FIG. 15. The photo-line of K 40 and Na 24 and Co 60 calibration lines. The source strength was less than 10~9 curie. The data were taken in about five hours. The K 40 energy is 1.48±0.02 Mev. 40 50 60 70 80 90 KX) PULSE HEIGHT(VOLTS)of the pulses in K 40 show that there is no other gammaray present with an intensity greater than about 10 percent of the 1.48-Mev line. Our success with the photographs of oscilloscope patterns suggests that this technique will be valuable for work with weak sources since it serves the purpose of a multichannel discriminator. Densitometer traces we have made of gamma-ray patterns resemble the discriminator curves very closely. For initial spotting of gamma-ray lines this technique will probably not be surpassed.We are greatly indebted to Drs. Dean Cowie and P. Abelson of the Carnegie Institution of Washington for supplying us with sources of Ga 66 . We wish to thank Drs. E. P. Tomlinson and J. L. Simons for sources of Au 198 .Photo-neutron yields from spheres of D 2 0 and Be containing Na 24 and Ga 72 gamma-ray sources were measured by spatial integration of induced foil activities in a large paraffin block. A standard neutron source was used to calibrate the apparatus.The Ga 72 and Na 24 sources were subsequently dissolved, and their radioactive strengths were measured by beta-counting of aliquots. The data so obtained, when combined with the known wall thickness of the spheres led to the following results after the application of appropriate corrections:(1) Photo-disintegration cross section of D at 2.76 Mev: 14.3X10~2 8 cm 2 (2) Photo-disintegration cross section of Be at 2.76 Mev: 6.74X10 -28 cm 2 (3) Cross section of D at 2.50 Mev multiplied by intensity of 2.50 Mev line in quanta per Ga 72 beta-emission: 2.81 X10-28 cm 2 (4) Sum of cross sections of Be at 1.84, 2.20 and 2.50 Mev weighted according to respective gamma-intensities in Ga 72 : 2.16 X10~2 8 cm 2These results are believed accurate within 8 percent. Agreement with theory is discussed for both D and Be.

PUBLICATIONof brief reports of important dis-JL coveries in physics may be secured by addressing them to this department. The closing date for this department is, for the issue of the 1st of the month, the 8th of the preceding month and for the issue of the 15th, the 23rd of the preceding month. No proof will be sent to the authors. The Board of Editors does not hold itself responsible for the opinions expressed by the correspondents. Communications should not exceed 600 words in length.Showers of Penetrating Particles at Altitude of 26,

SUMMARYA series of laboratory experiments were performed in an effort to improve our understanding of the physics of the echo-intensification phenomenon, in which the radar reflectivity of a region of a thundercloud increases rapidly immediately following the occurrence of lightning within it. Measurements were made of the charges, q, deposited on falling water drops when they engage in direct interactions with positive corona streamers in a uniform electric field, E, just above the limit for propagation, E,. As the drop radius increased from 12 to 950pm, q increased from about to about 10-'°C; and q increased as E was raised above E,. These measurements yielded an estimate of 30pm and lo8, respectively, for the radius of, and the number of elementary charges contained in, the tip of a propagating streamer at a pressure of one atmosphere.The collection efficiencies of drops in the radius range 40 to 120pm carrying charges appropriate to the direct interaction process, for uncharged droplets of radius around 12pm, were found to be about four times the non-electrical values. It was concluded that this enhancement in collection resulted largely from dipole rather than coulomb forces. A simple analysis indicates that reported echo-intensification observations cannot be explained in terms of these enhanced collection efficiencies, but could possibly be a consequence of the greatly enhanced velocities (and hence growth rates) of drops of radius -100pm highly charged by direct interaction with corona streamers in the intense electric fields of a thundercloud.

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