Ionization Maximum for Protons

IN a recent paper, Holloway and Moore1 have given a curve showing the average specific ionization, as a function of the residual range, for protons in air. On this curve the ionization maximum occurs at 1.9 mm. residual/range, and in the course of the paper the absolute value of the maximum specific ionization is quoted as 2,320 ion pairs per mm. The shape of the ionization/range curve was deduced from earlier measurements of Parkinson, Herb, Bellamy and Hudson2, and the absolute value of the ionization was obtained on the assumption that, in the region of 3 cm. residual range, the mean specific ionization produced by a proton is a quarter of that produced by an α-particle of the same velocity. On this basis Holloway and Moore inferred that the maximum specific ionization is about one third as great for protons as for α-particles in air (6,760 ion pairs per mm.). Now, while there is overwhelming evidence for the correctness of this last assumption (the 1:4 ratio of ionizations for particles of the same velocity, when the ratio of the effective charges is constant at 1:2), it also appears that the conclusions, regarding the position and height of the ionization maximum for protons, to which the subsequent argument leads, run contrary to accepted evidence from other experiments. If this be admitted, then one must clearly assume that the results of Parkinson, Herb, Bellamy and Hudson will not bear the detailed analysis which Holloway and Moore place upon them, when protons of the smallest energy are concerned. It is the object of the present note to point out the contrary evidence referred to above.