Where is the Scissors Mode Strength in Odd-Mass Nuclei?

Enders, J.; Huxel, N.; Neumann-Cosel, P. von; Richter, A.

doi:10.1103/physrevlett.79.2010

Cited by 62 publications

(34 citation statements)

References 32 publications

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“…The method was originally proposed to analyze β-delayed particle emission spectra [36], but later it was successfully adopted for the study of electron scattering data [37,38] and can be used in general for high-resolution spectra of nuclear reactions (see, e.g., Refs. [21,39,40]). Detailed descriptions of the method can also be found in Refs.…”

Section: A Fluctuation Analysismentioning

confidence: 99%

“…Differences of the characteristic scales between the investigated nuclei could be traced back to their low-energy collective structure. One exception is 40 Ca -the lightest nucleus studied so far with the wavelet technique -, where a recent random-phase approximation (RPA) calculation employing a realistic nuclear interaction derived with the Unitary Correlation Operator Method (UCOM) found that the characteristic scales result from Landau damping [10] in contrast to a large variety of previous RPA results, where the ISGQR strength is always concentrated in a single state.…”

Section: Introductionmentioning

confidence: 99%

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Fine structure of the isovector giant dipole resonance inPb208: Characteristic scales and level densities

et al. 2014

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Background: The electric isovector giant dipole resonance (IVGDR) in208 Pb has been measured with high energy resolution with the (p, p ) reaction under extreme forward angles [A. Tamii et al., Phys. Rev. Lett. 107, 062502 (2011)] and shows considerable fine structure.Purpose: The aim of the present work is to extract scales characterizing the observed fine structure and to relate them to dominant decay mechanisms of giant resonances. Furthermore, the level density of J π = 1 − states is determined in the energy region of the IVGDR.Methods: Characteristic scales are extracted from the spectra with a wavelet analysis based on continuous wavelet transforms.Comparison with corresponding analyses of B(E1) strength distributions from microscopic model calculations in the framework of the quasiparticle phonon model and the relativistic random phase approximation allow to identify giant resonance decay mechanisms responsible for the fine structure. The level density of 1 − states is related to local fluctuations of the cross sections in the energy region of the IVGDR, where contributions from states with other spin-parities can be neglected. The magnitude of the fluctuations is determined by the autocorrelation function.Results: Scales in the fine structure of the IVGDR in 208 Pb are found at 80, 130, 220, 430, 640, 960 keV, and at 1.75 MeV. The values of the most prominent scales can be reasonably well reproduced by the microscopic calculations although they generally yield a smaller number of scales.. The inclusion of complex configurations in the calculations changes the E1 strength distributions but the impact on the wavelet power spectra and characteristic scales is limited. The level density of 1 − states is extracted in the excitation energy range 9 − 12.5 MeV and compared to a variety of phenomenological and microscopic models. Conclusions:In both models the major scales are already present at the one-particle one-hole level indicating Landau damping as a dominant mechanism responsible for the fine structure of the IVGDR in contrast to the isoscalar giant quadrupole resonance, where fine structure arises from the coupling to low-lying surface vibrations. The back-shifted Fermi gas model parameterization of Rauscher et al., Phys. Rev. C 56, 1613 describes the level-density data well, while other phenomeological and microscopic approaches fail to reproduce absolute values or the energy dependence or both.

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Section: A Fluctuation Analysismentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Fine structure of the isovector giant dipole resonance inPb208: Characteristic scales and level densities

et al. 2014

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“…Nevertheless, the possibility of strong branching of the scissors mode to excited states was recently suggested from theoretical calculations [10]. To date, such branching of the scissors mode has not been investigated, and it was assumed by necessity to be negligible in previous attempts with NRF to investigate the total strength and fragmentation of the scissors mode in odd-mass nuclei [5]. The observation of additional scissorsmode strength using NRF, as branching or fragmentation, would not only challenge theoretical models of the scissors mode, but also impact proposed applications to use the NRF of scissors-mode resonances for detection or assay of odd-mass fissile nuclei [11,12].…”

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confidence: 99%

Branching and Fragmentation of Dipole Strength inTa181in the Region of the Scissors Mode

et al. 2016

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In recent measurements of the scissors mode in radiative decay experiments, transition strengths were observed that were double that expected from theory and systematics well established from measurements on the radiative excitation channel, that is, using nuclear resonance fluorescence (NRF). Additional strength as measured with NRF can only be present as heretofore unobserved branching or fragmentation of the scissors mode. Such possibilities were investigated in a transmission NRF measurement on the deformed, odd-mass 181 Ta, using a quasi-monoenergetic γ-ray beam at two beam energies. This measurement further influences applications using transmission NRF to assay or detect odd-mass fissile isotopes. A large branching, ≈75%, of small resonances to excited states was discovered. In contrast, previous studies using NRF of the scissors-mode strength in odd-mass nuclei assumed no branching existed.The presently observed branching, combined with the observed highly-fragmented elastic strength, could reconcile the scissors-mode strength observed in NRF measurements with the expectations for enhanced scissors-mode strength from radiative decay experiments. Recently, however, in a measurement probing the radiative-decay channel, where decay γ-rays were measured following excitation of the nucleus by light ion-scattering, double the expected scissors-mode strength was observed in both even and odd-mass actinide nuclei compared to expectations established for the radiative excitation channel, that is, using NRF [6]. The same phenomenon of increased strength was observed a decade prior in lanthanide nuclei using similar measurements of radiative decay following neutron capture [7,8] and ( 3 He, α) scattering [8], which observation has been reinforced in a contemporary measurement. This discrepancy in measured strength between the radiative excitation and radiative decay channels for the scissors mode remains its greatest mystery.Additional scissors-mode strength can only be present in the radiative excitation channel if it were to occur as heretofore unobserved fragmentation or branching, forbearing the alternative that the additional strength is confined solely to the radiative decay channel. This alternative could happen if states excited by ion-scattering at energies up to 10 MeV de-excite emitting 2-3 MeV γ-rays more strongly than would be expected from the ground state scissors-mode strength (see e.g. Krtička et al. [7]). Nevertheless, the possibility of strong branching of the scissors mode to excited states was recently suggested from theoretical calculations [10]. To date, such branching of the scissors mode has not been investigated, and it was assumed by necessity to be negligible in previous attempts with NRF to investigate the total strength and fragmentation of the scissors mode in odd-mass nuclei [5]. The observation of additional scissorsmode strength using NRF, as branching or fragmentation, would not only challenge theoretical models of the scissors mode, but also impact proposed applications t...

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“…Therefore, a large part of the strength remains unresolved in the background of the NRF spectra of the deformed odd-mass nuclei. With the available experimental techniques the detection of these weak transition strengths is impossible [20,21]. It was shown that the missing strength can be extracted from the background by means of a statistical fluctuation analysis method [22].…”

Section: Resultsmentioning

confidence: 99%

Investigation of low-lying magnetic dipole excitations in the $^{153}$Eu nucleus

Tabar¹

2017

Turk J Phys

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Abstract:The properties of low-lying magnetic dipole ( M 1) excitations in 153 Eu were studied within a rotational invariant quasiparticle phonon model that takes into account the symmetry-restoring as well as spin-spin residual forces.The calculations show that there are purely collective M 1 excitations lying at 2-4 MeV fragmented over orbital 1 + states of the 152 Sm core nucleus. The results were compared to the experimentally known M 1 excitations at 2-3 MeV.A reasonably good agreement in the total transition strength, the centroid energy, and the resonance width was obtained.

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Where is the Scissors Mode Strength in Odd-Mass Nuclei?

Cited by 62 publications

References 32 publications

Fine structure of the isovector giant dipole resonance inPb208: Characteristic scales and level densities

Fine structure of the isovector giant dipole resonance inPb208: Characteristic scales and level densities

Branching and Fragmentation of Dipole Strength inTa181in the Region of the Scissors Mode

Investigation of low-lying magnetic dipole excitations in the $^{153}$Eu nucleus

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