cated in the lower half of Canister 5. This signal was received, demodulated, and recorded by the ground station. 2.1 DETECTORS The detectors used in thig experiment were designed to give a quantitative measurement of the absolute neutron spectrum and the gamma-ray dosage produced by the nuciear device. ' Since the shot was to take place at a very-high altitude (about 90,000 feet), it was known that the neutron spectrum could straight forwardly be obtained by a time-of-flight measurement. This involved the assumption that, for the purposes of instrument design, the effect of the atmosphere for neutron energies in excess of 0.1 Mev, is small at these altitudes. The gamma-ray flux was measured in two ways—the dosage in the initial pulse, integrated for 10 usec, and the total dosage, integrated for 120 mseo. 2.1.1 Description. The detector used to measure the neutron flux employed a scintil- lation crystalofenriched (95 percent) Li‘! and a 926 photodiode to measure the light output of the crystal. The Li’ rendered the crystal sensitive to neutrons mainly through the Li‘(n, a@)H® reaction. In order to provide a suitable control for this crystal and, in particular, to observe its gamma response, a orystal of identical geometry composed of ordinary Lil was used aa a second detector. Since 7.5 percent of normal lithium is Li, the output of this detector due to neutrons should be only 7.9 percent of that of the Li'l. The difference in output of these two crystala, after a suitable correction for the presence of Li®, can therefore be taken to be due solely to neutron activity. The initial burst of gamma radiation was monitored by a cealum fodide crystal. In this case, the crystal was small (about 0.1 cc) and was taped directly to the face of a 926 phototube. The integral of the gamma dosage overthe first 10 usec was read from the output of this detector. In addition, the detector furnished a pulse to the associated electronic circuitry to initiate the time sequences necessary for the proper collection of data from all the detectors. The fourth detector was used to determine the total gamma dosage measured over a period of 120 msec. This detector consisted of a crystal of potassium bromide treated go as to color under the action of gamma rays. The resulting change in light transmission was measured by means of a lamp and a small (1P42) photodiode. The sensitivity of the crystal was such that dosages ranging from 50 to 2,000 r were easily detectable. 2.1.2 Calibration. The calibration of the neutron detectors was for the purpose of establishing the relationship between the neutron flux passing through the detectors and the corresponding output currents of the photodiodes. Since it was necessary to calibrate the detectors over a considerable range of energies, the NRL 2 Mev Van de Graaff generator was used to produce monoergic neutrons from the following reactions: Tip, n)He® Did, n)He? and T(d, n)He (0.15, 0.25 and 0.36 Mev neutrons) (4.28 Mev neutrons) (15.0 Mev neutrons) A photomultiplier was used to observe individual events in the crystals, since the neutron fluxes available were too small to permit direct observation by a photodiode of the light 15