Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1998. / Includes bibliographical references (leaves 88-92). / The first direct measurement of the rate of spin diffusion through a homogeneous sample was performed as an incoherent NMR scattering experiment. The experiment consists of a combination of pulsed gradient spin echo methods with multiple pulse/ pulsed gradient spatial encoding met.hods. The NMR scattering experiment involves the creation of an initial spatial magnetization grating, a period of spin evolution including the displacement of spin magnetization, followed by the detection of the residual magnetization grating. The essence of NMR scattering measurements is to record the extent of microscopic motion of spin magnetization through a sample by directly observing amplitude and phase changes of a well defined spin magnetization grating. The spin diffusion measurement records the rate of destruction of a magneti­zation grating by the random offset of spin magnetization associated with the flip-flop term of the homonuclear dipole-dipole interaction. Since the microscopic motion driven by di- polar coupling is very slow, only fine magnetization gratings are sensitive to the small spatial offsets. Strong pulsed mag­netic field gradient techniques were developed for these studies which generate switched gradients with strengths up to 103T /m (a factor of 100 stronger than those commer­cially available, and a factor of 25 stronger than the highest previously reported). These gradients are able to create a spatial magnetization grating with a pitch of from l[mu]m to 1nm for solid state NMR scattering experiments. Gradients on the order of 200T /m were applied in the spin diffusion measurement experiment. For single crystal CaF2, the measured parallel components of the spin diffusion rates are 7.1 x 10-12cm2 /s along the [0,0,1) direction and 5.3 x 10-12cm2/s along the [1,1,1) direction, in good agreement with theoretical predictions. Additional work has been done on flow measurement. A novel approach is intro­duced to measuring flow velocities using a probe with a spatially varying RF field, and without using other magnetic field gradients. The velocities of the spins are measured as a modulation of the NMR signal from the translatlation of a spatial mag­netization grating through a detection coil with a spatially periodic field profile. Since the same coil can be employed to create the initial magnetization grating, the overall measurement is as simple as recording the signal modulation frequency following a single excitation pulse. The design principles are discussed for a probe that has a spatially periodic field constructed from a series of lumped element x-circuits. Spa­tial modulation of the amplitude or the phase of the RF field can easily be achieved, and either of these may be used t.o characterize a flow field. Examples are shown of measurements of pipe flow using a probe with an amplitude modulated RF field. / by Wurong Zhang. / Ph.D.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1998. / "September 1997." Vita. / Includes bibliographical references (p. 124-132). / A methodology for evaluating environmental management programs using integrated risk communication, assessment and management tools is developed. The main novelty of the methodology lies in the use of decision analysis methods to integrate the wide range of decision objectives which characterize environmental management problems, and risk assessment for impact evaluations under uncertainty, in a framework that emphasizes and incorporates input from stakeholders in all aspects of the process. The outcomes of the analysis are then used to guide the behavioural deliberative process that is engaged to reach a consensual, defensible decision. The first step of the methodology is that of identifying all consequences relevant to the implementation of the decision, i.e. the performance measures. These are identified through a decomposition process based on the use of conditional influence diagrams which allow to incorporate and structure the quantitative a1.1d qualitative issues of the decision problem. Aggregation of the evaluations of the performance measures is done by means of an additive utility function in which single-attribute utilities for the various performance measures are weighed by appropriate measures of their relative importance. The weights of the performance measures are assessed by the pairwise comparison method of the Analytic Hierarchy Process (AHP) applied to the hierarchical structure of the influence diagram. For the determination of the single-attribute utilities we employ a novel approach based on the AHP in which the comparisons are made not on the actual numerical values of the performance measures but, rather, on more intuitive concepts such as 'worst', 'moderate' and 'best'. Within this approach, the innovative introduction of elements of fuzzy logic allows us to account for linguistic imprecision in the expression of the stakeholders' preferences ... / by Enrico Zio. / Ph.D.

In this dissertation, we introduce a new approach in building a hybrid nuclear model that combines some existing theoretical models and a \universal" approximator. The goal of such an approach is to obtain new predictions of nuclear masses and charge radii. We begin our study by investigating nuclear masses based on theoretical and experimental values. Nuclear masses are essential for astrophysical applications, such as r-process nucleosynthesis and neutron-star structure. To overcome the intrinsic limitations of the existing ``state-of-the-art" mass models, a renement is generated based on a Bayesian Neural Network (BNN) formalism. A novel BNN approach is applied with the aim of optimizing mass residuals between theory and experiment. A signicant improvement (of about 40%) in the mass predictions of existing models is obtained after BNN renement. Moreover, these improved results are accompanied by proper statistical errors. By constructing a \world average" of these predictions, we obtained a unied mass model that is used to predict the composition of the outer crust of a neutron star. In order to get a better description of nuclear structure, a similar procedure is also implemented in the nuclear charge radius. A class of relativistic energy density functionals is used to provide robust predictions for nuclear charge radii. In turn, these predictions are rened through the BNN approach to generate predictions for the charge radii of thousands of nuclei throughout the nuclear chart. The neural networks function is trained using charge radii residuals between theoretical predictions and experimental data. Although the predictions obtained with density functional theory provide a fairly good description of the experiment, our results show signicant improvement (better than 40%) after BNN renement. Despite the improvement and robust predictions, we failed to uncover the underlying physics behind the intriguing behavior of charge radii along the calcium isotopic chain. Overall, we have successfully demonstrated the ability of the BNN approach to signicantly increase the accuracy of nuclear models in the predictions of nuclear masses and charge radii. Extension to other nuclear observables is a natural next step in asserting the eectiveness of the BNN method in nuclear physics. / A Dissertation submitted to the Department of Physics in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / Fall Semester 2016. / November 9, 2016. / Bayesian Neural Network, Neutron Star, Nuclear Structure / Includes bibliographical references. / Jorge Piekarewicz, Professor Directing Dissertation; Washington Mio, University Representative; Harrison Prosper, Committee Member; Simon Capstick, Committee Member; Volker Cred´e, Committee Member.

Experimental results on the nuclear structure of 44S and 26Si will be reported in this thesis. 44S is studied because of its interest in understanding how nuclei behave far from stability. 26Si is studied because of the impact of understanding its nuclear structure can have on the astrophysical 25Al(p,γ) reaction rate. These are two very differently motivated studies and will be described separately in Chapters 2 and 3, respectively. Chapter 2 focuses on the exotic N=28 nucleus, in 44S. Previous experiments observed a 4+ state and suggested that this state may exhibit a hindered E2-decay rate, inconsistent with being a member of the collective ground state band. We populate this state via a two-proton knockout reaction from a beam of exotic 46Ar projectiles delivered from the coupled cyclotron facility and measure its lifetime using the recoil distance method with the GRETINA γ ray spectrometer. The result, 76(14) stat (20) syst ps, implies a hindered transition of B(E2; 4+ →2+1 ) = 0.61(19) single- particle or Weisskopf units strength and supports the interpretation of the 4 + state as a K = 4 isomer, the first example of a high-K isomer in a nucleus of such low mass. Chapter 3 focuses on resonances above the proton threshold in 26Si. Previous experiments have solidified the placement of 3 resonances thought to contribute to the 25Al(p,γ)26Si reaction. A fourth resonance has been suggested by various experiments, but more recent experiments have suggested that this level has been misidentified. We populate excited states in 26Si via the 24Mg(3He,n) reaction at 10 MeV at the John Fox Lab at FSU. Neutron time-of-flight spectroscopy is used to identify which resonance is populated in 26Si and the γ-array at FSU is used to determine how these levels de-excite. The γ ray sensitivity in this experiment is the highest sensitivity reached to date, but a 4th resonance above the proton threshold was not identified, giving further indication that this state may have been misidentified by past experiments. / A Dissertation submitted to the Department of Physics in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / Spring Semester 2017. / April 5, 2017. / Includes bibliographical references. / Ingo Wiedenhöver, Professor Directing Dissertation; Henry Fuelberg, University Representative; David Collins, Committee Member; Sam Tabor, Committee Member; Alexander Volya, Committee Member.

No description available.

The parity-violating asymmetry arising from inelastic electron scattering at backward angle (∼ 95??) near the Delta resonance has been measured for both hydrogen and deuterium targets as part of the G 0 experiment. For Q2 = 0.34 (GeV/c) 2 and W = 1.18 GeV, the asymmetries were found to be AHinel=-33.4+/-5 .3stat+/-5 .1sysppm, ADinel=-43.6+/-1 4.6stat+/- 6.2sysppm. .;From the hydrogen asymmetry, the axial transition form factor, GAND , can be extracted. GAND is related to probability of the quark spin-flip that occurs as the proton transitions to the Delta. From the measured asymmetry, the form factor is found to be GAND=-0.046+/- 0.35stat+/- 0.34sys+/- 0.06theory. .;Though GAND has been previously studied using charged current reactions, the G0 measurement represents the first measurement of the asymmetry in the neutral weak sector. These findings agree within errors with the theoretical predictions.

Cross sections for neutron induced alpha production in carbon were measured at seventeen energies between 18 and 22 MeV, using a deuterated anthracene crystal as both target and detector. Pulse shape discrimination was employed to separate the alphas and elastically scattered deuterons from the other reaction products. Published (n,d) elastic scattering data were used as a standard to obtain the alpha production cross sections. Comparison with available measurements shows good agreement

Background: Majority of patients with unilateral hydronephrosis (HN) detected on ultrasound (US) do not require pyeloplasty. Indications for pyeloplasty are in patients with symptomatic obstruction (recurrent flank pain), complications such as urinary tract infection, a drop in differential renal function (DRF) of more than 10% and a progressive increase in the anterior posterior diameter (APD) in subsequent studies. Schlotmann et al, Piepsz et al and Harper et al have demonstrated the measurement of the cortical transit time (CTT) to predict the need for patient who may require pyeloplasty. Objectives: To assess if the CTT would have predicted a drop in DRF in patients with unilateral HN on the affected side. In addition to assess whether the CTT would differ on the first renogram between those patients who had a pyeloplasty and those who did not have a pyeloplasty at the Red Cross War Memorial Children’s Hospital (RCWMCH). Methods: Sixty eight (68) patients with at least two renograms with unilateral HN with a normal contralateral kidney were observed retrospectively between December 2000 and May 2015. The CTT was recorded for the upper, middle and lower third of each kidney and the mean used as the CTT of the kidney. Each renogram was processed three times to measure the DRF using the Rutland Patlak and Integral methods. The mean of the three DRF measurements was used for analysis. Results: The mean CTT of the left and right hydronephrotic kidneys were 6.0minutes and 6.7minutes respectively. A significant relationship was demonstrated in the CTT and DRF as well as CTT and APD in the first renogram of those patients who did not have a pyeloplasty (p < 0.05). There was no difference between the DRF of the first and second renograms in those patients who did not have a pyeloplasty. In the 20 patients who had a pyeloplasty, there was a drop of more than 10% in the DRF of 3 patients. No difference was seen in the DRF or in the CTT between the first and second renogram. The CTT was shorter in the second renogram in 9 of the 20 patients who had a pyeloplasty. No significant difference was found in the CTT or DRF when comparing the group who had surgery against the group who did not have surgery. Conclusion: The current study was unable to demonstrate in our series of patients that CTT can predict those patients who would require pyeloplasty. This may be owing to the retrospective nature of the study and the reliance on the clinical notes for the US data and surgical notes. In future, a prospective study evaluating the relationship between CTT and a drop in the DRF should be undertaken in this unit.

Includes abstract. / Includes bibliographical references. / The iThemba LABS neutron beam facility is currently being used for various applications of fast neutron studies, such as measurements of fission cross sections, the biological effectiveness of high-energy neutrons, calibration of detectors used for dose monitoring in space and aircrafts, and the development of neutron dose monitors. Neutron beams with energies up to 200 MeV are produced at iThemba LABS by irradiating thin targets of 7Li and 9Be with protons from the separated-sector cyclotron. The neutrons are collimated to produce a beam with a diameter of about 50 mm at a flight path of 7.7 m from the target. The collimator geometry is designed to maximize the central part of the beam resulting in a beam with a uniform intensity throughout its diameter and a small penumbra. Secondary neutrons produced from the interactions of the primary charged particles with structural parts e.g. beampipes, shielding wall, target holder, etc. have been observed in the measured neutron fluence spectra. The Monte Carlo radiation transport code FLUKA were used to study the effects of secondary neutrons on the neutron fluence spectra. Results obtained from the calculations were compared with those obtained experimentally.

This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Thesis: Ph. D., Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, 2019 / Cataloged from student-submitted PDF version of thesis. / Includes bibliographical references (pages 205-208). / During the Fukushima Daiichi and Daini accidents, the interactions of multiple units at the same site made accident mitigation more difficult compared to single unit sites. The accidents revealed important multiunit risk sources that are not identified by risk assessments for single-unit sites. Therefore, it's important to obtain an integrated risk evaluation for multiunit sites. However, multiunit accident scenarios are difficult to analyze due to the complexity of multiunit interactions. In the work reported here, a framework capable of analyzing multiunit accident scenarios involving inter-unit interactions is presented. Our framework provides a structured method to analyze accident propagation events, which are not being studied much currently. In addition, our framework is capable of providing accident mitigation and site improvement suggestions that can help improve site safety. / The accident scenarios and risk contributors analyzed in our framework are developed based upon our interviews with Tokyo Electric Power Company (TEPCO) engineers concerning their experiences during the 2011 Fukushima accidents. This first-hand information helps us to better understand multiunit accident scenarios and difficulties in multiunit accident mitigations. In this work, the major steps of our framework are first explained by a simplified two-unit site. The simplified site structure is constructed such that the distractions from overly complex systems are minimized. Additionally, analyses of more risk contributors are illustrated using a relatively complex two-unit site, which illustrates the capability of our framework to analyze complex sites. Even though only a limited number of accident scenarios and risk contributors are illustrated in our work, the capability of the framework goes beyond that. / With proper input information, our framework can be adapted to sites and accident scenarios more complex than those illustrated in our work. Analyzing multiunit risks using our framework can help sites to refine expertise and data and to identify hidden multiunit vulnerabilities and eliminate them in advance. In addition, the risk assessment groups developed during this process can support emergency trainings and risk communications as well as provide risk assessment leadership for utilities. / by Yinan Cai. / Ph. D. / Ph.D. Massachusetts Institute of Technology, Department of Nuclear Science and Engineering