TABLE 795.— SECONDARY COSMIC RAYS J\ \ 



The secondary cosmic rays, which are due to the ionization and other actions of the high- 

 energy particles of the primary cosmic rays, have been studied by various methods for 

 various positions with respect to the geomagnetic latitude on the earth's surface and for 

 different elevations up to such heights that only about 0.5 percent of the atmosphere, by 

 weight, is above the measuring instrument. The secondary rays consist of all sorts of 

 particles such as electrons, both positive and negative ; protons, and other heavy particles ; 

 mesons ; neutrons, traveling with various speeds, and radiant energy of very short wave- 

 length. 



At the surface of the earth (sea level) the cosmic rays are of such intensity that they 

 produce 1.63 ion pair cm" 3 sec" 1 . The intensity is about constant, within a very few per- 

 cent, for geomagnetic latitudes higher than above 40 and from this point to the equator 

 the intensity drop-off is about 9 percent. 



The ionization increases with altitude up to about 16,000 m for geomagnetic latitudes 

 >40, where it is about 150-200 times as large as at sea level. Above this altitude the in- 

 tensity of ionization drops off until, at an elevation where the amount of the atmosphere 

 above the measuring instrument is only about 0.5 percent (35,300 m), the intensity is about 

 0.2 percent of that at the maximum, or about the same as that observed at 0.4 atmosphere 

 above the earth. The variation with altitude is much less at the geomagnetic equator. 



Cosmic rays react with the atoms of the atmosphere and produce a variety of effects ; 

 the production of a simple ion pair, the production of neutrons and electrons, the produc- 

 tion of mesons, the production of extensive showers, where the released energy is so great 

 that the cosmic ray must be only the cause of some explosion or some artificial disintegra- 

 tion. Mesons are particles that may have a unit positive or negative charge or they may be 

 neutral as to charge. The mass of the meson is about 200 times that of an electron ; it is 

 very penetrating and is radioactive, with a life of about 2XlO" 8 sec. Some evidence exists 

 for mesons with a mass of about 1000 m„. 



Thus, there are formed bursts, an extensive production of ionization, and stars when a 

 group of particles have a common origin as shown by cloud-chamber pictures. Stars are 

 probably so named because these pictures show a number of tracks that have a common 

 origin. These tracks vary from 2 to 10 with an average of about 4. The number of stars 

 increases with the elevation above sea level. At an elevation of about 4,500 m the average 

 energy ionization star particle was about 12 Mev. 



Cosmic-ray showers, extensive ionizations of exceedingly complex reactions taking 

 place in the atmosphere, extend over distances up to several hundred meters. These 

 showers contain millions of particles and represent a total of about 10 16 ev. 



These secondary rays may be roughly divided into a hard and a soft component. The 

 separation is generally made by filtering out the soft component with about 10 to 12 cm 

 of lead. The hard component consists of mesons, a small number of protons, possibly some 

 fast-moving electrons, and short-wavelength photons. The soft component consists of 

 electrons, photons, and some slow-moving mesons, protons, and neutrons. The number of 

 rays of the hard component does not reach a maximum with height but seems to increase 

 to as great a height as measurements have been made, i.e., up to a height where the pres- 

 sure is above 0.8 mmHg, where it is about 15 times as intense as at sea level. The soft 

 component increases in intensity down from the top of the atmosphere to a pressure of 

 75 mmHg, then decreases to sea level, where the intensity is about 1 percent of that at its 

 maximum. At its maximum intensity the soft component is about 5 times that of the hard 

 component, in the vertical direction. At the earth's surface this hard component makes up 

 about 75 percent of the cosmic radiation and a much smaller part at high altitudes. This 

 hard component is very penetrating, since it win pass through many meters of water or 

 lead. Cosmic rays have been detected in a mine at a depth of 384 meters, and by tipping 

 the apparatus, the thickness through which the cosmic rays passed was equivalent to 1,408 

 meters of water (about 124 meters of lead!). Another observer detected this radiation in 

 a coal mine at a depth of 610 meters, which is equivalent to 1,600 meters of water ! In this 

 case, the intensity measured at the depth corresponding to 1,600 meters of water was only 

 about 1/20000 of that at the surface! These highly penetrating rays are thought to be 

 mesons, produced by the primary cosmic rays. 



TABLE 796.— MEAN IONIZATION ENERGY OF -y-RAY NECESSARY TO 

 PRODUCE AN ION PAIR* 



(See Table 799.) 



Gas ev Gas ev Gas ev 



H, 33.0 N, 35.0 Ne 27.4 



He 27.8 O, 32.3 A 25.4 



For reference, see footnote 203, p. 624. 



SMITHSONIAN PHYSICAL TABLES 



