42 NEUTRONS AND THEIR SPECIAL EFFECTS 



lowest measured energy of 0.005 ev, corresponding to a temperature 

 of -216° C. 



Theory expects that for elastic scattering up to energies of about 20 

 mev the scattered particle would have no preferred direction, but would 

 have a perfectly symmetrical distribution in the center of mass system, 

 an expectation which has been verified experimentally. However, in 

 dealing with the total scattering cross section, theory, at first, agreed 

 well with experiment only for fast neutrons. For thermal neutrons, it 

 predicted a cross section of 2.4 barns, which is to be compared with the 

 observed experimental value of 58 barns. This discrepancy was over- 

 come by altering the theoiy in two ways: first, by considering the spin 

 relationship between the neutron and proton, and, second, by taking 

 into account the chemical binding of the proton. It was shown by 

 Fermi that the scattering cross section for protons bound in molecules 

 was about 4 times larger than that for free protons. Two reasons which 

 contribute to this higher cross section are: (1) the apparent mass of the 

 proton is increased because it is firmly tied to its molecule and (2) for 

 thermal-neutron energies, the molecular vibrational motion of the 

 proton cannot be neglected. 



Thus far we have a picture of a neutron entering paraffin or tissue 

 and slowing down in a series of collisions, each of which transfers part 

 of the neutron energy to another nucleus, causing chemical excitation 

 or ionization, but never transferring enough energy to cause nuclear 

 excitation of the struck nucleus. Furthermore, the cross section pre- 

 sented by the proton increases as the neutron slows down, making the 

 target larger, the slower the neutron. We have still to consider the 

 eventual fate of the neutron, the reaction by which it is removed from 

 circulation. Since the half life of a free neutron is of the order of 15 min, 

 whereas its half life in paraffin is about 10""* sec, we must look to a nuclear 

 reaction to account for its capture. 



There are only two possible nuclear reactions between a neutron 

 below 100 mev and a light nucleus: simple capture and inelastic scat- 

 tering. That is, a neutron may be captured by the nucleus, and the 

 excess energy given off as gamma radiation, or else the neutron may be 

 captured and re-emitted, leaving the struck nucleus in an excited state, 

 from which it returns by emission of a gamma ray. Since this process 

 of inelastic scattering always involves nuclear excitation, it can easily 

 be differentiated from elastic scattering. In the light elements, inelastic 

 scattering is very improbable for neutrons of 1-mev energy, which is 

 the range that interests us. 



It was shown in 1934 by Chadwick and Goldhaber that a natural 

 gamma ray from thorium C could cause a deuteron to disintegrate into 



