Atlas of Gamma-Ray Spectra from the Inelastic Scattering of Reactor Fast Neutrons

Compilation of the “Atlas of Gamma-rays from the Inelastic Scattering of Reactor Fast Neutrons” (1978DE41) by A.M. Demidov, L.I. Govor, Yu.K. Cherepantsev, M.R. Ahmed, S. Al-Najjar, M.A. Al-Amili, N. Al-Assafi, and N. Rammo

Motivation

Inelastic neutron scattering is the primary energy-loss mechanism for fast neutrons in heavy (A>12) nuclei. As such, a good knowledge of it is required for virtually all branches of applied nuclear science ranging from shielding calculations to the design of advanced nuclear-energy systems to international security and counter proliferation. The need for improved neutron-scattering data was explicitly stated in the recent white paper [1] from the Nuclear Data Needs and Capabilities for Applications Workshop. In addition to its utility for nuclear applications, (n,n’γ) provides unique insight into off-yrast nuclear structure due to the non-selective nature of the reaction (which can include a significant compound component) and the wide range of angular momentum states accessible to fast neutrons.

Angle-differential (n,n’) data is notoriously difficult to measure due to the difficulties involved in measuring the neutron energy and the lack of intense neutron beams. An alternate approach to determining (n,n’γ) cross sections involves measuring the prompt γ rays emitted from the excited states populated via inelastic scattering, e.g., (n,n’). While these measurements lack the angle-differential information needed to improve neutron transport, they can provide an important integral constraint to the nuclear-reaction evaluation process and can be used to improve modeling for nondestructive assay of materials using active neutron interrogation.

The “Atlas of Gamma-rays from the Inelastic Scattering of Reactor Fast Neutrons” published by A.M. Demidov et al., [2] is one of the most comprehensive compilations of data on (n,n’γ) in existence containing over 7000 γ rays from 76 natural and 29 isotopically-enriched targets. The data in the Atlas comes from a single Li-drifted Ge γ-ray detector located on a filtered “fast” neutron beam line at the IRT-5000 reactor formerly located at the Al-Tuwaitha research facility outside of Baghdad, Iraq. All of the data in the “Baghdad Atlas” was taken using the same Ge γ-ray detector and the same experimental configuration. Detailed information regarding the target areal density was provided, allowing all the transition intensities to be normalized to the partial γ-ray cross section for inelastic scattering to the first excited state in 56Fe at 847 keV. While the details of the neutron spectrum are not completely known, the experimental setup was designed to minimize the presence of low-energy thermal neutrons. Their success is evident in that there are only 30 transitions in the Atlas firmly assigned to (n,γ) reactions. The authors reference earlier work [3] indicating the energy dependence of the fast neutron spectrum is of the form exp(-βEn) where β=0.6-0.7 and En is the neutron energy (MeV).

In order to determine the best value for the flux-weighted 56Fe(n,n’γ847) cross section we convolved the spectrum for the IRT reactor shown in Figure 5 of the Atlas [2] with the value for the 56Fe(n,n’γ847) cross section taken from ENDF/B-VII.1 [4]:

$$ \langle \sigma \rangle = \frac{\int\limits_{0}^{E_{n}=20} \phi(E_{n}) \sigma(2^{+}_{1}) dE}{\int\limits_{0}^{E_{n}=20} \phi(E_{n}) dE}, \qquad\qquad (1)$$

where φ(En) is the flux from the Atlas and σγ(21+) is the partial γ-ray cross section for the 847-keV 21+ → 0gs+ transition.

There is an additional contribution to the 847 keV transition which can be attributed to the β– decay of 56Mn formed in the 56Fe(n,p) reaction, which produces the 56Fe 847-keV γ-ray transition with a 98.85% branching ratio [5]. This contribution can be calculated using the same approach given in Eq. (1) because the experiment was performed over a long period of time compared to the lifetime of 56Mn (t1/2 ≈ 2.6 h [4]), allowing it to come into secular equilibrium. This contribution has been estimated, using the ENDF/B-VII.1 value for the (n,p) cross section as 2.34% of the (n,n’γ847) value. Figure 1 shows the energy-differential cross sections for the 56Fe(n,n’γ847) and the 56Fe(n,p)56Mn cross sections from ENDF/B-VII.1 [4] used. Taking this into account together with the natural-abundance of 56Fe (91.754% [6]) we obtain a spectrum-averaged, decay-corrected value of 468 mb for ⟨σ⟩.

Figure 1: Cross-section data [3] for the 56Fe(n,n’γ847) and 56Fe(n,p) reactions.

The data in the Atlas is valuable for many applications ranging from nuclear nonproliferation, active neutron-interrogation studies and benchmarking nuclear-reaction models in the fast-fission neutron range. However, its use is limited by the fact that the data was only available in printed form. The nuclear data group at the Lawrence Berkeley National Laboratory and UC Berkeley have compiled the data into a set of comma separated value ASCII tables and an SQLite (structured query language) database for dissemination to increase the accessibility for the international community. The complete package may be downloaded and contains the following:

• Scripts to compile the data into a SQLite database for both Python 2 and Python 3. The Makefile provided will test for the appropriate version and run with the necessary settings.
• A C-code to produce the extension-functions library allowing for mathematical functions such as sine, cosine, logarithm, square root etc. to be invoked with SQL methods. These are not standard functions with the SQLite3 engine. The Makefile will establish the correct OS-kernel name and compile the library accordingly.
• The complete set of CSV data files compiled for 76 different nuclei in the range 3 ≤ Z ≤ 92. This includes data sets from 76 natural samples and 29 isotopically-enriched samples (105 data sets in total).
• An example Jupyter Notebook illustrating methods for interacting with and visualizing the data.
• Several SQL scripts showing database-interaction examples. The OS-dependent library will be defined in SQL methods where appropriate during the make run phase of the installation.
• A PDF of the ATLAS [2] provided for reference.
• An HTML manual describing the software installation procedure and data-access methods. An overview of the ATLAS data is also provided in this documentation.

This first release of the Atlas database is designed to serve the needs of the application community. The energy levels, γ-ray energies and intensities are those stated in the Atlas itself and have not been reconciled to match those in the current version of the Evaluated Nuclear Structure Data File (ENSDF), although state-of-the-art structure information has been used to identify γ-ray doublets (reported as unresolved doublets for now). Our intention is to issue periodic revisions where the γ-ray and level-scheme information for specific nuclei has been updated to match modern values in ENSDF, and we encourage users of the atlas to assist in this process by sending updated csv data to the Berkeley group <LABernstein@lbl.gov> for more general dissemination to the user community.

In addition to its immediate use to the application community, the Atlas can serve as a valuable resource to both the nuclear structure and reactions evaluations communities. Since (n,n’) at fast-reactor neutron energies proceeds via both direct and compound processes it provides a nonselective insight into the properties of off-yrast levels over a far wider range of spins than thermal and epithermal neutron capture. This property of (n,n’) has made it an attractive area of research to many research groups in the international nuclear science community. This is exemplified by the plethora of papers on 56Fe(n,n’γ) by groups from Russia [7], Los Alamos [8, 9], GELINA [10], and Dresden [11].

Lastly, we hope that this SQLite database will provide a useful tool for the reactions evaluation community by providing easy access to angle-integrated data to aid in the benchmarking process needed for the validation of Evaluated Nuclear Data File libraries.

Acknowledgments

This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003180. This work is also supported in part by the Lawrence Berkeley National Laboratory under Contract No. DE-AC02-05CH11231 for the US Nuclear Data Program. We would like to thank a number of people who have contributed to the production of the Baghdad Atlas database including Dr. Andre Trkov from the IAEA for “saving” the Atlas, Dr. Rick Firestone from LBNL/UC Berkeley who first brought the Atlas to our attention, and Drs. Bradley Sleaford and Sean Walston for the original OCR scan of the document. We would also like to thank Ms. Ivana Abramovic from Eindhoven University for her early work with the Atlas, and to James Bevins, Leo Kirsch, Eric Matthews, and Andrew Voyles, all from UC Berkeley, for their assistance with testing procedures.

Copyright

This manuscript has been authored by an author at Lawrence Berkeley National Laboratory under Contract No. DE-AC02-05CH11231 with the U.S. Department of Energy. The U.S. Government retains, and the publisher, by accepting the article for publication, acknowledges, that the U.S. Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for U.S. Government purposes.

Disclaimer

This document was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor the Regents of the University of California, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof, or the Regents of the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof, or the Regents of the University of California.

References