Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms
Activation cross-section measurement of fast neutron-induced reactions in Al, Au, Bi, Co, F, Na, and Y
Introduction
The optimization of contemporary nuclear systems and the further development of advanced ones require precise knowledge of excitation functions of neutron-induced reactions. Even though numerous cross-section measurements have been carried out over the last eight decades, there is still a lack of experimental data for many important nuclear reactions. In the energy range of tens of MeV and higher, there is a deficiency in experimentally determined cross sections for almost all reactions. The experiments focused on fast neutron-induced reaction cross-section determination were carried out on the accelerator-driven fast neutron source at the Nuclear Physics Institute of the Czech Academy of Sciences in Řež (NPI CAS). Samples of Al, Au, Bi, Co, NaF, and Y were irradiated. These materials are widely used as neutron monitors in fission, fusion, and spallation devices. Many of the studied reactions are also mentioned in the High Priority Nuclear Data Request List (HPRL) [1], which is a compilation of the highest priority nuclear data requirements maintained by the Nuclear Energy Agency under the Organisation for Economic Co-operation and Development (NEA OECD). From a methodological point of view, we are also focused on determining the cross sections of the production of isomeric states and radionuclides, which are pure positron emitters without gamma decay. Determining them has its specific challenges and there is usually a lack of measured data for such reactions.
Section snippets
Experimental set-up
The quasi-monoenergetic neutron generator at the NPI CAS is based on the p+Li(C) reaction. Protons are accelerated by the isochronous cyclotron U-120M to the energy of 20–35 MeV and directed to the 2 mm Li target. Protons that passed through Li foil are stopped in a 10 mm thick carbon beam stop. The detailed description of the NPI CAS experimental facility is presented in Ref. [2].
The irradiated samples were in the form of high purity thin foils with a dimension of . The thickness of the
Neutron source spectrometry
The neutron source used within the presented experiments is based on the reaction (threshold 1880.36(6) keV [3]). The neutron spectra of the p+Li(C) source consist of monoenergetic peak and low-energy continuum. The monoenergetic peak corresponds to the production of the ground and first excited states of . The width of the monoenergetic peak corresponds to proton stopping energy loss in 2 mm Li and is approximately 2 MeV, but the energy difference between ground and the first
Gamma-ray spectrometry and yield determination
The reaction yield, i.e. the number of studied radionuclides created in the irradiated foil during the whole irradiation, was determined by means of gamma-ray spectrometry. Standard equation and spectrometric corrections were used [11], [12], [13]: where is the gamma peak area, is the self-absorption correction, is the beam instability correction, is the gamma emission probability, is
Analysis of pure positron emitter
A pure positron emitter was studied using gamma-ray spectrometry within this work. It was produced in the reaction . The only response of the HPGe detector to the presence of is the increase in the annihilation peak area. When a studied sample contains only one pure positron emitter, the annihilation peak area corresponds only to the amount of this emitter. Detailed MCNPX model of the Canberra GC3018 was used to calculate detector efficiency [12]. For the purpose of MCNPX
Decay cascade
Some of the studied reactions led to the production of both the ground state (labeled ) and the metastable excited state (labeled ) of the observed nuclide. The deexcitation of the metastable state increases the number of ground state nuclei. The situation after the end of irradiation can be described by the following equations: where is the number of nuclei in time and represents decay constant. The solutions of Eqs. (3), (4) with initial
Neutron background
Some of the measured nuclear reactions, especially those with low threshold energies, might be strongly affected by the low-energy neutron continuum. One MCNPX calculated neutron spectrum used in this work is depicted in Fig. 7 along with the excitation functions of selected nuclear reactions calculated utilizing TALYS 1.95 [19] code. It is obvious that the low energy neutron continuum has a different impact on the yield of each reaction depending on the reaction threshold energy and the shape
Cross-section determination
Cross-section determination using the activation method requires experimental data from neutron source spectrometry, irradiated samples gamma-ray spectrometry and samples dimension and weight measurements: where is the mean value of reaction yield, is the low energy neutron continuum correction, is the irradiated foil area, is the molar weight, is the number of neutrons in peak, is the Avogadro’s number, and is the mass of the irradiated foil. The
Uncertainty budget
A detailed quality assurance analysis was performed in order to reasonably determine the uncertainties of the final results. The uncertainties are divided into several categories according to various parts of the measurement.
Results
For twenty nuclear reactions, new data were obtained. The experimental results are compared with the available data from both experimental and evaluated nuclear data libraries and model calculations using the TALYS 1.95 code. This comparison is provided in Fig. 9, Fig. 10, Fig. 11, Fig. 12, Fig. 13, Fig. 14, Fig. 15, Fig. 16, Fig. 17, Fig. 18, Fig. 19, Fig. 20, Fig. 21, Fig. 22, Fig. 23, Fig. 24, Fig. 25, Fig. 26, Fig. 27, Fig. 28. The numerical values of all the obtained cross sections are
Discussion and conclusions
An accelerator-driven neutron source based on p+Li(C) reaction at NPI CAS was used to irradiate Al, Au, Bi, Co, NaF, and Y samples. Cross sections of several neutron reactions were determined for energy of 17.5(10), 19.8(9) MeV and 27.5(7) MeV. The obtained cross sections were compared with available data from both experimental and evaluated nuclear data libraries. The new data are in good agreement with data in the EXFOR library. For several studied reactions, there are no or very few previous
CRediT authorship contribution statement
Jiří Jarošík: Conceptualization, Methodology, Investigation, Data curation, Validation, Writing – original draft, Gamma spectroscopic measurements and analysis, Cross-section determination. Vladimír Wagner: Supervision, Investigation, Writing – review & editing, Conceptual design of the work, Gamma spectroscopic measurements and analysis. Mitja Majerle: Resources, Data curation, Ensure irradiation at the neutron source and its error-free course, To obtain and provide all data necessary for the
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
We would like to express our thanks to the cyclotron staff at the NPI CAS for perfect cooperation. The irradiation experiments in the neutron field of accelerator-driven fast neutron source carried out on the CANAM infrastructure of the NPI CAS Řež were supported through the MŠMT project no. LM2015056. Computational resources were supplied by the project “e-Infrastruktura CZ ” (e-INFRA LM2018140) within the program Projects of Large Research, Development and Innovations Infrastructures. We are
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