10 RADIATION BIOLOGY 



each molecule can be activated by the impact of a particle only if this 

 particle has a kinetic energy equal to at least ^%3 = 2.2 ev. 



There is yet another method of expressing the "potency" of a radiation 

 with reference to familiar thermal concepts. Vapor molecules which 

 filter out of an oven at a temperature of T degrees absolute have a kinetic 

 energy of the order of magnitude of kT, where 



k = 1.4 X 10~i^ erg/degree = 8.5 X lO"" ev/degree (6) 



and is the "Boltzmann constant." The formula indicates that it 

 would take an oven at about 12,000°C to emit particles Avith a kinetic 

 energy of the order of 1 ev. Any radiation source may thus be character- 

 ized from the thermal standpoint by means of an "equivalent tempera- 

 ture" which is proportional to the kinetic energy of the radiation particles 

 at a rate of 12,000°C/ev. 



A comparison of the various scales of radiation potency is shown in 

 Fig. 1-5. 



1-2. PRODUCTION OF CORPUSCULAR RADIATIONS 



Electrically charged particles can be directed into a beam and then 

 accelerated to a desired velocity under the action of electric forces. 

 Neutral particles, on the contrary, cannot be controlled easily; the main 

 device to form a beam of neutral particles is to let the particles pass from 

 the space where they are produced to another portion of space through 

 collimating holes or slits. 



l-2a. Sources of Charged Particles. Electrons are easily produced in a 

 vacuum by heating a metal. The hot metal emits electrons in quantities 

 limited only by the mutual repulsion of the electrons in the surrounding 

 space ("space-charge effect"). Rates of emission of the order of 

 amperes, i.e., of 10^^ electrons per second, are practicable. 



Positrons have to be emitted by radioactive nuclei or produced by 

 X rays. Their rate of production is much smaller than that of electrons 

 from a hot wire. For example, a "strong" radioactive source, of the 

 order of curies, yields only 10^^ positrons per second, i.e., about 10~^ amp. 



Protons and other positive ions are produced by stripping electrons off 

 the atoms or molecules of a gas at low pressure. The stripping results 

 from the violent atomic collisions which accompany an electric discharge. 

 Usable ion-beam intensities of the order of a milhampere have been 

 obtained. 



Charged particles produced by these methods can be formed into a 

 beam by attracting them toward a conductor charged with electricity 

 of opposite sign. Thence they can be canalized into empty space. Suit- 

 ably distributed electric and magnetic forces also serve to concentrate or 

 "focus" beams of charged particles. Still, the achievement of good 

 focusing at a high intensity level offers difficulties. 



