526 L. H. GRAY 



2. Protons 



If a source of polonium is placed immediately behind a film of 

 paraffin wax about 40 m in thickness, which is sufficient to stop all the 

 a particles emitted by the polonium, a few protons resultmg from 

 headon colhsions between an a particle and a hydrogen nucleus will 

 be emitted by the wax. The maximum energy of such protons will 

 be 2.7 m.e.v. If aluminum foil is used instead of wax foil rather 

 more protons are obtained as a result of the disintegration of the 

 aluminum by the polonium a particles. These protons will have all 

 energies up to 7 m.e.v. but slow protons will predominate. It ap- 

 pears that a source consisting of 10 miUicuries of polonium on a 5 

 mm disc mounted 3 mm. behind a 25 ju aluminum foil gives a proton 

 dose rate of about 60 r.e.p. per day at 1 cm. from the foil surface. 

 With increasing thickness of specimen the dose rate falls roughly ex- 

 ponentially and is reduced to half value by about 30 m of tissue. 



Proton beams of sufficient intensity for most biological experunents 

 can only be obtained with the help of high tension equipment, either 

 directly as a result of the acceleration of hydrogen ions or indirectly 

 by the irradiation of tissues by fast neutrons. 



A minimum of 1 milhon volts energy will generally be needed for 

 the irradiation of biological spechnens by protons [cf. Fig. 13). Pro- 

 tons of this energy may be obtained by accelerating hydrogen ions 

 supphed from a low voltage arc or other suitable source, by means of 

 a high voltage generator and allowing them to pass through a very 

 thin window at the end of the evacuated accelerating tube. Expo- 

 sure to 5 rays from the window must be avoided as described in the 

 case of electron beams. The apparatus is considerably more com- 

 phcated than that required for the acceleration of electrons and will 

 be referred to again m connection with neutron sources. Protons 

 have not hitherto been accelerated in this way to greater than 4-5 

 m e V but 10 m.e.v. may be achieved. Proton beams capable of 

 penetrating about 3 cm. of tissue may be generated by means of cyclo- 

 trons of the ordinary type operated by continuous wave high fre- 

 quency supply. It should be possible to obtain proton beams having 

 a penetrating power of the order of 12 cm. by means of cyclotrons 

 having pole diameters of 90 inches or more operated on the tre- 

 quency-modulated" principle {57,58). Wilson {59) has pointed out 

 that the sharp rise in ionizing power of protons during the last lew 

 miUimeters of their range might be appUed to advantage m the radio- 



