Photometrical measurements of the photographic images of cloud tracks of the disintegration of boron by slow neutrons,have enabled the ranges of both the He and the Li particle to be determined. We find that the He particle has a range of 7·0 ± 0·3 mm. and the Li particle a range of 4·3 ± 0·2 mm. in standard air.We deduce that the 7Li nucleus is usually formed in an excited state, with energy of excitation of either 0·5 or 0·8 M.e.V. The wide latitude in excitation energy is due to the uncertain state of our knowledge of the energy-range relation for α-particles.

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Photometrical determinations of the variation of density along photographs of expansion champer tracks have enabled the positions of maximum ionizing efficiency for single α-particles, protons, deuterons, and 3H particles to be determinated.The maximum rate of ionization for an α-particle in air occure when it has a residual range of 3·7 mm. in standard air.The distance of the position of maximum ionizing efficiency from the end of the particle range is shown to be approximately in the ratio of the particle mass in the case of the particles 1H, 2H, and 3H.These experiments were carried out in the Cavendish High Tension Laboratory.

In an investigation of the disintegration ofby protons, resulting in the emission of three α-particles, Dee and Gilbert* found that the observed distributions of ranges and angles of these particles could be explained by assuming that one α-particle is first ejected, leaving a short-lived, excitednucleus which then decomposes into two more α-particles. The half-life of this excited beryllium nucleus was estimated to be about 10–21sec.

While a number of investigations have been carried out to determine the efficiency of emission of K series fluorescent radiation from various elements,* * * § only two determinations have been made of the L yield, that is the efficiency o f emission o f L series radiation. r Some years ago Auger,f using an expansion chamber, investigated the L yield for the two inert gases krypton and xenon, and recently Lay4 using a photographic method, has determined this quantity for certain elements between atomic number 40 and 92. The results of the two investigations are not consistent, the values found by Auger for the L yields of krypton and xenon being much greater (100% and more) than the values for these gases given by interpolating and extrapolating the data published by Lay for neighbouring elements.As pointed out in a previous paper, § the expansion chamber has the advantage that values o f the efficiency of excitation can be calculated by a direct count, the only problem being the identification of tracks. In the present problem this presents little difficulty as the tracks are o f two types, with and without a spur at their origins. Ionization and photo graphic methods yield values, the accuracy of which ultimately depends on the accuracy of existing absorption data, and in general where these exist the agreement between different workers leaves much to be desired. The difficulties of these methods will be discussed later.In the present work measurements o f the L yield for xenon and krypton have been repeated since these gases give spur tracks (due to internal conversion) which are sufficiently long to be readily recognized. Further, experience in the past has shown that they behave well in an expansion chamber.