The Gamow-Teller strength distribution covering the entire beta-decay window, up to 10.312(4) MeV, of 80g+mGa was measured for the first time in photofission of UCx 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 = 0.0530.008 0.009)] in 80Ge. To understand these data, we performed a realistic shell-model calculation and systematic analysis of log 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 B(GT) distribution and therefore the half-life of the precursor.

The pygmy dipole resonance (PDR) has been the subject of numer-ous studies, both experimental and theoretical. Indeed, the study of the PDR has been and still is of great interest since it allows to constrain the symmetry energy, an important ingredient of the equation of state of nuclear matter that describes the matter within neutron stars. Moreover, the PDR is predicted to play a key role in the r-pro cess via the increase of the neutron capture rate. However, despite numerous experiments dedicated to the study of the PDR, a consistent description is still missing. In this context, we have proposed to study the PDR using a new probe: the neutron inelastic scattering reaction (n,n'-y). An experiment to study the pygmy resonance in 140Ce using the (n,n'-y) reaction has been performed in Septem-ber 2022. This experiment has been made possible thanks to the high-intensity proton beam of the new accelerator SPIRAL2 at GANIL and the NFS (Neutron For Science) facility. The experimental setup was composed of the new generation multi-detectors PARIS, for the detection of-y-rays coming from the de-excitation of the PDR, and MONSTER, for the detection of scattered neutrons. In this article, the experiment motivation and description are presented.

Neutron dark decays have been suggested as a solution to the discrepancy between bottle and beam experiments, providing a dark matter candidate that can be searched for in halo nuclei. The free neutron in the final state following the decay of ^{6}He into ^{4}He+n+χ provides an exceptionally clean detection signature when combined with a high efficiency neutron detector. Using a high-intensity ^{6}He^{+} beam at Grand Accélérateur National d'Ions Lourds, a search for a coincident neutron signal resulted in an upper limit on a dark decay branching ratio of Br_{χ}≤4.0×10^{-10} (95% C.L.). Using the dark neutron decay model proposed originally by Fornal and Grinstein, we translate this into an upper bound on a dark neutron branching ratio of O(10^{-5}), improving over global constraints by one to several orders of magnitude depending on m_{χ}.

$^{225}\mathrm{Pa}$ and $^{221}\mathrm{Ac}$ were produced at the IGISOL facility through proton-induced fusion-evaporation reactions and have been studied using $\ensuremath{\alpha}$-particle spectroscopy, as well as $\ensuremath{\alpha}\text{\ensuremath{-}}\ensuremath{\gamma}$ and $\ensuremath{\alpha}$-electron coincidence spectroscopy. The level scheme of $^{221}\mathrm{Ac}$, daughter of $^{225}\mathrm{Pa}$, and of $^{217}\mathrm{Fr}$, daughter of $^{221}\mathrm{Ac}$ were reconstructed. An interpretation of $^{221}\mathrm{Ac}$ levels as $K=5/{2}^{\ifmmode\pm\else\textpm\fi{}}$ and $K=3/{2}^{\ifmmode\pm\else\textpm\fi{}}$ parity-doublet bands is proposed. Such bands appear in reflection-asymmetric models and would be an indication of a static reflection asymmetric shape for $^{221}\mathrm{Ac}$.

Abstract We predict the existence of medium spin PDR and find experimental evidence in 80Ge using β-decay of 80Ga. This nucleus was produced in the photo-fission of 238U. A hybrid γ-ray spectrometer was developed using a plastic detector for β-tagging, HPGe detectors for low-energy γ rays detection and phoswich detectors of PARIS for high-energy γ rays. The low-high energy coincidence spectrum shows two peaks at 7181(53) and 7337(53) keV in the β-decay of the 80mGa isomeric state only, in coincidence with the 21+ → 0+ transition of 80Ge. Combined with decay information, electric dipole character (E1) were assigned to these γ rays which de-excite 7.84 MeV and 7.996 MeV states. Based on this fact and the comparison with theoretical calculations, we conclude that these enhanced E1 transitions are driven by neutron-skin oscillation of 80Ge. The oscillation is triggered by a conversion of a deeply-bound neutron from the Z=N=28 proton-neutron saturated core into a proton in the Fermi surface. These results provide an evidence for a PDR state with spin different from 1− and built on a low-energy excited state. Consequently, it extends the PDR’s boundary along spin dimension for the first time and provides experimental data for testing the validity of the Brink-Axel hypothesis in the PDR region.

Excited states in the N = 50 nucleus Ge-82 have been investigated via beta decay of Ga-82 at the ALTO facility. More than 50 new gamma transitions were identified. The preliminary results are presented in this work.

Medium spin states of light N = 50 isotones have been populated using fast neutron-induced fission of ^232 Th. Online prompt γ spectroscopy has been performed using the hybrid γ spectrometer ν- Ball coupled to the LICORNE directional neutron source at the ALTO facility of IJCLab. Medium spin states of the neutron-rich nucleus ^82 Ge have been investigated using γ - γ and γ - γ - γ coincidence data to exploit the resolving power of ν- Ball. Two new transitions were assigned to this nucleus and a new level was placed in the level scheme. We tentatively assigned to this new state a ( 7^+ ) spin-parity, which is interpreted as a new N=50 core breaking state. This provides further insight into the energy evolution of the N=50 shell gap toward ^78 Ni.

The population of isomeric states in the prompt decay of fission fragments-so-called isomeric yield ratios (IYRs)-is known to be sensitive to the angular momentum J that the fragment emerged with, and may therefore contain valuable information on the mechanism behind the fission process. In this work, we investigate how changes in the fissioning system impact the measured IYRs of fission fragments to learn more about what parameters affect angular momentum generation. To enable this, a new technique for measuring IYRs is first demonstrated. It is based on the time of arrival of discrete gamma rays, and has the advantage that it enables the study of the IYR as a function of properties of the partner nucleus. This technique is used to extract the IYR of 134Te, strongly populated in actinide fission, from the three different fissioning systems: 232Th(n, f), 238U(n, f), at two different neutron energies, as well as 252Cf(sf). The impacts of changing the fissioning system, the compound nuclear excitation energy, the minimum J of the binary partner, and the number of neutrons emitted on the IYR of 134Te are determined. The decay code TALYS is used in combination with the fission simulation code FREYA to calculate the primary fragment angular momentum from the IYR. We find that the IYR of 134Te has a slope of 0.004 +/- 0.002 with increase in compound nucleus (CN) mass. When investigating the impact on the IYR of increased CN excitation energy, we find no change with an energy increase similar to the difference between thermal and fast fission. By varying the mass of the partner fragment emerging with 134Te, it is revealed that the IYR of 134Te is independent of the total amount of prompt neutrons emitted from the fragment pair. This indicates that neutrons carry minimal angular momentum away from the fission fragments. Comparisons with the FREYA+TALYS simulations reveal that the average angular momentum in 134Te following 238U(n, f) is 6.0 h over bar . This is not consistent with the value deduced from recent CGMF calculations. Finally, the IYR sensitivity to the angular momentum of the primary fragment is discussed. These results are not only important to help understanding the underlying mechanism in nuclear fission, but can also be used to constrain and benchmark fission models, and are relevant to the gamma -ray heating problem of reactors.

225Pa and 221Ac were produced at the IGISOL facility through proton-induced fusion-evaporation reactions and have been studied using & alpha;-particle spectroscopy, as well as & alpha;-& gamma; and & alpha;-electron coincidence spectroscopy. The level scheme of 221Ac, daughter of 225Pa, and of 217Fr, daughter of 221Ac were reconstructed. An interpretation of 221Ac levels as K = 5/2 & PLUSMN; and K = 3/2 & PLUSMN; parity-doublet bands is proposed. Such bands appear in reflection-asymmetric models and would be an indication of a static reflection asymmetric shape for 221Ac.

A research programme focused on the study of the nuclear structure of actinide isotopes has recently been implemented at the IGISOL facility, University of Jyväskylä. Within this scope, a new decay station named VADER (Versatile Actinides DEcay spectRoscopy setup) has been developed and commissioned. The system consists of a compact array of silicon detectors, a liquid-nitrogen-cooled silicon lithium (Si(Li)) detector and three broad energy germanium detectors (BEGe), placed around a thin implantation carbon foil. The combined use of different detectors allows the measurement of α particles, conversion electrons and de-excitation γ rays in coincidence, enabling a full reconstruction of nuclear decay schemes. The measurement of basic nuclear decay observables provides a picture of the nuclear shell evolution in neutron-deficient actinides, and highlights the possible emergence of reflection-asymmetric shapes in the region.

The structure of the unbound F-15 nucleus is investigated using the inverse kinematics resonant scattering of a radioactive O-14 beam impinging on a CH2 target. The analysis of H-1(O-14, p) O-14 and H-1(O-14, 2p) N-13 reactions allowed the confirmation of the previously observed narrow 1/2(-) resonance, near the two-proton decay threshold, and the identification of two new narrow 5/2(-) and 3/2(-) resonances. The newly observed levels decay by 1p emission to the ground of O-14, and by sequential 2p emission to the ground state of N-13 via the 1(-) resonance of O-14. Gamow shell model (GSM) analysis of the experimental data suggests that the wave functions of the 5/2(-) and 3/2(-) resonances may be collectivized by the continuum coupling to nearby 2p- and 1p-decay channels. The observed excitation function H-1(O-14, p) O-14 and resonance spectrum in F-15 are well reproduced in the unified framework of the GSM.

The $$\beta $$ -delayed $$\gamma $$ spectroscopy of $$^{81}$$ As has been performed using a purified beam of $$^{81}$$ Ge $$(9/2^+)$$ ground state at the Ion Guide Isotope Separator On-Line facility (IGISOL). The $$^{81}$$ Ge $$^+$$ ions were produced using proton-induced fission of $$^{232}$$ Th and selected with the double Penning trap JYFLTRAP for the post-trap decay spectroscopy measurements. The low-spin $$(1/2^+)$$ isomeric-state ions $$^{81m}\hbox {Ge}^+$$ were not observed in the fission products. The intrinsic half-life of the $$^{81}$$ Ge ground state has been determined as $$T_{1/2}=6.4(2)~\hbox {s}$$ , which is significantly shorter than the literature value. A new level scheme of $$^{81}$$ As has been built and is compared to shell-model calculations.

We report on spectroscopic information and lifetime measurements of even-even neutron-rich Te isotopes. Excited states were populated in fast-neutron induced fission of U-238 at the ALTO facility of IJCLab with the LICORNE neutron source and detected using the hybrid.-ball spectrometer. We provide first results on lifetimes of the 6(1)(+) state in Te-136 and the (6(1)(+)), (4(1)(+)), and (2(1)(+)) states in Te-138 and discuss the results in the context of large-scale shell-model calculations. The level schemes of Te-136 and Te-138 are revised in terms of lifetimes of their 2(1)(+), 4(1)(+) states and updated information on the (4(2)(+)) state in Te-136 is presented. In addition, previously reported data on spectroscopy and lifetimes in Te-134 are reexamined. This work provides new insights into the evolution of collectivity for Te isotopes with N = 82, 84, 86.

We report on spectroscopic information and lifetime measurements in the neutron-rich I 135 , 137 , 139 isotopes. This is the first lifetime data on iodine isotopes beyond N = 82 . Excited states were populated in fast neutron-induced fission of U 238 at the ALTO facility of IJCLab with the LICORNE neutron source and detected using the hybrid ν -ball spectrometer. The level schemes of the I 135 , 137 , 139 isotopes are revised in terms of excited states with up to maximum spin-parity of ( 33 / 2 + ) , populated for the first time in fast neutron-induced fission. We provide first results on the lifetimes of the ( 9 / 2 1 + ) and ( 13 / 2 1 + ) states in I 137 and I 139 , and the ( 17 / 2 1 + ) state in I 137 . In addition, we give upper lifetime limits for the ( 11 / 2 1 + ) states in I 135 − 139 , the ( 15 / 2 1 + ) state in I 137 , the ( 17 / 2 1 + ) state in I 139 , and reexamine the ( 29 / 2 1 + ) state in I 137 . The isomeric data in I 135 are reinvestigated, such as the previously known ( 15 / 2 1 + ) and ( 23 / 2 1 − ) isomers with T 1 / 2 of 1.64(14) and 4.6(7) ns, respectively, as obtained in this work. The new spectroscopic information is compared to that from spontaneous or thermal-neutron induced fission and discussed in the context of large scale shell-model (LSSM) calculations for the region beyond Sn 132 , indicating the behavior of collectivity for the three valence-proton iodine chain with N = 82 , 84 , 86 .

When a heavy atomic nucleus splits (fission), the resulting fragments are observed to emerge spinning1; this phenomenon has been a mystery in nuclear physics for over 40 years2,3. The internal generation of typically six or seven units of angular momentum in each fragment is particularly puzzling for systems that start with zero, or almost zero, spin. There are currently no experimental observations that enable decisive discrimination between the many competing theories for the mechanism that generates the angular momentum4–12. Nevertheless, the consensus is that excitation of collective vibrational modes generates the intrinsic spin before the nucleus splits (pre-scission). Here we show that there is no significant correlation between the spins of the fragment partners, which leads us to conclude that angular momentum in fission is actually generated after the nucleus splits (post-scission). We present comprehensive data showing that the average spin is strongly mass-dependent, varying in saw-tooth distributions. We observe no notable dependence of fragment spin on the mass or charge of the partner nucleus, confirming the uncorrelated post-scission nature of the spin mechanism. To explain these observations, we propose that the collective motion of nucleons in the ruptured neck of the fissioning system generates two independent torques, analogous to the snapping of an elastic band. A parameterization based on occupation of angular momentum states according to statistical theory describes the full range of experimental data well. This insight into the role of spin in nuclear fission is not only important for the fundamental understanding and theoretical description of fission, but also has consequences for the γ-ray heating problem in nuclear reactors13,14, for the study of the structure of neutron-rich isotopes15,16, and for the synthesis and stability of super-heavy elements17,18. γ-ray spectroscopy experiments on the origin of spin in the products of nuclear fission of spin-zero nuclei suggest that the fission fragments acquire their spin after scission, rather than before.

The 2019 Mazurian Lakes Conference on Physics was held in Piaski, a vacation resort located at the lake of Bełdany in the heart of the Great Mazurian Lakes District, from the 1 st to 7 th of September 2019.It was the 36 th meeting in a series initiated over 50 years ago, in 1968, by Professor Zdzisław Wilhelmi and his students and collaborators.Throughout these years, the Mazurian conferences have gained a world-wide reputation for their high scientific merit and a unique atmosphere.

The reaction of a pulsed 18 O beam on a self-supporting and gold-backed isotopically-enriched 164 Dy target of thickness 6.3 mg/cm 2 at separate primary beam energies of 71, 76 and 80 MeV was studied at the accelerator at the ALTO facility of the IPN Orsay. The γ rays produced were detected using the newly-constructed ν-Ball spectrometer which comprised of HPGe and LaBr 3 (Ce) detectors. This conference paper describes the methodology and effectiveness of multiplicity/sum-energy gating, for channel selection between fusion evaporation events and lower multiplicity/energy events from inelastic nuclear scattering and Coulomb excitation of the target, and from two-neutron transfer reactions to 166 Dy.

The nu-ballspectrometer is an hybrid array combining high purity co-axial germanium detectors from the french-UK loan pool, clover detectors from the GAMMAPOOL, lanthanum bromide (LaBr3:Ce) scintillator detectors belonging to the FATIMA collaboration and phoswitches from the PARIS collaboration. The aim was to couple the excellent energy resolution of germanium detectors to the excellent time resolution of the LaBr3 detectors. We achieved a total photopeak efficiency of 6.7% at 1.3 MeV, and peak-to-total ratio of 50% for the germanium part of the array. Using the digital acquisition system FASTER, we achieved time resolution of about 250 ps for LaBr3. This acquisition system made also possible the use of the calorimetry for reaction selection. It makes v-ball the first fully digital large fast timing spectrometer with time resolution similar to analogue electronics. The construction began in June 2017 and commissioning was performed in early November 2017. From November 2017 to June 2018, more than 3200 h of beam time were provided by the ALTO facility to perform eight experiments during the campaign. Among them, five weeks of beam time were dedicated to gamma spectroscopy of fast neutron induced reactions. In this paper all the technical details about the spectrometer are presented. First steps of the data analysis process are also discussed.

Excited states in the neutron-rich Se-85 nucleus have been studied using for the first time a fast neutron-induced fission of Th-232. The experiment was performed at the ALTO facility of the IPN Orsay. Coupling of the LICORNE directional neutron source with the nu-ball high-resolution gamma-ray spectrometer provided unique access to high-spin states in neutron-rich fission fragments from the Th-232(n,f) reaction. A preliminary level scheme of Se-85 was established by the analysis of prompt gamma-gamma-gamma coincidences. Identification of the all known yrast states in Se-85 is the first step towards studies of more neutron-rich Se isotopes.