XV. ELECTRONS, NEUTRONS, AND ALPHA PARTICLES 539 



This state of affairs is, of course, not even approximately true in 

 the case of megavoltage X radiation, and future work with this radia- 

 tion derived from betatrons, synchrotrons, and microwave hnear 

 accelerators will call for a reconsideration of the manner in which 

 dose can be most usefully defined. This problem has only just be- 

 come acute. 



Much earlier the use of neutrons in research and in therapy called 

 for a reconsideration of the problem of radiation dosimetry, on ac- 

 count of the fact that in the case of neutron radiation the second con- 

 dition upon which the usefulness of the roentgen hinges is completely 

 invalid. The absorption of neutron radiation per unit mass of tissue 

 is generally about five times that per unit mass of air and depends 

 critically on the hydrogen content of the tissue. 



More recently, in connection with the use of radioactive isotopes 

 in research and therapy, problems have arisen connected with the esti- 

 mation of the dose received by tissues on account of their radioactive 

 content. Although such cases may be covered by an obvious exten- 

 sion of the definition of the roentgen, this unit cannot properly be 

 applied as at present defined. 



Analyzing the present situation we see that the immense practical 

 value of the roentgen is that it specifies a quantity of energy too small 

 to be measured calorimetrically by the ionization that would be pro- 

 duced in air by the expenditure of this amount of energy by ionizing 

 particles; its present inadequacy arises from the fact that the 

 quantity defined is X- or 7-ray energy absorbed in air, whereas the 

 quantity in which we are interested is the energy expended by the 

 ionizing particles in tissue. There can hardly be any doubt that the 

 wise course is to define dose in terms of the energy lost per unit mass 

 of tissue by the ionizing particles that traverse the element of tissue 

 under consideration. Dose may then be expressed in the fundamental 

 units of physics, namely ergs per gram, for all types of ionizing radia- 

 tion. This introduces a great simplification into our conception of 

 dose. If it is desired to maintain a numerical correspondence with 

 dose expressed in roentgens, a new unit must be defined which is a 

 multiple of the natural and fundamental unit of 1 erg per gram. This 

 thought prompted the definition of the "energy unit" of dose by 

 Gray and Read {81) and of the "equivalent roentgen" (e.r.) or 

 "roentgen-equivalent-physical" (r.e.p.) which is attributed to Parker 

 (82). The energy unit and the equivalent roentgen only differ in 

 the quantity selected as the unit of energy. 



