R ECENTLY Dunlap 1 and DeSorbo 2 reported on resistivity measurements of heat-treated germanium single crystals at hydrogen temperatures. They found that the activation energy for thermally introduced acceptors is about 0.03 ev; i.e., considerably higher than for chemical acceptors. 3 Some time ago, 4 at the
ACKNOWLEDGMENTSWe are indebted to Dr. Messiah and Dr. Marshak for theoretical guidance, as we are to Dr. Kroll of Columbia University and to Professor Wentzel of Chicago. This research has been a group effort, and we are grateful to the members of our group, not primarily concerned with the work reported here, for many suggestions and much help. These men are Dr.The yields of the L-series x-ray lines from 7r-mesonic atoms have been measured for fourteen elements between Z=6 and 26. The total yield rises from 18% at carbon to a maximum of about 70% in the vicinity of aluminum, and then decreases. This decrease is caused primarily by nuclear absorption of pions from the 3d state; the 3d-*2p yield determinations thus measure the nuclear absorption rate from the 3d state. The low-Z drop-off is predicted qualitatively by theoretical calculations of the meson cascade scheme. The observed relative intensities of the different X-series emission lines are substantially independent of Z in the range 6^Z^16. The 3d-*2p transition energies are consistent with electromagnetically predicted values within the experimental error of 3 to 5% for Z <28.
T HE 130-inch cyclotron has accelerated protons out to a radius of 58i" (approximately the « = 0.2 point) or to an estimated energy of 240 Mev since the first of the year. Measurement of the C" activity of a carbon-tipped probe indicates average currents of about 0.1 jua at this radius and radio-autographs of the target show the beam to be about 1" in height and well centered vertically on the probe. Eastman Kodak NTB plates placed along the probe show, among many proton recoil tracks, w~ and ir + tracks.About 15 kw of r-f power for the Dee is furnished by a grounded grid oscillator with feedback through the Dee. The natural frequency of the Dee is varied from 26 to 18 megacycles by a rotating condenser. The condenser which is in the Dee tank consists principally of a rotor of copper-plated, thin-toothed zircon disks mounted on a shaft parallel to the back edge of the Dee and a Dee stator which is on the back edge of the Dee. The magnetic drag of this rotor up to 2000 r.p.m. is small compared to the power taken by the rotor shaft seals. The copper-plated ceramic disks already give promise of satisfactory life.Ion loading of the Dee is obviated by a negatively biased grid structure covering the linear above and below the Dee. Globar resistors break the grid into lengths too short to resonate to the Dee frequencies.
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