THE STORY OF COSMIC RAYS — SWANN 



251 



Figure 3. — The descendants of a high-energy particle may include those shown here. Any 

 one such primary particle does not, in general, give rise to all the types of mesotrons in 

 the second stage. 



pair production and loses energy only by ionization phenomena. As 

 a result of this, the mu-mesotron can travel very much farther through 

 matter than can an electron of the same energy. 



The mu-mesotron, when traveling with a velocity small compared 

 with that of light, has an average life of only about 2.3 microseconds. 

 Even if it traveled with the velocity of light and had only this average 

 lifetime, it could not go more than about 700 meters before death over- 

 took it. However, it results that the theory of relativity demands 

 and experiment confirms that the lifetime of the particle shall increase 

 with its energy. As a result of this, many cosmic-ray mesotrons have 

 lives hundreds or thousands of times 2.3 microseconds, so that, so far 



ELEMENTARY PARTICLES OF MATTER 



Mass in electron Average lifetime against Products of spontaneous 



Particle masses spontaneous decay decay* 



Electron 1 Stable 



Proton 1,845 Stable 



Neutron.. 1,848 18.5 ± 3.6 minutes Proton and electron 



Positron 1 Stable 



Positive mu-mesotron 210 2.3xl0-*seo. Electron and two neutrinos 



Negative mu-mesotron 210 2.3 x 10~* sec. Electron and two neutrinos 



Positive pi-mesotron 276 About 10 -8 sec. Mu-mesotron and neutrino 



Negative pi-mesotron. 276 About 10-» sec. Mu-mesotron and neutrino 



Neutral pi-mesotron 264 Less than 10-» sec. Two photons 



K-particle. About 1,100 About 10 - ' sec. One mu-mesotron and two 



neutral particles 



Tau-particle 977 Greater than 10-* sec. Three pl-mesotrons 



Chl-mesotronf SOO-1,500 Greater than 10 - ' sec. Pi-mesotron and one neu- 

 tral particle 



Kappa-mesotron t — Less than 1,400 Greater than 10-' sec. Mu-mesotron 



Zeta-particle 637 Greater than lO -10 sec. Two pl-mesotrons 



Positive V-particlet Uncertain Less than 10-» sec. Uncertain 



Negative V-particlet Uncertain Less than 10 - » sec. Uncertain 



Neutral V-particle Uncertain Less than 10-'« sec. Probably mesotrons and 



protons 



Photon Stable None 



Neutrino.. Stable None 



•In general, these products are accompanied with release of energy in kinetic form and in amount sufficient 

 to conserve mass. 



fit is uncertain how far the positive and negative V-partlcles are distinct from the chl- and the kappa- 

 particles. 



