D O C . 4 3 8 R A D I AT I O N B I O L O G Y A N D T H E R A P Y 4 1 7 438. “On Physical Aspects of Radiation Biology and Therapy” Received [before 28 December 1926][1] Dated 19 February 1927 I.[2] What happens to radiation from the source until absorption? Do the individual kinds of radiation (α, β, γ), and particularly those within the X-ray spectrum, have specific characteristics with respect to physico-chemical pro- cesses, or is the effect of radiation only dependent on the quantity per unit volume of the absorbed radiation dose? With respect to propagation, α-, β- and γ-rays, or X-rays, behave in the same way insofar as the propagation proceeds practically rectilinearly. With respect to absorption, α- and β-rays behave differently from γ- and X-rays. α- and β-rays (corpuscular rays) set off a series of ionization processes (like a string of pearls) along the path of each particle, whereby the particle gradually loses its velocity. The absorption processes occur most densely at the end of a particle’s path, the least densely at the beginning. Although γ- and X-rays are also comparable to cor- puscular rays because they are directed elementary processes and propagate recti- linearly, the elementary process does not lose its energy by generating a chain of ionizations along its path rather, it is lost as such by a single, entirely localized1) elementary event. There are three different kinds of such destructive elementary events: a) Absorption. Vanishing of the elementary γ- event with emission of an electron from the atom, which absorbs the elementary γ-event. This electron then continues to act in any direction as a β-ray and initiates an ionization chain, as an elementary β-ray does. b) Compton effect.[3] An atom takes up the whole energy of the elementary event and divides it into two parts. One part leaves as a β-ray with one electron, the other one leaves as a secondary, softer elementary γ-ray. Both secondary elementary acts have a practically random direction. c) Actual scattering. An impacted atom changes the elementary γ-act into another one with different direction and of the same hardness. 1) From among the abundance of radiation problems, the editor has selected the following 5 questions that to him appear to be of special importance in radiation biology therapy. Thankfully, Prof. Einstein has undertaken to answer these questions for our volume. [p. 1]