324 BELL SYSTEM TECHNICAL JOURNAL 



Now in trying to evaluate (7"o — Ti) one encounters two difficulties 

 which are of no great importance in this case, but may be serious in 

 others. First, the incident alpha-particles do not all have the same 

 speed and the expelled neutrons do not all have the same speed; it 

 may be that (To — Ti) is the same for each individual event, but so 

 long as we can only observe these events in multitudes we have to 

 tolerate a wide distribution of Tq and a wide distribution of Ti. 

 Second, the kinetic energy of the neutrons is not measured; what is 

 measured is the range of the particles (atom-nuclei) which they strike, 

 and from this the speed of these struck particles is deduced, and from 

 this the kinetic energy of the neutrons themselves, which thus is two 

 steps away from the data! Luckily it is the difference between To 

 and Ti which enters into the equation, and this is not nearly so large 

 as either; Chadwick estimates it as .0016 mass-unit, so obtaining: 



mass of neutron = (1.0067 ± .005). (6) 



The alteration seems so much smaller than the uncertainty as to be 

 not worth the making; but the latter again is Aston's extremely 

 generous estimate of the uncertainty, which may be three or four 

 times the probable error; so that perhaps the allowance for {To — Ti) 

 is worth while. Similar computations can be made for the neutrons 

 expelled from Be and Li, but perhaps had better be left for those who 

 have personal acquaintance with the problem of estimating their 

 kinetic energies. ^^ 



Mass-number 12 is the principal isotope of carbon; it would be the 

 only one known, were it not for observations made on band-spectra 

 of carbon compounds by King and Birge, who detected lines due 

 to C^^. This latter nucleus is presumably the residue of the trans- 

 mutation of B^" by the impact of an alpha-particle, which frees a 

 proton and merges with what is left. The process permits another 

 test of the mass-to-energy relation (not so good as the one described 

 above) which I have treated elsewhere. ^^ 



Mass-numbers 14 and 15 are isotopes of nitrogen, the former being 

 vastly the more abundant. 



Mass-numbers 16, 17 and 18 are isotopes of oxygen, the first being 

 much the most abundant. The other two were discovered (by 

 Giauque and Johnston) through observation of faint lines in absorp- 



1* Neutrons are reported to have been expelled from many of the more massive 

 elements by alpha-particle impact. It is interesting to notice that owing to the 

 trend of the packing- fraction curve (Fig. 8) the application of the foregoing reasoning 

 to these neutrons would lead to values of neutron-mass very much closer to 1.000, 

 unless {To — 7"i) were to amount to several millions of electron-volts. 



1' Review of Scientific Instruments, June, 1933. 



