190 Applied Biophysics 



relative measurements of the "strength" of a neutron beam, and 

 in discriminating between neutrons of different energy. 



An immediate extension of the definition of the rontgen to 

 include neutron radiation would not be very suitable for use in 

 biology and therapy because, as pointed out above, the energy 

 conversion of the neutrons varies rapidly with the atomic type, 

 even for "light" elements, in contrast to the energy conversion 

 of X- or gamma radiation. In other words, air is no longer a 

 satisfactory approximation to tissue (which contains so much 

 hydrogen in the form of water and various organic compounds). 

 For example. Gray and Read ^- have calculated that when soft 

 tissues are irradiated by fast neutrons, about 92% of the energy 

 converted goes to the recoil protons, 5% to the recoil oxygen 

 nuclei, 2% to the recoil carbon nuclei, and 1% to other effects, 

 and that 1 gram of average tissue would absorb seven times as 

 much energy as 1 gram of air for neutrons of particular energy 

 about 3 million electron volts. 



For reasons such as these. Gray and Read ^^ have proposed 

 that energy absorption in water should replace that in air for 

 the purpose of neutron dosimetry. The unit dose is then that 

 quantity of neutron radiation which communicates to unit volume 

 of water the same energy that is communicated by one rontgen 

 of gamma radiation, i.e., about 94 ergs. This unit may be 

 thought of as an "equivalent rontgen." 



For the actual measurement of energy absorption in a given 

 medium, use may be made of Gray's Principle of Equivalence. 

 In a hydrogenous material, the "corpuscular emission" is pre- 

 dominantly composed of recoil protons. The application of the 

 method has been treated in detail by Gray. A relative measure 

 of exposure that has been widely used in practice is the ioniza- 

 tion produced by the neutron beam in the Victoreen type of 

 X-ray "thimble" chamber dosemeter. This arbitrary unit is 

 known as the "n" unit. 



Charged-Partiole Radiations 



All charged-particle radiations may be considered together, 

 for they have this in common, that by virtue of their charge 



