arXiv subjects

V. Guadilla

Publications and source records attributed to V. Guadilla.

At least 19 recordsLinked to original sources

arXiv2026

Study of $\boldsymbol{\beta}$ Decay Shape Factors in First-Forbidden Transitions with $\boldsymbol{\Delta I^\pi = 0^-}$ for Reactor Antineutrino Spectra Predictions

The electron spectra of the $\beta$ decays of $^{92}$Rb and $^{142}$Cs, key contributors to the reactor antineutrino spectrum, were measured at the IGISOL facility using radioactive beams of high isotopic purity. The shapes of the measured $\beta$ spectra were compared with various $\beta$ shape models, including first-forbidden correction factors for $\Delta I^\pi = 0^-$ ground-state to ground-state transitions. Comparisons with previous experimental results are also provided. The shapes of the newly measured $\beta$ spectra are well reproduced employing feedings extracted from total absorption gamma spectroscopy measurements.

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arXiv2025

Data sorting modes of phoswich detector array

The different data-sorting modes of the phoswich detector array PARIS used for detecting high-energy (4$-$10 MeV) $\gamma$ rays are investigated. The characteristics including time resolution, energy resolution and detection efficiency under various modes are studied. The present study shows that PARIS has capabilities of rejecting escape and pileup events when used for decay spectroscopy. Notably, the methods presented in this work refer specifically to the $\beta$-decay experiment of $^{80g+m}$Ga conducted with three PARIS clusters comprising 27 phoswich detectors, rather than to a general report on the PARIS array or its overall performance for in-beam spectroscopy. Compared with the 2"$\times$2"$\times$2" LaBr$_3$(Ce) detector (Ciema{\l}a et al., 2009), even in individual mode, PARIS provides significant suppression of single- and double-escape peaks and reduces background via vetoing function of the outer-volume NaI(Tl) crystals. In contrast to the common approach of adding back the energies in LaBr$_3$(Ce) and NaI(Tl) to increase the detection efficiency of the full-energy peak, using NaI(Tl) as a veto shield provides a superior trade-off for applications where spectral purity is essential. Employing add-back analysis within each cluster of nine phoswiches or between all phoswiches could enhance full-energy peak efficiency and further suppress escape peaks and background. Applying a multiplicity condition provides a further suppression but simultaneously lowers the statistics of full-energy peaks.

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arXiv2025

The beta decay of Tz=-2 64Se and its descendants: the T=2 isobaric multiplet

In this paper we present our results on the decay of 64Se. It is the heaviest Tz=-2 nucleus that both beta decays and has a stable mirror partner Tz=+2, thus allowing comparison with charge exchange reaction studies. The beta decays of 64Se and its descendants were studied at the RIKEN Nishina Center (Tokyo, Japan) following their production in the fragmentation of 78Kr on a beryllium target. Beta-delayed gamma-ray and particle radiation was identified for each of the nuclei in the decay chain allowing us to obtain decay schemes for 64Se, 64As, and 63Ge. Thus new excited states could be found for the descendant nuclei, including the interesting case of the N=Z nucleus 64Ge. Furthermore we observed for the first time the beta-delayed proton emission of 64Se and 64As. Based on these results we obtained proton branching ratios of 48.0(9)% in 64Se decay and 4.4(1)% in 64As decay. We obtained a half-life value of 22.5(6) ms for 64Se decay and half-lives slightly more precise than those in the literature for each nucleus involved in the decay chain. Using our results on the excited levels of 64As and the mass excess in the literature for 63Ge we obtained -39588(50) keV for the mass excess of 64As. Then based on the IMME we obtained the mass excess of -27429(88) keV for 64Se by extrapolation. The mirror process of 64Se beta decay, the charge exchange reaction 64Zn(3He,t)64Ga, has already been measured allowing us to study the mirror symmetry through the comparison of the weak force (beta decay) and strong force (charge exchange reaction). An interpretation of the decay schemes based on the idea of the Anti Analogue State is proposed.

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arXiv2025

Decay study of the most neutron-deficient Zn isotopes with the Warsaw Optical TPC detector

Results of decay studies of nuclei in the vicinity of 54Zn, which is the most neutron-deficient isotope of zinc and undergoes ground-state two-proton radioactivity (2p), are presented. The measurements were performed with a gaseous time projection chamber with optical readout which allowed us to record tracks of protons emitted in the decays. A new method of data analysis was used to reconstruct energies and emission angles of low-energy protons that were stopped within the active volume of the chamber. Half-lives and branching ratios for \b{eta}-delayed proton emission channels were determined for 56Zn, 55Zn, and 55Cu. The \b{eta}-delayed emission of two protons for 55Zn was observed for the first time. Five events of 2p radioactivity of 54Zn were detected and reconstructed. The distribution of the opening angle between momenta of the two protons is consistent with the findings published in [Ascher et al. PRL 107, 102502 (2011)]. The combination of all results suggests a flat angular distribution, in contrast to the one measured for 45Fe.

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arXiv2025

Study of beta spectrum shapes relevant to the prediction of reactor antineutrino spectra

The shapes of the beta spectra of 92Rb and 142Cs, two of the beta decays most relevant for the prediction of the antineutrino spectrum in reactors, have been measured. A new setup composed of two dE-E telescopes has been used. High purity radioactive beams of the isotopes of interest were provided by the IGISOL facility using the JYFLTRAP double Penning trap. The resulting beta spectra have been compared with model predictions using beta decay feedings from total absorption gamma spectroscopy measurements and shape corrections employed in the calculation of the antineutrino spectrum, validating both further. The procedure can be extended to other relevant nuclei in the future, providing solid ground for the prediction of the antineutrino spectrum in reactors.

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arXiv2025

$\beta$-delayed spectroscopy of $^{80}$Ge$_{48}$ and competition between Gamow-Teller and first-forbidden transitions in $^{80g+m}$Ga$_{49}$ $\beta$ decay

The $\beta$-delayed spectroscopy of $^{80}$Ge has been studied using sources of ground and low-lying isomeric states of $^{80}$Ga. A hybrid $\gamma$-ray spectrometer was used, composed of high-purity germanium (HPGe) detectors for low-energy $\gamma$-ray detection, phoswich detectors from the PARIS array for high-energy $\gamma$ rays and a plastic detector for $\beta$ tagging. The new decay level schemes are presented, with 13 and 14 states $\beta$ populated by $^{80g}$Ga and $^{80m}$Ga, respectively, being reported for the first time. We quantitatively compare summed intensities of first-forbidden and previously reported Gamow-Teller $\beta$ transitions [R. Li et al., \href{https://doi.org/10.1103/PhysRevC.111.034303}{Phys. Rev. C 111, 034303 (2025)}]. The upper-limit fractions of first-forbidden transitions contributing to the decays of $^{80g}$Ga and $^{80m}$Ga are (48.0 $\pm$ 2.7)$\%$ and (47.9 $\pm$ 3.3)$\%$ of the observed total $\beta$ transition intensities of (78.2 $\pm$ 2.5)$\%$ and (82.2 $\pm$ 3.7)$\%$, respectively. Notably, the half-lives decrease in accordance with the upper limits of (48.0 $\pm$ 6.0)$\%$ [3.67(20) s $\rightarrow$ 1.91(3) s] and (47.9 $\pm$ 7.2)$\%$ [3.01(19) s $\rightarrow$ 1.57(1) s] for $^{80g}$Ga and $^{80m}$Ga, respectively, when including first-forbidden transitions, in contrast to those by Gamow-Teller transitions only.

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arXiv2024

Isospin breaking in the $^{71}$Kr and $^{71}$Br mirror system

Isospin symmetry is a fundamental concept in nuclear physics. Even though isospin symmetry is partially broken, it holds approximately for most nuclear systems, which makes exceptions very interesting from the nuclear structure perspective. In this framework, it is expected that the spins and parities of the ground states of mirror nuclei should be the same, in particular for the simplest systems where a proton is exchanged with a neutron or vice versa. In this work, we present evidence that this assumption is broken in the mirror pair $^{71}$Br and $^{71}$Kr system. Our conclusions are based on a high-statistics $\beta$ decay study of $^{71}$Kr and on state-of-the-art shell model calculations. In our work, we also found evidence of a new state in $^{70}$Se, populated in the $\beta$-delayed proton emission process which can be interpreted as the long sought coexisting 0$^+$ state.

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arXiv2024

Decay study of $^{11}$Be with an Optical TPC detector

The $β$ decay of one-neutron halo nucleus $^{11}$Be was investigated using the Warsaw Optical Time Projection Chamber (OTPC) detector to measure $β$-delayed charged particles. The results of two experiments are reported. In the first one, carried out in LNS Catania, the absolute branching ratio for $β$-delayed $α$ emission was measured by counting incoming $^{11}$Be ions stopped in the detector and the observed decays with the emission of $α$ particle. The result of 3.27(46)\% is in good agreement with the literature value. In the second experiment, performed at the HIE-ISOLDE facility at CERN, bunches containing several hundreds of $^{11}$Be ions were implanted into the OTPC detector followed by the detection of decays with the emission of charged particles. The energy spectrum of $β$-delayed $α$ particles was determined in the full energy range. It was analysed in the R-matrix framework and was found to be consistent with the literature. The best description of the spectrum was obtained assuming that the two $3/2^+$ and one $1/2^+$ states in $^{11}$B are involved in the transition. The search for $β$-delayed emission of protons was undertaken. Only the upper limit for the branching ratio for this process of $(2.2 \pm 0.6_{\rm stat} \pm 0.6_{\rm sys}) \times 10^{-6}$ could be determined. This value is in conflict with the result published in [Ayyad et al. Phys. Rev. Lett. 123, 082501 (2019)] but does agree with the limit reported in [Riisager et al., Eur. Phys. J. A (2020) 56:100]

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arXiv2024

Simultaneous impacts of nuclear shell structure and collectivity on $\beta$ decay: Evidence from $^{80}$Ga$_{49}$

The Gamow-Teller strength distribution covering the entire $\beta$-decay window, up to 10.312(4) MeV, of $^{80g+m}$Ga was measured for the first time in photo fission of UC$_x$ induced by a 50 MeV electron beam. The new data show significant enhancement in the high-energy region with a jump structure. Simultaneously, the $\gamma$ deexciting behavior of $\beta$-populated states presents a competition between deexcitation to 2$_1^+$ [$\beta_2$ = 0.155(9)] and to 2$_2^+$ [$\beta_2$ = 0.053$_{0.009}^{0.008}$)] in $^{80}$Ge. To understand these data, we performed a realistic shell-model calculation and systematic analysis of log $\it{ft}$ ratios between precursors' $\beta$ decay to 2$_2^+$ and to 2$_1^+$ of Ga isotopes. We conclude that these phenomena evidence simultaneous impacts of nuclear shell structure and collectivity on $\it{B}$(GT) distribution and therefore the half-life of the precursor.

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arXiv2023

First measurements with a new $β$-electron detector for spectral shape studies

The shape of the electron spectrum emitted in $β$ decay carries a wealth of information about nuclear structure and fundamental physics. In spite of that, few dedicated measurements have been made of $β$-spectrum shapes. In this work we present a newly developed detector for $β$ electrons based on a telescope concept. A thick plastic scintillator is employed in coincidence with a thin silicon detector. The first measurements employing this detector have been carried out with mono-energetic electrons from the high-energy resolution electron-beam spectrometer at Bordeaux. Here we report on the good reproduction of the experimental spectra of mono-energetic electrons using Monte Carlo simulations. This is a crucial step for future experiments, where a detailed Monte Carlo characterization of the detector is needed to determine the shape of the $β$-electron spectra by deconvolution of the measured spectra with the response function of the detector. A chamber to contain two telescope assemblies has been designed for future $β$-decay experiments at the Ion Guide Isotope Separator On-Line facility in Jyväskylä, aimed at improving our understanding of reactor antineutrino spectra.

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arXiv2022

Total absorption $γ$-ray spectroscopy of the $β$ decays of $^{96\text{gs,m}}$Y

The $β$ decays of the ground state (gs) and isomeric state (m) of $^{96}$Y have been studied with the total absorption $γ$-ray spectroscopy technique at the Ion Guide Isotope Separator On-Line facility. The separation of the 8$^{+}$ isomeric state from the 0$^{-}$ ground state was achieved thanks to the purification capabilities of the JYFLTRAP double Penning trap system. The $β$-intensity distributions of both decays have been independently determined. In the analyses the de-excitation of the 1581.6 keV level in $^{96}$Zr, in which conversion electron emission competes with pair production, has been carefully considered and found to have significant impact on the $β$-detector efficiency, influencing the $β$-intensity distribution obtained. Our results for $^{96\text{gs}}$Y (0$^+$) confirm the large ground state to ground state $β$-intensity probability, although a slightly larger value than reported in previous studies was obtained, amounting to $96.6_{-2.1}^{+0.3}\%$ of the total $β$ intensity. Given that the decay of $^{96\text{gs}}$Y is the second most important contributor to the reactor antineutrino spectrum between 5 and 7 MeV, the impact of the present results on reactor antineutrino summation calculations has been evaluated. In the decay of $^{96\text{m}}$Y (8$^{+}$), previously undetected $β$ intensity in transitions to states above 6 MeV has been observed. This shows the importance of total absorption $γ$-ray spectroscopy measurements of $β$ decays with highly fragmented de-excitation patterns. $^{96\text{m}}$Y (8$^{+}$) is a major contributor to reactor decay heat in uranium-plutonium and thorium-uranium fuels around 10 s after fission pulses, and the newly measured average $β$ and $γ$ energies differ significantly from the previous values in evaluated databases (...)

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arXiv2022

Beta decay of the very neutron-deficient $^{60}$Ge and $^{62}$Ge nuclei

We report here the results of a study of the $β$ decay of the proton-rich Ge isotopes, $^{60}$Ge and $^{62}$Ge, produced in an experiment at the RIKEN Nishina Center. We have improved our knowledge of the half-lives of $^{62}$Ge (73.5(1) ms), $^{60}$Ge (25.0(3) ms) and its daughter nucleus, $^{60}$Ga (69.4(2) ms). We measured individual $β$-delayed proton and $γ$ emissions and their related branching ratios. Decay schemes and absolute Fermi and Gamow-Teller transition strengths have been determined. The mass excesses of the nuclei under study have been deduced. A total $β$-delayed proton-emission branching ratio of 67(3)% has been obtained for $^{60}$Ge. New information has been obtained on the energy levels populated in $^{60}$Ga and on the 1/2$^-$ excited state in the $βp$ daughter $^{59}$Zn. We extracted a ground state to ground state feeding of 85.3(3)% for the decay of $^{62}$Ge. Eight new $γ$ lines have been added to the de-excitation of levels populated in the $^{62}$Ga daughter.

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arXiv2020

On the $β$-decay of $^{186}$Hg

The Gamow-Teller strength distribution of the decay of $^{186}$Hg into $^{186}$Au has been determined for the first time using the total absorption gamma spectroscopy technique and has been compared with theoretical QRPA calculations using the SLy4 Skyrme force. The measured Gamow-Teller strength distribution and the half-life are described by mixing oblate and prolate configurations independently in the parent and daughter nuclei. The best description of the experimental beta strength is obtained with dominantly prolate components for both parent $^{186}$Hg and daughter $^{186}$Au. The approach also allowed us to determine an upper limit of the oblate component in the parent state. The complexity of the analysis required the development of a new approach in the analysis of the X-ray gated total absorption spectrum.

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arXiv2020

Determination of Beta Decay Ground State Feeding of Nuclei of Importance for Reactor Applications

In $β$-decay studies the determination of the decay probability to the ground state of the daughter nucleus often suffers from large systematic errors. The difficulty of the measurement is related to the absence of associated delayed $γ$-ray emission. In this work we revisit the $4πγ-β$ method proposed by Greenwood and collaborators in the 1990s, which has the potential to overcome some of the experimental difficulties. Our interest is driven by the need to determine accurately the $β$-intensity distributions of fission products that contribute significantly to the reactor decay heat and to the antineutrinos emitted by reactors. A number of such decays have large ground state branches. The method is relevant for nuclear structure studies as well. Pertinent formulae are revised and extended to the special case of $β$-delayed neutron emitters, and the robustness of the method is demonstrated with synthetic data. We apply it to a number of measured decays that serve as test cases and discuss the features of the method. Finally, we obtain ground state feeding intensities with reduced uncertainty for four relevant decays that will allow future improvements in antineutrino spectrum and decay heat calculations using the summation method.

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arXiv2019

Total absorption $γ$-ray spectroscopy of the $β$-delayed neutron emitters $^{137}$I and $^{95}$Rb

The decays of the $β$-delayed neutron emitters $^{137}$I and $^{95}$Rb have been studied with the total absorption $γ$-ray spectroscopy technique. The purity of the beams provided by the JYFLTRAP Penning trap at the ion guide isotope separator on-line facility in Jyväskylä allowed us to carry out a campaign of isotopically pure measurements with the decay total absorption $γ$-ray spectrometer, a segmented detector composed of eighteen NaI(Tl) modules. The contamination coming from the interaction of neutrons with the spectrometer has been carefully studied, and we have tested the use of time differences between prompt $γ$-rays and delayed neutron interactions to eliminate this source of contamination. Due to the sensitivity of our spectrometer, we have found a significant amount of $β$-intensity to states above the neutron separation energy that de-excite by $γ$-rays, comparable to the neutron emission probability. The competition between $γ$ de-excitation and neutron emission has been compared with Hauser-Feshbach calculations, and it can be understood as a nuclear structure effect. In addition, we have studied the impact of the $β$-intensity distributions determined in this work on reactor decay heat and reactor antineutrino spectrum summation calculations. The robustness of our results is demonstrated by a thorough study of uncertainties, and with the reproduction of the spectra of the individual modules and the module-multiplicity gated spectra. This work represents the state-of-the-art of our analysis methodology for segmented total absorption spectrometers.

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arXiv2019

Updated Summation Model: An Improved Agreement with the Daya Bay Antineutrino Fluxes

A new summation method model of the reactor antineutrino energy spectrum is presented. It is updated with the most recent evaluated decay databases and with our Total Absorption Gamma-ray Spectroscopy measurements performed during the last decade. For the first time the spectral measurements from the Daya Bay experiment are compared with the detected antineutrino energy spectrum computed with the updated summation method without any renormalisation. The results exhibit a better agreement than is obtained with the Huber-Mueller model in the 2 to 5 MeV range, the region which dominates the detected flux. An unexpected systematic trend is found that the detected antineutrino flux computed with the summation model decreases with the inclusion of more Pandemonium free data. The detected flux obtained now lies only 1.9% above that detected in the Daya Bay experiment, a value that may be reduced with forthcoming new Pandemonium free data leaving less and less room to the reactor anomaly. Eventually, the new predictions of individual antineutrino spectra for the $^{235}$U, $^{239}$Pu, $^{241}$Pu and $^{238}$U are used to compute the dependence of the reactor antineutrino spectral shape on the fission fractions.

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arXiv2019

Total absorption $γ$-ray spectroscopy of niobium isomers

The $β$ intensity distributions of the decays of $^{100\text{gs},100\text{m}}$Nb and $^{102\text{gs},102\text{m}}$Nb have been determined using the Total Absorption $γ$-Ray Spectroscopy technique. The JYFLTRAP double Penning trap system was employed to disentangle the isomeric states involved, lying very close in energy, in a campaign of challenging measurements performed with the Decay Total Absorption $γ$-ray Spectrometer at the Ion Guide Isotope Separator On-Line facility in Jyväskylä. The low-spin isomeric state of each niobium case was populated through the decay of the zirconium parent, that was treated as a contaminant. We have applied a method to extract this contamination, and additionally we have obtained $β$ intensity distributions for these zirconium decays. The $β$-strength distributions evaluated with these results were compared with calculations in quasiparticle random-phase approximation, suggesting a prolate configuration for the ground states of $^{100,102}$Zr. The footprint of the Pandemonium effect was found when comparing our results for the analyses of the niobium isotopes with previous decay data. The $β$-intensities of the decay of $^{102\text{m}}$Nb were obtained for the first time. A careful evaluation of the uncertainties was carried out, and the consistency of our results was validated taking advantage of the segmentation of our spectrometer. The final results were used as input in reactor summation calculations. A large impact on antineutrino spectrum calculations was already reported and here we detail the significant impact on decay heat calculations.

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arXiv2018

First determination of $β$-delayed multiple neutron emission beyond A = 100 through direct neutron measurement: The P$_{2n}$ value of $^{136}$Sb

Background: $β$-delayed multiple neutron emission has been observed for some nuclei with A$\leq$100, with $^{100}$Rb being the heaviest $β$2n emitter measured to date. So far, only 25 P$_{2n}$ values have been determined for the $\sim$300 nuclei that may decay in this way. Accordingly, it is of interest to measure P$_{2n}$ values for the other possible multiple neutron emitters throughout the chart of the nuclides. It is of particular interest to make such measurement for nuclei with A$>$100 to test the predictions of theoretical models and simulation tools for the decays of heavy nuclei in the region of very neutron-rich nuclei. In addition, the decay properties of these nuclei are fundamental for the understanding of astrophysical nucleosynthesis processes such as the $r$-process, and safety inputs for nuclear reactors. Purpose: To determine for the first time the two neutron branching ratio, P$_{2n}$ value, for $^{136}$Sb through a direct neutron measurement, and to provide precise P$_{1n}$ values for $^{136}$Sb and $^{136}$Te. Method: Pure beams were provided by the JYFLTRAP at the IGISOL facility of the University of Jyväskylä, Finland. The purified ions were implanted into a moving tape at the end of the beam line. The detection setup consisted of a plastic scintillator placed right behind the implantation point, and the BELEN detector, based on neutron counters embedded in a polyethylene matrix. The analysis was based on the study of the $β$- and neutron- growth-and-decay curves and the $β$-one-neutron and $β$-two-neutron time correlations. Results: The P$_{2n}$ value of $^{136}$Sb was found to be 0.14(3)\% and the measured P$_{1n}$ values for $^{136}$Sb and $^{136}$Te were found to be 32.2(15)\% and 1.47(6)\%, respectively. The measured P$_{2n}$ value is a factor 44 smaller than predicted by the FRDM+QRPA model used for $r$-process calculations.

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