arXiv subjects

H. C. Berg

Publications and source records attributed to H. C. Berg.

9 recordsLinked to original sources

arXiv2026

Statistical properties of $^{133}$Xe and the $^{132}$Xe$(n,\gamma)$ cross section

$^{133}$Xe is an interesting case for plasma physics to explore nuclear excitation by electron capture, as the process can be studied using statistical properties of $^{133}$Xe. In this work we present results on $^{133}$Xe from the inverse-Oslo method where we extract the nuclear level density and the $\gamma$-strength function, which is used to calculate the (n,$\gamma$) cross section on $^{132}$Xe. The $\gamma$-strength function of $^{133}$Xe can constrain the estimated decay rate from nuclear excitation by electron capture. The $\mathrm{d}(^{132}\mathrm{Xe},\mathrm{p})^{132}\mathrm{Xe}$ reaction was used to create the compound nucleus $^{133}$Xe, which was recorded with an annular particle telescope and a scintillator array consisting of \la and BGO-shielded HPGe Clover detectors. With the inverse-Oslo method, it is possible to study nuclei that are impossible or unable to manufacture targets from, short lived isotopes, or as in this work, noble gases. We present the extracted nuclear level density, and $\gamma$-strength function for $^{133}$Xe, along with shell-model calculations of the statistical properties of $^{133}$Xe. These are the first statistical properties extracted below 6 MeV for any xenon isotope. We constrain the $^{132}$Xe(n,$\gamma$) $^{133}$Xe cross section and reaction rate using the TALYS reaction code.

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arXiv2025

Constraining the Synthesis of the Lightest p Nucleus 74Se

We provide the first experimental cross section of the $^{73}\text{As}(p,\gamma)^{74}\text{Se}$ reaction to constrain one of the main destruction mechanisms of the p nucleus $^{74}\text{Se}$ in explosive stellar environments. The measurement was done using a radioactive $^{73}\text{As}$ beam at effective center-of-mass energies of 2.9 and 2.3 MeV/nucleon. Along with the total cross-section measurement, statistical properties of the $^{74}\text{Se}$ compound nucleus were extracted, constraining the reaction cross section in the upper Gamow window of the $\gamma$ process. The impact of the experimentally constrained reaction rate on $^{74}\text{Se}$ production in Type II supernovae was investigated through Monte Carlo one-zone network simulations. The results indicate that the overproduction of $^{74}$Se by Type II supernova models cannot be resolved by nuclear physics alone and point toward the need for a more detailed understanding of the astrophysical conditions of relevance for the $\gamma$ process.

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arXiv2025

Beta-decay Half Lives beyond $^{54}$Ca: A Systematic Survey of Decay Properties approaching the Neutron Dripline

In an experiment performed at the Facility for Rare Isotope Beams (FRIB) using the FRIB Decay Station initiator (FDSi), 15 new half lives of isotopes near $^{54}$Ca were measured. A new method of extracting lifetimes from experimental data, taking into account the unknown $\beta$-delayed neutron emission branches of very neutron-rich nuclei, was developed to enable systematic uncertainty analysis. The experiment observed a dramatic change in the half-life systematics for the isotopes with neutron number N =34. Beyond N =34, the decline of nuclear lifetime is much slower, leading to longer than anticipated lifetimes for near-dripline nuclei. State-of-the-art shell-model calculations can explain the experimental results for Z$>$19 nuclei, revealing the imprint of shell effects and the need for modification of single-particle neutron states. The results from a newly developed QRPA model with potential for making global predictions were also tested against the experimental results and good agreement was found.

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arXiv2025

Universal Effective Charges in the $sd$ and $fp$ Shells

The 247-keV state in $^{54}$Sc, populated in the $β$ decay of $^{54}$Ca, is reported here as a nanosecond isomer with a half-life of 26.0(22) ns. The state is interpreted as the $1^+$ member of the $πf_{7/2}\otimesνf_{5/2}$ spin-coupled multiplet, which decays to the $3^+,πf_{7/2} \otimes νp_{1/2}$ ground state. The new half-life corresponds to a pure $E2$ transition with a strength of 1.93(16) W.u., providing the most precise, unambiguous $B(E2)$ value in the neutron-rich $fp$ region to date for a nucleus with valence protons above $Z=20$. Notably, it is roughly four times larger than the $B(E2; 1/2^{-} \rightarrow 5/2^{-})$ value in $^{55}$Ca. The results, as compared to semi-empirical and ab initio shell-model calculations, indicate (1) a weak $N=34$ sub-shell gap relative to $N = 32$, (2) a large $E2$ enhancement in Sc as compared to Ca due to $1p-1h$ proton excitations across $Z=28$, and (3) empirical effective proton and neutron charges, $e_π$ = 1.30(8)$e$ and $e_ν$ = 0.452(7)$e$, respectively, that are in contrast to reports of $e_π\approx 1.1-1.15e$ and $e_ν\approx 0.6-0.8e$ for $fp$-shell nuclei near $N = Z$. We demonstrate that these reports are erroneous and that, in fact, a universal set of effective charges can be used across the $sd$ and $fp$ shells.

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arXiv2025

A saga on the $γ$-decay branching ratio of the Hoyle state

The radiative branching ratio of the Hoyle state is crucial to estimate the triple-$α$ reaction rate in stellar environments at medium temperatures of $T=0.1$ to 2 GK. Knowledge of the $γ$-decay channel is critical as this is the dominant radiative decay channel for the Hoyle state. A recent study by Kibédi et al. [Phys. Rev. Lett. 125, 182701 (2020)] has challenged our understanding of this astrophysically significant branching ratio and its constraints. The main purpose was to perform a new measurement of the $γ$-decay branching ratio of the Hoyle state to deduce the radiative branching ratio of the Hoyle state, an additional objective was to independently verify aspects of the measurement conducted by Kibédi et al. For the primary experiment of this work the Hoyle state was populated by the $^{12}\textrm{C}(p,p')$ reaction at 10.8 MeV at the Oslo Cyclotron Laboratory. The $γ$-decay branching ratio was deduced through triple-coincidence events between a proton populating the Hoyle state and the subsequent $γ$-ray cascade. An independent analysis of the 2014 data published by Kibédi et al. has been carried out. From the main experiment of this work, a $γ$-decay branching ratio of the Hoyle state was determined as $Γ_γ^{7.65}/Γ^{7.65}=4.0(3)\times 10^{-4}$, yielding a radiative branching ratio of $Γ_{\textrm{rad}}/Γ=4.1(4) \times 10^{-4}$. The reanalysis of the 2014 experiment in this work yielded $Γ_γ^{7.65}/Γ^{7.65}=4.5(6)\times 10^{-4}$, with a radiative branching ratio of $Γ_{\textrm{rad}}/Γ=4.6(6) \times 10^{-4}$. The measurements of the radiative branching ratio of the Hoyle state in this work is in excellent agreement with several recent studies, as well as the previously adopted ENSDF average of $Γ_{\textrm{rad}}/Γ=4.16(11)\times 10^{-4}$.

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arXiv2023

Cross Section Measurement of the $^{82}$Kr(p,$γ$)$^{83}$Rb Reaction in Inverse Kinematics

The total cross section of the $^{82}$Kr(p,$γ$)$^{83}$Rb reaction was measured for the first time at effective center-of-mass energies between 2.4 and 3.0 MeV, within the relevant Gamow window for the astrophysical $γ$ process. The experiment took place at the National Superconducting Cyclotron Laboratory at Michigan State University using the ReA facility. A $^{82}$Kr beam was directed onto a hydrogen gas cell located at the center of the Summing NaI(Tl) (SuN) detector. The obtained spectra were analyzed using the $γ$-summing technique and the extracted cross section was compared to standard statistical model calculations using the \textsc{non-smoker} and \textsc{talys} codes. The comparison indicates that standard statistical model calculations tend to overproduce the cross section of the $^{82}$Kr(p,$γ$)$^{83}$Rb reaction relative to the experimentally measured values. Furthermore, the experimental data was used to provide additional constraints on the nuclear level density and $γ$-ray strength function used in the statistical model calculations.

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arXiv2023

Nuclear level densities and $γ$-ray strength functions in $^{120,124}$Sn isotopes: impact of Porter-Thomas fluctuations

Nuclear level densities (NLDs) and $γ$-ray strength functions (GSFs) of $^{120,124}$Sn have been extracted with the Oslo method from proton-$γ$ coincidences in the ($p,p^{\prime}γ)$ reaction. The functional forms of the GSFs and NLDs have been further constrained with the Shape method by studying primary $γ$-transitions to the ground and first excited states.The NLDs demonstrate good agreement with the NLDs of $^{116,118,122}$Sn isotopes measured previously. Moreover, the extracted partial NLD of 1$^{-}$ levels in $^{124}$Sn is shown to be in fair agreement with those deduced from spectra of relativistic Coulomb excitation in forward-angle inelastic proton scattering. The experimental NLDs have been applied to estimate the magnitude of the Porter-Thomas (PT) fluctuations. Within the PT fluctuations, we conclude that the GSFs for both isotopes can be considered to be independent of initial and final excitation energies, in accordance with the generalized Brink-Axel hypothesis. Particularly large fluctuations observed in the Shape-method GSFs present a considerable contribution to the uncertainty of the method, and may be one of the reasons for deviations from the Oslo-method strength at low $γ$-ray energies and low values of the NLD (below $\approx1\cdot10^{3}-2\cdot10^{3}$ MeV$^{-1}$).

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arXiv2022

Evolution of the $γ$-ray strength function in neodymium isotopes

The experimental gamma-ray strength functions (gamma-SFs) of 142,144-151Nd have been studied for gamma-ray energies up to the neutron separation energy. The results represent a unique set of gamma-SFs for an isotopic chain with increasing nuclear deformation. The data reveal how the low-energy enhancement, the scissors mode and the pygmy dipole resonance evolve with nuclear deformation and mass number. The data indicate that the mechanisms behind the low-energy enhancement and the scissors mode are decoupled from each other.

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arXiv2021

Comprehensive test of the Brink-Axel hypothesis in the energy region of the pygmy dipole resonance

The validity of the Brink-Axel hypothesis, which is especially important for numerous astrophysical calculations, is addressed for 116,120,124Sn below the neutron separation energy by means of three independent experimental methods. The $γ$-ray strength functions (GSFs) extracted from primary $γ$-decay spectra following charged-particle reactions with the Oslo method and with the Shape method demonstrate excellent agreement with those deduced from forward-angle inelastic proton scattering at relativistic beam energies. In addition, the GSFs are shown to be independent of excitation energies and spins of the initial and final states. The results provide a critical test of the generalized Brink-Axel hypothesis in heavy nuclei, demonstrating its applicability in the energy region of the pygmy dipole resonance.

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