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<div> <div> <div> <p>This dissertation describes work conducted in pursuit of interests in adapting Tension Metastable Fluid Detectors (TMFDs) for dosimetry-related applications with the specific intent of engineering a neutron ambient dose spectrometer. TMFDs possess several charac- teristics desirable for neutron spectrometry, including high efficiencies, complete blindness to gamma and beta radiation, and tailorable-threshold response functions. Prior spectro- scopic work with TMFDs, aptly named Single Atom Spectroscopy (SAS), was constrained to a specific subset of detection fluids who’s composition includes hydrogen and only one other higher Z element (e.g. hydrocarbons), where only one element is assumed capable of initiating a cavitation detection event (CDE). The present work alleviates these restrictions, enabling spectroscopy in detection fluids with multiple constituent elements. </p> <p>Simulating the detector’s response predicates knowledge of the energy necessary for ra- diation induced nucleation, which has been theoretically derived with nucleation theory for superheated fluids, but remains unbeknownst for tensioned metastable states. This limi- tation was overcome using MCNPX-PoliMI to model the spatial recoil nuclei spectra from isotope sources and coupled with SRIM to generate the ion energy deposition probabil- ity density within a critical length scale of each interaction event. Thereafter, the energy deposition threshold necessary to generate a detection event, and corresponding response matrix, was derived empirically by solving for the solution curve that minimizes the residual difference between the measured and simulated count rates. </p> <p>The accuracy of the derived response matrix was evaluated through comparisons with a 6LiI Bonner Sphere Spectrometer in which, for 252Cf and 239PuBe/241AmBe isotope source neutron spectra, the two systems offered results within ±10% of each other for ambient equivalent fluences on the order of 100 μRem/hr fields. Notably, when under ultra-low (10 μRem/hr) fields the Bonner spectrometer and other traditional detectors proved impractical. In contrast, the TMFD system was capable of resolving underlying spectral features and corresponding ambient dose rates within ±5% of MCNP predictions. </p> </div> </div> </div>

Neutron diagnostics will play a fundamental role in future fusion plasma machines,where the harsh environment will make the use of many other type of diagnos-tics practically impossible. Complex techniques to measure the neutron spectrumemitted from tokamk plasmas have been developed over the years, producing stateof the art neutron spectrometers. However, recently compact neutron spectrom-eters have been gaining the interest of the research community. They are muchsimpler to operate and maintain, have lower cost and they can be employed in thechannels of a neutron camera, providing profile measurements. The drawbacks arethat they have a worse resolution and a response to neutrons that is not optimalfor spectroscopy.The goal of the work presented in this thesis is to estimate to which extenta compact detector such as a NE213 liquid scintillator can be used to performneutron emission spectroscopy analysis.The detector used for this study was installed in the back of the MPRu spec-trometer at JET in 2012. The characterization of the response of the detector wasdone using a combination of MCNPX simulations and real measurements. Thedata analysis was performed using the forward fitting approach: a model of theneutron spectrum is produced, then folded with the response of the detector andfinally compared with the data. Two types of plasma scenarios were analyzed, onewith NBI heating only, and another with NBI and third harmonic radio-frequencyheating. In both cases the TOFOR spectrometer was used as a reference to esti-mate the parameters in the model of the neutron spectrum.The results are promising and suggest that neutron spectroscopy can be per-formed with NE213 scintillators although the quality of the results, as given byperformance indicators such as uncertainties, is much lower than the performanceof high resolution spectrometers.

<p>A MATLAB code for calculating neutron energy spectra from JET discharges was developed. The code uses the fuel ion distribution calculated by the computer code SELFO to generate the spectrum through a Monte-Carlo simulation. The calculated spectra were then compared against experimental results from the neutron spectrometer TOFOR. In the calculations, the exact orbits of the fuel ions are taken into account, in order to investigate what effects this has on the spectrum. The reason for this is that, for certain plasma heating scenarios, large populations of fast fuel ions are formed. These fast ions may have Larmor radii of the order of decimeters, which is comparable to the width of the sight line of TOFOR, and may therefore affect the recorded neutron spectrum. A JET discharge with both NBI and 3rd harmonic ICRF heating was analyzed. The results show that the details of the line of sight of the detector indeed affects the neutron spectrum. This effect is probably important for other diagnostics techniques, such as gamma-ray spectroscopy and neutral particle analysis, as well. Good agreement with TOFOR data is observed, but not for the exact same time slice of the discharge, which leaves some questions yet to be investigated.</p>

A MATLAB code for calculating neutron energy spectra from JET discharges was developed. The code uses the fuel ion distribution calculated by the computer code SELFO to generate the spectrum through a Monte-Carlo simulation. The calculated spectra were then compared against experimental results from the neutron spectrometer TOFOR. In the calculations, the exact orbits of the fuel ions are taken into account, in order to investigate what effects this has on the spectrum. The reason for this is that, for certain plasma heating scenarios, large populations of fast fuel ions are formed. These fast ions may have Larmor radii of the order of decimeters, which is comparable to the width of the sight line of TOFOR, and may therefore affect the recorded neutron spectrum. A JET discharge with both NBI and 3rd harmonic ICRF heating was analyzed. The results show that the details of the line of sight of the detector indeed affects the neutron spectrum. This effect is probably important for other diagnostics techniques, such as gamma-ray spectroscopy and neutral particle analysis, as well. Good agreement with TOFOR data is observed, but not for the exact same time slice of the discharge, which leaves some questions yet to be investigated.

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This thesis investigates Raman spectra of novel glasses and their correlation with structure for Raman gain applications. Raman gain for all-optical amplification by fibers depends significantly on the cross section for spontaneous Raman scattering allowing to compare signal strength and spectral coverage. We also investigate the relationship between glass structure and the Boson peak (enhancement of the low-frequency vibrational density of states) and report new inelastic neutron scattering spectra for niobium-phosphate glasses. Polarization resolved Raman spectra of glasses based on tellurite and phosphate formers have been measured from 6 – 1500 cm-1 using an excitation wavelength of 514 nm. The Tellurite glasses exhibit Raman Spectra at least 10 times more intense, are more spectrally uniform and possess spectral bandwidths more than a factor of two wider than fused silica. Assignments of the vibrational bands are presented and the compositional dependence of the spectra is discussed with respect to the molecular structure. Significantly high Boson peaks were found in the frequency range from 30-100 cm-1. The Raman gain curves were calculated from the polarized spontaneous Raman spectra. In particular, they show broad and flat band in the low frequency region (50-400 cm-1) suggesting that these glasses may be useful for Raman gain applications extending to very low frequencies. The inelastic neutron scattering spectra of the niobium-phosphate glasses display a pronounced low-frequency enhancement of the vibrational density of states. By averaging over the full accessible wavevector range we obtain an approximate spectral distribution of the vibrational modes. Through direct comparison with the Raman spectra we determine the Raman coupling function which shows a linear behavior near the Boson peak maximum. Possible mechanisms contributing to the low frequency Raman band such as disorder-induced irregular vibrational states are discussed.

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