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The total prompt -ray energy distributions for the neutron-induced fission of 235 U, 239,241 Pu at incident neutron energy of 0.025 eV -100 keV, and the spontaneous fission of 252 Cf were measured using the Detector for Advanced Neutron Capture Experiments (DANCE) array in coincidence with the detection of fission fragments by a parallel-plate avalanche counter. DANCE is a highly segmented, highly efficient 4 -ray calorimeter. Corrections were made to the measured distribution by unfolding the two-dimension spectrum of total -ray energy vs multiplicity using a simulated DANCE response matrix. The mean values of the total prompt -ray energy, determined from the unfolded distributions, are ~ 20% higher than those derived from measurements using single -ray detector for all the fissile nuclei studied. This raises serious concern on the validity of the mean total prompt -ray energy obtained from the product of mean values for both prompt -ray energy and multiplicity.

a b s t r a c tA compact gas-filled avalanche counter for the detection of fission fragments was developed for a highly segmented 4p g-ray calorimeter, namely the Detector for Advanced Neutron Capture Experiments located at the Lujan Center of the Los Alamos Neutron Science Center. It has been used successfully for experiments with 235 U, 238 Pu, 239 Pu, and 241 Pu isotopes to provide a unique signature to differentiate the fission from the competing neutron-capture reaction channel. It was also used to study the spontaneous fission in 252 Cf. The design and performance of this avalanche counter for targets with extreme a-decay rate up to $ 2.4 Â 10 8 /s are described.

The neutron spectrum from neutron-induced fission needs to be known in designing new fast reactors, predicting criticality for safety analyses, and developing techniques for global security application. The experimental data base of fission neutron spectra is very incomplete and most present evaluated libraries are based on the approach of the Los Alamos Model. To validate these models and to provide improved data for applications, a program is underway to measure the fission neutron spectrum for a wide range of incident neutron energies using the spallation source of fast neutrons at the Weapons Neutron Research (WNR) facility at the Los Alamos Neutron Science Center (LANSCE). In a double time-of-flight experiment, fission neutrons are detected by arrays of neutron detectors to increase the solid angle and also to investigate possible angular dependence of the fission neutrons. The challenge is to measure the spectrum from low energies, down to 100 keV or so, to energies over 10 MeV, where the evaporation-like spectrum decreases by 3 orders of magnitude from its peak around I MeV. For these measurements, we are developing two arrays of neutron detectors, one based on liquid organic scintillators and the other on 6Li-glass detectors. The range of fission neutrons detected by organic liquid scintillators extends from about 600 keV to well over 10 MeV, with the lower limit being defined by the limit of pulse-shape discrimination. The 6Li-glass detectors have a range from very low energies to about 1 Me V, where their efficiency then becomes small. Various considerations and tests are in progress to understand the important contributing factors to designing these two arrays and they include selection and characterization of photomultiplier tubes (PM), the performance of relatively thin (1.25 cm) 6Li-glass scintillators on 12.5 cm diameter PM tubes, use of 17.5 cm diameter liquid scintillators with 12.5 cm PM tubes, measurements of detector efficiencies with tagged neutrons from the WNRILANSCE neutron beam, and efficiency calibration with 252Cf spontaneous fission neutrons. In addition, significant modeling is underway to assess contributions from room-return neutrons and detector cross-talk. A new flight path is being constructed to reduce the effect of room-return neutrons. A data acquisition system based on wave-form digitizers is being developed to extract the maximum amount of information from the signals with the minimum amount of dead time. Design considerations and test results will be presented in this talk.