We measure the average spin of 144Ba, a common fragment produced in 252Cf(sf), as a function of the total kinetic energy (TKE). We combine a twin Frisch-gridded ionization chamber with a world-class gamma-ray spectrometer that was designed to measure high-multiplicity gamma-ray events, Gammasphere. The chamber, loaded with a 252Cf(sf) source, provides a fission trigger, the TKE of the fragments, the approximate fragment masses, and the polar angle of the fission axis. Gammasphere provides the total gamma-ray yield, fragment identification through the tagging of decay gamma rays, and the feeding of rotational bands in the fragments. We determine the dependence of the average spin of 144Ba on the fragments' TKE by correlating the fragment properties with the distribution of discrete levels that are fed. We find that the average spin only changes by about 0.5 h across the TKE range of 158-203 MeV. The virtual independence of the spin on TKE suggests that spin is not solely generated through the statistical excitation of rotational modes, and more complex mechanisms are required.

This paper presents recent experimental results on the yield of prompt fission $\gamma$ rays from the spontaneous fission of $^{252}$Cf. We use an ionization chamber to tag fission events and measure the masses and kinetic energies of the fission fragments and trans-stilbene organic scintillators to measure the neutrons and $\gamma$ rays emitted by the fission fragments. The combination of the ionization chamber and trans-stilbene scintillators allows us to determine the properties of neutrons and $\gamma$ rays in coincidence with the fragments. The yield of $\gamma$ rays is known to be influenced by the angular momenta (AM) of the fission fragments. We present new experimental evidence that indicates that the total $\gamma$-ray multiplicity, i.e., the sum of both fragments' emission, saturates at sufficiently high internal fragment excitation energies. We also observe distinct behaviors for the yield of $\gamma$ rays from the light and heavy fragment, which for certain mass and total kinetic energy (TKE) regions are weakly or anti-correlated, indicating the presence of complex AM generation modes. We also observed a mass- and TKE-dependent anisotropy of the $\gamma$ rays, which challenges and expands on the conventional notion that the fragments' AM are always aligned perpendicularly to the fission axis. Moreover, the dependence of the anisotropy on mass and TKE indicates a dependence of these properties on the specific fission channels, thus providing an insight into the deformations and dynamics in fission and their connection with experimentally observable quantities.

This paper presents recent experimental results on the yield of prompt fission $\gamma$ rays from the spontaneous fission of $^{252}$Cf. We use an ionization chamber to tag fission events and measure the masses and kinetic energies of the fission fragments and trans-stilbene organic scintillators to measure the neutrons and $\gamma$ rays emitted by the fission fragments. The combination of the ionization chamber and trans-stilbene scintillators allows us to determine the properties of neutrons and $\gamma$ rays in coincidence with the fragments. The yield of $\gamma$ rays is known to be influenced by the angular momenta (AM) of the fission fragments. We present new experimental evidence that indicates that the total $\gamma$-ray multiplicity, i.e., the sum of both fragments' emission, saturates at sufficiently high internal fragment excitation energies. We also observe distinct behaviors for the yield of $\gamma$ rays from the light and heavy fragment, which for certain mass and total kinetic energy (TKE) regions are weakly or anti-correlated, indicating the presence of complex AM generation modes. We also observed a mass- and TKE-dependent anisotropy of the $\gamma$ rays, which challenges and expands on the conventional notion that the fragments' AM are always aligned perpendicularly to the fission axis. Moreover, the dependence of the anisotropy on mass and TKE indicates a dependence of these properties on the specific fission channels, thus providing an insight into the deformations and dynamics in fission and their connection with experimentally observable quantities.

Recent theoretical and experimental results have brought renewed interest and focus on the topic of fission fragment angular momentum. Measurements of neutrons and γ rays in coincidence with fission fragments remain the most valuable tool in the exploration of fission physics. To achieve these scientific goals, we have developed a system that combines a state-of-the-art fission fragment detector and n−γ radiation detectors. A new twin Frisch-gridded ionization chamber has been designed and constructed for use with a spontaneous fission source and an array of forty trans-stilbene organic scintillators (FS-3) at Argonne National Laboratory. The new ionization chamber design we present in this work aims at minimizing particle attenuation in the chamber walls, and provides a compact apparatus that can be fit inside existing experimental systems. The ionization chamber is capable of measuring fission fragment masses and kinetic energies, whereas the FS-3 provides neutron and γ-ray multiplicities and spectra. The details of both detector assemblies are presented along with the first experimental results of this setup. Planned event-by-event analysis and future experiments are briefly discussed.

We present the results of a measurement of isotopic concentrations and atomic number ratio of a double-sided actinide target with alpha-spectroscopy and mass spectrometry. The double-sided actinide target, with primarily Pu-239 on one side and U-235 on the other, was used in the fission Time Projection Chamber (fissionTPC) for a measurement of the neutron-induced fission cross-section ratio between the two isotopes. The measured atomic number ratio is intended to provide an absolute normalization of the measured fission cross-section ratio. The Pu-239/U-235 atom number ratio was measured with a combination of mass spectrometry and alpha-spectroscopy with a planar silicon detector with uncertainties of less than 1%.

Most evaluated elemental fission product yield distributions are not experimentally measured. Instead, the majority of evaluated distributions are based on analytic expressions of the Zp-model for relevant cumulative yields. Here we report independent elemental fission product yield distributions of a 235U target for incident neutron energies ranging from 0.11 MeV through 92.4 MeV. Atomic numbers are calculated by an approach that combines a 2E analysis with a stopping force analysis method, developed within this paper. These analyses are applied to more than 6.1 × 106 fission fragment ionization tracks captured within the NIFFTE (Neutron Induced Fission Fragment Tracking Experiment) collaboration fission time projection chamber (fissionTPC). A 3-Z resolution was obtained with the fissionTPC spatial and energy resolutions. Tabulated results are presented for the atomic yield and experimentally derived Zp values as a function of pre-neutron-emission fragment masses for the complete range of incident neutron energies. The stopping-force-derived Zp values tend to support the unchanged charge distribution theory within uncertainty.

The $^{239}$Pu(n,f)/$^{235}$U(n,f) cross-section ratio has been measured with the fission Time Projection Chamber (fissionTPC) from 100 keV to 100 MeV. The fissionTPC provides three-dimensional reconstruction of fission-fragment ionization profiles, allowing for a precise quantification of measurement uncertainties. The measurement was performed at the Los Alamos Neutron Science Center which provides a pulsed white source of neutrons. The data are recommended to be used as a cross-section ratio shape. A discussion of the status of the absolute normalization and comparisons to ENDF evaluations and previous measurements is included.

The Neutron Induced Fission Fragment Tracking Experiment (NIFFTE) collaboration has performed measurements with a fission time projection chamber (fissionTPC) to study the fission process by reconstructing full three-dimensional tracks of fission fragments and other ionizing radiation. The amount of linear momentum imparted to the fissioning nucleus by the incident neutron can be inferred by measuring the opening angle between the fission fragments. Using this measured linear momentum, fission fragment angular distributions can be converted to the center-of-mass frame for anisotropy measurements. Angular anisotropy is an important experimental observable for understanding the quantum mechanical state of the fissioning nucleus and vital to determining detection efficiency for cross section measurements. Neutron linear momentum transfer to fissioning $^{235}$U, $^{238}$U, and $^{239}$Pu and fission fragment angular anisotropy of $^{235}$U and $^{238}$U as a function of neutron energies in the range 130 keV--250 MeV are presented.

Properties of fission in U-233 were measured at the Los Alamos Neutron Science Center at incident neutron energies from thermal to 40 MeV. Fragments were observed in coincidence by using a twin ionization chamber with Frisch grids. The average total kinetic energy released and fragment mass yields were determined by using the double energy analysis method based on conservation of mass and momentum. The experimental method was validated by using Th-232 and absolute energy was calibrated by using thermal-neutron-induced fission of U-235. This work incorporates novel applications of multi-chance fission channel cross sections and fission models to account for the complexities introduced by prompt neutron emission at high energy and extends results up to higher incident neutron energies than previously measured. Accurate experimental measurements of these parameters are necessary to better understand the fission process in isotopes central to the thorium fuel cycle.

Templates of uncertainties expected in specific measurement types were recently developed. One aim of these templates is to help evaluators in identifying (1) missing or suspiciously low uncertainties and (2) missing correlations between uncertainties of the same and different experiments, when estimating covariances for experimental data employed in their evaluations. These templates also provide realistic estimates of standard deviations and correlations for a particular uncertainty source and measurement type that can be used by evaluators in situations where they are not supplied by the experimenters. This information allows for a more comprehensive uncertainty analysis across all measurements considered in an evaluation and, thus, more realistic evaluated covariances. Here, we extend a template that is applicable to uncertainties expected in neutron-induced fission, (n,f), cross-section measurements. It is applied to improving covariances of 239Pu(n,f) cross-section measurements in the database underlying the Neutron Data Standards evaluations. This particular example was chosen since this evaluation is primarily based on experimental information. Also, some uncertainties of individual 239Pu(n,f) cross-section experiments in this database were suspected to be underestimated. The evaluated uncertainties obtained after updating the covariances in the database by means of the template indeed do increase compared to their original values. Even more importantly, the evaluated mean values change noticeably. These modified cross sections impact application calculations significantly, as is demonstrated by employing them in simulations of the effective neutron multiplication factor for a few selected critical assemblies. However, this updated evaluated 239Pu(n,f) cross section should not be interpreted as the final one that should replace values of the current Neutron Data Standards project. Evaluations for the Neutron Data Standards of the 239Pu(n,f) cross section must be linked to many other observables included in the associated database, most notably to cross sections for 235U(n,f), but also to those for 10B(n,α), 6Li(n,t), 238U(n,f), and 238U(n,γ), because of included measurements of the 239Pu(n,f) cross section that appear as ratios to these reactions. Some of these other reactions are correlated to further observables in the database. Hence, updating uncertainties of data sets of any of these observables can potentially impact the 239Pu(n,f) cross section. Uncertainties for all measurements of these linked physical observables have to be updated before a comprehensive evaluation of the 239Pu(n,f) cross section and its corresponding uncertainties can be provided.

Fission fragment angular distributions can provide an important constraint on fission theory, improving predictive fission codes, and are a prerequisite for a precise ratio cross section measurement. Available anisotropy data is sparse, especially at neutron energies above 5 MeV. For the first time, a three-dimensional tracking detector is employed to study fragment emission angles and provide a direct measurement of angular anisotropy. The Neutron Induced Fission Fragment Tracking Experiment (NIFFTE) collaboration has deployed the fission time projection chamber (fissionTPC) to measure nuclear data with unprecedented precision. The fission fragment anisotropy of $^{235}$U has been measured over a wide range of incident neutron energies from 180 keV to 200 MeV; a careful study of the systematic uncertainties complement the data.

Gas ionization detectors are very widely used for radiation detection and measurement, but for external sources an entrance window is used which can reduce particle energy. For heavy ions this energy loss can be significant enough to affect measurements and very thin windows, such as those composed of silicon nitride (SiN), may be utilized to minimize this effect. For fission spectroscopy, carbon conversion foils are also common in measurements. In the current work, energy losses were measured for Cf-252 spontaneous fission products passing through thin foils of carbon and of silicon nitride. The foils ranged in from 22.5 to 131.0 mu g/cm(2) for C and from 56.4 to 402.2 mu g/cm(2) for SiN. For comparison, simulations were performed with the TRIM program in SRIM-2013 and with MCNP6.2. To understand calculation differences, effective charge from partial ionization was predicted by several methods and compared with results directly using the Bethe stopping power formula.

The neutron spin rotation (NSR) collaboration used parity-violating spin rotation of transversely polarized neutrons transmitted through a 0.5 m liquid helium target to constrain weak coupling constants between nucleons. While consistent with theoretical expectation, the upper limit set by this measurement on the rotation angle is limited by statistical uncertainties. The NSR collaboration is preparing a new measurement to improve this statistically-limited result by about an order of magnitude. In addition to using the new high-flux NG-C beam at the NIST Center for Neutron Research, the apparatus was upgraded to take advantage of the larger-area and more divergent NG-C beam. Significant improvements are also being made to the cryogenic design. Details of these improvements and readiness of the upgraded apparatus are presented. We also comment on how recent theoretical work combining effective field theory techniques with the 1/Nc expansion of QCD along with previous NN weak measurements can be used to make a prediction for dϕ/dz in 4He. An experiment using the same apparatus with a room-temperature target was carried out at LANSCE to place limits on parity-conserving rotations from possible fifth-force interactions to complement previous studies. We sought this interaction using a slow neutron polarimeter that passed transversely polarized slow neutrons by unpolarized slabs of material arranged so that this interaction would tilt the plane of polarization and develop a component along the neutron momentum. The results of this measurement and its impact on the neutron-matter coupling gA2 from such an interaction are presented. The NSR collaboration is also preparing a new measurement that uses an upgraded version of the room-temperature target to be run on the NG-C beamline; and it is expected to constrain gA2 by at least two additional orders of magnitude for λc between 1 cm and 1 μm.

The MICROMEGAS (MICRO-MEsh GAseous Structure) charge amplification structure has found wide use in many detection applications, especially as a gain stage for the charge readout of Time Projection Chambers (TPCs). Here we report on the behavior of a MICROMEGAS TPC when operated in a high-energy (up to 800MeV) neutron beam. It is found that neutron-induced reactions can cause discharges in some drift gas mixtures that are stable in the absence of the neutron beam. The discharges result from recoil ions close to the MICROMEGAS that deposit high specific ionization density and have a limited diffusion time. For a binary drift gas, increasing the percentage of the molecular component (quench gas) relative to the noble component and operating at lower pressures generally improves stability.

In order to advance our understanding of the fission process we need to measure changes in fission product yields as a function of excitation energy of the fissioning system, as well as study the correlations between kinetic energy, mass and charge of the products. In addition, fission product yields are used for diagnostics in nuclear technology and there is therefore interest in reducing the uncertainties in their yield. The SPIDER instrument is based on the 2v-2E technique for measuring the mass of fission products, and has been used to study spontaneous fission of Cf-252, as well as thermal neutron-induced fission of U-233, U-235, and Pu-239. The current instrument has two spectrometer "arms", which provides sufficient detection efficiency for studying fission at thermal neutron energies. In order to study the change in fission yields at fast energies higher detection efficiency is needed, and a new version of SPIDER is therefore under construction. The new instrument will have a total of 16 individual arms, bringing the total solid angular coverage to about 1%.

Properties of fission in Th-232 and U-233 were studied at the Los Alamos Neutron Science Center at incident neutron energies from sub-thermal to 40 MeV. Fission fragments are observed in coincidence using a twin ionization chamber with Frisch grids. The average total kinetic energy released from fission and fragment mass distributions are calculated from observations of energy deposited and conservation of mass and momentum. Accurate experimental measurements of these parameters are necessary to better understand the fission process in isotopes relevant to the thorium fuel cycle, in which Th-232 is used as a fertile material to generate the fissile isotope of U-233. This process mirrors the uranium breeder process used to produce Pu-239 with several potential advantages including the comparative greater abundance of thorium, inherent nuclear weapons proliferation resistance, and reduced actinide production. Thus, there is increased interest in the thorium fuel cycle to meet future energy demands and improve safety and security while increasing profitability for the nuclear power industry. This research is ongoing and preliminary results are presented.

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.

The normalized U-238(n, f)/U-235(n, f) cross section ratio has been measured using the NIFFTE fission Time Projection Chamber (fissionTPC) from the reaction threshold to 30 MeV. The fissionTPC is a two-volume MICROMEGAStime 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 U-238(n, f)/U-235(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 U-238(n, f) cross section, an important standard used in neutron-flux measurements. Use of a development target in thiswork 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.beta 5 at 14.5 MeV.

Different aspects of the nuclear fission process have been studied at Los Alamos Neutron Science Center (LANSCE) using various instruments and experimental techniques. Properties of the fragments emitted in fission have been investigated using Frisch-grid ionization chambers, a Time Projection Chamber (TPC), and the SPIDER instrument which employs the 2v-2E method. These instruments and experimental techniques have been used to determine fission product mass yields, the energy dependent total kinetic energy (TKE) release, and anisotropy in neutron-induced fission of U-235, U-238 and Pu-239.