NUCLEAR PHYSICS — DTJBRIDGE 231 



The problem of detecting and measuring neutron beams is also an 

 interesting one. Since neutrons do not interact appreciably with 

 atomic electrons they produce almost no ionization along their paths. 

 Hence, all the ordinary methods of detecting radiations (ionization 

 chambers, counters, cloud chambers, etc.) which depend on ionization, 

 are at first sight apparently useless. This difficulty is avoided by 

 making use of collisions of neutrons with protons. The recoil protons 

 produced in hydrogen-containing materials produce intense ioniza- 

 tion, and the intensity of this can be taken as a measure of neutron 

 intensity. The length of the recoil proton tracks produced in a 

 hydrogen-filled cloud chamber can be used as a measure of the neutron 

 velocities. 



Still another method of detection is to make use of the disintegra- 

 tions produced by neutrons in which alpha-particles are ejected, the 

 alpha-particle ionization then being measured. The boron reaction 

 is particularly useful: 



B 10 +n 1 =Li 7 +He 4 . 



This reaction is particularly efficient for low energy neutrons, for 

 which the proton recoil ionization cannot be used. An ionization 

 chamber lined with boron or filled with boron fluoride gas is widely 

 used for the detection of neutrons. 



Finally neutrons may be detected and measured by the amount 

 of induced radioactivity they produce in certain materials such as 

 silver. 



It is evident from what has been said that the process by which a 

 neutron loses energy in passing through matter is quite distinct from 

 the processes by which gamma-rays or X-rays are absorbed. The 

 latter give their energies to electrons while neutrons give their energy 

 in appreciable quantities only to protons or other light nuclei. Elec- 

 trons produce relatively small ionization over a long path; protons 

 produce intense ionization over a short path; carbon nuclei, still 

 more intense ionization over a still shorter path. Since it is to be 

 expected that biological effects on individual cells will depend more 

 on ionization density than total number of ions, it will be rather 

 expected that neutrons will be more biologically effective than gamma- 

 rays or X-rays of the same intensity. Preliminary experiments by 

 Lawrence at California and by Zirkle of the Johnson Foundation 

 indicate that this is indeed the case. Certainly this matter should be 

 further investigated with great care and at once. As a matter of fact 

 many experiments on neutron effects on both plant and animal tissue 

 are now in progress, and there will be plenty of work in this field for 

 many years to come. 



