230 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 8 



What, then, are some of the properties of neutrons so produced? 

 In some ways a beam of neutrons is similar to a beam of gamma-rays. 

 They are undefiected by a magnetic field and have considerable pene- 

 trating power through heavy substances. They were, as a matter of 

 fact, mistaken for gamma-rays in the early experiments. A further 

 examination shows, however, profound differences between neutrons 

 and gamma-rays — and these differences may have biological impor- 

 tance. A gamma-ray loses energy in passing through matter largely 

 through the energy it imparts to the orbital electrons of the atoms 

 through which it passes. A gamma-ray, being an electromagnetic 

 radiation, has strong interaction with charged particles (electrons) 

 and these will sooner or later rob each gamma-ray of all its energy. 

 A neutron, however, being an uncharged particle, is not at all in- 

 fluenced by passing through an atmosphere of electrons. Its course 

 can be deflected or its velocity changed only by a close encounter 

 with the nucleus of an atom, and this is a rather rare occurrence. 

 Even when it does occur, if the nucleus against which the neutron 

 collides is heavy, the neutron will rebound without much loss of energy, 

 like a golf ball from a brick wall. The neutron is scattered but does 

 not slow down. 4 The neutron is completely absorbed only in case it 

 enters the nucleus, and is there captured or causes the ejection of 

 another particle. In most substances this absorption probability is 

 very small and the neutrons therefore penetrate great thickness — 

 e. g., several feet of lead. On the other hand some substances, for 

 example cadmium, have a very large selective absorption for neutrons 

 of a particular velocity — and so rapidly remove neutrons of this 

 particular velocity from the beam. 



When neutrons pass through light materials, especially materials 

 containing hydrogen, the situation is quite different. In collision with 

 a hydrogen nucleus, which has about the same mass as a neutron, the 

 neutron may lose a large fraction or all of its energy, imparting it to 

 the proton, as in the collision of two billiard balls. On the average 

 it gives up more than half its energy at each collision, and so in about 

 20 collisions a 5 Mev neutron will be slowed down to thermal energies 

 (0.1 electron volt or less). Thus a beam of fast neutrons after passing 

 through about 6 to 10 cm of paraffin will be largely slowed down to 

 thermal energies. At these low velocities neutrons are readily cap- 

 tured by a proton to form a deuteron, and so are quickly absorbed. 

 A block of paraffin or a water cell is thus an effective barrier for neu- 

 trons — though they could easily pass through several feet of lead. 

 The technique of handling neutron beams thus presents surprising 

 problems, which there is not time to discuss. 5 



* Inelastic scattering (slowing down) of fast neutrons by heavy nuclei has recently been discovered at 

 Cornell. 

 » For a full account see Rasetti, Nuclear, Physics, Prentice-Hall, 1936. 



