1,720,971 research outputs found
The Texas Cesium Iodide Array for Astrophysical Measurements
Full text linkA novel cesium iodide detector array as been designed for use at the Cyclotron Institute at Texas A&M University (TAMU). Officially named ���The Texas Cesium Iodide Array for Astrophysical Measurements,��� or TexCAAM, its design is intended for use in sub-Coulomb, alpha-transfer astrophysical experiments. Specifically, TexCAAM was designed to collect data in experiments that offer potential solutions to the Cosmological Lithium Discrepancy, as well as experimental observations of processes that could contribute to the formulation of zero metallicity, population-III stars in the early Universe. TexCAAM consists of 32 thallium-doped, cesium iodide scintillation detectors that are arranged to surround a mounted target. Its design has high geometric efficiency, possessing a solid angle coverage of ~90%. To reduce noise, TexCAAM was designed with a mounted 1500-um silicon charged-particle detector that functions as a coincidence gate. Rare isotope beams are available at the TAMU Cyclotron Institute using the Momentum Achromat Recoil Separator. As an efficient gamma spectrometer, TexCAAM can be used to fully characterize the reactions between a beam and a target material. TexCAAM���s construction is complete, and it has undergone energy calibrations for each of its constituent detectors. The efficiency calibration for the array is also complete for low energy gamma rays (< 1.5-2 MeV). TexCAAM has already been used successfully in several nuclear astrophysical experiments, and final calibrations are currently being conducted to characterize the detector setup for higher energy gamma rays. The physics and methodology for the apparatus characterization are presented in this thesis
Particle Detector for Low Energy Heavy Ions
Full text linkThe purpose of this research is to build, calibrate and test a parallel-plate avalanche counter (PPAC) detector, which can be used to detect low energy heavy ion particles. This detector will enable Texas A&M to conduct experiments using low energy heavy ions with the MDM magnetic spectrometer, which was not possible previously. It will be used primarily in experiments in nuclear astrophysics that utilize indirect methods such as the sub-Coulomb transfer reaction and the Trojan Horse method. The expected outcomes are that the detector can separate particles with equal magnetic rigidities, but different mass/charge ratios. It should be able to measure the time of flight for particles as they move from one end of the detector to the other. It should also be able to locate, with sufficient precision, the coordinates of the particle inside the detector so that the actual path which a particle takes can be found
An improved upper limit on the direct 3 α decay of the Hoyle state
Full text linkThe structure of the Hoyle state, a near-threshold 0+ state of extreme astrophysical significance in 12C has long been investigated. An experiment was performed to measure the branching ratio for the decay of this state directly into 3 α-particles. Such a branching ratio is expected to be a good observable for whether the resonance can be described as a dilute gas of α- particles known as an α-condensate. This experiment gave the best upper limits to date for this direct decay via the improvement of the traditionally used DDΦ model to isotropic decay to the available phase space. The new DDP2 model includes three-body penetrabilities and gives a limit of < 0.026% (95% C.L.), a factor of 5 improvement over the previous experimentally obtained limit.</p
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Mean-Field Investigation of Strength Functions of Giant Resonances Compared with the Unexpected Experimental Characteristics
Full text linkWe calculate properties of nuclear giant resonances using Hartree-Fock based Random Phase Approximation theory adopting a Skyrme-type effective interaction. Centroid energies for isoscalar and isovector giant resonances of multipolarities L = 0-3 in ^40,^48Ca, ^68Ni, ^90Zr, ^116Sn, ^144Sm and ^208Pb are obtained for 33 interactions found in the literature. We compare our theoretical results with experimental data and determine the correlation between theoretical centroid energies and each nuclear matter property related to each Skyrme interaction. We obtained strong correlations and agreement with experimental data for the isoscalar giant monopole and quadrupole resonances and the isovector giant dipole resonance (IVGDR).
We determined the best range for the incompressibility coefficient (KvNvM = 210-240MeV), the effective mass (m*/m = 0.7-0.9) and the enhancement coefficient of the energy weighted sum rule of the IVGDR (�� = 0.25-0.70). These constraints, valid across a wide range of masses, may be used in a fit to develop a new energy density functional with improved predictive power. We also performed a similar analysis for two mass regions: A = 90-100 and A = 44-68. Interest recently arose in these regions when we found significant disagreements between experiment and theory for certain isotopes. Therefore, we extended our investigation to determine if other interactions agreed with the experimental results. However, we found that none of the interactions considered reproduced the unexpected experimental characteristics
MEASUREMENT OF ��-DELAYED PROTONS FROM 35K RELEVANT TO THE 34Clg,m(p,��) 35Ar REACTION
Full text linkOne of the most challenging problems in nuclear astrophysics is answering a question about the origin and abundance of elements. There are numerous sites in the Universe where new elements can be created. In this particular work two types of extreme stellar environments are in focus: Novae and X-ray bursts. The peak temperatures achieved during a Nova explosion (ONe-type) is around 0.4*10^9 K, while for an X-ray burst this value can be of order of 10^9 K. Temperature is one of the major factors, which determines how far (by A mass number) the nucleosynthesis can go. While there are numerous reactions to consider in such environments, in this particular study the reaction ^34Cl(p,��)^35Ar was investigated. ^34Cl has a relatively short halflife (1.5264(14) s) and can quickly decay into 34S. However this process competes with a reaction of proton capture 34Cl(p,��)^35Ar, thus leaving a smaller amount of ^34Cl available for the decay to ^34S. The latter, on the other hand, is used for determining isotopic abundances in presolar grains that are extracted from meteorites. Typically, these grains have different isotopic ratios for sulphur than the one found in the Solar System. These differences can say something about the places where the pre-solar grains were synthesized. To be able to accurately predict isotopic abundances in Novae or X-ray bursts, the information about the rate of the reaction 34Cl(p,��)^35Ar needs to be evaluated. That, in turn, requires some knowledge about the resonances in ^35Ar that lie just above proton threshold separation energy (Svp). The fact that only those resonances are important is dictated by the temperatures of the previously mentioned stellar environments. The Gamow window for a temperature of 0.4*10^9 K is located around 433 keV above Svp level in ^35Ar. An indirect method was used for populating the states above the proton separation energy, Svp, in ^35Ar. ^35K undergoes ��+ -decay into ^35Ar and the Q-value of this reaction is sufficiently high to populate states above the Svp level. ^35Ar then decayed into ^34Cl + p. The spectroscopy of the levels decaying by protons was in the main focus of this work. The AstroBoxII is the detector used in this experiment. It allows low background noise measurements with high efficiency. Also HpGe detectors were used in coincidence with the AstroBoxII to be able to accurately distinguish between states in ^35Ar that decay either to the ground state or excited states in ^34Cl that then undergo a ��-emission. The major result of this thesis is the report of a new resonance at 6348(11) keV in ^35Ar. This resonance sits right in the middle of the Gamow window (for T=0.4*10^9 K) and potentially can have a big impact on the reaction rate of ^34Cl(p,��)^35Ar. The estimate for the reaction rate dependence as a function of temperature is given in the Conclusions chapter
Inverse Problems Arising in Medical, LIDAR, and Homeland Security Imaging
No full textSeveral tasks in the fields of medicine and homeland security require the ability to detect weak
signals under stymieing conditions. For instance, in medical imaging single photon emission computed tomography can be used for imaging tumors, but the dosage must be limited to minimize radiation damage to patient tissue. In the field of homeland security, there is significant interest in detecting shielded nuclear sources, e.g. illicit nuclear material being smuggled at a border crossing. Additionally, homeland security and defense agencies are interested in using light detection and ranging (LIDAR) for gathering geospatial intelligence on e.g. targets under forest cover. In this dissertation we discuss recent advances in the application and theory of Compton and LIDAR imaging which address these types of problems.
Compton imaging utilizes the Compton scattering effect to detect high energy photons using
devices called Compton cameras. Compton cameras measure the (weighted) conical Radon transform (CRT) of the radiative source distribution function. The conical Radon transform maps a function to its integrals over surface cones. Compton cameras thus provide partial direction information without any attenuation of the signal, which makes them especially suited for detection of weak signals in the presence of strong noise. Compton cameras therefore have significant potential for use in medical and homeland security applications. In particular we detail recently published results on a convolutional neural network which exhibits high sensitivity and specificity at detecting shielded Uranium-238 embedded in complex cargo configurations. We also obtain a range description of the CRT over the space of smooth functions with compact support, a result of great theoretical importance for future study of the CRT.
A coincidence processing algorithm for Geiger-mode LIDAR developed in collaboration with the Engineer Research and Development Center Geospatial Research Laboratory is detailed. Numerical implementation results of the algorithm are presented with real data collected by the Geospatial Research Laboratory. This algorithm is highly parallelizable and provides high quality 3D images of LIDAR targets
Fabrication of CDMS Dark Matter Detector Using Bi-layer Lift-off Technique and Detection of Axions/Axion Like Particles (Alps) Using CsI(Tl) Scintillator Detectors
No full textFinding the nature of dark matter is one of the biggest quest in physics today. One of the various methods for dark matter detection is direct detection in which the interaction between dark matter and the normal matter is explored. The extremely weak nature of dark matter particles makes direct detection very challenging thereby demanding highly sensitive detectors. SuperCDMS experiment uses cryogenic superconducting Ge and Si as target materials with transition edge sensors (TES) for direct dark matter detection. The classic fabrication technique of CDMS detectors uses chemical etching of aluminum and tungsten thin films deposited on Si and Ge single-crystals to photolitho-graphically fabricate TESs on the detectors. In this work, I have explored another technique called the bi-layer lift-off process which allows the deposition of phonon absorbing aluminum layer di-rectly on the substrate, thereby improving the phonon-collection efficiency in the detectors and allowing for low threshold detection. It also allows us to explore other target materials like sap-phire, GaAs, diamond, SiC, etc. for sub-GeV dark matter and CE��NS searches. Results from a new sapphire detector are discussed.
Null results so far for WIMP dark matter have resulted in increased interest in exploring other dark matter candidates i.e., axions or axion-like-particles (ALPs). Searches for pseudoscalar axion-like particles typically rely on their decay to photons in beam dumps or their conversion into photons in haloscopes and helioscopes. In this thesis, I present the preliminary results from the study of a 45 Kg prototype detector made out of thallium-induced cesium iodide CsI(Tl) scintillator carried out at a 1 MW TRIGA type nuclear reactor situated at the Nuclear Science Center (NSC) at Texas A&M University to search for ALPs which promises lower background using single scatter (veto) technique. Results from a scaled-up experiment consisting of a 30 kg fiducial mass of CsI(Tl) and the background analysis results and projected sensitivity for ALPs with photons, electrons, and nucleons are discussed. Close proximity to the core will have visibility to ALP decays and inverse Primakoff scattering inside the CsI(Tl) crystals, providing the world���s best laboratory-based constraints to the ALP-photon coupling over a wide sub-MeV ALP mass range
Development of RadSigPro - An Open Source Code for Fast and Real Time Radiation Detection
No full textIn this work, an open source code RadSigPro 1.0 is developed and used for fast processing
of nanosecond long pulses from scintillation detectors. The pulse processing and identification
involves pulse height distribution (PHD), pulse shape discrimination (PSD), and time-of-flight
(TOF). For the goal of better particle segregation, processed particle waveforms are supplied to
test machine learning techniques along with TOF labeled neutrons and gamma-rays to train the
data. The code is used to model the programmable logic design of an field programmable gate array (FPGA) design for on-the-fly processing of neutron and gamma-ray pulses, along with testing the results. Finally, a comparison between CAEN���s CoMPASS Data Acquisition (DAQ) Software and RadSigPro���s resulting tallies is attempted.
When trained on pulse waveform data, classification accuracy of 96% could be achieved with
less than 100 ns of data, but 400 ns were required to get the accuracy to 97%. This indicates the
information relevant to labeling a pulse as a neutron or gamma-ray is mostly found at the pulse���s start. Principle component analysis (PCA) extracts information from the entire pulse, so relevant information is not lost when the number of components is trimmed. As a result, support vector machine (SVM) models trained on two principal components could accurately classify pulses over 94% of the time. To achieve 97% accuracy, models with nonlinear kernels required fewer than 50 principal components for training. Misclassification results displayed a 1.97% false gamma-ray rate and a 2.27% false neutron rate.
A weighted average of the percent difference of the results for RadSigPro 1.0 implemented on a
central processing unit (CPU) and an FPGA logic design is calculated. This shows a 0% difference
for the PHD data sets, a 0.458% and 0.344% difference for the designated gamma-detector and
neutron-detector���s PSD data sets respectively, and a 0% difference for the TOF data set. When the FPGA logic design is applied and simulated, it computes the total and tail pulse areas within 5 nanoseconds of the arrival of the final data point used for accumulation and also captures the pulse height value within 2 nanoseconds of the arrival of the pulse maximum���s data point
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