 

 99 per cent, which would be very misleading). 



These are very important matters, for the chance of deciding whether 

 the cosmic rays are likest to gamma-rays or to material particles turns 

 largely upon differences such as these. Another detail is important: 

 the practice of stating values of m for standard materials, such as water 

 or lead. It is approximately true that slabs of various materials will 

 absorb the same fraction of an incident beam of gamma-rays (of a 

 single wave-length, be it understood) if their thicknesses are so adjusted 



