Disentangling decaying isomers and searching for signatures of collective excitations in β decay

This is a self-archived version of an original article. This version may differ from the original in pagination and typographic details. Author(s): Title: Year: Version: Copyright: Rights: Rights url: Please cite the original version: CC BY 3.0 https://creativecommons.org/licenses/by/3.0/ Disentangling decaying isomers and searching for signatures of collective excitations in β decay © Authors, 2020 Published version Guadilla, V.; Fallot, M.; Algora, A.; Tain, J. L.; Agramunt, J.; Äystö, J.; Briz, J. A.; Cucoanes, A.; Eronen, T.; Estienne, M.; Fraile, L. M.; Ganioǧlu, E.; Gelletly, W.; Gorelov, D.; Hakala, J.; Jokinen, A.; Jordan, D.; Kankainen, A.; Kolhinen, V.; Koponen, J.; Lebois, M.; Meur, L. L.; Martinez, T.; Monserrate, M.; Montaner-Pizá, A.; Moore, I.; Nácher, E.; Orrigo, S. E. A.; Penttilä, H.; Pohjalainen, I.; Porta, A.; Reinikainen, J.; Reponen, M.; Rinta-Antila, S.; Rubio, B.; Rytkönen, K.; Shiba, T.; Sonnenschein, V.; Sonzogni, A. A.; Valencia, E.; Vedia, V.; Voss, A.; Wilson, J. N.; Zakari-Issoufou, A. A. Guadilla, V., Fallot, M., Algora, A., Tain, J. L., Agramunt, J., Äystö, J., Briz, J. A., Cucoanes, A., Eronen, T., Estienne, M., Fraile, L. M., Ganioǧlu, E., Gelletly, W., Gorelov, D., Hakala, J., Jokinen, A., Jordan, D., Kankainen, A., Kolhinen, V., . . . Zakari-Issoufou, A.A. (2020). Disentangling decaying isomers and searching for signatures of collective excitations in β decay. In INPC2019 : 27th International Nuclear Physics Conference (Article 012134). IOP Publishing Ltd. Journal of Physics : Conference Series, 1643. https://doi.org/10.1088/1742-6596/1643/1/012134 2020 Journal of Physics: Conference Series PAPER • OPEN ACCESS Disentangling decaying isomers and searching for signatures of collective excitations in β decay To cite this article: V. Guadilla et al 2020 J. Phys.: Conf. Ser. 1643 012134 View the article online for updates and enhancements. This content was downloaded from IP address 130.234.243.86 on 19/01/2021 at 09:32 Content from this work may be used under the terms of theCreative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under licence by IOP Publishing Ltd 27th International Nuclear Physics Conference (INPC2019) Journal of Physics: Conference Series 1643 (2020) 012134 IOP Publishing doi:10.1088/1742-6596/1643/1/012134 1 Disentangling decaying isomers and searching for signatures of collective excitations in βdecay V. Guadilla1, M. Fallot1, A. Algora2,3, J. L. Tain2, J. Agramunt2, J. ¨ Ayst¨o4, J. A. Briz1, A. Cucoanes1, T. Eronen4, M. Estienne1, L. M. Fraile5, E. Ganio˘glu6, W. Gelletly7, D. Gorelov4, J. Hakala4, A. Jokinen4, D. Jordan2, A. Kankainen4, V. Kolhinen4, J. Koponen4, M. Lebois8, L. Le Meur1, T. Martinez9, M. Monserrate2, A. Montaner-Piz´a2, I. Moore4, E. N´acher10, S. E. A. Orrigo2, H. Penttil¨a4, I. Pohjalainen4, A. Porta1, J. Reinikainen4, M. Reponen4, S. Rinta-Antila4, B. Rubio2, K. Rytk¨onen4, T. Shiba1, V. Sonnenschein4, A. A. Sonzogni11, E. Valencia2, V. Vedia5, A. Voss4, J. N. Wilson8and A. -A. Zakari-Issoufou1 1Subatech, IMT-Atlantique, Universit´e de Nantes, CNRS-IN2P3, F-44307, Nantes, France 2Instituto de F´ısica Corpuscular, CSIC-Universidad de Valencia, E-46071, Valencia, Spain 3Institute of Nuclear Research of the Hungarian Academy of Sciences, Debrecen H-4026, Hungary 4Department of Physics, University of Jyv¨askyl¨a, 40014, Jyv¨askyl¨a, Finland 5Grupo de F´ısica Nuclear and IPARCOS, Universidad Complutense de Madrid, CEI Moncloa, E-28040 Madrid, Spain 6Department of Physics, Istanbul University, 34134, Istanbul, Turkey 7Department of Physics, University of Surrey, GU2 7XH, Guildford, UK 8Institut de Physique Nucl`eaire d’Orsay, 91406, Orsay, France 9Centro de Investigaciones Energ´eticas Medioambientales y Tecnol´ogicas, E-28040, Madrid, Spain 10Instituto de Estructura de la Materia, CSIC, E-28006, Madrid, Spain 11NNDC, Brookhaven National Laboratory, Upton, NY 11973-5000, USA E-mail: [email protected] Abstract. In this contribution we summarize the recent study of the βdecay of neutron-rich nuclei with isomeric states close in energy to the ground states. The disentanglement of each pair of β-decaying states was achieved by applying different strategies and using the purification capabilities of the JYFLTRAP double Penning trap system at the Ion Guide Isotope Separator On-Line facility in Jyv¨askyl¨a. The Total Absorption γ-ray Spectroscopy technique was employed to determine the βintensity probabilities populating the excited states in the daughter nuclei. Previously undetected βintensity was found and we have already evaluated the impact of part of these results on reactor summation calculations. The possibility to populate states associated with the Pygmy Dipole Resonance in the βdecay of 96gsY has also been investigated thanks to the sensitivity of our technique to high-lying strength in the daughter nuclei. 27th International Nuclear Physics Conference (INPC2019) Journal of Physics: Conference Series 1643 (2020) 012134 IOP Publishing doi:10.1088/1742-6596/1643/1/012134 2 1. Introduction In the region A≈100 some neutron-rich niobium and yttrium isotopes exhibit β-decaying isomeric states close in energy to the ground states. Due to the small energy difference and the similar half-lives, it is often difficult to separate them experimentally in order to study the βdecay of each decaying state. In addition to the scarcity of information for many of these cases, the βintensity probabilities for the known cases have usually been determined with HPGe detectors, which have a modest detection efficiency. This implies an underestimation of the βintensity at high-excitation energies in the daughter nucleus, due to the non-detection of part of the γcascades that de-excite these states. This is known as Pandemonium effect [1] and becomes more important when one moves away from stability and β-decay energy windows (Qβ) increase. The Total Absorption γ-ray Spectroscopy (TAGS) technique has been shown to be a powerful tool to obtain the β-intensity probabilities free from the Pandemonium effect [2]. It is based on the use of large scintillator crystals in almost full solid angle coverage, thus maximizing the detection efficiency. The sum of the γcascade is detected and the β-intensity distribution is obtained by means of a deconvolution process that uses the response function of the spectrometer. This response function is calculated with Monte Carlo (MC) simulations that are validated with standard calibration sources [3]. The decay data for some of these nuclei are specially important for nuclear reactor studies, since they are relevant fission products and their decay contributes: 1) to the energy released by the radioactive decay of fission fragments in a nuclear reactor, known as decay heat, and 2) to build up the reactor antineutrino spectrum with the antineutrinos emitted in their βdecay. The good prediction of the former is essential to safely operate nuclear reactors, while the understanding of the latter is needed for reactor-based antineutrino experiments. A summation approach has proven to be a suitable method to calculate both the reactor decay heat and the reactor antineutrino spectrum. This approach depends on the decay information available in the databases, and the inclusion of TAGS data, free from Pandemonium effect, has been shown to improve significantly these calculations [4, 5]. The recent observation of discrepancies in flux [6] and spectral shape [7, 8, 9] when reactor antineutrino experimental spectra are compared with calculations keeps the neutrino physics community on tenterhooks. A recent summation method study has shown the reduction of these discrepancies when the latest TAGS results are taken into account, specially for the flux anomaly [10]. In this work challenging measurements of the βdecays of 96Y and 98,100,102Nb have been performed at the Ion Guide Isotope Separator On-Line (IGISOL) facility in Jyv¨askyl¨a [11]. These cases are summarized in Table 1, where the energy of each isomer is presented. The JYFLTRAP double Penning trap system [12] was used for precision trap-assisted separation, and different strategies were followed to study each pair of β-decaying states separately, as will be commented later. The measurement of the present decays with TAGS technique has been encouraged by the International Atomic Energy Agency (IAEA), and they were assigned high priority for the improvement of the reactor decay heat and antineutrino spectrum summation calculations, as presented in Table 1. The segmented Decay Total Absorption γ-ray Spectrometer (DTAS) [16] composed of 18 NaI(Tl) crystals was employed in coincidence with a plastic detector for βparticles. The nuclei of interest were extracted from JYFLTRAP and implanted on a moving tape located at the center of DTAS and in front of the plastic detector (see Ref. [17] for more details). The analysis methodology of the Valencia group [3, 18, 19] has been applied to obtain the β-intensity distributions from the experimental total absorption spectra, which were reconstructed offline as described in Ref. [20]. 27th International Nuclear Physics Conference (INPC2019) Journal of Physics: Conference Series 1643 (2020) 012134 IOP Publishing doi:10.1088/1742-6596/1643/1/012134 3 Table 1. Cases with isomers studied with DTAS in the last experimental campaign at IGISOL. The β-decay energy window (Qβ) for the decay of the ground state is presented in the second column (from the Atomic Mass Evaluation (AME) 2016 [13]) and the energy of the isomeric state can be found in the third column (according to the NUBASE 2016 evaluation [14]). The priority of the TAGS measurement of these decays for the IAEA [15] is also included, both for reactor decay heat studies (columns fourth and fifth for U/Pu and Th/U fuels, respectively) and for reactor antineutrino spectrum studies (last column). Parent nucleus QβEnergy Priority Priority Priority [keV] [keV] U/Pu Th/U νe 96gsY 7103(6) 0 2 2 1 96mY 1540(9) - 1 - 98gsNb 4591(5) 0 1 1 1 98mNb 84(4) - - - 100gsNb 6396(8) 0 1 1 1 100mNb 313(8) - 1 - 102gsNb 7262(8) 0 2 2 1 102mNb 94(7) - 1 - 2. Decays of 98,100,102Nb Similar strategies were applied for the three niobium cases studied, motivated by the small energy difference between the isomeric state and the ground state in all of them and by the limited beam time for the experiment. Here we will explain the 98Nb system shown in the left panel of Figure 1. The βdecay of the low spin state was studied by measuring the βdecay of the zirconium parent, which decays only into 98gsNb and does not populate the isomeric state. The βdecay of 98Zr, for which no γrays are reported in ENSDF [21], was thus treated as a contaminant. We have confirmed this βpure character by setting different time windows offline to the measurement of 98Zr+98gsNb, in the line of our recent work for 100,102Nb [22]. Finally, a measurement of both decaying states together, 98gsNb+98mNb, allowed us to study the highspin component by considering the decay of 98gsNb as a contaminant. In order to illustrate this, we show in Figure 2 the experimental spectrum of 98gsNb+98mNb, where the 98gsNb spectrum coming from the 98Zr+98gsNb measurement is considered as a contaminant. The TAGS analyses of the decays of 100,102Nb have shown that the previous high-resolution spectroscopy data are affected by the Pandemonium effect, and the βintensities for the decay of 102mNb were obtained for the first time [22]. The impact of these results on reactor antineutrino summation calculations was significant in the region of the reactor antineutrino shape distortion, and they have contributed to the reduction of the discrepancy between the summation calculations and the measured antineutrino spectra [24]. Analogously, the impact of these results on reactor decay heat summation calculations was found to be noticeable, specially 10 s after fission [22]. In the construction of the response function for the TAGS analyses of 98gs,mNb we have paid special attention to the strong E0 transition that de-excites the 0+level at 734.6 keV excitation energy in 98Mo. Our preliminary results confirmed the dominance of the ground state to ground state transition in the decay of 98gsNb, while in the decay of 98mNb we determined a slight amount of previously undetected β-intensity above 4103.3 keV, the last level in 98Mo known to be populated in this decay. An example of the quality of the analyses is shown in Figure 2 27th International Nuclear Physics Conference (INPC2019) Journal of Physics: Conference Series 1643 (2020) 012134 IOP Publishing doi:10.1088/1742-6596/1643/1/012134 4 98Zr 0+30.7 s 5+84 keV 98Nb 1+0 98Mo 0+stable β− β− 2.86 s β−51.3 m 96Sr 0+1.07 s 8+1.54 MeV 96Y 0−0 96Zr 0+∼stable β− β− 9.6 s β−5.34 s Figure 1. Schemes for the decays of 98Nb (left panel) and 96Y (right panel). Each pair of β-decaying states is shown. The βdecay of the grandparents, as well as the final daughter nuclei are also shown. Spin-parity values and half-lives from ENSDF [21, 23] are included and the energy of the isomeric states is also presented [14]. 0 1000 2000 3000 4000 5000 Counts 1 10 2 10 3 10 β Q Nb 98m Nb 98gs Energy [keV] 0 1000 2000 3000 4000 5000 Residuals 0.5− 0 0.5 Figure 2. Relevant histograms for the analysis of the decay of 98mNb: experimental total absorption spectrum (solid grey), summing-pileup contribution (dashed blue), contamination of the 98gsNb low-spin component (dotted red) and reconstructed spectrum (solid black). The relative deviations between experimental and reconstructed spectra are shown below. for the decay of 98mNb. The experimental spectrum is compared with the reconstructed one obtained from the convolution of the βintensities determined in the analysis with the response function. 3. Decay of 96Y The large energy difference between the isomeric state and the ground state (see Table 1 and right panel of Figure 1) allowed us to distinguish them directly with JYFLTRAP. In the response functions for the TAGS analyses we took carefully into account the strong E0 transition from the 0+level at 1581.6 keV excitation energy in 96Zr, for which conversion electron emission 27th International Nuclear Physics Conference (INPC2019) Journal of Physics: Conference Series 1643 (2020) 012134 IOP Publishing doi:10.1088/1742-6596/1643/1/012134 5 competes with pair production. The results obtained confirmed the strong ground state feeding probability in the decay of 96gsY, and for the decay of the 8+isomer previously undetected β intensity was found above 6 MeV. The production of exotic nuclei with large β-decay energy windows opens the possibility to study collective excitations in β-decay experiments. In particular the search of hints of the Pygmy Dipole Resonance (PDR) has recently attracted a lot of attention [25, 26, 27]. This possibility can be investigated in the βdecay of 96gsY as suggested in [25], since it directly populates 1−levels, which are associated with low-lying pygmy dipole modes. The advantage of TAGS for these studies is twofold: the sensitivity to high-lying strength in the daughter nuclei [28, 29, 30, 31] and the possibility of detecting the decay of collective modes, preferentially by one or two very energetic γrays, without Pandemonium effect. In addition, the segmentation of DTAS may give us more information about the decay-pattern of such levels. In Figure 3 the high-energy region of the experimental TAGS spectrum for the decay of 96gsY is shown. We observed broad structures that could be related to the population of potential 1−levels in this region. The majority of this high-energy spectrum was found to be due to events where energy was deposited only in one crystal (module multiplicity Mm=1), which suggests a de-excitation pattern dominated by one γray. Energy [keV] 4000 5000 6000 7000 Counts 1 10 2 10 TAGS total =1 m TAGS M β Q Figure 3. Experimental TAGS spectrum for the decay of 96gsY zoomed at high energies (solid black). The Mm=1 gated TAGS spectrum is shown in solid grey. The possible 1−levels according to ENSDF [23] are depicted with arrows. 4. Conclusions In this work we have reviewed recent measurements of important fission products for reactor calculations that exhibit isomeric states. The separate study of the βdecays of the ground state and the isomeric state for each case required different strategies and was possible thanks to the purification capabilities of the JYFLTRAP system. The recently published results for the TAGS analyses of the decays of 100,102Nb showed a significant impact on reactor summation calculations [24]. The evaluation of the other two cases discussed, 98Nb and 96Y, is ongoing and was additionally complicated by the presence of strong E0 transitions. Following the recent interest for the potential study of the PDR in βdecay [25], we plan to take advantage of the segmentation of our spectrometer to constrain and characterize the βstrength observed at highexcitation energies. All these studies will be very useful for future experimental campaigns of 27th International Nuclear Physics Conference (INPC2019) Journal of Physics: Conference Series 1643 (2020) 012134 IOP Publishing doi:10.1088/1742-6596/1643/1/012134 6 neutron-rich cases with isomeric states. 5. Acknowledgments This work has been supported by the CNRS challenge NEEDS and the associated NACRE project, as well as by the Spanish Ministerio de Econom´ıa y Competitividad under Grants No. FPA2011-24553, No. AIC-A-2011-0696, No. FPA2014-52823-C2-1-P, No. FPA2015-65035P, No. FPI/BES-2014-068222, No. FPA2017-83946-C2-1-P and the program Severo Ochoa (SEV-2014-0398), by the Spanish Ministerio de Educaci´on under the FPU12/01527 Grant, by the European Commission under the FP7/EURATOM contract 605203 and the FP7/ENSAR contract 262010, and by the Junta para la Ampliaci´on de Estudios Programme (CSIC JAE-Doc contract) co-financed by ESF. References [1] Hardy J et al. 1977 Phys. Lett. B 71 307 [2] Rubio B et al. 2005 J. Phys. G: Nucl. Part. Phys. 31 S1477 [3] Cano-Ott D et al. 1999 Nucl. Instrum. and Methods A 430 333 [4] Algora A et al. 2010 Phys. Rev. Lett. 105 202501 [5] Fallot M et al. 2012 Phys. Rev. Lett. 109 202504 [6] Mention G et al. 2011 Phys. Rev. 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