NUCLEAR PHYSICS — DUBRIDGE 229 



penetrating power with increasing energy (decreasing wave length) 

 does not continue indefinitely. Above about 2.5 million volts (Mv) 

 the penetrating power of X-rays in lead begins to decrease again, so 

 that 3 Mv rays may actually be "softer" instead of "harder" than 2 

 Mv. For lighter materials than lead the maximum of penetrating 

 power occurs at rather higher energies (10-12 Mv for Al) but the 

 existence of this maximum must be taken into account, and may offer 

 difficulties in the development of absorption and filtering techniques 

 which will have to be used with these multimillion volt machines. 



But million-volt X-rays are really not new tools for biological work — 

 they are only more powerful forms of tools already familiar. We must 

 look to the results of experiments on the transmutation of elements 

 for the really new possibilities which nuclear physics will furnish to 

 biology. Among these the most promising are the last two listed 

 above, namely, the neutron and induced radioactivity, and to these 

 subjects I shall devote the remainder of this discussion. 



THE NEUTRON 



The neutron is a particle of mass 1 (actually 1.0090) and charge 

 zero. It is one of the two fundamental building stones from which all 

 nuclei are constructed, the other being the proton (mass 1.0076, 

 charge +1)- All nuclei are believed to be composed of these two 

 particles and no others. Neutrons may be ejected from various 

 nuclei by bombarding them with protons, deuterons, alpha-particles, or 

 gamma-rays. And this, in fact, is the only way in which neutrons 

 can be made available for use in experimental work. When radon 

 (radium emanation) is collected in a small capsule containing pow- 

 dered beryllium a convenient source of neutrons is obtained. The 

 neutrons are ejected by the bombardment of the beryllium nucleus 

 by the alpha-particles from radon according to the reaction 



Be 9 +He 4 =C 12 +n x . 



The neutrons emerge with energies up to 13.7 million electron 

 volts (Mev) though most of them have much lower energies. 3 



While the intensity of the neutron beam obtainable from a radon- 

 beryllium source is sufficient for many purposes, enormously more 

 intense beams have been produced by the cyclotron. Lawrence has 

 reported neutron beams equivalent to what would be produced only 

 by several hundred kilograms of radium and beryllium. The reaction 

 often used in this case is the bombardment of beryllium by deuterons, 

 according to the reaction 



Be»+H. 2 =B l0 +ri. 



» The kinetic energy which a particle of charge e (electrostatic units) acquires in falling through a potential 

 difference of V volts is given by the relation K. E. = Ve/300. An uncharged particle such as the neutron 

 which has the same kinetic energy which an electron (or proton or any other single charged particle) would 

 acquire in falling through a potential difference of 1 Mv is said to have an energy of 1 Mev. 



