Is The Generalized Brink-Axel Hypothesis Valid?

Guttormsen, M.; Larsen, A. C.; Görgen, A.; Renstrøm, T.; Siem, S.; Tornyi, T.; Tveten, G. M.

doi:10.22323/1.281.0062

Cited by 5 publications

(13 citation statements)

References 22 publications

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“…As shown in Ref. [13], Porter-Thomas fluctuations are not significant for 238 Np, but they are expected to be larger for lighter nuclei. It is therefore important to estimate more quantitatively if this is the case for 64,65 Ni before extracting conclusions regarding the validity of the gBA in these nuclei.…”

Section: Fluctuations Of the γ Sfmentioning

confidence: 68%

“…Following the procedure described in Ref. [13], we here assume that the fluctuations in the γ SF follow the χ 2 ν distribution, with ν equal to the number of transitions, n, included in the averaging for a specific E γ . For a χ 2 ν distribution we define the ratio r between the deviation σ and the average μ as r = σ/μ = √ 2/ν.…”

Section: Fluctuations Of the γ Sfmentioning

confidence: 99%

“…Furthermore, the results from Refs. [12,13] show that, for a heavy odd-odd nucleus such as 238 Np, the NLD is extremely high and averaging over many γ transitions can be performed. Under these circumstances, there is a significant suppression of Porter-Thomas [14] fluctuations, i.e, fluctuations in the individual partial radiative widths related to the complexity of the nuclear states involved in the decay.…”

Section: Introductionmentioning

confidence: 99%

“…Thus, the gBA hypothesis could be reliably tested in 238 Np. For lighter nuclei, however, the situation is normally rather different and large Porter-Thomas fluctuations are present [13,15]. Such fluctuations can be understood in terms of the wave functions of the various nuclear states involved in a given decay.…”

Section: Introductionmentioning

confidence: 99%

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Test of the generalized Brink-Axel hypothesis in Ni64,65

et al. 2018

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Previously published particle-γ coincidence data on the 64 Ni(p, p γ) 64 Ni and 64 Ni(d, pγ) 65 Ni reactions were further analyzed to study the statistical properties of γ decay in 64,65 Ni. To do so, the γ decay to the quasicontinuum region and discrete low-lying states was investigated at γ-ray energies of 2.0-9.6 and 1.6-6.1 MeV in 64 Ni and 65 Ni, respectively. In particular, the dependence of the γ-strength function with initial and final excitation energy was studied to test the validity of the generalized Brink-Axel hypothesis. Finally, the role of fluctuations in transition strengths was estimated as a function of γ-ray and excitation energy. The γ-strength function is consistent with the hypothesis of the independence of initial excitation energy, in accordance with the generalized Brink-Axel hypothesis. The results show that the γ decay to low-lying levels displays large fluctuations for low initial excitation energies.

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Section: Fluctuations Of the γ Sfmentioning

confidence: 68%

Section: Fluctuations Of the γ Sfmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Test of the generalized Brink-Axel hypothesis in Ni64,65

et al. 2018

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“…Oslo-method data have previously been used to study the shapes of the GSFs as functions of initial and final excitation energies to address the question on the validity of the gBA hypothesis [16,31,32,35]. With the exception of Ref.…”

Section: Porter-thomas Fluctuations and γ−Ray Strength Functionsmentioning

confidence: 99%

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

Markova,

Larsen,

von Neumann-Cosel

et al. 2023

Preprint

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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 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 ≈ 1 • 10 3 − 2 • 10 3 MeV −1 ).

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How effective is the Brink–Axel hypothesis for astrophysical weak rates?

Nabi¹,

Mazhar²,

Johnson³

2022

J. Phys. G: Nucl. Part. Phys.

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We explore the effectiveness of the Brink-Axel hypothesis for the computation of stellar electron capture and $\beta$-decay rates, namely that the transition strength function depends only upon the transition energy and not upon the details of the initial state. For this purpose, we calculated Gamow-Teller (GT) strength distributions for a selection of $sd$--shell nuclides, using two different microscopic models, namely the proton-neutron quasiparticle random phase approximation and the full configuration-interaction shell model, taking into account the first 100 states of both the initial and final nuclides. The GT transition strengths among these levels evolve with initial state energy. These transition strength functions we folded into %We later performed the calculation of weak-interaction mediated rates in stellar matter, specifically electron capture and $\beta$-decay rates, for a range of densities $10 \, \mathrm{g/cm^3} \leq\rho \leq10^{11} \, \mathrm{g/cm^3} $ and range of temperatures $1 \, \mathrm{GK} \leq T \leq 30 \, \mathrm{GK}$. When transitions from excited states were approximated using the Brink-Axel hypothesis, augmented by so-called `back-resonance' transitions, the rates were affected by up to 3 orders of magnitude or more at high temperatures and densities. Thus the Brink-Axel hypothesis is not a reliable approximation for the calculation of stellar rates, especially in high temperature--density environments.

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Is The Generalized Brink-Axel Hypothesis Valid?

Cited by 5 publications

References 22 publications

Test of the generalized Brink-Axel hypothesis in Ni64,65

Test of the generalized Brink-Axel hypothesis in Ni64,65

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

How effective is the Brink–Axel hypothesis for astrophysical weak rates?

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