arXiv subjects

I. Gheorghe

Publications and source records attributed to I. Gheorghe.

14 recordsLinked to original sources

arXiv2025

Precise measurement of the $\gamma$-decay probability of the Hoyle state with a new triple coincidence-detection method

We measured the $\gamma$-decay probability of the Hoyle state with a new method of triple coincidence detection of a scattered $\alpha$ particle, a recoil $\rm ^{12}C$ nucleus, and a $\gamma$ ray in inelastic alpha scattering on $\rm ^{12}C$. This method successfully enabled a low-background measurement and a precise determination of the $\gamma$-decay probability of the Hoyle state as $\Gamma_\mathrm{\gamma}/\Gamma=[4.00 \pm 0.22 \mathrm{(sta.)} \pm 0.18 \mathrm{(sys.)}]\times10^{-4}$, which is consistent with the previous literature value. Therefore, we concluded that the literature value can be reliably used in the study of nucleosynthesis in the universe.

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arXiv2025

ELIGANT-TN -- ELI Gamma Above Neutron Threshold: The Thermal Neutron setup

Here we present the thermal neutron counter from the ELI Gamma Above Neutron Threshold setup at the Extreme Light Infrastructure - Nuclear Physics. We describe the mechanical design of the setup, the properties of the ${}^{3}$He gas counters, and the hardware data-acquisition electronics and software digital signal processing. The performance of the complete detector array is demonstrated via Geant4 and MCNP simulations, and measurements with typical neutron sources. The analysis procedure for experimental measurements are outlined with a in-beam test experiment with an $\alpha$ beam to measure the ${}^{13}\mathrm{C}(\alpha,\mathrm{n}_{0}){}^{16}\mathrm{O}$ cross-section branching ratios.

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arXiv2025

Photoneutron reactions on gold in the giant dipole resonance region: reaction cross sections and average kinetic energies of $(\gamma,\,xn)$ photoneutrons

In this work, we present new data on the $^{197}$Au photoneutron reactions in and above the giant dipole resonance region, obtained by using 8 to 39~MeV quasi-monochromatic $\gamma$-ray beams produced at the NewSUBARU facility in Japan and a high-and-flat efficiency neutron detection system. We report absolute cross sections and mean photoneutron energies for the $^{197}$Au$(\gamma,\,inX)$ reactions with $i$~=~1 to 4. The photoabsorption cross section was obtained as the sum of the $(\gamma,\,inX)$ reaction cross sections. The giant dipole resonance parameter values were obtained by fitting the experimental photoabsorption cross sections. The present photoabsorption cross sections are in good agreement with the Saclay results of Veyssiere~\emph{et al.}. Thus, our study does not support the recommendation of Berman~\emph{et al.} of lowering the Saclay photoabsorption cross sections by 8$\%$. We observed a non-statistical high-energy neutron emission in the $(\gamma,\,n)$ reaction in the low-energy region between $S_n$ and 10~MeV. The present results are compared with data from the literature and statistical model calculations performed with the TALYS and EMPIRE codes.

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arXiv2024

Photoneutron cross section measurements on $^{208}$Pb in the Giant Dipole Resonance region

Photoneutron reactions on $^{208}$Pb in the Giant Dipole Resonance energy region have been investigated at the $\gamma$-ray beam line of the NewSUBARU facility in Japan. The measurements made use of quasi-monochromatic laser Compton backscattering $\gamma$-ray beams in a broad energy range, from the neutron threshold up to 38 MeV, and of a flat-efficiency moderated $^3$He neutron detection system along with associated neutron-multiplicity sorting methods. We report absolute cross sections and mean photoneutron energies for the $^{208}$Pb$(\gamma,\,inX)$ reactions with $i$~=~1 to 4. The fine structure present in the $^{208}$Pb$(\gamma,\,n)$ cross sections at incident energies lower than 13~MeV has been observed. The photoabsorption cross section has been obtained as the sum of the $(\gamma,\,inX)$ reaction cross sections. By reproducing the measured ring-ratio values at excitation energies below the two neutron separation energy, we were able to extract estimations on the $^{208}$Pb$(\gamma,\,n)$ photoneutron energy spectra and on the partial photoneutron cross sections for leaving the residual $^{207}$Pb in its ground and first two excited states. The present results are compared with data from the literature and statistical model calculations.

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arXiv2024

Giant Dipole Resonance photofission and photoneutron reactions in $^{238}$U and $^{232}$Th

New measurements of photofission and photoneutron reactions on $^{238}$U and $^{232}$Th in the Giant Dipole Resonance (GDR) energy region have been performed at the laser Compton-scattering $\gamma$-ray source of the NewSUBARU synchrotron radiation facility using a high-and-flat efficiency moderated $^3$He detection array. The neutron-multiplicity sorting of high-multiplicity fission neutron coincidence events has been performed using a dedicated energy dependent, multiple firing statistical treatment. The photoneutron $(\gamma,\,in)$ with $i$~=~1~--~3 and photofission $(\gamma,\,F)$ reactions have been discriminated by considering a Gaussian distribution of prompt-fission-neutron (PFN) multiplicities predicted by the theory of evaporation in sequential neutron emission from excited fission fragments. We report experimental $(\gamma,\,n)$, $(\gamma,\,2n)$, $(\gamma,\,3n)$ and $(\gamma,\,F)$ cross sections, average energies of PFNs and of $(\gamma,\,in)$ photoneutrons, as well as the mean number of PFNs per fission act and the width of the PFNs multiplicity distribution. Based on these primary experimental results and combined with reasonable assumptions, we extract also the first- and second-chance fission contributions. The new experimental results are compared with statistical-model calculations performed with the EMPIRE-3.2 Malta and TALYS-1.964 codes on the present data and with prompt fission emission calculations obtained with the Los Alamos model in the frame of the most probable fragmentation approach with and without sequential emission.

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arXiv2020

Shape Coexistence at Zero Spin in 64Ni Driven by the Monopole Tensor Interaction

The low-spin structure of the semimagic 64Ni nucleus has been considerably expanded: combining four experiments, several 0+ and 2+ excited states were identified below 4.5 MeV, and their properties established. The Monte Carlo shell model accounts for the results and unveils an unexpectedly complex landscape of coexisting shapes: a prolate 0+ excitation is located at a surprisingly high energy (3463 keV), with a collective 2+ state 286 keV above it, the first such observation in Ni isotopes. The evolution in excitation energy of the prolate minimum across the neutron N = 40 subshell gap highlights the impact of the monopole interaction and its variation in strength with N.

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arXiv2020

Fast-timing study of $^{81}$Ga from the $β$ decay of $^{81}$Zn

The $β^{-}$ decay of $^{81}$Zn to the neutron magic $N=50$ nucleus $^{81}$Ga, with only three valence protons with respect to $^{78}$Ni, was investigated. The study was performed at the ISOLDE facility at CERN by means of $γ$ spectroscopy. The $^{81}$Zn half-life was determined to be $T_{1/2}=290(4)$ ms while the $β$-delayed neutron emission probability was measured as $P_n=23(4)\%$. The analysis of the $β$-gated $γ$-ray singles and $γ$-$γ$ coincidences from the decay of $^{81}$Zn provides 47 new levels and 70 new transitions in $^{81}$Ga. The $β^-$$n$ decay of $^{81}$Zn was observed and a new decay scheme into the odd-odd $^{80}$Ga nucleus was established. The half-lives of the first and second excited states of $^{81}$Ga were measured via the fast-timing method using LaBr$_3$(Ce) detectors. The level scheme and transition rates are compared to large-scale shell-model calculations. The low-lying structure of $^{81}$Ga is interpreted in terms of the coupling of the three valence protons outside the doubly-magic $^{78}$Ni core.

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arXiv2019

Normal and intruder configurations in $^{34}$Si populated in the $β^-$ decay of $^{34}$Mg and $^{34}$Al

The structure of $^{34}$Si was studied through $γ$ spectroscopy separately in the $β^-$ decays of $^{34}$Mg and $^{34}$Al at the ISOLDE facility of CERN. Different configurations in $^{34}$Si were populated independently from the two recently identified $β$-decaying states in $^{34}$Al having spin-parity assignments $J^π= 4^-$ dominated by the normal configuration $π(d_{5/2})^{-1} \otimes ν(f_{7/2})$ and $J^π= 1^+$ by the intruder configuration $π(d_{5/2})^{-1} \otimes ν(d_{3/2})^{-1}(f_{7/2})^{2}$. The paper reports on spectroscopic properties of $^{34}$Si such as an extended level scheme, spin and parity assignments based on log($ft$) values and $γ$-ray branching ratios, absolute $β$ feeding intensities and neutron emission probabilities. A total of 11 newly identified levels and 26 transitions were added to the previously known level scheme of $^{34}$Si. Large scale shell-model calculations using the {\sc sdpf-u-mix} interaction, able to treat higher order intruder configurations, are compared with the new results and conclusions are drawn concerning the predictive power of {\sc sdpf-u-mix}, the $N=20$ shell gap, the level of mixing between normal and intruder configurations for the 0$_1^+$, 0$_2^+$ and 2$_1^+$ states and the absence of triaxial deformation in $^{34}$Si.

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arXiv2019

IAEA Photonuclear Data Library 2019

Photo-induced reaction cross section data are of importance for a variety of current or emerging applications, such as radiation shielding design and radiation transport analyses, calculations of absorbed dose in the human body during radiotherapy, physics and technology of fission reactors (influence of photo-reactions on neutron balance) and fusion reactors (plasma diagnostics and shielding), activation analyses, safeguards and inspection technologies, nuclear waste transmutation, medical isotope production and astrophysical applications. Since the release of the IAEA Photonuclear Data Library in 1999 however, new experimental data as well as new methods to assess the reliability of experimental cross sections have become available. Theoretical models and input parameters used to evaluate photo-induced reactions have improved significantly over the years. In addition, new measurements of partial photoneutron cross sections using mono-energetic photon beams and advanced neutron detection systems have been performed allowing for the validation of the evaluations and assessments of the experimental data. Furthermore, technological advances have led to the construction of new and more powerful gamma-beam facilities, therefore new data needs are emerging. We report our coordinated efforts to address these data needs and present the results of the new evaluations of more than 200 nuclides included in the new updated IAEA Photonuclear Data Library, where the photon energy goes up to 200 MeV. We discuss the new assessment method and make recommendations to the user community in cases where the experimental data are discrepant and the assessments disagree. In addition, in the absence of experimental data, we present model predictions for photo-induced reaction cross section on nuclides of potential interest to medical radioisotope production.

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arXiv2018

Time Projection Chamber (TPC) Detectors for Nuclear Astrophysics Studies With Gamma Beams

Gamma-Beams at the HIgS facility in the USA and anticipated at the ELI-NP facility, now constructed in Romania, present unique new opportunities to advance research in nuclear astrophysics; not the least of which is resolving open questions in oxygen formation during stellar helium burning via a precise measurement of the 12C(a,g) reaction. Time projection chamber (TPC) detectors operating with low pressure gas (as an active target) are ideally suited for such studies. We review the progress of the current research program and plans for the future at the HIγS facility with the optical readout TPC (O-TPC) and the development of an electronic readout TPC for the ELI-NP facility (ELITPC).

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arXiv2018

The gamma-ray strength function of $^{89}$Y and $^{90}$Y

In this work, we present new data on the $^{89}$Y($γ$,n) cross section studied with a quasi-monochromatic photon beam produced at the NewSUBARU synchrotron radiation facility in Japan contributing torwards resolving a long standing discrepancy between existing measurements of this cross section. Results for $γ$-ray strength function below threshold obtained by applying the Oslo method to $^{89}$Y($p,p'γ$)$^{89}$Y coincidences combined with the $^{89}$Y($γ$,n) data this providing experimental data for the $γ$-ray strength function of $^{89}$Y for $γ$ energies in the range of $\approx 1.6$ Mev to $\approx$ 20 MeV. A low-energy enhancement is seen for $γ$-rays below $\approx 2.5$ MeV. Shell-model calculations indicate that this feature is caused by strong, low-energy $M1$ transitions at high excitation energies. The nuclear level density and $γ$-ray strength function have been extracted from $^{89}$Y($d,p γ$)$^{90}$Y coincidences using the Oslo method. Using the ($γ,n$) and ($d,pγ$) data as experimental constraints, we have calculated the $^{89}$Y($n,γ$)$^{90}$Y cross section with the TALYS reaction code. Our results have been compared with directly measured (n,$γ$) cross sections and evaluations. The $N=50$ isotope $^{89}$Y is an important bottleneck in the s-process and the magnitude of the $^{89}$Y(n,$γ)$ cross section is key to understanding how s-process stars produce heavy isotopes.

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arXiv2018

Verification of detailed balance for $γ$ absorption and emission in Dy isotopes

The photo-neutron cross sections of $^{162,163}\rm{Dy}$ have been measured for the first time in an energy region from the neutron threshold ($S_n$) up to $\approx$ $13$~MeV. The ($γ$,n) reaction was induced with quasi-monochromatic laser Compton-scattered $γ$ rays, produced at the NewSUBARU laboratory. The corresponding $γ$-ray strength functions ($γ$SF) have been calculated from the photo-neutron cross sections. The data are compared to reanalyzed $γ$SFs of $^{160-164}\rm{Dy}$, which are measured below $S_n$. The excellent agreement with the photo-neutron data at $S_n$ confirms the principle of detailed balance. Thus, a complete $γ$SF is established covering in total the energy region of 1 MeV $\leq$ E$_γ$ $\leq$ 13 MeV. These mid-shell well-deformed dysprosium isotopes all show scissors resonances with very similar structures. We find that our data predict the same integrated scissors strength as ($γ,γ^\prime$) data when integrated over the same energy range, which shows that the scissors mode very likely is consistent with the generalized Brink hypothesis. Finally, using the $γ$SFs as input in the reaction code TALYS, we have deduced radiative neutron-capture cross sections and compared them to direct measurements. We find a very good agreement within the uncertainties, which gives further support to the experimentally determined $γ$SFs.

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arXiv2016

Experimental setup and procedure for the measurement of the 7Be(n,α)α reaction at n_TOF

The newly built second experimental area EAR2 of the n_TOF spallation neutron source at CERN allows to perform (n, charged particles) experiments on short-lived highly radioactive targets. This paper describes a detection apparatus and the experimental procedure for the determination of the cross-section of the 7Be(n,α) reaction, which represents one of the focal points toward the solution of the cosmological Lithium abundance problem, and whose only measurement, at thermal energy, dates back to 1963. The apparently unsurmountable experimental difficulties stemming from the huge 7Be γ-activity, along with the lack of a suitable neutron beam facility, had so far prevented further measurements. The detection system is subject to considerable radiation damage, but is capable of disentangling the rare reaction signals from the very high background. This newly developed setup could likely be useful also to study other challenging reactions requiring the detectors to be installed directly in the neutron beam.

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arXiv2015

Low-energy enhancement in the γ-ray strength functions of $^{73,74}$Ge

The $γ$-ray strength functions and level densities of $^{73,74}$Ge have been extracted up to the neutron separation energy S$_n$ from particle-$γ$ coincidence data using the Oslo method. Moreover, the $γ$-ray strength function of $^{74}$Ge above S$_n$ has been determined from photo-neutron measurements, hence these two experiments cover the range of E$_γ\approx$ 1-13 MeV for $^{74}$Ge. The obtained data show that both $^{73,74}$Ge display an increase in strength at low $γ$ energies. The experimental $γ$-ray strength functions are compared with $M1$ strength functions deduced from average $B(M1)$ values calculated within the shell model for a large number of transitions. The observed low-energy enhancements in $^{73,74}$Ge are adopted in the calculations of the $^{72,73}$Ge(n,$γ$) cross sections, where there are no direct experimental data. Calculated reaction rates for more neutron-rich germanium isotopes are shown to be strongly dependent on the presence of the low-energy enhancement.

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