arXiv subjects

D. Gjestvang

Publications and source records attributed to D. Gjestvang.

5 recordsLinked to original sources

arXiv2026

Statistical and non-statistical $\gamma$-decay properties of $^{64}$Zn

We present a study on the $\gamma$-decay properties of $^{64}$Zn using the Oslo method on $^{64}$Zn($p,p^\prime \gamma$) data combined with $^{64}$Zn$(\gamma,n)$ cross-section measurements at the NewSUBARU facility. With the Oslo method, we have measured the $\gamma$-ray strength function ($\gamma$SF) and the nuclear level density (NLD) below the neutron threshold. We observe that the NLD trend in the quasi-continuum region of $^{64}$Zn is best characterized by a constant-temperature-like model. %with temperature parameter $T_{\rm CT}=1.21(5)$ MeV. Surprisingly, we find that $\gamma$-ray transitions from the quasi-continuum decaying directly to the $0^+$ ground state seem to be strongly hindered with a hindrance factor of $\kappa \approx 0.5$, which could be an indication of non-statistical effects in the ground-state decay due to, \textit{e.g.}, differences in nuclear shapes. For $\gamma$ energies above the neutron separation energy, the NewSUBARU ($\gamma, n$) data set probes a significant part of the giant dipole resonance. Furthermore, we find that the Oslo-method $\gamma$SF shows a rather smooth behavior, with a clear low-energy enhancement (LEE) for $E_{\gamma} < 4$ MeV.

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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

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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