The hypothesis is suggested that initiation of high voltage breakdown in vacuum is due to traversal of the high voltage gap by a clump of loosely adhering material. The implication of this hypothesis for uniform-field gaps is that the breakdown voltage is proportional to the square root of the gap length. A summary the literature is presented which supports this conclusion for a range of voltages from 20 kilovolts to 7 megavolts, and for a range of gap distance from 0.2 mm to 6 meters. Additional qualitative evidence is presented which tends to support the proposed hypothesis.

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The neutron spectrum associated with the fission of U 235 induced by slow neutrons has been remeasured from 0.18 to 12 Mev. Two different experimental techniques were employed in this measurement: (1) the time-of-flight method covering the energy range from 0.18 to 2.7 Mev, and (2) the photographic plate method encompassing the energy range from 0.35 to 12 Mev. The combined results of these measurements fit the relation, N{E)^Ke^E' QMh sinh[(2.29£)»], where N(E) is the neutron flux, E is the neutron energy in Mev and K is a constant. The simpler expression, N(E) =KEh~°-17bE , also fits the data well for neutron energies below 9 Mev.f This work was performed under the auspices of the U. S.

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