arXiv subjects

A. Voinov

Publications and source records attributed to A. Voinov.

34 records · Page 2Linked to original sources

arXiv2006

Microcanonical entropies and radiative strength functions of $^{50,51}$V

The level densities and radiative strength functions (RSFs) of $^{50,51}$V have been extracted using the ($^3$He,$αγ$) and ($^3$He,$^3$He$^{\prime} γ$) reactions, respectively. From the level densities, microcanonical entropies are deduced. The high $γ$-energy part of the RSF is described by the giant electric dipole resonance. A significant enhancement over the predicted strength in the region of $E_γ \lesssim 3$ MeV is seen, which at present has no theoretical explanation.

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arXiv2006

Level densities and thermodynamical quantities of heated 93-98Mo isotopes

Level densities for 93-98Mo have been extracted using the (3He,alpha gamma) and (3He,3He' gamma) reactions. From the level densities thermodynamical quantities such as temperature and heat capacity can be deduced. Data have been analyzed by utilizing both the microcanonical and the canonical ensemble. Structures in the microcanonical temperature are consistent with the breaking of nucleon Cooper pairs. The S-shape of the heat capacity curves found within the canonical ensemble is interpreted as consistent with a pairing phase transition with a critical temperature for the quenching of pairing correlations at T_c ~ 0.7-1.0 MeV.

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arXiv2006

Primary versus secondary gamma intensities in 171Yb(n_th,gamma)

The two published literature values [Greenwood et al., Nucl. Phys. A252, 260 (1975) and Gelletly et al., J. Phys. G 11, 1055 (1985)] for absolute primary gamma intensities following thermal neutron capture of 171Yb differ in average by a factor of three. We have resolved this conflict in favor of Greenwood et al. by a measurement of primary versus secondary intensities.

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arXiv2004

Level densities and $γ$-ray strength functions in $^{170,171,172}$Yb

Level densities and radiative strength functions in $^{171}$Yb and $^{170}$Yb nuclei have been measured using the $^{171}$Yb($^3$He,$^3$He$^\primeγ$)$^{171}$Yb and $^{171}$Yb($^3$He,$αγ$)$^{170}$Yb reactions. New data on $^{171}$Yb are compared to a previous measurement for $^{171}$Yb from the $^{172}$Yb($^3$He,$αγ$)$^{171}$Yb reaction. Systematics of level densities and radiative strength functions in $^{170,171,172}$Yb are established. The entropy excess in $^{171}$Yb relative to the even-even nuclei $^{170,172}$Yb due to the unpaired neutron quasiparticle is found to be approximately 2$k_B$. Results for the radiative strength function from the two reactions lead to consistent parameters characterizing the ``pygmy'' resonances. Pygmy resonances in the $^{170,172}$Yb populated by the ($^3$He,$α$) reaction appear to be split into two components for both of which a complete set of resonance parameters are obtained.

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arXiv2004

Radiative strength functions in 93-98Mo

Radiative strength functions (RSFs) in 93-98Mo have been extracted using the (3He,alpha gamma) and (3He,3He' gamma) reactions. The RSFs are U-shaped as function of gamma energy with a minimum at around E_gamma=3 MeV. The minimum values increase with neutron number due to the increase in the low-energy tail of the giant electric dipole resonance with nuclear deformation. The unexpected strong increase in strength below E_gamma=3 MeV, here called soft pole, is established for all 93-98Mo isotopes. The soft pole is present at all initial excitation energies in the 5-8 MeV region.

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arXiv2004

Large enhancement of radiative strength for soft transisitons in the quasicontinuum

Radiative strength functions (RSFs) for the 56,57-Fe nuclei below the separation energy are obtained from the 57-Fe(3-He,alpha gamma)56-Fe and 57-Fe(3-He,3-He' gamma)57-Fe reactions, respectively. An enhancement of more than a factor of ten over common theoretical models of the soft (E_gamma ~< 2 MeV) RSF for transitions in the quasicontinuum (several MeV above the yrast line) is observed. Two-step cascade intensities with soft primary transitions from the 56-Fe(n,2gamma)57-Fe reaction confirm the enhancement.

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arXiv2004

Establishment of M1 multipolarity of a 6.5 mu_N^2 resonance in 172-Yb at E_gamma=3.3 MeV

Two-step-cascade spectra in 172-Yb have been measured after thermal neutron capture. They are compared to calculations based on experimental values of the level density and radiative strength function (RSF) obtained from the 173-Yb(3-He,alpha gamma)172-Yb reaction. The multipolarity of a 6.5(15) mu_N^2 resonance at E_gamma=3.3(1) MeV in the RSF is determined to be M1 by this comparison.

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arXiv2003

Thermal properties and radiative strengths in 160,161,162Dy

The level densities and radiative strength functions (RSFs) in 160,161Dy have been extracted using the (3He,alpha gamma) and (3He,3He' gamma) reactions, respectively. The data are compared to previous measurements on 161,162Dy. The energy distribution in the canonical ensemble is discussed with respect to the nucleon Cooper pair breaking process. The gross properties of the RSF are described by the giant electric dipole resonance. The RSF at low gamma-ray energies is discussed with respect to temperature dependency. Resonance parameters of a soft dipole resonance at Egamma = 3 MeV are deduced.

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arXiv2003

Evolution of level density step structures from 56,57-Fe to 96,97-Mo

Level densities have been extracted from primary gamma spectra for 56,57-Fe and 96,97-Mo nuclei using (3-He,alpha gamma) and (3-He,3-He') reactions on 57-Fe and 97-Mo targets. The level density curves reveal step structures above the pairing gap due to the breaking of nucleon Cooper pairs. The location of the step structures in energy and their shapes arise from the interplay between single-particle energies and seniority-conserving and seniority-non-conserving interactions.

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arXiv2002

Level density and $γ$-ray strength in $^{27,28}$Si

A method to extract simultaneously level densities and $γ$-ray transmission coefficients has for the first time been tested on light nuclei utilizing the $^{28}$Si($^3$He,$αγ)^{27}$Si and $^{28}$Si($^3$He,$^3$He'$γ)^{28}$Si reactions. The extracted level densities for $^{27}$Si and $^{28}$Si are consistent with the level densities obtained by counting known levels in the respective nuclei. The extracted $γ$-ray strength in $^{28}$Si agrees well with the known $γ$-decay properties of this nucleus. Typical nuclear temperatures are found to be $T\sim 2.4$ MeV at around 7 MeV excitation energy. The entropy gap between nuclei with mass number $A$ and $A\pm 1$ is measured to be $δS\sim 1.0 k_B$, which indicates an energy spacing between single-particle orbitals comparable with typical nuclear temperatures.

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arXiv2002

Measurements of Nuclear Level Densities and Gamma-Ray Strength Functions and their Interpretations

A method to extract primary $γ$-ray spectra from particle-$γ$ coincidences at excitation energies up to the neutron binding energy is described. From these spectra, the level density and $γ$-ray strength function can be determined. From the level density, several thermodynamical quantities are obtained within the microcanonical and canonical ensemble. Also models for the $γ$-ray strength function are discussed.

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arXiv2002

Determination of the electromagnetic character of soft dipole modes solely based on quasicontinuous gamma spectroscopy

We show that the combined analysis of the quasicontinuous gamma spectra from the (He-3,alpha) and the (n-thermal,2gamma) reactions gives the possibility to measure the electromagnetic character of soft dipole resonances. Two-step gamma-cascade spectra have been calculated, using level densities and radiative strength functions from the (He-3,alpha gamma) reaction. The calculations show that the intensity of the two-step cascades depends on the electromagnetic character of the soft dipole resonance under study. The difference reaches 40-100% which can be measured experimentally.

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arXiv2001

Level densities and $γ$-strength functions in $^{148,149}$Sm

The level densities and $γ$-strength functions of the weakly deformed $^{148}$Sm and $^{149}$Sm nuclei have been extracted. The temperature versus excitation energy curve, derived within the framework of the micro canonical ensemble, shows structures, which we associate with the break up of Cooper pairs. The nuclear heat capacity is deduced within the framework of both the micro canonical and the canonical ensemble. We observe negative heat capacity in the micro canonical ensemble whereas the canonical heat capacity exhibits an S-shape as function of temperature, both signals of a phase transition. The structures in the $γ$-strength functions are discussed in terms of the pygmy resonance and the scissors mode built on exited states. The samarium results are compared with data for the well deformed $^{161,162}$Dy, $^{166,167}$Er and $^{171,172}$Yb isotopes and with data from (n,$γ$)-experiments and giant dipole resonance studies.

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arXiv2000

Observation of the scissors mode in the quasicontinuum

The experimental resonance parameters of the pygmy resonance in rare earth nuclei are compared to global parameters of the scissors-mode states. It is argued that the pygmy resonance in rare earth nuclei can be described in terms of orbital M1 strength observed in (gamma,gamma') experiments. The pygmy resonance is therefore interpreted as the scissors mode in the quasicontinuum.

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arXiv2000

Thermal and electromagnetic properties of 166-Er and 167-Er

The primary gamma-ray spectra of 166-Er and 167-Er are deduced from the (3-He,alpha gamma) and (3-He,3-He' gamma) reaction, respectively, enabling a simultaneous extraction of the level density and the gamma-ray strength function. Entropy, temperature and heat capacity are deduced from the level density within the micro-canonical and the canonical ensemble, displaying signals of a phase-like transition from the pair-correlated ground state to an uncorrelated state at Tc=0.5 MeV. The gamma-ray strength function displays a bump around E-gamma=3 MeV, interpreted as the pygmy resonance.

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arXiv2000

Gamma-ray strength function and pygmy resonance in rare earth nuclei

The gamma-ray strength function for gamma energies in the 1-7 MeV region has been measured for 161,162-Dy and 171,172-Yb using the (3-He,alpha gamma) reaction. Various models are tested against the observed gamma-ray strength functions. The best description is based on the Kadmenskii, Markushev and Furman E1 model with constant temperature and the Lorentzian M1 model. A gamma-ray bump observed at E_gamma=3 MeV is interpreted as the so-called pygmy resonance, which has also been observed previously in (n,gamma) experiments. The parameters for this resonance have been determined and compared to the available systematics.

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