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Measurement of the 244Cm and 246Cm neutron-induced capture cross sections at the n_TOF facility
(2019)
The neutron capture reactions of the 244Cm and 246Cm isotopes open the path for the formation of heavier Cm isotopes and heavier elements such as Bk and Cf in a nuclear reactor. In addition, both isotopes belong to the minor actinides with a large contribution to the decay heat and to the neutron emission in irradiated fuels. There are only two previous 244Cm and 246Cm capture cross section measurements: one in 1969 using a nuclear explosion [1] and the most recent data measured at J-PARC in 2010 [2]. The data for both isotopes are very scarce due to the difficulties in performing the measurements: high intrinsic activity of the samples and limited facilities capable of providing isotopically enriched samples.
We have measured both neutron capture cross sections at the n_TOF Experimental Area 2 (EAR-2) with three C6 D6 detectors and also at Area 1 (EAR-1) with the TAC. Preliminary results assessing the quality and limitations (back-ground subtraction, measurement technique and counting statistics) of this new experimental datasets are presented and discussed.
Study of the photon strength functions and level density in the gamma decay of the n + 234U reaction
(2019)
The accurate calculations of neutron-induced reaction cross sections are relevant for many nuclear applications. The photon strength functions and nuclear level densities are essential inputs for such calculations. These quantities for 235U are studied using the measurement of the gamma de-excitation cascades in radiative capture on 234U with the Total Absorption Calorimeter at n_TOF at CERN. This segmented 4π gamma calorimeter is designed to detect gamma rays emitted from the nucleus with high efficiency. This experiment provides information on gamma multiplicity and gamma spectra that can be compared with numerical simulations. The code DICEBOXC is used to simulate the gamma cascades while GEANT4 is used for the simulation of the interaction of these gammas with the TAC materials. Available models and their parameters are being tested using the present data. Some preliminary results of this ongoing study are presented and discussed.
i-TED is an innovative detection system which exploits Compton imaging techniques to achieve a superior signal-to-background ratio in (n,γ) cross-section measurements using time-of-flight technique. This work presents the first experimental validation of the i-TED apparatus for high-resolution time-of-flight experiments and demonstrates for the first time the concept proposed for background rejection. To this aim both 197Au(n,γ) and 56Fe(n,γ) reactions were measured at CERN n\_TOF using an i-TED demonstrator based on only three position-sensitive detectors. Two \cds detectors were also used to benchmark the performance of i-TED. The i-TED prototype built for this study shows a factor of ∼3 higher detection sensitivity than state-of-the-art \cds detectors in the ∼10~keV neutron energy range of astrophysical interest. This paper explores also the perspectives of further enhancement in performance attainable with the final i-TED array consisting of twenty position-sensitive detectors and new analysis methodologies based on Machine-Learning techniques.
The idea of slow-neutron capture nucleosynthesis formulated in 1957 triggered a tremendous experimental effort in different laboratories worldwide to measure the relevant nuclear physics input quantities, namely (n,γ) cross sections over the stellar temperature range (from few eV up to several hundred keV) for most of the isotopes involved from Fe up to Bi. A brief historical review focused on total energy detectors will be presented to illustrate how, advances in instrumentation have led, over the years, to the assessment and discovery of many new aspects of s-process nucleosynthesis and to the progressive refinement of theoretical models of stellar evolution. A summary will be presented on current efforts to develop new detection concepts, such as the Total-Energy Detector with γ-ray imaging capability (i-TED). The latter is based on the simultaneous combination of Compton imaging with neutron time-of-flight (TOF) techniques, in order to achieve a superior level of sensitivity and selectivity in the measurement of stellar neutron capture rates.
Fission program at n_TOF
(2019)
Since its start in 2001 the n_TOF collaboration developed a measurement program on fission, in view of advanced fuels in new generation reactors. A special effort was made on measurement of cross sections of actinides, exploiting the peculiarity of the n_TOF neutron beam which spans a huge energy domain, from the thermal region up to GeV. Moreover fission fragment angular distributions have also been measured. An overview of the cross section results achieved with different detectors is presented, including a discussion of the 237Np case where discrepancies showed up between different detector systems. The results on the anisotropy of the fission fragments and its implication on the mechanism of neutron absorption, and in applications, are also shown.
Although the 12C(n,p)12B and 12C(n,d)11B reactions are of interest in several fields of basic and applied Nuclear Physics the present knowledge of these two cross-sections is far from being accurate and reliable, with both evaluations and data showing sizable discrepancies. As part of the challenging n_TOF program on (n,cp) nuclear reactions study, the energy differential cross-sections of the 12C(n,p)12B and 12C(n,d)11 B reactions have been measured at CERN from the reaction thresholds up to 30 MeV neutron energy. Both measurements have been recently performed at the long flight-path (185 m) experimental area of the n_TOF facility at CERN using a pure (99.95%) rigid graphite target and two silicon telescopes. In this paper an overview of the experiment is presented together with a few preliminary results.
Background: The photon strength functions (PSFs) and nuclear level density (NLD) are key ingredients for calculation of the photon interaction with nuclei, in particular the reaction cross sections. These cross sections are important especially in nuclear astrophysics and in the development of advanced nuclear technologies.
Purpose: The role of the scissors mode in the M1 PSF of (well-deformed) actinides was investigated by several experimental techniques. The analyses of different experiments result in significant differences, especially on the strength of the mode. The shape of the low-energy tail of the giant electric dipole resonance is uncertain as well. In particular, some works proposed a presence of the E1 pygmy resonance just above 7 MeV. Because of these inconsistencies additional information on PSFs in this region is of great interest.
Methods: The γ-ray spectra from neutron-capture reactions on the 234U, 236 U, and 238 U nuclei have been measured with the total absorption calorimeter of the n_TOF facility at CERN. The background-corrected sum-energy and multi-step-cascade spectra were extracted for several isolated s-wave resonances up to about 140 eV.
Results: The experimental spectra were compared to statistical model predictions coming from a large selection of models of photon strength functions and nuclear level density. No combination of PSF and NLD models from literature is able to globally describe our spectra. After extensive search we were able to find model combinations with modified generalized Lorentzian (MGLO) E1 PSF, which match the experimental spectra as well as the total radiative widths.
Conclusions: The constant temperature energy dependence is favored for a NLD. The tail of giant electric dipole resonance is well described by the MGLO model of the E1 PSF with no hint of pygmy resonance. The M1 PSF must contain a very strong, relatively wide, and likely double-resonance scissors mode. The mode is responsible for about a half of the total radiative width of neutron resonances and significantly affects the radiative cross section.
Presolar grains and their isotopic compositions provide valuable constraints to AGB star nucleosynthesis. However, there is a sample of O- and Al-rich dust, known as group 2 oxide grains, whose origin is difficult to address. On the one hand, the 17O/16O isotopic ratios shown by those grains are similar to the ones observed in low-mass red giant stars. On the other hand, their large 18O depletion and 26Al enrichment are challenging to account for. Two different classes of AGB stars have been proposed as progenitors of this kind of stellar dust: intermediate mass AGBs with hot bottom burning, or low mass AGBs where deep mixing is at play. Our models of low-mass AGB stars with a bottom-up deep mixing are shown to be likely progenitors of group 2 grains, reproducing together the 17O/16O, 18O/16O and 26Al/27Al values found in those grains and being less sensitive to nuclear physics inputs than our intermediate-mass models with hot bottom burning.
Asymptotic giant branch (AGB) stars are thought to be among the most important sources of fluorine in our Galaxy. While observations and theory agree at close-to-solar metallicity, stellar models overestimate fluorine production in comparison to heavy elements at lower metallicities. We present predictions for 19F abundance for a set of AGB models with various masses and metallicities, in which magnetic buoyancy induces the formation of the 13C neutron source (the so-called 13C pocket). In our new models, fluorine is mostly created as a consequence of secondary 14N nucleosynthesis during convective thermal pulses, with a minor contribution from the 14N existing in the 13C pocket zone. As a result, AGB stellar models with magnetic-buoyancyinduced mixing show low 19F surface abundances which agree with fluorine spectroscopic observations at both low and near-solar metallicity.
s-processing in asymptotic giant branch stars in the light of revised neutron-capture cross sections
(2021)
Current AGB stellar models provide an adequate description of the s-process nucleosynthesis that occurs. Nonetheless, they still suffer from many uncertainties related to the modeling of the 13C pocket formation and the adopted nuclear reaction rates. For many important s-process isotopes, a best set of neutron-capture cross sections was recently re-evaluated. Using stellar models prescribing that the 13C pocket is a by-product of magnetic-buoyancy-induced mixing phenomena, s-process calculations were carried out with this database. Significant effects are found for a few s-only and branching point isotopes, pointing out the need for improved neutron-capture cross section measurements at low energy.