414 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 64 



expression for the decay of that species is known, then the strength 

 of the interaction may be computed. 



This experimental evaluation of Fermi's interaction was made for 

 many different radioactive species. It was the same for each, and — 

 what was most surprising — it was found to be extremely small com- 

 pared with the other known nuclear force. For this reason, it has 

 become known as a second kind of nuclear force termed the "weak 

 interaction." 



As it is the field of force that a particle carries along that determines 

 how readily it will collide with other particles, the strength of this 

 force field determines how much matter a particle will penetrate 

 before it is stopped. Because of this, the neutrino described by the 

 theory of Fermi turns out to be an extremely penetrating particle. 

 All other particles known carry some or all of the other force fields 

 with them, and so they slow down quite readily when they enter a 

 thick layer of matter. The neutrino, on the other hand, carries only 

 this weak field with it and so sees other particles very poorly ; in fact, 

 hardly at all. 



We may give the value of the force numerically, but it might be 

 more comprehensible if we instead interpret it in terms of how deeply 

 a neutrino may be expected to penetrate matter. This can be done 

 by recounting a true story involving the author and his colleague. Dr. 

 Frederick Reines. In dreaming of ways to detect neutrinos from the 

 sun (for the sun should be making neutrinos as it generates its own 

 nuclear energy), we wondered how one might prove that such neu- 

 trinos actually came from the sun, once detected. The first thought 

 was simple: Observe the signal rate at noon and at midnight, then 

 compare the two rates. The one taken at night would require solar 

 neutrinos to have penetrated the earth, and so the signal would be 

 reduced by absorption in passing through the earth. We calculated 

 the reduction to be expected, and found that the midnight rate would 

 be indistinguishable from the noontime rate. We must have more ab- 

 sorber than the earth can provide ! Well, let's perform the experi- 

 ment during a solar eclipse, w^hen the moon would also be an absorber 

 for us. Still no change worth considering. Our curiosity aroused, 

 we then calculated how many moons, all eclipsing the sun at the same 

 time, would be required to reduce the signal by a detectable amount. 

 We found that there isn't enough room between here and the sun to 

 crowd in enough moons to do this ! It would take a line of moons some 

 3 or 4 light-yeai-s long to absorb only one neutrino of every two that 

 started through them. So small is the "weak interaction." 



Another way to visualize a neutrino is by a "size." If we relate 

 the penetrating ability of a neutrino to its size, in the sense that the 

 smaller it is, the less likely it will be to strike anything, then the neu- 

 trino which would penetrate our long line of moons would have a cross- 



