arXiv subjects

M. Guttormsen

Publications and source records attributed to M. Guttormsen.

At least 91 records · Page 5Linked to original sources

arXiv2008

Nuclear level densities and gamma-ray strength functions in 44,45Sc

The scandium isotopes 44,45Sc have been studied with the 45Sc(3He,alpha gamma)44Sc and 45Sc(3He,3He' gamma)45Sc reactions, respectively. The nuclear level densities and gamma-ray strength functions have been extracted using the Oslo method. The experimental level densities are compared to calculated level densities obtained from a microscopic model based on BCS quasiparticles within the Nilsson level scheme. This model also gives information about the parity distribution and the number of broken Cooper pairs as a function of excitation energy. The experimental gamma-ray strength functions are compared to theoretical models of the E1, M1, and E2 strength, and to data from (gamma,n) and (gamma,p) experiments. The strength functions show an enhancement at low gamma energies that cannot be explained by the present, standard models.

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arXiv2006

Breaking of nucleon Cooper pairs at finite temperature in 93-98Mo

The S shape of the canonical heat capacity is known as a signature of the pairing transition and along an isotopic chain it is significantly more pronounced for nuclei with an even number of neutrons than with an odd number. Although the heat capacities extracted from experimental level densities in 93-98Mo exhibit a clear S shape, they do not show such an odd-even staggering. To understand the underlying physics, we analyze thermal quantities evaluated from the partition function calculated using the static-path plus random-phase approximation (SPA+RPA) in a monopole pairing model with number-parity projection. The calculated level densities reproduce very well the experimental data and they also agree with estimates using the back-shifted Fermi-gas model. We clarify the reason why the heat capacities for Mo isotopes do not show odd-even staggering of the S shape. We also discuss thermal odd-even mass differences in 94-97Mo using the three-, four-, and five-point formula. These thermal mass differences are regarded as indicators of pairing correlations at finite temperature.

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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 density of $^{56}$Fe and low-energy enhancement of $γ$-strength function

The $^{55}$Mn$(d,n)^{56}$Fe differential cross section is measured at $E_d=7$ MeV\@. The $^{56}$Fe level density obtained from neutron evaporation spectra is compared to the level density extracted from the $^{57}$Fe$(^3$He,$αγ)^{56}$Fe reaction by the Oslo-type technique. Good agreement is found between the level densities determined by the two methods. With the level density function obtained from the neutron evaporation spectra, the $^{56}$Fe $γ$-strength function is also determined from the first-generation $γ$ matrix of the Oslo experiment. The good agreement between the past and present results for the $γ$-strength function supports the validity of both methods and is consistent with the low-energy enhancement of the $γ$ strength below $\sim 4$ MeV first discovered by the Oslo method in iron and molybdenum isotopes.

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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

Free energy and criticality in the nucleon pair breaking process

Experimental level densities for 171,172Yb, 166,167Er, 161,162Dy, and 148,149Sm are analyzed within the microcanonical ensemble. In the even isotopes at excitation energies E < 2 MeV, the Helmholtz free energy F signals the transition from zero to two quasiparticles. For E > 2 MeV, the odd and even isotopes reveal a surprisingly constant F at a critical temperature Tc of appr. 0.5 MeV, indicating the continuous melting of nucleon Cooper pairs as function of excitation energy.

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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

Caloric curves of atomic nuclei and other small systems

Caloric curves have traditionally been derived within the microcanonical ensemble via dS/dE=1/T or within the canonical ensemble via E=T^2*d(ln Z)/dT. In the thermodynamical limit, i.e., for large systems, both caloric curves give the same result. For small systems like nuclei, the two caloric curves are in general different from each other and neither one is reasonable. Using dS/dE=1/T, spurious structures like negative temperatures and negative heat capacities can occur and have indeed been discussed in the literature. Using E=T^2*d(ln Z)/dT a very featureless caloric curve is obtained which generally smoothes too much over structural changes in the system. A new approach for caloric curves based on the two-dimensional probability distribution P(E,T) will be discussed.

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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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arXiv2002

Model for pairing phase transition in atomic nuclei

A model is developed which allows the investigation and classification of the pairing phase transition in atomic nuclei. The regions of the parameter space are discussed for which a pairing phase transition can be observed. The model parameters include: number of particles, attenuation of pairing correlations with increasing seniority, single particle level spacing, and pairing gap parameter. It is argued that for nuclear models, the pairing phase transition has to be separated in temperature from the signal of the exhaustion of the finite model space.

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