The normalized $^{238}\mathrm{U}(n,f)/^{235}\mathrm{U}(n,f)$ cross section ratio has been measured using the NIFFTE fission Time Projection Chamber (fissionTPC) from the reaction threshold to $30\phantom{\rule{4pt}{0ex}}\mathrm{MeV}$. The fissionTPC is a two-volume MICROMEGAS time projection chamber that allows for full three-dimensional reconstruction of fission-fragment ionization profiles from neutron-induced fission. The measurement was performed at the Los Alamos Neutron Science Center, where the neutron energy is determined from neutron time of-flight. The $^{238}\mathrm{U}(n,f)/^{235}\mathrm{U}(n,f)$ ratio reported here is the first cross section measurement made with the fissionTPC, and will provide new experimental data for evaluation of the $^{238}\mathrm{U}(n,f)$ cross section, an important standard used in neutron-flux measurements. Use of a development target in this work prevented the determination of an absolute normalization, to be addressed in future measurements. Instead, the measured cross section ratio has been normalized to ENDF/B-VIII.$\ensuremath{\beta}5$ at 14.5 MeV.

We describe an apparatus used to measure the electron-antineutrino angular correlation coefficient in free neutron decay. The apparatus employs a novel measurement technique in which the angular correlation is converted into a proton time-of-flight asymmetry that is counted directly, avoiding the need for proton spectroscopy. Details of the method, apparatus, detectors, data acquisition, and data reduction scheme are presented, along with a discussion of the important systematic effects.

The average Total Kinetic Energy (TKE) release and fission-fragment yields in neutron-induced fission of 235U and 238U was measured using a Frisch-gridded ionization chamber. These observables are important nuclear data quantites that are relevant to applications and for informing the next generation of fission models. The measurements were performed a the Los Alamos Neutron Science Center and cover En = 200 keV – 30 MeV. The double-energy (2E) method was used to determine the fission-fragment yields and two methods of correcting for prompt-neutron emission were explored. The results of this study are correlated mass and TKE data.

Backscatter of electrons from a beta spectrometer, with incomplete energy deposition, can lead to undesirable effects in many types of experiments. We present and discuss the design and operation of a backscatter-suppressed beta spectrometer that was developed as part of a program to measure the electronantineutrino correlation coefficient in neutron beta decay (aCORN). An array of backscatter veto detectors surrounds a plastic scintillator beta energy detector. The spectrometer contains an axial magnetic field gradient, so electrons are efficiently admitted but have a low probability for escaping back through the entrance after backscattering. The design, construction, calibration, and performance of the spectrometer are discussed.

Submitted for the DNP15 Meeting of The American Physical Society Absolute Neutron Fluence Measurements at the NIST Center for Neutron Research A. YUE, M. DEWEY, D. GILLIAM, J. NICO, National Institute of Standards and Technology, E. ANDERSON, M. SNOW, Indiana University, G. GREENE, University of Tennessee / Oak Ridge National Laboratory, A. LAPTEV, Los Alamos National Laboratory — Precise, absolute fluence measurements of cold and thermal neutron beams are of primary importance to beam-type determinations of the neutron lifetime, measurements of standard neutron cross sections, and the development of standards for neutron dosimetry. At the National Institute of Standards and Technology (NIST), a totally absorbing neutron detector based on absolute counting of the B(n,α1) 7Li reaction 478 keV gamma ray has been used to perform fluence measurements with a precision of 0.06%. This detector has been used to improve the neutron fluence determination in the 2000 NIST beam neutron lifetime by a factor of five, significantly reducing the uncertainty in the lifetime result. Ongoing and possible future uses of the Alpha-Gamma device include 1) Calibration of the neutron fluence monitors that will be used in the upcoming NIST beam neutron lifetime measurement BL2; 2) The first direct, absolute measurement of the 6Li(n,t)4He neutron cross section at sub-thermal neutron energy; 3) Measurements of the 10B(n,γ)11B and 235U(n,f) neutron cross sections; 4) A re-calibration of the national neutron standard NBS-1. The apparatus, measurement technique, and applications will be discussed. Andrew Yue National Institute of Standards and Technology Date submitted: 01 Jul 2015 Electronic form version 1.4

Higher quality measurements of outgoing prompt neutron spectra from neutron-induced fission as a function of the incoming neutron energy are needed. These data can be used in designing new fast reactors, predicting criticality for safety analyses, and developing techniques for global security applications. As part of the program to measure the prompt fission neutron spectra (PFNS) from the fission of 239Pu at the Los Alamos Neutron Science Center, we are developing a new array of liquid-scintillator detectors. This array will be used to measure the PFNS over a range of outgoing neutron energies from approximately 600 keV to 12 MeV and incident neutron energies from 0.5 to 30 MeV. A complete characterization of the detectors and the array as a whole will be carried out, targeted at understanding the light-output curves, efficiencies, and the neutron multiple-scattering backgrounds.

The U-233,U-234,U-236,U-238 fission cross sections have been measured relative to U-235(n, f) for incident neutron energies from 200 keV to 200 MeV using neutron time-of-flight at the Los Alamos Neutron Science Center. The results are generally consistent with the current ENDF/B-VII evaluation, but some discrepancies with previous measurements above 20 to 30 MeV are observed. These measurements are part of a campaign to measure fission cross sections with high precision in support of fast reactor technology.

The prompt 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. A program to measure this spectrum for neutron-induced fission of 239Pu is underway at the Los Alamos Neutron Science Center. The goal is to obtain data on the shape of the spectrum with a small uncertainty over the emitted neutron energy range of 100 keV to 12 MeV with additional data below and above this range. The incident neutron energy range will be from 0.5 to 30 MeV. The status of this program including results of initial experimental measurements is described here.

The prompt γ-ray spectra from 235U(n,f) at incident energies of 1 to 20 MeV and for 235Cf(s.f.) were measured up to 4 MeV in coincidence with two parallel plate avalanche counters and the liquid scintillator array FIGARO. The unfolded γ-ray spectra from 1–4 MeV using the single value decomposition and iterative Bayesian techniques for the incident neutron energy regions corresponding to the first, second and third chance fissions were found to nearly identical and similar to the distribution from the spontaneous fission of 252Cf. General agreement with the γ-ray distributions from fission was also found with previous measurements.

The outgoing neutron energy spectra from neutron-induced fission of various actinides are important for basic understanding of the fission process near the scission point as well as playing a large role in neutron transport codes, which are heavily relied upon in the design of advanced nuclear reactors and simulations of critical assemblies. The reliability of the results of neutron transport models is a strong function of the quality of the nuclear data used as input. Currently, the world's experimental database of fission neutron spectra is severely incomplete (especially for higher incident neutron energies) with large uncertainties in key portions of the outgoing energy spectra. Many transport codes use evaluated data libraries, which are based on the approach of the Los Alamos model. Other theoretical models have been developed, but the available data cannot distinguish the results of different models (as is the case for $^{239}$ Pu). Better measurements are needed for all incident and outgoing neutron energies, but most urgently in the low-energy (below 1 MeV) and high-energy (above 6 MeV) portions of the outgoing spectra where theoretical model results differ greatly. We present the design considerations (and some characterization results) of the two Chi-Nu neutron detector arrays: one array of $^{6}$ Li-glass detectors and one array of liquid-scintillator detectors. These detector arrays are being constructed to meet the challenge of measuring the prompt fission neutron spectra (for a few common actinides) to a higher accuracy and precision than achieved previously and over a larger incident energy range than has been covered by previous experimenters. We see a significant reduction in neutron-scattering backgrounds with our new array designs.

In order to investigate the discrepancy in the available sets of data on Pu-239, Chi-Nu is a program to measure prompt-fission-neutron spectra at the Los Alamos Neutron Science Center (LANSCE). To meet the required accuracy based on the compilations among different theoretical models and sets of data, two different types of neutron detectors, liquid scintillators and Li-6-glass scintillators, are used in conjunction to the actinide Parallel Plate Avalanche Counter by measuring a time of flight. Monte Carlo simulations are used to study detector responses. In this manuscript, we discuss the status of the Chi-Nu project including the commissioning in 2012 summer.

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 1 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 MeV, where their efficiency then becomes small. Various considerations and tests are in progress to understand important contributing factors in designing these two arrays and they include selection and characterization of photomultiplier tubes (PM), the performance of relatively thin (1.8 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 WNR/LANSCE neutron beam, and efficiency calibration with 252Cf spontaneous fission neutrons. Design considerations and test results are presented.

The distributions of prompt γ rays from the spontaneous fission of 252Cf and neutron-induced fission of 235U were measured up to ∼4MeV using a liquid scintillator array. The unfolding of measured fission γ rays is presented using the Single Value Decomposition and iterative Bayesian methods. General agreement was found with comparisons made with previous measurements. The energy dependence of the prompt γ-ray distributions for the spontaneous fission of 252Cf and the neutron-induced fission of 235U from bombarding energies of 1-2, 5-10, and 10–20MeV were found to be almost identical in the γ-ray energy region 1–4MeV.

Prompt-fission-neutron spectra for neutron-induced fission reactions on uranium and plutonium isotopes are important for nuclear applications. We have used 6Li-glass scintillation detectors to measure outgoing neutron energies in the range from 10keV to 1MeV, where there is currently large uncertainty in nuclear data. To better understand the response of 6Li-glass detectors in this energy range, measurements of well-known spontaneous-fission neutrons from a 252Cf source were done in the neutron-beam flight path. Results were compared with Monte Carlo simulations and they show good agreement. Similar measurements with a 7Li-glass detector were used to assess gamma-ray background yields.