648 BELL SYSTEM TECHNICAL JOURNAL 



with our idea of a smash. It can be achieved by alpha-particles of 

 any speed, above (presumably) some minimum which in these experi- 

 ments was not attained; the energy of the ejected proton increases with 

 the energy of the projectile which brought about the crash. 



Pose's investigation is one of several which in the last three years 

 have been devoted to the ranges of the protons set free in transmuta- 

 tion. I chose to emphasize it because of the beauty and clearness of 

 the curves, their long horizontal segments and sudden steep ascents 

 which are the evidence for "groups" of protons; it is outstanding also 

 because of the extent to which Pose controlled and varied the speeds of 

 the alpha-particles. Other physicists, however, exposed a wide variety 

 of elements to the bombardment of the alpha-rays, and observed the 

 protons ejected at diverse angles to the direction along which the bom- 

 barding corpuscles came. These were Bothe and Franz of the Reich- 

 sanstalt, and Chadwick, Constable and Pollard of the Cavendish 

 Laboratory. 



Bothe and Franz attacked the light element boron; except for alu- 

 minium this is the element of which the transmutation has been most 

 studied, for it seems to be much more liable to disruption by the rela- 

 tively slow alpha-particles of polonium — those which have been used in 

 most of the newer work — than the others commonly tested. Observ- 

 ing the protons projected more or less nearly straight ahead, these 

 physicists plotted a curve comparable to those of Fig. 6; a curve, that 

 is to say, whereof any ordinate represents the number of protons having 

 ranges greater than the value given by the corresponding abscissa. 

 Their curve had a horizontal segment — the first ever observed, so far 

 as I am aware; for its historical interest I reproduce it here as the upper- 

 most curve in Fig. 8. The sloping part to the right of that segment 

 implies a group of protons having ranges between 2>d> and 23 cm.; the 

 sloping part to the left, a distribution of ranges extending from some 

 20 cm. downwards. These inferences stand out more clearly from an 

 inspection of the differential curve, which I exhibit here as Fig. 9. 



Returning to this field of research (or, more probably, continuing in 

 it uninterruptedly) Bothe and Franz in 1930 published separate further 

 papers. Bothe varied another factor — the angle between the direction 

 along which the alpha-particles were coming, and that along which the 

 particular protons which he observed were departing. In any one 

 experiment, as we shall see, this angle varies over a wide range; one 

 must specify its mean value, or some value near its mean; this I will 

 denote by 6. Bothe, then, adjusted his apparatus so that to 6 he 

 could successively give several values between 0° and 116°; and some 

 of the curves had a horizontal segment, or at least a fiattish gently- 



