The γ decay from the 2+2 near-threshold resonance in 14C, located 142 keV above the neutron emission threshold, was searched for in a fusionevaporation experiment at Argonne National Laboratory with the GODDESS setup, comprising the GRETINA γ-ray spectrometer coupled to the ORRUBA charged particle detector. The Shell Model Embedded in the Continuum predicts a significant enhancement of the 2+2 → 0 transition probability, owing to a collectivization of the

near-threshold state. The corresponding γ branch is expected to be of the order of 5 × 10−5, which is comparable with the sensitivity of this experiment

Bottoni, S.

Ciemala, M.

Corbari, G.

Crespi, F. C. L.

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DOI 10.1393/ncc/i2023-23090-1Communications: SIF Congress 2022IL NUOVO CIMENTO 46 C (2023) 90Searching for the γ decay from the near-neutron threshold 2+ statein 14C: A probe of collectivization phenomena in light nucleiG. Corbari(1)(2), S. Bottoni(1)(2), M. Ciemala(3), F. C. L. Crespi(1)(2),S. Leoni(1)(2), B. Fornal(3), R. V. F. Janssens(4)(9), S. D. Pain(5),M. Siciliano(6), E. Albanese(1)(2), A. D. Ayangeakaa(4)(9), G. Benzoni(2),S. Carmichael(7), M. Carpenter(6), K. Chipps(5), N. Cieplicka(3), P. Copp(6),J. Forson(8), E. Gamba(1)(2), L.W. Iskra(3), H. Jayatissa(6), F. Kondev(6),T. Lauritsen(6), B. Million(2), C. Müller-Gatermann(6), A. Palmisano(8),M. Polettini(1)(2), C. Porzio(1)(2), W. Reviol(6), N. Sensharma(4)(9),D. Seweryniak(6), C. Ummel(10), O. Wieland(2), G. Wilson(6)(11), S. Zhu(12)and S. Ziliani(1)(2)(1) Dipartimento di Fisica, Università di Milano - Milano, Italy(2) INFN, Sezione di Milano - Milano, Italy(3) Institute of Nuclear Physics, IFJ-PAN - Krakow, Poland(4) University of North Carolina at Chapel Hill - Chapel Hill, NC, USA(5) Oak Ridge National Laboratory - Oak Ridge, TN, USA(6) Argonne National Laboratory - Lemont, IL, USA(7) Notre Dame University - Notre Dame, IN, USA(8) University of Tennessee-Knoxville - Knoxville, TN, USA(9) Triangle Universities Nuclear Laboratory, Duke University - Durham, NC, USA(10) Rutgers University - Piscataway, NJ, USA(11) Louisiana State University - Baton Rouge, LA, USA(12) Brookhaven National Laboratory - Upton, NY, USAreceived 20 February 2023Summary. — The γ decay from the 2+2 near-threshold resonance in14C, lo-cated 142 keV above the neutron emission threshold, was searched for in a fusion-evaporation experiment at Argonne National Laboratory with the GODDESS setup,comprising the GRETINA γ-ray spectrometer coupled to the ORRUBA charged par-ticle detector. The Shell Model Embedded in the Continuum predicts a significantenhancement of the 2+2 → 0 transition probability, owing to a collectivization of thenear-threshold state. The corresponding γ branch is expected to be of the order of5× 10−5, which is comparable with the sensitivity of this experiment.Creative Commons Attribution 4.0 License (https://creativecommons.org/licenses/by/4.0) 12 G. CORBARI et al.1. – IntroductionLight nuclei, with mass A < 20, have proven to be optimal test benches for modernnuclear structure models (see, e.g., refs. [1-3]). Of particular interest are near-thresholdstates, i.e., narrow resonances lying in the proximity of particle-emission threshold, in-duced by the coupling of discrete bound states with particle-decay channels. Within theShell Model Embedded in the Continuum (SMEC) framework [4], which describes nucleias open quantum systems, the existence of near-threshold states emerges as a universalphenomenon. The structure of these states is expected to provide relevant informationon the microscopic mechanism leading to the onset of collectivization in light nuclei, suchas C, O, Ne isotopes, including clustering phenomena.In the present work, we studied the case of 14C, which presents a 2+2 near-thresholdstate at 8318 keV–142 keV above the neutron separation energy. This state is predi-cated by the SMEC model to gain collectivity when the coupling with the continuumis considered, in contrast with standard Shell-Model calculations, resulting in enhancedelectromagnetic transition probabilities. According to the SMEC model [5], the γ-decaybranch from the 2+2 , near-threshold state in14C is expected to be of the order of 5×10−5.Therefore, state-of-the-art arrays with improved sensitivity based on segmented HPGecrystals, such as GRETINA [6], are needed to possibly detect the 8315-keV γ-ray fromthe 2+2 narrow resonance, or to provide the best possible upper limit.2. – Experimental detailsIn this experiment, the 14C nucleus was populated using the 6Li + 9Be fusion-evaporation reaction, after the evaporation of a single proton from the 15N compoundnucleus. The 6Li beam was provided by the ATLAS facility of Argonne National Lab-oratory (ANL), at an energy of 7MeV, impinging on a 9Be target with a thickness of200 or 400 μg/cm2. The GODDESS setup (GRETINA ORRUBA: Dual Detectors forExperimental Structure Studies) was employed: evaporated protons were detected bythe ORRUBA charged particle silicon strip detectors, arranged in a barrel configura-tion [7], while γ-rays were detected by the GRETINA γ-ray spectrometer [6], comprising48 HPGe segmented detectors. A total detection efficiency of ∼10%, at 1.3MeV, and 55%are estimated for GRETINA and ORRUBA, respectively [8]. Data were acquired withtwo different triggers, a particle-γ coincidence and a particle only condition, downscaledby a factor of ≈ 100.3. – Data analysisThe initial stages of the data analysis were focused on the reconstruction of the reac-tion kinematics, which was a necessary step to obtain i) the excitation energy spectrum ofthe 14C nucleus, ii) perform an event-by-event Doppler correction and iii) build γ-particlecoincidences.By exploiting the highly segmented QQQ5 endcap detectors and the resistive stripsof the SX3 barrel detectors, it was possible to determine the interaction position of theevaporated protons in ORRUBA, with a resolution of Δθ ∼ 1◦. Furthermore, to preventα particle (and other evaporation channels) from reaching the ORRUBA detectors, Alabsorbers of about 200, 250 and 500μm were mounted. As shown in fig. 1, this allowedobtaining a rather clean kinematic line matrix, at the cost of worsening the energyresolution due to the straggling of protons in the absorbers. The reconstructed excitationSEARCHING FOR THE γ DECAY ETC. 3Fig. 1. – Left: proton energy E versus laboratory angle θ matrix. Superimposed black linescorrespond to expected kinematic lines for protons feeding both ground and excited states of14C. Experimental data correspond to runs with the 400μg/cm2 target. Calculations performedwith the nptool software [9]. Right: partial level scheme of 14C showing known (solid lines) andexpected γ decays (dashed lines) from the 2+1 , 2+2 and 2+3 states. Energies in keV from ref. [10].energy spectrum of 14C is presented on the left panel of fig. 2: the first excited 1− stateand the near-threshold 2+2 state of interest are well separated, while peaks in the 7MeVregion are not completely resolved due to the high density of states and the limited energyresolution, the latter being ∼400 and ∼500 keV for the 200 and 400μg/cm2 targets,respectively.Finally, GRETINA and ORRUBA data were merged, and γ-particle coincident spec-tra were built, after the reconstruction of γ events with the add-back procedure andthe application of an event-by-event Doppler correction. Prompt γ-rays were identifiedwithin a 200 ns coincidence window between GRETINA and ORRUBA. The energyand direction of incident γ-rays were determined with the add-back technique, by sum-Fig. 2. – Left: reconstructed excitation energy spectrum of 14C, in coincidence with γ-rays.The red region corresponds to the gate interval on the 7341-keV state of the excitation energyspectrum of 14C in coincidence with γ-rays. Right: projection of the background-subtractedγ-ray spectrum in coincidence with the 2− state. Transitions of 613, 1248, 6092, 6727 and7339 keV are observed, corresponding to the γ-decay towards the 3− and 1− excited states, aswell as the ground state. Single and double star symbols indicate 1st and 2nd escape peaks,respectively.4 G. CORBARI et al.ming up energy core signals within a spherical volume (r = 80mm) centered around themost energetic interaction, which was assumed as a first hit. An event-by-event Dopplercorrection was then applied to the reconstructed γ-ray energy, using the relative angleθγ,rec between the recoiling14C nucleus and the direction of the γ-ray. The former wasobtained from the energy and direction of the detected protons. Particle-γ coincidencespectra were initially analyzed in the energy region of discrete states of 14C, as shownin the left panel of fig. 2. The projection on the γ-ray energy axis, gated on the 7341-keV 2− excited state, is presented in the right panel of fig. 2. All of the γ transitionsdepopulating the 2− state were clearly identified, demonstrating the effectiveness of theexperimental technique.By assuming a similar cross section for the population of the 2+1 and 2+2 states, asobserved in [11] and in our work, a detection limit of 2.6 × 10−5 is obtained for the γ-decay branch from the 2+2 state. A detailed analysis of the 8-MeV region of the gammaspectrum, using different background subtraction techniques, is ongoing, and final resultson the 2+2 γ-decay branch will be reported in a future publication.4. – Conclusions and perspectivesThe γ-decay from the 2+2 , near-neutron-threshold state in14C was searched for in anexperiment performed at Argonne National Laboratory to investigate the impact of thecontinuum on the electromagnetic decay properties of this resonance. A detailed pre-analysis of the ORRUBA and GRETINA data was performed in order to reconstruct thereaction kinematics, obtain the excitation energy spectrum of 14C and build γ-particlecoincidence events. In the future, an estimate for the 8315-keV γ-decay branch from the2+2 state will be given and the results will be compared with SMEC model predictions.∗ ∗ ∗The authors acknowledge GRETINA and ORRUBA Collaborations and the staff ofthe Argonne National Laboratory for providing the beam and for the help given duringthe experiment.REFERENCES[1] Ciemala M. et al., Phys. Rev. C, 101 (2020) 021303.[2] Cieplicka-Oryńczak N. et al., Phys. Lett. B, 834 (2022) 137398.[3] Ziliani S. et al., Phys. Rev. C, 104 (2021) L041301.[4] Okolowicz J., Nazarewicz W. and Ploszajczak M., Fortschr. Phys., 61 (2012) 66.[5] Ploszajczak M. and Okolowicz J., J. Phys.: Conf. Ser., 1643 (2020) 012156.[6] Paschalis S. et al., Nucl. Instrum. Methods Phys. Res. A, 709 (2013) 44.[7] Pain S. D. et al., Phys. Procedia, 90 (2017) 455.[8] Lauritsen T. et al., Nucl. Instrum. Methods Phys. Res. A, 836 (2016) 46.[9] Matta A. et al., J. Phys. G: Nucl. Part. Phys., 43 (2016) 045113.[10] Ajzenberg-Selove F., Nucl. Phys. A, 523 (1991) 1.[11] Ajzenberg-Selove F., Bingham H. and Garrett J., Nucl. Phys. A, 202 (1973) 152.

Searching for the γ decay from the near-neutron threshold 2+ state in 14C: A probe of collectivization phenomena in light nuclei

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Searching for the γ decay from the near-neutron threshold 2+ state in 14C: A probe of collectivization phenomena in light nuclei

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