133 research outputs found
A segmented total energy detector (sTED) for (n, Îł) cross section measurements at n_TOF EAR2
This work was supported in part by the I+D+i grant PGC2018-096717-B-C21 funded by MCIN/AEI/10.13039/501100011033 and by the European Commission H2020 Framework Programme project SANDA (Grant agreement ID: 847552).The neutron time-of-flight facility n_TOF is characterised by its high instantaneous neutron intensity, high resolution and broad neutron energy spectra, specially conceived for neutron-induced reaction cross section measurements. Two Time-Of-Flight (TOR) experimental areas are available at the facility: experimental area 1 (EAR1), located at the end of the 185 m horizontal flight path from the spallation target, and experimental area 2 (EAR2), placed at 20 m from the target in the vertical direction. The neutron fluence in EAR2 is similar to 300 times more intense than in EARL in the relevant time-of-flight window. EAR2 was designed to carry out challenging cross-section measurements with low mass samples (approximately 1 mg), reactions with small cross-sections or/and highly radioactive samples. The high instantaneous fluence of EAR2 results in high counting rates that challenge the existing capture systems. Therefore, the sTED detector has been designed to mitigate these effects. In 2021, a dedicated campaign was done validating the performance of the detector up to at least 300 keV neutron energy. After this campaign, the detector has been used to perform various capture cross section measurements at n_TOF EAR2.MCIN/AEI/10.13039/501100011033 I+D+i
PGC2018-096717-B-C21European Commission H2020 Framework Programme
SANDA 84755
Measurement of the 244Cm and 246Cm neutron-induced capture cross sections at the n_TOF facility
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 di culties 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 C6D6 detectors and also at Area 1 (EAR-1)
with the TAC. Preliminary results assessing the quality and limitations (background
subtraction, measurement technique and counting statistics) of this new
experimental datasets are presented and discussed
(n,cp) reactions study at the n_TOF facilitty at CERN: results for the cosmological lithium problem
The Big Bang Nucleosynthesis describes the production of the lightest nuclides from deuterium to Li at the early stages of the Universe. While a general good agreement is found for most of the isotopes involved in the synthesis, a serious discrepancy between the predicted abundance of 7Li and the related experimental observations is still present. This discrepancy has been referred since several decades as Cosmological Lithium Problem. In one last attempt to find nuclear solutions to this longstanding conundrum, the 7Be(n,alpha)4He and 7Be(n,p)7Li reactions, that affect predominantly the production of 7Li via the destruction of his parent nucleus 7Be, have been studied. Here we present the 7Be(n,a)4He and 7Be(n,p)7Li reaction crosssection measurements performed at the high-resolution n_TOF facility using the time-of-flight technique and high purity samples. The result of the experiments definitely rules out neutron induced reactions as a solution to the puzzle, thus indicating that explanations have to be sought out in other Physics scenarios.Postprint (published version
Neutron capture measurements with high efficiency detectors and the Pulse Height Weighting Technique
Neutron capture cross section measurements in time-of-flight facilities are usually performed by detecting the prompt Îł-rays emitted in the capture reactions. One of the difficulties to be addressed in these measurements is that the emitted Îł-rays may change with the neutron energy, and therefore also the detection efficiency. To deal with this situation, many measurements use the so called Total Energy Detection (TED) technique, usually in combination with the Pulse Height Weighting Technique (PHWT). With it, it is sought that the detection efficiency depends only on the total energy of the Îł-ray cascade, which does not vary much with the neutron energy. This technique was developed in the 1960s and has been used in many neutron capture experiments to date. One of the requirements of the technique is that Îł-ray detectors have a low efficiency. This has meant that the PHWT has been used with experimental setups with low detection efficiencies. However, this condition does not have to be fulfilled by the experimental system as a whole. The main goal of this work is to show that it is possible to measure with a high efficiency detection system that uses the PHWT, and how to analyze the measured data.This work was supported in part by the I+D+i grant PGC2018-
096717-B-C21 funded by MCIN/AEI/10.13039/501100011033 and by
the European Commission H2020 Framework Programme project
SANDA (Grant agreement ID: 847552)
Measurement of the 154Gd(n,?) cross section and its astrophysical implications
The neutron capture cross section of 154Gd was measured from 1 eV to 300 keV in the experimental area located 185 m from the CERN n_TOF neutron spallation source, using a metallic sample of gadolinium, enriched to 67% in 154Gd. The capture measurement, performed with four C6D6 scintillation detectors, has been complemented by a transmission measurement performed at the GELINA time-of-flight facility (JRC-Geel), thus minimising the uncertainty related to sample composition. An accurate Maxwellian averaged capture cross section (MACS) was deduced over the temperature range of interest for s process nucleosynthesis modelling. We report a value of 880(50) mb for the MACS at keV, significantly lower compared to values available in literature. The new adopted 154Gd(n,Îł) cross section reduces the discrepancy between observed and calculated solar s-only isotopic abundances predicted by s-process nucleosynthesis models.Funded by SCOAP
Measurement of the 154Gd(n,Îł) cross section and its astrophysical implications
The neutron capture cross section of 154Gd was measured from 1 eV to 300 keV in the experimental area
located 185 m from the CERN n_TOF neutron spallation source, using a metallic sample of gadolinium,
enriched to 67% in 154Gd. The capture measurement, performed with four C6D6 scintillation detectors,
has been complemented by a transmission measurement performed at the GELINA time-of-flight facility
(JRC-Geel), thus minimising the uncertainty related to sample composition. An accurate Maxwellian
averaged capture cross section (MACS) was deduced over the temperature range of interest for s process
nucleosynthesis modelling. We report a value of 880(50) mb for the MACS at kT = 30 keV, significantly
lower compared to values available in literature. The new adopted 154Gd(n,Îł ) cross section reduces the
discrepancy between observed and calculated solar s-only isotopic abundances predicted by s-process
nucleosynthesis models
Recent highlights and prospects on (n,) measurements at the CERN n_TOF facility
Neutron capture cross-section measurements are fundamental in the study of
the slow neutron capture (s-) process of nucleosynthesis and for the
development of innovative nuclear technologies. One of the best suited methods
to measure radiative neutron capture (n,) cross sections over the full
stellar range of interest for all the applications is the time-of-flight (TOF)
technique. Overcoming the current experimental limitations for TOF
measurements, in particular on low mass unstable samples, requires the
combination of facilities with high instantaneous flux, such as the CERN n_TOF
facility, with detection systems with an enhanced detection sensitivity and
high counting rate capabilities. This contribution presents a summary about the
recent highlights in the field of (n,) measurements at n_TOF. The
recent upgrades in the facility and in new detector concepts for (n,\g)
measurements are described. Last, an overview is given on the existing
limitations and prospects for TOF measurements involving unstable targets and
the outlook for activation measurements at the brand new high-flux n_TOF-NEAR
station.Comment: 7 pages, 5 figures (8 panels). Proceedings of the CGS-17 conference.
To be published in EPJ Web of Conference
Constraints on the dipole photon strength for the odd uranium isotopes
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 Îł-ray spectra from neutron-capture reactions on the U, U, and 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
Measurement of the N(n,p)C cross section at the CERN n_TOF facility from sub-thermal energy to 800 keV
Background: The N(n,p)C reaction is of interest in neutron
capture therapy, where nitrogen-related dose is the main component due to
low-energy neutrons, and in astrophysics, where 14N acts as a neutron poison in
the s-process. Several discrepancies remain between the existing data obtained
in partial energy ranges: thermal energy, keV region and resonance region.
Purpose: Measuring the 14N(n,p)14C cross section from thermal to the resonance
region in a single measurement for the first time, including characterization
of the first resonances, and providing calculations of Maxwellian averaged
cross sections (MACS). Method: Time-of-flight technique. Experimental Area 2
(EAR-2) of the neutron time-of-flight (n_TOF) facility at CERN.
B(n,)Li and U(n,f) reactions as references. Two
detection systems running simultaneously, one on-beam and another off-beam.
Description of the resonances with the R-matrix code sammy. Results: The cross
section has been measured from sub-thermal energy to 800 keV resolving the two
first resonances (at 492.7 and 644 keV). A thermal cross-section
(1.8090.045 b) lower than the two most recent measurements by slightly
more than one standard deviation, but in line with the ENDF/B-VIII.0 and
JEFF-3.3 evaluations has been obtained. A 1/v energy dependence of the cross
section has been confirmed up to tens of keV neutron energy. The low energy
tail of the first resonance at 492.7 keV is lower than suggested by evaluated
values, while the overall resonance strength agrees with evaluations.
Conclusions: Our measurement has allowed to determine the N(n,p)
cross-section over a wide energy range for the first time. We have obtained
cross-sections with high accuracy (2.5 %) from sub-thermal energy to 800 keV
and used these data to calculate the MACS for kT = 5 to kT = 100 keV.Comment: 18 pages, 15 figures, 4 table
Neutron capture cross section measurements of Am-241 at the n_TOF facility
Neutron capture on Am-241 plays an important role in the nuclear energy production and also provides valuable information for the improvement of nuclear models and the statistical interpretation of the nuclear properties. A new experiment to measure the Am-241(n,gamma) cross section in the thermal region and the first few resonances below 10 eV has been carried out at EAR2 of the n_TOF facility at CERN. Three neutron-insensitive C6D6 detectors have been used to measure the neutron-capture gamma cascade as a function of the neutron time of flight, and then deduce the neutron capture yield. Preliminary results will be presented and compared with previously obtained results at the same facility in EAR1. In EAR1 the gamma-ray background at thermal energies was about 90% of the signal while in EAR2 is up to a 25 factor much more favorable signal to noise ratio. We also extended the low energy limit down to subthermal energies. This measurement will allow a comparison with neutron capture measurements conducted at reactors and using a different experimental technique
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