Three instrumEmt packages containing Geiger tubes ,were flown into the inner Van Allen belt to, altitudes up' to 1500 km. Various absorber thicknesses were used to obtain range ' speetra for penetrating particlel3. Magnetic ,separation .of electrons and protons was perfoOlled on ~ounters with absorber 'thicknesses ranging from 1.7' mg/cm" of mica to 36 mg/cm" of aluminum. The electron spectrum ' observed ' with counters having electron energy thresholds of 30, 160, 190, !lend 460 kev' appears 'considerably softer than a neutron beta-decay spectrum. Counts in cowrters with higher thresholds are a~med to, be due to trapped protons and the measurements are compare, d 'with those obtained with ,nuclear ' emufl3ion experiIDents on similar trajectories. Absolute flux v!1lues are reported.' '

The trapped protons in the outer zone of the Van Allen radiation belt were measured on October 4, 1960, by means of a Scout rocket payload containing a two‐crystal scintillation spectrometer. Protons with energies above 1 Mev were detected, the intensity rising above background at an altitude of 1200 km. Representative intensities for 1≤Ep≤0 Mev at 2, 3, 4, and 5 times; 03 km, through a range of 42°N to 30°N geomagnetic latitudes, were 0.27, 0.90, 2.02, and 4.56 times; 05 protons/cm2 sec ster. These intensities are averages for protons having pitch angles between 90° and ∼60°. The proton energy spectrum at 5000 km is J(E) = 2.0 × 106E−5.2 protons/cm2 sec ster Mev for 1.02≤Ep≤2.24 Mev, and J(E) = 0.71 × 106E−3.9 for 2.24≤Ep≤7.3 Mev. The pitch angle distributions for altitudes of 4600 km and 5150 km were also obtained for angles between 90° and 58°. There was no evidence for a proton intensity maximum coincident with the E2 maximum.

In the first of a series of experiments, an instrument package of eight Geiger counters, shielded and collimated in various ways, was carried aloft and released by an Atlas ICBM; its trajectory passed into the lower Van Allen radiation belt. Since there are variations in altitude at the isocount contours of the belt at several latitudes, the trajectory achieved its deepest penetration into the radiation belt somewhat after reaching its maximum height of 990 km (Fig. 1). Of the eight counters, two with relatively large area and low shielding were saturated by the high flux at deepest penetration and were used only as aids in determining the orientation of the instrument package. The remaining six counters, which provided the data discussed below, were shielded with five different absorber thicknesses (in milligrams per square centimeter of the material indicated) as follows: A, 1.7 mica; B and C, 30 aluminum (2 counters) ; D, 150 aluminum; E, 400 aluminum; and F, 2000 brass.

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