154 BELL SYSTEM TECHNICAL JOURNAL 



achieved the feat of taking and examining no fewer than 6900 cloud- 

 chamber photographs in order to deduce the distribution-in-speed of 

 neutron-streams from the tracks of the recoiUng nuclei of various kinds 

 of atoms. Most observers publish no curves, but give only verbal ac- 

 counts in which they state the thickness (in air-equivalent) of the 

 intercepting screens athwart the proton-beam, for which they observed 

 a notable falling-off of the strength of that beam; or else they state 

 what groups they believe in, inferring them presumably from observa- 

 tions of that type. This makes tiresome and unsatisfactory reading. 



Much of recent research is meant to detect the very fastest neutrons 

 emitted from a given element, for a reason which will presently be 

 obvious if it is not already. Chadwick gives 3.35 MEV for the energy 

 of the fastest neutrons ejected from boron by polonium alpha-particles, 

 and 12 MEV for those similarly ejected by beryllium, while Dunning 

 gives 14.3 MEV for those which beryllium emits when bombarded by 

 the somewhat faster alpha-particles from radon. 



Curves called "disintegration-functions," or more commonly "exci- 

 tation-functions," have been plotted several times for the neutrons 

 from beryllium and once at least for those from boron. One must 

 realize an important distinction between them and the curves obtained 

 when the fragments are alpha-particles or protons, as in Figs. 16 and 

 17. When the fragments are charged particles, it is practically certain 

 that all of them which reach the detector at all are duly detected. 

 When the fragments are neutrons it is certain that the only ones de- 

 tected are those which strike protons (or other nuclei) hard enough and 

 squarely enough to give them a considerable amount of energy and 

 enable them to produce a good many ions in the ionization-chamber; 

 and it is equally certain that those constitute but a small fraction of the 

 total number of neutrons, most of which go through the expansion- 

 chamber unperceived. Would that this were at least a constant frac- 

 tion! we could then rely on the shape of the so-called excitation-curve, 

 while realizing that all its ordinates must be multiplied by some un- 

 known but constant factor. But we must not suppose even this; it is 

 practically certain that the factor varies with the speed of the neutrons, 

 and hence in all probability with the speed of the primary alpha- 

 particles; and hence the so-called excitation-curve must be distorted 

 from the true curve of number-of-atoms transmuted versus energy-of- 

 alpha-particles. (Also the distribution-in-range curves must be dis- 

 torted.) 



With these severe limitations in mind, one may consider the pub- 

 lished excitation-functions. The most striking are those obtained with 

 very thin films of beryllium, one by Chadwick and one by Bernardini, 



