XV. ELECTRONS, NEUTRONS, AND ALPHA PARTICLES 529 



were accelerated l^y means of a 380 kv. constant potential generator 

 (62). The measured total neutron emission (63) was 10^ neutrons 

 per second per milliampere of 380 kv. deuterium ions incident on a 

 target of "heavy wax." The apparatus was usuallj' operated at ion 

 currents of 0.5 ma. giving 2 r.e.p. per minute at 1.5 cm. from the tar- 

 get (r.e.p. unit is defined in Sect. Dl). No y radiation is emitted 

 in this nuclear reaction, and all the neutrons emitted in a given direc- 

 tion have the same energy. At the voltages used bj^ Gray and Read 

 this energy is only slightly dependent on the accelerating voltage since 

 most of the energy with which the neutron is expelled is derived from 

 the energy released in the reaction listed to the right of the equation. 

 The recoil protons generated in tissue by these neutrons have a mean 

 energy of about 1.2 m.e.v. and a mean linear ion density of 1000 ions 

 per micron (o/. Table I), whicli is greater than the corresponding figure 

 for the other neutrons used hitherto; on this account the biological 

 results obtained have been more sharply distinguished from those 

 produced by X rays. 



On account of the sharp fall in neutron emission with decreasing 

 ion-accelerating potentials a minimum of about 300 kv. is required to 

 give useful dose rates for most biological work. As the accelerating 

 kilovoltage is increased there is a slow steady rise in neutron output 

 (Fig. 14) but above 800 kv. this is almost entirely due to the increased 

 penetration of the deuterium ions into the wax target and may be 

 estimated from the known relation between range and energy' of the 

 deuteron. 



The 3'ield of neutrons from the other reactions listed rises much 

 more steeply with voltage {cf. Fig. 14). The (D-Li) and (D-Be) re- 

 actions have both been used as a source of neutrons for biological re- 

 search (11, 24,64)-* The yield from both reactions is about the same 

 as from the (D-D) reaction at 700 kv. and exceeds it at all higher 

 voltages. There is little accurate information regarding the yields 

 at voltages above 1 m.e.v. but the curves of Figure 14 may serve as 

 a guide in the choice of reaction and voltage best suited to particular 

 needs. The apparatus used by Lea, Haines, and Bretscher (24) was 

 such that biological specimens could most conveniently be exposed 

 to neutrons emitted at 90° to the deuteron beam. When lithium 

 was bombarded by 100 hsl. of 900 kv. deuterons the dose rate at the 

 specimens 4 cm. from the center of the target was about 15 equivalent 



* The earliest investigations of biological offocts of neutrons were made with 

 the (D-Be) neutrons generated in the 37-iri. Berkeley cyclotron. 



