The Focal plane Detector Package on the TUNL Split-pole Spectrograph

A focal plane detector for the Enge Split-pole Spectrograph at Triangle Universities Nuclear Laboratory has been designed. The detector package consists of two position sensitive gas avalanche counters, a gas proportionality energy loss section, and a residual energy scintillator. This setup allows both particle identification and focal plane reconstruction. In this paper we will detail the construction of each section along with their accompanying electronics and data acquisition. Effects of energy loss throughout the detector, ray tracing procedures, and resolution as a function of fill pressure and bias voltage are also investigated. A measurement of the $^{27}\!$Al$(d,p)$ reaction is used to demonstrate detector performance and to illustrate a Bayesian method of energy calibration

MESA and NuGrid simulations of classical novae: CO and ONe nova nucleosynthesis

Classical novae are the result of thermonuclear flashes of hydrogen accreted by CO or ONe white dwarfs, leading eventually to the dynamic ejection of the surface layers. These are observationally known to be enriched in heavy elements, such as C, O and Ne that must originate in layers below the H-flash convection zone. Building on our previous work, we now present stellar evolution simulations of ONe novae and provide a comprehensive comparison of our models with published ones. Some of our models include exponential convective boundary mixing to account for the observed enrichment of the nova ejecta even when accreted material has a solar abundance distribution. Our models produce maximum temperature evolution profiles and nucleosynthesis yields in good agreement with models that generate enriched ejecta by assuming that the accreted material was pre-mixed. We confirm for ONe novae the result we reported previously, i.e.\ we found that $^3$He could be produced {\it in situ} in solar-composition envelopes accreted with slow rates (\dot{M} < 10^{-10}\,M_\odot/\mbox{yr}) by cold ($T_{\rm WD} < 10^7$ K) CO WDs, and that convection was triggered by $^3$He burning before the nova outburst in that case. In addition, we now find that the interplay between the $^3$He production and destruction in the solar-composition envelope accreted with an intermediate rate, e.g.\ \dot{M} = 10^{-10}\,M_\odot/\mbox{yr}, by the $1.15\,M_\odot$ ONe WD with a relatively high initial central temperature, e.g.\ $T_{\rm WD} = 15\times 10^6$ K, leads to the formation of a thick radiative buffer zone that separates the bottom of the convective envelope from the WD surface. (Abridged)Comment: 19 pages, 23 figures, 2 tables, accepted to publication by MNRA

Studies of 20 < A < 30 Nucleosynthesis in AGB Stars and Novae

In this thesis, a variety of topics are investigated. Part I discusses asymptotic giant branch (AGB) stars. We review their evolution and their contribution to the galactic chemical evolution. We particularly pay attention to the nucleosynthesis in different layers of the AGB stars, and discuss diverse chains of reactions that can happen under different circumstances. Out of many of such reactions, three are the subjects of our special attention. The 23Na(p,α)20Ne, 23Na(p, γ)24Mg and 26YAl(p, γ)27Si reactions are important reactions that are part of the NeNa and MgAl cycles. Their reaction rates used to be uncertain by orders of magnitude, and thus have been subjects of investigation. Recently, there has been new experimental information released on these reactions. In this project, we have used this new information, and have calculated the new reaction rates for those reactions. The results show less uncertainty range in all three reaction rates compared to the prior measurements. We then have used these new less uncertain rates to calculate the AGB yields of hydrogen through to 62Ni. However, these reaction rates only affect the yields of Ne to Si isotopes noticeably, which are presented in Appendix A. Dr. Karakas has calculated the AGB yields by computing stellar evolution and nucleosynthesis models for a 6 M (symbol) AGB star with three different metallicities (Z = 0.02, 0.004 and 0.008) using the new reaction rates. The results show that the changes in the yields due to individually using the updated 23Na(p, γ)24Mg or 23Na(p,α)20Ne reaction rate are noticeable for some isotopes. However, these new reaction rates result in completely opposite changes in most of the yields; moreover, the updated 26gAl(p, γ)27Si reaction rate has no effect on any of the stellar yields except on the yield of 28 Si obtained by the Z = 0.02 model. Thus, by using all three new reaction rates simultaneously in the nucleosynthesis network, we only see major changes for a few isotopes, e.g. significant destruction of 20Ne and considerable production of 23 Na, 24Mg and 28Si. There is no noticeable effect on any of the remaining AGB yields. Part II of this project discusses the significance of studying the nuclear structure of 26Si and 30S, which are not yet well understood. We discuss classical novae and their nucleosynthesis. We pay attention to some reactions, whose rates are still uncertain, e.g. the 25 Al(p, γ)26 Si, and 29 P(p, γ)30S reactions. To lower the uncertainty range in such reaction rates, the structure of 26Si and 30S should be better understood. We have carried out an experiment at Wright Nuclear Structure Laboratory (WNSL) at Yale University to be able to determine whether or not further studies of the structure of 26Si and 308 can be pursued by the (12C,6He) reaction mechanism. We investigated the 20 NeC2C,6He)26 Si and 12C(24 Mg,6He)30 S reactions. The time for collecting the data for the whole experiment was only about five days. Taking into consideration the number of experiments that were done in five days, some of them resulted in low statistics. The 20 NeC2C,6He)26 Si experiment gave a null result. This is due to the fact that the target that was used was old, and the 20 Ne in that target has been diffused out. Thus, we could not determine whether the (12C,6He) reaction mechanism proves to be a good method to study the structure of 26 Si. As for the nuclear structure of 30 8, we could see the ground state and the first excited state. The time was not enough to collect enough data to be able to determine this structure; however, the (12C,6He) reaction mechanism for studying the structure of 30 S looks promising. ThesisMaster of Science (MSc

Real-time spatial characterization of micrometer-sized X-ray free-electron laser beams focused by bendable mirrors

A real-time and accurate characterization of the X-ray beam size is essential to enable a large variety of different experiments at free-electron laser facilities. Typically, ablative imprints are employed to determine shape and size of μm-focused X-ray beams. The high accuracy of this state-of-the-art method comes at the expense of the time required to perform an ex-situ image analysis. In contrast, diffraction at a curved grating with suitably varying period and orientation forms a magnified image of the X-ray beam, which can be recorded by a 2D pixelated detector providing beam size and pointing jitter in real time. In this manuscript, we compare results obtained with both techniques, address their advantages and limitations, and demonstrate their excellent agreement. We present an extensive characterization of the FEL beam focused to ≈1 μm by two Kirkpatrick-Baez (KB) mirrors, along with optical metrology slope profiles demonstrating their exceptionally high quality. This work provides a systematic and comprehensive study of the accuracy provided by curved gratings in real-time imaging of X-ray beams at a free-electron laser facility. It is applied here to soft X-rays and can be extended to the hard X-ray range. Furthermore, curved gratings, in combination with a suitable detector, can provide spatial properties of μm-focused X-ray beams at MHz repetition rate

measurement of the 30 s α system for type i x ray bursts

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Electron population dynamics in resonant non-linear x-ray absorption in nickel at a free-electron laser

Free-electron lasers provide bright, ultrashort, and monochromatic x-ray pulses, enabling novel spectroscopic measurements not only with femtosecond temporal resolution: The high fluence of their x-ray pulses can also easily enter the regime of the non-linear x-ray–matter interaction. Entering this regime necessitates a rigorous analysis and reliable prediction of the relevant non-linear processes for future experiment designs. Here, we show non-linear changes in the L3-edge absorption of metallic nickel thin films, measured with fluences up to 60 J/cm2. We present a simple but predictive rate model that quantitatively describes spectral changes based on the evolution of electronic populations within the pulse duration. Despite its simplicity, the model reaches good agreement with experimental results over more than three orders of magnitude in fluence, while providing a straightforward understanding of the interplay of physical processes driving the non-linear changes. Our findings provide important insights for the design and evaluation of future high-fluence free-electron laser experiments and contribute to the understanding of non-linear electron dynamics in x-ray absorption processes in solids at the femtosecond timescale

Photon shot-noise limited transient absorption soft X-ray spectroscopy at the European XFEL

Femtosecond transient soft X-ray Absorption Spectroscopy (XAS) is a very promising technique that can be employed at X-ray Free Electron Lasers (FELs) to investigate out-of-equilibrium dynamics for material and energy research. Here we present a dedicated setup for soft X-rays available at the Spectroscopy & Coherent Scattering (SCS) instrument at the European X-ray Free Electron Laser (EuXFEL). It consists of a beam-splitting off-axis zone plate (BOZ) used in transmission to create three copies of the incoming beam, which are used to measure the transmitted intensity through the excited and unexcited sample, as well as to monitor the incoming intensity. Since these three intensity signals are detected shot-by-shot and simultaneously, this setup allows normalized shot-by-shot analysis of the transmission. For photon detection, the DSSC imaging detector, which is capable of recording up to 800 images at 4.5 MHz frame rate during the FEL burst, is employed and allows approaching the photon shot-noise limit. We review the setup and its capabilities, as well as the online and offline analysis tools provided to users.Comment: 11 figure

LEVEL STRUCTURE OF 30S AND THE 29P(p, gamma)30S THERMONUCLEAR REACTION RATE

In order to determine the parent stellar sites for the presolar grains of potential nova origin, it is crucial to know the rates of the thermonuclear reactions which affect the Si production and destruction in novae. One such reaction is the 29P(p, gamma)30S. This reaction also influences type I X-ray bursts. The energy generation and nucleosynthesis in the burst, along with its duration and light-curve structure, are very sensitive to the reaction flow through a few waiting-point nuclei along the rp- and ap-process paths. In particular, network calculations show that the waiting-point nucleus 30S (t1/2 = 1175.9(17) ms) is critical. The structure of proton-unbound 30S states strongly determines the thermonuclear 29P(p, gamma)30S reaction rate at temperatures characteristic of explosive hydrogen burning in classical novae and type I X-ray bursts (0.1 ≤ T ≤ 1.3 GK). Specifically, the rate had been previously predicted to be dominated by two low-lying, unobserved, Jpi = 3+ and 2+ levels in the Ex = 4.7 to 4.8 MeV region. The 3+ resonance was observed a few years ago via a 32S(p, t)30S measurement. However, the 2+ resonance remained unobserved. To search for it, we have performed a higher energy resolution charged-particle spectroscopy and an in-beam gamma-ray spectroscopy to investigate the level structure of 30S above the proton threshold via the 32S(p, t)30S and 28Si(3He, n-gamma)30S reactions, respectively. In this work we provide a description of the experimental setup, data analysis and results of both experiments. Moreover, we have calculated the 29P(p, gamma)30S reaction rate via the state-of-the-art Monte Carlo technique, and have investigated the impact of this updated rate on the abundances of elements synthesized in novae, including those of silicon isotopes.Doctor of Philosophy (PhD