252 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1956 



as life is concerned, they could travel right through the atmosphere 

 and to distances far below. 



The death of a mu-mesotron is accompanied by the birth of an 

 electron — positive or negative — and two neutrinos. 



Finally, in the realm of cosmic-ray particles there have appeared 

 the K-mesotron, of around 1,100 electron mass units, and another 

 heavy particle, the tau-mesotron (i-mesotron) of about 900 electron 

 mass units. The tau-mesotron is unstable and is believed to decay into 

 three pi-mesotrons. 



It is of interest to observe that even the neutron, when in free space, 

 is unstable and has a life expectancy of only about 181^ minutes, after 

 which it changes to a proton by ejecting an electron in the mysterious 

 process of dividing nothing into two halves, throwing out the negative 

 half (a negative electron) and retaining the positive half (a proton). 



Of the various mesotrons, only the mu-mesotron plays a significant 

 part in the phenomena observed in cosmic rays. However, from the 

 standpoint of what one may call cosmic-ray genealogy, a recognition 

 and understanding of the other particles is necessary. 



COSMIC RAYS IN THE ATMOSPHERE 



The primary cosmic radiation. — The primary cosmic rays appear 

 to contain samples of every kind of nonradioactive atom in the uni- 

 verse. The chief constituent is the proton, the nucleus of the hydrogen 

 atom. The percentages of the elements vary according to different 

 observers, but from representative data they are in the ratio of 4,000 

 particles of hydrogen to 1,000 of helium, to 35 of carbon, nitrogen, and 

 oxygen, to 10 of all nuclei with atomic numbers greater than 10. 



There can be no mesotrons hi the primary radiation, for the mean 

 life of the mu-mesotron, the longest-lived of the mesotron family, is 

 only 2.3 microseconds when the particle is at rest, and even with ener- 

 gies as great as 100 billion electron volts, the mean life is only % oo °^ 

 a second. There is no place from which the mesotrons could have 

 come, for the nearest body that could reasonably be a candidate for 

 supplying them is the sun, and a particle traveling with the velocity 

 of light itself would take more than 8 minutes to reach us from the 

 sun. 



Lifetime considerations rule out even neutrons as possible com- 

 ponents of the primary radiation, since neutrons have a mean life of 

 the order of 18.5 minutes. The sun is the only body from which they 

 could reach us, and if they came from the sun in appreciable amount, 

 there would be a much larger change in their intensity from day to 

 night than experiment reveals. Unlike charged particles, whose paths 



