Improved 235U, 238U, 239Pu, and 240Pu Photofission Cross Sections Across the GDR
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Silano, Jack [Lawrence Livermore National Laborato↗
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Detection of Prompt Photofission Neutrons from U-238 with a He-4 Scintillation Detector.
Abstract not provided.
Searfus, Oskar↗
Detection of Prompt Photofission Neutrons from U-238 with a He-4 Scintillation Detector.
Abstract not provided.
Searfus, Oskar↗
Detection of Prompt Photofission Neutrons from U-238 with a He-4 Scintillation Detector.
Abstract not provided.
Searfus, Oskar↗
Monoenergetic photon-induced fission cross-section ratio measurements for U 235 , U 238 , and Pu 239 from 9.0 to 17.0 MeV
Background: High-accuracy data are needed to advance microscopic descriptions of the nuclear fission process, to improve the predictive precision of phenomenological models, and for applications in nuclear energy and homeland security. Purpose: In this work, the main goal is to provide high-accuracy cross-section data for photofission of 235 U, 238 U, and 239 Pu in the energy range from the fission threshold to the high-energy tail of the giant dipole resonance. These new data should contribute significantly to the reduction of the systematic uncertainty in evaluated photofission cross-section databases. Method: Cross-section ratios for photofission of the “big three” actinide nuclei were measured using a quasimonoenergetic photon beam and dual-fission chambers. The measurements were performed at the HIγS facility using a photon beam produced from Compton backscattering of free-electron laser light. The dual-fission chamber enabled simultaneous counting of fission events from two targets. This method allows for cross-section ratio measurements with very small systematic errors. The largest source of systematic error in the determination of the cross-section ratios is the uncertainty in the ratio of the target thicknesses. Results: We report photon-induced fission cross-section ratios for σ[ 235 U(γ,f) / 238 U(γ,f)] , σ[235U(γ,f) / 239 Pu(γ,f)] , and σ[ 238 U(γ,f) / 239 Pu(γ,f)] at photon energies from 9.0–17.0 MeV. More than 20 data points were measured for each ratio, and the systematic uncertainties are less than 2%. Conclusion: The present photofission cross-section ratio data sets are compared to ratios computed from previous measurements, to the corresponding neutron-induced fission cross-section ratios, and to ratios computed from evaluated databases. Finally, the data obtained in this work for σ[ 235 U(γ,f) / 238 U(γ,f)] are consistent with the existing data, while for σ[ 235 U(γ,f) / 239 Pu(γ,f)] and σ[ 238 U(γ,f) / 239 Pu(γ,f)] the present data are systematically lower.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
FREYA Developments 2015-2016
We have been studying the sensitivity of neutron observables to the FREYA input distributions. Patrick Talou generated 1000 different yield functions, Y(A,Z,TKE), for 252 Cf(sf) based on fits to available data on the yields as a function of fragment mass, Y(A), and the total kinetic energy distribution as a function of heavy fragment mass, TKE(AH), and its width, σTKE(AH), to obtain Y(A,TKE). These results are augmented by Wahl systematics to include the fragment charge. We have so far found that the neutron observables do not depend strongly on the yield functions. We also found that the yield functions, constrained by data, are not sufficient to constrain the average neutron multiplicity within the evaluated uncertainty. A paper on this is in progress. We began working on a paper on photofission with the University of Michigan while on a Consortium for Verification Technology fellowship. This is an important paper because they have measured the neutron multiplicity distribution, the prompt fission neutron spectrum, and the energy correlation of two neutrons as a function of angle of separation for the first time in photofission of 235 U. We have made FREYA calculations to compare with the data, but the paper is still in progress since the student left and other researchers have had to take over the analysis. We expect to submit the paper in the next several months, after the analysis is complete.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
A Study of Fission Modes to Improve Nuclear Forensics
Obtain and evaluate charge and mass distributions from various databases and the literature Establish the mathematical formulism to decompose the selected mass distributions into their respective components of the multimodal fission models Compare and evaluate the results with prior work available from the literature Obtain, evaluate/validate experimentally-measured, photofission mass distributions using complementary INL-led research Apply decomposition methods to the photofission mass distributions Evaluate peak-to-valley ratios and specific isotopic ratios as a function of excitation energies Report and publish appropriate findings
38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗
High-fidelity photoneutron detection via neutron activation analysis
To interdict illicit radioactive sources and prevent nuclear terror, governments have deployed portal monitor systems at ports of entry. Such portal monitors rely upon the detection of spontaneously-emitted radiation signatures, however, detection scenarios become more challenging for highly enriched uranium, which emits relatively few spontaneous radiation signatures. Photon active interrogation techniques can augment current cargo inspection scenarios and improve detection capabilities for illicit nuclear material. A high-energy photon interrogation source can induce photofission in nuclear material, producing strong radiation signatures for detection. Prompt fission signals are of specific interest because prompt neutrons are greater than 99% of the fission signal. When using detectors capable of spectroscopy, e.g., pulse shape discrimination-capable organic scintillators, the intense interrogating source often will cause pulse pile-up, degrading particle classification. In this work, we demonstrate photoneutron detection via neutron activation of aluminum and iron. The experiments rely upon nuclear reactions that can only be induced by neutrons, and are robust against pile-up, thereby providing high-confidence neutron rates. In this work, we apply neutron activation analysis for the detection of high-energy neutrons produced by high-energy photon irradiation; the depleted uranium sample was irradiated with a 9-MeV bremsstrahlung beam. We find that the irradiated depleted uranium produced an aluminum activation flux of 1.77 × 10 5 n cm –2 s –1 and an iron-56 activation flux of 1.05 × 10 5 n cm –2 s –1 . Furthermore, the ability to detect radioactivity induced by neutrons created in (, n) and photofission reactions demonstrates a new approach for photon active interrogation to identify nuclear materials.
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
Detectors for Active Interrogation Applications
Active interrogation creates an environment that is particularly challenging from a radiation-detection standpoint: the elevated background levels from the source can mask the desired signatures from the SNM. Neutron based interrogation experiments have shown that nanosecond-level timing is required to discriminate induced-fission neutrons from the scattered source neutrons. Previous experiments using high-energy bremsstrahlung X-rays have demonstrated the ability to induce and detect prompt photofission neutrons from single target materials; however, a real-world application would require spectroscopic capability to discern between photofission neutrons emitted by SNM and neutrons emitted by other reactions in non-SNM. Using digital pulseshape discrimination, organic liquid scintillators are capable of reliably detecting neutrons in an intense gamma-ray field. Photon misclassification rates as low as 1 in 10 6 have been achieved, which is approaching the level of gaseous neutron detectors such as 3 He without the need for neutron moderation. These scintillators also possess nanosecond-timing resolution, making them candidates for both neutron-and photon-driven active interrogation systems. Lastly, we have applied an array of liquid and NaI(Tl) scintillators to successfully image 13.7 kg of HEU interrogated by a DT neutron generator; the system was in the direct presence of the accelerator during the experiment.
36 MATERIALS SCIENCE↗
Decisive role of wriggling vibrations in the formation of angular and spin distributions of products originating from binary and ternary fission of oriented nuclei
It is shown that the multiplicities and angular and energy distributions of neutrons and photons evaporated from thermalized fragments originating from the spontaneous and low-energy induced fission of nuclei, the relative yields of ground and isomeric states of final fragments, and the features of delayed neutrons emitted upon the beta decay of the above fragments can successfully be described by employing nonequilibrium distributions of spins and relative orbital angular momenta of fission fragments formed in the vicinity of the scission point for the fissile nucleus being studied. It is also shown that these distributions, which are characterized by large mean values of the spins and orbital angular momenta directed orthogonally to the symmetry axis of the fissioning nucleus are successfully constructed upon simultaneously taking into account zero-mode transverse wriggling and bending vibrations of a fissile compound nucleus in the vicinity of its scission point, the wriggling vibrations being dominant. It is confirmed that the zero-mode wriggling vibrations considered immediately above are directly involved in the formation of the angular distributions of fragments originating from the spontaneous and low-energy fission of nuclei. This makes it possible to describe successfully such distributions for photofission fragments.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Assessment of Impact of Monoenergetic Photon Sources on Prioritized Nonproliferation Applications: Simulation Study Report
Near-monoenergetic photon sources (MPSs) have the potential to improve sensitivity at greatly reduced dose in existing applications and enable new capabilities in other applications. MPS advantages include the ability to select energy, energy spread, flux, and pulse structures to deliver only the photons needed for the application, while suppressing extraneous dose and background. Some MPSs also offer narrow divergence photon beams which can target dose and/or mitigate scattering contributions to image contrast degradation. Current broad-band, bremsstrahlung photon sources (e.g., linacs and betatrons) deliver unnecessary dose that in some cases also interferes with the signature to be detected and/or restricts operations, and must be collimated (reducing flux) to generate narrow divergence beams. While MPSs can in principle resolve these issues, they are technically challenging to produce. Candidate MPS technologies for nonproliferation applications are now being developed, each of which have different properties (e.g. broad divergence vs. narrow). Within each technology, source parameters trade off against one another (e.g. flux vs. energy spread), representing a large operation space. To guide development, requirements for each application of interest must be defined and simulations conducted to define MPS parameters that deliver benefit relative to current systems. The present project conducted a broad assessment of potential nonproliferation applications where MPSs may provide new capabilities or significant performance enhancement (reported separately), which led to prioritization of several applications for detailed analysis. The applications prioritized were: cargo screening and interdiction of Special Nuclear Materials (SNM), detection of hidden SNM, treaty/dismantlement verification, and spent fuel dry storage cask content verification. High resolution imaging for stockpile stewardship was considered as a sub-area of the treaty topic, as it is also of interest for future treaty use. This report presents higher-fidelity calculations and modeling results to quantitatively evaluate the prioritized applications, and to derive the key MPS properties that drive application benefit. Simulations focused on the conventional signatures of radiography, photofission, and NRF to enable comparison to present methods and evaluation of benefit.
98 NUCLEAR DISARMAMENT, SAFEGUARDS, AND PHYSICAL P↗
Clinical 6 MV X-Ray Facility Photo-Neutron/Fission Interrogations with TMFD Sensors
Detection of several targets (U, Be, D2O) of interest to national security has been achieved at Purdue University’s 6 MV (~104 R/h) X-Ray clinical linear accelerator (CLINAC), from photoneutron/fission induced neutron signatures using a Centrifugally Tensioned Metastable Fluid Detector (CTMFD). Traditional neutron detection schemes were saturated and ineffective under the intense (to ~104 R/h) photon environment; however, the CTMFD sensor technology was capable of measuring the relatively insignificant (~10-12x lower intensity) neutron field produced from 6 MeV-end point X-Ray beam interactions with the CLINAC components and surroundings. This is in stark contrast to published results based on conventional belief for immeasurable neutron dose in 6 MV CLINAC facilities. Photoneutron and photofission neutron production rates were detected and analyzed at various standoffs from the isocenter, with and without special nuclear material (SNM) related targets ranging in mass from 5 g of Be, to ~0.6 kg of UO2. It was confirmed that 6 MV X-Ray photons do result in measurable neutron dose above normal cosmic background. Additionally, in threshold mode, the CTMFD operating under a tensioned metastable pressure of -5bar, enabled conclusive rejection of background photoneutrons, while enabling the rapid detection of: Be, D2O, C and U targets within seconds to minutes while operating in and around 104 R/h photon fields.
Ozerov, Stepan↗
12 C(p,p') 12 C Reaction ( E p =19.5–30 MeV) for Active Interrogation of Special Nuclear Material
Passive detection of special nuclear material (SNM) is challenging due to its inherently low rate of spontaneous emission of penetrating radiation, the relative ease of shielding, and the fluctuating and frequently overwhelming background. Active interrogation, the use of external radiation to increase the emission rate of characteristic radiation from SNM, has long been considered to be a promising method to overcome those challenges. Current active-interrogation systems that incorporate radiography tend to use bremsstrahlung beams, which can deliver high radiation doses. Low-energy ion-driven nuclear reactions that produce multiple monoenergetic photons may be used as an alternative. The 12 C ( p , p ' ) 12 C reaction is one such reaction that could produce large yields of highly penetrating 4.4- and 15.1-MeV gamma rays. This reaction does not directly produce neutrons below the approximately 19.7 MeV threshold, and the 15.1-MeV gamma-ray line is well matched to the photofission cross section of 235 U and 238 U . In this article, we report the measurements of thick-target gamma-ray yields at 4.4 and 15.1 MeV from the 12 C ( p , p ' ) 12 C reaction at proton energies of 19.5, 25, and 30 MeV. Measurements are made with two 3 -in. EJ-309 cylindrical liquid scintillation detectors and thermoluminescent dosimeters placed at 0 ° and 90 ° , with an additional 1.5 -in. Na I ( Tl ) cylindrical scintillation detector at 0 ° . We estimate the highest yields of the 4.4- and 15.1-MeV gamma rays of 1.65 × 10 10 and 4.47 × 10 8 sr - 1 μ C - 1 at a proton energy of 30 MeV, respectively. The yields in all experimental configurations are greater than in a comparable deuteron-driven reaction that produces the same gamma-ray energies— 11 B ( d , n γ ) 12 C . However, a significant increase of the neutron radiation dose accompanies the proton energy increase from 19.5 to 30 MeV.
38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗
Erratum: Monoenergetic photon-induced fission cross-section ratio measurements for U 235 , U 238 , and Pu 239 from 9.0 to 17.0 MeV
In this erratum we report a correction to the previously published photofission cross-section ratio data. The actinide fission-chamber foils utilized in this work were extensively characterized during a new campaign using α- and γ-ray spectroscopy, and 2π alpha counting. These three techniques agreed, resulting in an improved accuracy of the fission-chamber foil masses used in the original experiment. The foil masses measured by α spectroscopy are detailed in Table I. All foils were counted on the same α spectrometer, using an identical geometry, which reduces the systematic uncertainty in the calculated mass ratios since the α spectrometer efficiency cancels out in the ratio. This results in a relative uncertainty on the mass ratio of 1.0% for 235 U/ 238 U, 0.62% for 239 Pu/ 235 U, and 1.1% for 239 Pu/ 238 U. The published cross-section ratio data are re-calculated to reflect the updated target masses and uncertainties. The new mass values cause a 2.40% decrease for the σ( 235 U(γ, f)/ 238 U(γ, f)) ratio, a 5.10% decrease for the σ( 235 U(γ, f)/ 239 Pu(γ, f)) ratio, and a 2.77% decrease for the σ( 238 U(γ, f)/ 239 Pu(γ, f)) ratio. The updated cross-section ratios are presented in Tables II, III, and IV, which are corrected versions of Tables III, IV, and V in Ref. [1]
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Revision of forensics-relevant nuclear data in Sb 127 β decay
With a half-life of 3.85(5) d, the fission product 127 Sb is potentially useful for the characterization of fallout debris that would be collected for forensic analysis following a hypothetical nuclear event. The β decay of 127 Sb has many characteristic $\gamma$-ray lines, however, the majority of decay strength is concentrated in the three strongest lines near 686, 473, and 784 keV, based on low-precision spectroscopic measurements from decades ago. In a new measurement of 127 Sb β decay, obtained from radiochemical separations of U photofission products, we show that the energies of the 686 and 784 lines are actually shifted lower by 0.8 and 1.1 keV, respectively. Such large discrepancies in the nuclear data inhibit accurate isotopic analyses and have important implications, for example, on the assessment of fission debris that might be collected in the days following a nuclear event. Therefore, a new evaluation of 127 Sb β decay and appropriate revision to the various nuclear databases are warranted. This work reports a new set of $\gamma$-ray energies and relative intensities corresponding to 127 Sb decay.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Fission Gamma-Ray Spectra Relevant to Post-Detonation Debris Analysis
A terrorist-generated nuclear attack would invoke unprecedented pressure for a rapid and accurate nuclear forensics characterization of the event. In-situ fission product gamma-ray spectroscopy offers the potential for a significant reduction in analysis time while still providing high fidelity identification of the primary composition of the detonated device. The in-situ spectroscopic approach we are proposing can rapidly identify the species that underwent fission as well as the fission-inducing neutron energy spectrum. To demonstrate this goal, we have used the gammaray history from our previously obtained LLNL-LANL-TUNL fission product-yield (FPY) data to provide insights for post-detonation debris analysis. The proposed method relies on the ratio of photopeak areas for selected gamma-rays representing the FPYs of sixteen fission products generated using mono-energetic neutrons at energies between 0.5 MeV to 14.8 MeV on highlyenriched 239 Pu, 235 U, and 238 U sealed targets. In addition to the fission product ratio measurements we have applied a new regression analysis based on our experimental FPY data and validated the approach using relevant Godiva FPY data and (surrogate) photofission product yields of 240 Pu. The main advantage of the regression analysis is that it investigates the FPY data set as a whole, rather than focusing on individual FPY ratios.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Fission with Exotic Nuclei (Abbreviated Report)
Nuclear fission is a key mechanism involved in the synthesis of heavy elements in the Cosmos and is the primary explanation for the stability of superheavy elements. Nevertheless, our knowledge of fission remains extremely fragmented. Most experiments have been conducted only on a tiny number of stable actinide nuclei and are often incomplete, leading to gaps in our basic understanding of the process. For many radioactive isotopes, basic fission data such as the charge or mass distribution of the fragments is unknown. These gaps cannot always be filled by simulation alone. Common fission models contain too many free parameters and lack predictive power. In contrast, the fundamental theory of fission under development at LLNL is much more predictive, but its current computational cost is too high to be used extensively for data evaluations. A unique window of opportunity to resolve these limitations has recently opened: the U.S. nuclear science community is ramping up major experimental programs at the Facility for Rare Isotope Beams (FRIB, the DOE flagship facility in low-energy nuclear science), and techniques from machine learning have shown great potential to simplify the use of a fundamental, quantum-mechanical theory of fission. This project has two components. On the experimental side, we acquired and deployed at the HIGS facility a new dual Frisch-Grid ionization chamber to measure correlated fragment-mass, kinetic energy, and angular distributions of fission fragments from induced fission. This new device was used to perform measurements of charge, mass and total kinetic energy of fission fragments in the photofission of 238 U and eight gamma-ray beam energies between 6.2 and 13 MeV, which allowed extracting high-precision independent yields for this reaction. The device was also used to perform measurements of the same quantities in the neutron-induced fission of 234 U with monoenergetic beams of energy between 5 and 8 MeV. In parallel, we collaborated with a team at Commissariat à l’énergie atomique et aux énergies alternatives (CEA) to perform a series of measurements of fission yields in inverse kinematics for the two isotopes of 236 U and 240 Pu. The experiment took place at the Grand Accélérateur National d’Ions Lourds in France in June 2023. The deployment of the VAMOS spectrometer with a new array called PISTA allowed determining the excitation energy of the fissioning system within 1 Mega-electronvolts. The second component of the project involved using deep neural networks to build fast and reliable emulators of our current fission models. In an invited paper published in Frontier in Physics, we showed that autoencoders could successfully compress nuclear wavefunctions in nuclear density functional theory. We achieved a dimensionality reduction of the order of two orders of magnitude while keeping the error in the total energy to less than 0.01%. In a second paper submitted to Physical Review Letters in June 2023 with our collaborators at CEA, we showed that variational autoencoders can learn the collective degrees of freedom driving the fission process.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Cosmological constraints on the lifetime of massive particles
Particles with masses more than a few MeV, decaying into photons or electrons, can cause destruction by photofission of cosmologically produced light elements. A previous calculation of this effect is corrected and extended, and used to derive maximum lifetimes for massive neutrinos; these range from a few thousand seconds upward, depending on the particle mass. Some approximate expressions are given that enable lifetime limits to be obtained for other particles, with different masses and abundances, such as gravitinos. These limits are generally stronger than previously determined constraints, such as distortion of the microwave background by energetic photons.
Lindley, D.↗