Detector For Advanced Neutron Capture Experiments Research Articles

New Capabilities of the RPI γ-Multiplicity Detector to Measure γ-Production

Accurate modeling of γ-production in neutron capture reactions is critical for many applications including on-proliferation, safeguards and modeling nuclear reactors. To improve this work, the Rensselaer Polytechnic Institute (RPI) 16-segment γ-multiplicity NaI(Tl) detector at the Gaerttner Linear Accelerator (LINAC) Center has been upgraded by implementing a digital data acquisition system. The new digitized system records the γ-energy deposition distribution in each individual detector, and γ-multiplicity values as a function of neutron time-of-flight (TOF). With the new capabilities, high precision capture (and fission) yield measurements can be made, and the accuracy of simulation tools used to predict capture γ-cascades can be tested. To validate the updated system, an experiment was performed using a natural Ta sample to measure 181Ta and 180mTa resonance capture yield by detecting prompt γ-rays emitted from neutron capture interactions as a function of both neutron energy and measured γ-multiplicity of each capture event. The results confirm earlier measurements and agree with theoretical yield in the low energy resonance region from 1 to 20 eV. A 238U(n, γ) measurement was also performed to generate γ-spectra. For capture γ-cascades where the total γ-energy deposition is close to the neutron binding energy, γ-spectra were measured for individual resonance energies and observed γ-multiplicities. The results are comparable in shape to a recent measurement done using the Detector for Advanced Neutron Capture Experiments (DANCE) array at Los Alamos Neutron Science Center (LANSCE); however, differences need to be compared to Monte-Carlo n-particle simulations.

Spin assignment and statistical properties of neutron resonances from Dy161,163(n,γ) and Er167(n,γ) measured at the DANCE facility

Background: Average resonance spacing ${D}_{0}$ and spin dependence of nuclear level density (NLD) are essential quantities in nuclear physics, especially important for calculations of nuclear reactions and the normalization of NLD models.Purpose: Neutron resonances with different spins in odd-mass targets can form close doublets that are often difficult to resolve. These doublets are not corrected for when accounting for subthreshold resonances during ${D}_{0}$ determination. Moreover, different literature sources disagree on the isotopic assignment of some resonances.Methods: The $\ensuremath{\gamma}$ rays following radiative neutron capture on $^{161,163}\mathrm{Dy}$ and $^{167}\mathrm{Er}$ were measured with the highly segmented $\ensuremath{\gamma}$-ray calorimeter Detector for Advanced Neutron Capture Experiments (DANCE) at the Los Alamos Neutron Science Center. The analysis of spectra using the $\ensuremath{\gamma}$-multiplicity-based spin assignment method allows checking for the presence of the above-mentioned doublets. The calorimetric sensitivity provides unambiguous isotopic assignment.Results: We were able to assign spin to tens of resonances as well as to identify new ones in all three isotopes. Some isotope assignments from the literature were corrected. Detailed analysis of the number of unobserved resonances, assuming that the resonance positions obey predictions of the Gaussian orthogonal ensemble and reduced neutron widths Porter-Thomas fluctuations, allowed determination of ${D}_{0}$ with an uncertainty of a few percent, ${D}_{0}=2.15(5)$, 6.39(24), and 3.86(12) eV for $^{161,163}\mathrm{Dy}$ and $^{167}\mathrm{Er}$, respectively. Thanks to the spin assignment, the spacings for the resonances with the two spins formed in $s$-wave neutron capture, ${D}_{0}^{\ensuremath{-}}$ and ${D}_{0}^{+}$, were determined. Their ratio was compared to different NLD models.Conclusions: Our deduced ${D}_{0}$ for $^{163}\mathrm{Dy}$ is lower than any $^{163}\mathrm{Dy}$ value found in the literature. Good consistency was found with some literature values for $^{161}\mathrm{Dy}$ and $^{167}\mathrm{Er}$. The ratios ${D}_{0}^{\ensuremath{-}}/{D}_{0}^{+}$ are consistent with several models available in the literature, but in a clear contradiction with a few microscopic NLD models.

Statistical neutron capture in the limit of low nuclear level density

A major barrier in the study of neutron-induced nuclear reactions is the impossibility of direct measurements with short-lived radioactive isotopes. For these exotic nuclei, theoretical inputs such as the photon strength function (PSF) are poorly constrained. At Los Alamos National Laboratory, the Detector for Advanced Neutron Capture Experiments (DANCE) provides direct measurements of $\ensuremath{\gamma}$-ray cascades following neutron capture reactions on stable or long-lived radioactive nuclei. While Hauser-Feshbach calculations can provide reasonable predictions for neutron capture on heavy nuclei, their application to neutron-rich light nuclei with low nuclear level densities and low neutron separation energies is questionable. In this paper, we report on the $\ensuremath{\gamma}$-ray spectra from individual neutron resonances from the $^{96}\mathrm{Zr}(n,\ensuremath{\gamma})$ reaction, with an emphasis on the sensitivity of of the $\ensuremath{\gamma}$-ray spectra to different PSF models. The comparison of the measured $\ensuremath{\gamma}$-ray spectra with predicted spectra does not support the addition of a low-energy enhancement of the size reported in many charged-particle reaction measurements, but the sensitivity of the $\ensuremath{\gamma}$-ray spectra to different PSF models is weak.

Dependence of the prompt fission γ-ray spectrum on the entrance channel of compound nucleus: Spontaneous vs. neutron-induced fission

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.

Unifying measurement of Pu239(n,γ) in the keV to MeV energy regime

A single, unifying measurement of the $^{239}\mathrm{Pu}$ capture cross section from 1 keV to 1.3 MeV has been performed for the first time using the Detector for Advanced Neutron Capture Experiments (DANCE) at the Los Alamos Neutron Science Center (LANSCE). The experimental method combines a prior experiment's characterization of prompt fission $\ensuremath{\gamma}$ rays in conjunction with a fission tagging detector with a separate experiment using a thick $^{239}\mathrm{Pu}$ sample to extract the neutron capture cross section in ratio to $^{239}\mathrm{Pu}(n,f)$. We have made new predictions of the capture cross section taking into account recent results for the $M1$ scissors mode present in other actinides. The results show deviations from current evaluations which are 30% higher at the highest energies, and will be used to improve calculations relevant for several applications.

239Pu(n,γ) from 10 eV to 1.3 MeV

The 239Pu(n,γ) cross section has been measured from 10 eV to 1.3 MeV as part of an experimental campaign using the Detector for Advanced Neutron Capture Experiments (DANCE). The work represents a significant advance in experimental technique, with improved systematic uncertainties in key regions in the keV to MeV regime. In general the results of prior work are confirmed with improved uncertainties, particularly at the highest incident neutron energies.

Constraining the calculation of U234,236,238(n,γ) cross sections with measurements of the γ -ray spectra at the DANCE facility

The cross section for neutron capture in the continuum region has been difficult to calculate accurately. Previous results for $^{238}\mathrm{U}$ show that including an $M1$ scissors-mode contribution to the photon strength function resulted in very good agreement between calculation and measurement. This paper extends that analysis to $^{234,236}\mathrm{U}$ by using $\ensuremath{\gamma}$-ray spectra measured with the Detector for Advanced Neutron Capture Experiments (DANCE) at the Los Alamos Neutron Science Center to constrain the photon strength function used to calculate the capture cross section. Calculations using a strong scissors-mode contribution reproduced the measured $\ensuremath{\gamma}$-ray spectra and were in excellent agreement with the reported cross sections for all three isotopes.

Radiative neutron capture cross section from U236

The $^{236}\mathrm{U}(n,\ensuremath{\gamma})$ reaction cross section has been measured for the incident neutron energy range from 10 eV to 800 keV by using the Detector for Advanced Neutron Capture Experiments (DANCE) $\ensuremath{\gamma}$-ray calorimeter at the Los Alamos Neutron Science Center. The cross section was determined with the ratio method, which is a technique that uses the $^{235}\mathrm{U}(n,f)$ reaction as a reference. The results of the experiment are reported in the resolved and unresolved resonance energy regions. Individual neutron resonance parameters were obtained below 1 keV incident energy by using the $R$-matrix code sammy. The cross section in the unresolved resonance region is determined with improved experimental uncertainty. It agrees with both ENDF/B-VII.1 and JEFF-3.2 nuclear data libraries. The results above 10 keV agree better with the JEFF-3.2 library.

Total prompt γ-ray emission in fission

The total prompt γ-ray energy distributions were measured for the neutron-induced fission of 235U, 239,241Pu at incident neutron energy of 0.025 eV–100 keV, and the spontaneous fission of 252Cf using the Detector for Advanced Neutron Capture Experiments (DANCE) array in coincidence with the detection of fission fragments by a parallel-plate avalanche counter. Corrections were made to the measured distribution by unfolding the two-dimension spectrum of total prompt γ-ray energy vs multiplicity using a simulated DANCE response matrix. A summary of this work is presented with the emphasis on the comparison of total prompt fission γ-ray energy between our results and previous ones. The mean values of the total prompt γ-ray energy ⟨Eγ,tot⟩, determined from the unfolded distributions, are ∼20% higher than those derived from measurements using single γ-ray detector for all the fissile nuclei studied.

Neutron capture cross sections of Kr

Neutron capture and β− -decay are competing branches of the s-process nucleosynthesis path at 85 Kr [1], which makes it an important branching point. The knowledge of its neutron capture cross section is therefore essential to constrain stellar models of nucleosynthesis. Despite its importance for different fields, no direct measurement of the cross section of 85 Kr in the keV-regime has been performed. The currently reported uncertainties are still in the order of 50% [2, 3]. Neutron capture cross section measurements on a 4% enriched 85 Kr gas enclosed in a stainless steel cylinder were performed at Los Alamos National Laboratory (LANL) using the Detector for Advanced Neutron Capture Experiments (DANCE). 85 Kr is radioactive isotope with a half life of 10.8 years. As this was a low-enrichment sample, the main contaminants, the stable krypton isotopes 83 Kr and 86 Kr, were also investigated. The material was highly enriched and contained in pressurized stainless steel spheres.

Isomeric ratio measurements for the radiative neutron capture Lu176(n,γ) at the LANL DANCE facility

The isomeric ratios for the neutron capture reaction $^{176}$Lu($n,\gamma$) to the $J^{\pi}=5/2^-$ 761.7 keV, $T_{1/2}$ =32.8 ns and the $J^{\pi}=15/2^+$, 1356.9 keV, $T_{1/2}$=11.1 ns levels of $^{177}$Lu have been measured for the first time. The experiment was carried out with the Detector for Advanced Neutron Capture Experiments (DANCE) at the Los Alamos National Laboratory. Measured isomeric ratios are compared with talys calculations using different models for photon strength functions, level densities, and optical potentials. In order to reproduce the experimental γ-ray spectra, a low-energy resonance must be added in the photon strength function used in our Hauser-Feshbach calculations.

Measurement of neutron capture onXe136

$^{136}$Xe is a 0{\nu}{\beta}{\beta} decay candidate isotope, and is used in multiple experiments searching for this hypothetical decay mode. These experiments require precise information about neutron capture for their background characterization and minimization. Thermal and resonant neutron capture on $^{136}$Xe have been measured at the Detector for Advanced Neutron Capture Experiments (DANCE) at the Los Alamos Neutron Science Center. A neutron beam ranging from thermal energy to greater than 100 keV was incident on a gas cell filled with isotopically pure $^{136}$Xe. The relative neutron capture cross sections for neutrons at thermal energies and the first resonance at 2.154 keV have been measured. Additionally, the {\gamma} cascades for these captures have been measured, and cascade models have been developed which may be used by 0{\nu}{\beta}{\beta} experiments using $^{136}$Xe.

Total promptγ-ray emission in fission

The total prompt γ-ray energy distributions were measured for the neutron-induced fission of 235 U, 239,241 Pu at incident neutron energy of 0.025 eV–100 keV, and the spontaneous fission of 252 Cf using the Detector for Advanced Neutron Capture Experiments (DANCE) array in coincidence with the detection of fission fragments by a parallel-plate avalanche counter. Corrections were made to the measured distribution by unfolding the two-dimension spectrum of total prompt γ-ray energy vs multiplicity using a simulated DANCE response matrix. A summary of this work is presented with the emphasis on the comparison of total prompt fission γ-ray energy between our results and previous ones. The mean values of the total prompt γ-ray energy ⟨Eγ,tot⟩, determined from the unfolded distributions, are ∼20% higher than those derived from measurements using single γ-ray detector for all the fissile nuclei studied.

Assessing the role of the (n,γf) process in the low-energy fission of actinides

We review the role of the (n , γ f ) process in the low-energy neutron-induced fission reaction of 239 Pu. Recent measurements of the average total γ -ray energy released in this reaction were performed with the Detector for Advanced Neutron Capture Experiments (DANCE) at Los Alamos. Significant fluctuations of this quantity in the resonance region below 100 eV can be interpreted by invoking the presence of the indirect (n, γ f ) process. Modern calculations of the probability for such an event to occur are presented.

Measurement of theMo97(n,γ)reaction with the DANCEγcalorimeter array

Spectra of $\ensuremath{\gamma}$ rays following the $^{97}\mathrm{Mo}(n,\ensuremath{\gamma})$ reaction were measured as a function of incident neutron energy with the DANCE (Detector for Advanced Neutron Capture Experiments) array of $160 {\mathrm{BaF}}_{2}$ scintillation detectors at the Los Alamos Neutron Science Center using an enriched $^{97}\mathrm{Mo}$ target. These spectra were used for the assignment of spins of the $^{97}\mathrm{Mo}$ resonances up to neutron energy ${E}_{n}=1.7$ keV, as well as in the study of photon strength functions (PSFs) in $^{98}\mathrm{Mo}$. Analysis of the spectra with the nuclear statistical model showed that they can be well reproduced with the same PSF models which well described the $\ensuremath{\gamma}$ decay following slow neutron capture in $^{95}\mathrm{Mo}$. On the other hand, the spectra are inconsistent with PSFs describing some other experimental data in $^{98}\mathrm{Mo}$.

Current and Future Research at DANCE

An overview of the current experimental program on measurements of neutron capture and neutron induced fission at the Detector for Advanced Neutron Capture Experiments (DANCE) is presented. Three major projects are currently under way: 1) high precision measurements of neutron capture cross sections on Uranium isotopes, 2) research aimed at studies of the short-lived actinide isomer production in neutron capture on 235U and 3) measurements of correlated data of fission observables. New projects include developments of auxiliary detectors to improve the capability of DANCE. We are building a compact, segmented NEUtron detector Array at DANCE (NEUANCE), which will be installed in the central cavity of the DANCE array. It will thus provide experimental information on prompt fission neutrons in coincidence with the prompt fission gamma-rays measured by 160 BaF2 crystals of DANCE. Additionally, unique correlated data will be obtained for neutron capture and neutron-induced fission using the DANCE-NEUANCE experimental set up in the future.

Enhancing the Detector for Advanced Neutron Capture Experiments

The Detector for Advanced Neutron Capture Experiments (DANCE) has been used for extensive studies of neutron capture, gamma decay, photon strength functions, and prompt and delayed fission-gamma emission. Despite these successes, the potential measurements have been limited by the data acquisition hardware. We report on a major upgrade of the DANCE data acquisition that simultaneously enables strait-forward coupling to auxiliary detectors, including high-resolution high-purity germanium detectors and neutron tagging array. The upgrade will enhance the time domain accessible for time-of-flight neutron measurements as well as improve the resolution in the DANCE barium fluoride crystals for photons.

Total prompt γ -ray emission in fission of U235 , Pu239,241 , and Cf252

The total prompt $\ensuremath{\gamma}$-ray energy distributions for the neutron-induced fission of $^{235}\mathrm{U}$ and $^{239,241}\mathrm{Pu}$ in the neutron energy range of 0.025 eV -- 100 keV and the spontaneous fission of $^{252}\mathrm{Cf}$ were measured using the Detector for Advanced Neutron Capture Experiments (DANCE) in coincidence with the detection of fission fragments by a parallel-plate avalanche counter. DANCE is a highly segmented, highly efficient 4$\ensuremath{\pi}$ $\ensuremath{\gamma}$-ray calorimeter. Corrections were made to the measured distribution by unfolding the two-dimension spectrum of total $\ensuremath{\gamma}$-ray energy vs multiplicity using a simulated DANCE response matrix generated with a geometrical model of the detector arrays and validated with the $\ensuremath{\gamma}$-ray calibration sources. The mean values of the total prompt $\ensuremath{\gamma}$-ray energy, determined from the unfolded distributions, are $\ensuremath{\sim}$20% higher than those of early measurements for all the fissile nuclei studied. The implication for the $\ensuremath{\gamma}$ heating in nuclear reactors is discussed.

Neutron Capture Cross Section of 239Pu

The Detector for Advanced Neutron Capture Experiments (DANCE) has been used to measure the 239Pu(n,γ) cross section from 10 eV to the keV region. Three experimental run conditions were used to characterize the prompt fission γ-ray spectrum across the entire energy regime, measure the cross section in the resolved resonance region, and obtain necessary count rate well into the keV region. The preliminary cross sections are in good agreement with current evaluations from 10 eV to 80 keV.

Improved neutron capture cross section ofPu239

The Pu($n,\ensuremath{\gamma}$) cross section has been measured over the energy range 10 eV to 1 keV using the Detector for Advanced Neutron Capture Experiments (DANCE) at the Los Alamos Neutron Science Center as part of a campaign to produce precision ($n,\ensuremath{\gamma}$) measurements on Pu. Fission coincidences were measured with a parallel-plate avalanche counter and used to measure the prompt fission $\ensuremath{\gamma}$-ray spectrum in this region to accurately characterize background. The resulting ($n,\ensuremath{\gamma}$) cross section is generally in agreement with current evaluations. The experimental method utilizes much more detailed information than past measurements on Pu and can be used to extend the measurement to higher incident neutron energies.