Triangle Universities Nuclear Laboratory Research Articles

Ne21 energy levels approaching the α -particle threshold

Nuclei around Ne20 exhibit an interplay of different excitations caused by different aspects of nuclear structure, including single-particle and multiparticle configurations and collective rotations. One-nucleon transfer reactions selectively probe single-particle structures in these nuclei. These nuclei are also important to astrophysics, with a number of important reactions proceeding through this mass region. Energy levels approaching the α-particle threshold in Ne21 are of importance to nuclear structure. The Ne20(d,p)Ne21 reaction was measured and the corresponding spectroscopic nuclear information was extracted. States in Ne21 were populated using the Ne20(d,p)Ne21 reaction in forward kinematics. Protons were identified in the Triangle Universities Nuclear Laboratory (TUNL) Enge split-pole spectrograph and angular distributions were extracted. Spin-party assignments were made and neutron partial widths were determined based on distorted-wave Born approximation (DWBA) analysis. Several new energy levels were observed at energies of 7176, 7235, 7250, and 7337 keV, and spin-parities are reported which generally agree with previous results where literature was available. Spin and parity assignments are reported for several energy levels along with estimated neutron widths for those states above the neutron threshold (Sn=6761keV). Results from this study are placed in context with a review of the available literature on all known states in this energy region of Ne21.Published by the American Physical Society2024

New constraints on sodium production in globular clusters from theNa23(He3,d)Mg24reaction

The star-to-star anticorrelation of sodium and oxygen is a defining feature of globular clusters, but, to date, the astrophysical site responsible for this unique chemical signature remains unknown. Sodium enrichment within these clusters depends sensitively on reaction rate of the sodium destroying reactions $^{23}\mathrm{Na}(p,\ensuremath{\gamma})$ and $^{23}\mathrm{Na}(p,\ensuremath{\alpha})$. In this paper, we report the results of a $^{23}\mathrm{Na}{(^{3}\mathrm{He},d)}^{24}\mathrm{Mg}$ transfer reaction carried out at Triangle Universities Nuclear Laboratory using a $21\phantom{\rule{0.16em}{0ex}}\mathrm{MeV}\phantom{\rule{0.16em}{0ex}}^{3}\mathrm{He}$ beam. Astrophysically relevant states in $^{24}\mathrm{Mg}$ between $11<{E}_{x}<12\phantom{\rule{0.16em}{0ex}}\mathrm{MeV}$ were studied using high-resolution magnetic spectroscopy, thereby allowing the extraction of excitation energies and spectroscopic factors. Bayesian methods are combined with the distorted wave Born approximation to assign statistically meaningful uncertainties to the extracted spectroscopic factors. For the first time, these uncertainties are propagated through to the estimation of proton partial widths. Our experimental data are used to calculate the reaction rate. The impact of the new rates are investigated using asymptotic giant branch star models. It is found that while the astrophysical conditions still dominate the total uncertainty, intramodel variations on sodium production from the $^{23}\mathrm{Na}(p,\ensuremath{\gamma})$ and $^{23}\mathrm{Na}(p,\ensuremath{\alpha})$ reaction channels are a lingering source of uncertainty.

Studies of photo-nuclear reactions at astrophysical energies with an active-target TPC

An experiment was conducted at the High Intensity γ-ray Source (HIγS) facility at the Triangle Universities Nuclear Laboratory (TUNL) in Durham, NC, USA to measure the cross-section of the key astrophysical thermonuclear reaction 12C(α,γ)16O by means of its inverse photo-disintegration process. A high-intensity monochromatic γ-ray beam interacted with the CO2 gas in the active volume of the Warsaw active target TPC detector. The reaction products were detected and their momenta reconstructed, so to also determine angular correlations. Data were collected at 15 beam energies ranging from 8.51 to 13.9 MeV. Preliminary results are presented.

Investigation ofB11andCa40levels at 8–9 MeV by nuclear resonance fluorescence

We report on the measurement of $^{11}$B and $^{40}$Ca levels between excitation energies of 8 and 9 MeV using nuclear resonance fluorescence (NRF). The experiment was carried out with nearly-monoenergetic and linearly polarized photon beams provided by the High-Intensity $\gamma$-ray Source (HI$\gamma$S) facility at the Triangle Universities Nuclear Laboratory (TUNL). States in $^{11}$B are important for calibrations of NRF measurements, while the properties of $^{40}$Ca levels impact potassium nucleosynthesis in globular clusters. For $^{40}$Ca, we report on improved excitation energies and an unambiguous $2^-$ assignment for the state at 8425 keV. For $^{11}$B, we obtained improved values for $\gamma$-ray multipolarity mixing ratios and branching ratios of the 8920 keV level.

First measurement of the asymmetry and the Gerasimov-Drell-Hearn integrand from the He3⃗(γ⃗,p)H2 reaction at an incident photon energy of 29 MeV

The first measurement of the $\vec{^3He}(\vec{\gamma},p)d$ process was performed at the High Intensity $\gamma$-ray Source (HI$\gamma$S) facility at Triangle Universities Nuclear Laboratory (TUNL) using a circularly polarized, monoenergetic $\gamma$-ray beam and a longitudinally polarized $^3$He target. The spin-dependent asymmetry and the contribution from the two-body photodisintegration to the $^3$He Gerasimov-Drell-Hearn integrand are extracted and compared with state-of-the-art three-nucleon system calculations at the incident photon energy of 29.0 MeV. The data are in general agreement with the various theoretical predictions based on the Siegert theorem or on explicit inclusion of meson-exchange currents.

Characterization of stilbene’s scintillation anisotropy for recoil protons between 0.56 and 10 MeV

The scintillation anisotropy of the single-crystal organic scintillator trans-stilbene was characterized for recoil protons between 0.56 and 10 MeV. The light output and pulse shape anisotropies were measured at 11 distinct recoil proton energies for over 168 recoil proton trajectories relative to the crystal axes. The measurements were performed using a neutron scatter kinematic measurement system and quasi-monoenergetic neutron beams produced by the tandem Van de Graaff accelerator at Triangle Universities Nuclear Laboratory (TUNL). The extensive recoil proton directional coverage enables interpolation over both energy and direction to form a complete response function of stilbene’s scintillation anisotropy for the measured energy range.

Toward short-lived and energy-dependent fission product yields from neutron-induced fission

Fission product yields (FPYs) are an important source of information that are used for basic and applied physics. They are essential observables to address questions relevant to nucleosynthesis in the cosmos that created the elements from iron to uranium, for example, in energy generating processes from fission recycling in binary neutron star mergers; resolving the reactor neutrino anomaly; decay heat release in nuclear reactors; and many national security applications. While new applications will require accurate energy-dependent FPY data over a broad set of incident neutron energies, the current evaluated FPY data files contain only three energy points: thermal, fast, and 14-MeV incident energies.Recent measurements using mono-energetic and pulsed neutron beams at the Triangle Universities Nuclear Laboratory (TUNL) tandem accelerator and employing a dual fission ionization chambers setup have produced self-consistent, high-precision data critical for testing fission models for the neutron-induced fission of the major actinide nuclei. This paper will present new campaign just beginning utilizing a RApid Belt-driven Irradiated Target Transfer System (RABITTS) to measure shorter-lived fission products and the time dependence of fission yields, expanding the measurements from cumulative towards independent fission yields.

Fission Product Yield Measurements from Neutron-Induced Fission of 235,238 U and 239Pu

Fission product yields (FPY) are one of the most fundamental quantities that can be measured for a fissioning nucleus and are important for basic and applied nuclear physics. Recent measurements using mono-energetic and pulsed neutron beams generated using Triangle Universities Nuclear Laboratory’s tandem accelerator and employing a dual fission chamber setup have produced self-consistent, high-precision data critical for testing fission models for the neutron-induced fission of 235,238U and 239Pu between neutron energies of 0.5 to 15.0 MeV. These data have elucidated a low-energy dependence of FPY for several fission products using irradiations of varying lengths and neutron energies. This paper will discuss new measurements just beginning utilizing a RApid Belt-driven Irradiated Target Transfer System (RABITTS) to measure shorterlived fission products and the time dependence of fission yields, expanding the measurements from cumulative towards independent fission yields. The uniqueness of these FPY data and the impact on the development of fission theory will be discussed.

Measurement of EJ-228 plastic scintillator proton light output using a coincident neutron scatter technique

The light output function of the fast plastic scintillator EJ-228 was measured using a coincident neutron scatter measurement system and quasi-monoenergetic neutron beams produced by the tandem Van de Graaff accelerator at Triangle Universities Nuclear Laboratory. The measurement of scintillator light output using neutron scatter kinematics provides a model-independent determination of the light output function with quantifiable sources of uncertainty. Consequently, light output measurements performed using this method have a key advantage compared to other techniques: the characterization of the scintillating material depends only on the material itself and not on the size of the detector. This advantage is realized if the light output is defined as the absolute neutron light output relative to the absolute electron light output and two conditions are met: (1) the scintillator volume is fully illuminated and edge effects can be neglected during energy calibration measurements and light output measurements, and (2) the light attenuation length for scintillation photons in the material is long enough that the lowest energy interaction of interest produces a statistically significant number of scintillation photons for an interaction at any position in the scintillator volume. Commonly implemented measurement and analysis techniques for light output characterizations are explored and evidence of bias is provided for characterizations performed by analyzing the full-energy deposition edge of a light output spectrum.

Fission product yield measurements using monoenergetic photon beams

Background: High-accuracy and self-consistent fission product yield (FPY) data are needed to advance microscopic/macroscopic descriptions of the nuclear fission process, to improve the predictive power of phenomenological models, and for applications in nuclear energy, nuclear forensics, and homeland security.Purpose: In a collaboration between the Triangle Universities Nuclear Laboratory (TUNL), Los Alamos National Laboratory (LANL), and Lawrence Livermore National Laboratory (LLNL), the dependence of a number of cumulative FPYs on the incoming neutron energy has been measured and unexpected energy dependencies of certain fission products have been reported [M. E. Gooden, Nucl. Data Sheets 131, 319 (2016)]. To investigate whether this observation is unique to neutron-induced fission, a program has been initiated to measure FPYs in photon-induced fission.Method: The photon-induced FPYs were measured by a combination of fission counting using a specially designed dual-fission chamber and $\ensuremath{\gamma}$-ray counting. The measurements were carried out with a monoenergetic photon beam at the $\mathrm{HI}\ensuremath{\gamma}\mathrm{S}$ facility. Gamma-ray counting of the activated targets was performed with well-shielded high-purity germanium (HPGe) detectors over a period of two months after irradiation to properly identify the decay history of fission products.Results: We report on our photofission product yield measurements on $^{235}\mathrm{U}$, $^{238}\mathrm{U}$, and $^{239}\mathrm{Pu}$ using a monoenergetic photon beam of ${E}_{\ensuremath{\gamma}}=13\phantom{\rule{0.28em}{0ex}}\mathrm{MeV}$. More than 40 fission products were uniquely identified, and their yield values were computed. The use of the fission chamber with post-activation measurements has provided absolute fission product yield data with minimal uncertainties.Conclusion: The photon-induced cumulative fission product yields of $^{235}\mathrm{U}$, $^{238}\mathrm{U}$, and $^{239}\mathrm{Pu}$ are compared with previous photon- and neutron-induced fission measurements. In the near future data will be obtained at lower and higher photon energies.

Photoneutron reaction cross section measurements on Mo94 and Zr90 relevant to the p -process nucleosynthesis

The photodisintegration cross sections for the 94Mo({\gamma},n) and 90Zr({\gamma},n) reactions have been experimentally investigated with quasi-monochromatic photon beams at the High Intensity {\gamma}-ray Source (HI{\gamma}S) facility of the Triangle Universities Nuclear Laboratory (TUNL). The energy dependence of the photoneutron reaction cross sections was measured with high precision from the respective neutron emission thresholds up to 13.5 MeV. These measurements contribute to a broader investigation of nuclear reactions relevant to the understanding of the p-process nucleosynthesis. The results are compared with the predictions of Hauser-Feshbach statistical model calculations using two different models for the dipole {\gamma}-ray strength function. The resulting 94Mo({\gamma},n) and 90Zr({\gamma},n) photoneutron stellar reaction rates as a function of temperature in the typical range of interest for the p-process nucleosynthesis show how sensitive the photoneutron stellar reaction rate can be to the experimental data in the vicinity of the neutron threshold.

Near-barrier photofission in Th232 and U238

A study of photofission of $^{232}\mathrm{Th}$ and $^{238}\mathrm{U}$ was performed by using quasimonoenergetic, linearly polarized $\ensuremath{\gamma}$-ray beams from the High Intensity $\ensuremath{\gamma}$-ray Source at Triangle Universities Nuclear Laboratory. The prompt-photofission neutron polarization asymmetries, neutron multiplicities, and the photofission cross sections were measured in the near-barrier energy range of 4.3 to 6.0 MeV. This data set constitutes the lowest energy measurements of those observables to date using quasimonoenergetic photons. Large polarization asymmetries are observed in both nuclei, consistent with the $E1$ excitation as observed by another measurement of this kind made in a higher-energy range. Previous experimental evidence of a deep third minimum in the $^{238}\mathrm{U}$ fission barrier has been identified as an accelerator-induced background.

Partial cross section of neutron-induced reactions on 136Xe at En = 5 MeV for 0νββ background studies

Partial cross section measurements were carried out on 136Xe at Triangle Universities Nuclear Laboratory (TUNL) using 5 MeV monoenergetic neutrons. These results are important for estimating potential neutron-induced backgrounds in the nEXO and KamLAND2-Zen neutrinoless double-β decay searches that use highly enriched 136Xe. Pulsed neutron beams were produced via the 2H(d, n)3He reaction and the deexcitation γ-rays from 136Xe were observed with two 60% relative efficiency HPGe detectors.

High-precision characterization of the neutron light output of stilbene along the directions of maximum and minimum response

The scintillation light output response of stilbene crystals has been measured for protons recoiling along the a, b, and c’ crystalline axes with energies between 1.3 and 10 MeV using neutrons produced with the tandem Van de Graaff accelerator at Triangle Universities Nuclear Laboratory. The proton recoil energy and direction were measured using the coincident detection of neutrons between a stilbene scintillator and an array of EJ-309 liquid scintillators spanning arranged neutron recoil angles. The maximum light output was found to coincide with proton recoils along the a-axis, in disagreement with other published measurements, which reported the b-axis as the direction of the maximum light output. Additional measurements were conducted using two different stilbene crystals to confirm these results: a second measurement using the coincident detection of neutrons; measurements of neutron full energy deposition events along the a and b axes; and measurements of the count rate for 252Cf neutrons traveling along the a and b axes directions. All measurements found that recoils along the a-axis produce the maximum light output.

The Focal-Plane Detector Package on the TUNL Split-Pole Spectrograph

A focal plane detector for the Enge Split-pole Spectrograph at Triangle Universities Nuclear Laboratory has been designed. The detector package consists of two position sensitive gas avalanche counters, a gas proportionality energy loss section, and a residual energy scintillator. This setup allows both particle identification and focal plane reconstruction. In this paper we will detail the construction of each section along with their accompanying electronics and data acquisition. Effects of energy loss throughout the detector, ray tracing procedures, and resolution as a function of fill pressure and bias voltage are also investigated. A measurement of the $^{27}\!$Al$(d,p)$ reaction is used to demonstrate detector performance and to illustrate a Bayesian method of energy calibration.

Cross-section measurements of the 94Mo(γ,n) and 90Zr(γ,n) reactions using real photons at the HIγS facility

The photodisintegration reaction cross-sections for 94Mo(γ,n) and 90Zr(γ,n) have been experimentally investigated with quasi-monochromatic photon beams at the High Intensity γ-Ray Source (HIγS) facility, Triangle University Nuclear Laboratory (TUNL). The measurements were focused primarily on studying the energy dependence of the photoneutron cross sections, which is the most direct way of testing statistical models, and were performed close to the respective neutron thresholds and above up to ~ 20 MeV. Neutrons from the (γ,n) reactions were detected using a 4π assembly of 3He proportional counters developed at Los Alamos National Laboratory and presently available at TUNL. While the 94Mo(γ,n) cross section measurement aims to contribute to a broader investigation for understanding the γ-process (the mechanism responsible for the nucleosynthesis of the so-called p-nuclei), the information from the 90Zr(γ,n) data is relevant to constrain QRPA calculations of γ-ray strength functions in this mass region. In this contribution, we will present our preliminary results of the total (γ,n) excitation functions for the two photoneutron reactions on 94Mo and 90Zr.

Exploratory study of fission product yield determination from photofission of Pu239 at 11 MeV with monoenergetic photons

Measurements of fission product yields play an important role for the understanding of fundamental aspects of the fission process. Recently, neutron-induced fission product-yield data of $^{239}\mathrm{Pu}$ at energies below 4 MeV revealed an unexpected energy dependence of certain fission fragments. In order to investigate whether this observation is prerogative to neutron-induced fission, a program has been initiated to measure fission product yields in photoinduced fission. Here we report on the first ever photofission product yield measurement with monoenergetic photons produced by Compton back-scattering of FEL photons. The experiment was performed at the High-Intensity Gamma-ray Source at Triangle Universities Nuclear Laboratory on $^{239}\mathrm{Pu}$ at ${E}_{\ensuremath{\gamma}}=11$ MeV. In this exploratory study the yield of eight fission products ranging from $^{91}\mathrm{Sr}$ to $^{143}\mathrm{Ce}$ has been obtained.

Energy dependence of fission product yields from 235U, 238U, and 239Pu with monoenergetic neutrons between thermal and 14.8 MeV

Under a joint collaboration between TUNL-LANL-LLNL, a set of absolute fission product yield measurements has been performed. The energy dependence of a number of cumulative fission product yields (FPY) have been measured using quasi-monoenergetic neutron beams for three actinide targets, 235 U, 238 U and 239 Pu, between 0.5 and 14.8 MeV. The FPYs were measured by a combination of fission counting using specially designed dual-fission chambers and γ-ray counting. 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, thus reducing uncertainties. γ-ray counting of the activation target was performed on well-shielded HPGe detectors over a period of two months post irradiation to properly identify fission products. Reported are absolute cumulative fission product yields for incident neutron energies of 0.5, 1.37, 2.4, 3.6, 4.6, 5.5, 7.5, 8.9 and 14.8 MeV. Preliminary results from thermal irradiations at the MIT research reactor will also be presented and compared to present data and evaluations. This work was performed under the auspices of the U.S. Department of Energy by Los Alamos National Security, LLC under contract DE-AC52-06NA25396, Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344 and by Duke University and Triangle Universities Nuclear Laboratory through NNSA Stewardship Science Academic Alliance grant No. DE-FG52-09NA29465, DE-FG52-09NA29448 and Office of Nuclear Physics Grant No. DE-FG02-97ER41033.

Few-Nucleon Research at TUNL: Probing Two- and Three-Nucleon Interactions with Neutrons

The central goal of few-nucleon research at the Triangle Universities Nuclear Laboratory (TUNL) is to perform measurements that contribute to advancing ab-initio calculations of nuclear structure and reactions. The program aims include evaluating theoretical treatments of few-nucleon reaction dynamics through strategically comparing theory predictions to data, determining properties of the neutron-neutron interaction that are not accessible in two-nucleon reactions, and searching for evidence of longrange features of three-nucleon interactions, e.g., spin and isospin dependence. This paper will review studies of three- and four-nucleon systems at TUNL conducted using unpolarized and polarized neutron beams. Measurements of neutron-induced reactions performed by groups at TUNL over the last six years are described in comparison with theory predictions. The results are discussed in the context of the program goals stated above. Measurements of vector analyzing powers for elastic scattering in A=3 and A=4 systems, differential cross sections for neutron-deuteron elastic scattering and neutrondeuteron breakup in several final-state configurations are described. The findings from these studies and plans for the coming three years are presented in the context of worldwide activities in this front, in particular, research presented in this session.

Energy Dependence of Fission Product Yields from 235U, 238U and 239Pu for Incident Neutron Energies Between 0.5 and 14.8 MeV

Fission Product Yields (FPY) have historically been one of the most observable features of the fission process. They are known to have strong variations that are dependent on the fissioning species, the excitation energy, and the angular momentum of the compound system. However, consistent and systematic studies of the variation of these FPY with energy have proved challenging. This is caused primarily by the nature of the experiments that have traditionally relied on radiochemical procedures to isolate specific fission products. Although radiochemical procedures exist that can isolate all products, each element presents specific challenges and introduces varying degrees of systematic errors that can make inter-comparison of FPY uncertain. Although of high importance in fields such as nuclear forensics and Stockpile Stewardship, accurate information about the energy dependence of neutron induced FPY are sparse, due primarily to the lack of suitable monoenergetic neutron sources. There is a clear need for improved data, and to address this issue, a collaboration was formed between Los Alamos National Laboratory (LANL), Lawrence Livermore National Laboratory (LLNL) and the Triangle Universities Nuclear Laboratory (TUNL) to measure the energy dependence of FPY for 235U, 238U and 239Pu. The measurements have been performed at TUNL, using a 10 MV Tandem Van de Graaff accelerator to produce monoenergetic neutrons at energies between 0.6 MeV to 14.8 MeV through a variety of reactions. The measurements have utilized a dual-fission chamber, with thin (10-100 μg/cm2) reference foils of similar material to a thick (100-400 mg) activation target held in the center between the chambers. This method allows for the accurate determination of the number of fissions that occurred in the thick target without requiring knowledge of the fission cross section or neutron fluence on target. Following activation, the thick target was removed from the dual-fission chamber and gamma-ray counted using shielded HPGe detectors for a period of 1-2 months to determine the yield of various fission products. To the extent possible all irradiation and counting procedures were kept the same to minimize sources of systematic errors. FPY have been determined at incident neutron energies of 0.6, 1.4, 2.4, 3.5, 4.6, 5.5, 8.9 and 14.8 MeV.