Welcome and introduction

• Alejandro Algora (IFIC (CSIC-Univ. Valencia))
• Paraskevi Dimitriou (IAEA)

Room: Salón de Actos del Edificio de Cabecera (PCUV)

Pandemonium free data for reactor applications

• Alejandro Algora (IFIC (CSIC-Univ. Valencia))

Room: Salón de Actos del Edificio de Cabecera (PCUV) Valencia-Nantes-Surrey-Warsaw-Jyväskylä Collaboration Nuclear structure and practical applications require nuclear data free from the Pandemonium effect [1,2]. For both applications we need information on the Q value of the decay, the half-life and the beta decay probabilities to the different levels in the daughter nucleus. Pandemonium, in this context, means that the beta decay probabilities might be distorted because of the technique used for their determination. Two techniques are considered free from the Pandemonium distortion: the total absorption gamma-ray spectroscopy study of the beta decay and the measurement of the shape of the beta spectrum. In this contribution I will introduce how this problem is addressed experimentally and show some examples of recent measurements. [1] J. Hardy, et al., Phys. Lett. B 71 (1977) 307 [2] A. Algora, et al., Eur. Phys. J. A (2021) 57:85 [3] G. Alcalá, et al., Phys. Rev. Letts. 135, 142502 (2025)

β decay of fission fragments with total absorption gamma-ray spectroscopy at IGISOL

• V. Guadilla (Univ. of Warsaw)

Room: Salón de Actos del Edificio de Cabecera (PCUV) In this contribution we will present the study of beta decays of interest for reactor antineutrino spectrum calculations using the Total absorption gamma-ray spectroscopy (TAGS). Measurements of the beta decays of several fission fragments including 103,108Tc, 103,108Mo and 95,96Sr were done at the Ion Guide Isotope Separator On-Line facility in Jyväskylä [1] using the Decay Total Absorption gamma-ray Spectrometer (DTAS) [2]. The complete beta intensity distributions have been obtained based on the analysis approach developed at IFIC [3, 4, 5] and they show that previous beta intensities determined with germanium detectors were incomplete due to the Pandemonium effect [6]. In the present contribution we will stress the usefulness of a segmented spectrometer to evaluate the consistency of the results looking at the spectra with multiplicity conditions. [1] I. D. Moore et al., Nucl. Instrum. Methods Phys. Res., Sect. B 317, 208 (2013) [2] J. L. Tain et al., Nucl. Instrum. Methods Phys. Res., Sect. A 803, 36 (2015) [3] D. Cano-Ott et al., Nucl. Instrum. Methods Phys. Res., Sect. A 430, 333 (1999) [4] J. L. Tain and D. Cano-Ott, Nucl. Instrum. Methods Phys. Res., Sect. A 571, 728 (2007) [5] J. L. Tain and D. Cano-Ott, Nucl. Instrum. Methods Phys. Res., Sect. A 571, 719 (2007) [6] J. Hardy et al., Phys. Lett. B 71, 307 (1977)

Update of the summation calculations for reactor antineutrino spectra

• M. Estienne (SUBATECH, CNRS/IN2P3)
• M. Fallot (SUBATECH, CNRS/IN2P3)

Room: Salón de Actos del Edificio de Cabecera (PCUV) SUBATECH, CNRS/IN2P3, Nantes Université, IMT Atlantique, F-44307 Nantes, France Contact: estienne@subatech.in2p3.fr Key words: reactor antineutrinos, summation calculations, nuclear data, TAGS Abstract: The accurate determination of reactor antineutrino spectra remains a very actual research topic for which interrogations have emerged in the past decade. Indeed, after the “reactor anomaly” (RAA) [1] – a deficit of measured antineutrinos at short baseline reactor experiments with respect to spectral predictions – the three international reactor neutrino experiments Double Chooz, Daya Bay and Reno have evidenced spectral distortions in their measurements w.r.t the same spectral predictions (Shape Anomaly)[2]. This puzzle is called the “shape anomaly”. The latter predictions were obtained through the conversion of integral beta energy spectra obtained at the ILL research reactor[3]. Several studies have shown that the underlying nuclear physics required for the conversion of these spectra into antineutrino spectra is not totally under control [4]. The unique alternative to converted spectra is a complementary approach consisting in determining the antineutrino spectrum through nuclear data [5,6]. In the past years, the reactor neutrino experiments such as Prospect [7], STEREO [8] and Daya Bay [9] have published in 2023 their analysis with the complete statistics of the measured data. The outcome of these analyses, combined with the work carried out in experimental nuclear physics with the TAGS measurements in particular [10, 11, 12], is that the sterile neutrino hypothesis is strongly disfavored to explain the RAA, but that further efforts remain to be made both theoretically and experimentally to fully understand the origin of RAA and shape anomaly, and that accurate predictions of antineutrino fluxes and spectra are still needed for future discoveries. Indeed the Daya Bay collaboration provided the first measurement of the high energy part of the reactor antineutrino spectrum above 8 MeV. In addition, the Juno-Tao [13] experiment will perform a measurement of reactor antineutrino spectra with unprecedented energy resolution that will allow to tackle the contribution of specific fission products and constrain potentially nuclear data with the measured antineutrinos. The summation method based on the nuclear data will be the privileged tool to interpret these measurements. At this workshop, we propose to present an update of our summation calculations with recently published TAGS results for 96,96mY [14], 95Rb and 137I [15], as well as new TAGS results for 138I, 142Cs and 99Y. The new SM2026 predictions will be compared with the most recent results from Daya Bay [16]. [1] G. Mention et al. , Phys. Rev. D 83 , 073006 (2011) [2] Double Chooz and Reno Collab. in Proceedings of the Neutrino 2014 Conf., http://neutrino2014.bu.edu/; Daya Bay Collab. in Proceedings of the ICHEP 2014 Conf., http://ichep2014.es/. [3] P. Huber, Phys. Rev. C 84, 024617 (2011). [4] A. C. Hayes et al., Phys. Rev. Lett. 112, 202501 (2014). [5] M. Fallot et al., Phys. Rev. Lett. 109 , 202504 (2012). [6] A A. Sonzogni et al. , Phys. Rev. C 91 , 011301 (R) (2015). [7] M. Andriamirado et al., Prospect Collab. Phys. Rev. Lett. 131, 021802 (2023) [8] F.P. An et al. Daya Bay collab., Phys. Rev. Lett. 130 (2023) 211801 and F.P. An et al. Daya Bay collab., Phys. Rev. Lett. 129 (2022) 041801). [9] H. Almazan et al., STEREO collab. Nature 613, 257–261 (2023). https://doi.org/10.1038/s41586-022-05568-2. [10] M. Estienne et al. Phys. Rev. Lett. 123, 022502 (2019). [11] A. Algora, B. Rubio, J.-L. Tain, M. Fallot, W. Gelletly Review Paper Eur. Phys. J. A 57, 85 (2021) [12] Chao Zhang a, Xin Qian a, Muriel Fallot bProgress in Particle and Nuclear Physics Vol. 136, (2024), 104106 [13] A. Abusleme, et al., TAO Conceptual Design Report. arXiv:2005.08745. [14] V. Guadilla et al. Phys. Rev. C 106, 014306 (2022) [15] V. Guadilla et al. Phys. Rev. C 100, 044305 (2019) [16] DB collaboration, Phys. Rev. Lett. 134, 201802 (2025)

Progress in accounting for the antineutrino spectrum generated by nuclear reactors

• A. A. Sonzogni (Brookhaven National Laboratory)

Room: Salón de Actos del Edificio de Cabecera (PCUV) Understanding the magnitude and detailed features of the highly precise IBD antineutrino spectra reported by the Daya Bay, NEOS, and RENO collaborations has been both a challenging and rewarding endeavor. Relative to the Huber–Mueller model, these spectra exhibit an overall ~5% excess in the predicted total yield—commonly referred to as the Reactor Antineutrino Anomaly (RAA)—as well as a deficit in the 4–6 MeV region, often called the “bump.” The origin of the RAA has been linked to a normalization issue in the 235-Uranium electron spectrum measurements, while the “bump” is suspected to arise from efficiency effects in the magnetic spectrograph used in those measurements. In addition, the assumption of allowed spectral shapes in the conversion method has been increasingly questioned. Summation (ab initio) calculations, while valuable for tracing antineutrino production, have also faced limitations. These stem primarily from incomplete nuclear database information, particularly for the short-lived fission products that dominate the IBD spectrum. Until recently, decay heat calculations represented the only other application requiring similar data, although the corresponding standards are less precise than the IBD measurements discussed here. The JUNO collaboration released through arXiv their first results in November 2025. At 53 Km from the reactors, the oscillation pattern induced by theta-13 - not yet apparent but expected with a larger number of events - as well as the shifting of the spectrum at higher energies due to a non-zero value of theta-12, will reinforce even further the need for better nuclear decay data and the electron spectra for the actinide nuclides undergoing fission. Acknowledgement: This work was supported in part by the U.S. Department of Energy Office of Science, Office of Nuclear Physics, under Contract No. DE-AC02-98CH10886.

Precision measurements of beta-particle energy spectra with a calorimetric technique

• Oscar Naviliat-Cuncic (FRIB)

Room: Salón de Actos del Edificio de Cabecera (PCUV) Sensitive searches for new physics have motivated the exploration of new techniques for the measurement of the beta-particle energy spectra. We describe here several experiments which use a calorimetry technique with inorganic scintillating crystals along with implanted radioactive beams to produce a 4pi geometry. We will discuss technical details including major advantages but also current limitations and possible perspectives.

Study of Beta Decay Spectra for Predictions of Reactor Antineutrino Spectra

• Alcalá Gustavo (IFIC)

Room: Salón de Actos del Edificio de Cabecera (PCUV) There are two discrepancies between measured and predicted reactor antineutrino spectra, a flux difference called the "Reactor Antineutrino Anomaly" and a shape deformation at 5-7 MeV named the spectral "bump". The use of the summation method as an alternative for calculating reactor antineutrino spectra predictions, together with beta feedings from Total Absorption Gamma Spectroscopy experiments, reduces the flux anomaly to a level of statistical insignificance. Due to this result, the e-shape collaboration developed telescopic electron detectors to directly measure the beta spectra of relevant nuclei expected to be associated with the bump, to improve the nuclear data and models employed to obtain reactor antineutrino spectra predictions. Measurements were performed at the IGISOL facility due to its capacity to generate highly pure radioactive beams. The $^{92}$Rb and $^{142}$Cs beta decays are the first and third contributors to the reactor antineutrino spectra in the region of the bump, and their measurements are presented in this talk along with predictions generated with different beta shapes and feeding models.

Detector calibration studies for the e-Shape experiment

• S. Durand (Subatech)

Room: Salón de Actos del Edificio de Cabecera (PCUV) Forbidden β decays are of particular interest for both nuclear physics and astrophysics. In the context of the astrophysical r-process, they govern the matter flow along neutron-rich nuclei and strongly impact the final abundance distribution of heavy elements. In nuclear reactors, an incomplete description of forbidden transitions has been identified as one of the leading hypotheses behind the reactor antineutrino shape anomaly, which complements the global reactor antineutrino anomaly. Despite their importance, forbidden β decays remain poorly understood due to the complexity of their spectra and the challenges in modeling their shape factors. This contribution focuses on the detector calibration work carried out after the 2023 e-Shape experimental campaign at the IGISOL facility of Jyväskylä, Finland. A detailed detector geometry has been implemented in Geant4 in order to reproduce the experimental setup as accurately as possible. Energy calibration has been performed using several radioactive sources and nuclei, together with a dedicated study of the detector energy resolution. The resulting calibrated simulations are compared with experimental data in order to assess the level of agreement between measurement and Monte Carlo spectra. A good agreement has already been obtained for the 114Ag beta decay energy spectrum, providing a first validation of both the detector calibration and the resolution convolution procedure. These developments constitute an essential step toward future precision analyses of forbidden β spectra and the extraction of reliable shape factors. Future efforts will focus on the analysis of β spectra of selected nuclei in order to extract shape factors, and on the benchmarking of these results against theoretical predictions. Ultimately, this work will contribute to more reliable modeling of forbidden transitions, with applications to both r-process nucleosynthesis simulations and reactor antineutrino spectrum predictions.

Latest from MTAS Including High-Energy Beta and Long-Lived Reactor Beta Spectra

• B Rasco (ORNL)

Room: Salón de Actos del Edificio de Cabecera (PCUV)

The (NA)2STARS: Neutrinos, Applications and Nuclear Astrophysics with a Segmented Total Absorption with higher Resolution Spectrometer, a combination of calorimetric and spectroscopic tools for beta decay and in-beam measurements

• Muriel Fallot (SUBATECH)

Room: Salón de Actos del Edificio de Cabecera (PCUV) The international collaboration constituted based on the Total Absorption Gamma-ray Spectroscopy technique (TAGS) in Europe is aiming to build a Total Absorption Spectrometer (TAS) of the next generation. TAGS is a calorimetric technique using large monolithic or segmented scintillators that cover more than 80% of 4p, but with limited energy resolution. It complements high-resolution spectroscopy using Germanium crystals. It is particularly well suited to physics themes requiring the detection of high-energy or multiple gamma photons, as in the case of beta decay of short-lived nuclei, or the measurement of reaction cross sections useful in certain nucleosynthesis processes. Indeed, in the case of beta decay of nuclei with large Q-values, the excitation energy states of the daughter nucleus are located at high energy and de-excited by multiple gamma lines or very energetic gamma-rays. A systematic error known as the Pandemonium effect [1] can affect data due to the low intrinsic or geometric efficiency of devices based on HPGe-type detectors. This effect results in poor determination of beta intensity distributions, and has far-reaching consequences for topics involving good knowledge of these intensity distributions. The new instrument, called STARS (Segmented Total Absorption with higher Resolution Spectrometer), will ally efficiency with a higher segmentation and energy resolution than the existing spectrometers thanks to the addition of 16 LaBr3 crystals. The two segmented TAS that exist in Europe that will benefit from this upgrade are DTAS detector (18 NaI crystals [2]) and the Rocinante detector (12 BaF2 crystals [3]). The scientific advances that will be made possible will concern nuclear structure, nuclear astrophysics, neutrino and reactor physics, topics to which the TAGS technique has proven to bring significant advances [4]. The research objectives span a wide physics program. The first experiment with the STARS is planned in GANIL in July. It will act at the time as a commissioning of the advanced TAS that will be then be available for new nuclear data measurements. [1] J. C. Hardy et al., Phys. Lett. 71 B, 307 (1977). [2] V. Guadilla et al., Nucl. Instr. Meth. A910, 79-89 (2018). [3] E. Valencia et al., Phys. Rev. C 95, 024320 (2017). [4] A. Algora, B. Rubio, J.-L. Tain, M. Fallot, W. Gelletly, Eur. Phys. J. A 57, 85 (2021) and references therein.

Status of the RENE Experiment

• Byeongsu Yang

Room: Salón de Actos del Edificio de Cabecera (PCUV) The Reactor Experiment for Neutrinos and Exotics (RENE) is a new short-baseline reactor antineutrino experiment designed to explore the parameter space relevant to the reactor antineutrino anomaly. Its primary objective is to test the existence of sterile neutrino oscillations around Δm412∼2 eV2, a region where previous experiments have reported both exclusions and possible hints. RENE combines a dedicated detector setup with detailed simulation studies to achieve high sensitivity in this domain. By providing fresh measurements, the experiment aims to clarify outstanding questions in reactor antineutrino physics and contribute to the broader understanding of neutrino properties. This presentation will summarize the current status and scientific goals of RENE.

Uncertainty Quantification in FPDH using 235U(nth,f) and 239Pu(nth,f) JEFF-4 fission yield evaluations

• Oscar Cabellos (Universidad Politécnica de Madrid, Madrid)

Room: Salón de Actos del Edificio de Cabecera (PCUV) This work investigates the impact of the JEFF-4.0 fission yield evaluation, including associated uncertainties and covariance data on Fission Pulse Decay Heat (FPDH). Using thermal fission of ²³⁵U and ²³⁹Pu as case studies, uncertainty quantification is performed through a Monte Carlo approach based on random sampling of correlated fission yield datasets. Calculations of Fission Pulse Decay Heat (FPDH) are carried out using the ACAB code. The results show that the inclusion of fission yield correlations leads to a strong and systematic reduction of propagated uncertainties, both in short-term and long-term decay heat quantities. A detailed analysis of decay chains and dominant contributors highlights the role of correlations—particularly negative correlations—in significantly reducing variance. However, the magnitude of this reduction raises questions about a possible underestimation of uncertainties when full covariance information is considered. These findings emphasize the importance of consistent and comprehensive covariance data in fission yield evaluations, with direct implications for uncertainty propagation in reactor observables and antineutrino spectrum predictions. In addition, calculations of depletion-related isotopes (134Cs, 135Xe and 149Sm) are carried out using the WIMSD5 codes for a typical 17x17 PWR fuel assembly as a function of the burnup. The results show also a strong and systematic reduction of propagated uncertainties.

Latest News from Chooz: Double Chooz & AntiMatter-OTech

• A Cabrera

Room: Salón de Actos del Edificio de Cabecera (PCUV) The EDF Chooz nuclear reactor (France) is the most renowned historical source of antineutrinos in Europe for fundamental research, with so far three generations of IBD-based antineutrino experiments: the Chooz and Double Chooz experiments, and, most recently, the AntiMatter-OTech project (under preparation). The latter includes the direct involvement of the EDF reactor operators. The AntiMatter-OTech, funded by and for innovation in Europe (EIC and UKRI), builds on the experience of the Double Chooz’s world highest single-detector reactor precision to explore new boundaries of antineutrino detection in the context of reactor instrumentation and monitoring, using the novel LiquidO detection technology — also invented (2012) in the perimeter of Double Chooz. In this talk, I shall summarise the status of the latest, most pertinent Double Chooz results and key elements of the preparation of the AntiMatter-OTech project.

Measurements of cumulative fission product yields and their impact on reactor antineutrino spectra

• Sean Finch (Duke University)

Room: Salón de Actos del Edificio de Cabecera (PCUV) A TUNL-LLNL-LANL collaboration has employed a rapid target transfer system to measure cumulative fission product yields (FPYs) following neutron-induced fission of $^{235}$U, $^{238}$U, and $^{239}$Pu at a neutron energy of 2.0 MeV. This work expands our previously published measurements of end-chain FPYs to include fission products up to four nuclides away from stability. The cyclic activation technique was used with a variety of irradiation and counting times to probe fission products with half-lives as low as 0.5 s. Using literature $\gamma$-ray intensity values, cumulative FPYs were determined for approximately 50 short-lived fission products from each of the three actinides. These include most of the key fission products expected to contribute to the structure observed in the reactor antineutrino spectrum around 5 MeV. Overall, comparison of our FPY data with the ENDF/B-VIII.0 and JEFF-3.3 evaluations shows very good agreement; however, there exist differences in some cumulative FPYs, such as in cases where the yield is split between the ground state and a long-lived isomeric state. Substituting the present FPY data into the ENDF/B-VIII.0 and JEFF-3.3 evaluations and using the summation model to calculate the reactor antineutrino energy spectra does not eliminate the 5 MeV bump feature. Instead, it provides stronger evidence for the bump and improves agreement with the Daya Bay data. We also present preliminary FPY data at lower incident neutron energies, 60 and 560 keV, for $^{235}$U and $^{239}$Pu.