COSMIC RADIATION — JOHNSON 199 



are about 300 a minute, practically no simultaneous discharges occur 

 when the tubes are separated, but if one counter tube is placed 

 directly above the other such simultaneous discharges or "coinci- 

 dences " are frequent. In the latter position single rays can pass 

 through both tubes. The advantage of this arrangement for the 

 cosmic ray measurements is that the radioactive radiations have 

 no effect. Such rays, if of the penetrating gamma type, excite dis- 

 charges only when they convert themselves into the nonpenetrating 

 beta type within the counter cylinder. One gamma ray can never 

 excite more than a single com iter. The coincidence counters thus 

 select the cosmic rays for their recording and, furthermore, they 

 pick out only the rays which are coming from within a small range 

 of directions. 



A third method of investigation consists of measuring the current 

 of ions produced directly by the cosmic rays in a vessel filled with 

 gas. To increase the effect the vessel is usually filled to a high pres- 

 sure and often the current measuring instrument is placed inside, 

 Kadioactive radiations can be eliminated, if necessary, by lead shields 

 on the outside. These instruments, called electroscopes, have been 

 used to a large extent in cosmic ray intensity surveys. Though ex- 

 tremely accurate and reliable they suffer a disadvantage in not being 

 able to determine from what direction the rays are coming. 



3. EVIDENCE THAT RAYS ARE OF COSMIC ORIGIN 



Experiments with all three types of instrument have built up 

 convincing proof that the rays are of cosmic origin. By rotating 

 the line of two coincidence counters, the intensity of cosmic rays 

 can be studied as a function of the direction. This directional dis- 

 tribution favors the vertical and very little intensity is incident from 

 the horizontal. Most of the cosmic rays are coming down from 

 above at steep angles. This type of angular distribution would be 

 expected if the rays originate outside the atmosphere, for they would 

 be absorbed at the low angles in proportion to their longer paths 

 in the air. A source outside the earth was proposed as early as 1913 

 by Hess who carried apparatus in a balloon and found the intensity 

 increasing with elevation. At a height of 4,300 meters four times 

 as many rays strike a square centimeter of surface per second as at 

 sea level, and a 300-fold increase over sea-level intensity has been 

 found in the stratosphere. All evidence points to the regions be- 

 yond the atmosphere as the source of the cosmic radiation. 



4. PROBLEMS FOR INVESTIGATION 



What are these rays which come to us from the depths of cosmic 

 space? How, where, and under what conditions are they produced? 



