CONTEMPORARY ADVANCES IN PHYSICS 139 



scribed by the increase of Tq in equations (1) and (2); and so should 

 the right-hand end of the sloping part of the curve in Fig. 9, and the 

 extremity of the curve of Fig. 11. There is an indication of the first 

 of these expected changes in the observation already quoted from 

 Lewis Livingston and Lawrence, of 14.8-cm. fragments ejected from 

 lithium by 1.33-MEV protons (page 133). We must wait for future 

 data to test the others, and to see what happens to the heights of the 

 steps and the general shape of the uninterpreted parts of the curves. 

 Already however we have data bearing on the so-called "disintegration- 

 function," or the relation of the total number of emitted fragments 

 to the energy of the bombarding particles. 



To speak of "total number of fragments" is to suggest too much. 

 The present knowledge suffers from two limitations: the counts of 

 fragments are made with apparatus which does not enclose the target 

 completely and must be separated from the target by a screen, so that 

 the fragments counted are only those which start off within a limited 

 solid angle of deflections and have sufficient range to penetrate the 

 screen. One generally makes a tentative correction for the former 

 limitation, by assuming that the fragments go off equally in all 

 directions and multiplying the number observed by the factor 4x/co, 

 where w stands for the solid angle subtended by the detector as seen 

 from the target. This factor may well be wrong, but perhaps does 

 not vary seriously with the energy of the bombarding particles, so 

 that at least the trend of the curve may not be distorted. For the 

 latter limitation we have not the knowledge to make any allowance; 

 it must always be stated that the count is of fragments having more 

 than such-and-such a range, or such-and-such an energy. Every kind 

 of device for observing transmutation suffers from some such lower 

 limit, set either by the sensitivity of the device itself, or by the stopping- 

 power of the wall which bounds it. 



With their dense streams of protons and exceedingly thin films 

 (page 113) Oliphant and Rutherford obtained the curves of Fig. 16: 

 the disintegration-functions of lithium and boron, with respect to 

 incident protons, up to proton-energies of some 200,000 electron-volts. 

 The wall between the target and the gas of the ionization-chamber had 

 an air-equivalent of 2.50 cm., and consequently the curves pertain only 

 to fragments having ranges greater than this.^^ The rise from the axis 

 is gradual, not abrupt; one might say that the shape of the curves 

 suggests that the protons have, not a definite capahility for transmuting 

 which begins suddenly at a critical energy, but a probability of trans- 



-^ I hear from Dr. Oliphant that the trend of the curve for the short-range frag- 

 ments is just the same. 



