Direct mass measurements of neutron-deficient nuclides around the N = 50 shell closure below 100 ... more Direct mass measurements of neutron-deficient nuclides around the N = 50 shell closure below 100 Sn were performed at the FRS Ion Catcher (FRS-IC) at GSI, Germany. The nuclei were produced by projectile fragmentation of 124 Xe, separated in the fragment separator FRS and delivered to the FRS-IC. The masses of 14 ground states and two isomers were measured with relative mass uncertainties down to 1×10 −7 using the multiple-reflection time-of-flight mass spectrometer of the FRS-IC, including the first direct mass measurements of 98 Cd and 97 Rh. A new Q EC = 5437 ± 67 keV was obtained for 98 Cd, resulting in a summed Gamow-Teller (GT) strength for the five observed transitions (0 + −→ 1 +) as B(GT) = 2.94 +0.32 −0.28. Investigation of this result in state-of-the-art shell model approaches sheds light into a better understanding of the GT transitions in even-even isotones at N = 50. The excitation energy of the long-lived isomeric state in 94 Rh was determined for the first time to be 293 ± 21 keV. This, together with the shell model calculations, allows the level ordering in 94 Rh to be understood.
We present first preliminary results of a novel method for measuring independent isotopic fission... more We present first preliminary results of a novel method for measuring independent isotopic fission yields (IIFYs) of spontaneous fission (SF) via direct mass measurements, at the FRS Ion Catcher (FRS-IC) at GSI. Fission products were generated from a 252Cf source installed in a cryogenic stopping cell, and were identified and counted with the multiple-reflection time-of-flight mass spectrometer (MR-TOF-MS) of the FRS-IC, utilizing well-established measurement and data analysis methods. The MR-TOF-MS resolves isobars unambiguously, even with limited statistics, and its non-scanning nature ensures minimal relative systematic uncertainties amongst fission products. The analysis for extracting IIFYs includes isotope-dependent efficiency corrections for all components of the FRS-IC. In particular, we applied a self-consistent technique that takes into account the element-dependent survival efficiencies in the CSC, due to chemical reactions with the buffer gas. Our IIFY results, which cove...
A very intense γ beam of the Gamma Factory facility proposed at CERN can be used to generate radi... more A very intense γ beam of the Gamma Factory facility proposed at CERN can be used to generate radioactive ion beams (RIBs) with high production yields and study the structure of exotic neutron-rich nuclei. The radioactive nuclides are generated via photofission in several actinide targets and thermalized in high-purity cryogenic helium, filling a gas cell which is enclosing the targets. Electric fields are used to extract heavy ions and form RIBs which can be send to various selection and measurement stations. Estimates for the production and extraction yields of exotic neutron-rich nuclei with such a setup are provided. A study of the impact of space charge, build-up inside the gas cell, on the extraction properties is presented and it is demonstrated that the beam needs to be chopped for achieving optimal extraction yields.
The design and commissioning of a new detector array within the ELIGANT set-up, with the aim of m... more The design and commissioning of a new detector array within the ELIGANT set-up, with the aim of measuring weak second and third order γ-ray emitting quantum-electrodynamical processes, is reported. This ELIGANT-GG set-up is discussed in terms of technical specifications, time resolution, and efficiency of the set-up itself. The long-term stability and linearity of the detectors are investigated as important factors for the feasability of long-running experiments. Furthermore, the capabilility of the set-up for measuring angular distributions is shown. Finally, it is demonstrated that indeed very rare γ-coincidence signals with a relative branching as low as 10−6 can be measured.
Here we report on the characterization of one of the large-volume LaBr_3:Ce detectors for the ELI... more Here we report on the characterization of one of the large-volume LaBr_3:Ce detectors for the ELIGANT project at ELI-NP. The main focus of this work is the response function for high-energy γ rays of such detectors. In particular, we compare a selection of unfolding methods to resolve small structures in γ-ray spectra with high-energies. Three methods have been compared using γ-ray spectra with energies up to 12 MeV obtained in an experiment at the 3 MV Tandetron facility at IFIN-HH. The results show that the iterative unfolding approach gives the best qualitative reproduction of the emitted γ-ray spectrum. Furthermore, the correlation fluctuations in high-energy regime from the iterative method are two orders of magnitude smaller than when using the matrix inversion approach with second derivative regularization. In addition, the iterative method is computationally faster as it does not contain large matrix inversions. The matrix inversion method does, however, give more consistent...
A significant experimental and theoretical research program is ongoing worldwide on second-order ... more A significant experimental and theoretical research program is ongoing worldwide on second-order processes in physics. For the electromagnetic nuclear interaction, the observation of the competitive double-γ decay from ^137mBa has opened up the nuclear structure field for detailed investigation of second-order processes through the manifestation of off-diagonal nuclear polarizability. Here we confirm this discovery with an 8.7σ significance, and an improved value on the double-photon versus single-photon branching ratio as 2.62×10^-6(30). Our results, now covering a larger angular range, contradict conclusions from the discovery experiment, where the decay was interpreted to be dominated by a quadrupole-quadrupole component. Here, we find a substantial enhancement in the energy distribution consistent with a dominating octupole-dipole character and a rather small quadrupole-quadrupole element in the decay, hindered due to an evolution of the internal nuclear structure. The implied s...
Multi-nucleon transfer (MNT) reactions give access to heavy exotic neutron-rich nuclei which are ... more Multi-nucleon transfer (MNT) reactions give access to heavy exotic neutron-rich nuclei which are difficult to be produced by fission or in fragmentation reactions. The U+Ni MNT reactions taking place inside a gas cell, is simulated with the GRAZING code. Simulations of the release of reaction product from the target and of their slowing down in the He gas of the cell are performed with the GEANT4 software. The space charge effects induced by gas ionization are studied with the SIMION program. The implications for an experimental program using such a setup are discussed.
A significant experimental and theoretical research program is ongoing worldwide on second-order ... more A significant experimental and theoretical research program is ongoing worldwide on second-order nuclear transitions, recently triggered by the observation of the competitive double-$\gamma$ decay in $^{137}$Ba. Here we unambiguously confirm this discovery with an improved value on the double-photon versus single-photon branching ratio as $2.62\times10^{-6}(30)$. Our results, now covering a larger angular range, contradict conclusions from the discovery experiment, where the decay was interpreted to be dominated by a quadrupole-quadrupole component. Here, we find a substantial enhancement in the energy distribution consistent with a dominating octupole-dipole character and a rather small quadrupole-quadrupole element in the decay. We investigate these results in terms of both the quasiparticle phonon model built on energy-density functionals and the Monte Carlo shell model. We find that both these approaches give a consistent and satisfactory reproduction of the octupole-dipole coup...
The production of very neutron-rich nuclides heavier than fission fragments is an ongoing experim... more The production of very neutron-rich nuclides heavier than fission fragments is an ongoing experimental challenge. Multi-nucleon transfer reactions (MNT) have been suggested as a method to produce these nuclides. By thermalizing the reaction products in gas-filled stopping cells, we can deliver them as cooled highquality beams to decay, laser and mass spectrometry experiments. High precision mass spectrometry will allow for the first time to universally and unambiguously identify the atomic and proton numbers of the ions produced in MNT reactions. In this way their ground and isomeric state properties can be studied in high-precision measurements. In experiments at IGISOL, Finland and at FRS Ion Catcher, Germany, we have done and will perform broadband measurements of the reaction products, with the aim to improve the understanding of the reaction mechanism and to determine the properties of the ground and isomeric states of the produced nuclides. First results and preparations for u...
Mass measurements of the $^{69}$As, $^{70,71}$Se and $^{71}$Br isotopes, produced via fragmentati... more Mass measurements of the $^{69}$As, $^{70,71}$Se and $^{71}$Br isotopes, produced via fragmentation of a $^{124}$Xe primary beam at the FRS at GSI, have been performed with the multiple-reflection time-of-flight mass spectrometer (MR-TOF-MS) of the FRS Ion Catcher with an unprecedented mass resolving power of almost 1,000,000. For the $^{69}$As isotope, this is the first direct mass measurement. A mass uncertainty of 22 keV was achieved with only 10 events. For the $^{70}$Se isotope, a mass uncertainty of 2.6 keV was obtained, corresponding to a relative accuracy of $\delta$m/m = 4.0$\times 10^{-8}$, with less than 500 events. The masses of the $^{71}$Se and $^{71}$Br isotopes were measured with an uncertainty of 23 and 16 keV, respectively. Our results for the $^{70,71}$Se and $^{71}$Br isotopes agree with the 2016 Atomic Mass Evaluation, and our result for the $^{69}$As isotope resolves the discrepancy between previous indirect measurements. We measured also the mass of $^{14}$N$^...
Within the ELI-NP (Extreme Light Infrastructure-Nuclear Physics) project developed in Magurele, R... more Within the ELI-NP (Extreme Light Infrastructure-Nuclear Physics) project developed in Magurele, Romania, an IGISOL (Ion Guided Isotope Separator On Line) facility is being designed in which exotic nuclei will be produced by the photo-fission of thin uranium foils then extracted and measured. The targets will be placed in a Cryogenic Stopping Cell (CSC) filled with He gas, in which the fission fragments will be thermalized and extracted, using both electric fields and gas flow. The optimization of the target system, aiming at maximizing the rate of ions released in the CSC gas, determining the optimum thickness, geometry and tilting angle of the targets and the optimum gamma beam energy range, is reported.
Second-order processes in physics is a research topic focusing attention from several fields worl... more Second-order processes in physics is a research topic focusing attention from several fields worldwide including, for example, non-linear quantum electrodynamics with high-power lasers, neutrinoless double-β decay, and stimulated atomic two-photon transitions. For the electromagnetic nuclear interaction, the observation of the competitive double-γ decay from 137mBa has opened up the nuclear structure field for detailed investigation of second-order processes through the manifestation of off-diagonal nuclear polarisability. Here, we confirm this observation with an 8.7σ significance, and an improved value on the double-photon versus single-photon branching ratio as 2.62 × 10−6(30). Our results, however, contradict the conclusions from the original experiment, where the decay was interpreted to be dominated by a quadrupole-quadrupole component. Here, we find a substantial enhancement in the energy distribution consistent with a dominating octupole-dipole character and a rather small q...
Here we report on the characterization of one of the large-volume LaBr 3 :Ce detectors for the EL... more Here we report on the characterization of one of the large-volume LaBr 3 :Ce detectors for the ELIGANT project at ELI-NP. The main focus of this work is the response function for high-energy γ rays of such detectors. In particular, we compare a selection of unfolding methods to resolve small structures in γ-ray spectra with high-energies. Three methods have been compared using γ-ray spectra with energies up to 12 MeV obtained in an experiment at the 3 MV Tandetron™ facility at IFIN-HH. The results show that the iterative unfolding approach gives the best qualitative reproduction of the emitted γ-ray spectrum. Furthermore, the correlation fluctuations in highenergy regime from the iterative method are two orders of magnitude smaller than when using the matrix inversion approach with second derivative regularization. In addition, the iterative method is computationally faster as it does not contain large matrix inversions. The matrix inversion method does, however, give more consistent results over the full energy range and in the low-statistics limit. Our conclusion is that the performance of the iterative approach makes it well suitable for semi-online analysis of experimental data. These results will be important, both for experiments with the ELIGANT setup, and for on-line diagnostics of the energy spread of the γ-ray beam which is under implementation at ELI-NP.
Direct mass measurements of neutron-deficient nuclides around the N = 50 shell closure below 100 ... more Direct mass measurements of neutron-deficient nuclides around the N = 50 shell closure below 100 Sn were performed at the FRS Ion Catcher (FRS-IC) at GSI, Germany. The nuclei were produced by projectile fragmentation of 124 Xe, separated in the fragment separator FRS and delivered to the FRS-IC. The masses of 14 ground states and two isomers were measured with relative mass uncertainties down to 1×10 −7 using the multiple-reflection time-of-flight mass spectrometer of the FRS-IC, including the first direct mass measurements of 98 Cd and 97 Rh. A new Q EC = 5437 ± 67 keV was obtained for 98 Cd, resulting in a summed Gamow-Teller (GT) strength for the five observed transitions (0 + −→ 1 +) as B(GT) = 2.94 +0.32 −0.28. Investigation of this result in state-of-the-art shell model approaches sheds light into a better understanding of the GT transitions in even-even isotones at N = 50. The excitation energy of the long-lived isomeric state in 94 Rh was determined for the first time to be 293 ± 21 keV. This, together with the shell model calculations, allows the level ordering in 94 Rh to be understood.
We present first preliminary results of a novel method for measuring independent isotopic fission... more We present first preliminary results of a novel method for measuring independent isotopic fission yields (IIFYs) of spontaneous fission (SF) via direct mass measurements, at the FRS Ion Catcher (FRS-IC) at GSI. Fission products were generated from a 252Cf source installed in a cryogenic stopping cell, and were identified and counted with the multiple-reflection time-of-flight mass spectrometer (MR-TOF-MS) of the FRS-IC, utilizing well-established measurement and data analysis methods. The MR-TOF-MS resolves isobars unambiguously, even with limited statistics, and its non-scanning nature ensures minimal relative systematic uncertainties amongst fission products. The analysis for extracting IIFYs includes isotope-dependent efficiency corrections for all components of the FRS-IC. In particular, we applied a self-consistent technique that takes into account the element-dependent survival efficiencies in the CSC, due to chemical reactions with the buffer gas. Our IIFY results, which cove...
A very intense γ beam of the Gamma Factory facility proposed at CERN can be used to generate radi... more A very intense γ beam of the Gamma Factory facility proposed at CERN can be used to generate radioactive ion beams (RIBs) with high production yields and study the structure of exotic neutron-rich nuclei. The radioactive nuclides are generated via photofission in several actinide targets and thermalized in high-purity cryogenic helium, filling a gas cell which is enclosing the targets. Electric fields are used to extract heavy ions and form RIBs which can be send to various selection and measurement stations. Estimates for the production and extraction yields of exotic neutron-rich nuclei with such a setup are provided. A study of the impact of space charge, build-up inside the gas cell, on the extraction properties is presented and it is demonstrated that the beam needs to be chopped for achieving optimal extraction yields.
The design and commissioning of a new detector array within the ELIGANT set-up, with the aim of m... more The design and commissioning of a new detector array within the ELIGANT set-up, with the aim of measuring weak second and third order γ-ray emitting quantum-electrodynamical processes, is reported. This ELIGANT-GG set-up is discussed in terms of technical specifications, time resolution, and efficiency of the set-up itself. The long-term stability and linearity of the detectors are investigated as important factors for the feasability of long-running experiments. Furthermore, the capabilility of the set-up for measuring angular distributions is shown. Finally, it is demonstrated that indeed very rare γ-coincidence signals with a relative branching as low as 10−6 can be measured.
Here we report on the characterization of one of the large-volume LaBr_3:Ce detectors for the ELI... more Here we report on the characterization of one of the large-volume LaBr_3:Ce detectors for the ELIGANT project at ELI-NP. The main focus of this work is the response function for high-energy γ rays of such detectors. In particular, we compare a selection of unfolding methods to resolve small structures in γ-ray spectra with high-energies. Three methods have been compared using γ-ray spectra with energies up to 12 MeV obtained in an experiment at the 3 MV Tandetron facility at IFIN-HH. The results show that the iterative unfolding approach gives the best qualitative reproduction of the emitted γ-ray spectrum. Furthermore, the correlation fluctuations in high-energy regime from the iterative method are two orders of magnitude smaller than when using the matrix inversion approach with second derivative regularization. In addition, the iterative method is computationally faster as it does not contain large matrix inversions. The matrix inversion method does, however, give more consistent...
A significant experimental and theoretical research program is ongoing worldwide on second-order ... more A significant experimental and theoretical research program is ongoing worldwide on second-order processes in physics. For the electromagnetic nuclear interaction, the observation of the competitive double-γ decay from ^137mBa has opened up the nuclear structure field for detailed investigation of second-order processes through the manifestation of off-diagonal nuclear polarizability. Here we confirm this discovery with an 8.7σ significance, and an improved value on the double-photon versus single-photon branching ratio as 2.62×10^-6(30). Our results, now covering a larger angular range, contradict conclusions from the discovery experiment, where the decay was interpreted to be dominated by a quadrupole-quadrupole component. Here, we find a substantial enhancement in the energy distribution consistent with a dominating octupole-dipole character and a rather small quadrupole-quadrupole element in the decay, hindered due to an evolution of the internal nuclear structure. The implied s...
Multi-nucleon transfer (MNT) reactions give access to heavy exotic neutron-rich nuclei which are ... more Multi-nucleon transfer (MNT) reactions give access to heavy exotic neutron-rich nuclei which are difficult to be produced by fission or in fragmentation reactions. The U+Ni MNT reactions taking place inside a gas cell, is simulated with the GRAZING code. Simulations of the release of reaction product from the target and of their slowing down in the He gas of the cell are performed with the GEANT4 software. The space charge effects induced by gas ionization are studied with the SIMION program. The implications for an experimental program using such a setup are discussed.
A significant experimental and theoretical research program is ongoing worldwide on second-order ... more A significant experimental and theoretical research program is ongoing worldwide on second-order nuclear transitions, recently triggered by the observation of the competitive double-$\gamma$ decay in $^{137}$Ba. Here we unambiguously confirm this discovery with an improved value on the double-photon versus single-photon branching ratio as $2.62\times10^{-6}(30)$. Our results, now covering a larger angular range, contradict conclusions from the discovery experiment, where the decay was interpreted to be dominated by a quadrupole-quadrupole component. Here, we find a substantial enhancement in the energy distribution consistent with a dominating octupole-dipole character and a rather small quadrupole-quadrupole element in the decay. We investigate these results in terms of both the quasiparticle phonon model built on energy-density functionals and the Monte Carlo shell model. We find that both these approaches give a consistent and satisfactory reproduction of the octupole-dipole coup...
The production of very neutron-rich nuclides heavier than fission fragments is an ongoing experim... more The production of very neutron-rich nuclides heavier than fission fragments is an ongoing experimental challenge. Multi-nucleon transfer reactions (MNT) have been suggested as a method to produce these nuclides. By thermalizing the reaction products in gas-filled stopping cells, we can deliver them as cooled highquality beams to decay, laser and mass spectrometry experiments. High precision mass spectrometry will allow for the first time to universally and unambiguously identify the atomic and proton numbers of the ions produced in MNT reactions. In this way their ground and isomeric state properties can be studied in high-precision measurements. In experiments at IGISOL, Finland and at FRS Ion Catcher, Germany, we have done and will perform broadband measurements of the reaction products, with the aim to improve the understanding of the reaction mechanism and to determine the properties of the ground and isomeric states of the produced nuclides. First results and preparations for u...
Mass measurements of the $^{69}$As, $^{70,71}$Se and $^{71}$Br isotopes, produced via fragmentati... more Mass measurements of the $^{69}$As, $^{70,71}$Se and $^{71}$Br isotopes, produced via fragmentation of a $^{124}$Xe primary beam at the FRS at GSI, have been performed with the multiple-reflection time-of-flight mass spectrometer (MR-TOF-MS) of the FRS Ion Catcher with an unprecedented mass resolving power of almost 1,000,000. For the $^{69}$As isotope, this is the first direct mass measurement. A mass uncertainty of 22 keV was achieved with only 10 events. For the $^{70}$Se isotope, a mass uncertainty of 2.6 keV was obtained, corresponding to a relative accuracy of $\delta$m/m = 4.0$\times 10^{-8}$, with less than 500 events. The masses of the $^{71}$Se and $^{71}$Br isotopes were measured with an uncertainty of 23 and 16 keV, respectively. Our results for the $^{70,71}$Se and $^{71}$Br isotopes agree with the 2016 Atomic Mass Evaluation, and our result for the $^{69}$As isotope resolves the discrepancy between previous indirect measurements. We measured also the mass of $^{14}$N$^...
Within the ELI-NP (Extreme Light Infrastructure-Nuclear Physics) project developed in Magurele, R... more Within the ELI-NP (Extreme Light Infrastructure-Nuclear Physics) project developed in Magurele, Romania, an IGISOL (Ion Guided Isotope Separator On Line) facility is being designed in which exotic nuclei will be produced by the photo-fission of thin uranium foils then extracted and measured. The targets will be placed in a Cryogenic Stopping Cell (CSC) filled with He gas, in which the fission fragments will be thermalized and extracted, using both electric fields and gas flow. The optimization of the target system, aiming at maximizing the rate of ions released in the CSC gas, determining the optimum thickness, geometry and tilting angle of the targets and the optimum gamma beam energy range, is reported.
Second-order processes in physics is a research topic focusing attention from several fields worl... more Second-order processes in physics is a research topic focusing attention from several fields worldwide including, for example, non-linear quantum electrodynamics with high-power lasers, neutrinoless double-β decay, and stimulated atomic two-photon transitions. For the electromagnetic nuclear interaction, the observation of the competitive double-γ decay from 137mBa has opened up the nuclear structure field for detailed investigation of second-order processes through the manifestation of off-diagonal nuclear polarisability. Here, we confirm this observation with an 8.7σ significance, and an improved value on the double-photon versus single-photon branching ratio as 2.62 × 10−6(30). Our results, however, contradict the conclusions from the original experiment, where the decay was interpreted to be dominated by a quadrupole-quadrupole component. Here, we find a substantial enhancement in the energy distribution consistent with a dominating octupole-dipole character and a rather small q...
Here we report on the characterization of one of the large-volume LaBr 3 :Ce detectors for the EL... more Here we report on the characterization of one of the large-volume LaBr 3 :Ce detectors for the ELIGANT project at ELI-NP. The main focus of this work is the response function for high-energy γ rays of such detectors. In particular, we compare a selection of unfolding methods to resolve small structures in γ-ray spectra with high-energies. Three methods have been compared using γ-ray spectra with energies up to 12 MeV obtained in an experiment at the 3 MV Tandetron™ facility at IFIN-HH. The results show that the iterative unfolding approach gives the best qualitative reproduction of the emitted γ-ray spectrum. Furthermore, the correlation fluctuations in highenergy regime from the iterative method are two orders of magnitude smaller than when using the matrix inversion approach with second derivative regularization. In addition, the iterative method is computationally faster as it does not contain large matrix inversions. The matrix inversion method does, however, give more consistent results over the full energy range and in the low-statistics limit. Our conclusion is that the performance of the iterative approach makes it well suitable for semi-online analysis of experimental data. These results will be important, both for experiments with the ELIGANT setup, and for on-line diagnostics of the energy spread of the γ-ray beam which is under implementation at ELI-NP.
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Papers by Dragos Nichita