IONIZATION AND BIOLOGICAL EFFECTS 105 



sufficient thickness to absorb the soft radiation passing through the tin. 

 Of course, copper has its own characteristic radiation, which is still 

 softer than that of tin, and to remove it a sheet of aluminum is added 

 to the filter. The characteristic radiation of this metal is so soft that a 

 few centimeters of air suffice to eliminate it. It is important to note that 

 a composite filter of this sort must always be used in such a way that the 

 radiation traverses it from the material of highest to that of lowest 

 atomic number. 



A filter removes from a heterogeneous beam of radiation its softest 

 components, reduces considerably the intensity of the components of 

 medium hardness, and reduces somewhat the intensity of the hardest 

 components. Thus the transmitted radiation, while much harder on 

 the average, is not monochromatic. The minimum wave-length is the 

 same in the filtered as in the unfiltered beam. What degree of filtration 

 one should use depends on the purpose for which the radiation is to be 

 employed. In radiotherapy with 200-kv. X-rays, a filter of 0.5 mm. Cu 

 and 0.5 mm. Al is satisfactory. Filters of 2 or 2.5 mm. Cu or equivalent 

 composite filters are also used. With 200-kv. X-rays the increase in 

 hardness with increase in filter beyond 0.5 mm. Cu is slight, while the loss 

 of radiation is considerable. The only way to obtain monochromatic 

 X-rays is by means of a spectrometer. Unfortunately, however, the 

 intensity of a monochromatic beam thus produced is usually too low for 

 most biological investigations. Beams of considerable homogeneity may 

 be produced in certain wave-length ranges by taking advantage of the 

 characteristic radiation and anomalous transmission of suitable materials. 

 For this purpose the metal used as a target in the X-ray tube, the voltage 

 applied to the latter, and the filter material, must be properly chosen. 



In the case of radium, filtration is essential if one wishes to use only 

 the gamma radiation. It may be recalled that radium in equilibrium 

 with its disintegration products emits three types of ionizing radiations: 

 (a) The alpha particles, or rays, which are helium nuclei projected at 

 high speed. They are completely absorbed by a sheet of paper of 

 ordinary thickness. (6) The beta particles, or rays, which are electrons 

 traveling at very high speed. It takes approximately 2 mm. of brass, or 

 1 mm. Pb, or 0.5 mm. Au or Pt to stop all the primary beta rays, (c) 

 The gamma rays, which are photons having on the average much higher 

 energies than are found in X-ray photons. Those of highest energy can 

 traverse many centimeters of lead. The spectrum of gamma rays is very 

 complex and is not very well known on account of experimental diffi- 

 culties arising from the short wave-lengths involved and lack of small 

 sources of very high power. However, it may be mentioned that the 

 same filtration which is required to eliminate the primary beta rays 

 also reduces effectively the soft gamma-ray components. For most 



