Session LK: Nuclear Reactions: Heavy-Ions/Rare isotope Beams (Fission)

Absolute Integral Fission Product Chain Yield Measurements at NCERC

Fission product chain yields were historically determined by chemical separation and beta counting of fissile samples irradiated in carefully controlled fission chamber experiments. These measurements provided the means to extract the absolute fission product yields (yield per fission, or Y_i/f for the i_th  fission product) that are included in the international nuclear data libraries, and ultimately used to model and characterize multiplying systems. However, re-evaluations conducted in the late 2010s for neutron-induced fission of ²³⁵U and ²³⁹Pu highlighted disagreement between several important measurements used in the data library evaluations.

In this presentation we will highlight recent activities to address discrepancies and data gaps using energy-integral measurements for ²³⁵U and ²³⁹Pu. This work relies on the historical combination of fission chambers and radiochemical analyses of fissile samples irradiated on the critical assemblies at the National Criticality Experiments Research Center (NCERC) at the Nevada National Security Site. Details will be provided on the design and performance of the fission chambers, along with the results from irradiations on the Flattop and Godiva critical assemblies. We will conclude with an outline of our plans to complete the work over the next several years.   

Measurement of the $^{239}$Pu(n,f)$/^{235}$U(n,f) Cross-Section Ratio with the NIFFTE fission Time Projection Chamber

The NIFFTE collaboration's fission time projection chamber (fissionTPC) is a $2\times2\pi$ charged particle tracker designed for measuring neutron-induced fission. The fissionTPC provides three-dimensional reconstruction of fission-fragment ionization profile enabling evaluation of systematic effects and corresponding uncertainties which are less directly accessible by other measurement techniques. Preliminary results of $^{239}$Pu(n,f)$/^{235}$U(n,f) cross-section ratio, for incident neutron energies from 100 KeV to 100 MeV will be discussed. The measurement was performed at the Los Alamos Neutron Science Center which provides a pulsed white source of neutrons.   

The Energy Dependence of Fission Product Yields

Under a collaboration between Los Alamos National Laboratory, Lawrence Livermore National Laboratory and the Triangle Universities Nuclear Laboratory, a set of absolute cumulative fission product yield measurements has been performed. The energy dependence of cumulative fission product yields (FPY) have been measured using quasi-mono-energetic neutrons for three actinide targets, ²³⁵U, ²³⁸U and ²³⁹Pu, between 0.5 and 14.8 MeV. The FPYs were measured by a combination of direct fission counting using specially designed dual-fission chambers and γ-ray spectroscopy. Each dual-fission chamber is a back-to-back ionization chamber encasing an activation target in the center with thin deposits of the same target isotope in each chamber. This method allows for the direct measurement of the total number of fissions in the activation target with no reference to the fission cross-section, thereby reducing the uncertainties. Data will be presented showing fission yield results for the energy range covering 6.5 - 11 MeV, fully covering the second chance fission region. Previous data from this work for 0.5 - 5.5 MeV and 14.8 MeV, that has already been published, will now be combined with the newly finalized data from the second chance region. This will provide a complete picture on the energy dependence of the cumulative fission product yields for more than a dozen peak yield fission products from the 3 major actinides.   

233U(n, γ) DANCE and NEUANCE measurements at LANSCE

Uranium-233 has played an important role in critical experiments in the 50's and 60's. More recently, management of legacy ²³³U material across the DOE complex has led to a requirement for improved nuclear cross sections. 

The experimental 233U(n,γ) cross section data available in the literature are scarce. An accurate measurement of the 233U(n,γ) cross section is required by the NCSP (National Critically Safety Program) to complete the neutron-induced cross section data. Measurements of the ²³³U(n,γ) reaction data were made in 2020 and 2021 at the Los Alamos Neutron Science Center (LANSCE) using the Detector for Advanced Neutron Capture Experiments (DANCE) combined with the NEUtron detector array at dANCE (NEUANCE).

Because ²³³U fission is around one order of magnitude more likely than capture, an accurate measurement of the capture cross section relies on the discrimination between fission and capture gammas. NEUANCE tagged fission neutrons while DANCE detected capture and fission gammas. Coincidences between them are reconstructed during analysis. This measurement will provide the capture to fission ratio, reducing the uncertainties with respect to an absolute measurement by eliminating experimental complications like self-absorption, beam/target overlap and non-uniformities.

    

Measurement of Independent Fission Yields in Direct and Inverse Kinematics

Fission product yields are an important observable for fundamental and applied nuclear physics. Recent advances in radioactive beam technology have allowed measurements of fragments from fission induced by transfer reactions and virtual photons in inverse kinematics with superb charge and mass resolution. We are beginning a program to study the independent fission yields before beta decay in both direct and inverse kinematics to benchmark these new techniques. We will discuss an experiment to measure photofission of ²³⁸U in direct kinematics using a position-sensitive Frisch-grid ionization chamber. This detector allows determination of the distributions of the independent fission yield mass, total kinetic energy, and azimuthal angle as well as correlations between these values. These distributions will be compared to measurements of fission induced by transfer reactions and virtual photons in inverse kinematics. Preliminary data from recent measurements in direct kinematics using a Frisch-grid ionization chamber will be presented.

    

Measuring the 235U(n,f)/6Li(n,t) cross section ratio in the NIFFTE fissionTPC

Neutron-induced reactions are important to our understanding of fundamental nuclear physics as well as nuclear applications. Due to their impact on a wide range of nuclear physics applications, it has become important to better understand the probability such reactions occur and try to minimize their uncertainties. In particular, there is a need for precision neutron-induced fission cross section measurements on actinides. Neutron-induced fission cross sections are typically measured as ratios, relative to a well-known cross section standard. The ²³⁵U(n,f) is a well measured standard, often used as a reference on cross section ratio measurements of other actinides. However, some light particle reactions are also well-known and their use as reference can provide information to remove shared systematic uncertainties that are present in an actinide-only ratio measurement. The NIFFTE collaboration's fission time projection chamber (fissionTPC) is a charged particle tracker designed for precision measurements of neutron-induced fission reactions. Detailed 3D track reconstruction of the reaction products enables evaluation of systematic effects and corresponding uncertainties which are less directly accessible by other measurement techniques. This work focuses on the recent measurement of the ²³⁵U(n,f) using as a reference the light-ion standard ⁶Li(n,t) reaction. Preliminary data of the ²³⁵U(n,f)/⁶Li(n,t) measurement conducted at the Los Alamos Neutron Science Center will be presented.

LLNL-ABS-836692: This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

    

The generation of angular momentum in fission fragments and its energy dependence

We present new experimental results that show the existence of an excitation energy dependence of fission fragment angular momenta. The setup used for this experiment is the FS-3 detection system, an array of forty trans-stilbene scintillators sensitive to neutrons and gamma rays, and a twin Frisch-gridded ionization chamber, capable of measuring fragment masses, kinetic energies, and polar emission angles relative to the source. We have measured mean gamma-ray and neutron multiplicities in coincidence with fragment masses and kinetic energies. We have determined that the gamma-ray multiplicity, the primary information carrier of fission fragment angular momenta, increases with decreasing total kinetic energy, eventually reaching a saturation plateau. Interestingly, the total number of gamma rays at high excitation energies is independent of the fragment mass split. However, the saturation energy, i.e., the excitation energy required to reach this plateau, changes significantly with mass split, being highest for very asymmetric and very symmetric fission events. In fact, the energies of the gamma rays correlating with excitation energy are characteristic of quadrupole band transitions and are thus strongly correlated with fragment angular momenta. The results are interpreted in terms of an angular momentum saturation model: in this model, the fragment angular momenta are primarily determined at scission and they strongly couple to their relative orbital angular momentum. Because of the lowering of the fission barrier with increasing orbital angular momentum, the saturation of fragment angular momenta is observed. 

    

A Twin Frisch-gridded Ionization Chamber for Correlated Fragment Emission in Fission.

A new twin Frisch-gridded ionization chamber (TFGIC) has been designed and constructed for fission fragment detection. The first experimental run was performed with a  ²⁵²Cf spontaneous fission source located in the middle of the chamber and an array of trans-stilbene organic scintillators surrounding the chamber. The TFGIC determines the masses, angles of emission and kinetic energy of the fragments using the 2E method with estimated resolutions of 4-5 amu, cosθ < 0.07, and  < 3 MeV, respectively. The scintillator array provides neutron and gamma-ray multiplicities and spectra. The experiment aims to study neutron and gamma-ray emission for different fragment excitation energies and masses, and thus could shed light on the role of angular momentum in nuclear fission. Details of the detector assembly and first performance tests of the setup will be presented, as well as plans for a future experimental program with the Gammasphere detector at the Argonne Tandem-Linac Accelerator System (ATLAS).   

Measurements of Differential (γ,fn) and (γ,xn) Cross Sections from Photofission of 235U, 238U, and 239Pu with Nearly Mono-Energetic Photons Between 5.3 and 10 MeV

There is strong interest in developing new technologies for detecting hidden fissile materials using the unique signature of neutrons produced by photon induced fission. An important aspect of guiding this effort is the measurement of neutron production rates from isotopes of interest using gamma rays in the energy range relevant to compact bremsstrahlung sources.  An array of liquid scintillators with pulse shape discrimination capability was used to measure neutrons emitted from ²³⁵U, ²³⁸U, and ²³⁹Pu from (γ,fn) and (γ,n) reactions between 5.3 and 10 MeV.  The incident photons were produced by inverse compton scattering at the High Intensity Gamma-ray Source. Neutron energies above around 0.8 MeV were measured by time-of-flight over a 1 m distance. Preliminary results will be presented.