The intensity of low-energy events in the cosmic radiation has been examined as a function of altitude with an energy gated scintillation counter. Events selected were those giving pulses the equivalent of 0.2-Mev to 2.0-Mev electron energy loss in the scintillation crystal. Compared with data for total radiation, the present results show a shift toward lower altitudes by an almost constant fraction of a radiation length. low-energy gamma-ray component 2 for which the crystal has high detection efficiency. 3 A smaller contribution is due to grazing collisions with energetic charged particles.The gamma rays represent the low-energy end of the cascade process further degraded by multiple Compton scattering. It is of interest to note that the Pfotzer maximum for low-energy events occurs at 1.08 meters of water equivalent or approximately 51 900 feet pressure altitude, while the data of Millikan indicate a maximum for total radiation at 0.76 meters of water equivalent or 60 000 feet pressure altitude. This shift in the position of the maximum is considered to be well established since the mean deviation for six flights was only 1500 feet.In essence, the low-energy distribution retains all features of the gross radiation curve and is of almost identical shape, but is shifted deeper into the atmosphere by slightly less than one radiation length. Such behavior is indicative of the very small dependence of the degradation of the soft component on the history of the cascade. It is presumed that a cascade theory including multiple Compton scattering and possibly the energy dependence of ionization loss would account for these results. The rise in counting rate between eight and ten meters is, of course, due to the natural radioactivity of the earth and lower atmosphere.The authors are indebted to Mr. J. C. Beynon for assistance in these experiments.2 Rest, Reiffel, and Stone, Phys. Rev. 81, 894 (1951). 3 ReifTel, Stone, and Rest, Phys. Rev. 82, 121 (1951). 1294 O BSERVATIONS have been made on the distribution with altitude of low-energy interactions between cosmic rays and a Harshaw Nal(Tl) scintillation crystal. Six balloon flights at geomagnetic latitude 39.8° north were made during a 14-day period. The resulting altitude curve obtained by averaging all data is shown in Fig. 1 together with a normalized curve for the omnidirectional total flux at 38.5° north as registered by a balloon electroscope. 1 The low-energy events in the 1^-in. diameter by 2 in. long crystal were selected by means of fixed differential pulse-height discriminators in the balloon units. The circuitry was set to accept pulses corresponding to between 0.2-Mev and 2.0-Mev energy loss. The counting rate so obtained results primarily from the intense LOW ENERGY EVENTS • NORMALIZED TOTAL INTENSITY (MILLIKAN, ET. AL.) 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 METERS OF H20 EQUIVALENT FIG. 1. Low-energy events vs altitude.1 Millikan, Neher, and Pickering, Phys. Rev. 66, 295 (1944).

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A radiation monitoring instrument based on the scintillation counter is described which measures γ- (or β-) radioactivity levels from 1 milliroentgen/hour to 100 roentgens/hour without changing scale. The instrument may thus be entirely unattended even in the vicinity of widely varying sources. Its performance and limitations are discussed, together with means for increasing its ultimate sensitivity.

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