XV. ELECTRONS, NEUTRONS, AND ALPHA PARTICLES 535 



The decay is by positron emission and the positrons by annihilation 

 give 0.5 m.e.v. 7 radiation. A nuclear 7 ray is also emitted. By the 

 end of relatively short bombardments, therefore, each neutron will be 

 accompanied by more than 1 m.e.v. of 7 radiation. Despite these 

 disadvantages neutrons from this reaction may have a place in bio- 

 logical research as providing neutrons of uniform but low energy. 

 For example, when the accelerating voltage is 800 kv. the neutrons in 

 the forward direction have about 350 kv. energy (74); at 600 kv. 

 and 90° to the beam the neutrons have about 150 kv. energy. The 

 mean energy of the recoil protons will be half these values, which, as 

 pointed out in earlier sections, is an interesting energy region since the 

 protons are too slow to form any 8 rays and the ratio of excitation to 

 ionization may be different from that characteristic of faster particles. 



5. Alpha Particles 



Polonium with a half-life of 13G days emits a particles having a 

 range of 4 cm. in air and about 40 n in tissue unaccompanied bj' ^ or 

 7 radiation. A small disc on which polonium is deposited is therefore 

 an extremely convenient source of a radiation for the irradiation of 

 bacteria {75), spores (76), and other small objects {cf. 29). All the 

 a particles are emitted with the same energy (5.3 m.e.v.) and all are 

 slowed down to approximately the same extent by foils of mica or 

 other material placed in front of the source. Zirkle {76) took ad- 

 vantage of this fact to study the relative effectiveness of a particles 

 of different speed in inhibiting division in fern spores. The a-ray 

 dose rate at 1 cm. from a point source of 1 millicurie is given by a 

 similar formula to that for jS radiation {cj. page 546), namely: 



dose rate = 1.20 A r.e.p./min. 



A — ion density of a particles in ions/ju tissue. Thus for a parti- 

 cles emitted by polonium the dose rate is about 2700 r.e.p./min. ris- 

 ing to 3100, 3800, 5000, and 7500 r.e.p. per minute as the energy is 

 reduced by foils successively to 4, 3, 2, and 1 m.e.v. RaC sources, 

 prepared by exposing a metal disc to radon and allowing half an hour 

 to elapse for the decay of RaA, emit a particles of 7.68 m.e.v. energy. 

 Although /3 and 7 rays will also be emitted from the source prepared 

 in this way the ionization per unit volume due to the /3 radiation in any 

 specimens small enough to be traversed b}^ the a particle will be only 

 about 1% of the a-ray ionization and the effect of the 7 radiation will 



