A model is presented which describes the geometrical effects in the radiolysis of water as a result of the diffusion of free radicals. The motion of secondary electrons immediately following the passage of the ionizing particle is discussed in an appendix, and it is concluded that radicals are most likely formed in pairs at the approximate sites of the original ionizations. Models for the diffusion of these radicals are shown to result in a definite fraction of radicals which undergo initial recombination for gamma‐ and fast beta‐rays, for which the spurs are considered as diffusing independently. For alpha‐rays a connected‐track model is used. For the intermediate case of tritium beta‐rays, a two‐stage model is constructed. In each case the comparative yields of the ``forward'' and ``radical'' reactions (GF and GR) are calculated. Subsequent chemical effects in pure water and solutions are also considered briefly.

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J. L.
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2.
D. E. Lea, Actions of Radiations on Living Cells (Cambridge University Press, Cambridge, 1947), pp. 1–68.
3.
Our term “spur” corresponds to the “cluster” in the terminology of
E.
Kara‐Michailova
and
D. E.
Lea
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36
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We have preferred to use the term “spur” in order to avoid confusion with the cluster theory of Lind, The Chemical Effects of Alpha‐Particles and Electrons (Reinhold Publishing Company, New York, 1935).
The term also corresponds to the “hot spot” of
Allen
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79
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1952
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A qualitative description along the same lines has already been given by
A. O.
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,
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See, for instance,
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12.
This value is rather lower than the average number of ion pairs per primary ionization in the gas phase. If a function is chosen which gives a higher value of this average, the result will, in general, be a higher value of GF/(GF+GR) for a given value of x. The lower average was chosen because it seemed to accord with the larger number of small spurs which would be expected in the liquid phase, where the effective ionization potential is lowered. In this connection it should be mentioned that the value of GF/(GF+GR) is not very sensitive to the actual spur size distribution function as long as a constant average number of radical pairs per spur is maintained.
13.
This formula is derived from the requirement that νVmtm/Z = 1 where ν is the number of tracks falling on 1 cm2 of the medium, Vm is the final volume, and tm the lifetime of a track of length Z.
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H. S. W. Massey and E. H. S. Burhop, Electronic and Ionic Impact Phenomena (Clarendon Press, Oxford, 1952), p. 279.
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Sir J. Townsend, Electrons in Gases (Hutchinson, London, 1947).
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