Browsing by Author "Alhassid, Yoram"

Now showing 1 - 12 of 12

Citation - WoS: 5
Citation - Scopus: 7
Calculating Level Densities of Heavy Nuclei by the Shell Model Monte Carlo Method
(Academic Press Inc Elsevier Science, 2014) Alhassid, Yoram; Özen, Cem; Nakada, Hitoshi
The microscopic calculation of nuclear level densities in the presence of correlations is a difficult many-body problem. The shell model Monte Carlo method provides a powerful technique to carry out such calculations using the framework of the configuration-interaction shell model in spaces that are many orders of magnitude larger than spaces that can be treated by conventional methods. We present recent applications of the method to the calculation of level densities and their collective enhancement factors in heavy nuclei. The calculated level densities are in close agreement with experimental data.
Citation - WoS: 0
Citation - Scopus: 0
Collective Enhancement of Nuclear State Densities by the Shell Model Monte Carlo Approach
(IOP Publishing Ltd, 2015) Özen, Cem; Alhassid, Yoram; Nakada, Hitoshi
The shell model Monte Carlo (SMMC) approach allows for the microscopic calculation of statistical and collective properties of heavy nuclei using the framework of the configuration-interaction shell model in very large model spaces. We present recent applications of the SMMC method to the calculation of state densities and their collective enhancement factors in rare-earth nuclei.
Citation - WoS: 1
Citation - Scopus: 1
Collectivity in Heavy Nuclei in the Shell Model Monte Carlo Approach
(EDP Sciences, 2014) Özen, Cem; Alhassid, Yoram; Nakada, Hitoshi
The microscopic description of collectivity in heavy nuclei in the framework of the configuration-interaction shell model has been a major challenge. The size of the model space required for the description of heavy nuclei prohibits the use of conventional diagonalization methods. We have overcome this difficulty by using the shell model Monte Carlo (SMMC) method which can treat model spaces that are many orders of magnitude larger than those that can be treated by conventional methods. We identify a thermal observable that can distinguish between vibrational and rotational collectivity and use it to describe the crossover from vibrational to rotational collectivity in families of even-even rare-earth isotopes. We calculate the state densities in these nuclei and find them to be in close agreement with experimental data. We also calculate the collective enhancement factors of the corresponding level densities and find that their decay with excitation energy is correlated with the pairing and shape phase transitions.
Citation - WoS: 49
Citation - Scopus: 54
Crossover From Vibrational To Rotational Collectivity in Heavy Nuclei in the Shell-Model Monte Carlo Approach
(Amer Physical Soc., 2013) Özen, Cem; Alhassid, Yoram; Nakada, Hitoshi
Heavy nuclei exhibit a crossover from vibrational to rotational collectivity as the number of neutrons or protons increases from shell closure towards midshell but the microscopic description of this crossover has been a major challenge. We apply the shell model Monte Carlo approach to families of even-even samarium and neodymium isotopes and identify a microscopic signature of the crossover from vibrational to rotational collectivity in the low-temperature behavior of < J(2)>(T) where J is the total spin and T is the temperature. This signature agrees well with its values extracted from experimental data. We also calculate the state densities of these nuclei and find them to be in very good agreement with experimental data. Finally we define a collective enhancement factor from the ratio of the total state density to the intrinsic state density as calculated in the finite-temperature Hartree-Fock-Bogoliubov approximation. The decay of this enhancement factor with excitation energy is found to correlate with the pairing and shape phase transitions in these nuclei. DOI: 10.1103/PhysRevLett.110.042502
Citation - WoS: 0
Citation - Scopus: 0
Level Densities of Heavy Nuclei in the Shell Model Monte Carlo Approach
(EDP Sciences, 2016) Alhassid, Yoram; Bertsch, George F.; Gilbreth, Christopher N.; Nakada, Hitoshi; Özen, Cem
Nuclear level densities are necessary input to the Hauser-Feshbach theory of compound nuclear reactions. However the microscopic calculation of level densities in the presence of correlations is a challenging many-body problem. The configuration-interaction shell model provides a suitable framework for the inclusion of correlations and shell effects but the large dimensionality of the many-particle model space has limited its application in heavy nuclei. The shell model Monte Carlo method enables calculations in spaces that are many orders of magnitude larger than spaces that can be treated by conventional diagonalization methods and has proven to be a powerful tool in the microscopic calculation of level densities. We discuss recent applications of the method in heavy nuclei.
Microscopic Nuclear Level Densities by the Shell Model Monte Carlo Method
(Cern, 2015) Alhassid, Yoram; Bertsch, George F.; Gilbreth, Christopher N.; Nakada, Hitoshi; Özen, Cem
The configuration-interaction shell model approach provides an attractive framework for the calculation of nuclear level densities in the presence of correlations but the large dimensionality of the model space has hindered its application in mid-mass and heavy nuclei. The shell model Monte Carlo (SMMC) method permits calculations in model spaces that are many orders of magnitude larger than spaces that can be treated by conventional diagonalization methods. We discuss recent progress in the SMMC approach to level densities and in particular the calculation of level densities in heavy nuclei. We calculate the distribution of the axial quadrupole operator in the laboratory frame at finite temperature and demonstrate that it is a model-independent signature of deformation in the rotational invariant framework of the shell model. We propose a method to use these distributions for calculating level densities as a function of intrinsic deformation. © 2015 CERN. All rights reserved.
Citation - Scopus: 0
Microsopic Nuclear Level Densities by the Shell Model Monte Carlo Method
(CERN, 2015) Alhassid, Yoram; Bertsch, George F.; Gilberth, Nichole C.; Nakada, Hitoshi; Özen, Cem
The configuration-interaction shell model approach provides an attractive framework for the calculation of nuclear level densities in the presence of correlations, but the large dimensionality of the model space has hindered its application in mid-mass and heavy nuclei. The shell model Monte Carlo (SMMC) method permits calculations in model spaces that are many orders of magnitude larger than spaces that can be treated by conventional diagonalization methods. We discuss recent progress in the SMMC approach to level densities, and in particular the calculation of level densities in heavy nuclei. We calculate the distribution of the axial quadrupole operator in the laboratory frame at finite temperature and demonstrate that it is a model-independent signature of deformation in the rotational invariant framework of the shell model. We propose a method to use these distributions for calculating level densities as a function of intrinsic deformation.
Citation - WoS: 1
Citation - Scopus: 1
Nuclear Level Density of 161dy in the Shell Model Monte Carlo Method
(EDP Sciences, 2012) Özen, Cem; Alhassid, Yoram; Nakada, Hitoshi
We extend the shell-model Monte Carlo applications to the rare-earth region to include the odd-even nucleus Dy-161. The projection on an odd number of particles leads to a sign problem at low temperatures making it impractical to extract the ground-state energy in direct calculations. We use level counting data at low energies and neutron resonance data to extract the shell model ground-state energy to good precision. We then calculate the level density of Dy-161 and find it in very good agreement with the level density extracted from experimental data.
Citation - WoS: 18
Citation - Scopus: 22
Nuclear State Densities of Odd-Mass Heavy Nuclei in the Shell Model Monte Carlo Approach
(Amer Physical Soc., 2015) Ozen, Cem; Alhassid, Yoram; Nakada, Hitoshi
The shell model Monte Carlo (SMMC) approach enables the microscopic calculation of nuclear state densities in model spaces that are many orders of magnitude larger than those that can be treated by conventional diagonalization techniques. However it has been difficult to calculate accurate state densities of odd-mass heavy nuclei as a function of excitation energy. This is because of a sign problem that arises from the projection on an odd number of particles at low temperatures making it difficult to calculate accurate ground-state energies of odd-mass nuclei in direct Monte Carlo calculations. Here we extract the ground-state energy from a one-parameter fit of the SMMC thermal energy to the thermal energy that is determined from experimental data. This enables us to calculate the state densities of the odd-even isotopes Sm149-155 and Nd143-149 as a function of excitation energy. We find close agreement with state densities extracted from experimental data. Our results demonstrate that the state densities of the odd-mass samarium and neodymium isotopes can be consistently reproduced using the same family of Hamiltonians that describe the neighboring even-mass isotopes within the configuration-interaction shell model approach.
Citation - WoS: 3
Citation - Scopus: 3
Recent Advances in the Application of the Shell Model Monte Carlo Approach To Nuclei
(IOP Publishing Ltd, 2015) Alhassid, Yoram; Bonett-Matiz M.; Mukherjee, A.; Nakada, Hitoshi; Özen, Cem
The shell model Monte Carlo (SMMC) method is a powerful technique for calculating the statistical and collective properties of nuclei in the presence of correlations in model spaces that are many orders of magnitude larger than those that can be treated by conventional diagonalization methods. We review recent advances in the development and application of SMMC to mid-mass and heavy nuclei.
Citation - WoS: 6
Citation - Scopus: 7
Recent Developments in the Shell Model Monte Carlo Approach To Nuclei
(IOP Publishing Ltd, 2012) Alhassid, Yoram; Mukherjee, A.; Nakada, Hitoshi; Özen, Cem
The shell model Monte Carlo (SMMC) approach provides a powerful method for the microscopic calculation of statistical and collective nuclear properties in model spaces that are many orders of magnitude larger than those that can be treated by conventional methods. We discuss recent applications of the method to describe the emergence of collectivity in the framework of the configuration-interaction shell model and the crossover from vibrational to rotational collectivity in families of rare-earth nuclei. We have calculated state densities of these rare-earth nuclei and find their collective enhancement factors to be correlated with the pairing and shape phase transitions. We also discuss an accurate method to calculate the ground-state energy of odd-even and odd-odd nuclei circumventing the sign problem that originates in the projection on an odd number of particles. We have applied this method to calculate pairing gaps in families of isotopes in the iron region.
Citation - WoS: 0
Citation - Scopus: 0
Signatures of Phase Transitions in Nuclei at Finite Excitation Energies
(Amer Inst Physics, 2012) Alhassid, Yoram; Özen, Cem; Nakada, Hitoshi
The mean-field approximation predicts pairing and shape phase transitions in nuclei as a function of temperature or excitation energy. However in the finite nucleus the singularities of these phase transitions are smoothed out by quantal and thermal fluctuations. An interesting question is whether signatures of these transitions survive despite the large fluctuations. The shell model Monte Carlo (SMMC) approach enables us to calculate the statistical properties of nuclei beyond the mean-field approximation in model spaces that are many orders of magnitude larger than spaces that can be treated by conventional diagonalization methods. We have extended the SMMC method to heavy nuclei and used it to study the transition from vibrational (spherical) to rotational (deformed) nuclei in families of rare-earth isotopes. We have calculated collective enhancement factors of level densities as a function of excitation energy and found that the decay of the vibrational and rotational enhancements is well correlated with the pairing and shape phase transitions respectively.