NEUTRON INDUCED FISSION CROSS SECTIONS FOR $sup 235$U FROM THE PERSIMMON EVENT.

The U(n,f) cross section was measured for incident neutron energies of from 20 to 1000 eV relative to the 6Li(n,a)T and ■*He(n,p)T reaction cross sections.Neutron energies were separated by time of flight from the single burst of an underground nuclear detonation in an ~300-m evacuated flight path.Results of this ex periment are compared with previous measurements of the 23^U(n,f) cross section.The U(n,f) cross section has been measured on these three single-pulse experiments.The re sults on the Persimmon event are reported here and compared with previous results of the Petrel experi ment.'*'The characteristics of each of these measure ments tend to complement one another, and provide added support for the capabilities of this experi mental technique.A detailed description of the use of under ground nuclear detonations for neutron time-of-2-4 flight studies has been previously published and will only be summarized in this report.II.EXPERIMENTAL TECHNIQUE Neutrons from the moderators, near the under ground detonation, traverse an evacuated line-ofsight pipe to the surface where a circular beam of approximately 1 cm diam is defined with iron colli mators.This intense beam is allowed to impinge on the target material where the fission reaction rate is determined by measuring the current outputs from solid-state detectors at various positions relative to the incident beam direction.The relative posi tions of the beam, target, and detectors used in the Persimmon experiment are shown in Fig. 2. The tar get was fabricated by evaporating approximately 6 mg of uranium oxide onto 0.3-mil platinum backing.Signals from the solid-state detectors are terminated in 51 S! at the input of four-decade logarithmic amp lifiers.The amplified signals are transmitted on coaxial cable to oscilloscopes in the recording sta tion located approximately 1000 ft from the position on the ground directly above the detonation.Moving film records of the oscilloscope traces provide the information storage system for these experiments.<7 (BARN) ERROR E (EV) a(BARN) error E (EV) (t(BARN) error <r(BARN) ERROR E (EV)