254 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1956 



the rate of production of mesotrons balances the rate of decay and 

 there is a maximum in the mesotron intensity. Below this point, the 

 disappearance of mesotrons (as a result of mean-life considerations 

 enhanced by energy loss) exceeds the rate of formation, and the meso- 

 tron intensity declines and continues to decline with further descent. 

 The high-energy mesotrons penetrate far, while the low-energy meso- 

 trons, either by decay or by coming to rest as a result of energy loss, 

 travel only shorter distances. 



The production of electrons. — How does the number of electrons 

 change with height? Near the extreme upper limits of the atmos- 

 phere cosmic-ray electrons are very few, because the mesotrons that 

 breed them have not yet been produced by the primaries in sufficient 

 number. The mu-mesotrons that are produced with low energy in the 

 first tenth of the atmosphere die very near the places where they are 

 born, giving rise to electrons. Higher-energy mesotrons, which can 

 travel farther, do not die as rapidly and therefore do not produce 

 electrons as copiously as do the low-energy mesotrons. Therefore, 

 many more electrons will be formed in the higher regions of the at- 

 mosphere than in the lower. 



Each electron that is produced, however, goes through the process 

 of pair production (discussed earlier, on pp. 248-249) and gives rise to 

 progeny which increase in numbers as we descend from the point 

 where the original parents were formed. The increase continues until 

 it becomes balanced by the loss of electrons, when the ionization that 

 they produce reduces their energy below the point at which the elec- 

 trons can reproduce themselves. Thus, each electron formed has its 

 own little genealogical history involving rise in progeny to a maxi- 

 mum with descent, followed by a decline in numbers with further 

 descent. A combination of the life histories of all the parent elec- 

 trons produced near the top of the atmosphere thus results in a rise 

 of the electron intensity as we descend, a maximum being reached at 

 a depth corresponding to about y 10 of the atmosphere, after which 

 the electron intensity diminishes rapidly with further descent. 



The situation is such that practically none of the parent electrons 

 produced in the higher regions of the atmosphere can succeed in hav- 

 ing any progeny at the earth's surface. In fact, a parent electron 

 would have to have an energy of some 10 trillion electron volts in the 

 outer regions of the atmosphere to have a single offspring at the earth's 

 surface ! 



Such electrons as we find in the lower atmosphere come partly from 

 the relatively rare deaths of fast-moving mesotrons and from the 

 deaths of mesotrons that have been slowed down by ionization energy 

 losses. Another source of electrons is closely related to ionization 

 itself : If a mesotron passes sufficiently near an electron in an atom, 



