similar correlation has been observed by Pierce and Blann for lighter projectiles. 5 There is not enough evidence to say whether this correlation is more than a useful fortuitous fact since the concept of an effective charge for an energetic ion in a solid is vague at best. If the electronic stopping power does follow the dependence of the mean charge of ions in gases, then differences, similar to the differences in the mean charges between I and U ions in air reported by Betz et a/. 16 (see Fig. 5), should be observable in the electronic slowing down process. Unfortunately, the present data do not cover a large enough range of reduced velocities to be sensitive to such deviations. More accurate data, taken over a wider range of energies of very heavy ions are needed in order to detect any possible dependence of the stopping process on the mass of the projectile.It is worth remarking that if the effective charge deduced from dE/dX measurements is indeed a physically meaningful quantity, then, perhaps, the potential charge experienced by the ions on their passage from 1 W. H. Zachariasen, quoted by A. W. Lawson and T. Tang, Phys. Rev. 76, 301 (1949).The 7 -> a phase transition of cerium has been investigated by measurement of the positron lifetime and angular correlation of positron-annihilation radiation in the two phases. The lifetime in the a phase was observed to be 8% shorter than the lifetime in the 7 phase when the a phase was produced by the application of pressure; and it was observed to be 5% longer when the a phase was produced by lowering the temperature. The angular correlation from a-cerium produced by pressure was observed to be slightly broader than that from 7-cerium, in contrast to the narrowing previously observed when a-cerium was produced by lowering the temperature. Since the observed changes are much smaller than predicted by models which explain the 7 -> a: transition by assuming a transfer, or partial transfer, of the 4/ electron to the conduction band, it is concluded that such electronic promotion does not occur. GUSTAFSON,McNUTT, AND ROELLIG

No abstract

The pressure dependence of the angular correlation of positron-annihilation radiation in sodium was measured in the range from 4 to 68 kbar. Below 55 kbar the measured value of 8+ was consistent with the predictions of the nearly-free-electron model. Above 55 kbar, 8z deviated from the nearly-free-electron model, indicating that above 55 kbar the electronic structure of sodium metal may no longer be free-electron-like. The percentage pf core annihilations varied smoothly from 30% at 4 kbar to 42% at 68 kbar. As pointed out by Kubica and Stott this incr~a= in the core-annihilation percentage with pressure allows the measurements of the pressure dependence of the positron lifetime in sodium made by MaeKenxie, LeBlanc, and McKee to be explained. The pressure dependence of the enhancement of the electrons at the positron was also ex~~ined. Reasonable absolute agreement with the theories of Kahana and Carbotte was observed, but the variation in the ratios of the enhancement coe%cients with pressure was not in agreement with the theoretical predictions.

An equilibrium method for the determination of the vacancy formation volume in many metals is provided by the trapping of positrons at vacancies. Utilizing a Bridgman press and angular-correlation counting techniques, the vacancy formation volume in metallic indium has been determined to be 6.1 + 0,2 cm'/mole, or 39+ 1% of the molar volume.

Positronium formation in muscle at +4 degrees C and -4 degrees C was examined by the measurement of the angular correlation of positron annihilation radiation. Since the positronium formation rate in ice is considerably higher than it is in water, there should be a comparable increase in the positronium formation rate in muscle tissue if recent speculation that cellular water is ordered in a semicrystalline icelike state is correct. Comparison of the angular correlation from muscle at +4 degrees C with that from water at +4 degrees C shows no enhancement of the positronium formation rate. Frozen muscle at -4 degrees C shows an enhancement of the positronium formation rate of approximately half that found in ice at -4 degrees C, indicating that most cellular water undergoes a normal water-ice transition when frozen. It is concluded therefore that cell water in muscle is not ordered in a hexagonal icelike structure. While the results are consistent with the hypothesis that cell water is in the liquid state, the hypothesis that cell water is ordered in an undetermined close packed structure which transforms to the hexagonal ice structure at or near 0 degrees C cannot be ruled out.