528 I- H. G R A Y 



greatly exaggerated in the frequency-modulated cyclotron, in which 

 the radiation is delivered in pulses, probably about a microsecond in 

 duration, and repeated 250 times per second. The mean currents 

 obtained with such instruments will probably not exceed 1% of those 

 obtained with ordinary cyclotrons. 



3. Fast Neutrons 



Two years before the identification of the neutron by Chadwick 

 {60), Bothe and Becker {61) had investigated some of the properties 

 of neutron radiation resulting from the disintegration of beryllium 

 by the a particles of polonium under the mistaken impression that it 

 was a very high energy 7 radiation. This original method still pro- 

 vides the strongest source of neutrons available to laboratories not 

 equipped \\dth high voltage apparatus, but for biophysical investiga- 

 tions it suffers from the twofold disadvantage that the total neutron 

 emission is low and that the neutrons are accompanied by 7 rays. 

 The neutron ionization produced in tissue at 1 cm. from a source con- 

 sisting of 10 milhcuries of polonium mixed with beryllium would only 

 be about the same as results from exposure to 12 r. per day of 7 radia- 

 tion. There would, moreover, be the problem of filtering out the 

 7 radiation resulting from the disintegration. For physical investi- 

 gations radium or radon may be substituted for polonium as the a- 

 ray source, thereby increasing the neutron emission about fivefold, 

 but this is inadmissible for radiobiological experiments on account of 

 the strong 7-ray emission from RaB and RaC in equilibrium with 



radium or radon. 



Except in rare circumstances, therefore, neutrons for radiobio- 

 logical research must be obtained from nuclear reactions in which the 

 bombarding particle is accelerated by some form of high voltage 

 equipment. The following nuclear reactions may be utiHzed : 



j)2 _|_ D2 > He' + »i + 3.18 m.e.v. (1) 



J32 4_ Li' > He^ 4- He^ + ai^ + 15 m.e.v. 



or less frequeutly > Be* + n' + 14.9 m.e.v. (2) 



Q2 _(. Be« > B'" + n^ + 4.2 m.e.v. (3) 



JJ2 _|_ c'2 > N'» -{- n^ - 0.28 m.e.v. (4) 



Reaction (1) above has been used by Gray, Read, and collaborators 

 for a variety of radiobiological investigations. The deuterium ions 



