232 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1938 



There are difficulties in comparing neutron and X-ray effects which 

 ought to be pointed out. X-ray doses are commonly measured in 

 terms of the ionization produced in a standard chamber. Certain 

 types of X-ray ionization chambers, however, may show almost no 

 ionization for a neutron beam. A chamber used for neutrons must 

 be lined with paraffin or Bakelite or other hydrogen-containing ma- 

 terials. The amount of ionization produced by a given beam will 

 then depend greatly on the amount and arrangement of the materials 

 introduced. Thus an X-ray and a neutron beam which produce equal 

 ionization in one chamber will produce quite different effects in 

 another one differently constructed. And the difference will depend 

 on the energy (voltage) of the X-ray beam and the speed of the neu- 

 trons. On just what basis then can one say that neutrons are more 

 effective than X-rays, or vice versa, unless there is some way of com- 

 paring intensities? Up to the present time each worker in this field 

 has in general used a different type of ionization chamber, but in each 

 case one has been chosen in which it was believed that the proton- 

 recoil ionization would be comparable to what would be produced by 

 the neutrons in biological materials. The results of different workers 

 may be expected to be qualitatively comparable but it will soon be 

 desirable to develop methods of quantitative measurement of neutron 

 intensities relative to some arbitrary standard. 



There is another important effect of neutrons which may be of 

 biological interest. Both slow and fast neutrons are very effective 

 in producing nuclear disintegrations, and in a large number of cases 

 the disintegrations lead to products which are radioactive. (We 

 shall have something to say about such materials later on.) In 

 some cases also these disintegration reactions are accompanied by 

 penetrating gamma-rays. A neutron is thus a "triple- threat man" 

 since in passing through biological materials it may simultaneously 

 (1) produce recoil protons or other nuclei, (2) create radioactive 

 atoms, (3) excite gamma-rays. It may be necessary to treat indi- 

 vidually the biological results of these three processes. In most 

 cases it seems likely that the first will be most important since disin- 

 tegration probabilities are less than collision probabilities in general. 

 And the elements for which the disintegration probability (giving 

 rise to gamma-rays or radioactive products) is high are not materials 

 commonly present in biological tissues, e. g., cadmium, samarium, 

 and boron. Nevertheless, one should be on the lookout for cases in 

 which neutrons produce effects on materials containing very little 

 hydrogen, which may be due to disintegrations rather than collisions. 

 In the case of very slow neutrons (energies of a few electron volts or 

 less) recoil ionization will be absent and their effects (if any) will be 

 due solely to disintegrations. 



