Browsing by Author "Gary E. Mitchell, Committee Chair"

Now showing 1 - 3 of 3

Level Densities and Radiative Strength Functions in 56Fe and 57Fe
(2002-11-13) Tavukcu, Emel; Gary E. Mitchell, Committee Chair
Understanding nuclear level densities and radiative strength functions is important for pure and applied nuclear physics. Recently, the Oslo Cyclotron Group has developed an experimental method to extract level densities and radiative strength functions simultaneously from the primary γ rays after a light-ion reaction. A primary γ-ray spectrum represents the γ-decay probability distribution. The Oslo method is based on the Axel-Brink hypothesis, according to which the primary γ-ray spectrum is proportional to the product of the level density at the final energy and the radiative strength function. The level density and the radiative strength function are fit to the experimental primary γ-ray spectra, and then normalized to known data. The method works well for heavy nuclei. The present measurements extend the Oslo method to the lighter mass nuclei ⁵⁶Fe and ⁵⁷Fe. The experimental level densities in ⁵⁶Fe and ⁵⁷Fe reveal step structure. This step structure is a signature for nucleon pair breaking. The predicted pairing gap parameter is in good agreement with the step corresponding to the first pair breaking. Thermodynamic quantities for ⁵⁶Fe and ⁵⁷ are derived within the microcanonical and canonical ensembles using the experimental level densities. Energy-temperature relations are considered using caloric curves and probability density functions. The differences between the thermodynamics of small and large systems are emphasized. The experimental heat capacities are compared with the recent theoretical calculations obtained in the Shell Model Monte Carlo method. Radiative strength functions in ⁵⁶Fe and ⁵⁷Fe have surprisingly high values at low γ-ray energies. This behavior has not been observed for heavy nuclei, but has been observed in other light- and medium-mass nuclei. The origin of this low γ-ray energy effect remains unknown.
Neutron Capture Reactions on Gadolinium Isotopes
(2010-02-02) Baramsai, Bayarbadrakh; Gary E. Mitchell, Committee Chair; Undraa Agvaanluvsan, Committee Co-Chair; Mohamed A. Bourham, Committee Member; John H. Kelley, Committee Member; Christoper R. Gould, Committee Member
The neutron capture reaction on 155Gd, 156Gd and 158Gd isotopes has been studied with the DANCE calorimeter at Los Alamos Neutron Science Center. The highly segmented calorimeter provided detailed multiplicity distributions of the capture gamma-rays. With this information the spins of the neutron capture resonances have been determined. The new technique based on the statistical pattern recognition method allowed the determination of almost all spins of 155Gd s-wave resonances. The generalized method was tested for s- and p-wave resonances in 94Mo and 95Mo isotopes. The results were compared with previous resonance data as well as results from other methods. The 155Gd(n,gamma)156Gd cross section has been measured for the incident neutrons energy range from 1 eV to 10 keV. The results are in good agreement with other experiments. Neutron resonances parameters were obtained using the multilevel R-matrix code SAMMY. The fitted radiation and neutron widths were compared with the nuclear data library ENDF/B-VII.0 and with a recent experiment at RPI. With the new spin assignments and resonance parameters, level spacings and neutron strength functions were determined for s-wave resonances in 155Gd. The Monte Carlo code DICEBOX was used to simulate the gamma-decay of the compound nuclei 156Gd, 157Gd and 159Gd. The DANCE detector response was taken into account with a GEANT4 simulation. The simulated and experimental spectra were compared to determine suitable model parameters for the photon strength functions (PSFs) and the level density (LD). The shape of the photon strength function which gave the best agreement with the DANCE spectra contained four low-lying Lorentzian resonances, two for the scissors mode and two for the M1 spin flip mode.
Photon Strength Functions and Spin Assignments in 95,96Mo from radiative neutron capture
(2007-05-08) Sheets, Steven; Gary E. Mitchell, Committee Chair
The $ˆ{94,95}$Mo(n,$gamma$) reaction was studied for neutron energies from thermal to 16 keV. The research was performed at the Detector for Advanced Neutron Capture Experiments (DANCE) at the Los Alamos National Laboratory. The primary motivation for these measurements was to test an enhancement observed in the low energy behavior of the photon strength function in $ˆ{93-98}$Mo by the Olso Cyclotron group. The gamma ray distribution and average gamma cascade multiplicity were measured following resonant neutron capture. These spectra were compared to simulations performed with the statistical model code DICEBOX. Results for a variety of photon strength function and level density models were simulated and compared to experiment. Models of the photon strength function with an Oslo-type enhancement below 1 MeV were unable to reproduce experimental spectra. However, good agreement was found between DANCE spectra and simulations that postulate a low-lying resonance in the photon strength function intermediate in strength between the KMF model and an Oslo-type photon strength function. A second experiment to measure the gamma ray spectra from thermal neutron capture at the Rez facility in Prague was performed with results consistent with the DANCE experiment. New spin assignments were made for resonances in $ˆ{95,96}$Mo using the multiplicity of gamma ray cascades and the gamma ray spectral shape. In $ˆ{96}$Mo 56 resonances were observed with 17 resonances given a new spin assignment and 22 resonances given a tentative spin assignment. For $ˆ{95}$Mo a total of thirty nine resonances were observed with 21 given a parity assignment.