XV. ELECTRONS, NEUTRONS, AND ALPHA PARTICLES 531 



of lead effectively eliminated the 7 rays from the source. In the 

 article referred to, Aebersold gives very useful practical information 

 concerning the manner in which the (D-Be) neutrons from a 37 inch 

 cyclotron may be collimated so as to give well defined neutron beams 

 suitable for the exposure of parts of large animals, or for therapy. 

 This requires essentially that a wall of wax or tanks of water at least 

 60 cm. thick be erected close to the cyclotron target to slow down 

 and absorb all neutrons except those passing through the defining 

 channel. This channel has to be lined, preferably with lead, to ab- 

 sorb the 2.2 m.e.v. 7 rays resulting from the combination of slow neu- 

 trons with protons taking place in the wax or water. These 7 rays 

 are in fact more troublesome than those coming from the target 

 (see above) . With an arrangement that defined a neutron beam 7X7 

 cm. at 70 cm. from the target the 7-ray ionization in an amber cham- 

 ber of 1.75 cc. volume was about the same within and outside the neu- 

 tron beam, and was about 5% of the neutron ionization within the 

 beam. The neutron ionization fell to 3, 2, and 1%, respectively, at 5, 

 10, and 20 cm. from the edge of the beam. When a second arrange- 

 ment was used that permitted a neutron beam in a direction of the deu- 

 teron beam, instead of at 30° as previously, to be employed, the neu- 

 tron dose rate in the beam was doubled, but fell off rather more slowly 

 outside the beam. The 7 ray dose rate was unaltered and so was 

 only 2.5% of the neutron dose rate within the beam. 



The ionization in the amber chamber at the outlet, which was 

 70 cm. from the target, was observed to be 5.4 electrostatic units per 

 cubic centimeter per microampere hour of 6 m.e.v. deuterons. Since 

 the ordinary operating current is stated to be 60 fxa. the normal values 

 of the dose rate at 70 cm. may be reckoned as 5 equivalent roentgens 

 per minute. 



In 1942 Stone and Larkin (66) reported upon the treatment of 

 120 cancer patients by neutrons from the 60 inch Berkeley cyclotron. 

 The collimation sj^stem was similar to that described by Aebersold. 

 The energy of the deuterons was 16 m.e.v. and the neutron energy 

 spectrum extended up to 21 m.e.v. The neutron dose rate in the 

 beam at 100 cm. from the target was about 12.5 equivalent roentgens 

 per minute. The increase in deuteron energy from 6-16 m.e.v, 

 made possible by the use of the larger cyclotron, thus increased the 

 actual yield of neutrons under normal operating conditions about 

 fivefold, and considerably improved the penetrating power of the neu- 

 trons so that deep lesions could be more easily reached. At the same 



