The Chi-Nu project to measure prompt fission neutron energy spectra for the major actinides has now completed two measurements, for neutron-induced fission of 239 Pu and 235 U for incident neutron energies from 1 to 20 MeV, and has almost completed the prompt fission neutron spectrum measurement for 238 U. In addition, similar data have been taken for spontaneous fission in some other isotopes, and a measurement of fission neutron spectra from 240 Pu( n , f ) is in progress. These measurements are done at the same facility, with the same equipment and analyzed in a similar way. A useful way to look at such data is to examine ratios of the prompt fission neutron spectra among actinides, as some of the experimental uncertainties involved in these data are the same from isotope to isotope, making the ratios of fission neutron spectra less dependent on the experimental and analysis details. We discuss here the ratios of prompt fission neutron spectra between 239 Pu( n , f ) and 235 U( n , f ) , as well as the evolution of the mean fission neutron energy for 239 Pu( n , f ) and 235,238 U( n , f ) with increasing incident neutron energy.

Prompt fission neutron spectrum (PFNS) evaluations use provide nuclear data for the PFNS across a wide range of incident and outgoing neutron energies. However, experimental data underlying the evaluation are sparse, inconsistent, and incomplete with respect to the desired incident and outgoing energy coverage. As such, evaluations sometimes predict features of the PFNS, such those relating to multi-chance fission and pre-equilibrium pre-fission neutron emission, without any experimental validation. The Chi-Nu experiment at Los Alamos National Laboratory has recently obtained high-precision results for the 239Pu and 235U PFNS which, for the first time in both cases, have shed light on multi-chance fission and pre-equilibrium contributions to the observed fission neutron spectrum. In addition to providing the first experimental data on some of these fission properties, the angular coverage of the Chi-Nu experiment allows for the extraction of angular distributions of pre-equilibrium pre-fission neutrons. PFNS results of multi-chance fission and pre-equilibrium pre-fission neutron emission are discussed in this proceedings in terms of the observed neutron spectrum and the average PFNS energies.

Although the prompt fission neutron spectrum (PFNS) is an essential component of neutron-driven systems that has been measured for decades, there are still multiple glaring unknowns regarding the PFNS of major actinides in the fission neutron incident energy range, specifically with regard to multichance fission and preequilibrium neutron emission processes. The only impactful experimental Pu-239 PFNS measurements included in recent nuclear data evaluations were measured over a limited outgoing neutron energy range at thermal and 1.5-MeV average incident neutron energy, while other potentially impactful measurements have been shown to contain errors that resulted in either large uncertainty increases or in complete exclusion from nuclear data evaluation. We report here a measurement of the Pu-239 PFNS over a wide range of incident neutron energy (1-20 MeV) and three orders of magnitude in outgoing neutron energy (0.01-10 MeV) resulting from the Chi-Nu experiment at the Los Alamos Neutron Science Center. These results are the combination of separate PFNS measurements in the same experimental area, one using a Li-glass and the other a liquid scintillator detector array. Covariances between all PFNS data points from each detector and within each incident energy range were generated between all other data in both detector arrays and within all other incident neutron energy bins, yielding a single covariance matrix for all 1300 PFNS data points reported here. These covariances are based on a thorough assessment of systematic bias and uncertainties associated with the measurement, PFNS extraction technique, combination of data from each detector type, and other aspects of the analysis. The existence of covariances between PFNS data points in different incident neutron energy ranges yielded covariances between average PFNS energy values at each incident energy to be reported here as well, which allowed for firm statements to be made regarding a shape of a purely experimental mean PFNS energy trend for the first time. Although minor PFNS shape differences exist between the results reported here and recent nuclear data evaluations, the ENDF/B-VIII.0 and JEFF-3.3 PFNS evaluations agree reasonably well with the present results from 1-to 10-MeV incident neutron energy, which spans the well-measured 1.5-MeV incident neutron energy PFNS from Lestone and Shores as well as the onset of second-chance fission. However, while the pre-equilibrium component of the PFNS above 12-MeV incident neutron energy roughly agrees in position and magnitude with ENDF/B-VIIL0 and JEFF-3.3, clear differences relating to the relative magnitude of third-chance fission PFNS features are present in the PFNS shape and in the mean PFNS energy trends.

We report the current results of a large effort to accurately measure the Prompt Fission Neutron Spectra (PFNS) for neutron-induced fission of 235U and 239Pu for incident neutrons with energies from 1 to 20 MeV. The Chi-Nu experiment at the Los Alamos Neutron Science Center used an unmoderated, white spectrum of neutrons to induce fission in actinide samples that were placed inside a parallel plate avalanche counter to provide a fast fission signal. A double time-of-flight technique was used to determine the incoming and outgoing neutron energies. Two neutron detector arrays, one with 54 liquid scintillators and another with 22 lithium glass detectors, were used to detect the outgoing neutrons and measure the PFNS distributions over a wide range in outgoing neutron energy, from below 100 keV to 10 MeV. Extensive Monte Carlo modeling was used to understand the experiment response and extract the PFNS. Systematic errors and uncertainties in the method have been examined and quantified. A summary of these results for incoming energies from 1 to 5 MeV is presented here.

Ruotsalainen, P.; Henderson, J.; Hackman, G.; Sargsyan, G.H.; Launey, K.D.; Saxena, A.; Srivastava, P.C.; Stroberg, S.R.; Grahn, T.; Pakarinen, J.; Ball, G.C.; Julin, R.; Greenlees, P.T.; Smallcombe, J.; Andreoiu, C.; Bernier, N.; Bowry, M.; Buckner, M.; Caballero-Folch, R.; Chester, A.; Cruz, S.; Evitts, L.J.; Frederick, R.; Garnsworthy, A.B.; Holl, M.; Kurkjian, A.; Kisliuk, D.; Leach, K.G.; McGee, E.; Measures, J.; Mücher, D.; Park, J.; Sarazin, F.; Smith, J.K.; Southall, D.; Starosta, K.; Svensson, C.E.; Whitmore, K.; Williams, M.; Wu, C.Y.

The physical properties of neutrons emitted from neutron-induced fission are fundamental to our understanding of nuclear fission. However, while state-of-the-art fission models still incorporate isotropic fission neutron spectra, it is believed that the preequilibrium prefission component of these spectra is strongly anisotropic. The lack of experimental guidance on this feature has not motivated incorporation of anisotropic neutron spectra in fission models, though any significant anisotropy would impact descriptions of a fissioning system. In the present work, an excess of counts at high energies in the fission neutron spectrum of ^{239}Pu is clearly observed and identified as an excess of the preequilibrium prefission distribution above the postfission neutron spectrum. This excess is separated from the underlying postfission neutron spectrum, and its angular distribution is determined as a function in incident neutron energy and outgoing neutron detection angle. Comparison with neutron scattering models provides the first experimental evidence that the preequilibrium angular distribution is uncorrelated with the fission axis. The results presented here also impact the interpretation of several influential prompt fission neutron spectrum measurements.

The T-z = -3/2 nucleus Mg-21 has been studied by Coulomb excitation on Pt-196 and Pd-110 targets. A 205.6(1)-keV gamma-ray transition resulting from the Coulomb excitation of the 5/2(+) ground state to the first excited 1/2(+) state in Mg-21 was observed for the first time. Coulomb excitation cross-section measurements with both targets and a measurement of the half-life of the 1/2(+) state yield an adopted value of B(E2; 5/2(+) -> 1/2(+)) = 13.3(4) W.u. A new excited state at 1672(1) keV with tentative 9/2(+) assignment was also identified in Mg-21. This work demonstrates a large difference in the B(E2; 5/2(+) -> 1/2(+)) value between T = 3/2, A = 21 mirror nuclei. The difference is investigated in the shell-model framework employing both isospin conserving and breaking USD interactions and using modern ab initio nuclear structure calculations, which have recently become applicable in the sd shell.

Prompt γ-ray spectra were measured for the spontaneous fission of 240,242Pu and the neutron-induced fission of 239,241Pu with incident neutron energies ranging from thermal to about 100 keV. Measurements were made using the Detector for Advanced Neutron Capture Experiments (DANCE) array in coincidence with the detection of fission fragments using a parallel-plate avalanche counter. The unfolded prompt fission γ-ray energy spectra can be reproduced reasonably well by Monte Carlo Hauser–Feshbach statistical model for the neutron-induced fission channel but not for the spontaneous fission channel. However, this entrance-channel dependence of the prompt fission γ-ray emission can be described qualitatively by the model due to the very different fission-fragment mass distributions and a lower average fragment spin for spontaneous fission. The description of measurements and the discussion of results under the framework of a Monte Carlo Hauser–Feshbach statistical approach are presented.

New prompt fission neutron spectrum measurements are reported for 235U(n,f) reactions induced by neutrons with energies from 0.7 to 20 MeV. These measurements cover outgoing neutron energies from 2.5 MeV down to 10 keV, using an array of 6Li-glass scintillators for neutron detection and a double time-of-flight technique. The neutrons were produced at the Weapons Neutron Research facility of the Los Alamos Neutron Science Center. A detailed MCNP® model of the experimental equipment and the surrounding room was used to interpret the experimental results. Backgrounds were measured in situ, making use of the time-dependent singles rates of the various detectors with asynchronous readout from waveform digitizers. The results presented here have been included in a re-evaluation of the fission neutron spectra for this fissioning system, a description of which is presented elsewhere in this issue.

The goal of the Chi-Nu experiment at the Los Alamos Neutron Science Center is to measure the prompt fission neutron spectra from major actinides using a double time-of-flight method with a pulsed, white incoming neutron source. Fission events are detected with a parallel-plate avalanche counter and outgoing neutrons are detected with either a 6 Li-glass or liquid scintillator detector array for low- or high-energy neutrons, respectively. A detector response matrix for the interaction of neutrons with the experimental environment for neutrons measured with the Chi-Nu 6 Li-glass detector array has been calculated to obtain a full understanding of the measured Chi-Nu data and also to allow for nearly instantaneous production of simulated Chi-Nu data spectra. Prompt fission neutron spectra corresponding to 19 incoming neutron energy ranges from 0.7-20 MeV have been extracted using the ratio-of-ratios method with Chi-Nu 6 Li-glass data on the neutron-induced fission of 235 U.

The Chi-Nu experiment aims to accurately measure the prompt fission neutron spectrum (PFNS) for the major actinides. At the Los Alamos Neutron Science Center (LANSCE), fission can be induced using the white neutron source. Using a two arm time of flight (T.O.F) technique; Chi-Nu presents a preliminary result of the low energy component of the U-235 PFNS measured using an array of 22-Lithium glass scintillators.

The octupole strength present in the neutron-rich, radiocative nucleus Ba-146 has been experimentally determined for the first time using Coulomb excitation. To achieve this, A=146 fission fragments from CARIBU were post accelerated by the Argonne Tandem Linac Accelerator System (ATLAS) and impinged on a thin Pb-208 target. Using the GRETINA gamma-ray spectrometer and the CHICO2 heavy-ion counter, the reduced transition probability B(E3; 3(-)-> 0(+)) was determined as 48((+21)(-29)) W.u. The new result provides further experimental evidence for the presence of a region of octupole deformation surrounding the neutron-rich barium isotopes.

Isospin symmetry was investigated in the lower $sd$ shell by studying the $T_z$ = $-frac{3}{2}$ nucleus ${}^{21}$Mg in Coulomb excitation on ${}^{196}$Pt and ${}^{110}$Pd targets. A 205.6(1)-keV $gamma$-ray transition resulting from the Coulomb excitation of the $frac{5}{2}^+$ ground state to the first excited $frac{1}{2}^+$ state in ${}^{21}$Mg was observed for the first time. Coulomb excitation cross-section measurements with both targets and a measurement of the half-life of the $frac{1}{2}^+$ state yield an adopted value of $B(E2;frac{5}{2}^+rightarrowfrac{1}{2}^+)$ = 12.7(4) W.u. A new excited state at 1672(1) keV with tentative $frac{9}{2}^+$ assignment was also identified in ${}^{21}$Mg. This work demonstrates the large deviation of the $B(E2;frac{5}{2}^+rightarrowfrac{1}{2}^+)$ values between $T$ = $frac{3}{2}$, $A$ = 21 mirror nuclei, predicted by a shell-model calculation employing a modified USD interaction. The new results are also compared to modern ab initio nuclear structure calculations, which have recently become applicable in the $sd$ shell.

The neutron-induced reaction cross sections of (242)mAm were measured at the Los Alamos Neutron Science Center using the Detector for Advanced Neutron-Capture Experiments array along with a compact parallel-plate avalanche counter for fission-fragment detection. A new neutron-capture cross section was determined, and the absolute scale was set according to a concurrent measurement of the well-known (242)mAm(n, f) cross section. The (n,gamma) cross section was measured from thermal energy to an incident energy of 1 eV at which point the data qualitywas limited by the reaction yield in the laboratory. Our new (242)mAmfission cross sectionwas normalized to ENDF/B-VII. 1 to set the absolute scale, and it agreed well with the (n, f) cross section reported by Browne et al. (1984) from thermal energy to 1 keV. The average absolute capture-to-fission ratio was determined from thermal energy to E-n = 0.1 eV, and it was found to be 26(4)% as opposed to the ratio of 19% from the ENDF/B-VII. 1 evaluation.

The absolute neutron-capture cross section of 242Pu was measured at the Los Alamos Neutron Science Center using the Detector for Advanced Neutron-Capture Experiments array along with a compact parallel-plate avalanche counter for fission-fragment detection. During target fabrication, a small amount of 239Pu was added to the active target so that the absolute scale of the 242Pu(n,γ ) cross section could be set according to the known 239Pu(n,f) resonance at En,R = 7.83 eV. The relative scale of the 242Pu(n,γ ) cross section covers four orders of magnitude for incident neutron energies from thermal to ≈ 40 keV. The cross section reported in ENDF/B-VII.1 for the 242Pu(n,γ ) En,R = 2.68 eV resonance was found to be 2.4% lower than the new absolute 242Pu(n,γ ) cross section.